Studies in Surface Science and Catalysis
Advisory Editors: B. Delmon and J.T. Yates
VOl. 55
NEW DEVELOPMENTS IN
SELECTIVE OXIDATION
Proceedings of an InternationalSymposium, Rimini, Italy,
September 18-22,1989
Editors
G. Centi and F. Trifiro
Department of Industrial Chemistry and Materials, University of Bologna,
V.le Risorgimento 4, 40 136 Bologna, Italy
ELSEVlER
Amsterdam
- Oxford - New York -Tokyo
1990
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L I b r a r y o f C o n g r e s s Cataloging-In-Publication
Data
New d e v e l o p m e n t s in s e l e c t i v e o w l d a t l o n
proceedlngs o f an
international sympostum. Rimtni, Italy. S e p t e m b e r 18-22. 1989 /
editors. G. Centi and F. Trifiri.
p.
cn. -- ( S t u d i e s in s u r f a c e s c i e n c e and c a t a l y s i s ; 55)
" P a p e r s p r e s e n t e d at t h e I n t e r n a t i o n a l S y m p o s i u m on New
D e v e l o p m e n t s in S e l e c t i v e O x i d a t i o n
o r g a n i z e d by t h e D e p a r t m e n t
of I n d u s t r i a l C h e m i s t r y and M a t e r i a l s o f t h e U n i v e r s i t y of B o l o g n a
in c o l l a b o r a t i o n With t h e C a t a l y s i s G r o u p of t h e I t a l i a n C h e m i c a l
S o c i ety"--Pref .
I n c l u d e s blbllographical references.
I S B N 0-444-88694-X
1 . Oxidation--Congresses.
I. Centl. G. (Gabrielel. 1955- .
11. Trifirb. F. (Ferruccio), 1938. 111. International Symposium
or; PJzh D o v e l o o n e n t s I n S e l c c t i v c C x ! d s t i o n flgP9 R : m * n 1 . I t a l y )
IV. U n i v e r s i t i di Bologna. Dept. o f Industrtal C h e m i s t r y and
Materials. V . S o c i e t i c h i m i c a italiana. C a t a l y s i s Group.
VI. S e r i e s .
T P 1 5 6 . 0 9 N 4 8 1990
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...
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XIV
New Developments in Selective Oxidation
Ximini, Italy, September 18-22,1989
ORGANIZED BY
G m p p Interdivisionale di Catalisi d e b Societa' Chimica Italiana (GIC-SCI)
Consiglio Nazionale delle Ricerche (CNR)
Progetto Finalizzato "Chimica Fine IY del CNR
Universita' di Bologna, Dipartimento di Chimica Industride e dei Materiali
INTERNATIONAL ADVISORY BOARD
B. Dehorn (Belgium)
R.K. Grasselli (USA.)
J. H a k r (Poland)
W. Htilderich (Germany)
0. Krylov ( U S S R . )
H. Mimoun (France)
M. Misono (Japan)
I. Pasquon (Italy)
R.A. Sheldon m e Netherlands)
F. Trifiro' (Italy)
ORGANIZING COMMllTEE
G. Busca, University of Genova, Italy
G. Centi, University of Bologna, Italy
P. Forzatti, Politecnico of Milano, Italy
A. Riva, University of Bologna, Italy
P. Ruiz, University of Louvain-la-Neuve, Belgium
F. Trifiro'. University of Bologna, Italy
A. Vaccari, University of Bologna, Italy
P. Villa, Politecnico di Milano, Italy
SPONSORING
The Organizing Committee gratefully acknowledges fmancial support from:
Air Liquide (France)
Alusuisse Italia (Italy)
BP America (USA)
Carlo Erba Strumentazione (Italy)
Degussa (BRD)
Dutral (Italy)
Enimont (Stabilimento di Ravenna) (Italy)
Eniricerche (Italy)
Hellma Italia Srl (Italy)
* IGI Italiana Gas Industriali (Italy)
Interox (U.K.)
Mitsubishi Kasei Corporation (Japan)
Mobil R&D Corporation (USA)
Monsanto (USA)
* Montedipe (Italy)
National Research Council (CNR) (Italy)
Norsolor - Groupe Orkem (France)
Progetto Finalizzato "Chimica Fine II" of
Repsol Petroleo (Spain)
Rhone Poulenc (France)
-
CNR (Italy)
XI11
Preface
This Volume is a collection of the invited and research papers presented at the International
Symposiumon New Developments in Selective Oxidation held in Rimhi, Italy, Sepember 18-22,1989.
The Symposium was organized by the Department of Industrial Chemistry and Materials of the
Univaity of Bologna in collaboration with the Catalysis Group of the Italian Chemical Society and
under the auspicies of the Italian National Research Council (CNR) and of the project "Chimica Fine
II" of the CNR.
The objectives of the Symposium were to present new developmentsin fundamental research and
in industrial applications of selective oxidation processes. At this meeting various trends were
reflected:
- a wide interest in the selective oxidation of sophisticated substrates, both in the liquid and
vapour phase for the synthesis of fine chemicals;
- growing possibilities offered by the use of light alkanes as feedstocks in selective oxidation
processes;
- new opportunitiesfor fundamental research created by new concepts in reactors;
- promising industrial prospects for the application of new zeolites in the liquid phase;
- a continuing search for new catalytic systems and nontraditional reactions for the
functionalization of substrates by selective oxidation.
Further aims of the Symposium were to (i) bring together specialists of various origins and
backgrounds working in the fields of homogeneous or heterogeneous catalysis and in photo-,
elecauchemical- or more traditional oxidation, to exchange ideas and experiences regarding the use of
different oxidizing agents such as Hz@, 02,NO, as well as of type of substrates (alkanes, alkenes,
aromatics,etc.) ,(ii) discuss and disseminate knowledge in specialized areas of selective oxidation and
(iii) serve as a springboard for new ideas as well as to foster innovation and creativity.
The symposium was attended by over 300 researchers from 30 counmes. More than 50% of the
participants came from the major industries operating in the field, providing a further opportunity for
interchange and cross-fertilization between academic and industrial points of view.
The Editors would like to thank the Authors for the quality of their presentations and for contributing
to rhis Volume. Thanks also are extended to the International Advisory Board and to all referees for the
time and effort spent to ensure the highly scientific level of this Volume.
The Editors also thank the Organizing Committeeand all the Chairmen of the Sessionsfor willingly
giving their time and experhse to the Symposium.
A special thank you is due to Professor Angelo Vaccari and Professor Alfred0 Riva as well as to
all researchers in the Department of Industrial Chemistry and Chemical Engineering (Politecnico
Milano) and of the Institute of Chemistry (Llniversity of Genova), whose invaluable efforts made
possible the conmte realization of the Symposium.
G. Centi and F. Trifiro', Editors
Bologna, December 1989
G. Centi and F. Trifiro’ (Editors), New Developments in Selective Oxidation
1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
1
CATALYTIC OXIDATIONS IN THE MANUFACTUREOF FINE CHEMICALS
Roger A. Sheldon
Andeno B.V.. P.O. Box 81,5900AB VENLO. The Netherlands
SUMMARY
In
recent
years
increasingly
a burdgeoning interest
stringent
in the
environmental
application
of
constraints
catalytic
oxidation
have led to
methods
to
fine chemicals manufacture. Whereas with bulk chemicals the choice of oxidant
is often limited to molecular oxygen the economics of fine chemicals manufacture
allow for a broader choice of primary oxidant. For example, relatively inexpensive
single oxygen donors, such as H202, R02H and NaOCI, in combination with a
variety of metal catalysts provide a wide range of synthetically and economically
useful oxidants. The various types of oxidation processes are outlined on the
basis of
type of
transformation.
catalyst,
primary oxidant,
mechanism and functional group
Since fine chemicals are often relatively complex, multifunctional
molecules the chemo- regio- and stereoselectivity of such processes are emphasized.
Recent developments in catalytic
such
as
(a)
the
use
of
oxidations
with
transition
metal-substituted
liquid
phase
H202 and
oxidation,
oxidations
phase
transfer
in the liquid phase are reviewed,
catalysts
NaOCl in biphasic.
(redox)
(c)
zeolites
as
heteropolyanions
to
facilitate
catalytic
aqueous-organic
mixtures,
(b)
heterogeneous
catalysts
for
as
novel,
oxidatively-stable
ligands, (d) new developments in catalytic asymmetric oxidation.
INTRODUCTION - WHY CATALYTIC OXIDATION?
As a result of increasingly stringent environmental constraints
it is becoming
more and more difficult to carry out industrial scale oxidations with traditional
stoichiometric
oxidants,
such as dichromate,
permanganate. etc.
Consequently,
there is a general trend towards the development of catalytic processes which
do not generate aqueous effluents containing large quantities of inorganic salts.
An illustrative example is the industrial synthesis of hydroquinone (see scheme 1).
2
1
FeIHC1
i"""
I (H+l
H2S04
OH
0
ROUTE
-
CLASSICAL
kg SALTS per kg PRODUCT
CATALYTIC
(1 (Hydroquinone 1 i g h t )
>10
SCHEME 1. Two routes to hydroquinone.
Traditionally
hydroquinone was
manufactured
by
oxidation
of
aniline
with
stoichiometric amounts of manganese dioxide to give p-benzoquinone, followed
by reduction with iron and hydrochloric acid. The aniline was derived from
benzene via nitration and reduction. In the overall process more than 10 kg of
inorganic salts (MnSO4. FeCIp Na2S04, NaCI) are produced per kg of hydroquinone.
In contrast, a more modern route to hydroquinone involves the catalytic oxidation
of p-diisopropylbenzene followed by acid-catalyzed rearrangement of the bis-
hydroperoxide. and produces < l kg of inorganic salts per kg of hydroquinone.
3
In the bulk chemical industry traditional environmentally unacceptable processes
have long been
oxidation
replaced with
cleaner
catalytic
oxidations.
Indeed.
catalytic
is the most widely applied technology for the conversion of
base
chemicals such as olefins and aromatics to commercially important oxygenated
products (see table 1 for examples). Both heterogeneous, gas phase oxidations
and homogeneous, liquid phase oxidations are applied (see table 1).
TABLE 1. Bulk Chemicals manufacture USA (1987).
CHEMICAL
VOLUME
REACTION
CATALYST
I106 tons)
Terephthalic acid
4.0
Ox idat i on
Styrene
CO
Formaldehyde
4.0
3.7
3.0
Ox idat ion
Heterogeneous
Ethylene o x i d e
2.8
Oxidation
Heterogeneous
Phenol
1.6
1.6
Oxidation
Homogeneo us
Carbonyl a t ion
Homogeneous
Ox i d a t ion
Homogeneous
Acryl oni t r i 1e
1.3
1.3
Amnoxldation
Heterogeneous
Vinyl acetate
1.2
Oxi da t ion
Homogeneous
Methanol
Acetic acid
Propylene oxide
Homogeneous
Dehydrogenation Heterogeneous
+
H2
Heterogeneous
In the fine chemicals industry, on the other hand, much smaller volumes are
involved and there has been much less pressure in the past to replace traditional
stoichiometric
oxidants.
But
times
are
rapidly changing.
The fine
chemicals
industry is also under increasing pressure to develop cleaner, more efficient
processes. This is not so surprising when one considers that although the
absolute volumes are significantly less, the number of kilo’s byproducts per kilo
product are generally much higher.
This is partly due to the fact that fine chemicals are often produced via multistep syntheses.
The simplest, cheapest and cleanest oxidant of all is molecular oxygen (dioxygen).
However. the reaction of dioxygen with organic molecules is fraught with several
difficulties :
~Dioxygenhas a triplet ground state which means that its reaction with most organic
molecules is a spin-forbidden process. Consequently, although its reactions with
4
hydrocarbons are thermodynamically favored, they generally exhibit high activation
energies. Once underway, however, they are difflcult to control and the thermodynamically most favored products are carbon dioxide and water.
0
Primary oxidation products (alcohols, aldehydes, epoxides. etc.) are generally more
easily oxidized than the hydrocarbon substrate. Reactions are therefore, often carried
out to low conversions necessitating recycling of large quantities of substrate.
0
Dioxygen is largely indiscriminate, i.e.
shows llttle chemo- or regioselectivity
in its reactions with organic substrates.
Catalytic oxidations of organic molecules with dioxygen can, therefore, be typified
by two extremes (see scheme 2). One extreme is complete oxidation which is of
importance in the context of control of automobile exhaust emissions. The other
extreme is exemplified by the chemo- regio-
and enantioselective hydroxylation
of progesterone mediated by the microorganism, Rhizopus nigricans.
1. COMPLETE OXIDATION
CnH2n+2
+
CPtl
(n+1)02
nC02
+
(n+l)H20
C a t a l y t i c conversion o f exhaust gases
2. SELECTIVE
(PARTIAL) OXIDATION
[Rhizopus nigricens]
>
Chemo-, regio- and s t e r e o s e l e c t i v e 0
0
SCHEME 2. Two extremes.
CHARACTERISTICS OF FINE vs BULK CHEMICALS MANUFACTURE
Although they obviously have many things in common there are several basic
differences
between fine
and bulk chemical processing which can influence
process selection :
0
Substrates are generally complex. multi-functional molecules with limited thermal
stability thus necessitating reaction in the liquid phase at moderate temperatures.
5
Chemo-
regio-
and
stereoselectivity
are
often
important
requirements.
Processing is multi-purpose and batch-wise in contrast to dedicated and continuous
in bulk chemicals. This means that not only raw materials costs but also simplicity
of operation and multi-purpose
character of the installations are important
economic considations (i.e. different ratio of variable t o fixed costs).
TYPES OF OXIDANT
A consequence of the last point is the fact that hydrogen peroxide is, in
principle, the oxidant of choice even though it is more expensive than dioxygen.
Moreover, because of the higher price commanded by the products the choice of oxidant
available (see table 2) t o the fine chemist is obviously much larger than that to
the bulk chemist who is largely limited to dioxygen. Next to price and ease of
handling the two important economic considerations are the nature of the byproduct
and the percentage available oxygen. The former is obviously important in the
context of environmental considerations and the latter generally has a direct
influence on the volume yield (kg product per unit reactor volume per unit time).
Hydrogen peroxide is obviously ’Mr. Clean’, its by-product being water. We note,
however, that the by-product from organic oxidants, such as tert-butylhydroperoxide
(TBHP) and amine oxides, is readily recycled via reaction with hydrogen peroxide.
The overall process produces water as the by-product,
but requires one extra
chemical step compared to the corresponding reactions with hydrogen peroxide.
TABEL 2. Oxygen donors.
DONOR
H202
t-BUOpH
Y. A C T I V E OXYGEN
47.0
BY PRODUCT
17.8
H20
t-BUOH
NaClO
21.6
NaCl
NaCl O2
35.6
13.4
13.7
10.5
29.9**
7.3
NaC 1
NaBrO
‘gH1 lN02*
KHS05
NaI04
PhIO
NaBr
‘gH1 lN0
KHS04
Na I
PhI
*N-Methylmorpholine-N-oxide
**Assuming a l l f o u r oxygen atoms a r e u t i l i z e d
6
With other
inorganic oxygen donors environmental considerations are relative.
Thus, sodium chlorlde (from NaClO or NaC102) and potassium sulfate (from KHS05)
are obviously preferred above chromium, manganese or lead salts.
In addition to the standard examples compiled in table 2 other
interesting
oxygen donors have been described in the recent literature. For example, sodium
perborate (Na2B2[02]2(0H)4.nH20) is an inexpensive bulk chemical (ca. I million
tons per annum) which is used primarily in detergents, bleach and antiseptic
mouthwash. Recently. McKillop and coworkers have reported its use as a selective
oxidant
in organic
corresponding
synthesis.' 82
nitrobenzenes,
For example, aniilnes were oxidized to the
sulfides
to
sulfoxides
or
sulfones,
ketones
to
esters and phenols or hydroquinones t o the correspondlng 1,4-benzoquinones.
Similarly, the use of 'sodium percarbonate' (Na2CO3.3/2HZO2) as a selective oxidant
coworker^.^
has been described by Ando and
Two interesting classes of organic single oxygen donors are the dioxiranes (lJ4
and the oxoammonium salts
(g).5a6 The
former are prepared from KHSOS and an
appropriate ketone (reaction 1) and the latter from a dialkylhydroxylamine and
aqueous NaOCl (reaction 2).
R
\
C=O
R/
t
KHS05
R.(
KHS04
R
R
'N-OH
+
+
+ C10-
+
R\+
c1-
Since the oxidation of organic substrates with
of
(1)and (2)leads
t o the formation
the corresponding ketone and dialkylhydroxylamine. respectively, the latter
may be considered as organic catalysts for oxygen transfer processes with KHS05
and NaOCI, respectively.
TYPES OF CATALYTIC OXIDATION PROCESSES
Catalytic oxidations
may be basically divided into three types based on the
type of reaction involved in the key oxidation step.'18
a. Free radical (aut)oxidation
Catalysis involves the metal ion-induced
decomposition of
H202 or R02H.
In reactions of hydrocarbons with dioxygen this is followed by the classical
autoxidation scheme :
Metal
catalysis
in
R02'
+
RH
+
R02H
R'
+
02
+
R02'
these
reactions
results
+
R'
(31
(41
in
rate
acceleration
but
has
little or no effect on the selectivity.
b. Oxygen transfer
This involves the reaction of an oxygen donor (see above) with an organic
substrate in the presence of a metal (or an organic) catalyst according t o
scheme 3.
CATALYST
OXYGEN DONOR
f
I
ACTIVE OXIDANT
CATALYST
+
SUBSTRATE (S)
PRODUCT
+
REDUCED OXYGEN DONOR
m m
SCHEME 3.Catalytic oxygen transfer.
8
The active oxidant in these processes can be an oxometal or a peroxometal
c
species (see scheme 4). Some metals (e.g. vanadium) can, depending on the
substrate, operate via either mechanism.7
- HX
S
- MOR + SO
M-02R
PEROXOMETAL PATHWAY
MX
+
RO2H
OXOMETAL PATHWAY
S
M=O
-ROH
I
- MX + SO
X
SCHEME 4.
c. Metal ion oxidations
In this class the key step involves the oxidation of
metal ion. Examples include the palladium (11)
the substrate by a
catalyzed oxidation of olefins
(Wacker process) and the oxidative dehydrogenation of alcohols where the
key steps are reactions (5) and (6),
respectively.
RCH-CH2
+
Pd"X2
+
H20
-
RCOCH3
+
Pdo
+
2HX
(5)
The oxidized form of the metal ion is subsequently regenerated by reaction
of the reduced form with the terminal oxidant which could, in principle. be
dioxygen or an oxygen donor. In the latter case this is merely a third type
of oxygen transfer process.
9
Although reactions of the first
applied
in
bulk
type (free radical autoxidation) are widely
chemicals they
are
largely
molecules with one reactive group (e.g. ArCH3
+
confined
to
relatively
simple
ArC02H). The methods of choice
in fine chemicals are, therefore, those involving catalytic oxygen transfer.
EXAMPLES OF CATALYTIC OXYGEN TRANSFER
Catalytic oxygen transfer is a reaction with tremendous scope.'
In addition to
the substantial number of relatively inexpensive oxygen donors which are available
(see earlier)
virtually
all
of
the
transition
metals and several
main group
elements (e.g. Sn, As, Se) can be used as catalysts. Hence, the number of
permutations and combinations is enormous. Probably the most well-known example
is
'
the
catalytic
epoxidation
of
olefins
with
alkyl
hydroperoxides
(reaction
7).94
(Catalyst )
+
CH~CH-CHZ
/O\
+ ROH
: Movl, Wvl, V",
T i I V (Arco)
ROzH
+
Catalyst : Homogeneous
CH~CH-CHZ
Heterogeneous : T i IV/SiO2
R = (CH3)$-
(7)
(Shell)
o r PhCH(CH3)-
The reaction is catalyzed by compounds of high-valent metals such as MovI, WvI,
Vv and Ti".
Molybdenum compounds are particularly effective as homogeneous
catalysts.
heterogeneous
highly
A
effective
and
can
Til'lsilica
be
catalyst
developed
used in continuous,
by
fixed-bed
is
also
operation.
Shell
The
economic importance of reaction 7 is underscored by the fact that it accounts
for more than one million tons annual production of propylene oxide worldwide.
Analogous epoxidations of a wide variety of olefins are readily performed in
hydrocarbon solvents at moderate temperatures (generally 80-120').These R02H-metal
catalyst reagents are particularly useful for chemo-, regio-
and stereoselective
epoxidation~.~The reactions proceed via a peroxometal mechanism (see earlier)
involving
rate-limiting
oxygen
species to the olefin (reaction 8).
transfer
from
an
electrophilic
alkylperoxometal
10
These reagents (metal cataIyst-ROzH or H202) have in recent years been widely
applied to the chemoselective oxidation of alcohols and the regioselective oxidation
of diols. They constitute environmentally attractive alternatives t o the classical
'~,
stoichiometric reagents based on chromium (W). Thus, m ~ l y b d e n u m - ~ ~ -vanadiuml8
and titaniumlg-based
catalysts in combination with TBHP mediate the selective
Oxidation of secondary alcohols. Zirconium, on the other hand, catalyzes the
selective oxidation of primary alcohols to aldehydes (without further oxidation
to
carboxylic
acids) and the chemoselectlve Oxidation of
the corresponding a.8-unsaturated
catalysts
TBHP".
also
mediate
the
allylic alcohols
and cerium21 *23
aldehydes.2o
selective
oxidation
to
of
secondary
alcohols
using
peracetic acid22 or NaBr0321J23 as the oxygen donor. Another excellent
catalyst for both primary and secondary alcohol oxidation is rutheniumz4 which
has been used in conjunction with H2Ozz5, R02H25-27, NaOC128*29, NaBrOgNO. NaIOd3l
N - r n e t h y l m o r p h ~ l i n e - N - o x i d e ~ ~KzS20834,
~~~
Ph10=
and even d i ~ x y g e n ~ ~as
. ~ the
'
terminal oxidant. A few illustrative examples are shown below (reactions 9-13).
TBHP
*
RCH20H
[ZrO (acac) 2]
Ph
4
(11p
RCHO
65-953 y i e l d
TBHP
*
[CrlI1/NAFK]
P
h
81% y i e l d
d
(12)"
POH
NAFK = Nafion
11
TBHP
(13)*'
4
[Ce I "/ NAFK]
98% y i e l d
511 perfluorinated ion exchange r e s i n .
( 14)26
RFCN
0
77-99% y i e l d
In some instances the use of different oxygen donors with the same metal catalyst
can lead t o dramatic changes in chemoselectivity. e.g., 38
[ T i O( acac) 2]
0
[ T i O( acac) 2]
TBHP
(161
I
H
A possible explanation is that the water present in aq. H202 seriously inhibits
epoxidation of the double bond.
The alcohol oxidations outlined above can proceed via peroxometal or oxometal pathways
depending on the catalyst used (see scheme 5). Thus, metals which are strong oxidizing
agents in their highest oxidation state (e.g. Cr"',
Vv.
Gel", Ruv"')
react via
oxometal species whilst weakly oxidizing metal ions (e.g. Mo"', ZrlV, Ti")
involve
peroxometal species in the key oxidative dehydrogenation step. Which mechanism is
operating can be easily demonstrated by carrying out the reaction stoichiometrically
in the absence of terminal oxidant. Systems involving peroxometal species as the
active oxidant will obviously give no reaction under these conditions.
12
-HzO
-HzO
0
H-C-
Rt)
I
H
&
-ROH
PEROXOMETAL
(Movl, T i ' " ,
ZrrV, etc.)
SCHEME 5.
A further variation on this theme is the use of organic oxygen transfer catalysts.
For example, the oxoammonium salts
(2)referred
of primary and secondary alcohols with NaOCI.'
t o earlier catalyze the oxidation
The reactions are carried out in
a two-phase CH2C12-H20 system at O'C and primary alcohols afford the corresponding
aldehydes in high yield. In the presence of a quaternary ammonium salt, as a
phase transfer catalyst, the aldehyde undergoes rapid further oxidation to the
corresponding carboxylic acid. The proposed mechanism'
for the key oxidation
step (scheme 6) in these alcohol oxidations is completely analogous to the
oxometal mechanism outlined in scheme 5.
t
)N=O
+
SCHEME 6.
Another
reaction
of
practical
interest
diols (reaction 17) which is traditionally
is
the
oxidative
cleavage of
vinical
carried out using the stoichiometric
13
reagents, periodate or lead tetra-acetate.
Some of the catalytic oxygen transfer
reagents described above, e.g. V O ( ~ C ~ C ) ~ / T BW042-/P043-/
HP~~,
H20z4’,
H3PW12O40/
HzOZ’~, R u C I ~ / N ~ O Cand
I ~ ~R U C I ~ / H ~ O
have
~ ~ been
~ , successfully applied t o this
reaction,
thus
providing
attractive
alternatives
to
the classical
stoichiometric
reagents.
HP,,c-c
P”/
\
-
Oxygen donor
[Catalyst]
\
,c=o
t
0-c
/
\
An interesting variation on this theme is the recently reported4’
(171
use of ruthenium
pyrochlore oxides (A2+XRuz-X07-y where A is Pb or Bi) as heterogeneous catalysts
for
the
liquid
phase oxidative
*c
cleavage of
vinical diols with
dioxygen. e.g.
in the conversion of cyclohexane-1 ,2-diol to adipic acid :
aq. NaOH
+ 1.5 02
C02Na
C02Na
[Catalyst]
81-873 y i e l d
The reactions were carried out in batch autoclaves or in continuous trickle
bed reactors.
In addition to the epoxidation of olefins mentioned earlier metal catalyst-oxygen
donor
reagents can effect
a variety of
potentially useful transformations
of
olefins (see scheme 7).’s4*
Similarly, reactions of aromatics with oxygen donors can, in principle, afford
products derived from nuclear hydroxylation, side-chain
cleavage of
the aromatic ring. Catalytic oxidation of
the corresponding carboxylic acids (ArCH3
+
oxidation
or oxidative
substituted toluenes to
ArC02H) is relatively straightforward.
There is still a need, however, for good methods (see later) for selective
oxidation t o the corresponding aldehydes (ArCH3
+
ArCHO). The great remaining
challenge in this area is the development of good methods for regioseiective
nuclear hydroxylation (but see later).
14
R
RCH2CHO
ALLYLIC OXIDATION
I
I
t
0
HYDROXYLATION
Oxygen donors o f choice : (a) TBHP ( b ) oZ or TBHP43 (c) N-methylmorpholineN - o ~ i d e( ~d )~ H2OzZ4 (e) NaOClZ4 ( f ) TBHP45
SCHEME 7. Oxidative transformations of olefins.
HYDROGEN PEROXIDE AS OXIDANT-PHASE TRANSFER CATALYSIS
As noted earlier the oxidant of choice in the fine chemicals industry is 30%
H202. Unfortunately, H202 (in common with other useful oxygen donors such as
NaOCI) is insoluble in many common organic solvents. This practical problem
has been overcome by the application of phase transfer catalysis. This involves
the transfer of a water-soluble
ammonium salt.
terminal oxidant
In catalytic
(&a. CIO-.
anion to the organic phase as a quaternary
oxidations
this can involve the transfer
S2OS2-) or the catalyst (Scheme 8).
TRANSFER OF TERMINAL OXIDANT AS ANION
of
the
15
TRANSFEROFCATALYSTASANION
+
R3NO(aq)
-
Q+Ru03-
H ~ o ~ +( ~Q+HMOO~~ )
-
+
R3N
(oxometal )
Q+Ru04
H ~ O + Q+HMOO~-
(peroxometal)
TRANSFEROFCATALYSTlREAGENTASNEUTRALSPECIES
0
e.g.
R4NX'H202;
O\Il/O
L = R3P0, R3N0
O/T\O
L
SCHEME 8. Phase transfer ca alysis in catalytic oxidati ns.
The first example of the application of phase transfer catalysis in a catalytic
oxidation is the ruthenium-catalyzed cleavage of olefins with NaOCl (reaction 19)
reported by Foglia and
coworker^.^'
[RuC 13 / B ~ 4 N Br]
ArC02Na
ArCH3
NaOC1, NaOH,
ClCH2CH2C1/H20
92-98% yield
25"C, p H = 9
More recently Sasson and coworkers4'
applied this technique to the selective
oxidation of deactivated methylbenzenes to the corresponding carboxylic acids
(reaction 20). The same group used a H2O2/RuCI3 system under phase transfer
conditions
aromatics4'
for
the
oxidation
of
alcoholsa.
and the oxidative cleavage of
the
ole fin^.^'
side-chain
oxidation
of
Reaction of styrene with
16
H202/RuC13. for example. afforded benzaldehyde in 64% yield.50 With PdC12 as
catalyst. under the same conditions acetophenone was the major product (56%)50 :
CR4NBrI
PhCHO
[ R u C ~ ~ ] (64%)
PhCH=CH2
+
(21)
H202
P hCOCH3
[PdCl2]
(56%)
The first example of a succesful catalytic epoxidation with aqueous H202 under
phase transfer conditions was reported by Venturello and coworkers5’ :
\ I
+
, C=C ,
[H+/ W042-/P0,3-/QX]
H202
H20/C1 CH2CHzCl
-
\ /O\ /
/c-c\
QX = onium s a l t
Subsequently this and analogous tungsten and molybdenum-based catalysts have
been widely applied to the epoxidation of
ole fin^^'-^^,
the oxidation of alcohols
and the oxidative cleavage of diols54*59i60 in aqueous/organic biphasic systems.
Both simple molybdate and tungstate as well as
Mo- and W-based heteropolyanions
have been employed as catalysts. A typical example of the latter is the H3PM120a
(M-Ma or W)/cetylpyridinium chloride combination which catalyzes the efficient
epoxidation
of olefins and allyllc alcohols under biphasic c o n d l t i o n ~ . The
~~
analogous oxidations of secondary alcohols to ketones and oxidative cleavage
of 1.2-diols. on the other hand, gave the best results under homogeneous conditions
in tert-butanol as solvent54, e.g.
95% y i e l d
17
LIGAND STABILITY, BlOMlMETlC OXIDATION AND HOMOGENEOUS vs HETEROGENEOUS
CATALYSIS
As noted above two factors which have an important influence on the efficiency
and selectivity
of
catalytic
oxidations are the nature of
the metal catalyst
and the primary oxidant. A third important factor is the nature of the ligands
surrounding
the metal ion.
In principle, the steric and electronic
properties
of catalysts can be finely tuned by an appropriate choice of ligand. This is
particularly
important
in
asymmetric
oxidations
(see
later).
Unfortunately,
most organic ligands are unstable in strongly oxidizing media. This is nowhere
more apparent than in the cytochrome P450-dependent monooxygenase enzymes which
catalyze a wide variety of in vivo oxidative biotransformations.61 The prosthetic
group of these enzymes contains an iron (111) porphyrin complex and the active
oxidant is generally accepted to be a high-valent oxoiron (V)porphyrin species.
However, this powerful oxidant is not only capable of oxidizing a wide variety
of organic substrates it can also self-destruct
by oxidative degradation of its
own porphyrin ligand. Hence, cytochrome P450-dependent enzymes are not stable
for
any significant
length of time outside the cell. Because this is a great
disadvantage in the context of practical applications, there have been numerous
studies62
aimed at designing simple model systems capable of effecting the
same, often highly regio-
and stereoselective oxidations. Most of these model
systems involve iron or manganese porphyrin catalysts in combination with single
oxygen donors such as NaOC163. KHS0564 and in a few cases H202.=
Unfortunately, virtually all of these systems suffer from the same disadvantage
as the natural enzyme, i.e. they contain expensive, unstable ligands. There is
a need, therefore, for oxidatively resistant ligands which can stabilize high-valent
oxometal species in the same way that porphyrin ligands can. In principle, this
can be achieved by 'fixing'
the appropriate metal ion in an inorganic matrix
such as a heteropolyacid(anion) or zeolite lattice.
HETEROPOLYACIDS AS OXIDATION CATALYSTS
Heteropolyacids ( H P A ' s ) ~and
~ their salts are polyoxocompounds incorporating anions
(heteropolyanions) having metal-oxygen octahedra (M06) as the basic structural units.
They contain one or more heteroatoms (Si, Ge, P. As, etc.) which are usually located
at the centre of the anion. The M06 octahedra are linked together to form an
18
extremely stable and compact structure for the heteropolyanion. One of the most
-
-
common types of HPA comprises the so-called Keggin anions, XMnl M212-n0aX- (where
M1 Mov', WvI and M2 V"). Despite their rather complex formulae HPA's are very
easy to synthesize by acidification of aqueous solutions containing the heteroelement
and the alkali metal molybdate, tungstate or vanadate. They possess several
rather unique properties whlch make them interesting in the context of (oxidation)
catalysis :
Strong Br4nsted acids
1
Bif unctlonal
Multi-electronxoxidants
catalysts
Soluble in water and oxygenated organic solvents ('soluble oxides')
Transition metal substituted HPAs can be considered as oxidatively resistant
analogues of metalloporphyrins. The HPA anion functions as a multi-electron
ligand and is able to stabilize reactive high-valent oxometal species.
Up till fairly recently applications have been largely limited t o heterogeneous
gas phase transformations6'
but it is becoming increasingly apparent that HPA's
are very useful catalysts for heterogeneous and homogeneous liquid phase oxidations.
In fact they may be considered as 'soluble oxides' and as such form a bridge between
heterogeneous gas phase oxidations and liquid phase homogeneous oxidations.
As discussed earlier some Mo and W-based heteropolyacids have already been used,
in combination with H202 as the primary oxidant under phase transfer conditions,
for a variety of oxidative transformations. Some HPA's. e.g. H 3 P M ~ v ' ~ 2 - n V n v O ~
(PMoV-n). are strong oxidants in their own right and can be used in combination
(PMoV-2) complexed
with dioxygen as the primary oxidant. For example, H5PMol&0a
with tetragiyrne catalyzes the oxidative bromination of organic substrates with
HBrlOp at ambient temperatures in chlorocarbon solvents.68 This reagent was
used for the regioselective para-bromination of phenol (reaction 25).
&
[PMoV-21
+ HBr + Ho2
Tetraglyme
ClCH2CH2C1, 20°C
Br
99% yield
19
These reactions proceed via the following steps :
ZHBr
t E02
PMoV-2
+
H20
t
PMoV-Z(,,)
PMoV-2 also catalyzes the homogeneous liquid phase oxidation of organic sulfides
to the corresponding sulfoxides and sulfones by dioxygen at 100-150' and 9-80
bar .69
The scope of HPAs as oxidation catalysts is further extended by incorporation
of other redox metals. In these systems the HPA anion functions both as a
(multi-electron)
ligand and a co-oxidant.
been used for the Wacker oxidation of
For example, Pdll-HPA catalysts have
ole fin^^^,^^
and the nuclear hydroxylation/
acetoxylation of aromatics with NaOAc and dioxygen in aqueous acetic acid.71
Similarly.
Mnll
and
(R4N)4HMP.W1
Coil-substituted
Coil)
(M=Mnff,
heteropolytungstates
of
general
formula
catalyze the epoxidation of olefins with Ph10"
and the hydroxylation of alkanes with TBHP.73 These reactions bear a close
resemblance t o the cytochrome P450 model systems referred to earlier and the
transition metal substituted polyoxometalates may be considered as oxidatively
resistant inorganic analogues of metalloporphyrins.
On the basis of the above examples we. cor)clude that heteropolyacid-based catalysts have
a very promising future in the synthesis of fine chemicals via selective oxidation.
REDOX ZEOLITES AS SELECTIVE OXIDATION CATALYSTS
Another way of 'fixing'
creating
them
catalysts
into
titanium
variety of
oxidation
a
with
zeolite
silicalite
redox metal ions in stable inorganic matrices, thereby
interesting
For
lattice.
(TS-1).
activities
and
example, the
developed
by
selectivities,
synthetic
is
to
build
titanium(1V) zeolite,
Enichem
catalyzes
a
useful oxidations with 30% H202 such as olefin epoxidation 76,77
,
of
primary alcohols
to
aldehydes7',
aromatic
ammoxidation of cyclohexanone t o cyclohexanone oximem
TS-1-catalyzed
hydroxylation
of
phenol
to
a
1:l
h y d r o ~ y l a t i o n ~ ~and
,
(see Scheme 9). The
mixture
hydroquinone has already been commercialized by E n i ~ h e r n . ~ ~
of
catechol
and
20
OH
OH
R\
R'
c=o
/
RCHO
SCHEME 9. Oxidations catalyzed by titanium silicalite (TS-1).
The TS-1
catalyst
exhibits
some quite
remarkable activities
and selectivities.
Thus, ethylene is epoxidized with 30% H202 in tert-butanol at ambient temperature,
giving ethylene oxide in 96% selectivity at 97% H202 conver~ion.'~Interestingly
TS-1
also catalyzes the rearrangement of styrene oxides to the corresponding
beta-phenyl-acetaldehydes (reaction 29).*l
Ar
R
\ /O\
/
C-CH2
[TS-11
Ar
R
'CHCHO
/
(291
90-98% y i e l d
From a mechanistic viewpoint it is noteworthy that the TS-1
catalyst contains
the same chemical elements in roughly the same proportions as the Shell Ti1"/Si02
catalyst referred to earlier. In the latter case we postulatedg the formation
21
of
catalytically
active,
isolated
titanyl
(Ti-0)
species t o
explain the
unique
activity of this catalyst :
0
The formation of isolated titanyl groups is presumably an important prerequisite
for catalytic performance since TiiV has a strong tendency to assume a high
coordination number via the formation of Ti-0-Ti
bonds. This presumably leads
to the formation of titanium centres which are only capable of catalyzing the
homolytic
decomposition
of
hydrogen
peroxide.
Despite
their
similarities
the
TS-1 catalyst displays a broader range of activities than the Tilv/SiOz catalyst.
The paramount question is then : what is the essential difference between these
catalysts? A possible explanation is that the TS-1 contains more (or more active)
isolated titanyl centres than the T#"ISiOp.
Based on the quite remarkable results obtained with TS-1 we predict a bright future
for the use of redox zeolites, i.e. zeolites modified via isomorphous substitution
of Silv with redox metals in the crystal lattice, as selective oxidation catalysts.
WHAT DETERMINES THE CHEMO- AND REGIOSELECTIVITY?
An understanding of the factors which determine chemo-
and regioselectivities
is of paramount importance in the context of designing selective oxidation catalysts.
As discussed earlier chemoseiectivities are influenced by the nature of the metal,
its surrounding ligands and the primary oxidant.
Regioselectivity is of particular importance in oxidations of hydrocarbons (alkenes,
arenes and alkanes). For example, what determines the extent of allylic vs vinylic
(double bond) attack in olefin oxidations? In the first place, this is influenced
by the nature of the metal (see Scheme 10). High-valent oxometal complexes such
as FeV-O and MnV-O are very strong electrophiles and give predominantly attack
22
at the (nucleophilic) double bond. High-valent oxometal complexes such as MoVl-0
and SelV-O. on the other hand, are weaker electrophiles and give predominantly
attack at the allylic C-H bond. Examples are the gas-phase, bismuth molybdate-catalyzed
oxidation of propylene t o acrolein and the liquid-phase.
Se02-catalyzed
allylic
hydroxylation of olefins with TBHP discussed earlier.
-0
[B i2 M o O c l
>300"C
+
H20
CHO
to2
-
NADH2, 25°C
The key selectivity-determining step :
SCHEME 10. What determines allylic vs vinylic attack?
As has recently been pointed out by Lyons'l
both the Oxidation state of the metal
and the nature of the surrounding ligands are critical factors in determining allylic
vs vinylic attack. Thus. the pailadlum(1l)-catalyzed oxidation of olefins (Wacker
process) t o give aldehydes, ketones or vinyl esters, involves nucleophilic attack
of water on a palladium(l1)-olefin
n
complex. In these reactions carbon-hydrogen
bond activation (8-hydrogen elimination) follows nucleophilic attack.
Pdo catalysts
afford n-allyipalladium(1l) species via oxidative
In
addition
contrast.
of
the
allylic C-H bond to the coordinatively unsaturated palladium (0) centre. In other
words, C-H activatlon precedes nucleophilic attack (see Scheme 11).
23
/Pd\
1
x
1
X
pd\
-HX
H
ROH
OR
I
A
Pd
/
H
OR
\
-HPdX
OR
I
H
SCHEME 11. Allylic vs vinylic oxidation of olefins.
Such mechanistic insights led to the development of methods for the preparation
of
acrylic acid
and
ally1 acetate
via
liquid phase, Pd/C-catalyzed
of propylene. in water and acetic acid respectively, under mild conditions.
H20
6
C02H
88% selectivity
[lo% Pd/C]
t
02
65"C/5 bar
MOAC
HOAc
90-99% selectivity
oxidation
24
In order t o achieve selective allylic oxidation it was necessary t o preactivate
the catalyst by treatment with propylene in the absence of oxygen, presumably
to generate active Pd(0) centres.
Similarly, the oxidation state of the catalyst is a crucial factor in determining
ring vs side-chain
oxidation of aromatics."
Thus, palladium (11) catalyzes the
nuclear oxidative acetoxylation of aromatics by strong oxidants (Cre07'-,
or S208*-)
Mn04-
in acetic acid. In contrast 10% Pd/C is a very effective catalyst for
the side-chain oxidation of aromatics under mild conditions :
I-
(33)
[lo% Pd/C]
1 0 0 W 5 bar
@CH20Ac
(34)
HOAc
THE ULTIMATE CHALLENGE IN SELECTIVITY-ENANTIOSELECTIVE CATALYSIS
As noted earlier much effort has been devoted in recent years towards designing
simple chemocatalysts which are able to emulate Nature's selective and versatile
biocatalysts, the monooxygenases. This has led to the development of a variety
of relatively simple metal catalyst/oxygen donor reagents which can mediate the
same
reactions,
e.g.
olefin
epoxidation.
alkane
hydroxylation.
etc..
as
the
monooxygenases. The ultimate challenge in biomimetic oxidations is. however,
the design of relatively simple chemocatalysts able to emulate the enantioselectivity
characteristic of
the monooxygenase-mediated transformations.
In this context
it is worth noting, however, that Nature is far from perfect. Thus, microbial
expoxidation of propylene mediated by a Nocardia coralline species, for example,
affords R-propylene oxide with 'only' 83%e.e.83
CH$H=CH2
02
L
Nocardia
b..,
0
coral 1 ine B-276
(R) 83% e.e.
(35)
25
Although attempts to achieve biomimetic enantioselective epoxidation of
functionalized
olefins
have so far
met with
non-
little success, quite spectacular
results have been obtained in two related catalytic asymmetric oxidations of
olefinic substrates.
The first
well-known
example of a highly enantioselective catalytic oxidation is the now
catalytic
asymmetric
epoxidation
of
allylic
alcohols
(Scheme
12)
developed by Sharpless and coworkers.84B85 Ttie same titanium(1V)ITBHP reagent
was subsequently applied, by Kagan and coworkersa6, to the enantioselective
catalytic oxidation of sulfides to the corresponding sulfoxides.
D-(-)-DIETHYLTARTRATE
CH,CL,, ( OPr
TBHP/Ti
-20" ) C
b
:LO
R3
70
(1
..
:0 :
-
80% YIELD
90% e.e.
It
L-( +)-DIETHYLTARTRATE
(NATURAL)
SCHEME 12. Catalytic asymmetric epoxidation.
The second important example, also developed by Sharpless and coworkers
87-91,
is the asymmetric vicinal-dihydroxylation of olefins by N-methyl-morpholine-N-oxide
(NMO) in the presence of an Os04 catalyst (0.2-0.4% m) and dihydroquinine or
dihydroquinidine esters as chiral ligands (Scheme 13).
26
DIHYDRoQUINIDINE ESTERS
OH"
"HO
I
w N \ o
70
-
95% YIELD
2
DIHYDRGQUININE ESTERS
-
OH
0
I OH"
"Ho
I
f-b
I
I
R'
1
P-CHLOROBENZOYL; Ar
-
SCHEME 13. Catalytic asymmetric vicinal dihydroxylation.
An
important
prerequisite
for
high
enantioselectivity
in
such
processes
is
that coordination of the metal ion to the chiral ligand results in a substantial
rate acceleration.
Sharpless8'
coined the term ligand-accelerated
catalysis to
describe this phenomenon. Thus, if the metal-chiral ligand complex (M-L chiral)
rapidly
exchanges its
llgands
in solution
then
high
enantioselectivitles
will
be possible only when M-L is a much more active catalyst than M (see Scheme 14).
M
+
L
t
-
chiral
Lchiral
t
achiral catalyst
c h i r a l catalyst
SCHEME 14. Ligand accelerated catalysis.
27
Application
of
this
principle
to
other
transition-metal
catalyzed
oxidations
should lead to the development of other catalytic asymmetric oxidations in the
future. Tartaric acid derivatives and cinchona alkaloidsg2 appear to constitute
attractive ligands in such processes. An interesting variation on this theme
is the use of
quaternary
derivatives
of
cinchona alkaloids as chiral phase
transfer catalysts in the base-catalyzed autoxidation of ketones t o the corresponding
a-hydroxyketones (reaction 36).93
0
0
02, 50% aq. NaOH
(36)
t o 1 uene, (Et0)3P,
Ca t a 1y s t
94% y l e l d
73% e.e.
Catalyst :
CONCLUDING REMARKS
Based on its wide choice of catalysts and oxygen donors, its diversity of mechanism
and its scope in organic synthesis catalytic oxidation is surely the most fascinating
and versatile of all catalytic processes. Moreover, with the added stimulation
of
the need to
replace environmentally inefficient stoichiometric
procedures,
we expect that the application of catalytic oxidation techniques to the manufacture
of fine chemicals will continue to be a very fruitful area of research in the future.
In particular, we have great expectations regarding the broader application of
transition metal substituted zeolites and heteropoly acids as liquid phase oxidation
catalysts and last but not least the development of more catalytic asymmetric oxidations.
28
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29
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63. For leading references see : J.P. Collman, J.I. Brauman, B. Meunier. T.
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4459-4462 (1985).
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P. Battioni. J.P. Renaud. J.F. Bartoli. M. Reine-Artiles. M. Fort and D. Mansuy,
._
J.
Am.
Chem.
SOC.,
110,
8462-8470
1988).
66. For recent reviews see : I.V. Kozhevnikovxuss. Chem. Rev..
8 1-825
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67.M. Misono, Catal. Rev. Sci. Eng., 29, 269 (1987).
68. R. Neumann and I. Assael, J. Chem. SOC. Chem. Commun.. 1285-1287 1988);
see also T.A. Gorodetskaya, I.V. Kothevnikov and K.I. Matveev. Kinet.
Katal. (Engl. Transl.). 23, 842-844 (1982).
69.I.V. Kozhevnikov. V.I. Simagina. G.V. Varnakova and K.I. Matveev. Kinet.
Catal. (Engl. Transl.). 20 416-419 (1979).
S.
E,
30
70. H. Ogawa, H. Fujinami, K. Taya and S. Teratani, J.C.S. Chem. Commun..
1274-1275 (1981).
71. J.E. Lyons, Catalysis Today, 2,245-258 (1988).
72. C.L. Hill and R.B. Brown, J. Am. Chem. SOC.. 108, 536-538 (1986).
73. M. Faraj and C.L. Hill, J. Chem. SOC.Chem. f i m u n . . 1487-1489 (1987).
74. B. Notari. Stud. Surf. Sci. Catal., 37,413-425 (1988).
75. G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito.
129-136 (1986).
Stud. Surf. Sci. Catal..
76. C. Nerl. B. Anfossi. A. Esposito and F. Buonomo, Eur. Pat. Appl., 100.119
(1984) to ANIC; Chem. Abstr., 101,38336f (1984).
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78. A. Esposito, C. Nerl and F. Buonomo, Eur. Pat. Appl.. 102.655 (1984) t o
ANIC; Chem. Abstr.. =,209167n
(1984).
79. A. Esposito, M. Taramasso, C. Neri and F. Buonomo, Br. Pat., 2,116.974
(1985) to ANIC; G. Belussl. M. Clerici. F. Bwnomo, U. Romano, A. Esposito
and B. Notari, Eur. Pat. Appl., 200.260 (1986) t o Enichem.
80. P. Roffia. M. Padovan, E. Morelti and 0. De Alberti. Eur. Pat. Appl., 208.311
(1985) te Montedipe.
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100,209389 (1984).
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Lyons, Homogeneous and Heterogeneous Catalysis". Y. Yermakov and V.
Likholobov, Eds.. VNU Science Press, Utrecht, 1986, pp. 117-138.
(7), 21-26 (1986).
83. K. Furuhashi. Chem. Econ. Eng. Rev.,
84. T. Katsuki and K.B. Sharpless. J. Am. Chem. SOC.. 102. 5976-5978 (1980).
85. For recent reviews see : K.B. Sharpless, Chem. Brit., 38-44 (1986); K.B.
Sharpless, S.S. Woodward and M.G. Finn, Pure Appl. Chem.. 55. 1823 (1983);
M.G. Finn and K.B. Sharpless in 'Asymmetric Synthesis', J.D. Morrison, Ed.,
Academic
Press.
New
York.
1985,
Vol.
5.
Chapter
8.
86. H.B. Kagan, Phosphorus and Sulfur, 27, 127-132 (1986); H.B. Kagan. E. Dunach.
C. Nemecek. P. Pitchen. 0. Samuel and S.H. Zhao. Pure Appl. Chem., 57.
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(1984).
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Am. Chem. SOC.. 110,1968-1970 (1988).
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Am. Chem. SOC.,111.737-739 (1989).
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Chem. SOC.,111,1123-1 125 (1989).
90. I.E. Mark6 lecture presented at the Chiral Synthesis Symposium and
Workshop, organized by Spring Innovations Ltd., in Manchester, April 1989.
91. For further examples of stoichiometric asymmetric dihydroxylatlons with
OsO4 and chirat diamine iigands see : M. Tokles and J.K. Snyder, Tetrahedron
Lett., 27. 3951-3954 (1986); K. Tomioka, M. Nakajima and K. Koga, J. Am.
Chem.xoc., E, 6213-6215 (1987); K. Tomioka, M. Nakajima. Y. litaka and
K. Koga. Tetrahedron Lett., 2,
573-576 (1988).
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in "Topics in Stereochemistry", E.L. Eliel and S. Wilen. Eds., Wiley. New
York. 1986, Voi. 16.
93. M. Masul, A. Ando and T. Shioirl, Tetrahedron Lett., 29.2835-2838 (1988).
a.
18
For further background reading on the subject see the following excellent review
articles :
F.di Furia and 0. Modena, Pure Appl. Chem.,
-6, 51 (1985).
H. Mimoun. Angew. Chem. Int. Ed. Engl.,
54.
U ,734 (1982):
1853 (1982); Rev. Chem. Interm.
31
J. Ruiz (Universith Catholique de Louvain. Belgium) :
Grafting
of
organometallic
compounds
(porphyrins.
phthalocyanines.
etc.)
into
acid supports (zeolites, pillared clays) seems to be a promising approach to
obtain
active
and selective
catalysts
for
the
preparation
of
fine chemicals.
What is your opinion concerning these new materials?
R.A. Sheldon (Andeno B.V., Venlo, The Netherlands) :
As I mentioned in my lecture the problem associated with the use of metal
porphyrins
and
related
complexes
is
the
limited
stability
of
such
ligands
under oxidizing conditions. If their stability could be improved by immobilization
in the pores of zeolites or pillared clays this could be a useful approach.
However,
my
inorganic
matrices
promising
approach.
which
are
personal
opinion
such
as
This
potentially
is
that
zeolites
allows
for
more active
'fixing'
or
redox
metal
heteropolyacids
the
creation
since deactivation
of
of
ions
in
represents
isolated
active
stable
a
more
metal
sites
intermediates
(oxometal. peroxometal) via dimerization is precluded. Moreover, heteropolyanions
(oxometalates) are
multi-electron
ligands
and may be considered as
stable,
inorganic equivalents of porphyrins.
J. Kiwi (Fed. Inst. Technology, Lausanne, Switzerland) :
You have stated that active intermediates in oxidative transformations mediated
by cytochrome P450-dependent mono-oxygenases are oxoiron
(V) species.
Are these
stable, isolated iron (V) species characterized by physical techniques such as
EPR? Or are they hypothetical cyt. P450-Fev-0
species where porphyrin ligands
mediate charge transfer? In such cases only iron (IV) species have been verified
experimentally.
R.A. Sheldon (Andeno B.V.. Venlo, The Netherlands) :
Although the active intermediate in cytochrome P450-mediated oxidations is often
described as a formally oxoiron (V) porphyrin I agree that it is now generally
accepted that this intermediate is more correctly described as an oxoiron (1V)porphyrin radical cation complex (P+.FelV-O). However, it is worth pointing out
that from the point of view of electron counting in oxidation processes it is
often
convenient
to
regard the
intermediate
as being formally oxoiron (V).
Moreover, an oxoiron (IV)-porphyrin radical cation complex is obviously different
to a simple oxoiron (IV) species.
32
J. Haber (Institute of Catalysis and Surface Science, Polish Academy of Sciences,
Krakow, Poland) :
When discussing the prospects of catalytic oxidations we should consider not
only the development of
in your
novel catalytic systems,
review, but also the possibilities of
so spectacularly presented
modifying the electron density
at the metal centre via photostlmulatlon or the application of electric potential.
I think that the combination of organometallic chemlstry with photoelectrocatalysis may open exciting new fields in both research and technology.
and
R.A. Sheldon (Andeno B.V., Venlo. The Netherlands) :
I agree wholeheartedly. Although I did not mention these aspects in my talk
photo- and electrocatalysis are potentially very useful techniques for generating
active catalysts, particularly in the context of fine chemicals manufacture.
0. Kryiov (Institute of Chemlcal Physics, Academy of Sciences of the USSR, Moscow) :
What do you think about the possible future application of intermediate systems
between homogeneous and heterogeneous catalysts. such as catalysts in reverse
micelles?
R.A. Sheldon (Andeno B.V.. Venlo. The Neterlands) :
I think that there is potentially a great future for such systems in catalytic
processes in general. The key advantage of heterogeneous over homogeneous
catalysts is facile recovery and recycling of the catalyst. Much effort has been
devoted in the last two decades, therefore, to immobilizing homogeneous catalysts
on Insoluble, solid supports. This approach has met with little success. There
are other promising approaches, however, which up till recently have not received
much attention. For example. the use of two-phase liquid systems in combination
with a phase transfer
catalyst or the use of catalysts attached t o soluble
polymers, catalysts in reverse micelles, catalytic membranes, etc. I think that
we shall see much more effort devoted to these approaches in the future. Finally,
I would like t o note that we are always talking about immobilizing homogeneous
catalysts; maybe we should think more about 'mobilizing' heterogeneous catalysts.
G. Centi and F. Trifiro’ (Editors), New Developments in Selective Oxidation
1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SELECTIVE OXIDATION
33
WITH TI-SILICALITE
U.Romano,
A.Esposito,
F.Maspero,
C.Neri,
ENICHEM SYNTHESIS, Milano (Italy)
and M.G.Clerici,
ENIRICERCHE, Milano (Italy)
by
* * *
The synthesis and crystal structure of Titanium silicalite-1
(TS-1). a new synthetic zeolite of the ZSM family, has been
reported -‘(Ref.l). Characteristic features of this zeolite are:
the total absence of A 1 atoms and strong acidic sites: the vicariant positions assumed by the Ti atoms in the silicalite framework: the uniform distribution of Ti random in the crystal,
so
that each tetrahedrally coordinated Ti atom is surrounded by four
-0-Si units. This type of distribution has been suggested to be
responsible for the typical reactivity of this material (Ref.2).
- TS-1 is an efficient and selective catalyst for oxidations
with H202. No oxidative reaction was detected using other
peroxides (e.g. tert-butyl , cumene hydroperoxide).
The most important catalytic reactions observed are:
-Alcohol oxidation (Ref.3);
-0lefin epoxidation (Ref.4,5,6,7);
-Aromatic nuclear hydroxylation (Ref.8);
-Amine oxidation:
-Cyclohexanone ammoximation (Ref.9).
Moreover, a competitive pathway of H202 decomposition is normally present during the course of other catalytic reactions: its
amount depends on the type of reaction and experimental conditions.
Further oxidation steps take place in more drastic conditions:
aldehydes a r e oxidized to carboxylic acids; hydroxylated aromatic
compounds may undergo ring opening cleavage.
REACTION OF TS-1 AND H202
After interaction with H202, TS-1 exhibits
a yellow colour,
a modified IR spectrum, with disappearing of the original band at
980 cm-1, and a new, intense EPR signal (Ref.10).
These modifications persist f o r some time, even in the separated zeolite: they cannot be reversed by addition of polar,
donor or protic compounds ( e - g . H20), even in a large excess. The
original material can be obtained upon interaction with oxidable
species (propylene, phenol), or by prolonged staying, whereby a
slow 02 development takes place.
The dependence of the H202 decomposition rates on catalyst
concentration and temperatures has been found.
34
ALCOHOL OXIDATION
The oxidation of methanol at low conversions yields selectively CH20 and dimethyl formale:
no formic acid nor its ester
were detected in these conditions.
The H202 decomposition to 02 is normally about 20-30 % of the
total H202 consumption.
The influence of kinetic parameters (temperature, catalyst
concentration, H202 concentration) has been studied, obtaining an
activation energy of about 20 Kcal/mole, a simple first order
dependence on the catalyst concentration , and a kinetic order
respect to the H202 concentration in the range 0
1.
This
kinetic behaviour is in accord with a 2-steps consecutive process:
-
(1)
(2)
+
--- >
Ti
+
[Ti,H202]
H202
+ CH30H
The formation of
reaction :
(3)
+
CH20
[Ti,H202)
Ti + CH20
+
2 H20
the acetale is a catalytic
->
2 CH30H
CH2(OCH3)2
not
+
oxidative
H20
Primary alcohols have been tested up to C8.
The selectivity to aldehyde is lower than for methanol, depending on the alcohol conversion. At low conversions, 1-propanol
shows a high selectivity to aldehyde and acetale (up to 95 % ) :
+
(4) EtCH2OH
+
(5) Et'CHO
H202
>-
2 EtCH2OH
+
EtCHO
2 H20
EtCH(OCH2Et)2
+
H20
As expected, the selectivity to aldehyde is lower at high
conversions, with increasing contribution of the consecutive
reaction :
+
( 6) EtCHO
H202
->
EtCOOH
+
2 H20
The ester formation appears to be independent on the acid
concentration; possibly, it could be due to a very fast oxidation of the hemiacetalic intermediate:
~
(7) EtCHO + EtCH2OH
->
EtCH(OH)OCH2Et
(+H202)->
EtCOOCH2Et
+ 2
H20
35
The H 2 0 2 decomposition to 0 2 in primary alcohol solution is
in general lower than for methanol ( < 10 % ) .
The influences of kinetics parameters on the oxidation rates
(temperature, catalyst and H 2 0 2 concentrations) were similar to
those found for methanol.
All the primary alcohols tested are oxidized faster than
methanol. Ethanol is the most easily oxidized, and the rates
decrease regularly with increasing chain
length:
while
the
isobutyl term shows a a lower oxidation rate due to the branched
chain.
Secondary alcohols are very selectively oxidized to ketones.
Except for some terms reacting very slowly (cyclohexanol), no
other product can be detected, and also the H 2 0 2 decomposition
is negligible.
Runs at different alcohol concentrations in methanol solutions
gave a first order dependence on the secondary alcohol.
Competition kinetics have been used to compare the rates of
different alcohols. While the oxidation rates decrease regularly
with increasing chain length (2-Butanol : 2-Pentanol = 1.2),
there is a striking effect of the position of the OH group in the
the
chain, (2-Pentanol : 3-Pentanol = 1 3 ) , suggesting that
accessibility of the reactive groups to the
catalytic sites
plays a role in kinetically important step.
Also the very low
rate of cyclohexanol is probably due to its large size, which
makes it difficult to approach a catalytic sites. All these facts
are in accordance with an inner-channel catalysis.
OLEFIN EPOXIDATION
Propylene is selectively epoxidized, since all
found are consecutive ring opening products:
+
(8)
C3H6
(9)
CH3CH-CH2
‘O/
(10)
CH3CH-CH2
(11)
CH3CH-CH2
\O/
b’
H202
->
side-products
C H 3 HH20
%OFH2
+ CH30H
-> C H 3 H- H 2 + CH3CH-CH2
EH g C H 3
bCH3 b H
+
H20
>
CH3qH-CH20H
OH
+
H 2 0 2 ->
C H 3 H-CH20H
etc.
$OH
+
-
In particular, no allylic oxidation product can be detected.
The final selectivities are normally about 98 %
at high H 2 0 2
conversions ( 9 5 % )
36
A solvent is needed to make the solutions homogeneous with
H202. If methanol is used, its oxidation is negligible, as well
as the H202 decomposition to 02.
Similarly to the alcohol oxidation, rates are first order
respect to the catalyst concentration, and of order < 1 respect
to H202.
Unlike in the alcohol oxidation, where no or little
product inhibition effect is observed,
added propylene oxide
does retard the epoxidation process.
The side-products have no
significant inhibition effect.
Olefins higher than C3 exhibit the same reactivity
feature
with regard to selectivity and kinetic effects as propylene
formed (i.e.,
does.
Moreover, no isomerization product is
cis epoxide only is formed from cis olefin):
(12)
R1,
R
P2
8=\4
+
H202
-’
R1,
82
R&
k4
\
,0
+
H20
Competition kinetics were performed in order to compare oxidation rates of different olefins within a strictly homogeneous
series, using methanol as solvent. The reactivity ratios,
as
obtained from the product ratios, are:
cis-2-Butene : 1-Butene : iso-Butene : trans-2-Butene =
= 13 : 4 . 5 : 3.6 : 1
This trend is different from what expected with homogeneous
electrophylic catalysis, which is: iso-Butene > cis-2-Butene >
trans-2-Butene > 1-Butene
Olefins having particularly hindered structures react more
slowly
than expected with homogeneous electrophylic
catalysis: the reactivity ratio of 1-hexene : cyclohexene is about 30.
Unsaturated compounds bearing a second functional group, even
if oxidable, are normally epoxidized. Allyl alcohol and esters
give glycidyl derivatives. Allyl methacrylate is epoxidized in
the ally1 group
A special regioselectivity is exhibited by several poliunsaturated compounds.
In particular,
while the selectivity
in
monoepoxidation in butadiene is not much higher than found
with other catalytic systems, those exhibited by diallyl carbonate
and diallyl ether
are higher than expected with homogeneous catalysis.
.
AROMATIC COMPOUNDS
Aromatic substrates are generally oxidized to phenols or
nol derivatives.
phe-
37
Both regio and chemio selectivity of this reaction depend on
the structure of the substrates and particularly on the presence
of substituent of the aromatic ring.
In the case of benzene, phenol is obtained at low conversions,
otherwise
the
hydroxylation process proceeds
further
to
dihydroxybenzenes. On t h e other side, the hydroxylation in
substituted activated aromatic compounds is selective towards
mono substitution. Products derived from side chain reactions
(toluene, ethyl benzene) have been detected.
A remarkable difference between TS1 and homogeneous catalysis
(Refs.ll,l2) in the isomeric distribution has been found with a
prevalence of para substitution which indicates the existence of
a "restricted transition state selectivity".
In competition tests toluene and benzene have shown similar
reactivities, while a reactivity ratio of i 10 : 1 has been
reported for homogeneous hydroxylation (Ref.13).
Particularly bulky substituents have a considerable retarding
effect (isopropyl). All the deactivated aromatic substrates (e.g.
benzonitrile, clorobenzene, benzoic acid, nitrobenzene) appear to
be non reactive independently on the bulkness of the substituent.
Cresols show the expected prevalence of hydroxy group in
orienting effect.
Aromatic compounds bringing a reactive substituent, hydroxyalkyl or unsaturated, react selectively at the side chain: styrene
is oxidized to phenyl acetaldehyde probably via styrene oxide,
which
is fastly isomerized by TS 1 in
same
conditions
(Refs.13,14). Benzyl alcohol follows the usual alcohols oxidation
pathway as well as 1 phenyl-and 2 phenyl-ethanol.
PHENOL HYDROXYLATION PROCESS
Phenol hydroxylation has been studied in deptht due to the industrial interest for the hydroquinone-cathecol production process.
This reaction is performed in excess of phenol in the presence
of an organic cosolvent and water which is in any case present in
the H202 feed, as well as a reaction coproduct.
Beside hydroxylation, tars are produced in the course of this
reaction together with minor amounts of 02, C02 and organic
acids, due to competitive coupling reactions and consecutive
oxidation of products.
The
dependences of selectivity and para/orto ratio
on
reaction conditions, catalyst concentration, solvent, temperature
and phenol conversion are in accordance with a shape selective
catalysis.
38
A new process for cathecol and hydroquinone production from
phenol has been developed by ENICHEM SYNTHESIS and an industrial
plant of 10.000 tons/year total capacity has been started up in
1986, in the Ravenna factory.
The new process displays a high selectivity at high phenol
conversions, achieved in selected reaction conditions with a high
stationary catalytic efficiency of TS1.
REFERENCES
( 1 ) M.Taramasso, G.Perego, B.Notari, US.Pat.4,410,501 (1983);
M.Taramaso, G.Manara, V.Fattore, B.Notari, U.S.Pat. 4,666,692
(1987); G.Perego,
G.Bellussi, C.Corn0, M.Taramasso, F.Buonuomo,
A.Esposito, in Y.Murakami, A.Tijima, J.W.Ward (Eds.), Proc.
Seventh Int. Conf. on Zeolites, Tokyo 1986, Tonk Kodansha
Amsterdam Elsevier, p.129.
(2) B-Notari, Stud. Surf. Sci. and Catal., 37,(1988), 37.
(3) A.Esposito, C.Neri, F.Buonuomo, U.S.Pat,4,480,135
(1984).
(4) C.Neri, A.Esposito,
B.Anfossi, F.Buonuomo, Eur.Pat. 100.119
(1984).
( 5 ) C.Neri, B.Anfossi, F.Buonuomo, Eur.Pat. 100.118 (1984).
(6) F.Maspero, U.Romano, Eur.Pat. 190.609 (1986).
( 7 ) M.G.Clerici, U.Romano, Eur.Pat. 230949 (1987).
( 8 ) A.Esposito,
M.Taramasso,
C.Neri,
F.Buonuomo, U.K.Pat.
2.116.974 (1985);
A.Esposito, M-Taramasso, C.Neri, U.S.Pat.
4,396,783.
(9) P.Roffia,
M.Padovan, E.Moretti, G.De Alberti, Eur.Pat.
208.311 (1987).
(10) G.Bellussi, G.Perego, A.Esposito, C.Corno, F.Buonuomo, Proc.
of Sixth Con. on Catal., Cagliari 1986, p.423.
(11) R.O.C.Norman, R.Taylor, Electrophylic Substitution in Benzenoid Compounds, ed. C.Eaborn, Elsevier, (1965).
(12) J.Varagnat, 1nd.En.Chem.Prod.Res.Dev. 15-3,(1976),212.
(13) C.Neri, F.Buonuomo, U.S,Pat. 4,495,371 (1985).
(14) C.Neri, f.Buonuomo, U.S.Pat. 4,609,765 (1986)
J.HABER (Institute of Catalysis and Surace Chemistry, Krakow,
Poland):
An obvious question which arises is as to what extent the TS-1
catalyst in oxidation of complex molecules behaves as a zeoliyte.
In homogeneous oxidation you have the solvent, the oxidant and
the oxidized molecule. Can the molecules of the solvent in a
narrow pore of the zeolite be considered as being in the liquid
phase? What happens when in this narrow channel you have a big
molecule of the reactant in vicinity of the molecule of hydrogen
peroxide and the molecule of the solvent. Certainly the degrees
of freedom will be diferent from those in the liquid phase. Or it
is only the outermost surface of the zeolite grains which is
involved in the reaction?
Unlike in homogeneous catalysis, where both H20 and
F-MASPERO :
methanol exhibit a strong inhibitory effect due to the formation
of stable metal complexes, the reactions catalized by TS-1 can be
advantageously carried out in these solvents: this fact strongly
suggests that H20 and CH30H do not form stable complexes with the
metal center, as they do in homogeneous catalysis.
On the other hand, some results are influenced by the nature
of the solvent. An example is the selectivity in the phenol
hydroxylation, which
is higher in water/acetone than in the
aqueous solution. Obviously the zeolitic catalysis is influenced
by the different diffusion parameters for H20, acetone and
substrate, which depend on the organophylic character of TS-1,
similarly to what happens with silicalite. Thus, the actual
dilution of reactants within the pores of zeolite will depend
very much on the nature of the solvent.
J.C.VEDRINE (Institut de Catalyze, Villeurbane, France):
A question already asked yesterday (A2 paper) is related to
the reaction occurring with all reactants within the pores or on
the external surface. If the reaction occurs within the pores the
selectivity in para isomer in phenol hydroxylation should greatly
depend on crystallite size. Did you check it and what was the
result?
Obviously one has to avoid isomerisation (thermodynamical equilibrium) on the surface site after reaction inside in a
batch type reactor.
F.MASPER0 (Enichem Synthesis, S.Donato Milanese, Italy):
Not only the para/orto ratio, but the products yield and the
tars selectivity, and also the overall reaction rate, are greatly
dependent on the crystal size. The change of the para/orto ratio
is mainly due to the decomposition effect on the catechol to form
tars. In the conditions of our experiments no isomerisation is
observed.
Generally speaking, all evidences indicate that the reaction
takes place mainly inside the zeolite pores: but we cannot rule
out the possibility to have an outer surface reaction at a lower
extent.
40
J.M. HERRMANN (Ecole Centrale de Lyon, Ecully Cedex, France) :
I have.found in your results several similarities with hetherogeneous photocatalytic reactions carried out on illuminated
Ti02 at room temperature with air. They concern the oxidation of
alcohols, the epoxidation of propene and the hydroxylation of
aromatic compounds. We proposed that the photocatalytic epoxidation of propene in the gas phase was due to a dissociated atomic
species (see ref. J.M.Herrmann et al. in this book), whereas the
hydroxylation in the aqueous phase was preferentially due to OH.
radicals.
Perhaps, the active species involved in your system
are of the same nature.
Concerning the influencew of temperature, you mention a decrease of activity for T > 90-C. We have observed the same phenomenon f o r the rates of various photocatalytic oxidation reactions
presented in an Arrhenius plot. We have attributed this decrease
to te act that the reactions became limited by the adsorption of
the substrate. Perhaps the same phenomenon occurs in your system.
F.MASPER0:
No doubt that several indications of a radicalic
reactivity can be found in the hydroxylation of aromatic hydrocarbons (see the side-chain oxidation), but the general pattern
of aromatic nuclear hydroxylation is similar to the acid catalyzed reaction (no resorcine in the phenol hydroxylation), with
the superimposed effect of shape-selectivity The fact that some
radicalic species of the aromatic ring are actually observed (epr
spectra) are strongly suggestive, but not conclusive of a radicalic mechanism.
All the general question about mechanism is not completely
defined at this stage.
The lowering of the selectivity at T > 100-C is due to a
decomposition step, rather than to a slower reaction.
.
M.HADDAD (Amoco Chemical Company, Naperville, Illinois,USA):
To shed more light on whether the molecular sieve structure
and the location of the Ti are important for the reported catalysis, have you tested the following catalysts:
1. Titanium Impregnated on sllicalite;
2. The hydrothermal reaction product of Titanium with amorphous silica under conditions which do not promote the formation
of the sieve structure.
F.MASPER0 :
Catalysts containing supported Ti02 both on amorphous silica and on silicalite give much more tars and H202
decomposition in the case of aromatic hydroxylation, and poor
selectivites of epoxidation in the conditions of our experiments
(protic solvents, H202 as oxidant).
41
J.F.BRAZDIL (BP Research, Cleveland, Ohio, USA):
Do Ti-Silicalites have catalytic activity for the oxidation of
C2-C4 paraffins (ethane, propane, butane) with H202? If so, what
are the major products and yields?
F.MASPER0 :
This reactivity has been studied, and will be
published soon.
A.J.PAPA (Union Carbide Corp., Charleston WV, USA):
Were any catalyst spectral structural changes observed during
product inhibition in propylene epoxidation?
F.MASPER0 : Structural changes are totally absent; otherwise,
these could be easily detected after catalyst recovering.
Spectral changes, other than those cited (i.e. after interaction of TS-1 with H202 and after subsequent reaction with the
olefine) were not observed until now; the spectral properties of
the catalyst are unchanged after recovering, except the powder Xray spectra which reveal some pores occlusion, and turn back
identical to the original material ater catalyst regeneration.
J.KIW1 (EPFL, Lausanne, Switzerland):
Do you have any experimental evidence for the Ti(H202) initial
reaction you postulate in your catalytic hydroxylation process?
Have you used o-toluidine or infrared techniques to measure the
amount and type of peroxytitanate formed in the initial step of
hydroxylation?
We have no direct IR evidence of the intermediate
F.MASPER0 :
Ti + H202, just the disappearing of the typical bands present in
TS-1, followed by reappearing after H202 decomposition or after
reaction with oxidable species.
Our hypothesis is based on the general kinetic pattern, which
is consistent with the intermediate formation of an adduct, but
we cannot define it more exactly at this stage.
G.BELLUSS1 (Eniricerche, S.Donato Milanese, Italy):
I would like to add a comment about the question whether the
reactivity takes place on external surface or inside the pores of
TS-1. One of the major properties of TS-1 catalyst is to prevent
secondary reactions. For instance in the phenol hydroxylation the
amount of heavy polinuclear compounds produced by TS-1 is very
low in comparison with Ti-supported silicalite or silica gel.
This is a proof that reactions must take place inside the pores
of TS-1.
G. Centi and F. Trifiro' (Editors),New Developments in Selectioe Oxidation
01990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
CYCLOHEXANONE AMMOXIMATION:
A BREAK THROUGH I N THE
6-CAPROLUCTAM
43
PRODUCTION
PROCESS
P. ROFFIA~, GLEOFANTI~,
A. CESANA
s. TONTIP, P. GERVASUTTI~
'Montedipe S.p.A., Research
Bol 1ate, M i 1ano, I t a l y .
Unit
1
1
, M. MANTEGAZZA , M.
of
Bollate,
Via
PADOVAN',
S. P i e t r o
G. PETRINI
n.
1
,
50, 20021
'Montedipe S.p.A.,
Research Center o f P.to Marghera, V i a D e l l ' E l e t t r i c i t 2 41,
30175 P o r t o Marghera, Venezia, I t a l y .
SUMMARY
As a p a r t o f a r e s e a r c h devoted t o s e l e c t i v e o x i d a t i o n o f o r g a n i c
compounds,
we have s t u d i e d t h e s y n t h e s i s o f cyclohexanonoxime by d i r e c t
ammonia o x i d a t i o n i n presence o f cyclohexanone. By u s i n g t i t a n i u m s i l i c a l i t e
as c a t a l y s t t h e r e a c t i o n t o o k p l a c e i n a smooth and s e l e c t i v e way. The e f f e c t
o f t h e most i m p o r t a n t experimental c o n d i t i o n s was i n v e s t i g a t e d .
The r o l e o f
t h e c a t a l y s t and some p o s s i b l e r e a c t i o n mechanisms were discussed. Our r e s u l t s
p r o v i d e d a p r o m i s i n g s t a r t i n g p o i n t f o r t h e development o f a new t e c h n o l o g y
f o r cyclohexanonoxime s y n t h e s i s .
INTRODUCTION
The development o f t e c h n i q u e s f o r t h e s e l e c t i v e o x i d a t i o n o f p e t r o l e u m
d e r i v a t i v e s under m i l d r e a c t i o n c o n d i t i o n s i s one o f t h e most i m p o r t a n t t a s k s
i n Chemical Science b o t h f o r i t s economic and environmental i m p l i c a t i o n s .
A
number o f commodity and s p e c i a l t y chemicals have been manufactured by
o x i d a t i o n u s i n g t r a n s i t i o n metal compounds as s p e c i f i c c a t a l y s t s ,
which make
s e l e c t i v e r e a c t i o n s o t h e r w i s e i n d i s c r i m i n a t e ( r e f s . 1-5).
P a r t o f o u r r e s e a r c h i n t h e s e l e c t i v e o x i d a t i o n was devoted t o an once
through
synthesis
production,
of
cyclohexanonoxime,
an i n t e r m e d i a t e f o r
caprolactam
by c a u s i n g cyclohexanone and ammonia t o r e a c t i n presence o f an
o x i d i z i n g agent.
Our aim was t o c a r r y o u t a r e a c t i o n ,
( r e f . 61, r e p r e s e n t e d by t h e f o l l o w i n g e q u a t i o n :
known as ammoximation
44
Only two main r e f e r e n c e s r e g a r d i n g ammoximation processes a r e f o u n d i n t h e
literature.
The f i r s t one concerns ammoximation i n t h e l i q u i d phase c a t a l y z e d
by p h o s p h o t u n g s t i c a c i d . T h i s process, developed by Toagosei ,does however have
a number o f disadvantages,
i.e.
s u b s t a n t i a l l o s s o f t h e hydrogen p e r o x i d e due
t o oxygen f o r m a t i o n and r a p i d decomposition o f t h e c a t a l y s t ( r e f . 7 ) .
second process,
c l a i m e d by A l l i e d ,
oxygen on a s i l i c a g e l c a t a l y s t
concerns t h e
( r e f . 6).
gas phase
ammoximation
The
by
The low y i e l d and s e l e c t i v i t y
c a l c u l a t e d on cyclohexanone, as w e l l as t h e r a p i d f o u l i n g and low p r o d u c t i v i t y
o f t h e c a t a l y s t i t s e l f , exclude v a l i d a p p l i c a t i o n s o f t h e c l a i m e d r e s u l t s ,
On choosing t h e o x i d a n t , o u r p r e f e r e n c e was g i v e n t o hydrogen p e r o x i d e be-
cause o f i t s a b i l i t y t o p e r f o r m o x i d a t i o n under m i l d c o n d i t i o n s and i t s c o s t ,
c a l c u l a t e d f o r c a p t i v e use,acceptable f o r t h e cyclohexanonoxime s y n t h e s i s .
employing hydrogen
peroxide
as
oxidant,the
r e a c t i o n model
could
By
be t h e
p e r o x i d i c oxygen t r a n s f e r t o a n u c l e o p h i l i c substrate.Under t h i s p o i n t o f view
the role o f the catalyst
should be t h e weakening o r t h e p o l a r i z a t i o n o f t h e
p e r o x i d i c bond c o n f e r r i n g e l e c t r o p h i l i c p r o p e r t i e s t o t h e o x i d a n t ( r e f .8).
Another p o s s i b i l i t y s h o u l d be t h e c o n f e r r i n g t o t h e hydrogen
peroxide t h e
p r o p e r t i e s o f oxene which i s a b l e t o produce e l e c t r o p h i l i c i n s e r t i o n on t h e
s u b s t r a t e t o be o x i d i z e d ( r e f . 9 ) .
oxygen
causes
two-electrons
I n b o t h cases t h e t r a n s f e r o f a p e r o x i d i c
oxidation
as
required
by
the
ammoximation
reaction.
Among t h e t r a n s i t i o n metal s,used
o r g a n i c s u b s t r a t e s (ref.101,
for
transferring
peroxidic
oxygen t o
t i t a n i u m a t t r a c t e d o u r i n t e r e s t because t i t a n i u m
c a t a l y s t s had been used f o r amines o x i d a t i o n w i t h h y d r o p e r o x i d e s ( r e f s . 11-12)
and t h e oxygenated t i t a n i u m compounds showed d e f i n i t e r e a c t i v i t y t o hydrogen
peroxide (ref.13).
I n p a r t i c u l a r we t r i e d t o use t i t a n i u m s i l i c a l i t e , a
zeolite
d e r i v e d f r o m s i l i c a l i t e by isomorphous s u b s t i t u t i o n o f s i l i c o n by t i t a n i u m
atoms,
because t h i s c a t a l y s t has r e c e n t l y been s u c c e s s f u l l y used f o r o l e f i n s
epoxidation,
a r o m a t i c hydrocarbons h y d r o x y l a t i o n and a l c o h o l s o x i d a t i o n u s i n g
hydrogen p e r o x i d e , even d i l u t e d , as o x i d a n t ( r e f s . 14-15).
45
EXPERIMENTAL
Samples o f t i t a n i u m s i l i c a l i t e were s y n t h e s i z e d a c c o r d i n g t o p a t e n t l i t e r a
t u r e ( r e f . 1 6 ) and c a l c i n e d a t 500"C.The cristallinity,determined by X-ray d i f f r a c t i o n was h i g h e r t h a n 95%;
t h e percentage o f t i t a n i u m determined by atomic
a b s o r p t i o n was i n t h e 1,4+1.6% (wt.)
range and t h e I R s p e c t r a c l e a r l y showed
t h e c h a r a c t e r i s t i c a b s o r p t i o n band produced by t h e i n t r o d u c t i o n o f t i t a n i u m
atoms i n t o t h e z e o l i t e framework.Samples o f H-ZSM-5 and s i l i c a l i t e were s y n t h e
s i z e d f o l l o w i n g t h e procedure r e p o r t e d i n p a t e n t l i t e r a t u r e ( r e f s . 1 7 - 1 8 ) .
The
s i l i c a supported t i t a n i a was o b t a i n e d by i m p r e g n a t i n g a commercial m i c r o 2 -1
s p h e r o i d a l s i l i c a (420 m g ) w i t h a diisopropyl-bistriethanoloamino-titanate
s o l u t i o n and c a l c i n i n g t h e p r o d u c t a t 200°C.
The ammoximation r e a c t i o n was c a r r i e d o u t by d i s p e r s i n g t h e c a t a l y s t i n an
ammonia-cyclohexanone aqueous-organic l i q u i d phase and by f e e d i n g t h e hydrogen
p e r o x i d e t o t h e w e l l - m i x e d s l u r r y . More p r e c i s e l y a g l a s s r e a c t o r , equipped by
a s t i r r e r and a h e a t i n g j a c k e t was p r e l i m i n a r i l y p r e s s u r i z e d by an i n e r t gas.
A f t e r l o a d i n g c a t a l y s t , water, ammonia and
stirred,
s o l v e n t , t h e whole was
vigorously
t h e temperature r a i s e d t o t h e d e s i r e d v a l u e and t h e cyclohexanone
i n t r o d u c e d by a s y r i n g e .
Hydrogen p e r o x i d e was t h e n added u s i n g a m e t e r i n g
pump. A t t h e end, a f t e r c o o l i n g , t h e l i q u i d was analysed by gaschromatography.
RESULTS AND DISCUSSION
Catalysts evaluation
R e s u l t s o b t a i n e d i n ammoximation experiments by d i f f e r e n t c a t a l y s t s a r e r e p o r t e d i n Table 1.
As shown, t i t a n i u m s i l i c a l i t e s u p p l i e d e x c e l l e n t c a t a l y t i c
properties i n the
ammoximation r e a c t i o n . A l s o t i t a n i u m supported on amorphous
s i l i c a showed good c a t a l y t i c a c t i v i t y ,
even though t h e b e s t performances were
o b t a i n e d by t i t a n i u m s i l i c a l i t e .
On t h e c o n t r a r y ,
a l o n e o r non c o n t a i n i n g t i t a n i u m
zeolites,the
by u s i n g amorphous s i l i c a
ammoximation o c c u r r e d o n l y t o a
n e g l i g i b l e e x t e n t , as w i t h o u t any c a t a l y s t , even i f t h e ammonia-cyclohexanone-hydrogen p e r o x i d e system showed a h i g h s e l f r e a c t i n g b e h a v i o u r .
46
TABLE 1
Ammoximation i n water/t-butanol by d i f f e r e n t c a t a l y s t s : c a t a l y s t concentration
2% (wt.); temperature 8OOC; NH /H 0
molar r a t i o 2.0;
r e a c t i o n time 1.5 hours
3 2 2
f o r a l l the runs except f o r t h e starred one e f f e c t e d i n 5 hours
Catalyst
Ti
%
None
-
amorphous
s i 1f c a l it e
H-ZSM-5
T i 02/Si 0
T i l l /Si02*
T i t ? S i 1igal it e
SiO
0
0
0
1.5
9.8
1.5
H 0 /Cy-hexanone
2 2
molar r a t i o
1.07
1.03
1.09
1.08
1.04
1.06
1.05
Cyclohexanone
Oxime y i e l d
Conv. Oxime Select. based on H202
%
%
w
53.7
55.7
59.4
53.9
49.3
66.8
99.9
0.6
1.3
0.5
0.9
9.3
85.9
98.2
0.3
0.7
0.3
0.4
4.4
54.0
93.2
Choice o f solvent
A s u i t a b l e r e a c t i o n medium should be a good solvent f o r both t h e reagents
and the r e a c t i o n product and have a good s t a b i l i t y t o hydrogen peroxide.
The r e s u l t s obtained operating by d i f f e r e n t solvents considered s u i t a b l e
for our r e a c t i o n are reported i n Table 2.
our requirements,even
T-butanol proved t o f i t very w e l l
i f other solvents were used w i t h s i m i l a r r e s u l t s .
water-alcohol mixture (weight r a t i o 1 : l ) showed a good
A
solvent power f o r both
cyclohexanone and cyclohexanonoxime and was stable during t h e reaction.
By
working i n such a s o l u t i o n t h e conversion (about 90%) and the s e l e c t i v i t y
(96-99%) o f t h e cyclohexanone t o cyclohexanonoxime were very high f o r a l l
t e s t e d c a t a l y s t s , as well as t h e oxime y i e l d s based on the oxidant (89-95%).
The r e p r o d u c i b i l i t y o f the r e s u l t s was very good.
47
TABLE 2
Ammoximation in d i f f e r e n t so1vents:temperature 80°C; NH3/H202 molar r a t i o 2.
Catalyst H 0 /Cy-hexanone
Cyclohexanone
Oxime y i e l d
Conv. Oxime S e l e c t . based on H,O,
'mglar r a t i o
Solvent
Benzene
To1 uene
t-aniyl alcohol
H O/t-butanol
2
,I
I,
8,
1.03
1.07
0.86
0.95
0.94
0.95
0.97
A 28
A 28
A 28/2
A 28/2
A 30/1
A 30/2
A 30/3
99.7
99.8
94.5
88.8
89.7
87.8
89.6
95.0
97.0
95.6
99.5
99.5
99.5
96.4
91.7
90.0
94.0
93.4
94.9
92.1
89.4
Choice of temperature
The r e s u l t s obtained a t temperatures between 60 and 95°C a r e recorded in
Table 3. A t 80 and 95°C t h e cyclohexanone s e l e c t i v i t y t o oxime and i t s y i e l d
based on hydrogen peroxide were similar and in both cases very high.
When
the reaction was performed a t 70°C a reduction in the oxidant y i e l d was
observed,
while a t 60°C
a considerable competition of non c a t a l y t i c
reactions decreased both s e l e c t i v i t y and y i e l d .
TABLE 3
Ammoximation i n water/t-butanol
a t d i f f e r e n t temperatures:
NH3/H202 molar
ratio 2.
Catalyst H 0 /Cy-hexanone Temperature
2 2
A 2812
A 28/2
k 28/2
A 2812
molar r a t i o
"C
0.93
0.97
0.83
0.88
60
70
ao
95
Cyclohexanone
Oxime y i e l d
Conv. Oxirne Select. based on H 0
I
ol
a,
2 2
81.5
90.2
80.1
83.0
87.0
96.4
98.8
99.9
76.4
89.4
95.0
94.0
Concentration of the c a t a l y s t
As i n any catalyzed reaction, t h e c a t a l y s t concentration revealed a great
importance i n determining t h e
ammoximation trend,
The useful
catalyst
concentration i s determined by t h e need t o produce s i g n i f i c a n t reaction r a t e
with low reagents concentration t o avoid s i d e reactions.
Tests performed w i t h d i f f e r e n t c a t a l y s t concentrations
Table 4:rhe
a r e recorded i n
r e s u l t s obtained using a 30 g / l concentration o f titanium s i l i c a
48
l i t e were e x c e l l e n t f o r
b o t h s e l e c t i v i t y and yield,whereas
the results
o b t a i n e d w i t h a 10 g / l c o n c e n t r a t i o n i n d i c a t e d a c o n s i d e r a b l e decrease i n
t h e cyclohexanone s e l e c t i v i t y and i n t h e o x i d a n t y i e l d t o t h e oxime.
TABLE 4
Ammoximation
in
water/t-butanol
temperature 8C"C;
Catalyst
g/l
A 28
A 28
A 28
A 30
A 30
with d i f f e r e n t
catalyst
concentrations:
NH3/H202 m o l a r r a t i o 2.
H 0 /Cy-hexanone
2mhar r a t i o
30
15
10
10
10
Cyclohexanone
Oxime y i e l d
Conv. Oxime S e l e c t . based on H202
%
%
%
98.2
96.9
86.0
84.0
82.6
99.9
98.9
81.4
1.05
1.05
1.04
1.05
1.05
78.4
76.3
93.2
91.6
67.0
62.5
60.0
AMMOXIMATION MECHANISM
The use o f t i t a n i u m s i l i c a l i t e as c a t a l y s t ,
has been t h e s p e c i f i c and
d e t e r m i n i n g f a c t o r f o r p e r f o r m i n g e x c e l l e n t ammoximation y e l d . I n o u r o p i n i o n
t h e a c t i v i t y and s e l e c t i v i t y o f t h i s c a t a l y s t i n t h e s t u d i e d r e a c t i o n a r e
the result
3f
a synergism between t h e presence o f i s o l a t e d t i t a n i u m i n a
c o o r d i n a t i v e s t a t e d i f f e r e n t t h a n usual and t h e z e o l i t e framework o f which
t i t a n i u m i s a c o n s t i t u e n t component and a c a t a l y t i c a l l y a c t i v e s i t e .
Tests performed w i t h s i l i c a l i t e h a v i n g t h e same s t r u c t u r e as t h e t i t a n i u m
s i l i c a l i t e showed t h e c a t a l y t i c a c t i v i t y i n ammoximation was n o t caused by
the
zeolitic
H-ZSM-5,e
structure.
A
similar
negative
r e s u l t was
obtained using
z e o l i t e derived from s i l i c a l i t e by p a r t i a l s u b s t i t u t i o n o f s i l i c o n
atoms w i t h aluminum atoms.
On t h e o t h e r hand,samples
p r e p a r e d by s u p p o r t i n g
t i t a n i u m d i o x i d e on s i l i c a w i t h a l a r g e s u r f a c e area p o i n t e d o u t a good
. c a t a l y t i c a c t i v i t y a l t h o u g h lower t h a n t h a t o f t h e t i t a n i u m s i l i c a l i t e .
The e x p e r i m e n t a l
evidence ( s e e T a b l e 11 suggests t h a t t h e t i t a n i u m main
r o l e i n amnoxination i s t o promote t h e s e l e c t i v e o x i d a t i o n o f t h e ammonia
n i t r o g e n t h r o u g h a c t i v a t i o n o f t h e hydrogen p e r o x i d e .
itself
The hydrogen p e r o x i d e
i s n o t a b l e t o produce ammoximation i f n o t i n t r a c e q u a n t i t i e s ,
because i n t h e b a s i c
r e a c t i o n environment
by-produces n i t r i t e s and n i t r a t e s ( r e f . 8 ) .
i t decomposes t o oxygen
and
It i s p o s s i b l e t h e i n t e r a c t i o n
49
between o x i d a n t and c a t a l y s t i s
s i m i l a r t o t h a t suggested f o r t h e r e a c t i o n
between p o r p h y r i n i c t i t a n i u m compounds and hydroperoxides i n t h e o x i d a t i o n
o f o l e f i n s o r d i a l k y l s u l f i d e s (refs.19-20).
i n t e r a c t i o n and
activation o f
It i s p o s s i b l e t h e r e f o r e t h i s
hydrogen p e r o x i d e can t h u s o c c u r t h r o u g h
s u b s t i t u t i o n o f t h e t i t a n o l groups.
The d i f f e r e n t
a c t i v i t y among t i t a n i u m c o n t a i n i n g c a t a l y s t s can n o t be
a t t r i b u t e d t o a d i f f e r e n t number o f t i t a n i u m atoms which a r e
Greater
in
the
silica
supported t i t a n i a
than
in
equal o r
titanium silicalite.
However i n t h e t w o t y p e s o f compared c a t a l y s t s t i t a n i u m i s m o s t l y p r e s e n t i n
a completely
different
s t a t e o f a g g r e g a t i o n and c o o r d i n a t i o n which can
reasonably e x p l a i n such a d i f f e r e n c e i n c a t a l y t i c a c t i v i t y .
The t e c h n i q u e
of s u p p o r t i n g o r a n c h o r i n g t e t r a v a l e n t t i t a n i u m t o an amorphous s i l i c a
p r o b a b l y cannot f i x f i r n i l y i n a t e t r a h e d r a l
most
s t r u c t u r e i s o l a t e d atoms o f
t i t a n i u m t o t h e surface o f the s i l i c a i t s e l f , but instead
produces o r leads
r a p i d l y t o t h e formation o f surface c l u s t e r s o f t i t a n i u m dioxide i n t h e
o c t a h e d r a l c o o r d i n a t i o n even a t low t i t a n i u m c o n c e n t r a t i o n ( r e f .21).
aggregations o f
t i t a n i u m atoms a r e n o t e v i d e n t l y as a c t i v e
These
as t h e i s o l a t e d
ones i n t h e c r y s t a l l i n e s t r u c t u r e o f t h e t i t a n i u m s i l i c a l i t e ( r e f . 2 2 ) .
In
order
silicalite,
to
explain
the
high
selectivity
achieved
with
i t must be assumed t h a t t h e f i r s t r e a c t i o n step,
titanium
i.e.
the
i n t e r a c t i o n o f hydrogen p e r o x i d e w i t h t h e t i t a n i u m atoms i n t h e z e o l i t e
framework,
t a k e s p l a c e f a i r l y r a p i d l y t o suppress a l l t h e r e a c t i o n s which
occur i n t h e absence o f t h e c a t a l y s t .
The second s t e p o f t h e r e a c t i o n i s t h e t r a n s f e r o f t h e p e r o x i d i c oxygen
added t o t h e t i t a n i u i n t o t h e s u b s t r a t e t o be o x i d i z e d .
The h i g h s e l e c t i v i t y
o b t a i n e d i n t h e cyclohexanone ammoximation i m p l i e s t h i s r e a c t i o n s t e p o c c u r s
through a
c o n c e r t e d mechanism
r e a g e n t s on
the
active
sites
involving the
of
f o r m a t i o n o f t h e cyclohexanonoxime.
the
interaction
catalytic
of
system and
all
the
three
direct
T h i s h y p o t h e s i s would appear t h e most
r e a l i s t i c and p r e f e r a b l e t o a h y p o t h e s i s o f s y n t h e s i s t h r o u g h a sequence o f
r e a c t i o n stages which would r e q u i r e t h e f o r m a t i o n o f i n t e r m e d i a t e s .
It i s
u n l i k e l y t h a t t h e s e would be a b l e t o c o e x i s t i n s o l u t i o n w i t h t h e v e r y
reactive
system
s e l e c t iv i t y .
and
this
is
not
consistent
with
the
high
reaction
50
CONCLUSIONS AN0 PERSPECTIVE
The r e s u l t s obtained using t i t a n i u m s i l i c a l i t e were very promising.
The
cyclohexanone s e l e c t i v i t y t o oxime was g e n e r a l l y higher than 99%,only t r a c e s
of
organic by-products were formed.
s t o i c h i o n e t r i c molar r a t i o ,
P r o v i d i n g a H 0 /cyclohexanone near
2 2
t h e hydrogen peroxide l o s s was very small and
mainly determined by t h e formation o f some i n o r g a n i c by-products.
Moreover our new ammoxiination r o u t e allows t o widen t h e a p p l i c a t i o n f i e l d
o f t h i s catalyst for
alkaline solution,
a c t i v a t i n g d i l u t e d hydrogen peroxide,
even i n an
and f o r e x p l o i t i n g i t s o x i d i z i n g power through s e l e c t i v e
oxygen t r a n s f e r t o ammonia n i t r o g e n atom.
REFERENCES
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
G.W. Parshall, Homogeneous Catalysts, Wiley, New York, 1980
R.A. Sheldon, J. Mol. Catal., 20 (1983) 1-26
J.E. Lyons, Hydr. Proces. (1980) 107-119
I . V . S p i r i n a , V.P. Maslennikov, Yu. A. Aleksandrov, Russ. Chem. Rev., 56
(Engl. T r a s l . ) (19871, 670
681
F.Cavani, G.Centi, F . T r i f i r 6 , R.K.Grasselli ,Catal .Today 3, (1988) 185-198
J.N. Armor, i n J.R. Kosak (Ed.), C a t a l y s i s o f Organic Reactions, Vol. 18,
Marcel Dekker, New York and Basel, 1984
S. Tsuda, Chem. Econ. Eng. Rev., (1970) 39-41
J.P. Schirmann, S.Y. Delavarenne, Hydrogen Peroxide i n Organic Chemistry,
E d i t i o n e t documentation I n d u s t r i e l l e , Paris, 1979
O.T. Sawyer, Chem. Tech., (1988) 369-375
R. Sheldon, B u l l . SOC. Chim. Belg., 94 (1985) 651-670
J.L. Russel, J. K o l l a r , US Pat. 1100672 (1965)
G.N. Koshel, M.I. Farberov, L.L. Zalygin, G.A. Krushinskaya, J.Appl.Chem.
USSR, 44 (1971 ) 885
J.A. Connor, E.A.V.
Ebsworth i n N.J. Emeleus, A.G. Sharpe ( E d i t o r s ) , Adv.
Inorg. Chem. Radiochem., Vol. 6, Academic Press, N.Y. and London, 286
G.
Perego, G, B e l l u s s i , C. Corno, M. Taramasso, F. Buonomo, A. Esposito,
Titanium s i l i c a l i t e : Proc. 7 t h I n t . Z e o l i t e Conference, Tokyo, August
17-22, 1986, E l s e v i e r , Amsterdam, 129-136
B. N o t a r i , Stud. S u r f . Sci. Catal., (1988) 413-25
M.Taramasso, G. Perego, B. N o t a r i , US Pat. 4410501 (1983)
R.J. Argauer, G. R. Landolt, US Pat. 3702886 (1972)
E. M o r e t t i , M. Padovan, M. S o l a r i , C. Marano, R. Covini, I t a l . Pat. Appl.
19238 A/82 (1982)
H.J. Ledon, F. Varescon, Inorg. Chem. 23 (1984) 2735
0. B o r t o l i n i , F. D i F u r i a , G. Modena, J . Mol. Cat. 33 (1985) 241-244
J. Chen, Ph.0. Thesis, Carnegie-Mellon Univ. (1986) and r e f s . i n c l u d e d
M.R.Boccuti,
K.M.Rao, A.Zecchina, G.Leofanti, G.Petrini, Proc.Eur. Conf.
T r i e s t e , September 13-16, 1988, i n
on S t r u c t . and React. o f Surface,
press
-
51
J. C.
VEDRINE ( I n s t i t u t de Recherches s u r l a C a t a l y s e )
69626 Villeurbanne (France):
You have
c l e a r l y shown
p r o p e r t i e s f o r oxime
T i 0 2 / S i 0 2 Catalyst.
channels
(5. 5
A
in
how T i - s i l i c a l i t e
gives high
catalytic
and ammoxime f o r m a t i o n w i t h r e s p e c t t o
Zeolite type materials exhibit
narrow
Ti-silicalite)
which
should
hinder
the
reactants t o reach a l l t o g e t h e r t h e inner a c t i v e s i t e s .
Moreover
i n l i q u i d s o l u t i o n t h e r e i s no d r i v i n g f o r c e f o r t h e r e a c t a n t t o
e n t e r t h e p o r e s p r e f e r e n t i a l l y t o t h e s o l v e n t molecules.
s i t e s on
r a t h e r t h a n i n s i d e t h e pores.
1i m i t ed.
opinion a r e
active
PAOLO ROFFIA
L a b o r a t o r y t e s t s have
the
s u r f a c e of
the
I f not, i s t h e r e a c t i o n
shovn t h a t a l l
I n your
cristallites
diffusion
t h e reagents a r e
rapilidy
a b s o r b e d i n t o t h e c h a n n e l s of t h e t i t a n i u m s i l i c a l i t e , s o i n o u r
o p i n i o n t h e ammoximation t a k e s
place inside the
catalyst
channels.
I f so, I do n o t exclude,
- t r a n s f e r problem.
At
a s you s u g g e s t , t h e e x i s t e n c e of
this
time
we
did
not
make
mass-
specific
e x p e r i m e n t s aimed t o e v i d e n c e t h e p r e s e n c e of t h i s problem b u t we
have i n program t o do it.
G.M.
PAJONK
69622 Villeurbanne {France):
According t o one s l i d e it seemed i m p o r t a n t t o p o u r
cyclohexanone
a l l a t once i n your r e a c t o r , why?
When NH3 r e a c t s o n l y w i t h
c a t a l y s t s (Ti02/SiOZ a s
H 2 0 2 over t h e titanium ion
well
Ti-Silicalite)
what
containing
were
the
reaction products?
I n p a r t i c u l a r i s NO d e t e c t e d i n t h i s c a s e ?
PAOLO ROFFIA
The a l l a t once a d d i t i o n
of cyclohexanone i s
only required
to
s e m p l i f y t h e way of performing t h e ammoximation.
The k e t o n e c a n be advantageously f e d
i n a c o n t i n u o u s way a s
for
t h e hydrogen p e r o x i d e .
To answer t h e second q u e s t i o n , I can s a y t h a t t h e main p r o d u c t i n
t h e ammonia o x i d a t i o n i s n i t r o g e n .
52
L.
-
ZULIANI
Chimica d e l F r i u l i
-
33050 T o r v i s c o s a (Udine):
My q u e s t i o n i s : Do you know t h e r o l e of
suppress undesirable s i d e reactions.
Thank you.
excess
ammonia
to
PAOLO R O F F I A
I n o u r ammoximation experiments w e used an e x c e s s of ammonia
(molar r a t i o NH3/cyclohexanone >2). However we have observed a t
This
l o w e r r a t i o s t h e r e a c t i o n begins t o become l e s s c l e a n .
p a r a m e t e r i s s t i l l under i n v e s t i g a t i o n .
P.
JIRU
-
Dolejskova 3, 1 8 2 2 3 Prague 8, Czechoslovakia:
is
lower ( * 2 - 7 % )
in
Also t h e s e l e c t i v i t y
of t r a s f i r m a t i o n of H202 t o oxime i n your p a t e n t s i s always lower
( 8 0 - 9 0 % ) .What a r e t h e reasons:
decomposition of H202, f o r m a t i o n
of o t h e r p r o d u c t s ( o r g a n i c p e r o x i d e s , N H 2 0 H , NO, N 2 . . . ) .
The oxime
yield
based
an
H202
always
comparison w i t h cyclohexanone s e l e c t i v i t y .
PAOLO ROFFIA
As I s a i d i n
my r e p o r t t h e
comparison w i t h
peroxide i s
t h a t of
H202 selectivity i s
cyclohexanone.
mainly determined
The l o s s
by i n o r g a n i c
and i n s m a l l amount n i t r a t e s and n i t r i t e s .
a b i t lower
of
products
in
hydrogen
formation
The hydrogen p e r o x i d e
decomposition t o oxigen does not occurs a t a l l .
A s f o r a s t h e claimed s e l e c t i v i t y i n o u r p a t e n t i s concerned,
lower v a l u e s a r e j u s t i f i e d by
lower c a t a l y t i c performances.
using a titanium s i l i c a l i t e
the
with
G. Centi and F. Trifiro' (Editors), New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
53
MODIFIED ZOLITES FOR OXIDATION REACTIONS
Cristina Ferrini and Herman W. Kouwenhoven
Technisch Chemisches Laboratorium, ETH-Zentrum, Universitatsstrasse 6, 8092 Zurich,
Switzerland
SUMMARY
Synthetic Ti-silicates with the MFI structure-type (Ti-silicalites or TS-1) have been reported to be
selective catalysts in (ep) oxidation reactions of aqueous H202 with unsaturated compounds. We have
applied secondary synthesis to introduce Ti into various zeolites by reaction with Tick. It appears that
Ti is incorporated into the zeolite framework. The catalytic activity of these modified zeolites was
screened using the oxidation of phenol with H202 as a test reaction. Their performance depends on
crystal structure, crystallite size, Ti content and preparation procedure and is compared with that of
the non-modified acidic zeolites The product composition is solvent dependent. Deactivated catalysts
may be regenerated by conventional techniques.
INTRODUCTION
TS-1, a titanium silicate with the MFI structure, is a selective catalyst for the (ep) oxidation of various
aromatic and unsaturated compounds using aqueous H202 as an oxidant. This very interesting
invention was reported by scientists from Enichem and affiliated companies (refs. 1,2,3,4). TS-1 is
shape selective in phenol oxidation, favouring the formation of catechol and hydroquinone (ref. 5). In
this reaction TS-1 is supposedly a better catalyst than H-mordenite and H-zeolite Y (ref. 6) or H-ZSM5 (ref. 7) which are all strongly acidic solids. TS-1 is synthesized directly from Si02 and Ti02
precursors by hydrothermal reaction. Secondary synthesis of Ti-modified ZSM-5 by reaction of
gaseous T i c 4 with acid-extracted Z S M J (Si/A1>2000) has been described and the properties of the
product are equal to those characteristic for synthetic TS-1 (ref. 8). Moreover, as a catalyst in the
hydroxylation of phenol with aqueous H202, its performance is comparable to that of TS-1 (ref. 9). It
is assumed that Ti occupies tetrahedral lattice sites.
In the present contribution we report on the secondary synthesis of Ti-modified ZSM-5, zeolite Beta
and zeolite Y, by reaction with Tic14 or Ti-tetraisopropylate with H-zeolite samples. Compared to
those earlier reported, the present materials contain after secondary syEthesis both A1 and Ti and are
therefore expected to be different catalysts. Catalyst performance was screened in the oxidation of
phenol with aqueous H202 as a test reaction in comparison with TS-1, silicalite and the H zeolites.
The effect of the addition of various solvents on product composition and conversion after 4 h was
investigated.
EXPERIMENTAL
Materials
TS-1 was synthesized according to (ref. 2) example 1. Zeolite Beta was synthesized following the
recipe given in (ref. 10). Samples of zeolite Y, (type FAU PY-32/Fl), H-ZSM-5, (type FZ 21/G) and
54
amorphous silica, (type C gel C-560) were obtained from the Chemische Fabrik Uetikon. Silicalite
was synthesized according to (ref. 10). small crystals, 2 rnm and (ref. 1l), large crystals, 25 mm.
Activation and Secondarv SYnthesis
Zeolite Beta and Y were converted into the H-form according to ammonium exchange techniques
usually used for the preparation of USY. H-ZSM-5 was exchanged further with 1 molar aqueous HC1
for deep Na+ removal. Reaction conditions for the T-site substitution reaction with T i c 4 are given in
Table 1.
TABLE 1
Secondary synthesis of Ti(A1)silicates by
gas phase reaction with Tic14
Reaction step
Temp.K
Time.Hr
Drying
Tic14 Reaction
Stripping
Steaming
Calcination
523
473-113
623
573
813
1
1-2
1
1
1
Characterization
Materials were characterized by the following methods: Mid IR spectra, using a Perkin Elmer IR
spectrometer 983. XRD, using a Guinier -de Wolff camera, Unit-cell size, using a Scintag
Diffractometer PAD-X. N2 adsorption, using a Micromeretics ASAP-2000. Elemental analysis by
AAS or ICP after dissolution in HF.IR traces are reproduced in Fig 1, other analytical data are
collected in Tables 2 and 3.
Test Reaction
Test reactions were performed in a stirred, closed glass reactor with and without added solvent at
temperatures between 353 and 400 K in N2 atmosphere, using 35%aq H202 and phenol, (Fluka
purum). H202 was added dropwise over 10 min. The mixture was sampled 4 h after finishing H202
addition. The products were analysed by standard GC and HPLC techniques. Product analyses are
given in Tables 4,5 and 6. The analysis is not quantitative, it was however verified that there is no
appreciable phenol loss (>lo%) in the experiments using solvents.
RESULTS AND DISCUSSION
Characterization
TS-1 and Ti modified ZSM-5 (Si/A1>2OOO) are usually identified by their mid IR spectra and XRD. IR
shows an absorption at 960 cm-1, the intensity of this absorption is dependent on the Ti content of the
sample.The interpretation of spectroscopic data from TS-1 samples is discussed in (ref. 12). The IR
spectra collected in Fig 1 show that the 960 cm-l band is present in the samples after reaction with
T i c k , indicating that indeed secondary synthesis may be applied to prepare Ti substituted zeolites. It
55
is interesting to note that also in amorphous silica Ti appears to be incorporated in similar sites. The
question remains however what type of Ti distribution is obtained by secondary synthesis.
Structural integrity was inspected by routine XRD using a Guinier-de Wolff camera. Crystallinity and
changes in unit-cell size were measured by quantitative XRD methods. A linear increase with Ti
content was reported (ref. 3) for Al-free samples with the MFI structure. Introduction of Ti into ZSM5 with a Si/Al ratio of 25 by reaction with Tic14 results in an increase in unit-cell volume. The Si/AI
ratio of the zeolite is not changed by the secondary synthesis (Table 2) which indicates that either
substituted A1 remains in the structure as extra framework alumina or that the reaction with TiQ is not
a substitution of A1 in T-sites by Ti, but rather a reaction of Tic1 4 with functional OH groups. The
change in unit-cell volume is, however, an indication that Ti is eventually incorporated into the
framework, in tetrahedral sites. In Table 3 it is shown that a loss of 10-20% in N2 adsorption capacity
occurs, possibly due to a comparable loss in crystallinity.
TABLE 2
Unit Cell Parameters of H-ZSM-5 and Ti-H-ZSM-5
Properties
H-ZSM-5
Ti-H-ZSM-5
Si/Al
Sirri
25
2800
25
36
:3yw
orth.
b (A)
c (A)
Unit Cell Vol.(A3)
20.11 l(6)
19.916(6)
13.401(4)
5367
orth.
20.16(1)
19.96(1)
13.447(8)
5415
TABLE 3
Properties of catalyst samples
Material
H-ZSM-5
Ti-H-ZSM-5
Ti-H-ZSMJ
Ti-H-ZSMJ
H-Beta
Ti-H-Beta
TS- 1
Tic1 4
Kl
Surface area
[m21gl
473
673
773
523
390
360
360
370
630
530
400
Sfli
SVAl
2800
36
42
58
25
25
25
25
46
all samples are crystalline
Using our preparation method the Si/Ti ratio of the product increases with the temperature of the
secondary synthesis. This result differs from those obtained wiih acid-extracted ZSM-5 having a Si/AI
ratio of >2000 (ref. 8)
56
I-
.
I
.I"
1
-
..,
I
I
,.
- -- -
".I
.
I
-
...
Fig. 1: IR Traces: a) TS-1, b) Ti-H-ZSM-5, c) H-ZSM-5, d) Beta, e) Ti-Beta, f) Ti-SiO2 (amorph)
Spectra a, b and c between 1600 - 200 cm-*,spectra d, e and f between 4000 - 200 cm-1
In the literature it is stressed that samples used in catalytic reactions with H2Q should not contain free
Ti02 (ref. 9), since this causes in H202 decomposition and poor catalyst performance. In fact phenol
hydroxylation may be applied as an elegant test for the presence of free Ti02 in Al-free catalyst
57
samples (ref. 9) and it is detected by a quick dark discoloration of the reaction mixture. This method
does not apply to the Al-containing samples of solid acids such as H-ZSM-5, since these generally
show a brown discoloration of the reaction mixture. Free Ti02 is conveniently identified by its IR
absorption at 380 cm-1. Ti02 has a poorly resolved IR spectrum in the range 400 - 800 cm-1, the
absorption at 380 cm-1 is however a relatively sharp peak, Fig 2, amorphous Ti02 gel also shows this
380 cm-1 absorption. Small quantities of free Ti02 (>OS%w) can thus be detected in zeolites,
provided that no zeolitic absorption occurs between 320 and 400 cm-1 (Fig. 2).
Fig 2: IR Traces: Mixtures of Ti@ and ZSM-5 (wt.% Ti02) a) 30%, b) 14%,c) 8%, d) 0.5%
Catalvtic tests
Apart from the recent studies mentioned before, hydroxylation of aromatics using a zeolitic catalyst is
hardly mentioned in the literature. Data on the application of acidic zeolites are conflicting,H-ZSM-5
is claimed to be an active and selective catalyst in (ref. 7), while (ref. 13) states that H-ZSM-5 is not a
58
good catalyst for this reaction. Conditions mentioned in the references differ appreciably as far as the
use of diluentdsolvents is concerned, but high conversions and high selectivities are reported in all
cases. It may be expected that the solvent will have an effect on the reaction depending on the nature of
the surface: high Si/AI ratio zeolites are hydrophobic, low Si/AI ratio zeolites are hydrophilic.
Accordingly we have tested the catalysts with and without addition of a solvent and we have also
varied the type of solvent in a limited number of exploratory experiments.
1) Tests without additional solvent, Table 4. In these experiments water is always present since it is
added with H202. Results confirm that H-ZSM-5 is a catalyst for the test reaction. Secondary
synthesis with TiCh improves its performance. Silicalite (Si/A1>500) has no catalytic activity, neither
is it active after reaction with T i c k unless it is given an additional thermal treatment at 1070K.
Crystallite size appears to have a strong influence, the activity of a material with u)mm particle size is
very low. The selectivity of the catalysts for the para product is >5 indicating that Ti atoms are most
probably located on the inner surface of the catalyst. Ti-USY did not show any activity in this test
although it has an IR absorption at 960 cm-1 after treatment with TiCb.
TABLE 4
Reaction of H202 and Phenol using various catalyst in the
absence of a solvent.
catalyst
H-ZSM-5
Ti-H-ZSM-5*
Silicalite
Ti-SilicaLite*
Ti-Silicalite
calc. 1073 K*
Ti-HZSM-5 (25pm)*
Ti-Y*
conv.after 4 hr.
Para
ortho
11
8
12
<1
<1
11
1
1
<1
<1
<1
<1
<1
<1
<1
<1
9
13
<1
<1
<1
*Tic14 at 473K, 2hrs.
smp at 350K, 1hrs.
steam at 573K, lhrs.
Conversion: mol productdmol H202 in
Conditions: Temperature 353 K, 2 ml35% H202,7.36 g
Phenol, 0.15 g Catalyst
2) Tests using a 1/1 methanoywatermixture as a solvent, Table 5. Both H-USY and H-Beta are active
and selective catalysts, the product p/o ratio is 0.7 for USY and 0.8 for Beta, which might be due to
the pore size of these materials (> 0.65 nm). TS-I is the most active catalyst, the p/o ratio in the
product is 1. Ti-modified H-ZSM-5 is less active than TS-1, its product p/o ratio is about 2. After
modification with Ti zeolite Beta has a lower conversion than the parent material and its product p/o
ratio is high and similar to that of the equally active Tick treated amorphous S i q . The last sample in
this series is a catalyst made by reaction of Ti-tetraisopropylate in P A at 348 K with a silicalite having
59
2 pm crystallites. The activity of this sample is very low and the initial conversion after 4 h is below
detection limit. After 23 h however a high conversion is observed and the product p/o ratio is 0.5. The
low initial activity of this sample might indicate that the Ti atoms are mainly located on the outer
surface of the silicalite crystals (<lm2/g). Because of its size Ti tetraisopropylate is not expected to
enter the pores of silicalite
TABLE 5
Comparison of Catalysts in the reaction of phenol and H202
(Solvent CH3OwH20 1/1 w/w)
catalyst
T i c 4 2h at K
Ti- H-ZSM-5
Ti-SiO2
H-USY
H-Beta
Ti-Beta
TS-1
Ti-Silicalite
Ti-Silicalite*
673
573
523
47 3
Conv after 4hrs
product p/o ratio
13
4
25
16
4
28
<1
30**
1.9
no ortho
0.7
0.8
no ortho
1
0.5
*prepared by reaction of Ti-tetraisopropylate (in P A ) with acidtreated Silicalite,Temp. 348 K, time 21 hrs.
**after 23 hrs. reaction
Conversion: mol products/mol H 2 Q in
Conditions: Temperature 353 K, 2 ml35% H202,5 g Phenol, 0.2 g catalyst, 4 g solvent
3) Comparison of solvents, Table 6. Using a sample of Ti-silicate prepared according to the recipe
given for TS-1 in (ref. 2) example 1, acetone, methanol and water and methanoVwater mixtures were
used as solvents. It appeared that under our conditions the highest conversion was obtained using
water even with this low-polarity catalyst and that activity does not vary much for water/methanol
ratios higher than 1.
TABLE 6
Reaction of H202 and Phenol over TS-I*:
Solvent effect.
Solvent
water
watedmethanol
watedmethanol
watedmethanol
watedmethanot
methanol
acetone
Ratio w/w
Conv.(%) after 4h
4/1
3/1
1/1
1/3
34
31
26
28
20
4
2
Conditions: Temperatur 353 K, 2 d 3 5 % H202,5 g Phenol, 0.2 g
catalyst, 4 g solvent
Conversion: mol products/mol H2@ in
*Ref. (2) example 1
product p/o
1
1
1
1
1
2.3
no ortho
~
60
CONCLUSIONS
Although the present results leave many questions open the following conclusions are drawn:
* Treatment of H-zeolites with T i c 4 as described here is a method to insert Ti atoms into the zeoli
framework.
* H-zeolite Y and H-zeolite Beta are active catalysts for the hydroxylation of phenol.
* The effect of the T i c 4 treatment on product catalytic activity depends on the zeolite structure.
* Ti deposited on the outside of zeolite crystals is active in the hydroxylation of phenol although at
low activity level and shows a lower selectivity for p substitution than a material having the actii
sites located in the zeolite pores.
We would like to thank Ciba-Geigy for financial support for C.F.
REFERENCES
1
2
3
4
5
6
7
8
9
10
11
12
13
M.Taramass.0, G.Perego and B.Notari, DEOS 3.047.798, 1981, Snamprogetti SPA.
M.Taramasso, G.Perego and B.Notari, USPat. 4.410.501, 1983, Snamprogetti SPA.
G.Perego, G.Bellusini, C.Corno, M.Taramasso, F.Buonomo and E.Esposito in
Y.Murakami, A.Iijima, J.W.Ward (eds), Proc.Seventh Int.Conf. on Zeolites, Tokyo
1986, Tonk Kodanska Elsevier, Amsterdam, p 129.
B.Notari, in P.J.Grobet, W.J.Mortier, E.P.Vansant, G.Schulz-Ekloff (eds), Innovation
in Zeolite Materials, Elsevier, Amsterdam, 1988, p 413.
A.Esposito, C.Neri and F.Buonomo, DEOS 3.309.669, 1983 Anic SPA.
H.S.Bloch, USPat. 3.580.956, 1971, UOP Corp.
C.D.Chang and D.Hellring, USPat. 4.578.956, 1986, Mobil Oil.
B.Kraushaar-Czametzki and J.C.vanHooff, CataLLett. 1,( 1988),81.
B.Kraushaar-Czametzki and J.C.vanHooff, Catal.Lett.2,( 1989),43.
P.A.Jacobs and J.A.Martens, Synthesis of High Silica Aluminosilicate Zeolites, Elsevier,
Amsterdam, 1987.
J.L.Guth, H.Kessler and R. Wey, in ref. 3, p. 121.
M.R.Boccuti, K.M.Rao, A.Zecchina, G.Leofanti and G.Petrini in C.Morterra,
A.Zecchina and G.Costa (eds), Structure and Reactivity of Surfaces, Elsevier,
Amsterdam 1989, p. 133.
GBellusi, M.G.Clerici, A.Giusto and F.Buonomo, EPA 226.258, 1986, Eniricerche.
61
G . BELLUSSI (ENIRICERCHE San Donato Milanese, Italy) : The dimensions of Tic14 are in
the range betwcen 5 and 6 A depending whether one considers the molecule is rotating
around its geometrical center or is in a fixed position. This dimension is close to the
pore diameter of TS-1. Considering that Tic14 can easily react with the surface silanol
group it seems unlikely that this molecules can penetrate inside the pore structure.
More likely they can react on the surface as in the case of chemical vapor deposition of
silicon compounds described by Prof. Murakami. Have you any direct evidence of Ti
framework insertion as for example from IR spectra?
H.KOUWENHOVEN (ETH Zurich, Switzerland) : Reaction products were characterized by
IR, XRD and BET surface area measurements (as reported in the paper) and the results
are very similar to those published for TS-I. Additionally we applied the hydroxylation
of phenol as a yardstick for the position of titanium. We have compared the
performance of samples without titanium with that of a sample which contains
titanium on the outer surface (sample treated with Ti-tetraisopropylate), a sample
prepared by repeating the synthesis of TS-1 (Pat. Nr. 4.410.501, Example 1) and finally
with that of
titanium zeolites prepared by sccondary synthesis. We measured the o/p
ratio of the products and checked that the presence of tar products was less than 10% of
the phenol reacted. The o/p ratio of the reaction product obtained using a sample
containing Ti in the outer surface is higher than that obtained with "secondary
synthesis Ti-zeolites'' and over a TS-1 sample. The good agreement of the results
obtained with TS-1 with those obtained over the "Ti-Silicates" prepared from H-ZSM-5
and the observation that the catalyst activity decreases with increasing crystal size
indicate that the conclusions are most probably correct. We are aware that more data
are required to establish the titanium distribution in the samples.
P. JACOBS (Lab. Oppcrvlaktechemie Leuven, Belgium) : From the results in your paper
and from your presentation, it seems to me that your data on the preparation of Tizeolites with Tic14 arc at variance with those of Kraushaar et al. (ref. 5 and 7 in your
paper). Indeed, thc latter author claims that it is essential to start with a Al-zeolite and
to dislodge A1 from the framework by steaming and/or acid treatment. Apparently, you
start off with a pure H-form and do not use such treatments. Could you elaborate on this
apparent contradiction.
H.KOUWENHOVEN (ETH Zurich, Switzerland): For our prcparation of Ti-zeolites using
T i c 1 4 we always started with an acid treatment in order to introducc hydroxyl groups
for the subsequent reaction with TiC14. The acid treatment was performed with 1N HCI
during one hour at reflux temperature (50 ml pro gram). Thc product was subsequently
filtered and washcd with demi. water. This treatment was repeated three times.
Elemental analysis by ICP of the pretreated H-ZSM-5 samples showed only little
dealumination: Si/AI ratio increased from 21 to 25. This is in contrast with the results of
ref. 5 and 7 where a higher degree of dealumination is observed (Si/AI 2000) and could
be a reason for the apparent contradiction. During the subsequent reaction with Tic14
we did not observe a substitution of Al by Ti (after Tic14 reaction the Si/AI ratio is still
25), we prefer accordlingly a mechanism involving the reaction of surface -OH groups
with TiCla.
J. VEDRINE (Institut de Catalyse, CNRS, France): You have answered to Bcllussi that your
conclusion for Ti incorporated in the latticc at framework position was based on Ti salt
interaction effect on unit cell volume increase. I do not agree with the latter statement,
IR spectroscopy of vibrational mode characterization would have been better. As a
matter of fact unit cell volume is greatly dependent on water content and you should
absolutely compare samples with the same water (or any adsorbate) content.
H.KOUWENHOVEN (ETH Zurich, Switzerland): The XRD measurements arc only one of our
characterization methods. The materials were characterized also by IR and BET surface
area measurement and by the test reaction. These data are for our "secondary synthesis
62
catalyst" in agreement with those published for TS-1. XRD data were obtained on
samples having the same Si/AI ratio, which were carefully equilibrated under identical
conditions. The differences which were observed in the diffraction patterns are
significant for c and have an acceptable reliability for a and b (table 2).
J.KIWI (EPFL Lausanne, Switzerland). The high degree of catalysis observed when Tiisopropylate is used on the zeolite surface has been explained by you on grounds of
different Ti-deposits than when TIC14 is used. Is it not due to the fact that with Tic14 you
affect profoundly the acid character of the zeolite surface which is not the case when
Ti-isopropylate is used? Have you measured the acid character of the surface in both
cases?
H.KOUWENHOVEN (ETH Zurich, Switzerland): The acidity of the various materials was not
measured. We observed however that a Tic14 treated silicalite as a catalyst is comparable
to TS-I and Tic14 treated H-ZSM-5 and shows a relatively high p/o ratio in the reaction
product. The silicalite treated with Ti-tetraisopropylate is different and has a much
lower p/o ratio in its reaction product and we assume that this difference is caused by
the fact that Ti in this case is deposited on the outside of the crystals. According to our
IR and XRD results for samples with the MFI structure, the Ti is located in sites having a
coordination comparable to that of Ti in TS- 1.
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
01990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Mn(II1)-TETRAARYLPORPHYRINS
BEARING
A
ANCHORED
COVALENTLY
63
AXIAL
LIGAND:
EFFICIENT CATALYSTS I N OLEFIN E P O X I D A T I O N S UNDER TWO-PHASE CONDITIONS.
P.L. ANELLI, S . BANFI, F. MONTANARI*,
and S. Q U I C I *
D i p a r t i m e n t o d i Chimica Organica e I n d u s t r i a l e d e l l ' U n i v e r s i t i e Centro CNR,
V i a G o l g i 19, 20133 M i l a n o , I t a l y .
SUMMARY
Imidazole o r p y r i d i n e " t a i l e d " Mn-tetraarylporphyrins are e f f i c i e n t
c a t a l y s t s f o r o l e f i n e p o x i d a t i o n s c a r r i e d o u t a t 0°C w i t h NaOCl o r 30%-H 0
2
under aqueous-organic two-phase c o n d i t i o n s . Terms b e a r i n g C1 atoms i n $he
o , o ' - p o s i t i o n s of t h e a r o m a t i c r i n g s a r e a l s o more r e s i s t a n t t o o x i d a t i v e
demolition.
INTRODUCTION
F e ( I I 1 ) and M n ( I I 1 ) complexes o f
mimic
the
catalysts
biological
for
the
with
of
epoxidation o f
s a t u r a t e d hydrocarbons
conditions
activity
NaOCl
(refs.
or
synthetic
natural
olefins
1-41,
30%-H 0
2 2
t e t r a a r y l p o r p h y r i n s which
monoxygenases
and f o r
the
are
efficient
hydroxylation
of
Reactions c a r r i e d o u t under two-phase
as
oxygen
donors
are
particularly
a t t r a c t i v e ( r e f . 5 ) . I n b o t h cases r e a c - t i o n r a t e s a r e g r e a t l y enhanced by t h e
presence o f l i p o p h i l i c h e t e r o c y c l i c bases ( a l k y l s u b s t i t u t e d i m i d a z o l e s o r
p y r i d i n e s ) as
e l e c t r o n donating
axial
ligands coordinated
to
t h e metal
centre.
1
2
M o n o l i g a t e d species 2 a r e h i g h l y r e a c t i v e ,
3
whereas 1 and 3 a r e p o o r l y
r e a c t i v e and c o m p l e t e l y i n a c t i v e species, r e s p e c t i v e l y ( r e f . 6 ) .
The o b j e c t o f t h e p r e s e n t paper i s t h e i n v e s t i g a t i o n o f t h e c a t a l y t i c
a c t i v i t y and chemical s t a b i l i t y of M n ( I I 1 ) p o r p h y r i n s b e a r i n g an i m i d a z o l e o r
64
p y r i d i n e , as a x i a l l i g a n d , c o v a l e n t l y bonded t o t h e p o r p h y r i n r i n g t h r o u g h an
a p p r o p r i a t e spacer c h a i n . Examples o f t h e s e c a t a l y s t s a r e Mn( 1 I I ) p o r p h y r i n s
0
CI
4
6a, n = 5
6b, n
= 10
0
CI
7a, n = 5
7b, n
5
= 10
The c a t a l y s t s , e s p e c i a l l y 6 and 7, have been s y n t h e t i z e d i n t h e l i g h t o f
previous i n v e s t i g a t i o n s
on o l e f i n e p o x i d a t i o n c a t a l y z e d by Mn( I I 1 I p o r p h y r i n s
and promoted by NaOCl and 30%-H 0
22
( r e f s . 6-8).
We had f o u n d ( r e f . 61 t h a t t h e e f f e c t o f t h e l i g a n d / p o r p h y r i n r a t i o (L/P)
on t h e r e a c t i o n r a t e depends o f b o t h t h e n a t u r e o f t h e p o r p h y r i n and o f t h e
I n t h e case o f Mn(III)-tetra-(2,6-dichlorophenyl ) p o r p h y r i n 8 and
substrate.
N - h e x y l i m i d a z o l e as t h e a x i a l l i g a n d , t h e maximum r e a c t i o n r a t e i s observed
for
L/P
=
1,
which
corresponds
to
the
monoligated species 2 a t t h e equilibrium.
maximum c o n c e n t r a t i o n
of
the
By i n c r e a s i n g t h e L/P r a t i o t h e
r e a c t i o n r a t e s p r o g r e s s i v e l y decrease; f u r t h e r m o r e t h e y a r e s t r o n g l y slowed
down i n t h e absence o f a x i a l l i g a n d .
The optimum c o n d i t i o n s
for
oxidations
carried out
with
NaOCl
in a
65
CH C1 -H 0 two-phase system a r e achieved by b u f f e r i n g t h e pH o f t h e aqueous
2 2 2
phase a t 10.5 ( r e f s . 6,9-11).
C a t a l y t i c amounts o f q u a t e r n a r y onium s a l t s as
p h a s e - t r a n s f e r c a t a l y s t s , c u r r e n t l y used by o t h e r a u t h o r s i n e p o x i d a t i o n s
promoted by NaOCl a t pH 12.7 ( r e f s . 1,2),
accelerate the reaction r a t e but
f a v o u r t h e o x i d a t i v e d e g r a d a t i o n o f t h e p o r p h y r i n and o f t h e a x i a l l i g a n d .
In t h e e p o x i d a t i o n r e a c t i o n s promoted by hydrogen p e r o x i d e ( r e f s . 7,8),
CH C1 -H 0 two-phase c o n d i t i o n s , t h e pH o f 30%-H202
2 2 2
commercial s o l u t i o n ( + 2.5) must be a d j u s t e d i n t h e 4.5-5.0 range. Lower pH
carried
values
out
under
decrease
decomposition.
the
reaction rates
whereas
higher
values
promote
H 0
2 2
R e a c t i o n r a t e s a r e s t r o n g l y a c c e l e r a t e d by t h e presence o f
v e r y small amounts o f benzoic a c i d .
RESULTS AND DISCUSSION
I n t h e s y n t h e s i s o f p o r p h y r i n s 4-7 t h e f o l l o w i n g parameters have been
c o n s i d e r e d : i )t h e l i n k a g e t o t h e p o r p h y r i n r i n g and t h e l e n g t h o f t h e spacer
chain;
i i ) t h e nature o f
porphyrin.
Porphyrins
the
bearing
axial
ligand;
iii)
c o v a l e n t l y attached
l i g a n d s have been d e s c r i b e d by s e v e r a l
authors
the
structure
imidazole
(ref.
or
of
the
pyridine
12) and t h e metal
complexes o f some have been used as c a t a l y s t s i n hydrocarbon o x y g e n a t i o n
r e a c t i o n s . However, i n t h i s l a s t case, t h e a x i a l l i g a n d was p l a c e d i n t h e
middle
of
a
bridge
connecting
two
opposite
aromatic
rings
of
t e t r a a r y l p o r p h y r i n ( r e f . 1 3 ) . We chose t h e more easy t o s y n t h e s i z e p o r p h y r i n s
4-7, c o n f i d e n t t h a t t h e v e r y h i g h complexation c o n s t a n t s between i m i d a z o l e s
o r p y r i d i n e s w i t h Mn(II1)tetraarylporphyrins
(refs.
6,141
would a l l o w t h e
spontaneous c o o r d i n a t i o n o f t h e metal and t h e a x i a l l i g a n d hung t h r o u g h che
f l e x i b l e c h a i n ( r e f . 1 5 ) . The c h a i n s have been l i n k e d t h r o u g h e t h e r o r amido
bonds i n t h e o r t h o o r meta p o s i t i o n s o f a meso phenyl r i n g o f the p o r p h y r i n .
The number o f t h e l i n e a r l y disposed atoms i n t h e c h a i n was 6-14.
The p o r p h y r i n r i n g s a r e t h o s e o f m e s o - t r i s ( p - t o l y 1 )
phenyl p o r p h y r i n 9
and tetra-(2,6-dichlorophenyl) p o r p h y r i n 8 ; t h e l a t t e r b e i n g p a r t i c u l a r l y
s t a b l e under o x i d a t i v e c o n d i t i o n s
(ref.
6).
I n catalysts
p y r i d i n e has been a t t a c h e d t o t h e spacer c h a i n .
7 only
Indeed, when i m i d a z o l e o r
p y r i d i n e a r e used as e x t r a bases t h e y a r e b o t h o x i d i z e d ,
olefins;
6 and
b u t w h i l e i m i d a z o l e s a r e c o m p l e t e l y demolished,
together w i t h t h e
pyridines give,
at
66
l e a s t i n t h e f i r s t step,
t h e c o r r e s p o n d i n g N-oxides which s t i l l behave as
e f f i c i e n t axial ligands ( r e f . 6).
P o r p h y r i n s 4 and 5 have been used i n a p r e v i o u s i n v e s t i g a t i o n ( r e f . 9).
E p o x i d a t i o n s promoted by H O C l and c a t a l y s e d by 4 and 5 a r e e x t r e m e l y f a s t and
a t 0°C a r e o v e r i n a few minutes.
However p o r p h y r i n s
undergo o x i d a t i v e
d e g r a d a t i o n i n t h e c o u r s e o f t h e e p o x i d a t i o n and a t t h e end o f t h e r e a c t i o n
a r e c o m p l e t e l y bleached.
behaviour,
since
Present day knowledge
porphyrins
without
substituents i n t h e o,o'-positions
bulky
led
and/or
us
to
electron
expect
this
withdrawing
o f t h e meso phenyl r i n g s a r e e x t r e m e l y
u n s t a b l e under o x i d a t i v e c o n d i t i o n s ( r e f s . 10,111.
Conv %
Conv 96
I,,/
P
30
Fig.1.
NaOCl c y c l o o c t e n e e p o x i d a t i o n
Fig.2.
60
90
120
150 t [rninl
NaOCl 1-dodecene e p o x i d a t i o n
c a t a l y z e d by Mn ( III) - p o r p h y r i ns : 6a
c a t a l y z e d by Mn ( I I I) - p o r p h y r i ns :
(01, 6b (01, 7a (01, 7b ( H I , and
6a (01,
6b (01, 7a (01,
7b ( H I ,
8 ( A ) . Reactions c o n d i t i o n s : CH2C12-
and 8 ( A ) . R e a c t i o n c o n d i t i o n s as
-H20, O"C,
i n F i g . 1.
pH 10.5;
Mn(II1)-porphy-
r i n : c y c l o o c t e n e : 0.35M-NaOCl= 1:200:
700 m o l a r r a t i o s . W i t h M n ( I I 1 ) p o r p h y r i n 8 and N-hexyl i m i d a z o l e , L/P = 1.
67
30%-H 0
and c a t a l y z e d by
2 2
M n ( 1 I I ) p o r p h y r i n s 6 and 7 have been performed by u s i n g c y c l o o c t e n e and
E p o x i d a t i o n r e a c t i o n s promoted by H O C l o r
1-dodecene as models o f r e a c t i v e and p o o r l y r e a c t i v e s u b s t r a t e s , r e s p e c t i v e l y .
The r e a c t i o n c o n d i t i o n s and t h e most s i g n i f i c a n t r e s u l t s a r e r e p o r t e d i n F i g .
1-4.
Unexpectedly,
p o r p h y r i n s 7a and 7b, i n which o n l y one o u t o f e i g h t
c h l o r i n e atoms o f p o r p h y r i n 8 has been r e p l a c e d by t h e amido group b e a r i n g
t h e f l e x i b l e chain,
proved t o be p o o r l y s t a b l e under b o t h H O C l
and H 0
2 2
oxidation conditions.
Conv %
Conv %
looif l
F i g . 3 . H 0 c y c l o o c t e n e e p o x i d a t i o n ca2 2
t a l y z e d by M n ( I I 1 ) - p o r p h y r i n s : 6a (01,
F i g . 4 . H 0 1-dodecene e p o x i d a t i o n
2 2
c a t a1yzed by Mn ( I I I - p o r p h y r ins : 6a
6b ( O ) , 7a ( O ) , 7b ( M I , and 8 ( A ) .
(01, 6b
c o n d i t i o n s : CH C1 -H 0, O"C, pH 4.5;
2 2 2
M n ( I I 1 ) - p o r p h y r i n : benzoic a c i d : c y c l o -
and 8 ( A ) . R e a c t i o n c o n d i t i o n s :
octene: 30%-H 0 = 1:1:200:400 molar
2 2
r a t i o s . With M n ( 1 I I ) p o r p h y r i n 8 and N- h e x y l i m i d a z o l e , L/P = 1.
( a ) , 7a
(01, 7b
(m ,
CH C1 -H 0, O ' C , pH 5.0; M n ( I I 1 )
2 2 2
- p o r p h y r i n : benzoic a c i d : l-dodecene:
30%-H 0 = 1:4:200:400 molar
2 2
r a t i o s . With M n ( I I 1 ) p o r p h y r i n 8 and
N-hexyl i m i d a z o l e , U P = 1.
68
Chemical s t a b i l i t y i s s t r o n g l y enhanced i n t h e case o f p o r p h y r i n s 6a and
6b i n which t h e amido group i s i n t h e meta p o s i t i o n o f t h e phenyl r i n g s , thus
, around
leaving unaffected
t h e metal, t h e s t r u c t u r e o f M n ( I I 1 ) - p o r p h y r i n 8.
M n ( I I 1 ) - p o r p h y r i n s 6 are more e f f i c i e n t c a t a l y s t s then 7, independently o f
t h e s u b s t r a t e and o f t h e o x i d a n t used. R e a c t i v i t y o f 6a i s equal o r h i g h e r
than t h a t o f M n ( I I 1 ) - p o r p h y r i n
8 i n t h e presence o f N-hexylimidazole
or
4-tert-butylpyridine
(L/P = 1 ) . I t must be stressed t h a t ,
complete conversion,
i n t h e HOCl epoxidations o f 1-dodecene w i t h 8 a h i g h e r
i n order t o get
amount o f a x i a l l i g a n d i s r e q u i r e d , due t o t h e o x i d a t i v e degradation o f t h e
l a t t e r ( r e f . 6).
C a t a l y t i c a c t i v i t y o f t a i l e d porphyrins gets worse i n t h e epoxidations
promoted by 30%-H 0 w i t h b o t h o l e f i n s ( F i g . 3, 4 ) . I n t h i s case o l e f i n
2 2
o x i d a t i o n i s accompanied by an e x t e n s i v e decomposition o f t h e c a t a l y s t s ,
m o s t l y concerning t h e c h a i n b e a r i n g t h e a x i a l l i g a n d .
CONCLUSIONS
The aim o f o b t a i n i n g M n ( I I 1 ) p o r p h y r i n s b e a r i n g a c o v a l e n t l y
anchored
a x i a l l i g a n d , which are e p o x i d a t i o n c a t a l y s t s a t t h e same t i m e e f f i c i e n t and
s u f f i c i e n t l y s t a b l e , has been reached i n t h e case o f p o r p h y r i n s 6a and 6b.
Unexpectedly, t h e l e s s s t e r i c a l l y demanding d e r i v a t i v e s 7a and 7b are poorer
c a t a l y s t s . The reasons o f t h i s behaviour a r e s t i l l unknown: f u r t h e r e f f o r t s
should be made i n o r d e r t o i d e n t i f y a l l t h e parameters i n v o l v e d t o b u i l d up
" t a i l e d " Mn-porphyrins w i t h h i g h e r c a t a l y t i c a c t i v i t y .
REFERENCES.
1 B. Meunier, B u l l . SOC. Chim. France, 578 (1986).
2 P.R.
Ortiz
de
Montellano,
Cytochrome P-450,
Structure,
Mechanism
and
Biochemistry, Plenum press, New York and London (1986).
3 I . Tabushi, Coordination Chem. Rev.,
4 0. Mansuy, Pure Appl. Chem.,
86, 1
2,
759
(1988).
(1987).
61,1631 (1989).
54, 1850 (1989).
5 F. Montanari, S. Banfi, S. Q u i c i , Pure Appl. Chem.,
6 S. Banfi, F. Montanari, S. Q u i c i , J. Org. Chem.,
7 P.L.
Anelli,
Commun.,
S.
Banfi,
779 (1989).
F.
Montanari,
S.
Quici,
J.
Chem.
SOC.,
Chem.
69
8 S. B a n f i , A. Maiocchi, F . Montanari, S. Q u i c i , Gazz. Chim. I t a l . ,
i n press.
9 F. Montanari,
50,
M.
Penso,
S.
Quici,
J.
Viganb,
P.
Org.
Chem.,
4888
(1985).
10 S. B a n f i , F. Montanari, M. Penso,
Ital.,
117,689
V. Sosnovskikh, P . Viganb, Gazz. Chim.
(1987).
53,
11 S. B a n f i , F. Montanari, 5. Q u i c i , J . Org. Chem.,
12 a ) J.E. Baldwin, P. P e r l m u t t e r , Top. Curr. Chem.,
Morgan, 0. Dolphin, i n " S t r u c t u r e and Bonding",
2863 (1988).
121, 181
J.W.
(1984); b ) B.
64,
B u c k l e r (Ed.),
115 (1987), and r e f e r e n c e s t h e r e i n .
13 a ) M. Momenteau, Pure Appl. Chem.,
2,1493
(1986); b ) B. Meunier, M.-E.
De Carvalho, 0. B o r t o l i n i , 14. Momenteau, I n o r g . Chem.,
I,161
(1988).
14 Unpublished r e s u l t s f r o m t h i s l a b o r a t o r y .
15 An e x t e n s i v e NMR i n v e s t i g a t i o n i s c u r r e n t l y under way i n o r d e r t o i d e n t i f y
t h e p o s s i b l e presence o f i n t e r m o l e c u l a r c o m p l e x a t i o n s .
ALAN J . CHALK (Givaudan Carp.,
Clifton,
N.J. 07014 USA): These c a t a l y s t s have
shown a r e v e r s a l o f t h e normal o l e f i n r e a c t i v i t y towards e p o x i d a t i o n and t h i s
has been a s c r i b e d t o s t e r i c b u l k of
t h e l i g a n d s about t h e manganese,
what
s t e r i c e f f e c t s o f t h i s n a t u r e have y o u seen?
F . Montanari ( D i p a r t i m e n t o Chimica o r g a n i c a I n d u s t r i a l e , U n i v e r s i t l d i M i l a n o ,
o f the
I t a l y ) : I t i s n o t t r u e t h a t t h e s e c a t a l y s t s have shown a r e v e r s a l
normal o l e f i n r e a c t i v i t y towards o x i d a t i o n ,
because t h e e p o x i d a t i o n r a t e o f
c y c l o o c t e n e i s always f a s t e r t h a n t h a t o f 1-dodecene.
J . HABER ( I n s t i t u t e o f C a t a l y s i s and S u r f a c e Chemistry,
Sciences,
Krakow,
Poland):
Our
quantum
chemical
P o l i s h Academy o f
calculations
of
electron
d i s t r i b u t i o n i n peroxide o r peracid l i n k e d t o a metal c e n t r e i n t h e porphyrin
have shown t h a t
t h e d e n s i t y o f e l e c t r o n s dn t h e
terminal
oxygen o f
the
p e r o x i d e g r o u p , i .e. i t s e l e c t r o p h i l i c p r o p e r t i e s and hence r e a c t i v i t y towards
o l e f i n s , s t r o n g l y depends on t h e e l e c t r o n d e n s i t y on t h e metal c e n t r e ,
obviously i s a f u n c t i o n o f t h e a x i a l ligand.
activity
which
Could y o u r r e s u l t s o f l e s s e r
o f t h o s e p o r p h y r i n s which have s m a l l e r s t e r i c h i n d r a n c e be e x p l a i n e d
by d i f f e r e n t
e l e c t r o n w i t h d r a w i n g p r o p e r t i e s o f t h e l i g a n d s as t h e second
important f a c t o r besides s t e r i c p r o p e r t i e s ?
70
F. Montanari ( D i p a r t i n e n t o Chimica Organica I n d u s t r i a l e , U n i v e r s i t a d i M i l a n o ,
I t a l y ) : There i s n o t d o u b t t h a t e l e c t r o n i c e f f e c t s
o f substituents i n the
p o r p h y r i n r i n g a f f e c t t h e e l e c t r o n d e n s i t y on t h e m e t a l c e n t r e and as a
consequence e l e c t r o p h i l i c p r o p e r t i e s and r e a c t i v i t y o f t h e i n t e r m e d i a t e metal
oxene.
Our d a t a (J.
Org. Chem.,
4,
1850,
1989 and u n p u b l i s h e d r e s u l t s )
o b t a i n e d f r o m a s e r i e s o f M n - t e t r a a r y l p o r p h y r i n s showed a s t r o n g dependence o f
mono and b i s - c o o r d i n a t i o n
constants
K
s u b s t i t u t e d i m i d a z o l e s and p y r i d i n e s ) .
and K
o f axial ligands ( a l k y l
1
2
These same e f f e c t s s h o u l d o p e r a t e i n
" t a i l e d " p o r p h y r i n s 6 and 7. However we f e e l t h a t t h e main d i f f e r e n c e between
6 and 7 s h o u l d be a t t r i b u t e d t o t h e e a s i e r c l e a v a g e o f t h e amido bond o f t h e
o r t o s u b s t i t u t e d 7 w i t h r e s p e c t t o t h e meta substict'uted 6 ( t h i s p o i n t i s under
c u r r e n t i n v e s t i g a t i o n ) . The more d i f f i c u l t i n t r a m o l e c u l a r c o o r d i n a t i o n i n 6 i s
l i k e l y balanced by a e a s i e r i n t e r m o l e c u l a r c o o r d i n a t i o n i n 7.
F 7
(b)
B.R.
JAMES
(Department
of
Chemistry,
University
of
British
Columbia,
Vancouver, BC, Canada, V6T 1Y6): Do you have d a t a r e g a r d i n g s t e r e o s e l e c t i v i t y
f o r t h e s e Mn systems; f o r example e p o x i d a t i o n o f c i s o r t r a n s s t i l b e n e ?
F. Montanari ( D i p a r t i n e n t o Chimica o r g a n i c a I n d u s t r i a l e , U n i v e r s i t a d i M i l a n o ,
I t a l y ) : We do n o t have y e t s t e r e o s e l e c t i v i t y d a t a f o r Mn-porphyrins 6 and 7.
However
in
the
e p o x i d a t i on
catalyzed
by
M n - t e t r a k i s ( 2 , 6 - d i c h l orophenyl ) -
p o r p h y r i n and c a r r i e d o u t w i t h H 0 - b e n z o i c a c i d , c i s - s t y l b e n e a f f o r d e d o n l y
2 2
cis-epoxide,
whereas
trans-stylbene
d i d not react.
The
less
rigid
trans-oct-4-ene
a f f o r d e d t h e c o r r e s p o n d i n g t r a n s e p o x i d e i n f a i r l y good y i e l d
(J.C.S. Chem. Corn., 779, 1989).
G. Centi and F. Trifiro’ (Editors), New Developments in Setective Oxidation
0 1990 Elsevier Science Publishers B.V.,Amsterdam - Printed in The Netherlands
XLKENE EPOXIDATION WITH HYDROGEN PEROXIDE,
CATALYZED B Y METALLOPORPHYRINS
G o o r , G. Prescher and M. Schmidt
Degussa A G , Forschung Organische Chemie
Rodenbacher Chaussee 4 , 0-6450 Hanau 1 IFRG)
G.
SUMMARY
Metalloporphyrins o f the type O-M(P)X, where M = M o , W , ( P ) =
tetraarylporphyrinato, octaethylporphyrinato, and X = halogene,
O H , O R , O A c , S C N , O C l O 3 , are used as homogeneous catalysts i n
the epoxidation of alkenes by hydrogen peroxide. Selectivities
and yields strongly depend on the nature of the alkene. In some
cases considerable amounts o f 1 , 2 - d i o l are formed which strongly
depends on the solvent used. 2-Hydroxyalkyl hydroperoxides are
intermediates i n the diol formation.
INTRODUCTION
Direct epoxidation o f olefins by hydrogen peroxide has been
a long-standing goal in oxidation chemistry. Contrary to oxid a n t s such as organic peraclds, hydroperoxides, chlorine via
chlorohydrines (ref.1) as w e l l as iodosylbenzene (ref.2) and
hypochlorite (ref. 3 ) hydrogen peroxide i s attractive from the
standpoint o f c o s t , and because water is its only reduction
product.
Unfortunately the oxidizing power o f hydrogen peroxide itself 1 s
rather l o w . Epoxidation reactions can be achieved only by means
of
suitable catalysts which are based mainly on group 5 A , B and
6 A , B metal oxides (ref. 4 , 5 1 . To d a t e , however, valuable
results in the case o f simple olefins have been obtained only by
working under virtually anhydrous conditions (ref. 6 , 7 ) or by
applying highly pretentious catalyst systems respectively
(ref. 8 ) .
The objective o f our research is the development
of
selective
homogeneous catalysts for epoxidation reactions with hydrogen
peroxide a s oxidant.
In this paper w e present the results o f studies o n the
catalytic activity of metal-ion porphyrin(V1 complexes
(Scheme 1 ) i n the epoxidation o f various alkenes. B o t h , aqueous
and anhydrous solutions o f hydrogen peroxide have been applied.
71
72
RESULTS AND DISCUSSION
S v n t h e s i s and C h a r a c t e r i ~ a t i o n o f M e t a l l o n o r D h v r i n p
I n g e n e r a l m e t a l l o p r o p h y r i n s a r e s y n t h e s i z e d by m e t a l
i n s e r t i o n ( r e f . 91. O x o m o l y b d e n u m ( V ) p o r p h y r i n s . 8.g. t e t r a p h e n y l p o r p h y r i n s d e r i v a t i v e , M o O ( T P P ) X , a r e e a s i l y a c c e s s i b l e by
r e f l u x i n g a m e t a l - f r e e p o r p h y r i n , 8.9. H 2 ( T P P ) . w i t h M o ( C O ) s i n
d e c a l i n e (ref. 1 0 ) .
The homologous oxotungsten(V) complexes, WOlTPPlX are formed
f r o m W ( C O l 6 o n l y o n p r o l o n g e d r e f l u x i n g i n DMF ( r e f . 1 1 ) .
To
prepare oxotungsten ( V ) octaethylporphyrin derivatives, K ~ W ~ C ~ Q
in refluxing benzonitrile or H z W O ~ in molten phenol have been
u s e d , y i e l d s h o w e v e r w e r e low ( r e f . 1 2 ) .
SCHEME 1
R
Fi
Q
Fi
-
M O ( OEP )X
MO TRP 1X
Specification of
a ) t h e porphyrins
OCHg
c1
H
CHj
(TAP)
( TClP I
( T P P1
(TTP)
b) the metals M
c )
the axial ligands X
M
Mo
W
O1za)
OCHg
OClOS
c1
Br
OAC
F
OH
a ) p-0x0 c o m p l e x .
73
T h e m e t a l i n s e r t i o n p r o c e d u r e c o u l d be i m p r o v e d f o r o x o t u n g s t e n ( V ) c o m p l e x e s r e s u l t i n g in n e w tetra p o r p h y r i n
derivatives, WO(TRP)X (ref. 13, Scheme 1 ) .
The n e w p o r p h y r i n c o m p l e x e s a r e v i o l e t - g r e e n g l i t t e r i n g ,
a i r s t a b l e c r y s t a l s and f o r m g r e e n s o l u t i o n s i n o r g a n i c s o l v e n t s
that display UV/VIS absorption spectra with three to four bands
t h e w a v e l e n g t h s o f w h i c h a r e very c h a r a c t e r i s t i c f o r t h e
c o o r d i n a t e d a n i o n X. The c o m p o s i t i o n a r e d e t e r m i n e d by e l e m e n t a l
a n a l y s i s and field i o n d e s o r p t i o n m a s s s p e c t r a . T h e a x i a l
l i g a n d s X and t h e t e r m i n a l WO
("tungstyl") group produce
specific vibrations in the I R spectra. The WO frequency is
s u b j e c t t o a t r a n s - e f f e c t o f X ( r e f . 1 4 ) : as t h e T - d o n o r
c a p a c i t y o f X i n c r e a s e s , t h e WO f r e q u e n c y d e c r e a s e s .
S C R E E N I N G OF T H E C A T A L Y T I C A C T I V I T Y
0 x 0 - o o r a h v r i n a t o m o l v b d e n u m ( V ) c o m o l e xes a s c a t a l w s t s
C a t a l y t i c a l a c t i v i t y o f t h e s e m e t a l l o p o r p h y r i n s in
epoxidation reactions was tested with different olefins. The
results of epoxidation with hydrogen peroxide in the presence o f
~ 5 , 1 0 , 1 5 , 2 0 ~ - t e t r a p h e n y l p o r p h y r i n a t o o x o m o l y b d e n u m c~ oVm~p l e x e s
(ref. 15, 16, 17) are summarized in Table 1.
The epoxide yields strongly depend upon the alkene applied.
For 1 , 5 - c y c l o o c t a d i e n e . both i n a q u e o u s and a n h y d r o u s h y d r o g e n
p e r o x i d e h i g h y i e l d s o f e p o x i d e a r e o b t a i n e d , e v e n w h e n using a
s u b s t r a t e t o c a t a l y s t r a t i o as high as 3 0 0 0 ( r u n 5 ) . When
c y c l o h e x e n e o r 2 - m e t h y l - 2 - b u t e n e i s used as s u b s t r a t e , a
r e l a t i v e l y l a r g e a m o u n t o f t h e r e s p e c t i v e 1 , Z - d i o l is formed
( r u n 2 , r u n 7). w h e r e a s i n t h e e p o x i d a t i o n o f 1 , 5 - c y c l o o c t a d i e n e
t h e f o r m a t i o n o f t h e c o r r e s p o n d i n g 1 , 2 - d i o l is not o b s e r v e d .
Furthermore, in the epoxidation of cyclohexene a third compound
w a s formed in a d d i t i o n t o e p o x i d e and 1 , 2 - d i o l . W i t h G C - M S t h i s
c o m p o u n d c o u l d be i d e n t i f i e d a s 2 - h y d r o x y c y c l o h e x y l h y d r o p e r o x i d e , A s n o 1 , 2 - d i o l is formed b e f o r e any 2 - h y d r o x y c y c l o h e x y l
h y d r o p e r o x i d e , it c a n b e assumed t h a t t h e l a t t e r c o m p o u n d i s an
intermediate in the 1.2-diol formation.
T h e o x o p o r p h y r i n a t o m o l y b d e n u m ( V ) c o m p l e x e s a r e not d e s t r o y e d
by t h e o x i d a n t and r e m a i n a c t i v e a f t e r r e c y c l i n g ( r u n 5 and 6 ) .
At h i g h e r i n i t i a l h y d r o g e n p e r o x i d e c o n c e n t r a t i o n s a l a r g e r part
74
of
the original catalyst i s converted into t h e blue trans-diper-
0x0-complex 102)2MolVI)1TPP)
s t r o n g a b s o r p t i o n i n t h e i.r.
( r e f . 181. T h i s i s i n d i c a t e d b y a
spectrum at 9 5 9 cm-1 which can be
In t h e U V / V I S s p e c t r u m t h e S o r e t p e a k a t
4 4 4 nm and w e a k absorptions at 573 nm and 6 1 3 n m belong to t h e
a s s i g n e d to vCMo(OzI1.
transdiperoxo-compound which d o e s not catalyse t h e epoxidation
of
alkenes anymore.
TABLE 1
Epoxidation of various alkenes with hydrogen peroxide in the presence o f
oxo15,10,15,20-tetraphenylporphyrinatoImolybdenum1VI complexes.',
*
Run
Catalyst
Immol)
Alkene
1mmol)
p-O[OMoITPP)I~
(24x10 I'
4
6
0 5 1 aq.
Yielda
1
l.2-Dio14
1
18
90.2
0
(h)
(24)
25.92 in nPAC6
120)
6
89.9
46.9
0MoITPP)SCN
( 1 2x 1 0 . 2 I
l15-C00
136)
25.92 in nPAC
10)
6
87.5
0
OMo(TPPIC1
196x10 * 2 1
1,S-COO
121~)
25.9% in nPAC
79 1
12
93.3
0
OMO(TPPIC~~~
148x10 -31
t .5-C00
I144 I
25.91 in nPAC
(481
24
95.7
0
OMOITPP)C~~~
196x10 .2 I
1 5-COD
(2881
25.92 in nPAC
1791
6
80.3
0
1
30.8
29.3
OMo1TPP)Me
[ 12x10 -')
1
I
Time
2-methyl-2-butene
1721
p-O[OMo1TPP)Iz
124x10 z ,
3
1 5-COD5
172)
H202
(rrmol)
CYC ohexene
136)
8 5 1 aq.
112)
'
Runs 1-6 in n-propyl acetate, run 7 in tert.-butanol:
T = 60°C;
Yield of (epoxide + d i o l ) relative to hydrogen peroxide:
Diol to (epoxide + diol) ratio x 100 Z:
1.5-COD = 1.5-cycloctadiene;
nPAC=n-propyl acetate; 7a Catalyst recyclecf once: 7b Catalyst recycled twice.
0x0-PORPHYRINATO TUNCSTEN(V) COMPLEXES AS CATALYSTS
On account of t h e marked catalytic activity of the molybdenum
c o m p l e x e s , it w a s , t h e r e f o r e , o f i n t e r e s t t o i n v e s t i g a t e t h e
analogous reactions catalyzed by t u n g s t e n porphyrins ( T a b l e 2 ) .
( r e f . 19).
1 1 1
75
[able 2: Epoxidatron o f alkenes catalyzed b y tungsten-porphyrin complexes')
Olefin
Catalyst
Immol)
ImmOlI
1, 5-COO3)
96.1
86.5
97.2
98.9
I
63.6
51.1
1.13
6
92.8
75.3
0.65
1721
I S-COO~~
no diol
I211
1721
8 5 i!
2-Methyl-2-bu-
0WlTPP)Br
tenel' 172)
121x10-21 1 2 0
85 I 1
1 -0ctene"
6
1
96.4
I
52.3
I
0.54
172)
77.75)
~ i i y iaicohoi"
1361
!5,9 l
Cyclohexene 3 1
83.2
81.6
0.32
83.3
81.1
0.27
E6.8
67.7
(721
0WlTPP)Br z 5 . 9 1
Cycloherene
85 I
Cyclohexene6)
172)
Cyclohexene
I ) At 6 O o C ;
11
85
only
epoxidi
is
I
69.6
55.3
1.6
112)
" Selectlvlty to fepoxlde
in n-propylacetate;
included;
6
124)
136)
')
1,5
121x1!l-2~ 1 2 4 1
1721
'I
+
d l o l l rel. to converted H 2 0 2 ;
')
glycerol-2-glycldol ether
in tert. butyl alcohol:
acetonitrileldloxane = 2 1 1 :
In acetonltrlle.
H i g h s e l e c t i v i t i e s t o g e t h e r w i t h very short r e a c t i o n t i m e s
a r e r e a c h e d w i t h 1 . 5 - c y c l o o c t a d i e n e , both w i t h 8 5 l a q u e o u s and
waterfree hydrogen peroxide [runs 1 ,
2). W i t h i n t e r n a l - and
t e r m i n a l o l e f i n s l i k e 2 - m e t h y l - 2 - b u t e n e and 1 - o c t e n e ,
respectively, the tungsten porphyrin catalysts seem to be less
s e l e c t i v e . A l t h o u g h t h e h y d r o g e n p e r o x i d e c o n v e r s i o n is h i g h ,
t h e yield o f e p o x i d e i s e v e n l o w e r t h a n i n t h e c o r r e s p o n d i n g
reactions catalyzed with molybdenum porphyrins. With ally1
a l c o h o l , no g l y c i d o l i s o b s e r v e d . G l y c e r i n - 2 - g l y c i d o l e t h e r ,
f o r m e d i n t h e c o n s e c u t i v e r e a c t i o n i s t h e o n l y product.
C o n c e r n i n g t h e e p o x i d a t i o n o f 1 , 5 - c y c l o o c t a d i e n e , almost
exclusively monoepoxide, 1,2-epoxycyclooctene-5, is formed.
Regarding the epoxide/diol ratio, a marked dial formation is
obvious in all cases. except for 1,5-cyclooctadiene. Furthermore, as mentioned for the molybdenum porphyrin catalyzed
epoxidation reaction with cyclohexene, 2-cyclohexyl hydropero x i d e a g a i n w a s f o r m e d in a d d t i o n t o e p o x i d e and 1 , 2 - d i o l .
T h e f o r m a t i o n o f t h i s i n t e r m e d i a t e and t h e d e p e n d e n c e o f t h e
e p o x i d e f o r m a t i o n w i t h t i m e i s shown i n F i g u r e 1 .
- intermediate
9
- epoxde
fig. 1
+
A f t e r a s l i g h t e x c e s s o f e p o x i d e at t h e b e g i n n i n g o f t h e
reaction. the formation o f the intermediate incereases until 3
t o 4 h o u r s , w h e r e a s t h e e p o x i d e s t a y s at a very l o w c o n c e n t r a t i o n . A f t e r t h i s t i m e ( m a x i m u m o f i n t e r m e d i a t e ) t h e yield o f
e p o x i d e s u d d e n l y i n c r e a s e s by a f a c t o r o f 5 w i t h i n o n e h o u r ,
with a concomitant decrease o f the hydroxohydroperoxide
i n t e r m e d i a t e , T h e c o u r s e o f t h i s r e a c t i o n i s i n very g o o d
a g r e e m e n t t o a m e c h a n i s m first p r o p o s e d by M a t t u c i et a l . ( r e f
20)
(Scheme 2) for the epoxidation of alkenes with hydrogen
p e r o x i d e i n t h e p r e s e n c e o f i n o r g a n i c peracids.
77
SCHEME 2
0
d+
0
2.
H2G2
8
HO
3.
GGH
OW( TPP 1 Br
0
Ho?300H
OWlTPPlBr
At f i r s t t h e e x p e c t e d e p o x i d a t i o n r e a c t i o n t a k e s p l a c e l e q n . 1 ) .
H o w e v e r , in t h e second s t e p t h e epoxide reacts w i t h further
hydrogen peroxide t o the hydroxo-cyclohexyl hydroperoxide
intermediate (eqn. 2 1 which in turn further reacts with alkene
t o yield e p o x i d e and d i o l ( e q n . 3 ) . When acetonitrile and
methanol is used as a solvent mixture ( l / l ) ,
2-hydroxycycloheyl
hydroperoxide can be obtained as main product.
Attempts were made t o increase the l o w epoxide/diol-ratio:
a ) aceotropic removal of water
b ) epoxidation at different pH values
cl e p o x i d a t i o n a t d i f f e r e n t
ti202
concentrations
d ) epoxidation at different olefin :
ti202
ratios
e ) variation of the solvent/-mixture.
However, none of the experiments a ) to d ) were suited t o
increase the epoxide portion.
The l o w epoxide/diol ratio was markedly increased when acetonitrile/dioxane
( 2 / 1 ) w a s used
as solvent mixture (run 8 1 .
Although acetonitrile alone is able t o react with alkenes ( r u n
9 ) ,
( r e f . 211. t h e a d d i t i o n o f t h e o x o t u n g s t e n ( V ) p o r p h y r i n
catalyst distinctly increases the reaction rate.
CONCLUSION
The observed activity of Mo(V1 porphyrins in catalytic
epoxidation with hydrogen peroxide has enormously stimulated the
investigation o f t h e s e complexes and has demonstrated that
hydrogen peroxide can be a very selective oxidizing agent w h e n
proper conditions a r e used.
78
REFERENCES
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
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R e i c h , F . Chow a n d S . L . P e a k e , S y n t h e s i s 1 9 7 8 ,
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Lecoq and J . P . Schirmann, Fundamental
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P l e n u m , New Y o r k , V o l . 3 , 1 9 7 9 . p p . 3 2 7 - 3 4 3 a n d r e f e r e n c e s
therein.
R . A . S h e l d o n and J.K. K o c h i i n M e t a l - c a t a l y s e d O x i d a t i o n s o f
O r g a n i c Compounds, A c a d e m i c P r e s s , New Y o r k , 1 9 8 1 . p p . 2 7 5 - 2 8 8
J.-P.
Renaud, P . B a t t i o n i , J . F . B a r t o l i and D. Mansuy, J .
Chem. S O C . , Chem. Commun., 1 9 8 5 , 8 8 8 .
J.W. B u c h l e r , i n : D . D o l p h i n ( E d . ) , The P o r p h y r i n s , V o l . 1 ,
A c a d e m i c P r e s s , New Y o r k , 1 9 7 8 . pp. 4 3 9 - 4 4 7 a n d r e f e r e n c e s
cited therein.
E.B. F l e i s c h e r and T . S . S r i v a s t a v a . I n o r g . C h i m . A c t a 5
(1971) 151.
E . B . F l e i s c h e r , R . D . Chapman a n d M. K r i s h n a m u r t h y . I n o r g .
Chem., 1 8 ( 1 9 7 9 ) 2 1 5 6 .
J.W. B u c h l e r . L . P u p p e , K . Rohbock a n d H . H . S c h n e e h a g e , Chem.
Ber., 106 ( 1 9 7 3 ) 2 7 1 0 .
J . B u c h l e r , G . H e r g e t . M . S c h m i d t a n d 6 . P r e s c h e r , DE 3 8 0 0 9 7 3
A l , Degussa A G .
B . P l e s n i c a r , i n : W.S. Trahanovsky ( E d . ) , O x i d a t i o n i n
O r g a n i c C h e m i s t r y , p a r t C , A c a d e m i c P r e s s , New Y o r k , 1 9 7 8 ,
p. 211.
M. S c h m i d t , 6 . P r e s c h e r a n d H . H u l l e r , E u r . P a t . 0 2 2 2 1 6 7 ,
Degussa A G .
M. S c h m i d t a n d 6 . P r e s c h e r , E u r . P a t . 0 2 8 2 7 0 8 A l , D e g u s s a A G
G . L e g e m a a t , W. D r e n t h , M. S c h m i d t , G . P r e s c h e r a n d G . G o o r ,
submitted f o r publication.
B . C h e v r i e r , T . D i e b o l d and R . W e i s s , I n o r g . Chim. A c t a , 19
(1976) L57.
M . S c h m i d t , G . P r e s c h e r , J. B u c h l e r a n d A . K l e e m a n n ,
DE 3 8 0 0 9 7 4 C 1 , Degussa A G .
A . M . M a t t u c i , E . P e r r o t t i a n d A . S a n t a m b r o g i o , J . Chem. S O C . ,
Chem. Commun., ( 1 9 7 0 ) 1 1 9 8 .
J.-P.
Schirmann and S . Y . Delavarenne, Hydrogen P e r o x i d e i n
O r g a n i c C h e m i s t r y , Ed. D o c u m e n t a t i o n I n d u s t r i e l l e , P a r i s ,
1980, p . 23.
S . R . JAMES I U n i v e r s i t y o f B r i t i s h C o l u m b i a , V a n c o u v e r , C a n a d a ) :
Could you p r o v i d e d e t a i l s on t h e p r e p a r a t i o n o f anhydrous
HzDz-solutions?
G.
GOOR
(DEGUSSA AG:
FRGI:
Preparation
n f anhydrous
solutions
of
H ~ O Zi n o r g a n i c s o l v e n t s i s d e s c r i b e d i n p a t e n t l i t e r a t u r e . U s e
lot' b o t h ' ; o l v e n t s
w h i c h f o r m an a z e o t r o p i c m i x t u r e w i t h w a t e r ,
whlch azeotropic m i x t u r e b o i l s below t h e b o i l i n g p o i n t o f H ? O z ,
e . g . n . p r o p y 1 d c e t a t e , ( r e f . 1 ) and o f high b o i l i n g s o l v e n t s
( r e f . 2 ) have boen d e s c r i b e d . However, i n t h e c o u r s e o f an
e x t e n s i v e s a f e t y e x a m i n a t i o n on anhydrous s o l u t i o n s o f H 2 0 2 i n
l o w b o i l i n g s o l v e n t s . 1 . e . t h o s e d e s c r i b e d i n ( r e f . 1 1 , it was
o b s e r v e d a t Degussa t h a t i f t h e s o l u t i o n c a t c h e s f i r e t h e
s o l v e n t w i l l e v a p u i a t s . A s a ~ o n ~ e q u a n chei g h l y c o n c e n t r a t e d
sriJutions o f H z O z i n o r g a n i c s o l v e n t r e s u l t w h i c h can d e t o n a t e .
T h e r e f o r e , u s e o f H ~ O ~ - s o l u t i o ni sn l o w b o i l i n g s o l v e n t s was
(I1 s c o n t iri u e d
.
( 1 )
(2)
Degussa A G .
Degussa A G ,
EP 9 8 4 2 7
EP 1 2 1 G G O .
R . A . S H E L D O N ( A n d r ? n u B V . The N e t h e r l a n d s ) :
P n r p h y r i n l i g a n d ? can be q u i t e e a s i l y exchanged or o x i d d t i v e l y
d e s t r c i y e d u n d e r t h e s e r e a c t i o n c o n d i t i o n s . How d o y o u know f o r
5 u r e t h a t t h e a c t i v e c a t a l y s t is a n d r e m a i n s t h e H o - o r W - p o r FI ti y I'.L n c o m p l e x ?
600R ( D E G U S S A A t : F R G ) : D u r i n g t h e c o u r s e o f t h e e p o x i d a t i o n
r t s a ~ : t i o n a n d a F t e r t h e r e a c t i o n was F i n i s h e d s a m p l e s w e r e t a k e n
and a n a l y z e d by U V - V I S s p e c t r o s c o p y : n e i t h e r d a m e t a l l a t i o n n o r
oxitJdti.ve d e g r d d a t i o n o f t h e porphyr1.n s k e l e t o n c o u l d be
observed.
Ci.
R.K.
I:RA!.CLLI
( M o b i l e R b D C o r p . , L I S A ) : C o u l d y o u comment o n t h e
enormous d i f f e r e n c e i n r e a c t i v i t y w h i c h you o b s e r v e d between
( I W L P J X a n d O M o ( P ) X in u p o x i d a t i n n r e a c t i o n s u s i n g H 2 O Z 7
1,.
G O O R I D E G U S S A A G , F R G ) : We h a v e n o e x p l a n a t i o n f o r t h e much
h i g h e r a c t i v i t y o f O W ( P 1 X a s compound t o O W o ( P ) X . It i s
i n t e r e s t i n g t o n o t e t h a t O W ( P ) X a n d O M u ( P ) X d l s o show different
b Q h a v i 0 r a g a i n s t a c t l o n nC H z 0 2 : w i t h O W ( P ) X t h e c i s . 0 ~ 0p e r o x n
t omplex
o f W i s f o r m e d ( r e f . 1 1 , w h e r e a s w l t h OMo(P)X t h e
t r a n b - d i p e r o x u c o m p l e x o f Ho 1 s o b t a i n e d ( 2 ) .
V . L . Goedkcn c t
1425 2G.
( ? I B. C h e v i e r , Th.
1 9 7 f i . 1 3 . 1.57.
( 1 J
al.,
J.
Chem.
D i d c r l d arid R .
3oc.,
Chem.
COmmUn..
W ~ i s s ,I r i o r g . C h i m
(19R5)
Acta,
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
USE ?F RiMETACLlC SYSTEMS FOR THE SELECTIVE OXIDATION
81
OF OLEFINS WITH HYDROGEN
P E ROX I DE
Giorgio STRUKUL*, Andrea ZANARDO and Francesco PINNA.
Uipartimento d, Chimica, Universita di Venezia, Dorsoduro 21 37, 301 23 Venezia
- ITALY
SUMMARY
The oxidation of olefins wit.h hydrogen peroxide catalyzed b y a Pt( I I V M bimetallic system ,s
reported where I*I = Rh, I r , Pd complexes. While w it h the former two met.als no okidatiori is
obtained, w i t h (dppe)Pd(CF3)(solv)
the reaction selectively produces ketones. This appears
to be a genuine example of bifunctional catalysis.
+
INTRODUCTION
Although hydrogen peroxide is a major chemical commodity, i t s direct use as oxidant i n
!ransition metal catalyzed reactions for industrial organic chemistry has iound, to date, only
limited success. This i s mainly due to the unavoidable presence of the co-product water which is
otherwise v ery appealing for environmental reasons.
We have recently reported ( r e f . 1 i on the epoxidation of simple olefins wit h H2O? catalyzecl
by a variety of P t ( I I ) complexes of the type: P*Pt(CF3)X ( P 2 = various diphosphines; X =
solvent, -OH). These catalysts are v e r y effective and versatlle for a number of reasons: ( i ) they
give high reaction rates, complete conversion of the oxidant into products and recovery of ?he
catalyst; (ii)they can be easily modified w i t h c h i r a l diphosphines to give asymmetric epoxidation ( r e f . 2 ) ; (iii) they allow also the use of other hydroperoxidic oxidants like
tert- butylhydroperoxide and potassium rnonoperoxysulfate ( r e f . 3 ) w i t h which different
selectivities in the oxidation of olefins are obtained
I n a kinetic studv of this system ( r e f . 4) we have been able to demonstrate that the oxygen
!ransfer step i s a bimolecular reaction involving the nucleophilic attack of a PtOOH species onto
an ulefin activated
Pt(ol)+
+
on a different Platirium center (reaction 1 1.
PtOOH
4
epoxide
+
Pt'
+
PtOH
(1)
This observation illustrates an example of bifunctional catalysis, where the activation of +he
two reactants takes place on two distinct metal centers which combine together i n the rate
determining step While this behavior i s typical of rriost heterogeneous catalysts, to our
knowledge i t has v e r y l i t t l e counterpart in homogeneous catalysis, the only p r i o r example in the
field of oxidation being due to Mares and coworkers ( r e f 5 ) who suggested that some Co-nitro
mmplexes can transfer an oxygen atom to alkenes activated by T I ( i l l ) The abilit y of Platinum t o
increase the nucleophilicity of hydrogen peroxide through formation of PtOOH complexes i s
82
rather uncommon and i s shared only w i t h some Pd species (ref s. 6 and 71, on the other hand the
role of olefin activator performed b y Platinum in this system could be in principle carried out
more efficiently b y a different group VIII transition metal. I n this work we report the results
obtained i n the oxidation of olefins w i t h H202 as the oxi&nt, in the presence of a bimetallic
catalyst system PtOOH/M(olefin) where M = Rh, Ir, Pd. Other very efficient olefin activators
such as cyclopentadienyl Iron( 1 1 ) complexes have not been considered because of the ease wi!h
which Fe( I I ) species undergo one-electron redox processes which led to the decomposition of
hydrogen peroxide
RESULTS AND DISCUSSION
Rhodium
All reactions were carried out i n a stoichiometric fashion F i r s t , a very simple Rh-olef,n
complex has been considered
Since t.his reaction may indicate that the Ft oxidant i s killed b y exchangeof the chloride present
we have attempted the approach w i t h a different Rh complex
((dppe)Rh(C@D)]ClO4
+
t diphoe)Pt(CF3)(00Hi
THF
is!
No P.eactiorl
The lack of reactivity might be due to the difficulty w i t h which COD i s oxidized by the Pt-OOH
oxidant. I n fact, because of the ilominant r o l e of steric effects i n this class of platinum catalysts
( r e f . 8 ) even w i t h (dlphoe)Pt(CF3)(solv)
as olefin activator no oxidation i s observed w i t h
+
internal olefins ( r e f . 1 ). We have therefore devised a way to introduce 1 -0ctene as the olefin
which i s v e ry reactive i n the case of the Pt+/PtOOH system, according to the following
procedure:
(dppe)Rh(COD)+
1 -octene
+
H2
DCE
-.
----f
(dppe)P.h( 1 -octene)2+
VE1CUUfrI
(dppe)Rh(solv)2H2+
- HZ
( diphoe)P t( CF 3 ) ( OOH)
24 hr
(diphoe)Pt(CF3)(0H)
+
"Rh" brown No Oxidation Product
However even i n this case, no organlc oxidation products were observed All the Rhodicm
complexes used are described i n the literature i r e f Sri
Iridium
The stoichiometric oxidation of 1 ,S-cylooctadiene coordinat.ed to the [(dppe)lr(COD)JCl@,
83
complex (ref. 10) was tested:
[ ( dppell r ( C0D)lClO 4
f diphoe)Pt( CF3N OOH 1
+
THF
( dip hoe)Pt ( CF 3 )( OH
(41
+
"I r dark - gr een
"
GL analysis shows free COD and a small amount (corresponding to about 3% of the P t
introduced) of an unknown new product This behavior is reminiscent of some previously
observed reactions involving Iridium complexes and hydroperoxides (ref 1 I )
(PPh3)21r(CO)(OH)
+
H202
+
IPPh3)2lr(CO)Cl
t-BuOOH
4
[(COD)lr(OH)I 2
+
+
H202
-+
dark-green solution
blue-green solution
(7)
All the above reactions have been recognized to lead to decomposition of the hydroperoxide
through Haber-Weiss mechanism promoted by the Ir( I ) / I d I I 1 redox couple
The conclusion that emerges is that, albeit for opposite reasons, neither Rh nor I r may be
employed as cocatalysts i n this oxidation system
Palladium
Since Pd( ! 1 ) centers are known to promote nucleophilic attack on coordinated olefins more
efficiently than P t ( l l ) (ref. 12). we have tried t o carry out catalytically the epoxidation
reaction with H202 using a bimetallic catalyst system consisting of (diphoe)Pt(CF3)(OH) and
the homologous complex f(dppe)Pd(CF3)(CH-,C12)1DF4.The latter complex was described by us
some years ago (ref. 6 ) and i s known to react both with H20 and H2% and therefore i s likely to
be sufficiently stable under the catalysis conditions.
The system was tested i n the oxidation of a variety of simple and substituted olefins and a
summary of the results obtained i s reported i n Table 1 . The reactions were carried out i n a one
phase THF/H20 medium at 65°C. Attempts to work at room temperature either i n THF/H20 or i n
a two phase DCE/H20 medium Qave modest yields and selectivities. As shown i n Table ! i n these
reactions the selectivity is inverted with respect to the analogous system consisting only o i
Platinum, the ketone being the major oxidation product. Significant epoxidation i s evident only
i n the case of 1 -w;tene, while other oxidation products are formed i n all cases, which include
t.he iso-alcohols and species which may arise from further reaction of the epoxides formed, l i k e
glycols o r benzaldehvde i n the case of styrene. In the oxidation o i butylvinyl ether partid;
hydrolysis of the ketonization product butylacetate leading to butanol and acetic acid i s observed
As a general trend, this system gives good amounts of oxidation products i n the case of
terminal olefins like styrene, butylvinyl ether, 1 -octene and allylacetate, while very modest
yields in ketones are observed with Internal olefins like cyclohexene, cyclouctene and
cis-4-methyl-2-pentene. With respect to the same substrates, Platinum alone is reactive only
84
TABLE 1
Oxidation of different olefin;
i n the presence of a 1 1 1 [(dppe)Pd)(CF,)(CH,Cl~)+/
(drphoe)Pt( CF 3)( OH) catalyst mixtures
Olefin
% Product&
Time ( t i )
ketone
other &
epoxide
Ally1 Alcohol
No reaction
-
4
24
0.5
1.5
6.4
Styrene
2
6
24
56
19 2
26 2
Cyclohexene
2
6
24
03
04
03
03
24
-
1.0
52
2
6
24
02
06
33
84
2
6
0.1
0.2
23
04
9.5
-
A l l v l Acetate
Cis- 4 Methyl-2-Pentene
Cvc I octene
2
24
6 uty lv in y 1 ether
6
I -0ctene
3
7
24
01
2.4
-
14.4
benzaldehyde 2.6
0.5
03
04
26.0
38 1
44
1"
22 4
38
4.5
0.8
10
06
15
06
43
58
90
-
-
10
-
72
30
08
64
54
36
60
a Experimental Conditions [Pd+]=[F'tvH] 2x10-3M. [ I -octenej 1 4 M, [H202] 0 7 M , salvmt
THF, T 65°C
b Yields calculated w i t h respect to H202
c Mainly iso-alcohols, glycols and C( n- 1 1 aldehydes
toward 1 -octene and v e r y slightly toward styrene, producing epoxldes as the exclusive uxidatiori
products ( r e f . 1 ), The increased reactivity of this bimetallic system seems ?n reflect ?he
expected order for nucleophilic attack onto a coordinated olefin.
I n order to get better insight into the r o l e o f Palladium on the activity and selectivity of this
bimetallic system, we have studied the oxidation of 1 -oc,tene as a function of the catalyst
composition by varying the Pd/Pt molar r a t i o The resultsare summarized i n Table 2. Again, i n
addition to 2-octanone and I ,2-epoxy-octane other oxidation products are formed including
2-octanol and 1 ,2-octandiol as the major components. Their total yield varies between 2-6.5%
depending on the amount of Palladium. A representative reaction p r o f i l e is shown in Fig. 1 .
85
TABLE 2
Catalytic activity of the Pd+/PtOH system i n the oxidation of 1 -octene as a functiori of the Pd/Pt
rati&.
Pd/Pt
Time ( h )
% Product&
ketone
epoxide
1 OSxRate
(Ms-1)
others
Cat 1ifetimeC
(h)
0.5
35
8
24
9.2
24
23
32
3.5
2.4
1.8
3.6
5.6
1.91
6
1 .o
3
7
24
4.4
12
22.4
3.0
3.6
6.0
0.8
6. 4
5.4
0.54
>8
3.0
0.5
3
8
24
1.8
9.4
15.6
15.0
4.2
5b
3.4
03
0.2
3.5
5.2
6.0
124
4
7
2
3
24
11.5
15 4
15.5
3.8
32
-
3.5
2.06
3
Pd+ a l o n d 3
6
5.3
18.2
17.5
0.3
0.8
0.4
2.be
1.15
>8
PtOHalone 2 4
-
2.4
-
0.07
24
62
6. 4
6.0e
6.4C
24
a Experimental Conditions. [PtOH] ~ x ~ O - ~[ 1-octenel
M ,
1.4 M, [H2021 0 7 M , solvent THF,
T 65'C
b Calculated wit h respect to H2%.
c Taken when the colorless reaction m i x t u r e fades to brown
d [ P d + l 2xlO-3M.
e about 60%heptanal
Typically, the formation of the ketone i s i n i t i a l l y slower than that of the epoxide, while the
epoxide formed after reaching a maximum i s p a r t l y destroyed. This behavior seems to suggest,
at least i n part , the occurrence of two consecutive reactions, where the epoxide i s part ly
isarnerized to ketone and p a r t l y converted into other oxidation products. Another important
difference of this system w i t h respect t o the analogous system consisting only of Platinum is the
catalyst
lifetime
that
[(dppe)Pd(~F3)(solv)1
[(dppe)Pt(CF,)(solv)I
+
+.
is
rather
complex
limited
due
compared
to
to
the
lower
that
of
stability
the
of
the
homologous
When Pd(0) starts forming the reaction practically stops while the
remaining hydrogen peroxide i s v e r y rapidly destroyed.
An analysis of the effect of the Pd/Pt r a t i o on the catalytic activity i s not easy. Table 2 shows
that the maximum amount of products does not significantly change when increasing Pd
concentration, since an increase i n the rate of conversion i s balanced by the lower stability of
epoxide
ketone
others
0
100
200
300
400
500
time ( m i n )
Fig. I . Reaction p r o f i l e for the oxidation of 1 -octene w i t h the Pdt/PtOH catalytic system.
Reaction conditions as i n Table 2, Pd/Pt = 3
the catalyst. Moreover the maximum rates of olefin conversion are observed both at the lowest
and highest Pd concentrations
In Table 2 the two blank reactions employing either Pd alone
or
P t alone as catalysts are
reported. While the latter i s a v e r y Door catalyst under the usual experimental conditions, Pd
shows an activity s i m i l a r to the bimetallic m i x t u r e but producing negligible amounts of epoxide
The data obtained i n the blank reactions seem to suggest that w i t h the bimetallic system the
two metal are actually working independently So i t seems important t o determine whether there
i s actually a cooperative effect between the two metals. Indeed according to the mechanism
determined for Pt only ( r e f . 4) reaction 1 i n the case of the bimetallic system should read as
Pd(ol)+
t
PtOOH
+
oxidation products
PdOH
+
Pt+
Pt(ol)+
+
PdOOH
+
oxidation products + PtOH
+
Pd+
Pd(ol)+
+
PdOOH
-t
Oxidation products
+
Pd'
PdOH + P t +
*
+
+
PdOH
Pd+ + PtOH
reaction 8 I n p r i n c j p l e i n the next catalytic cycle the two roles would be inverted ( reaction
10)
However, t@gether w i t h the two heterobimetalliG oxidation steps even the two
homobimetallic oxidation steps are possible
I
e reaction 1 and reaction 10 Of course the
relative weight of these four possibilities on the overall catalytic activity w i l l depend on the
-0HexchangeequiIibrium
1
e reaction 1 1
The position of equilibrium 1 1 was determined by 19F NMR spectroscopy The complexes
(dppe)Pd(CF3)(0H) ( 0 05 mmOl, 6(CF3) -29 71 ppvi (dd). 3 J ~ p c l s24 7 Hz, 3J~pirans63 4
Hz) and I(diphoe)Pt(CF,)(CH*CI2)1BF4
il) 05 mmol, S(CF3) -28 58 ppm (dd). 3JFpCls 8 6
87
Hz, 3
J ~ p t 56.7
~ - ~Hz,
~ ~2 d ~ p t 518
Hz) Were dissolved in CD$12
( 1 mL) and a 19F NMR
spectrum of the m i x t u r e was r u n showing the presence of only [(dppe)Pd(CF3)(CH2Cl,)1BF4
(S(CF3): -26.97 ppm (dd), 3 J ~ p 23.0
~ i ~Hz, 3 J ~ p 63.7Hz)
t ~ ~ and
~ ~(diphoe)Pt(CF3)(OH)
~
(S(CF3): - 2 7 . 4 6 ppm (dd), 3 J ~ 9.7
p Hz,
~ ~3 ~ J
~ 57.1
p Hz,
~ 2~J ~~565
p ~~ Hz)~ indicating that
equilibrium 1 1 i s completely shifted to the right. Identical 19F NMR results were obtained
starting from [ (dppe)Pd( CF3)( CH2C12)IBF 4 and (diphoe)Pt( CF3 )( OH 1. These experiments
suggest reaction 8 as the main oxidation pathway in t his bimetallic system.
In conclusion the data obtained w i t h Palladium as olefin activator prove that the bimetallic
catalytic system in these oxidation reactions i s indeed involved and that the activity and
selectivity of the system can be modified. Although the search for the appropriate combination of
metal complexes was not straightforward the results here reported show that bifunctionalbimetallic catalysis i s indeed possible and t h i s i s i n principle another possible way in which a
homogeneous catalytic reaction can be "tuned" to the achievement of the desired properties.
EXPERIMENTAL SECT ION.
Atmaratus.
IR spectra were taken on a Perkin-Elmer 597 spectrophotometer either in Nujol mulls (Csl
plates) or i n solution ( NaCl windows) 19F NMR spectra i n CD2C12 were recorded on a Varian FT
80 A spectrometer operating in FT mode, using as reference external CFC13. Negative chemical
shifts are upfield from the reference GLC measurements were taken on a Hewlett-Packard
5890 A gas chromatograph equipped w i t h a Hewlett-Packard 3 3 9 0 A integrator. Identification
of products was made w i t h GLC by comparison w i t h authentic samples.
Materials.
Solvents were dried and purified acvording to standard methods. Olefins were purified b y
passing through neutral alumina, distilled and stored under N2 i n the dark. Hydrogen peroxide
( 3 4 % )(Fluka) was acommercial product and used without purification. The preDaration of the
complexes was performed under d r y N: b y conventional Schlenk and syringe techniques.
The following compounds were prepared by literature methods: [(COD)RhC112 (ref . 91,
[ ( dppe)Rh( C0D)lClO 4 ( r e f . 91, [(dppe)Rh( 1 -octene)2)1C104 ( ref. 91, [ ( dppe)lr( COD)IC104
( ref.
1 0)
~
idip hoe )P t( CF3)(
OH) ( ref.
1 3 1,
I( dip hoe)P t( CF3 )( CH2CI2 11BF
( diphoe)Pt( CF 3)( OOH) ( ref. 61, [ (dppe)Pd( CF3 )( CH2C12)I BF
( ref. 13 ) ,
( ref. 6 ) , ( dppe)Pd( CF3)( OH)
(ref. 6).
Abbreviations:
dppe = 1,2-diphenylphosphinoethane; diphoe = cis- 1,2-diptienylphos-
phinoethylene; COD = 1 ,5-cyclooctadiene; 01 = olefin; solv = solvent; THF = tetrahydrofuran;
DCE = 1.2-dichloroethane.
Reactivitv.
Catalytic Reactions were carried out in a 25 mL round-bottomed flask equipped wit h a
stopcock for vacuum/N2 operations, a r e f l u x condenser and a side-arm fitted w i t h a
screw-capped silicone septum to allow sampling. Constant temperature (65°C) was maintained
by an external o i l bath equipped w i t h heating coil and a Vertex thermometer for temperature
control. S t i r r i n g was performed b y a teflon-coated bar driven externally by a magnetic s t i r r e r .
The general procedure here reported was followed in a l l cases. In a typical experiment the
appropriate amounts of complexes were placed solid i n the reactor which was evacuated and
placed under N2 atmosphere. D r y , N2-satIJrated THF was added, followed by the olefin t.o be
oxidized After s t i r r i n g the m i x t u r e up to the desired temperature the H202 solution was
injected and the time was started. The conversion was monitored by sampling periodically the
reaction mix t ure wi t h a microsyringe.
ACkNOWLEDGEMENTS
This work was supported j o i n t l y by the European Economic Community (Brussels) and
Degussa AG (F rank f u r t ) through the special program BRlTE Special thanks are expressed to
D r s G Goor and M Schmidt (Degussa AG) to Professor W Drenth (University of Utrecht) and to
Professor J W Buchler (University of Darmstadt) f o r stimulating discussions
REFERENCES.
1 G. S t ruk ul arid R.A. Michelin, J . Chem. Soc. Chem. Commun., ( 1984) 1538; G. Strukul and
R.A. Michelin, J . Am. Chem. Sac., I07 ( 1985) 7563.
2 R. Sinigalia, R.A. Michelin, F. PinnaandG. Strukul, Organometallics, 6 ( 1987) 728.
3 G. Strukul, R. Sinigalia, A. Zanardo, F. Pinna and R.A. Michelin, Inorg. Chem., 28 ( 1989)
554.
4
5
A. Zanardo, F. Pinna, R.A. Michelin and G. Strukul, Inorg. Chem., 27 ( 1988) 1966.
S.E. Diamond, F. Mares, A. Szalkiewicz, D.A. Muccigrosso and J.P. Solar, J. Am. Chem. SOC.,
6
7
8
G. S t ruk ul, R. R0sandR.A. Michelin, Inorg. Chem., 21 (1982) 495.
M. Roussel and H. Mimoun, J. Org. Chem., 45 ( 1980) 5387.
A. Zanardo, R.A. Michelin, F. PinnaandG. Strukul, Inorg. Chem., 28 ( 1989) 1648.
J . Chatt and L.M. Venanzi, J. Chem. Soc. A, ( 1957) 4735; R.R. Schrock and J.A. Osborn,
J. Am. Chem. Soc., 93 ( 1973) 2327.
M. Green, T.A. Kuc and S.H. Tatlor, J. Chem. Sac. A, ( 1971 2334.
G. Strukul, Unpublished results; B.L. Booth, R.N. Hasze1dineandG.R.H. Neuss, J. Chem. SOC.
Dalton Trans , 37 ( 1982).
R.A. Sheldon and J.K. Kochi, Metal Catalyzed Oxidations of Organic Compounds, Academic
Press, New York, 1981, p. 191; F . R . Hartley,Chem. Rev., 69 (1969) 799; F.R. Hartley.
J Chem. Educ., 50 ( 1973) 263.
R.A. Michelin, M. Napoli and R. Ros, J. Organomet. Chem., 175 ( 1979) 239.
9
10
11
12
I3
104 ( 1982) 4266.
Vmc~weI, Lariacia,. i t is possible that !.he
compiexe.; catalyze, i n solution, the isornet-ization of epoxides to ketones, a
factor which could affect dlrectlv the observed selectivities Have you checked this possibility?
..i,4;-lEi, 1
- Iat1riurri;F’aliadirlrri
F a
6. :TRWKIJL.
r z i t y of bri!:sr~ 5;liwmd,
(University of Venice.
It.air). The Fiatinurn comp1e.f used does not cata!yre the
!:.l:irriei-izstil?n lit’epoxides t o ketones. however, as you suggest, thls IS 3 Ilkel\/ possibility I n ir~e
C ~ S Bof Palladium that we w e rlot. checked A implemerttal-y and/or alter native pilssitiliity i s
!h31 t.he Palladii.lm i:ornplek prnrriotes farther reactioq of the epoxide wit h H?02/H2r3 3s seen.15 to
be suggested bv t h e reaction p r o f i l e ( Fi g I j dnd by the nature of the other products reported Ir.
Tatlles 1 arid 2
G . Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
89
PLATINUM CATALYSTS FOR CYCLOHEXEmE EPOXIDATION WITH AN
OXYGEN-HYDROGEN MIXTURE
N.I. KUZmTSOVA, A.S. LISITSYN, A.I. BORONIN and V.A.LIKHOLOBOV
Institute of Catalysis, Novosibirsk 630090 (USSR)
SUlynraARY
Upon simultaneous oxidation of c-hexene and hydrogen with molecular oxygen, epoxide of c-hexene is formed on Pt catalysts
in absence of a porphyrin-like co-catalyst, provided that a part
of Pt in the catalysts is in a metallic state and (i) the catalysts were reduced at moderate temperature or (ii) special combination of two Pt catalysts was used or (iii) hydrogen chloride was added into the reactor. The nature of the active sites
responsible for the epoxide formation is discussed.
INTRODUCTION
Co-oxidation of two or eeveral substrates sometimes allows
one to obtain desirable product under milder conditions and/or
with the use of a simpler oxidant. In particular, when olefins
interact with O2 in the presence of NaBH4 (ref. 11, sodium ascorbate (ref. 21, %I/CH~COOH(ref. 3 ) or H2'(refs. 4-6), such valuable compounds as epoxidea are formed:
\
/
c=c,
/
+ oz
When R is hydrogen? metallic Pt together with the Mn or Fe porphyrins is used to catalyze the process (refs. 4-6). Recently it
has been found, however, that the porphyrin complexes are not necessary component of the catalytic mixture and epoxide can form
on Pt catalysts alone (ref. 7). Here we present new data on properties of the catalysts, with main attention to factors affecting the catalytic activity in epoxidation.
EXPERIMENTAL
i) Preparation of catalysts. Silica (ZOO m2/g, BET) was impregnated with aqueous solutions of H2PtC16, K2PtC16 or K2PtC14
(commercial substances) to provide a Pt content in the catalysts
of 2.0 wt.%. The samples were dried in air for 20 h at ambient
temperature and then treated in a flo w of hydrogen (0.5-1 cm3/8/d
90
at a stepwise increase in temperature (0.5 h at 320, 370 and
420 K, 4h at 520 K and 7 h at 720 K). Before contact with air,
they were cooled in H2 and flushed with N2. For designation of
catalysts see Table 1. With the exception of 1-720, studied by
XPS were earlier prepared catalysts (ref. 7). Pollowing impregnation, they were dried at 330 K and treated in H2 at heating at
once to desired temperature.
ii) XPS-study
was performed on a YO ESCALAB electron spectrometer. Samples were supported on a nickel net and spectra caequal to 103.6 eV.
librated relative to % Sipp
iii) Catalytic runa were carried out following the same technique as described earlier (ref. 7). A static two-neck reactor
was supplied with a magnetic stirrer and connected with a volumetric burrete filled with dibutylphthalate. After loading a catalyst (usually 20 mg) into the reactor, acetonitrile (1 m l ) was
introduced to cover the catalyst, then the system was flushed
with premixed O2 and H2 and experiment started by introducing chexene (10 pl). In special experiments, a freshly prepared solution of HC1 in acetonitrile (via mixing acetonitrile and concentrated aqueous HC1) was added (several pl) before c-hexene. Products were analyzed chromatographically (sampling with a syringe
through a silicon gasket in one of the reactor necks).
The laboratory system was placed behind a protective screen.
( A l l other precautions necessary for operations with explosive
mixtures should be followed if repeating the experiments).
RESULTS
Treatments of the samples with supported Pt chlorides at progressively higher temperature8 led to decrease in intensity and
then disappearance of C1
lines in photoelectron spectra, which
2P
was accompanied by a shift of the Pt4f lines to EB value typical
for metallic Pt (71.0 eV for Pt 4f7,2,
(ref. 8 ) ) . Nevertheless,
ion etching of the samples (Ar+, -10-20 nm) resulted in a significant back shift of the Ptqf lines to high binding energy
(cf. spectra 5 and 4 in Fig. 1) and reappearance of the C1
2P
lines.
Catalytic properties of the samples prepared in different conditions and from different Pt compounds are given in Table 1. In
this case, only on the Pt(1V)-derived catalyst8 reduced in H2 in
a narrow range of temperature (320 S T 5 4 2 0 K) waa formation
of c-hexene epoxide observed. To some but minor extent, the up-
91
h
Fig. 1. X-ray
pretreated in
brackets near
ded after ion
photoelectron spectra of H2PtC16-derived catalysts
hydrogen under different conditions (indicated in
each spectrum; in case (5) the spectrum was recoretching the sample).
per and lower values of feasible temperature and catalytic properties were influenced by such parameters as temperature of drying the samples, duration of reduction, etc. (cf. data for newly
and earlier prepared H2PtC16-based catalysts in Table 1). The
main alteration during catalytic runs was suppression of the
side-hydrogenation of c-hexene, so that the epoxide to c-hexane
ratio increased with time.
It has been found, however, that epoxide is formed when "overreduced" catalysts are combined with those non-subjected to prereduction and contained Pt(I1) or Pd(I1) chlorides (Table 2).
Although each component of the catalytic mixtures appeared nearly or entirely inactive if being tested separately, the velocity
and selectivity of epoxide formation at the combined action were
sometimes higher than even for best catalysts in Table 1.
Meanwhile, in case of (CH3CNI2PdCl2 as second component, an
elemental analysis showed rapid decomposition of the complex
92
TABLE 1
Conditions of preparation and catalytic properties of Pt catalysts
in co-oxidation of c-hexene and hydrogen under standard conditions (K2/02 = 1 v/v, 293 K, 1 atm, catalyet amount 20 mg, 0.1
mmol c-hexene in 1 ml acetonitrile)
Designation of
catalyst
;:y:r-
Pt corn- Treat Colour 02/H2
pound
ment
of
ueed
in He, sample
Tmax
( mmol) a
(K)
I-init
1-320
1-370
1-420
1-520
I-320b
I-370b
I-420b
11-init
11-320
11-370
11-420
111-init
111-320
111-370
111-520
H2PtC16 no
- I / - 320
-
-
-
-
- 370
11
420
- 520
- 320
-
II
I1
,I
II
-
- 370
- 420
K2PtC16 no
- 11 - 320
- - 370
- 420
K2PtC14 no
- 11 - 320
- 11 - 370
- 11 - 520
a over 1 h;
II
yellow 0.2
yellow 0.25
grey
0.45
grey
1.1
grey
1.25
lightgrey
grey
grey
yellow 0.15
yellow 0.15
grey
0.2
0.3
grey
pink
0.1
grey
0.3
0.2
grey
0.9
grey
-
-
c-Hexene conversion
( ,umolIa
total into
c-hexane
into epoxide
25
20
50
7
10
l5
17
60
15
10
none
trace
7
trace
none
10
65
40
13
9
12
25
10
6
4
none
none
a
8
10
13
l3
20
10
4
12
3
1
1
4
8
none
none
none
none
11
17
5
prepared and tested earlier (ref. 7)
TABLE 2
Properties of some binary catalytic mixtures in c-hexene epoxidation (standard consitione)
No
1
2
3
4
Composition
1-420 + 1-297
1-420 + K2PtC14/Si02
1-420 + H2PdC14/Si02
1-420 + (CH3CB)2PdC12
c-Hexene conversion (%%)
over 1 h
total
into epoxide
18
69
14
38
24
63
63
17
a by 10 mg of each su ported catalyst, with an equimolar Pt to
Pd ratio in case (47
under experimental conditions and absence of Pd in the solution
93
TABLE 3
Properties of Pt catalyete In co-oxidation of a-hexene and hydrogen when HC1 (5 ,umol) I s added to the reaction mixture (0.1
mmol c-C6HI0 in 1 m l acetonitrile, 20 mg catalyst, 293 K)
OdH2
Catalysta consuption
(moll
1-320
1-370
1-420
1-520
1-720
11-320
11-370
11-420
I1I-lnit
111-320
111-370
111-520
0.15
0.15
O m 25
0.35
0.35
0.1
0.25
0.4
0.05
0.05
0.3
0.3
Total
c-hexene
conv ersion
( p m o l )b
c-Hexane
formed
(prnol~~
trace
25
50
50
50
55
4
25
50
6
6
7
7
none
1
8
none
2
10
20
35
5
7
45
a as designated in Table 1;
c-Hexene
epoxide
formed
( pmolJb
over 1 h
after catalytic run. But when the solid was filtered off from
the reaction mixture and w e d in new catalytic run without adding (CH3C8)2PdC12, epoxide of c-hexene was not formed.
An attempt has also been made to recreate the active sites reeponsible for epoxidation through a back treatment of the "overreduced" samples with hydrogen chloride. When being pretreated
with an acetonitrile solution of HC1, such samples remained inactive in epoxidation; however, the epoxidation took place if an
appropriate quantity of HC1 was added immediately into the reactor for catalytic testluge. The main results obtained in this latter case are present i n Table 3 and Figs. 2-5. As under standard
conditions, other products were c-hexane, c-hexenol and preeumably c-hexenon (mass-spectrometry data), with combined yield near
50% on a converted c-hexene basis. Also detected were c-hexane
chlorhydrine and some heavy unidentified products. Addition of
HBr turned out less effective and that of HC104 or CF3COOH noneffective.
DISCUSSION
The preeent study reveals aome new conditions under which one
94
n
2 ao
a
000-
5
6
0
i
16
40
a
a,
20
cl
I
0
80
40
time
(min)
HC1
(pmol)
Fig. 2. Typical catalytic performance of Pt catalysts in c-hexene epoxidation with an oxygen-hydrogen mixture upon addition of
HC1 (12 p o l at 0.1 mmol c-C6H10 in 1 ml acetonitrile, catalyet
1-720).
.and3. Fraction
of c-hexene converted over 1 h into all products
('I7
into epoxide (El as a function of the HC1 amount added
to the reaction mixture (0.1 mmol c-C6H10 in 1 m l acetonitrile,
Pi
02/H2 = 1, catalyst 1-520).
-
-
Fig. 4. Formation of c-hexene epoxide
: 1
in usual catalytic run with addition of HC1; 2 when equimolar amount of c-hexene epoxide is also introduced to consume the HC1 added. Other
conditions: 10 pnol HC1, 0.1 mmol c-hexene, 1 m l acetonitrile,
catalyst 111-520.
-
Fig. 5. Decomposition of c-hexene epoxide in acetonitrile solution under 02/H2 gas phaee: 1
in presence of Pt catalyst
(1-720, 20 mg); 2 under the action of H C 1 (was added to provide
in presence of both the catalyst and
[HCl], 3 12 mmol/l); 3
HC1.
-
-
should expect for the epoxide formstion at interaction of c-hexene with 02/H2 mixture on Pt catalysts. It seems more reliable to
use binary catalytic compositions (Table 2) or promote Pto-con-
95
taining samples with HCl (Table 3). In opposite case (table 1)
the Pt catalystsmust be prepared by very specific method, and it
makes understandable why their capability of catalyzing epoxidation ha6 been revealed only recently (ref. 7).
Based on some literature data and inactivity (under standard
conditions) of 'both initial and "overreduced" samples, a twostage scheme of the catalytic process has been proposed (ref. 7 )
which assumes peroxide formation f r o m O2 and H2 on metallic Pt
and subsequent interaction of the peroxide and olefin in presence
of Pt ions. It is supported by the results obtained in thie work
with binary catalytic mixtures (Table 2). Presence of metallic Pt
in a l l the active catalysts is clearly indicated by their colour
and consistent with the XPS data (Table 1, Fig. 1). Besides,
tests with K I have been made, which did witness for the peroxide
formation during catalytic m a .
Although bifunctional action of the catalysts can, in general,
be accepted, there is uncertainty in exact nature o f the necessary active sites. At the moment, it s e e m most probable that f o r mation of both peroxide and epoxide takes place on the surface of
metallic Pt, but with chlorine-containing species serving a6 a
necessary modifier of the surface. These surface chloride adducts
can form in presence of HCl or the metal chlorides which are
specially added or present in the sample8 treated in H2 under
mild conditions. (The Pt chlorides which remain in the catalysts
after a high-temperature reduction can hardly participate in the
catalytic process. In this case they are probably incapsulated
in micropores of the support or inside crystallites of metallic
Pt (in accord with TEM data on the 1-470 and 1-720 catalysts
Pt crystallites of 2-20 and more nm in size). It expthere are
lains the inactivity of such "overreduced" catalysts in epoxidation and why the oxidized Pt is hardly developed in photoelectron spectra ( 3 ) and (4) in Fig. 1, recorded without ion etching
the samples.) Perhaps, the role of metal chlorides is limited
simply to supplying HC1,through their partial or complete reduction during catalytic 2 ~ 1 6 ,as it has been observed for the Pd
complex as the second component (case 4 in Table 2).
It should be pointed out, however, that H C l reacts rapidly
with c-hexene epoxide and promotes not only its formstion but
decomposition as well (Fig. 5). Obvioualy, it is responsible for
an apparent induction period in c-hexene epoxide formation seen
in Fige. 2,4 (note that it is absent in case (2) in Fig. 4) and
96
a volcano-like plot of the dependence of epoxide yields VS.
amount of HC1 added (Fig. 3).
The unsuccessful attempts to activate the "overreduced" catalysts with HC1 in other way than in situ show the surface chloride species to be of labile nature and exist in equilibrium with
some chlorine-containing compounds in the reaction solution. Because HC1 is rapidly consumed with epoxide, the role of such compounds during catalytic runs is played, presumably, by c-hexane
chlorhydrine
At last, it is not excluded that acetonitrile participates in
the process not as a solvent o n l y , and more detailed information
on mechanisms of epoxide and by-products formation would be valuable for improving the rate and selectivity of epoxidation.
.
REFEREMCES
1 H. Sakurai, Y. Hataya, T. Goromaru and H. Illatsuura, J. Mol.
Catal., 29 (1985) 153.
2 D. Mansuy, Id. Fontecave and J.F. Bartoli, J. Chem. SOC. Chem.
Commun. (1983) 253.
3 P. Battioni, J.F. Bartoli, P. Ledue, K. Fontecave and D. Mansuy, J. Chem. SOC. Chem. Comun. (1987) 791.
4 I. Tabushi and R. Morimiteu, J. her. Chem, SOC., 106 (1984)
6871.
5 I. Tabushi, 96. Kodera and 1. Yokoyama, J. Amer, Chem. SOC.,
107 (1985) 4466.
6 Van Beijnum, A . I . van Dillen, I.W. GeU8 and W. Drenth, in:
Yu.1. Yemakov and V.A. Likholobov (Eds. ), Homogeneous and
Heterogeneous Catalyeis, Proc. 5th Intern. Symp. on Relat.
between Homogeneous and Heterogeneous Catalysis, VNU Sci.
Press, Utrecht, 1986, p. 293.
7 N.I. Kuanetsova, A.S. Lisitsyn and V.A. Likholobov, React. Kinet. Catal. Lett., 38 (1989) 205.
8 G.E. Moilenberg (Ed.), Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer, Minnesota, 1979.
J . K I W I (Federal I n s t . o f Techn., EPEL Chimie Phys., S w i t z e r l a n d ) : I f y o u combine 0 + H ( 1 : l ) over P t / S i O c a t a l y s t as y o u have done, you have a h i g h r i s k o f
e x p l o2 s i o n ? To a v o i d t h i s i n which
2
o r d e r do y o u mix, t h e c a t a l y s t , s o l v e n t and
r e a c t a n t s gases w i t h cyclohexene ?
N . I . KUZNETSOVA ( I n s t . of C a t a l y s i s , USSR) : To m i n i m i z e t h e r i s k o f e x p l o s i o n ,
t h e c a t a l y s t was always b r o u g h t i n c o n t a c t w i t h t h e 0 IH m i x t u r e a f t e r i t had
a l r e a d y been c o v e r e d w i t h s o l v e n t ( a c e t o n i t r i l e ) and presence
2 2
o f dry catalyst
on t h e r e a c t o r w a l l s was t h o r o u g h l y e l i m i n a t e d . A l l experiments (more t h a n 100)
have gone smoothly when we f o l l o w e d t h e s e p r e c a u t i o n s .
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
97
SELECTIVE OXIDATION OF ALCOHOLS TO CARBONYL COMPOUNDS BY MANGANESE (111) ~ - 0 X CARBOXYLATO
0
COMPLEXES
Hubert MIMOUN", Lucien SAUSSINE', StCphane MENAGE' and Jean-Jacques GIRERD'
'Institut FranGais du PCtrole, B.P. 311,92506 Rueil Malmaison (France)
'Laboratoire de Chimie Inorganique, UA CNRS 420, Institut de Chimie moltkulaire d'Orsay, UniversitC Paris-Sud, Bat 420,91405 Orsay (France)
Summary
Novel manganese (IIU p-oxo complexes with the formula Mn~'O(~F,CO,),(bipy),(1)were
synthesized from the oxidation of Mni (C,F,C0,)3(bipy), Q) by potassium persulfate. The X-ray
crystal structure of a)revealed a [Mn, O(RC0,)J + p-0x0 core, with one C,F,CO, group linked to
each Mn(1II) ion in amonodendate fashion. The X-ray structure of (2) indicated acupric acetate type
structure with four binucleating carboxylato groups. Compound a) selectively oxidizes alcohols to
carbonyl compounds, giving Q) in quantitative yields. Kinetic studies of this reaction, followed bX
visible spectroscopy in CH@ solution, suggested that the Mn"-O-Mnm core dissociates into Mn
and active Mn"=O (or Mn '-0')species which react with alcohols in a way similar to highvalent
metal 0x0 species.
Introduction
The active oxygenated species involved in catalytic oxidations have been to date almost
exclusively limited to metal peroxides and highvalent metal 0x0 species (Scheme 1). [l]
-
Scheme 1
M
+o2
__c
MOz
+M
MQM -2M=O
+M
--C
MOM
98
Reactive binuclear y o x o complexes are rare. To our knowledge, there only exists one reported
example of allylic alcohol oxidation by p-0x0-bis(chlorotriphenylbismuth(v) [ 2 ] . In fact, p-0x0
complexes are generally considered as dead-end species in various oxidative processes such as the
catalytic oxidation by O2of phosphines in the presence of Mo(V1) cisdioxo dialkyldithiccarbamates
[3],andthe catalytic epoxidationofolefins by02in thepresenceofrutheniumtetramesityl porphyrins.
[41
Binuclear Mn(1II) p-0x0 complexes bearing tridendate nitrogeneous bases, [5] or bipyridine
(bipy) [6] as terminal ligands have recently been prepared, but their reactivity is not known.
We report here the synthesis and characterization of novel Mn(IlI) p o x 0 complex with the
formula Mn2mO(qF7COa,(bipyX
and its reactivity with alcohols to give selectively
MnP((;F7C0,),(bipy)2 a)and the corresponding carbonyl compound, according to scheme 2.
u),
D
R
Scheme 2
RESULTS AND DISCUSSION
Oxidized Complex
This purple complex was prepared by precipitation from the reaction of manganese(II) sulfate
with heptafluorobutyric acid, 2,2'-bipyridine, and potassium persulfate K,S,O, in water during 30
min at 85'C (see Experimental Section). Crystals were obtained by slow evaporation of a CH,Cl,
solution of a).
The X-ray crystal structure of
consists of dimeric units (Fig 1) built by a diad axis: an
oxygen atom lies on this axis and is shared by two manganese atoms ( Mn-01=1.75(3) A,
Mn-OI-Mn=130(4)'). The environment of each Mn atom is completed by 2 oxygen atoms of 2
bridging carboxylate groups, one oxygen atom of a monodendate carboxylate group and 2 nitrogen
atoms of the bipy ligand. The coordination polyhedron is then an octahedron. Complex
contains
ru
99
a,
the shorter Mn-0 bond compared to other p o x 0 Mn(III) complexes
(4) and c5)having bridging
acetates, and the larger Mn-0-Mn angle (see Table 1) . Details of the X-ray difhction data and
structure determination of (I> are described elsewhere.[7]
N1
w
Figure 1. Structure of Mn20(qF,C02),(bipy)2
The infrared spectrum of a) exhibits an intense absorption at 1700 cm'l with a shoulder at
1670 cm", which can be assigned to the asymmetric v(C=O) vibrations of the bidendate and the
monodendate heptafluorocarboxylate groups, respectively. A medium absorption at 730 cm-' has
v,(Mn-0-Mn) bridge vibration. Treatment of a) with a CH2CI2-H2O'*
been atmbuted to the
mixture results in an isotopic exchange of the oxygen atomof the Mn-0-Mn bridgs, and the apparition
of a new band at 690 cm".
The UV-visible spectrum of a) in acetone consists of a broad absorption at 710 nm (E= SO),
and three bands at 520 (~=204),
500 (~=210)and 480 nm (~=220).
The three absorptions around 500
nm are also found in the other p o x 0 Mn(1II) complexes u),W, and G)(Table I), and c o n f i i the
p-0x0 k-carboxylato structure of (13
The magnetic susceptibility of (13 x ~ was
T found invariant at 5.86 cm3mor' from 50 to 250
K, with no antiferromagnetic coupling between the two hign spin Mn(II1) atoms. This value is close
to that found for (5) (5.98 cm3mor' at 300 K), for which no magnetic exchange was found (ref 4).
but contrasts with that of compounds
and @) for which a an antiferromagnetic coupling was
a)
evidenced.
in deuterated acetone exhibits eight resonances at 56.5, 17.0, 1.2,
The NMR spectrum of
-8.8, -28.2, -28.9. -79.5, and -103.0 ppm corresponding to the protons of bipyridine ,shifted due to
the S=2 electronic spin of Mn(III). The observation of the 8 resonances is in agreement with the C2
type structure found in the XRD structure, where one the nitrogen atoms of the bipy ligand is trans
to the p-0x0 group.
100
Table 1. XRD (Distances and angles Mn-0-Mn), Infrared and UV-Visible properties, and
magnetic susceptibilities of pox0 bis pcarboxylato bridged dimanganese (In)species.
Compounds
c1)
Q)
0
(5)
d(Mn-O)/A
1.75
1.783
1.81
1.780
Mn-0-Mn r
130
122.9
120.9
125.1
Infrared
v(Mn-O-Mn)/cm-'
730
730
712
UV-Vis
480(220)'
500(210)
520(200)
7 lO(80)
490(340)'
520(300)
535(280)
640(200)
486b
52 1
486(337)'
503( 190)
524( 175)
540(165)
582(95)
5.86
not coupled
5.46
J=-6.8 cm.'
6.69
J=+18 cm-'
5.98
not coupled
L(E)/nm
Magnetic susceptibility
xU.T(287 K)/cm3mol-'K
Compound
Q):
Mn20(OAc),(bipy~(PFs),0,,,
(ref
5);
Compound
a):
[~Mn,O(OAc)J(C1O4),.H2O where L= N,N',N"-trimethyl-1,4.7-triazacyclononane (ref 4a);
Compound (5): Mn20(OAc),(HB(pz),k where HB(pz),= hydrotris(1-pyrazoly1)borate (ref 4c).
'Solvent=acetone (this work and ref 5); bSolvent=CH,CN (from ref 4b). 'Solvent=CH,Cl, (from ref
4)
This whitecomplex was prepared from thereaction of manganese(II)carbonate with C,F,CO,H
and bipy in water at 70°C for 1 hour, or as pure crystals from the reaction of a) with ethanol (see
Experimental Section).
The X-ray crystal structure of
consits of centrosymmetric dimeric units in which each
manganese atom is bonded to four carboxylate oxygen atoms and two bipy nitrogen atoms in a skew
trapezoidal bipyramidal configuration (see Fig.2). The four carboxylate groups are bidendate,
bridging the two manganese atoms with a Mn-Mn distance of 3.679(3) A. The four oxygen atoms
arising from the carboxylate groups and adjoining the the manganese atoms are coplanar with
maximum deviations from the least-squares plane of + 0.002(15) A. The coordination of the h h
atom is completed by the two nitrogen atoms of the bipy molecule on the same side of the square
constructed from the oxygen atoms of the carboxylate groups. The manganese atom lies at 0.737(2)
A of this plane and is directed towards the N atoms. The two pyridine rings of the bipy ligand are
coplanar (maximum deviation from the best plane : + 0.05(2) A) and almost perpendicular to the
oxygen planes (87.8(4)') with the NLN2 line parallel to the 02-03 and 01-04 edges of the oxygen
squares.
u)
101
The tetra-p-carboxylate bridged structure is novel for manganese,but rather common for other
metal ions such as CuoI), Cr(II),Rh(II), Mo(II) [81. In W, the Mn-Mn distance is particularly long
(3.679 A) compared to the other metal-metalbonds.Details of the X-ray diffraction data and structure
determination of (2) are described in ref 7.
caz
cia
Figure 2. Smcture of Mnz(C&7C0J4(bipy)z0
Reaction with okfins.
Olefins such as cyclohexene, tetramethylethyleneand isoprene were found unreactive towards
a)in dichloromethane and no oxidation products were detected. However, when the reaction of (JJ
with 1-pentenc was carried out in acetone, an acetonylation reaction occurred, giving rise to the
formation of 2-octanone in c a 50% yield based on complex
according to equation (1).
u,
-+*lL&
(1)
This homolytic addition of acetone to the double bond in the presence of Mn(III) species has
already been previously described.[9]
Oxidhtion of alcohols to carbongl compounds
Addition of alcohols such as ethanol to a solution of complex in acetone or M,Cl, results
in a progressive decoloration of the solution, and the almost quantitative formation of the corresponding carbonyl compound together with the reduced colorless complex
according to the
stoichiometry of equation (2)
a
a,
102
h4n20(~F7C02),(bipyX
+ RCH,OH
d
Mn2(C,F7CO&bipy),
+ RCHO + H20
(2)
Table 2 shows that primary alcohols are more reactive than secondary alcohols, and that
amongst primary alcohols, benzylic and allylic alcohols are very easily oxidized by (1).This is
probably due to the conjugative stabilisation between the developing carbonyl group and the olefin
or the aromatic x-system. The same reactivity order of alcohols was previously observed in the
reaction of chromic compounds and manganese dioxide.[101
Table 2. Stoichiometric Oxidation of Alcohols by (I)'
I
Substrate
Yield (%)'
Product(s)b
Reaction timed
bin)
+ CH,CH(OEt),"
100
340
100
120
C&-CH=CH-CHO
95
10
CH,=C(CHJ-CHZOH
CH,=C(CH,)-CHO
97
50
1-octanol
octanal
40
200
isopropanol
acetone
40
250
2-octanol
2-octanone
10
250
CH,CHO
CbHs-CHO
I
I
I
'Reacn'on conditions: Temperature= 30T, solvent= CH2Cl,, alcohol concentration= 0.72 mol.l',
l.22.10-3mol.I-'. bProductswere identified by gcms coupling. 'Yields are based on initial (I).
dReactiontime for which the yield was obtained.cCH,CHO=86%, acetal=14%.
a)=
Kinetic study of the reaction of (awith EtOH in CH2Ci2.
The reaction was monitored by UV-Vis photometry at 485 nm in CH2C12at 30°C. This
wavelength corresponds to the maximum of absorption for u), but to a weak absorption of a).We
used the formula :
where [C] is the Concentration of binuclear species, DO the optical density of the solution,(C]othe
initial concentration of a),and el and 6 the extinction coefficients of u) and Q), respectively.
Figure 3 shows the variation of concentrationvs. time of a).A striking result is obtained when
the ethanol concentration is kept constant and when [C], is varied (see Figure 4). The half reaction
time increases when [C], increases. This strongly suggests a dissociative step in the mechanism, as
shown in equations 4-6 ( X= C3F,C0,, L=bipy).
103
xEtOH + MnmzOX,,L,
Mn”OX,L
K1
+ EtOH -%
MnnXzL(EtOH),
Mn”X,L(EtOH), + MnWOX,L
+ MnnX,L + H,O
(5)
Mnuz&L, + xEtOH
(6)
CH,CHO
+ Mn”X,L -+
(4)
If we assume that Mn”0 species react very quickly and are in a stationary concentration, and
that the concentration of ethanol, used in large excess, remains constant, we can resolve the following
simplified scheme:
k
A&B+C
k-l
a,
where A stands for
B for the Mn(I1) intermediate, C for the Mn”0 one, and D for the reduction
product of C. The following kinetic expression has been derived for the relation : concentration of
U) versus time:
where k,= k’,[RO€IIX,kl=k’.l, k,= k’,[ROH] and K = k,/k’,
which gives an increasing half reaction time when [C], increases. The theoretical curves are represented on figures3 and 4 and are in good agreement with the experimentalvalues. The corresponding
parameters Kk, and k, are listed in Table 3.
The variation of k, and Kk2 in function of the initial concentration of ethanol [ROH], are
represented in Figure 5 and give the following expressions :
k,= k’,[ROH],2’ and Kk,= K’k’z[ROH]:5
which are coherent with the value of x = 2.5. The origin of this non-integervalue is not really known,
but it might be due to the initial step involving the coordination of the alcohol to the Mn(1II) p-oxo
complex 0,which was not included in our treatment. This might be responsible for the discrepancy
existing between experimental and theoretical values at the beginning of the r e d o n .
104
I
I
5D
I
1M
I
I
IP
ZOO
I
2%
I
tllC/”1*
yx)
Fig.3. concentration vs. time dependence for the
reaction of (1) with EtOH in CH,Cl,, followed
by spectrometry at 485 nm and 30’C. The
curves correspond to the best tit to the expression (9). [C],=1.04 M. [EtOH],= (a) 0.71 M; (b)
1.18 M; (c) 1.76M.
Fig.4. Concentration vs. time dependence for
the reaction of (1)with EtOH in CH,C1, at
30’C. The curves correspond to the best fit to
the expression (9). [EtOH],=2.17 M; [C],= (a)
0.37 mM; (b) 0.47 mM; (c) 0.61 mM; (d) 0.78
mM.
-.i
-7
-9
Figure 5. Variation of log k, and log Kk, in function of log [EtOH],. a, and a, refer to the slopes.
105
Table 3. Rate constants for the oxidation of ethanol by complex
u)at 30'C in CH,Cl,.
2.17
7.4
2.0
1.76
2.2
2.95
1.18
1.0
7.35
0.7 1
0.35
0.1
Isotopic Effects.
Isotopic substitutionof ethanol by deuterium slows down the oxidation by (1).A k&,, isotopic
ratio of 2.3 was obtained when W , O D is used instead of C&OH as substrate. This value is close
to that observed in the oxidation of ethanol by acid permanganate (kfiD=2.6).[1 1J This indicates
that hydrogen abstractionby W from the hydroxyl group of the alcohol represents a rate-determining
step in the reaction.
When GD,OD was used, a differentreaction occurred:apartial decolorationwas first observed,
but then the optical density of the solution raised again and no oxidation of the wholly deuterated
alcohol took place.
CONCLUSION
The kinetic studies of the oxidation of alcohols by Mn(II1) p-0x0 species strongly suggest that
i) a dissociation of u) into a reactive MnW=O(or Mn"'-0') and an unreactive Mn(II) complex occm.
ii) This dissociation is induced by the presence of alcohols. N M R studies of acetone solutions of (1)
showed that the proton resonances are not affected by the presence of paramagnetic Mn(I0 species.
iii) The formation of Mn"=O species reactive towards alcohols accelerate the disproportionation.
Whether the active species is Mnw=O acting a two-electron heterolytic oxidant, or Mnm-O'
acting as an homolytic two consecutive one electron oxidant is not clear at the moment. Although
the acetonylation of olefins suggest an homolytic mechanism, the reactivity order of alcohols and
the isotopic effect arc closer to the reactivity of MnO, or permanganates which are known to be
heterolytic in nature. We therefore suggest the mechanism shown in Scheme 3. which involves the
formation of an hydroxy-alkoxyMn(1V) intermediate,which decomposesin aconcerted two-electron
transfer reaction to give the carbonyl compound; water and the reduced Mn(Q species.
106
Scheme 3
- Mn"
I
Experimental Section
Svnthesls.
Synrhesis of Mn20(C3F&02),(bipy), (a.To a solution of 1.1 g of MnSO,.H,O (5 mmol) in water
(25 ml) were added 1.3 ml(10 mmol) of GF,C02H (n-heptafluorobutyric acid) and 2.5 g of K2S208.
The mixture was heated for 3 min at W C , after which a dark brown color developed. 0.8 g of solid
2,2'-bipyridine were then added, and the resulting mixture was heated at W C during 30 min. During
this time, a purple solid precipitated off, which was filtered, washed with distilled water and diethylether, and dried in vacuo.Yield = 1.36 g. Anal. Calcd for C,,N8N20,,F,,Mn: C, 33.51; H, 1.25;
N, 4.34; F, 41.23. Found: C, 33.61; H, 1.30; N, 4.20; F, 40.61. Crystals were obtained by slow
evaporation of a dichloromethane solution of
under air.
Synthesis ofMns(C3F&OJJbipy), (2). Method 1 . By refluxing II)in ethanol, the reduced complex
(2) was quantitatively obtained as a white cristalline powder. Anal. Calcd for C,8H8N20,F,,Mn: C,
33.91; H, 1.26; N, 4.40, F, 41.76. Found: C, 33.46; H, 1.25; N, 4.32, F, 39.74. Method2. 1.2 g (10
mmol) of manganese (II) carbonate MnC0,.H20 were dissolved in degassed water (25 ml); 2.6 ml
(20 mmol) of C,F,CO,H were added to the solution, and the mixture was heated at 70'C for 15 min.
After C02gas has evolved, the brownish solution was filtered, and 1.57g of bipy were added to the
filtrate. A yellow solution was obtained and heated for 1 hour at 7072. Evaporation of water under
vacuum gave complex
as a white solid.
a)
ADDaratus
UV-Visible spectra were recorded on a varian 2300 spectrometer. Infrared spectra were
recorded on a Perkin-Elmer infrared spectrometer and NMR spectra on a Bruker AM 250 MHz
instrument.Magneticmeasurementsin the 3-300K temperature range werecaniedout with aFaraday
type magnetometer equipped with a helium continuous-flow cryostat. HgCo(NCS), was used as a
susceptibility standard.
107
The formation of carbonyl compounds was followed by gas chromatography (DEGS column
10%4m) and by UV-Vis spectrometry of CH2Cl,solutionin thermostated cells.Alcoholsand solvents
were purified by standard procedms before use.
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Oxford.
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Kates, J . Org. Chem.; 1985,50,3659.
10. P. Muller in “The chemistry of rhefirnctional groups, Ethers, Crown Ethers and Hydroxyl
Groups”, 1980, 1.469-538. S . Patai, ed. Wiley. Chichester (U.K) and references therein.
11. K.K. Banerji, Bull. Chon.SOC.Japan, 1973,46,3623.
108
Y . Moro-oka (Tokyo I n s t . o f Techn., Japan): I t h i n k y o u r work i s v e r y i m p o r t a n t
c o n c e r n i n g u n d e r s t a n d i n g t h e mechanism o f s e v e r a l k i n d s o f monooxygenase, such
as methane monooxygenase and t y r o c y n a s e .
( 1 ) It i s g e n e r a l l y known t h a t secondary a l c o h o l i s more e a s i l y o x i d i z e d t h a n
p r i m a r y one. 00 you have any e x p l a n a t i o n why p r i m a r y a l c o h o l i s o x i d i z e d f a s t e r
t h a n secondary one by y o u r complex ?
( 2 ) I f metaloxo complex i s i n v o l v e d t h e r e a c t i o n , do you have any i d e a why
o l e f i n i s not oxidized t o corresponding epoxide ?
H. Mimoun (IFP, Rue Malmaison, France): ( 1 ) There i s a s t r o n g commitement of
s t e r i c e f f e c t s i n t h i s r e a c t i o n . F u r t h e r t h i s r e a c t i v i t y i s n o t unusual w i t h
r e s p e c t t o a l c o h o l o x i d a t i o n by MnO
2’
( 2 ) O l e f i n e p o x i d a t i o n r e q u i r e s a v a i l a b l e c o o r d i n a t i o n s i t e s on t h e m e t a l
which a r e l a c k i n g here.
R . A . Sheldon (Andeno 6.V., Venlo, The N e t h e r l a n d ) : What i s t h e m e c h a n i c i s t i c
e x p l a n a t i o n f o r t h e d i s s o c i a t i o n o f t h e ,u-oxo complex ( M n t I I I ) - O - M n ( I I I )
i n t o M n ( I 1 ) and M n ( I V ) = O i n t h e presence o f e t h a n o l ?
H. Mimoun: I b e l i e v e t h a t t h e o x i d a t i o n o f a l c o h o l i s t h e d r i v i n g f o r c e f o r
t h i s dissociation.
G. Centi and F. Trifiro' (Editors),Neur Developments in Selective Oxidation
0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
109
SELECTIVE OXIDATIONS CATALYZED BY DIOXO(PORPHYRINATO)RUTHENIUM(VI) SPECIES
NIMAL RAJAPAKSE, BRIAN R. JAMES* and DAVID DOLPHIN
Department of Chemistry, University of British Columbia, Vancouver, British
Coluiibia, Canada V6T 1Y6
SUMMARY
The
complexes trans-Ru(porp)(O),,
where porp = the dianion of
or
5,10,15,20-tetra(2.6-dichloropheny1)porphyrin (OCP), are readily formed in benzene by treatment of the
Ru(:I)
bis(acetonitri1e) precursors with 0, or air. Such dioxo species in
solution utilize both oxygen atoms for oxygenation of thioethers to sulfoxides,
phenol to hydroquinone, and (as noted by other groups) olefins to epoxides;
2-propanol is also dehydrogenated to give acetone. Catalytic 0,-oxygenation
has been demonstrated for the thioether and olefinic substrates.
5,10,15,20-tetramesitylporphyrin
(TMP)
INTRODUCTION
There remains extreme interest in selective, catalytic oxidation of
organics, especially using 0, or air, the cheapest oxidant (refs. 1-31. Some
enzyme systems of the mono- and dioxygenase type, where one or both oxygen
atoms of O , , respectively, are incorporated into a substrate molecule, function
via an iron-porphyrin centre.
These monooxygenases, such as cytochrome P450,
utilize reductive activation of 0,, where one 0-atom is reduced to H,O and the
second 0-atom becomes available within a high-valent metal 0x0 species
V
+
O=FeI"(porp.) 1 for the oxygenation process; the dioxygenases
(O=Fe (porp)
-+
are much less well-defined (refs. 1-61.
With the aim of mimicking the Fe-porphyrin systems and to learn more of
the mechanisms, work on the second-row analogues (Ru-porphyrins) was initiated
in this department (UBC) some 15 years ago, and this included both protein and
non-protein work (refs. 5, 9-13). In attempts to attain higher oxidation state
Ru-0x0 species for emulating P450 systems, by using what has become a standard
procedure, namely, addition of an 0-atom donor to a suitable metalloporphyrin
precursor, Groves and Quinn (ref. 141 and the UBC group (ref. 15) independently
synthesized trans-Ru(TMP) ( O I z , 2 , by reaction of meta-chloroperbenzoic
The presence of the ortho-methyl groups of the TMP
acid with Ru(TMP)(CO).
ligand is advantageous in preventing sterically formation of p-0x0 species
(Ru-0-Ru),
which are inactive, thermodynamic sinks in Ru-oxygen chemistry
(refs. 5, 15-17).
110
It soon became evident that 2 was readily formed in situ on exposing solutions of Ru(TMP)L, ( L = THF, MeCN, or vacant) to air or 0, (refs. 182 0 ) . and that the system catalyzes stereospecific 0,-oxidation of olefins to
epoxides (refs. 18, 21). oxidation of tertiary phosphines to the oxides (ref.
20) and thioethers to the sulfoxides (ref. 22).
Both 0-atoms of 2 are
transferred to substrate, with formation of two mole-equivalents of monooxygenated substrate per mole of 2; thus, in effect, 2 acts as a dioxygenase
while the substrate undergoes monooxygenase conversion.
More generally, the use of porphyrin and nonporphyrin 0x0 complexes of
metals in solution (with Ru dominating the more recent literature) for stoichiometric and catalytic oxidation reactions is extensively documented (refs. 2330); the systems include 0-atom transfer, and dehydrogenation with concomitant
formation of H,O, but the use of 0, as the primary oxidant in the absence of
a sacrificial added reductant is rare, and dioxygenase-type activity is thus
far unique for the title trans-Ru(porp) (0) complexes.
The present paper gives further details of our thioether oxidations reported recently (ref. 221, as well as results on the use of 2 for oxygen
incorporation into phenol, and for conversion of 2-propanol to acetone; some
data on another analogous, sterically hindered, octachloroporphyrin system,
trans-Ru(OCP)(O),,
are presented also.
EXPERIMENTAL METHODS
The carbonyl Ru(TMP)(CO) was prepared from Ru,(CO),, and the free-base
porphyrin H,TMP (ref. 31) according to a literature procedure (ref. 32). The
carbonyl was converted to the bis(acetonitri1e) complex Ru(TMP) (MeCN),,
1,
by a standard photolysis method described elsewhere (refs. 9, 19) using a 450W
Hanovia Hg vapour lamp: 1, characterized previously by
lH
NMR and IR data
(ref. 1 9 ) , has now been subjected to X-ray crystallographic analysis, the
structure (Fig. 1) being solved by conventional heavy-atom methods (ref. 33).
Addition of 0, to benzene or toluene solutions of 1 rapidly generates in
situ solutions of trans-Ru(TMP)(O),,
2 (refs. 18, 1 9 1 , and complete
removal of solvent from such solutions gives quantitative yields of spectroThe corresponding trans-Ru(OCP)(O),,
3 , was
scopically pure 2.
synthesized by
precursors.
analogous routes via the RU,(CO)~,
and H,OCP
(ref. 34)
The dioxo species are formed also by addition of meta-chloroper-
benzoic acid to the carbonyl derivatives (refs. 14, 15).
The above synthetic reactions, and subsequent reactions of 2 and 3
with tliioethers, phenol, and 2-propanol, are readily monitored by UV/vis and 1H
NMR spectroscopy, using specially designed optical cells (ref. 35) or NMR tubes
that could be attached to a vacuum line, and/or fitted with serum caps via
111
which the substrate could be added.
Kinetic data were obtained using a
thermostatted Perkin-Elmer 552A spectrophotometer, and 1H NMR on C,D, solutions
using a Varian XL-300 instrument.
The Rucporp) (OSR,) , complexes containing S-bonded sulfoxide ligands (R, =
n
n
Pr,, Bu,, Me(n-decyl)), the final Ru(I1) diamagnetic products for-
Me,, Et,,
med from reaction of 2 or 3 with the R,S
this type of reaction (20°C, in C,H,
thioethers, are isolated from
for 12h) as described previously for
R,=Et, (ref. 22), o r by reaction of the Ru(porp)(MeCN), species with the appropriate R,SO sulfoxide in excess: the latter process is rapid in C,H, at room
temperature, and the bis(su1foxide) either precipitates directly, or is obtained following chromatography on Activity I1 alumina using CH,C9, as elutant
following hexane to remove excess sulfoxide.
Reaction of 2 with excess phenol under 0, at 20'C
gives the bis (quinolato) complex Ru(TMP) (OC,H,OH)
,,
Zc,
fo:: 12h in C,H,
which
is eluted
with benzene as a brown band following chromatography of the reaction mixture
on Activity I alumina.
All isolated complexes give satisfactory elemental analyses, and have been
characterized especially using 'H NMR and W/vis spectroscopy (Tiible 1).
Fig. 1
n
(a) Diagram of Ru(porp) moiety; for TMP. X=Y=Me; for OCP. X=C9, Y=H.
(b) AE ORTEP view of the Ru(TMP) (MeCN) , molecule. Some dimensions (Ao
or " ) are: Ru-N(3) 2.027, Ru-N(2) 2.052, Ru-N(l) 2.051, N(3)-C(29)
1.126, C(29)-C(30) 1.457, Ru-N(3)-C(29) 172.5, N(3)-C(29)-C(30)
179.5,
N ( 2)-Ru-N (3 92.8 1, N ( 1)-Ru-N (3 86.16.
RESULTS AND DISCESSION
As noted in the Introduction, the presence of the ortho-substituents (Me
or CE) wi-chin the TMP and OCP systems (Fig. 1) appears to be critical for
successful generation of easily handled
trans-dioxo species such as 2
and 3 which have been shown to transfer both oxygens to substrates.
-
In the
presence of 0,, the systems become catalytic for the following processes: olefins
epoxides, PR,
-+
OPR, and R,S
-+
R,SO (refs. 18, 20-22).
112
Addition of thioethers to 2 in benzene, under Ar or 0, from 10-30°C,
results initially in production of the 0-bonded Ru(TMP)(gSR,),
2a, via a
process that is kinetically first-order in both Ru and R,S (ref. 2 2 ) : clean
isosbestic points are observed in the UV/vis, and detailed 'H NMR studies show
unambiguously that 2a is the product, eqn. (1)
(&
=
Ru(TMP), ref. 2 2 ) :
Species 2a subsequently converts slowly to the mixed F&(QSR,)
and then to the bis(S-bonded) derivative RJ(OSR,),,
(OSR,) species
(ref. 2 2 ) ; these latter
species (R,
= dialkyl), which have been isolated ( s e e Experimental), are
substitution-inert and are unreactive toward 0,. An observed, limited cataly-
tic 0,-oxidation of Et,S to the sulfoxide using 2 at 6 x lO-3M (a maximum
of 15 turn-overs over 15h at 65'C)
is thus pictured as occurring via 2a
with 0, replacing the more labile 0-bonded sulfoxides (ref. 36) to regenerate
2.
At the end of this catalysis, the TMP ligand has clearly undergone
degradation (perhaps via reactivity with a Ru-(di)oxo species) as indicated by
l o s s of the Soret absorption maximum in the 410-42Onm region and the 1H NMR
signal for the pyrrole protons.
Species 3 , containing the oxidant-resistant octachloroporphyrin
(ref. 3 4 ) , is a much more effective catalyst for the thioether oxidation: at
2 x 10-3 M in C,D,, 3 under 0, (-1 atm at 20°C) in a sealed NMR tube
effects close to complete conversion of 0.035M Et,S to the sulfoxide and s u l fone Et,SO, (4:l mixture) in a few hours at 100OC. There is no evidence for
decomposition of the Ru(0CP) moiety which at the end of the catalysis is prespecies (Table 1 ) :
further 1H NMR selective
sent as two Ru(OCP)(OSEt,),
decoupling experiments are required to elucidate whether the sulfoxide ligands
are 0- and/or S-bonded. Conditions are being sought for the selective 0,-oxidation of thioethers to the corresponding sulfoxide, this being a reaction of
industrial importance (refs. 25, 37).
Some kao values (eqn. (1)) together with the corresponding activation
parameters are given in Table 2. The rates increase with increasing alkyl
chain length within R,S, the differences perhaps being reflected more in
differences in AS' than in AH';
the 0-atom transfer, if induced by strong
u(Ru=O) vibrational motion (ref. 251, might be more efficient on encountering
a bulkier substrate and this would be reflected in relatively higher AS $
values, although as expected (and seen) the coupling reaction is entropically
unfavourable.
Corresponding data for 0-atom transfer from [Ru(bipy),(py)O] I+
to Me,S are AH' -34 kJ mol-l and AS* --110 JK-lmol-* (ref. 25). Of interest,
diphenylsulfide and methyl p-tolylsulfide do not react with benzene solutions
113
of 2 at 20"C, implying that n-acceptor aromatic groups on the thioether
impede 0-atom transfer via electronic effects. The selectivity for oxidation
of dialkylsulfides contrasts with that found for an FeC!?(TPPI/FhIO system that
utilizes the iodosobenzene as the 0-donor and effects catalytic formation of
sulfoxides from dialkyl-, alkylaryl- and diarylsulfides (ref. 38); the intermediate proposed was "C!?Fe(TPP)O". Consideration of this, and our data, which
show that (Et,Sg)Ru(TMP)O
is a more effective 0-atom donor than 2 (eqn.
(111, and those o f Groves and Ahn (ref. 201, which show that 2 is a more
potent 0x0-transfer agent than 5-coordinate Ru(TMP)O (for oxidation of PPh,),
demonstrates the likely critical role of the ligand trans to the 0x0 ligand, a
key factor in biologically important oxoiron(1V) porphyrin systems (refs. 4-6).
TABLE 1
b
1H NMR datag (and sone UV/vis absorption maxima-) for selected Ru(THP) (=I&)
and
Ru(0CP) (=Ru') species.
o-Me-
2.50
2.50
1.70
-30.45
-12.50
7.23
7.23
7.25
7.63
7.52
2.10
2.22
2.50
2.90
2.90
8.54
8.90
8.66. 8.48
7.42
7.85
7.44m. 7.36d
6.90
7.75
Hpyrrole
Ru(OznPr 1
lf
R~(o$Bu,) a2b.
&(HO-@OHhzg
2 ~ . )
H
O
@
-(
&
1R ~ ( o H ) , ~?
3.
a
Ru' (MeCN)21
k
Ru'(O),Ru'(OSEt,),- ?!
d
p-Me
Complex
8.60
8.64
8.64
d ppm from TMS in C,D,,
m-HC
3.00
2.85
7.241~1,
6.52t
unless stated otherwise: NMR data for &(MeCN),,
and the three &(OSEt,),
species are given in refs. 19, 22. All
resonances integrate correctly and are singlets unless stated otherwise.
b In C,H,; UV/vis data for &(O), (refs. 18, 22) and &(OSEt,), (ref. 22) have
been reported.
Meta-proton; single peak shows presence of a porphyrin mirror plane.
4 Ortho-methyl; single peak shows presence of a porphyrin mirror plane.
In toluene-d,. For axial ligends: b -0.20 (t. CH,), -0.43 and -0.70
(m, 8-CH,), -1.62 and -1.97 (m, a-CH2).
For axial ligands: 6 0.18 (m, CH,CH,), -0.40 and 0.60 (m, p-CH,), -1.56 and
-1.78 ( m , a-CH,).
g For axial ligands: 6 5.85 (=-HI, 5.76 (g-H).
In toluene-d,. For axial ligands: d 49.68, -68.19, -71.85 (unissigned as yet).
$ For axial ligands: d -0.35 (OH?).
1 A max at 408, 507nm. d -1.44 (MeCN).
k A niax at 420, 510nm.
a Mixture of 2 isomers with 0- and/or 5 bonded Et,SO.
Ru(O),
114
TABLE 2
Kinetic data for 0-atom transfer from Ru(TMP)(O),
Me (n-decyl)S
k20°,
0.11
M-ls-I
AH',
k J mol-l
AS',
JK-1 mol-1
56.5 ?r 1.7
-70 2 6
to alkyl thioethers, eqn. (1).
"Bu,S
Et,S
0.012
47.4 f 7.1
-120 f 30
0.0075
58.3 i 2.7
-86 k 9
The reaction of 3 with Et,S (cf. eqn. (1)) is again cleanly firstorder in 3 and in Et,S, with k = 0.072 M-ls-l at 2OoC. This value is about
10 times that for the TMP system (Table 2). and shows that the electronwithdrawing chlorine substituents favour at least the first 0-atom transfer,
presumably by increasing the electrophilicity of the coordinated 0x0 ligands(s).
Preliminary data suggest that benzene solutions of Ru(TMP)(O),,
2,
react with phenol under 0, according to the steps and stoichiornetry shown in
eqn. (21 (& = Ru(TMP)l:
2
Complex 2c has been isolated; a solution magnetic moment (ref. 39) of
-3.01.1 is consistent with the Ru(IV) formulation with S=l, exactly analogous to
B
dihalogenoruthenium(1V) porphyrin complexes; the lH NMR chemical shift data
(Table l ) , which vary almost linearly with inverse temperature, are also typical of such paramagnetic species (refs. 11, 40-42).
0.7
Abs,
in)
0.01
Fig. 2
'
380
410
nm
440
UV/vis spectral changes observed in C,H, at 20DC for reaction of 2
with phenol to give Zc, eqn. ( 2 ) ; [Rul = 2.5 x 10-6M, [phenol] =
3.72 x 10-ZM. Inset shows pseudo-first order plot for the disappeararlce
of
2
(At and A, are absorbances at 420nm at times t and m , respectively).
115
Monitoring the reaction by UV/vis reveals a clean conversion of 2
-.
2c (Fig. 21, with a rate given by k"21 [PhOH]: k'(= 0.069 t!:-ls-l at 20'C)
is assigned to the step shown in eqn ( 2 ) . which corresponds to that invoked by
Meyer's group for attack of phenol on [Ru(bipy),(py)O]l+
diamagnetic, bis(hydroquinone)
(ref. 26).
Ru(I1) intermediate 2b is detected by
The
1H
NMR
(Table 1). when the reaction of 2 with phenol is carried out under 0,-free
conditions.
Conditions for effective catalytic hydroxylation of phenol to give hydroquinone using 2 under 0, have yet to be realized, but can bs envisioned to
occur via 2b if its axial ligands, the hydroquinone product, can be removed
and isolated from the oxidizing medium
thioethers via 2a. eqn. ( 1 ) ) .
(cf. catalytic oxidation of the
Benzene solutions of 2 under 0, react with 2-propanol and W/vis
changes akin to those shown in Fig.
2 are observed at corresponding condi-
tions, although the rates are -500 times slower. 1H NMR data for in situ
reactions show that as 2 is consumed a paramagnetic Ru porphyrin product
and acetone, as well as H,O, are generated:
coordinated propoxide ligands are
not
observed. The preliminary data suggest a non-catalytic conversion of
2-propanol to acetone and water, with concomitant loss of the Ru-dioxo complex,
Further studies are in progress to
perhaps to give Ru(TMP) (OH), (Table 1).
isolate and characterize the Ru product.
SUMMARY
In summary, dioxo(porphyrinato)ruthenium(VI)
species are capable of
transferring 0-atoms to, or abstracting H-atoms from, several diverse types of
substrates; as 0, is the 0-atom source, the systems represent a major advance
in 0,-oxidation chemistry and offer an excellent opportunity for detailed
mechanistic insight into oxidations of biological and industrial importance.
The scene is comparable to that of catalytic hydrogenation in the early 1960's.
when many transition metal hydrides were being synthesized using H,, and their
catalytic properties were being discovered (ref. 4 3 ) .
ACKNOWLEDGEMENT
We thank the Natural Sciences and Engineering Research Council of Canada
(B.R.J.) and the U.S. National Institute of Health (Grant AM17989 to D.D.) for
financial support, and Johnson Matthey Ltd. for the loan of Ru.
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117
U. SCHUCHARDT (University of Estadual de Campinas. Brasil): Have you tried
to oxidize saturated hydrocarbons with your systems and could you say
something about the turnover numbers?
B.R. JAMES (University of British Columbia, Canada): Reports on reactivity
of the Ru(porp)(O), species toward saturated hydrocarbons have not appeared.
We find that benzene or toluene solutions of Ru(TMP) (0) and Ru(0CP) (0) are
stable for long periods at room temperature, showing that the aromatic and
activated methyl C-H bonds are not hydroxylated at the ambient conditions.
At > 6OoC, the Ru(TMP)(O), solutions (but not those of Ru(OCP)(O),) are
bleached, showing destruction of the TMP ligand, presumably due to oxidation
by a Ru=O moiety. We plan to test a substrate such as cyclooctane under mild
conditions (1 atm 0, is sufficient to form the dioxo species). but are not
hopeful for postive results.
,
,
F. MONTANARI (University of Milan, Italy): Sulphides are oxidized at room
temperature. What are the conditions for the oxidation of alkenes catalyzed
by the Ru-octachloroporphyrin dioxo species?
B.R. JAMES (University of British Columbia, Canada): We have not studied
alkene oxidation by Ru(OCP)(O),.
With the Ru(TMP)(O), species. catalytic
olefin epoxidation occurs at ambient conditions with 1 atm 0,; turnover
numbers are in the range of 15-45 per day, depending on the olefinic
substrate (see refs. 18. 21 in our paper). Based on commonly observed
increased activity f o r halogenated porphyrins (e.g. ref. 11, somewhat higher
activity is predicted for the octachloro system.
1 P.E. Ellis Jr., J.E. Lyons, J. Chem. SOC. Chem. Commun., 1989, 1189.
J.M. BREGEAULT (University of P. and M. Curie, France):
species formed from the bis(acetonitrile1 complex and O,?
How is rhe Ru(O),
B.R. JAMES (University of British Columbia, Canada): We find that the
reaction of Ru(TMP)(?teCN), with 1 atm 0, in toluene is fast, but probably
amenable to study by the stopped-flow technique. In an NMR-tube, the
reaction is possibly diffusion controlled, because Groves and Ahn (ref.20)
were able to detect, via NMR studies (stated t8 be in C,D,, although the
spectral data shown appear to be in CD,Cl,),Ru
(TMP)O enroute to the dioxo
specie and they suggested formation of the latter by disproportionation of
the Ruq'=O
intermediate. Thus, a plausible route for the reaction is as
follows (& = Ru(TMP), MeCN and possib!e tcluene ligends Gmitted):
11
0
RuI' A [ & O z ] g &11102&111--+
2&IVO
&"(0)2
f &I1
-
It should be noted that we have isolated the 4-coordinate Ru(TMP) complex
(ref. 19).
G. Centi and F. Trifiio’ (Editors), New Developments in Selective Oxidatwn
Q 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
RUTHENIUM
(11) CATALYSTS
FOR
OF ALIPHATIC HYDROCARBONS AND
M.
BRESSAN and A.
119
THE HOMOGENEOUS OXYGENATION
ETHERS.
MORVILLO
Dipartimento di Chimica Inorganica and Centro
Universita’ di Padova, v. Marzolo 1, 35131 Padova
C.N.R.,
(Italy)
ABSTRACT
Hydroxylation or ketonization of alkanes and selective
conversion of ethers to esters or p - k e t o - e t h e r s are
achieved with hypochlorite and a choice of ruthenium(I1)
complexes
as
catalysts,
in
a
biphasic
waterdichloromethane system.
INTRODUCTION
There is a constant
selective systems for
compounds
various
catalysts
1)-
by
the
need to develop
the preparation
direct
oxidation
of
transition metal
complexes
have
in conjunction with
Ruthenium
tetroxide was
stoichiometric
reagent
and
new mild and
of oxygenated
hydrocarbons
been
tested
a number of oxidants
early
ever
introduced
since
only
and
as
as
(ref.
useful
scattered
reports have appeared, dealing with ruthenium-catalyzed
oxidations of aliphatic ethers (ref. 2) and saturated
hydrocarbons
(ref.3). We previously
reported
that
variety of ruthenium(I1) complexes mediate the transfer
a
of
oxygen from hypochlorite and other single-oxygen donors
to
alkenes
(ref.4a.b)
and other
organic
substrates
(ref .4c).
In this study we describe their catalytic behavior
oxygenation
presence
of
of
inexpensive
alkanes
and
ruthenium(I1)
and
towards ethers
easy
to
aliphatic
ethers.
complexes,
handle,
shows
In
the
hypochlorite,
high
reactivity
(up to 2 turnovers per minute), thus making
this procedure a possible synthetic method.
examined
in the
The
complexes
are [RuCl (DPP)2]PPS, trans- [RuClz(DPP)zI (ref. 5a)
{DPP=1,3-bis(diphenylphosphino)
propane)
and cis[RuClz(MezSO)4 1 (ref-6a). [OsCl (DPP)Z] PF6 (ref-5b) and cis-
120
[RuC12 (phen)21 (ref.6b)
(phen=l,10-phenanthroline)
also tested, but gave negligible yields.
were
RESULTS AND DISCUSSION
In a typical experiment, the substrate ( l ~ l O moll
-~
and
the catalyst ( 2 ~ 1 0 .mol)
~
were dissolved in CHzC1, (1 mL)
and stirred vigorously with 1 mL of aqueous LiClO (1 - 1.4
M , as determined by iodometric titration). Aliquots of the
organic layer were periodically analyzed by glc, and the
products were identified by mass spectrometry. The use of
dilute solutions of LiClO (down to 0.2 M ) results in a
proportional decrease in the oxidation rate.
The ruthenium-complexes examined here promote the
oxidation of saturated cyclic hydrocarbons (up to ca 3
turnovers per hour were achieved; see Table 1): each yield
increases rapidly in the initial stage and shows a
tendency to be saturated after more than 2 days reaction.
Tertiary CH groups give the corresponding tertiary
alcohols, whereas methylene groups are mainly converted to
ketones, with minor amounts of secondary alcohols being
detected. Since secondary alcohols are very effectively
converted into ketones by the present catalytic system
(ref- 4c) , it is reasonably to postulate a two - stage
oxidation of CHz groups: first, and relatively slow, to
alcohol and then, much faster, to ketone (eq.1).
-CH2- - >
-CH(OH)- - >
- C O - + HzO
(1)
Oxidation rate ratios for cyclohexane/cyclohexane-dlz
are in the 5+6 range, in agreement with a significant C - H
bond breaking in the transition state. The large ratio of
oxidation of the tertiary position to the secondary, in
adamantane and methylcyclohexane, also implies that the
catalytic reaction follows a free-radical path, where
oxidation happens in preference on the hydrogen carried by
tertiary carbons. A l s o the methylene and methine groups of
the alkyl-chains of alkyl-aromatics (ethylbenzene, cumene)
are effectively transformed into ketones and ter-alcohols
respectively. Aliphatic linear alkanes undergo selective
oxidation at the secondary positions only, following the
w - l rule (ref.7). In all examined cases little or no
oxidation of methyl groups is observed.
121
Ethers,
converted
both cyclic and
to o-lactones
linear,
and
were
esters,
selectively
(>99%)
respectively,
and
these latters were no further oxidized to anhydrides, thus
indicating that the methylene group adjacent to the alkyl
oxygen in esters, unlike the a-methylene groups of ethers,
is strongly deactivated.
The most
striking result consists in the fact that n o t
only the a-methyl groups (see methyl-n-butyl-ether), b u t
also the tertiary a-carbon atom in ethers
(see 2,5dimethyl- and 2-methyl-tetrahydrofuran), contrary to the
secondary ones, remain unaffected by the oxidation. This
strongly suggests that the reaction does not proceed by a
simple hydrido or hydrogen atom transfer involving
carbon atom only.
Indeed, in those cases, where a-methine
and p-methylene groups
and
the a-
are present,
as
in
2-methyl-tetrahydrofuran, unexpected
2, 5 - d i m e t h y lp-ketonization
occurs. Nevertheless, the sizeable kinetic isotope effect,
resulting from complete deuteration of the substrates (for
tetrahydrofuran, krr/ko = 5+6) , points to carbocationic or
carbon radical
intermediates.
OH. .R
0
ox
In the proposed
intermediates are
well-characterized
mechanistic pathway, ruthenium(1V)-oxo
responsible for the oxygen transfer: a
0x0-derivative of
ruthenium
has
been
122
reported to convert tetrahydrofuran into y-butyrolactone
(ref. 8 ) and we previously reported positive evidence of
0x0-species during the PhIO-epoxidation of alkenes by
[RuC1(DPP)2]PF6 (ref.4b). The oxidation of ethers might be
somewhat different from that commonly proposed for the
oxidation of the alkanes, although both reactions appear
to be radical in nature, as shown by the
sizeable
deuterium kinetic effects observed for cyclohexane and
tetrahydrofuran oxidation.
A first distinctive feature is the anomalous reactivity
order of the a-carbon in ethers, i-e. sec > > tert, prim =
0, whereas the tert > see > prim reactivity pattern is
conventionally associated to non-activated alkanes. A
simple explanation, accounting the lack of reactivity of
tertiary ethers to the presence of the heteroatom:
however, hardly agrees with the remarkably high rates
observed for methylene groups of ethers, either in a- o r
$-positions, which are some orders of magnitude larger
than those in unactivated alkanes (see Table 1 and 2). It
is therefore possible that the oxygenation of ethers
proceeds in a different way, similar, for example, to that
previously suggested by Meyer (ref.9) for the oxidation of
alcohols by ruthenium-oxo complexes, and involving a
simultaneous abstraction of two hydrogen atoms from both
the a - and the p-position, with formation of an a - $
unsaturated ether. The latter can undergo
further
oxidation (ref.lO), but in the cases of a-tertiary ethers,
$-keto-ethers, and not a-alcohol-ethers, are likely to be
formed.
A final remark deals with the consumption of the oxidant
during the reactions. Inspection of the aqueous phase
revealed complete exhaustion of the oxidant, once ca 200
cycles have been completed. By
addition
of fresh
hypochlorite, oxygenation of the substrates starts again
and the conversions can be made practically quantitative,
in the presence of a large excess of oxidant (about 10
times, as calculated from the stoichiometric requirements
of eq.2 and 3 ) .
R'COOR
R'CHaOR + 2 [O] - >
+ H2O
(2)
+ 2 [O] - > R'COR
R'CH2R
+ H20
(3)
123
1
Ruthenium-catalyzed oxygenation of alkanes by LiCl0.a
TABLE
Substrates
Products
Catal.activity
I
I1
I11
adamantane
1-adamantanol
adamantanone
1.5
0.13
1.8
0.16
3
0.3
cyclo-octane d
cyclo-octanone
cyclo-octanol
0.7
0.02
1.6
0.13
2.2
0.7
cyc lo hex ane
cyclohexanone
0.3
1.2
1.2
methylcyclohexane
1-methylcyclohexanol
methylcyclohexanones
0.3
0.3
0.1
0.07
0.6
1.3
hexane
2-hexanone
3-hexanone
0.1
0.01
0.14
0.15
0.04
ethyl-benzene
acetophenone
0.1
0.1
0.5
cumene
2-phenylpropan-2-01
0.04
0.1
0.4
d
e
0.04
b
Catalyst, 2 mM, and substrate, 1 M , in CH,Cl,;
LiC10,
1.4 M, in H,O; 22OC.
Mol of product per mol of catalyst, formed in 1 hour:
I, cis- [RuCl,( D M S O ) ,I ; 11, trans- IRuC1, (DPP)2 1 ; 111,
[RuCl(DPP)2 ] PF,.
0.5 M.
LiClO 1 M .
Together with traces of cyclohexanol
2 M.
TABLE 2
Ruthenium-catalyzed oxygenation of ethers by LiC1O.n
a
Substrates
Products
.
Catal activitp
I
I1
I11
di-n-propyl-ether
propyl-propionate
0.8
1.1
0.4
methyl-n-butyl-ether
methyl-hutyrate
0.4
0.4
0.3
tetrahydropyran
I-valerolactons
0.3
0.2
0.2
tetrahydrofuran
I-hutyrolactone
0.7
0.9
0.4
2.5-diHe-tetrahydrofuran
2.5-diMe-dihydrofuran-3-0ne
2.9
1.6
0.5
2-He-tetrahydrofuran
2-He-dihydrofuran-3-one
I-valerolactone
0.2
0.2
0.2
0.1
0.1
0.1
~~-~
a
Catalyst, 2 mM, and substrate, 1 M, in CH,Cl,;
LiC10,
1 M, in H,O; 22OC.
Mol of product per mol of catalyst, formed in 1 minute:
I, cis - [RuCl,( D M S O ) , I ; 11,
trans-[RuCl,( D P P ),] ; 111,
[RuCl(DPP), I PF,.
124
The ruthenium-complexes clearly promote also the
dismutation of hypochlorite (to chloride and oxygen),
which is a well-established process, commonly triggered by
metal ions. Independent experiments showed that 2 m M
solutions of [RuCl (DPP)2 1 PF6 in dichloromethane catalyze
the dismutation of 1 M aqueous solutions of LiC10, at a
rate of ca 0.5 turnovers per min. The process i s
unaffected by the presence of alkanes, but usually
accelerated by the presence of small amounts of ethers,
irrespective of their reactivity. It is therefore likely
that ethers act as phase transfer agents, by complexing
the lithium cations and making the hypochlorite soluble in
the organic phase.
REFERENCES
1
2
3
4
5
6
7
8
9
10
W.J. Mijs and C.R.H.I. deJonge, Organic Syntheses by
Oxidation with Metal Compounds, Plenum Press, New
York, 1986.
(a) A.B. Smith, and R.M. Scarborough, Synth. Commun.,
1980, 10, 205; (b) P.H.J. Carlsen, T. Katsuki, V.S.
Martin, and K.B. Sharpless, J. Org. Chem., 1981, 46,
3936.
(a) D.Dolphin, B.R. James, and T.Leung, Inorg. Chim.
Acta, 1983, 79, 25 and 180;
(b) M.M. Taqui Khan, H.C.
Bajaj, R.S. Shukla, and S.A. Mirza, J. Mol.
Catal., 1988, 45, 51; (c) T.C. Lau, C.M. Che, W.0.
Lee, and C.K. Poon, J. Chem. SOC. Chem. Commun., 1988,
1406; (d) G. Barak, and Y. Sasson, J. Chem. SOC. Chem.
Commun., 1988, 637.
(a) M. Bressan, and A. Morvillo, J. Chem. SOC. Chem.
Commun, 1988, 650;
(b) Inorg. Chem., 1989, 28, 950;
(c) J. Chem. SOC. Chem. Commun., 1989, 421.
(a) M. Bressan, and P. Rigo, Inorg. Chem., 1975, 14,
2286; (bl M. Bressan, R. Ettorre, and P. Rigo, Inorg.
Chim. Acta. 1977, 24, L57.
(a) I.P. Evans, A. Spencer, and G. Wilkinson, J. Chem.
SOC. Dalton Trans., 1973, 204; (b) F.P. Dwyer, H.A.
Goodwin, and E.C. Gyarfas, Aust.J. Chem, 1963, 16, 42.
J - March, Advanced Organic Chemistry, 3rd ed., Wiley
Interscience, New York, 1986, p.621.
V.W.W. Yam, C.M. Che, and W.T. Tang, J. Chem. SOC.
Chem. Commun., 1988, 100.
M.S.
Thompson, and T.J. Meyer, J. Am. Chem. SOC.,
1982, 104, 4106.
G. Piancatelli, A.
Scettri,
and
M.
D’Auria,
Tetrahedron Lett., 1977 , 3483.
G. Centi and F. Trifiro' (Editors),New Developments in Selectwe Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
125
DIRECT CONVERSION OF BENZENE TO PHENOLS UNDER AMBIENT CONDITIONS
KAZUO SASAKI, SOTARO I T 0 AND ATSUTAKA KUNAI
Department of A p p l i e d C h e m i s t r y , H i r o s h i m a U n i v e r s i t y
S a i j o - c h o , H i g a s h i - H i r o s h i m a 724 ( J a p a n )
SUMMARY
A new c a t a l y t i c r e a c t i o n s y s t e m f o r p r o d u c i n g p h e n o l s f r o m b e n z e n e i s
d e s c r i b e d . The s y s t e m is b a s i c a l l y composed of t w o g a s e o u s r e a c t a n t s (H2 and
02) and s i l i c a s u p p o r t e d Pd-Cu c a t a l y s t and o p e r a t e s i n n e a t b e n z e n e u n d e r
ambient c o n d i t i o n s .
E i t h e r phenol o r hydroquinone can be obtained w i t h a
remarkably h i g h s e l e c t i v i t y .
T h i s s y s t e m may be u t i l i z e d as o n e o f t h e
g e n e r a l method o f a r o m a t i c h y d r o x y l a t i o n .
INTRODUCTION
T h e r e i s a s t r o n g demand i n c h e m i c a l i n d u s t r y t o c o n v e r t b e n z e n e d i r e c t l y
t o p h e n o l a s a n a l t e r n a t i v e t o t h e most e f f i c i e n t p r o c e s s c u r r i m t l y o p e r a t i n g
I n fact the latter process
i n t h e c h e m i c a l i n d u s t r y , t h e Cumene P r o c e s s .
n e c e s s i t a t e s multi-step
production of
o p e r a t i o n s t h r o u g h o u t t h e whole p r o c e s s . and g i v e s by-
e q u i - m o l a r amount of
acetone,
which m i g h t become a s e r i o u s
drawback i n t h e f u t u r e .
During t h e l a s t d e c a d e , w e h a v e engaged i n a new p r o c e s s c a p a b l e t o r e p l a c e
t h e Cumene P r o c e s s .
Our m e t h o d b a s i c a l l y u t i l i z e s t h e c h a r g e t r a n s f e r
-
r e a c t i o n between monovalent c o p p e r i o n s and g a s e o u s oxygen, e q . (1):
+
O2
H202
2 Cu'
+
Cu+
+
+
2
H+
H+
H202
OH
+
+
2 Cu2+
H20
+
Cu2'
OH r a d i c a l s t h u s produced a t t a c k b e n z e n e n u c l e u s t o y i e l d e i t h e r p h e n o l
(PhOH) o r h y d r o q u i n o n e (HQ).
The s e l e c t i v i t y t o w a r d s PhOH o r HQ c a n r e a d i l y
be a l t e r e d a t w i l l (eq. 3 ) .
The r e a c t i o n mechanism h a s been s t u d i e d i n some
d e t a i l and r e p o r t e d i n our p r e v i o u s p a p e r s ( r e f s . 1-3).
T h e r e a c t i o n s e q u e n c e e x p r e s s e d by e q s . ( 1 ) a n d ( 2 ) p r o d u c e s b i v a l e n t
copper ion i n t h e r e a c t i o n system and i t is necessary t o r e t u r n it t o
monovalent s t a t e i n o r d e r t o set up a c o n t i n u o u s p r o d u c t i o n s y s t e m .
f i r s t l y s t u d i e d t h e e l e c t r o l y t i c r e g e n e r a t i o n of Cu(1) s p e c i e s .
h a s been proved t o be q u i t e a t t r a c t i v e p a r t i c u l a r l y
benzoquinone (BQ) ( r e f . 4 ) .
We have
T h i s method
f o r p r o d u c i n g K-
126
In particular, we were able to establish an ideal electrolysis system,
which we named "duet electrolysis", where a single product, p-benzoquinone, is
produced both at cathode and anode electrodes simultaneously from a single
starting material, benzene (refs. 5 - 7 ) .
Electrochemical method seems,
however, to be not always advantageous, if one aims at phenol as the final
product.
We thus attempted to develop another possibility that is the
chemical reduction of bivalent copper to monovalent one.
be appropriate for
Hydrogen seemed to
reducing chemicals because proton is a necessary reagent
for hydrogen peroxide.
In order to perform reduction of Cu(I1)
with
hydrogen, however, we had to prepare some suitable heterogeneous catalyst and
Pd loaded on silica surface was employed tentatively. This paper deals mainly
with the use of palladized silica catalyst.
EXPERIMENTS
Catalyst
A proper amount of palladium chloride was precipitated on commercial silica
gel (Merck No. 9385, 230-400 mesh ASTM) and dried by heating.
In some of
catalysts, to be used in neat benzene, several types of copper(I1) salt were
coprecipitated on the silica support.
Oxidation reaction
A desired amount of catalyst was put into an Erlenmeyer flask (50 ml)
containing 20 ml of neat benzene and reaction was started by feeding the
reacting gases.
Two different modes of gas feeding were employed, i.e.,
alternate and simultaneous feedings.
In the former mode, hydrogen and oxygen
(normally air) were fed alternately in a programmed manner. In this case, the
catalyst is activated during the hydrogen stage and promotes the oxidation of
benzene during the oxygen stage. In the latter mode, no particular treatment
for catalyst activation was made and both hydrogen and oxygen were fed
simultaneously.
127
RESULTS
Reaction of benzene
In Table 1,
are shown the results obtained with a catalyst composed of
CuSO4 and PdC12 supported on silica.
The catalyst was first activated by
streaming hydrogen for 2 h and then hydrogen was switched to air stream and
continued f o r 1 h.
The data show that a combined use of Pd and Cu is
essential for realizing a high catalyst activity (runs 1 to 3 ) .
Silica
support seems to play an important role (runs 8 and 9) and cannot be
eliminated.
Table 2 shows other data which were obtained with the
simultaneous gas feeding.
Except for the initial period of reaction, phenol
accumulates linearly in benzene with increasing reaction time.
TABLE 1
Effect of catalyst composition on the phenol yield.”
Run
1
2
3
4
5
6
Catalyst composition
Si02/g CuS04/mmol PdC12/mmol
2
2
2
2
10
2
2
2
0
0
2
11
C
7
8
9
Additive
CH,COOH/g
Product
Phenol/pol
83
51
50
2
2
0.1
1
0.1
2
2
2
0
0.2
0
0
0.1
0
0.1
0
0
1
0
1
0
2
0.1
1
2
2
0.1
b
2
0.1
0
0
0
0
0
3
tract!
tract!
trace
4
5
aThe reactions were performed using the catalysts correspondirtg to type A by
applying alternate feeding of H for 2 h and air for 1 h. In runs 8 and 9 ,
powdery mixture of CuSO,t, and Pd312 were used without support.
bH PtC16 (0.1 mmol) was used instead of PdC12.
‘Alumina (29) was used instead of silica.
TABLE 2
Catalytic oxidation of benzene by simultaneous feeding of hydrogen and oxygen .a
Reaction time/h
PhOH / p o l
0.5
4.7
1
15
2
48
3
77
4
5
6
114
141
167
7
8
9
183 205 229
aThe reaction was carried out using the catalyst C by simultaneous feeding of
H2 and 02.
128
Fig. 1 shows the effect of repetition of the alternating feeding.
In this
case, hydrogen and air feedings were alternated for every 30 min and the
alternation was repeated up to six times.
Three lines stand for three
catalysts different in their process of preparation but have the same
composition (see ref. 8). Although the slope of lines are different depending
on the process of catalyst preparation, it is clearly indicated that the
reaction continues steadily without any l o s s of catalyst activity.
It should be noted that not only the process of catalyst preparation but
also the nature of counter ions of copper salt being supported on the silica
surface affects the catalyst performance.
Roughly speaking, the acid strength
of corresponding free acid of counter ions determines the relative selectivity
between benzoquinone and phenol (Q/P) : the stronger is the acid strength, the
higher is the selectivity of phenol relative to quinones.
The difference in
counter ion in copper salt also affects the reaction rate.
An example is
shown in Fig. 2 , where phenol yield is plotted as a function of time elapsed
for activating the catalyst.
The figure indicates that cupric acetate is more
slowly activated than cupric sulfate does.
The
selectivity is also affected by the oxygen partial pressure in the
surrounding gas.
The effect i s appreciable when
This is shown in Table 3.
the catalyst is made from cupric acetate.
A s far as the oxygen source is
ordinary air, the product ratio, Q/P, is only 0.2 even in the highest case.
At 5% level of acetic acid added deliberately, the value increases from 0.16
to 1.53 corresponding to
atm, respectively.
the change in oxygen pressure from 0.2 (air) to 3
At 6 atm, the value tend to saturate.
When the catalyst
is made from cupric sulfate, the Q/P ratio never exceeds 0 . 1 5 even if
pressurized oxygen is supplied.
This is because, at lower pH, the saturation
of pressure effect appears at lower pressure.
A similar observation was
obtained also in aqueous phase reaction (ref. 7 ) .
TABLE 3
Effect of oxygen pressure on the product selectivity .a
Cu Salt ACOH/VO~%
CU(OAC)~ 2.5
CU(OAC)Z
5.0
CU(OAC)~ 10.0
CU(OAC)~ 20.0
cuso4
cuso4
0
10.0
Ratio of BQ/PhOH (Total products/pmol)
air, 1 atm
0 2 , 1 atm
0 2 , 3 atmb 0 2 , 6 atmb
-
0.79
0.63
0.61
0.45
0.11
0.15
1.50 ( 2 9 . 6 )
1.53 ( 5 7 . 6 )
0.84 (45.8)
0.16 ( 2 4 . 6 )
0.20 ( 4 1 . 0 )
-
0.10 ( 5 6 . 2 )
0.01 ( 3 1 . 2 )
(47.1)
(44.2)
(62.6)
(43.4)
(37.0)
(39.0)
0.12 ( 7 0 . 6 )
-
-
1.39 ( 2 9 . 6 )
-
-
-
a The reactions were carried out in the same manner as Table 1.
Hydrogen reduction was also done under the same pressure as the oxidation.
129
1
2
4
3
5
6
Number of r e p e t i t i o n
Fig. 1.
ChanRe i n c a t a l y t i c a c t i v i t y d u e t o t h e method of c a t a l y s t
p r e p a r a t i o n . A r t e r n a t e g a s f e e d i n g f o r e a c h 30 min was a p p l i e ?
.
r
1
2
3
Time e l a p s e d f o r c a t a l y s t a c t i v a t i o d h
F i g . 2. Time e l a p s e d f o r c a t a l y s t a c t i v a t i o n and i t s e f f e c t on t h e p h e n o l
y i e l d . C o o r d i n a t e a x i s r e p r e s e n t s t h e y i e l d of p h e n o l p r c d u c e d i n l h o f
oxidation reaction.
( 0 ) PdClZ 0.lmmol.
( A ) PdCIZ 0.2mm01,
( 0 ) PdCIZ O.lmmo1,
( A ) PdCIZ O.Zmmo1,
( 0 ) PdC12 0.3mmol.
CuSOb 2mmol/Si02 2g.
CuSO4 2mrnol/SiOZ 2g.
C U ( O A C ) ~ 2mmol/Si02 2g.
Cu(0Ac)z 2mmol/SiOz 2g - AcOH ( l g ) was added.
C U ( O A C ) ~ 2mmol/Si02 2g.
130
Reaction
of naphthalene
In principle, there is a possibility that our
reaction system can be
utilized as the general method of aromatic hydroxylation.
We have thus
studied tentatively the reaction of naphthalene in place of benzene.
In this
case, however, we have to find a suitable solvent which is inert to the attack
of hydroxyl radicals. Several candidates were tested including cyclohexane,
acetone, ethyl acetate as well as some aliphatic alcohols.
cyclohexane was found most
promising when
benzene was
Among
these,
reacted in it.
Unfortunately, however, the reaction of naphthalene in this solvent was much
slower than that of benzene.
retards the chance of
surface.
Probably, hydrophobic nature of
encounter between solute molecule and
cyclohexane
the catalyst
Accordingly, the use of acetic acid was finally examined.
catalyst used in this system contained only palladium ( 0 . 1 mmol Pd
and
cupric acetate was dissolved in the solution phase (40 mM).
/g
SiO,)
Gases were
supplied through a sintered glass-ball disperser at a rate of 7 . 5 ml/min
both hydrogen and oxygen.
Solid
for
Both the alternate and simultaneous feeding were
tested for comparison. Results obtained are listed in Table 4 .
TABLE 4
Reactions in acetic acid.a
Reactant
Feeding
mode
Catalystb
Simultaneous
Simultaneous
Simultaneous
A1 ternate
1g
1g
4 g
4R
Naphthalene Simultaneous
Naphthalene Alternate
4 g
4 g
Benzene
Benzene
Benzene
Benzene
Productlpmol
(Phenol)
169
186
212
164
(1-NpOH‘)
215
172
(HQ)
379
349
437
112
(BQ)
126
115
137
232
(2-NpOH) (1,4-NQ)
<121
649
< 88
320
(Total)
674
650
785
508
(Total)
985
580
aCu(OAc)2 was dissolved in solution phase (40mM).
bComposition was 0.lmmol Pd /g SiO
‘NpOH and NQ stand for naphthol an3 naphthoquinone, respectively.
.
In this experiment, total reaction time was fixed at 2 h irrespective of
the feeding modes.
Since in the alternate mode, the two gases were altered at
every 15 min, effective reaction time elapsed was one half of that of the
simultaneous feeding.
If this is taken in mind, the data suggest that the
simultaneous feeding is not always superior in the production rate and the final judgment is still a subject of debate. In any case, Table 4 clearly indicates that naphthalene is oxidized at a rate comparable with that of benzene.
131
REFERENCES
A.Kunai, S.Hata, S.Ito, and K.Sasaki, J. Orn. Chem.. 11_ ( 1 9 8 6 ) 3471-3474.
A.Kunai, S.Hata, S.Ito, and K.Sasaki, J. Am. Chem. SOC., 108 (1986) 6012-
6016.
S.Ito, T.Yamasaki, H.Okada, S.Okino, and K.Sasaki, J. Cheni. SOC., Perkin
Trans. 2, ( 1 9 8 8 ) 285-293.
S.Ito, H.Okada, R.Katayama, A.Kunai, and K.Sasaki, J. Electrochem. %.,
135 ( 1 9 8 8 )
2996-3000.
S.Ito, R.Katayama, A.Kunai, and K.Sasaki, Tetrahedron
206.
&.,
30
( 1 9 8 9 ) 205-
S.Ito, N.Fukumoto, A.Kunai, and K.Sasaki, Chem. Lett., ( 1 9 8 9 ) 745-746.
S.Ito, A.Kunai, H.Okada, and K.Sasaki, J. Orn. Chem., 53 (1988) 296-300.
A.Kunai, T.Wani, F.Iwasaki, Y.Kuroda, S.Ito, and K.Sasaki, Bull. &em. SOC.
k.,
62 (1989) 2613-2617.
D.Olivier ( Institut de Recherche sur la Catalyse, France ) :
What is the proof that the reaction is not performed in liquid phase ? Have
you checked the supported metal loading after reaction ?
K.Sasaki ( Dept. Applied Chem., Hiroshima University, Japan ) :
No. We haven't studied the catalyst composition after use yet. However, we
believe that no appreciable amount of Pd had been lost in the solution phase
during a limited time of reaction.
B.R.James (University of Brit. Columbia, Canada ) :
1. Use of H2-02 mixtures for monooxygenase-type reaction was first
demonstrated, to my knowledge, by my group in 1969 (Can.J.Chem.).
systems suffer from competing direct hydrogenolysis of 02 to water.
Such
Do your
simultaneous feeding system suffer by competing water production and, if
so,
what is the ratio of phenol : water production at various conditions ?
2.
I n your simultaneous feeding experiments, what 0 2 : H ~ratios were used and
were explosion limits carefully avoided ?
K.Sasaki : 1. Yes. An appreciable
part of hydrogen seem to yield water
directly. Although no material balance has been studied in the reaction system
here reported, we have made a separate experiment to study the percentage
conversion of hydrogen to phenol. According to that, the conversion was ca.10
% at the highest.
2.
In the present report, the two
gases
through the space over benzene layer at an equal rate, 7.5 ml/min.
were flown
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in T h e Netherlanda
133
A NOVEL AND EFFECTIVE OXYGENATION OF 2,3,6-TRMTHYLPHENOL TO
TRIMETHYL-p -BJZNZOQUINONE BY DIOXYGEN WITH COPPERm) CHLORIDE/
AMINE HYDROCHLORIDE SYSTEM CATALYST
Katsuomi Takehim,+ Masao Shimizu, Yoshihito Watanabe, Hideo Orita and Takashi Hayakawa
National Chemical Laboratory for Industry, Tsukuba Research Center, Tsukuba, Ibaraki 305, Japan
SUMMARY
2,3,6-Trimethylpheno1 was selectively and quickly oxygenated to trimethyl -p-benzoquinoneby
molecular oxygen in the presence of catalytic amount of copper(II) chloriddamine hydrochloride
system in alcoholic solvent at ambient temperature.
INTRODUCTION
Trimethyl-pbenzoquinonc (TMQ) is a key compound for the synthesis of Vitamin E, etc., and
the current method of its production on an industrial scale is p -sulfonation of 2,3,6-trimethylphenol (TMP) followed by oxidation with MnO2. The methods for one step synrhesis by the
oxidation of TMP have been investigated so far using several oxidizing reagents,such as hydrogen
peroxide,l) etc. Use of molecular oxygen as the oxidizing reagent seems the most promising
method where cobalt(II)-Shiff base?) and copper (II) chloride3) have been tested as the catalyst.
Usually, the latter catalyst was used as the binary system combined with LiCI, resulting in high
selectivity for TMQ formation?) The catalyst life of the former is very short though its activity is
high, while the catalytic activity of the latter is so low even when coupled by LiCl that an almost
stoichiometric amount of copper(II) salt is required to complete the oxidation?) We herein
communicate a novel and effective oxygenation of TMP to TMQ by copper (II) chloriddaxnine
hydrochloride system catalyst.
Me&
.:
/$Me
Me
-m
VitaminE
0
EXPERIMENTAL
The oxidation of TMP (1, 2 mmol) was carried out using a mpper compound (0.2 mmol) and an
additive (0.2 mmol) as the catalyst in alcohol (2 ml) as the solvent at 40-6O'C under latm. of
oxygen atmosphere, where the amount of oxygen consumed was measured by a gas burette. TMQ
(2), 4-~hl0ro-2,3,6-trimethylphenol
(3) and 4,4'-dihydmxy-2,2', 3,3', 5,5'-hemcthylbiphenyl ( 4)
134
as the products were determined by GLC and HF'LC method using a Thermon 3000 and an Inertsil
ODS as columns, respectively.
RESULTS AND DISCUSSIONS
The results of the oxidation of 1 for 5 h with several copper-amine system catalysts are shown in
Table 1. The maximum rate of 0 2 consumption is shown as "do 2/dt." CuC12.2H20 alone showed
a very low activity for the production of 2, while Cu(OAc)2*H20 afforded a substantial
TABLE 1
Oxidation of I with the copper - additive system catalyst.
Cu compound
(0.2 mmol)
Additive
(0.2 mmol)
dO2/dt
(mmovh)
~~
CuC12*2H2 0
Cu(0Ach.H 2 0
CuC12.2H 2 0
Cu(OAc)2*H2 0
CuC12-2H20
CuC12*2H 2 0
CuC12-2H20
CUCl
cuc12
CuC12-2H20
0.062
0.225
0.353
0.910
0.162
0.038
0.527
1.54
1.44
1.45
Conversion
of 1 (%)
2
Yield (%) of
3
4
~
24.9
79.4
90.7
100
54.3
12.6
100
100
100
100
8.9
0
68.8
6.5
28.4
1.5
81.8
29.3
83.7
80.5
8.9
0
6.7
0
7 .O
5.1
1.6
0
0
0
7.5
36.6
16.5
14.6
12.0
2.9
14.7
0
12.3
13.5
amount ofthe dimer 4. An addition of LiCl to the CuC12 system caused an increase in the activity
of 2 production as claimed in the several patents.4) It took 7 - 8 h for completing the oxidation of I
by the CuC1202H20 - LiCl system in the present conditions. When the amount of LiCl was
changed, its use of 0.2 mmol resulted in the highest activity suggesting an l/1 active complex
formation between CuC12 and LiCI. A replacement of LiCl by NH4Cl or (CH3)4NCI caused a
decrease in the activity, while (C2H 5)4NCI was effective as the co-catalyst for the production of 2.
It is noteworthy that (C2H5)3N*HCIwas the most effective among the additives tested and
shortened the reaction time to 2 - 3 h for completing the oxidation of 1. An use of CuCl caused
generally rapid consumption of 1, where the amount of 2 formed was however substantially
lowered.
The results of the oxidation of 1 with CuCl2*2H20- amine system catalyst are shown in Table 2.
Many kinds of mines were effective when used as the salt form with inorganic proton acid,
among which not only hydrochloric acid but also sulfuric and hydrobromic acids were included.
135
TABLE 2
Oxidation of 1 with the coppcra) - amine system catalyst.
Amine
(0.2 mmol)
d02/dt
(mmobh)
C H ~ N H ~ - H C ~ 0.715
1.28
(CH3NH2)2*H2!@)
(CH3)zNH-HCI
1.60
(cH3)3N-HCI
1.13
Cm5NH2-HCI
1.27
C2H5NHpHBr
1.2 1
(C2H 5)2NH=HCl
1.55
(CzH 5)3NmHCI
1.45
R - C ~ H ~ N H ~ D H C2.05
I
(n-C 3H7) 2NH
1.07
(n-C3H7) 2NH*HCI
1.46
(n-C3H7)3N
1.13
( n -C3H7) 3N + HCIC)
0.990
(i-C3H3)2NH-HCl
0.720
n -cQH9NHz-HCl
1.59
Conversion
of 1 (%)
100
95.8
100
100
100
100
100
100
100
100
100
100
100
100
100
Yield (%) of
2
3
4
79.5
71.3
84.2
82.6
79.2
83.3
84.2
80.5
80.5
50.0
79.8
26.0
71.5
84.4
85.5
0
3.4
0
0
0
0
0
0
0
1.9
0
0
0
0
0
21.6
21.4
13.3
20.4
13.3
15.4
13.3
13.5
15.5
0
17.3
RKCt.
time(h)
0.7
19.1
16.8
15.5
5
5
5
5
4
4
4
4
4
4
4
4
4
5
4
a h C I 2 - 2 H 2 0 ( 0.2 m o l ) was used. b)O. 1 mmol was used. Ch.2 mmol of HCl was used in
the form of its 36 % aqueous solution.
Secondary amine, such as ( M 3 ) 2NH-HCI or (C2H5) 2NH-HCI was prcfelable than primary or
tertiary amine in the case with short alkyl chams, while the primary amine also cawed a high
activity in the case with long alkyl chains. When the amount of the amine (C2H5) 2NH-HCI was
changed, the highest activity was obtained by its use of 0.2 mmol, suggesting formation of V1 active
complex between CuCl2 and (C2Hj)zNH. Cyclic amine, such as morpholine, aromatic amine, such
as aniline, and amindcohol, such as ethanolamine,were effective as the co-catalyst in the presence
of hydrochloric acid.
Table 3 shows the results obtained with the CuC12-2H2 0 - amino acid system catalyst. Amino
acid was also useful when the ability of bidentate coordination was weakened by the alkyl
substitution at the amino group or by the esterification of the carboxyl group: glycine was not
effective even when coupled by hydrochlodc acid, while N,N-dimethylglycineor glycine ethyl
coupled by hydrochloric acid showed high activity. QL-Alanine was effective as the cu-catalyst,
though it has both fke amino and &xyl
groups: this may be due to the steric hindnnce which
weakens the coordinating ability of the amino group to copper. Both of the two optical isomers,
de ., D- and Lalanine,
showed similar activity to that obtained by the racemic mixture, ie.,
136
TABLE 3
Oxidation of 1 with the coppera) - amino acid system catalyst.
2
Yield (%) of
3
4
4.8
12.4
3.8
0
2.8
6.3
0
7.0
3.6
17.7
15.0
14.5
14.2
20.6
5
4
100
7.2
3.2
79.1
78.5
36.2
31.2
74. I
100
18.2
0
16.3
5
Amino acid
(0.2 mmol)
d02/dt
(mmovh)
Conversion
of 1(%)
NH2CH2COOH
NH2CH2COOH*HCI
(CH3) 2NCH2COOH.HCI
NH2CH2COOC2H5*HCI
NH2(CH2)2COOH
NHz(CH2)2COOH + HClb)
D,L-CH3$HCOOH
0.078
0.130
0.670
1.48
0.200
0.274
0.825
24.9
25.2
D,L-CH3FCOOH + HClb)
NH2
0.810
NHz
100
100
60.4
62.7
Reaction
time (h)
5
5
5
5
5
a)CuC12*2H20 ( 0.2 mmol ) was used. b10.2 mmol of HCI was used in the form of its 36%
aqueous solution.
D,L-alanine. p-Alanine oppositely showed very low activity even when coupled by hydrochloric
acid this can be reasoned by its strong bidentate coordination because of the absence of steric
hindrance. Neither diamine, such as ethylenediamine,nor dipyridyl was effective as the co- catalyst.
It seems thus that the bidentate ligand occupies the active site of the copper complex resulting in the
low activity.
All the amine compounds tested did not reveal the high activity in the form of free amine, i.e ., in
the absence of inorganic proton acid the addition of the acid caused an increase in the activity of
copper - m i n e system catalyst. When the amount of HCI added to CuC12-2H20 (0.2 mmol) (C2H 5)3N (0.2 mmol) system was changed, its optimum amount was not observed at a fixed value:
the addition of 0.2 mmol of HCl was sufficient to obtain a good yield of 2 (> 80 %) and the further
addition did not affect substantially the mte and the yield of 2 production. This result and the
effectiveness of Hl3r and H2SO4 as the additive as shown in Table 1 suggest that the acids works as
the proton source during the oxidation of 1 to 2.
The effect of the solvent on the yield of 2 in the oxidation with CuCl2-2H20 - (C2H5)2NH*HCI
or (CH3)2NH*HCIsystem is shown in Table 4. Use of alcohol of low molecular weight. i.e.,
ethanol, propanol or isopropanol, as the solvent resulted in a decrease in the yield: this might be
partly due to the low solubility of the amine in these solvents. When an aromatic compound, ie.,
benzene or toluene, was mixed in the mtio of V1 into alcohol as the solvent, the mte of the
oxidation became two times higher compared to the case without the aromatic solvent and the
137
TABLE 4
Oxidation of 1with copper - amine system catalyst.
Amine
A
A
A
A
A
A
A
B
B
B
B
B
B
Solvent
(ml)
a/dt
(mmovh)
EtOH(2)
Bz( l)+EtOH(1)
1-PIOH(2)
Tol( l)+l-P10H(l)
i -PIOH(2)
Bz( l)+i -PIOH( 1)
Tol( l)+i-PIOH(1)
EtOH(2)
Bz( l)+EtOH(1)
l-PrOH(2)
Tol(1) + 1-PrOH(1)
i -PIOH(2)
Bz( 1) + i -P10H(1)
0.452
0.700
0.565
1.01
0.575
0.975
0.955
0.555
1.33
0.770
0.970
0.525
1.02
Conversion
of 1 (%)
83.0
96.9
92.4
100
81.6
100
100
96.8
100
100
100
100
100
Yield
(1%)
of
2
3
4
60.2
78.8
71.3
86.4
64.9
81.6
89.4
73.9
83.6
82.6
85.4
86.4
90.0
5.4
2.7
3.7
0
5.4
1.4
0
2.9
0
0
4.1
17.6
13.0
10.5
6.8
3.1
1.8
18.5
0
1.3
0
1.7
14.9
11.8
1.7
2.5
a)l, 2 mmol; CuC12*2H2 0 0.2 mmol; (C2H5)2NH*HCI(A)or (CH3)2NH*HCl(B),0.2 mmol;
Reaction temperature, 40'C; Reaction time, 5 h.
yield of 2 increased up to about 90 %. It seems that the mixed solvent can work as a good medium
for the present oxidation by dissolving well each component of the reaction: 1 and the amines are
well soluble in the aromatics, while CuC12.2H 2 0 is in the alcohols.
The activity of the CuC12 -2H 2 0 - (C2H5)2NH*HClsystem catalyst thus obtained was 5 - 6 times
higher than that of the well known CuC12.2H20 - LiCl system catalyst and the yield of 2 reached a
Scheme I
+
Cl
3
138
value of about 90 % in a
few hours of the reaction
with the former catalyst.
Time course of the reaction with this catalyst
system (Fig. 1) suggests
a plausible oxidation
scheme as follows: a
main pathway may be a
direct oxidation of 1 to 2
accompanied by a forma-
U
0
1
-2
tion of small amount of
Y
3
Reaction time (h)
Fig. 1
Oxidation of I with CuCI2*2H20- (C2HJ2NH*HC1 catalyst.
TMP, 2 mmol; CuClZWp, 0.2 mmol; (CP@-HCl,
0.2 mmol
n-Hexanol, 2 ml; Reaction temperature,60’C; pOaS60-
4. Apartof2 canbe
formed via 3, i.e, by p chlorination of 1.
(Scheme I) Attempts to
increase the catalytic activity of the copper system catalyst and to estab-
lish a more complete view of the mechanism of the TMP oxidation are under way.
REFERENCES
M. Shimii,H. Orita, T. Hayakawa and K. Takehira, Tetrahedron Lett.,30,47 1 (1989) and
references cited therein.
R. A. Sheldon and J. K. Kochi, ”Metal Catalpd Ornilations oforganic Cowunds,” p.373.
Academic Press, New York, 1981.
Ref.2, p.369.
Japan Patent 225,137(1984) to Mitsubishi Gas Chemical Co.
G. Centi and F. Trifiuo' (Editore), New Developments in Selective Oxidatwn
0 1990 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands
139
PHENOL OXIDATION WITH MOLECULAR OXYGEN IN THE PRESENCE
METALLIC COPPER
OF
N. RAVASIO1, M. GARGANOl and M. ROSS12
1 Centro C.N.R. sulle Metodologie Innovative di Sintesi Organiche,
Dipartimento di Chimica dell'universita, via Amendola 173, 1-70126 Bari
2Dipartimento di Chimica lnorganica e Metallorganica, UniversitP di Milano,
via Venezian 21, 1-20133 Milano
SUMMARY
The reactivity of phenol, a - and p-naphtol with molecular 0 2 in the
presence- of metallic copper in methanoVpyridine solution has been
investigated and the results obtained compared with those aYready reported
with Cu(l) and Cu(ll) amine homogeneous systems.
Phenol gave 4,5-dimethoxy-l,2-benzoquinone 1 with fairly good yield (up
to 46%) while the reaction of a-naphtol afforded up to 43% of 2-methoxy1,4-naphtoquinone 2.
This reaction represents a new and simple route for the synthesis of
methoxyquinones, useful intermediates for the production of drug and
photosensitive materials.
Moreover, the total selectivity exhibited towards ortho-hydroxylation in
the reaction of phenol, mimics the monophenolase activity of copper enzyme
tyrosi nase.
P-naphtol gave mainly [l,l'-binaphtalene]-2,2'-diol.
INTRODUCTION
The activity of copper complexes in promoting oxygen activation in
chemical and biological systems has been widely investigated -2Particular attention has been devoted to the oxidation of phenols due to
either industrial applications of polyphenylene ethers and quinones or
interest in modelling the action of oxygenases such as tyrosinase and
pyrocathecase.
Thus the oxidative coupling of 2,6-dialkyl and 2,6-diaryl substituted
phenols in the presence of homogeneous catalysts derived from copper salts
and amines, is a well established method for the synthesis of polyphenylene
e t hers3 which requires, however, continuous efforts to enhance the
spec if icity4.
140
Concerning the industrial production of hydroquinone, the direct air
oxidation of phenol to p-benzoquinone in polar aprotic solvent, in the
presence of copper ions and inorganic bases, followed by catalytic
hydrogenation developed by Lyons and Hsu5, represents a quite actractive
alternative to the hydrogen peroxide oxidation of phenol.
An unusual high selectivity to p-benzoquinones has also been claimed
with CuC12 and NEb in acetonitrile as a catalytic systems.
A more fundamental research concerns mechanistic studies on the copper
mediated oxygenolysis of phenol to cis,cis-muconic acid monornethylester
that mimics the action of pyrocathecase enzyme718 and the selective
oxidation of catechols to the corresponding o-quinones which represents a
model for the diphenolase activity of the copper enzyme tyrosinaseg-11.
It is generally accepted that homogeneous systems containing the redox
couple Cu(l)/Cu(ll) are responsible for the oxygen activation and therefore
catalytic systems are prepared by using either mono or divalent copper
compounds.
The activation of 0 2 towards phenols on metallic Cu has been scarcely
investigated. An interesting example however, is reported by Capdevielle
and Maumy which deals with the synthesis of copper catecholates from
metallic Cu, phenols and oxygen, catalyzed by CuCll*, while 1,2-dicarbonyls
undergo oxidative C-C bond scission to give carboxilate Cu(l I) complexes by
reaction with Cu(0) and aminesl3.
Our recent work on the activation of molecular oxygen on metallic Cu14
lead to the conclusion that organic Bronsted acids such as methanol,
nitromethane and benzoic acid easily react with Cu and 0 2 according to the
following scheme (X=-OMe,-OCOPh):
Cu
+ 112 0 2 -, Cu-0
Cu-0
Cu<OH
X
+ HX
+ HX+
+
(surface)
Cu<OH
X
CuX2 +H20
Scheme 1
Thus, the interaction of molecular 0 2 with Cu forms a layer of atomic
oxygen, as supported by different surface analytical techniques
(EELS,XPS,UPS)15; this oxygen covered surface can interact with proton
donor molecules to give an hydroxylation reactionl6.
141
This latter surface species can evolve only in the presence of a base, to
give Cu(ll) complexes, this evolution being responsible of the bulk reaction
of copper.
As an extension of this research, we have investigated the possibility to
use metallic Cu as a catalyst for the oxidation of monohydroxilated
aromatics as an alternative to the use of CuCl and CuC12.
As previously observed a Cu(ll) complex should be expected in solution
after interaction of Cu(0) with 0 2 and the organic reagent.
However, a different catalytic behaviour can be suspected starting from
metallic Cu, respect to classical Cu(l) and Cu(ll) salts, on the basis of
either
a different mechanism for oxygen activation or stabilization of
different oxidation states.
We here report preliminary results obtained by reaction of phenol, a- and
p-naphtol with molecular oxygen in the presence of metallic copper in
methanollpyridine solution and compare them with those already reported
with Cu(l) and Cu(ll) homogeneous systems.
RESULTS
Phenol reacts smoothly with oxygen at atmospheric pressure and room
temperature in the presence of metallic Cu, methanol and pyridine. From the
resulting solution orange-yellow needles of 4,5-dimethoxy-l,2benzoquinone 1 precipitate. Best yields were obtained with a phenoI/Cu
ratio of 6 and an Oelphenol ratio of 1.5. Under these conditions 46% of 1
was collected.
When the solution was allowed to adsorb a higher amount of oxygen,
products derived from Cl-C2 bond cleavage began to form.
In every case, no para-products could be identified at any stage of the
reaction.
We can assume that an o-hydroxylation reaction followed by oxidation to
o-quinone takes place; nucleophilic addition to the activated 4 and 5
positions and reoxidation of the substituted catechols formed, give account
of the observed products.
1
Starting from catechol, oxidation and dimethoxylation are also observed,
under the conditions employed for phenol, and 1 can be obtained beside a
catecholate Cu(ll) pyridine complex.
142
The hydroxylation-oxidation of naphtols in the presence of Cu(0) shows a
quite different behaviour in comparison with phenol. The a isomer readily
consumes 1 mole of oxygen to give 2-methoxy-1,4-naphtoquinone2 (43% at
90% conversion) besides unsubstituted 1,4-naphtoquinone (15%).
Therefore, a selective para-oxidation takes place in this case, whereas
only monomethoxylation was observed.
2
The reaction of p-naphtol shows a more different pathway. After
adsorption of 1 equivalent of 0 2 we could isolate [l
,l'-binaphtalene]-2,2'diol 3 (41%) and the fission product 3-(2-carboxyphenyl)-2-propenoic acid
monomethylester 4 (9%), besides non reacted naphtol (1go/,).
The presence of 3 and 4 suggests activation of o-position in p-naphtol
and in particular the formation of an o-quinonic intermediate as precursor
of 4 . The oxidation of phenol under similar conditions but using Cu(ll) salts
as catalysts has been already studied. In particular, according to Brackrnan
and Havinga" phenol does not react in the presence of pyridine and
methanol. The reaction occurs in the presence of a secondary amine as
morpholine (M): o-hydroxylation followed by oxidation takes place and 4,5dimorpholino-l,2-benzoquinonecan be obtained.
143
On the other hand, 1 can be obtained according to Rogic and Demmin in a
similar way but starting from the already o-functionalized catechol by
using C12Cupy27.
A few reports deal with the oxidation of naphtols in the presence of Cu
salts. The Brackman-Havinga system transforms both a- and p-naphtol in
4-rnorpholino-l,2-naphtoquinone; however dinaphtol derivatives are also
formed, particularly during reaction of the a isomerl7.
It is worth noting that alcohols do not normally add to quinones, metal
ions catalysis being always neededI8n19 and only a few examples of direct
conversion of phenols and a-naphtol to alkoxyquinones are known20121.
In particular, the reaction of phenol here reported represent!; a novel and
simple route for the synthesis of 1.
o-Benzoquinone derivatives are conventionally prepared by oxidation of
the corresponding catechols. The preparation of 1 , used in the synthesis of
redox polymers, has been achieved by oxidation of catechol with PbOz in
methanol and in the presence of CH30Na 2Zl23 or with NalO3 in methanol24,
yields never exceeding 60%.
The easy hydroxylation observed when phenol reacts with oxygen in the
presence of metallic Cu can be rationalized on the basis of the following
model. The acidic phenol interacts with the oxygen-covered Cu surface
according to Scheme 1 to produce a copper phenolate intermediate.
According to EELS determinations, the oxygen covered Cu surface
produces different species15 and in particular the existence of peroxo
groups can be inferred by the presence of a stretching vibration band around
880 cm-1.
Therefore, the coordinated phenolate can react with the p~sroxospecies
owing to partial electrophilic character of one oxygen atom 01 the peroxide
unit as suggested by Solomon25 to explain the monophenolase activity of
tyrosi nase.
This model agrees with the well accepted existence of a p-1,2-peroxo
species as active intermediate of oxy-tyrosinase.
6-
144
REFERENCES
1 - Houben-Weyl, "Methoden der Organische Chemie", IV/1b, Georg Thieme
Verlag, Stuttgart, 1976, p.55-67
2 - K.D.Karlin and J.Zubieta (eds.), "Copper Coordination Chemistry:
Biochemical and Inorganic Perspectives", Adenine Press, Guilderland,
N.Y., 1983
3 - a) General Electric Co., Neth. Appl. 295,748, C.A. 64:9843a; b) Dynamit
Nobel A.G., Neth. App1.6,610,017, C.A. 66:116152t; c) Hay,A.S.,
U.S.Patent 3,432,466, C.A. 70:97406t; d) Toyo Rayon Co. Ltd.,
Fr.1,523,821, C.A. 71 :13525r; e) Hori,R., Kataoka,T., Kodama,H., Japan
7001,633, C.A. 72:112034g; f) Hay,A.S., Polym.Eng.Sci. 1 6 (1976)l.
4 - C.E.Koning, G.Challa, F.B.Hulsbergen and J.Reedjik, J.Mol.Catal. 34
(1986) 355 and references therein
5 - C.Y.Hsu and J.E.Lyons, Eur. Pat. Appl.EP 93,540, C.A. 100:67996f; EP
104,937, C.A. 101 :54719p; EP 105,067, C.A. 101 54721 h; EP 107,427,
C.A. 1 0 1 :170888s; U.S.Patent 4,442,036, C.A. 101 :6829c; U.S.
4,482,756, C.A. 102:113013z.
6 - Showa Denko K.K., Jpn. Kokai Tokkyo Koho JP 60,123,440 (85,123,440),
C.A. 1045640j.
7 - T.R.Demmin and M.Rogic, J.Org.Chem. 45 (1980) 4210
8 - J.Tsuji and H.Takayanagi, Tetrahedron 34 (1978) 641
9 - J.S.Thompson and J.C.Calabrese, J.Am.Chem.Soc. 108 (1986) 1903
10 - G.Speier, J.Mol.Catal. 37 (1986) 259
11 - S.Tsuruya, H.Kuwahara and M.Masai, J.Catalysis 108 (1987) 369 and
references therein
12 - P.Capdevielle and M.Maumy, Tetrahedron Lett. 23 (1982) 1577
13 - G.Speier and Z.Tyeklar, J.C.S.Dalton Trans. (1988) 2663
14 - M.Gargano, N.Ravasio, M.Rossi, A.Tiripicchio and M.Tiripicchio
Camellini, J.C.S. Dalton Trans. (1989) 921
15 - K.Prabhakaran, PSen and C.N.R.Rao, Surface Sci. 177 (1986) L971
16 - K.Prabhakaran, PSen and C.N.R.Rao, Surface Sci. 169 (1986) L301
17 - W.Brackman and E.Havinga, Rcl. Trav.Chim. Pays-Bas, 7 4 (1955) 937,
1021, 1070, 1100, 1107
18 - Y.Kitayama and T.Sato, Nippon Kagaku Kaishi 9 (1980) 1309, C.A.
94:14755c
19 - A.Takuwa, O.Soga, H.lwamoto and K.Maruyama, Bull.Chem.Soc.Jpn. 59
(1986) 2959
20 - Showa Denko K.K., Jpn. Kokai Tokkyo Koho JP 60,123,441 (85,123,441),
C.A. 104:5641 k.
145
21 - O.Reinaud, P.Capdevielle and M.Maumy, Tetrahedron Lett. 26 (1985)
3993
22 - H.W.Wanzlich and U.Jahnke, Chem.Ber. 101 (1968) 3744; Ger. Pat. No.
1294969, C.A. 71 :030237
23 - A.I.Zvonok, P.Matusevich, N.M.Kuz'menok, A.I.Kumachev, USSR Pat.
638,537, C.A. 90:87050W
24 - Y.ltoh, T.Karuta, M.Hirano and T.Marimoto, Bull. Chem. SOC. Jpn. 5 2
(1979) 2169
25 - D.E.Wilcox, A.G.Porras, Y.T.Hwang, K.Lerch, M.E.Winkler arid E.I.Solomon,
J.Am.Chem.Soc. 107 (1985) 4015
U. TAKAKI (Mitsui Toatsu Chem. Inc., Japan): 1) You get binaphthyl
compounds from p-naphtol. Why you do not get p,p'-biphenol from phenol,
since you use the same experimental conditions?
2) Why don't you synthesize chiral Binap using chiral ligands by your
coupling technique?
N. RAVASIO (C.N.R. MISO, Universita di Bari, Italy): 1) Minor amounts of
diphenoquinones and other polymeric products are in fact present in the
residue of phenol reaction mixture. The products distribution found in the
described reactions depends on the substrates oxidation potential, the
presence of different copper oxidation states and reciprocal orientation of
the intermediates coordinated to the metal.
2) Work is in progress on the synthesis of chiral binaphtol.
G. Centi and F. Trifiio' (Editors),New Developments in Selective Oxidatinn
0 1990 Elsevier Science PublishersB.V., Ameterdam - Printed in The Netherlands
147
PLATINUM CATALYZED OXIDATION OF 5-HYDROXYMETHYLFURFURAL
P. VINKE, H.E. van DAM" and H. van BEKKUN
Laboratory f o r Organic Chemistry, D e l f t U n i v e r s i t y o f Technology, P.O.
2600 GA D e l f t , The Netherlands.
'Present address: N o r i t N.V.,
The Netherlands.
P.O.
Box 5045,
Box 105, 3800 AC Amersfoort,
SUMMARY
The 1 i q u i d phase o x i d a t i o n o f 5-hydroxymethylfurfural (HMF) over platinum on
alumina c a t a l y s t s i s described. The main intermediate i s 5-formyl-2-furani n d i c a t i n g t h a t t h e hydroxymethyl group of HMF i s
c a r b o x y l i c a c i d (FFCA),
o x i d i z e d f i r s t i n the presence o f the aldehyde group. The c a t a l y s t i s not
deactivated by oxygen, due t o a strong metal/substrate i n t e r a c t i o n v i a the
x - e l e c t r o n s o f the furan nucleus.
INTRODUCTION
I n recent years
the
interest
i n the
use o f
carbohydrates
as
chemical
feedstock i s growing considerably (1, 2, 3 , 4). A t t e n t i o n i s given both t o the
use o f renewables as s t a r t i n g m a t e r i a l f o r e x i s t i n g products as w e l l as t o the
replacement
of
o i l - d e r i v e d chemicals by new products made from renewables. One
o f the options of the second category
i s t h e a c i d catalyzed dehydration of
carbohydrates (e.g. fructose) y i e l d i n g 5-hydroxymethylfurfural (HMF) (5). HMF
may serve as a s t a r t i n g material i n several i n d u s t r i a l a p p l i c a t i o n s (6). For
example, t h e o x i d a t i o n products o f HMF (see scheme 1) are p o t e n t i a l b u i l d i n g
u n i t s f o r polymers.
OH
FDC
HMF
OH
FFCA
HFCA
OH
OH
FDCA
Scheme 1. Oxidation
products derived from HMF: 2,5-furandicarboxaldehyde (FDC),
acid
(HFCA) , 5-formyl-2-furancarboxyl i c a c i d (FFCA) and 2,5-furandicarboxyl i c a c i d (FDCA).
5-hydroxymethyl-2-furancarboxyl i c
Several methods have been described t o synthesize FDC, HFCA and
FDC can
be
FDCA.
permanganate (8). HFCA i s formed by o x i d a t i o n o f HMF w i t h molecular oxygen
a
combined
Thus,
prepared by o x i d a t i o n o f HMF w i t h manganese d i o x i d e (7) o r barium
silver
oxide/copper
oxide
catalyst
over
( 9 ) . FDCA can be obtained by
148
oxidation with oxygen over a palladium on carbon catalyst (9). Up to now the
preparation of FFCA from HMF has not been described.
Noble metals (e.g. platinum and palladium) on a carrier are widely used as
catalyst in oxidation reactions. Very little is known about the deta 1 ed
mechanism of the oxidation reaction which takes place at the noble metal
surface. Some mechanistic considerations on the noble metal catalyzed oxidation
sequence, primary alcohol aldehyde * carboxylate, are given below.
+
SUPPORTED NOBLE METAL OXIDATION CATALYSTS
The platinum catalyzed oxidation of primary alcohol groups can be described
as a stepwise oxidative dehydrogenation as shown in Scheme 2. During the
oxidation the metal surface is largely covered by hydrogen which is oxidized by
adsorbed atomic oxygen.
RCH,OH
RCHO
RCH(OH),
RCOO-
+
H++
[ ]
+
[RCOOH]
[OI
+[ I
+ KO,
Scheme 2. Mechanism of the oxidative dehydrogenation o f alcohols over a platinum
catalyst in aqueous media. (From ( l o ) , with permission.)
[ ] metal surface site, < > (sub-surface) hydrogen site.
In non-aqueous media the reaction stops in the aldehyde stage. If water is
present the aldehyde is hydrated to a geminal diol and further dehydrogenated
yielding a carboxylate group.
When the rate of dehydrogenation of the substrate is lower than the rate of
oxidation of adsorbed hydrogen the catalyst is deactivated due to the excessive
adsorption of oxygen. Then, the metal is covered by chemisorbed oxygen, probably
as hydroxyl species, and sub-surface hydrogen can not be formed anymore. Upon
longer exposure to oxygen, the metal surface is covered with an amorphous oxide
layer and all catalytic activity is lost. Thus, the catalyst surface can appear
in three forms, (i) active catalyst covered with sub-surface hydrogen, ( i i )
149
deactivated c a t a l y s t w i t h low a c t i v i t y covered w i t h chemisorbed hydroxyl
species, and (iii) poisoned c a t a l y s t w i t h an amorphous oxide l a y e r . If
necessary, t h i s d e a c t i v a t i o n by oxygen can be prevented by applying low oxygen
p a r t i a l pressures o r by using ' d i f f u s i o n s t a b i l i z e d ' c a t a l y s t s (11).
In p r i n c i p l e ,
all
noble metals
which
are
able
to
perform
(i) the
dehydrogenation o f t h e substrate and (ii)t h e o x i d a t i o n o f adsorbed hydrogen a t
t h e same time, are s u i t a b l e c a t a l y s t s f o r t h e o x i d a t i o n o f HMF. I n p r a c t i c e only
the
platinum group metals ( P t , Pd, Rh, Ru and Ir) can be used. However, l a r g e
d i f f e r e n c e s i n turnover number (TON) and s e n s i t i v i t y f o r oxygen d e a c t i v a t i o n
have been found (12). I n the case o f methanol oxidation, platinum Is the l e a s t
s e n s i t i v e f o r oxygen and has the highest TON o f the metals mentioned before.
The
present
paper
describes the platinum-catalyzed o x i d a t l o n o f HMF w i t h a
focus on the s e l e c t i v e formation o f FFCA. A model f o r t h e o x i d a t i o n mechanism i s
proposed.
The influences o f r e a c t i o n conditions, such as temperature and pH, on
t h e s e l e c t i v i t y and r a t e o f the o x i d a t i o n were studied.
MATERIALS AND PROCEDURES
Oxidation eauioment
Experiments were performed i n a thermostatted glass batch r e a c t o r o f 300 m l ,
equipped w i t h a glass g a s t i g h t s t i r r e r (1500 rpm). The pH was kept constant
using a pH meter (Metrohm 6 5 4 ) coupled t o a pH c o n t r o l u n i t (Hetrohm 6 1 4 ) and an
automatic b u r e t t e (Metrohm 655,
10 m l
piston)
containing
2.00
M potassium
hydroxide. The oxygen p a r t i a l pressure o f the gas phase could be adjusted t o any
desired value between 0.05 and 1 and was kept constant d u r i n g the r e a c t i o n using
an
automatic
oxygen
supply
system.
This system consisted o f a motor b u r e t t e
f i l l e d w i t h water as d i s p l a c i n g l i q u i d , a thermostatted
(30
'C)
gas
burette
f i l l e d w i t h oxygen, and a d l f f e r e n t i a l pressure sensor, which operated t h e motor
burette. The oxygen and hydroxyde uptakes were recorded d u r i n g t h e
reactions.
The oxygen concentration i n t h e l i q u i d phase could be monitored too, by using an
Orion 970899 oxygen electrode. The o x l d a t i o n set-up i s shown i n Figure 1.
Oxidation orocedure
(i)Reduction o f t h e standard c a t a l y s t . 1 g o f d r y powdered c a t a l y s t (5%
Janssen Chimica, Beerse, Belgium)
was introduced i n the reactor, 50 m l o f water was added and t h e system was
flushed w i t h n i t r o g e n (ca. 500 ml/min) t o remove oxygen from the r e a c t o r . Then,
hydrogen was conducted through the r e a c t o r f o r 5 mln a t high f l o w and l o w
s t i r r i n g speed, followed by an a d d i t i o n a l 25 min a t low f l o w and high s t i r r i n g
speed. F i n a l l y t h e hydrogen was removed from t h e gas phase by f l u s h i n g w i t h
n i t r o g e n f o r 5 min.
platinum on alumina, platinum dispersion 0.30,
150
I
I
t
r----------
Figure 1. Batch oxidation equipment. 1 thermostatted batch r e a c t o r , 2 motor
b u r e t t e , 3 gas burette, 4 d i f f e r e n t i a l pressure sensor, 5 gas b u r e t t e ,
6 manually operated piston, 7 pH meter, 8 pH control u n i t , 9 automatic
motor b u r e t t e with storage vessel, 10 oxygen sensor, 11 recorders, 12
sample tube.
( i i ) S t a r t i n u the reaction. 8 mmol of s u b s t r a t e in 30 m l of water was added
t o t h e reduced c a t a l y s t under a low nitrogen flow, t o prevent introduction of
oxygen. After the system was e q u i l i b r a t e d a t t h e preset temperature, the desired
oxygen p a r t i a l pressure was s e t by sucking a calculated amount o f gas out o f t h e
r e a c t o r with piston 6 (see Figure l ) , which was automatically replaced by pure
oxygen. The reaction s t a r t e d a f t e r t h e pH was adjusted by a c t i v a t i n g t h e pH
control system. After 3 minutes t h e f i r s t sample was drawn.
( i i i ) Samole oreoaration f o r HPLC. Samples of ca. 0 . 4 m l were spinned in a
small tube (V= 2 ml) f o r 1 min t o allow t h e c a t a l y s t t o s e t t l e down. The c l e a r
s o l u t i o n was c o l l e c t e d and stored a t -20 'C. J u s t before HPLC a n a l y s i s 200 pl of
s o l u t i o n was added t o 200 pl of 1,5-pentanediol solution (20 mg/ml), which was
used a s internal standard.
HPLC analvsi s
The system consisted of a Waters
d i f f e r e n t i a l refractometer and a
590 chromatography pump, a Waters R401
Perkin-Elmer ISS-100 autosampler. A Biorad
151
HPX87H column (strong cation exchange resin in the Ht form) was used with 3 * 1 ~ 1 - ~
M trifluoroacetic acid (TFA) as mobile phase at 60 'C.
For HPLC-MS measurements a similar system with a Waters 510 chromatography
pump was used, connected to a VG 70-SE mass spectrometer. The ionisation was
accomplished by plasma spray.
NMR measurements
13C NMR spectra were recorded on a Varian VXR-400s spectrometer. Sample
concentrations were ca. 0.3 M and at a pH o f 9. By applying long relaxation
times the spectra could be interpreted quantitatively. Deuterium oxide was added
to lock the signal. Proton spectra were recorded on a Nicolet NT-200 WB
apparatus. Sample concentrations were 0.2 M in deuterium oxide. No internal
standard was applied.
RESULTS AND DISCUSSION
Selective oxidation of HMF to FFCA
In Figure 2 the reaction mixture composition o f a typical oxidation
experiment of HMF is shown as a function of time. In every oxidation experiment
the final oxidation product was FDCA, which was formed in quantitative yields.
conc.
0.10
(rnrnot/rnl)
0.08
0.06
0.04
0.02
0.00
0
80
160
240
320
400
t (rnin)
Figure 2. Oxidation of HMF over a platinum catalyst. Reaction co.nditions: T 60
'C, pH 9.0, p(02) 0.2 atm, p(tota1) 1.0 atm, Co(HMF) 0.1 M, 1.00 g 5%
platinum on alumina powdered catalyst, V(H20) 80 ml. V.= HMF, u= FDC,
A= HFCA, o= FFCA and += FDCA.
152
At
this
stage, the c a t a l y s t was deactivated by oxygen chemisorption because o f
t h e low r e a c t i v i t y o f FDCA. This paper w i l l focus on t h e s e l e c t i v e formation
of
the intermediate FFCA and the f a c t o r s determining t h e s e l e c t i v i t y .
A t t h e present conditions, aldehydes u s u a l l y are more r e a c t i v e than primary
alcohols. So, t h e intermediate formed w i t h a maximum y i e l d a t t= 160 min, was
expected t o be HFCA. However, the product formed i n high y i e l d s
proved
to
be
FFCA. I t s s t r u c t u r e was determined by HPLC coupled t o a mass spectrometer (HPLC-
MS). The molecular mass peak o f t h e intermediate was 141 (MFFCA+
fragmentation
1). The
p a t t e r n o f the intermediate was i n accordance w i t h t h a t o f FFCA.
The 13C NMR spectrum of t h e r e a c t i o n mixture a t t= 160 min confirmed t h a t FFCA
was the intermediate (main peaks a t 6= 183.1 ppm (-C=O), 6= 166.5 ppm (-CO;),
6=
155.5 ppm (C5 furan), 6= 153.3 (CZ),
Comparison
of
the
experimental
6= 126.5
ppm
(C4),
6= 117.5
ppm
and c a l c u l a t e d oxygen uptake a l s o showed t h a t
FFCA i s the main intermediate.
Scheme 3. Main o x i d a t i o n sequence o f HMF upon platinum catalyzed oxidation.
-H,O
1
(C3).
1 H,O
OH
Scheme 4. Resonance s t r u c t u r e s and e q u i l i b r i u m hydration o f HMF.
I
153
I n Scheme 3 t h e main o x i d a t i o n r e a c t i o n sequence
initial
selectivity
is
shown.
The
unusual
f o r alcohol instead o f aldehyde o x i d a t i o n may be caused by
t h e conjugation o f the aldehyde group w i t h the aromatic nucleus ( c f . Scheme 4).
This c o u l d prevent hydration o f t h e aldehyde t o the r e a c t i v e geminal d i o l .
Indeed, 'H NMR measurements i n d i c a t e d t h a t the aldehyde group i s hydrated f o r
l e s s than 1%i n aqueous s o l u t i o n s a t a temperature range o f 30-70 'C and a pH o f
8-11.
The e f f e c t o f v a r v i n s r e a c t i o n conditions on t h e r a t e o f r e a c t i o n
I n Figure 3 t h e r a t e s o f r e a c t i o n a t low conversion and the maximum y i e l d s o f
FFCA are shown f o r several d i f f e r e n t r e a c t i o n conditions. The r e a c t i o n r a t e s are
determined a t low conversion (ca. 5%) and d i f f e r s l i g h t l y from the t r u e
initial
rates.
(i)
V a r i a t i o n o f oxvsen D a r t i a l Dressure (not shown i n Figure . 3 ) . The r a t e o f
r e a c t i o n i s e s s e n t i a l l y f i r s t order i n oxygen p a r t i a l pressure i n the gas phase.
U
0 - 04
20
60
40
T
80
0"
40
'
7
9
8
("3
10
11
12
PH
-n
e
v
l4.
._
%
100
d
75
50
(D
0 - 0 4
0.00
0.05
0.10
c,(HMF)
0.15
(MI
0.20
0.25
d
E
25
0
I
II
111
N
v
catalyst type
!/I
Figure 3. The r a t e o f r e a c t i o n a t low conversion and the maximum concentration
o f FFCA versus (a) temperature, (b) pH, (c) i n i t i a l HMF concentration,
111 5%
and (d) c a t a l y s t type. I 5% Ptjalumina, I 1 1% Pt/alumina,
Pt/alumina extrudates, I V 5% Pt/carbon, V Adams' c a t a l y s t , and V I 5%
Pd/alumina. Standard conditions: T 60 'C,
pH 9.0, p(0 ) 0.2 atm,
p(tota1) 1.0 atm, CO(HMF) 0.1 M, 1.00 g 5% Pt/A1203 powdeped c a t a l y s t ,
V(H20) 80 m l .
154
30 'C and a p(02) o f 0.2 atm t h e oxygen concentration i n t h e l i q u i d phase
d u r i n g r e a c t i o n i s 6 - 7 ppm ( s a t u r a t i o n 7.5 ppm). Therefore t h e g a s / l i q u i d mass
At
t r a n s f e r of oxygen i s n o t r a t e l i m i t i n g , a t l e a s t n o t a t low temperatures. The
d i f f u s i o n i n t o t h e c a t a l y s t p a r t i c l e s i s u n l i k e l y t o be r a t e l i m i t i n g as can be
seen f o r t h e o x i d a t i o n w i t h Adam' c a t a l y s t (powdered pure platinum) which has a
comparable r a t e of r e a c t i o n . Other studies a l s o i n d i c a t e t h a t oxygen d i f f u s i o n
i n t o t h e c a t a l y s t p a r t i c l e i s n o t r a t e l i m i t i n g (13). Thus, t h e chemisorption of
oxygen on t o t h e platinum surface has t o be t h e r a t e determining step.
Jii)V a r i a t i o n o f t e m e r a t u r e . I n the temperature range studied, the r a t e o f
o x i d a t i o n f o l l o w s an exponential curve as a f u n c t i o n of temperature. Assuming
(see above) t h e r e a c t i o n t o be f i r s t order i n [ O 2 I L , r a t e constants can be
c a l c u l a t e d and t h e Arrhenius parameters can be obtained. I t has t o be taken i n t o
account
that
t h e s o l u b i l i t y o f oxygen i s temperature dependent, so t h e r a t e o f
r e a c t i o n has t o be corrected f o r these differences. The
this
reaction
a c t i v a t i o n energy
for
appears t o be 37.2 kJ/mol which i s i n good accordance w i t h other
platinum catalyzed oxidations (e.g. glucose oxidation,
EA=
40 kJ/mol) (14).
jiii) V a r i a t i o n o f DH. The r a t e of r e a c t i o n i s independent o f t h e pH i n the
range o f pH 8-11, which r e s u l t i s i n c o n t r a s t t o o t h e r studies i n t h i s f i e l d ,
r e p o r t i n g increasing
r e a c t i o n r a t e s a t higher pH values (14). These authors
explained the increase i n r e a c t i o n r a t e by assuming a higher degree o f
i o n i s a t i o n of t h e hydroxyl ( o r hydrated aldehyde) group o f t h e substrate.
Apparently, t h i s process i s o f
no
importance
for
the
of
rate
the
present
o x i d a t i o n reaction.
A t low pH values (pHs 8) t h e r e a c t i o n r a t e decreases somewhat because
dioxide,
which
is
formed
in
small
amounts
by
oxidative
cleavage
substrate, evolves from solution, thus lowering the oxygen p a r t i a l
the
gas
phase.
o f the
pressure
in
higher pH values the carbon d i o x i d e i s kept i n s o l u t i o n as
At
( b i )carbonate.
j i v ) Variation
carbon
of
initial
substrate
concentration. The i n i t i a l substrate
concentration d i d n o t i n f l u e n c e t h e r e a c t i o n r a t e
significantly
(zero
order),
except f o r very low values o f t h e concentration. I n t h a t case t h e dehydration o f
t h e substrate i s slower than the
oxidation
of
chemisorbed
c a t a l y s t i s deactivated.
j v ) V a r i a t i o n o f c a t a l v s t tvoe. The r e a c t i o n r a t e i s
the
type
o f c a t a l y s t used.
This
hydrogen
strongly
and t h e
dependant
on
confirms the f a c t t h a t t h e g a s / l i q u i d mass
t r a n s f e r o f oxygen i s n o t r a t e l i m i t i n g . The r e a c t i o n r a t e f o r palladium i s much
h i g h e r than t h a t f o r platinum. Even a t a high oxygen concentrations i n the
l i q u i d phase ( [ O P l L 6-7
~ ppm) the palladium c a t a l y s t remains a c t i v e , i n contrast
t o o x i d a t i o n o f methanol where the c a t a l y s t i s poisoned a t [O2IL= 1 ppm (12).
155
I n general, noble metal
catalyst
are
often
deactivated when
the
oxygen
concentration i n t h e l i q u i d phase i s too high. I n t h e case o f HHF, however, the
c a t a l y s t remains a c t i v e and stable, even a t very high oxygen concentrations i n
solution.
This
can
be explained
by
assuming
strong metal/substrate
i n t e r a c t i o n , i n which t h e substrate i s adsorbed s t r o n g l y onto t h e metal surface.
The i n t e r a c t i o n o f a hydroxyl o r aldehyde group w i t h t h e metal .is probably n o t
strong enough t o prevent oxygen chemisorption, as
oxidation
of
a
can
be concluded
from the
methanol o r glucose where c a t a l y s t d e a c t i v a t i o n occurs. Therefore
t h e i n t e r a c t i o n o f t h e r - e l e c t r o n system o f t h e aromatic furan r i n g i s
to
be responsible
fact
that
the
for
believed
t h i s strong adsorption. This model i s supported by the
rates o f
r e a c t i o n are
zero
order
in
initial
substrate
concentration. So, a t any time during r e a c t i o n t h e platinum surface i s l a r g e l y
covered w i t h substrate molecules. I n t h i s way the oxygen coverage i s kept low
and t h e c a t a l y s t remains a c t i v e .
The r a t e s o f r e a c t i o n o f FDC and HFCA are d i f f e r e n t from HMF (02 uptake a t
standard c o n d i t i o n s 2.71*10m2 m o l / m i n f o r FDC, and 3.87*10-2 mmol/min f o r
HFCA). This could be caused by d i f f e r e n c e s i n r a t e s o f dehydrogenation,
leading
t o a d i f f e r e n c e i n hydrogen occupation o f t h e platinum. Due t o the d i f f e r e n c e i n
hydrogen coverage, t h e chemisorption o f oxygen i s affected.
At
low degree o f
conversion (5 t o 25%) t h e r a t e o f r e a c t i o n i s decreased,
compared t o t h e i n i t i a l r a t e . A t t h e s t a r t o f t h e o x i d a t i o n the platinum surface
i s covered w i t h HMF. Even a t very low conversions, p a r t o f tht! metal surface
w i l l be occupied
interaction o f
by
the
intermediate
first
FDC,
due t o
tht!
very
t h i s causes a decrease i n o v e r a l l r e a c t i o n r a t e , which i s experimentally
At
higher
strong
FDC w i t h the metal. Because FDC has a lower ratc! o f oxidation,
conversions,
when
the
amount
of
found.
FDC i s almost zero, t h e r a t e o f
r e a c t i o n i s increasing again. The adsorption o f t h e second intermediate FFDC on
to
the
platinum
surface
is
less
strong and t h e o x i d a t i o n r e a c t i o n proceeds
faster.
The s e l e c t i v i t v towards FFCq
I n Figure 3 t h e maximum y i e l d s
of
FFCA
are
shown
for
several
different
r e a c t i o n conditions.
J i l V a r i a t i o n o f oxyqen
oartial
Dressure
(not
shown
in
Figure 3 ) .
The
v a r i a t i o n o f oxygen p a r t i a l pressure does n o t have any e f f e c t on t h e s e l e c t i v i t y
o f t h e reaction, which i s i n accordance w i t h t h e model presented above.
..
Var i a t i o n of t emDerat u re. A change i n temperature has l i t t l e e f f e c t on
t h e s e l e c t j v i t y . A t lower temperatures t h e maximum y i e l d o f FFCA i s somewhat
111
less.
This
may
be caused by a change i n adsorption c h a r a c t e r i s t i c s o f HMF and
FDC a t t h e metal surface.
156
Jiii)
Variation
of
initial
substrate
concentration.
The
substrate
concentration has l i t t l e i n f l u e n c e on t h e s e l e c t i v i t y o f t h e o x i d a t i o n reaction,
which i s i n accordance w i t h the model o f t h e r e a c t i o n .
J i v ) V a r i a t i o n o f t h e DH. A t high pH values the
selectivity
decreases
s i g n i f i c a n t l y . The l o s s o f s e l e c t i v i t y i s caused by the concurrent formation o f
HFCA as intermediate and n o t by d i r e c t o x i d a t i o n o f FCD t o FDCA. Apparently, a t
h i g h pH values (210) the o x i d a t i o n o f the aldehyde group proceeds more e a s i l y .
Possibly, the hydrated aldehyde i s s t a b i l i z e d a t t h e platinum surface by
ionization o f
the
geminal
diol.
Because
a gerninal d i o l i s more r e a c t i v e i n
o x i d a t i o n r e a c t i o n s than an alcohol, a l a r g e amount o f HFCA i s formed.
I n principle,
the
oxidation o f
HFCA
FFCA i n h i g h y i e l d s , but
can g i v e
experimentally t h i s i s only 40%. This low s e l e c t i v i t y f o r FFCA upon o x i d i z i n g
HFCA can be explained by assuming t h a t t h e adsorption o f HFCA and FFCA on t o the
metal surface w i l l n o t d i f f e r s i g n i f i c a n t l y . Consequently, t h e two substrates
w i l l be o x i d i z e d simultaneously.
The h i g h s e l e c t i v i t y f o r FFCA upon o x i d a t i o n o f HMF a t moderate pH values can
be
explained
too.
The hydrated aldehyde i s n o t ' s t a b i l i z e d '
by i o n i z a t i o n and
t h e r e f o r e t h e alcohol group w i l l be oxidized s e l e c t i v e l y , y i e l d i n g
oxidized
to
FFCA.
The
ionized carboxylate group
metal/substrate i n t e r a c t i o n , so FDC
is
adsorbed
FDC, which i s
o f FFCA w i l l decrease the
predominantly,
even
i n the
presence o f FFCA. Experimentally, o x i d a t i o n o f FDC i s g i v i n g FFCA i n 95% y i e l d .
J v ) V a r i a t i o n o f t h e c a t a l v s t tvoe. The s e l e c t i v i t y i s dependant on the type
o f platinum c a t a l y s t used. Possibly t h e d i s p e r s i o n o f t h e c a t a l y s t influences
t h e i n t e r a c t i o n o f t h e substrate w i t h t h e platinum. A t h i g h dispersions the
larger
density
o f steps and edges on t h e noble metal c r y s t a l l i t e surface could
decrease t h e i n t e r a c t i o n o f t h e substrate w i t h
the
metal,
thus
lowering
the
selectivity.
CONCLUSIONS
The
oxidation
of
HMF over platinum on alumina c a t a l y s t s proceeds w i t h high
s e l e c t i v i t y towards t h e intermediate FFCA. This i s believed t o be caused by t h e
conjugation o f the carbonyl bond w i t h t h e aromatic furan nucleus. Thus, the
aldehyde i s o n l y s l i g h t l y hydrated t o a geminal d i o l , which i s t h e r e a c t i v e
species i n t h e o x i d a t i v e dehydrogenation t o the corresponding carboxyl i c acid.
The oxygen concentration i n t h e l i q u i d phase i s
oxidation,
although
rate
determining
for
this
no d i f f u s i o n l i m i t a t i o n i s observed. This can be explained
by assuming a strong metal/substrate i n t e r a c t i o n , which also prevents oxygen t o
d e a c t i v a t e t h e c a t a l y s t . The aromatic n u c l e i are believed t o be responsible f o r
t h i s i n t e r a c t i o n , which i s supported by the f a c t t h a t t h e r e a c t i o n i s zero order
i n substrate concentration but dependant on type o f substrate.
157
ACKNOWLEDGEMENTS
f o r generously p r o v i d i n g a sample o f
We wish t o thank Siiddeutsche Zucker A.G.
HMF, and D r . Jan Oouwstra o f
Netherlands
for
TNO,
p r o v i d i n g FDC.
Division
The
of
Technology
i n v e s t i g a t i o n was
for
Society,
supported
by
The
the
Netherlands Organization f o r S c i e n t i f i c Research (NWO).
REFERENCES
1
2
3
4
5
6
7
8
9
10
11
12
13
14
A. Fuchs, Starch/Stlrke, 10, (1987), 335-43.
A.J.J. Straathof, A.P.G. Kieboom, and H. van Bekkum, Carbohydr. Res., 146,
(1986), 154-9; A.J.J. Straathof, A.P.G. Kieboom, and H. van Bekkum,
Starch/Stlrke, 40, (1988), 229-34; A.J.J. Straathof, A l k y l glucoside
s u r f a c t a n t s from starch and sucrose, Thesis D e l f t U n i v e r s i t y o f Technology,
The Nether1 ands, (1988).
H. Schiweck, K. Rapp, and M. Vogel, Chem. Ind., 4, (1988), 22e-34.
J.L. Hickson ( e d i t o r ) , Sucrochemistry, ACS Symposium Series 41, Amerlcan
Chemical Society, Washington D.C., (1977).
H.E. van Dam, A.P.G. Kiebaom, and H. van Bekkum, Starch/Starke, 3, (1986),
95-101.
A. Faury, A. Gaset, and J.P Gorrichon, I n f . Chim., 214, (198l), 203-9.
A.F. O l e i n i k , and K.Y. N o v i t s k i i , J. Org. Chem. USSR, 6, (1971), 2643.
T. E l - H a j j , J.-C. Martin, and G. Descotes, J. Heterocyclic Chem., 20,
(1983), 233-235.
B.W. Lew, US Patent 3.326.944, (1967).
H.E. van Dam, A.P.G. Kieboom, and H. van Bekkum, Appl. Catal., 33, (1987),
361-72.
H.E. van Dam, P. Duijverman, A.P.G. Kieboom, and H. van Bekkum, Appl.
Catal., 33, (1987), 373-82.
H.E. van Dam, Carbon supported noble metal c a t a l y s t s i n the o x i d a t i o n o f
glucose-1-phosphate and r e l a t e d alcohols, Thesis D e l f t U n i v e r s i t y o f
Technology, The Netherlands, (1989).
P.J.M. D i j k g r a a f , H.A.M. Duisters, B.F.M. Kuster, and K. van der Wiele, J.
Catal., 112, (1988), 337-44.
P.J.M. D i j k g r a a f , Oxidation o f glucose t o g l u c a r i c a c i d by Pt/C c a t a l y s t s ,
Thesis Eindhoven U n i v e r s i t y o f Technology, The Netherlands, (1!389).J
158
B. DELMON ( U n i v e r s i t e Catholique de Louvain, Belgium): A t non-perfect c o n d i t i o n s
(inadequate support o r 02 pressure) you observe a d e a c t i v a t i o n o f your c a t a l y s t .
There are, i n p r i n c i p l e , two reasons a t l e a s t why such a d e a c t i v a t i o n could
occur:
- o x i d a t i o n o f t h e P t surface
- polymerization o f t h e aldehyde group
You n i c e l y solved t h e problem. Nevertheless, i t would be i n t e r e s t i n g t o i d e n t i f y
t h e o r i g i n o f d e a c t i v a t i o n (and, thus, the r e a l r o l e o f t h e favorable
m o d i f i c a t i o n s you make).
One can n o t i c e t h a t both possible causes o f d e a c t i v a t i o n can be a f f e c t e d by O z ,
( i ) t h e o x i d a t i o n o f t h e P t surface, which i s obvious, and ( i i ) t h e condensation
o f t h e aldehyde through the e f f e c t on a c i d i t y o f t h e support by s p i l l - o v e r
oxygen. Do you have physico-chemical information on t h e p o s s i b l e cause o f
d e a c t i v a t i o n , and d i d you t r y other supports?
P. VINKE ( D e l f t U n i v e r s i t y o f Technology, The Netherlands): F i r s t o f a l l I have
t o emphasize t h a t i n t h e case o f o x i d a t i o n o f aromatic compounds such as
5-hydroxymethylfurfural (HMF) t h i s d e a c t i v a t i o n does n o t occur u n t i l 1 the
o x i d a t i o n i s completed. This i s probably caused by a p r o t e c t i v e i n t e r a c t i o n o f
t h e n - e l e c t r o n s o f t h e aromatic nucleus w i t h t h e noble metal surface. However,
i n many other cases d e a c t i v a t i o n o f the c a t a l y s t i s a serious problem. I n our
l a b o r a t o r y t h e d e a c t i v a t i o n o f t h e c a t a l y s t i s studied using a c t i v a t e d carbon as
c a r r i e r and methanol as substrate (1). We found t h a t t h e electrochemical
p o t e n t i a l o f the c a t a l y s t p a r t i c l e s i s changing d u r i n g d e a c t i v a t i o n , i n d i c a t i n g
a change i n chemical s t r u c t u r e o f the noble metal. These p o t e n t i a l measurements
l e a d t o t h e conclusion t h a t t h e metal i s changing from t h e reduced s t a t e i n t o
t h e o x i d i z e d s t a t e during deactivation. This c l e a r l y shows t h a t d i r e c t o x i d a t i o n
o f the noble metal surface causes t h e c a t a l y s t poisoning.
1. H.E. van Dam and H. van Bekkum, Recl. Trav. Chim. Pays-Bas, i n press.
H.E. van Dam, Carbon supported noble metal c a t a l y s t s i n t h e o x i d a t i o n
glucose-1-phosphate
and
r e l a t e d alcohols, Thesis D e l f t U n i v e r s i t y
Technology, The Netherlands, (1989).
of
of
D. ARNTZ (Degussa A.G., Hanau, BRD): The comparison on a c t i v i t y was made o n l y i n
view o f precious metal content. Because o f t h e d i f f e r e n t dispersions due t o
d i f f e r e n t preparation methods a b e t t e r c h a r a c t e r i z a t i o n would be a c o r r e l a t i o n
between a c t i v i t y and number o f a c t i v e centers. Are t h e r e measurements on the
dispersions o f t h e a c t i v e phase and d i d you c o r r e l a t e them t o t h e a c t i v i t y ?
P. VINKE ( D e l f t U n i v e r s i t y o f Technology, The Netherlands): Indeed, i t i s
i n t e r e s t i n g t o r e l a t e the a c t i v i t y w i t h t h e amount o f exposed noble metal.
Therefore, I w i l l g i v e you t h e TON's (turnover numbers) as mol 02/mol metal
exposed/min f o r the d i f f e r e n t c a t a l y s t s as described i n Figure 3.
Table. TON's f o r the s i x c a t a l y s t s t e s t e d (see Figure 3).
c a t a l y s t code
I
d i spersi on2
5% P t /
A1 2 0 3
0.30
TON I l m i n )
extrudates,
measured as mol
c a t a l y s t type
0.71
I1
111
1% P t /
A1203
0.15
5% P t /
A1203'
0.07
3.02
1.31
IV
5% P t /
C
V
VI
0.51
Pt
black
0.02
5% Pd/
A1203
0.07
0.61
2.46
3.28
CO adsorbed per mol noble metal
As can be seen from these r e s u l t s , t h e TON's d i f f e r s i g n i f i c a n t l y . Not o n l y the
two noble metals show d i f f e r e n t values, but t h e platinum c a t a l y s t s used are not
comparable e i t h e r . A t r e n d can be observed towards higher TON's a t lower
dispersions. Therefore i t can be concluded t h a t t h e dispersions a l s o i n f l u e n c e
t h e TON f o r t h i s o x i d a t i o n r e a c t i o n .
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE USE OF PRECIOUS METAL CATALYSTS SUPPORTED ON ACTIVATED
CARBON I N OXIDATION REACTIONS FOR THE SYNTHESIS OF F I N E
CHEMICALS, ESPECIALLY FOR THE SELECTIVE OXIDATION OF GLUCOSE T O
GLUCONIC ACID
8,_.ML-P.e.spe~.r-~~
K. D e l l e r , E . P e l d s z u s
Oegussa A G , G e s c h a f t s b e r e i c h A n o r g a n i s c h e C h e m i e p r o d u k t e
Abt.
AC-AT 3-CK,
P o s t f a c h 13 4 5 , D - 6 4 5 0 Hanau 1
ABSTRACT
For t h e o x i d a t i o n o f glucose t o gluconic a c i d i n t h e
l i q u i d phase a t r i m e t a l l i c c a t a l y s t P d - P t - B i on a c t i v a t e d
c a r b o n has been developed. A c t i v i t i e s o f more t h a n 4000 g
[ g l u c o n i c a c i d l / g [ p r e c i o u s m e t a l 1 x h w e r e f o u n d . The
s e l e c t i v i t y o b t a i n e d i s h i g h e r t h a n 96 m o l l .
A d e t a i l e d i n v e s t i g a t i o n o f t h i s c a t a l y s t as w e l l a s
P d - B i o n a c t i v a t e d c a r b o n . P t on a c t i v a t e d c a r b o n a n d P t - B i o n
a c t i v a t e d c a r b o n has been c a r r i e d o u t showing t h e e f f e c t o f
e a c h m e t a l component o r t h e c o m b i n a t i o n o f them.
The r o l e o f P t a s a b o o s t e r f o r a c t i v i t y a n d B i a s a
b o o s t e r f o r s e l e c t i v i t y i s shown.
T h e r e i s n o c o r r e l a t i o n b e t w e e n t h e r e s u l t s o b t a i n e d by
c a r r y i n g out usual physico-chemical characterization techniques
and t h e c a t a l y t i c b e h a v i o u r o f t h i s P d - P t - B i on a c t i v a t e d
c a r b o n system.
INTRODUCTION
The u s e o f p r e c i o u s m e t a l c o n t a i n i n g s u p p o r t e d c a t i i l y s t s
f o r t h e synthesis o f f i n e chemicals under o x i d a t i v e c o n d i t i o n s
i n t h e l i q u i d phase i s m a i n l y r e p o r t e d i n t h e l i t e r a t u r e f o r
t h e o x i d a t i o n o f a l c o h o l s , t h e o x i d a t i o n o f a l k e n e s and t h e
o x i d a t i o n o f sugars.
Especially f o r t h e c a t a l y t i c o x i d a t i o n o f glucose, the main
p r o d u c t s a r e g l u c o n i c a c i d ( i n d u s t r i a l l y u s e d as c h e l a t i n s
agent f o r cleaning a p p l i c a t i o n s )
and/or g l u c a r i c acid.
159
160
A n o t h e r r e a c t i o n product i s 2 - k e t o - g l u c o n i c a c i d ( u s e d in
t h e m a n u f a c t u r e o f Vitamin C ) . By-products a r e f r u c t o s e
( i s o m e r i z a t i o n o f g l u c o s e ) and other c a r b o x y l i c acid products
(cleavage oxidation reaction).
C a t a l y s t s w i t h high a c t i v i t y . high selectivity and high
stability a r e d e s i r a b l e t o e n a b l e s u c h a process t o c o m p e t e
economically w i t h t h e i n d u s t r i a l f e r m e n t a t i v e synthesis o f
g l u c o n i c acid. Different c a t a l y s t s a r e known a l r e a d y , most o f
them based on Pd o n activated c a r b o n ( r e f s . 1 - 5 ) . The u s e o f
Bi resp. Pb. l e a d s t o t h e s e l e c t i v e f o r m a t i o n o f g l u c o n i c a c i d
f r o m g l u c o s e e s p e c i a l l y under a l c a l i n e reaction c o n d i t i o n s .
W e h a v e i n v e s t i g a t e d h o w t h e different possible c a t a l y s t
s y s t e m s P d - B i o n activated c a r b o n , Pt o n activated c a r b o n and
P t - B i on a c t i v a t e d c a r b o n i n f l u e n c e activity a n d selectivity
for t h e o x i d a t i o n o f g l u c o s e t o g l u c o n i c a c i d . Based o n t h e s e
r e s u l t s w e h a v e f i n a l l y developed a t r i m e t a l l i c catalyst
P d - P t - B i o n activated c a r b o n .
W e will a l s o r e p o r t r e s u l t s o n t h e i n f l u e n c e o f r e a c t i o n
a n d c a t a l y s t parameters both on activity and selectivity.
EXPERIMENTALS
M a te r i~ 1 2
G l u c o s e a s g l u c o s e m o n o h y d r a t e f r o m Fluka ( " p . a . " q u a l i t y )
r e s p . R i e d e l - d e Haen ( " r e i n " q u a l i t y ) w a s used a s r e c e i v e d .
P u r e O 2 w a s used.
T h e f o l l o w i n g Oegussa catalysts w e r e used:
C E F 196 RA/W 4 7: P d , 1 X P t , 5 X B i ( P d - P t - B i o n a c t i v a t e d
carbon t
C F 196 RA/W 5 X P t , 5 Z B i
( P t - E i o n activated c a r b o n )
C E 196 RA/W 5 1: P d , 5 X B i
( P d - B i on activated c a r b o n )
F
196 RA/W 5 X Pt
( P t o n activated c a r b o n )
F
196 B / W 5 Z P t
( P t on activated c a r b o n )
_--I.
161
c 3 ~.al~.S.t_RXePar.a.~.O~
An a c t i v a t e d carbon powder ( B E T s u r f a c e area: 1000 m 2 / g )
w i t h high m a c r o p o r e content w a s used as support. T h e m e t a l l i c
phase on t h e catalyst w a s prepared using a s o l u t i o n o f Bi20s
( d i s s o l v e d in H C l conc.) a n d / o r h e x a c h l o r o p l a t i n i c a c i d and/or
p a l l a d i u m ( I I l c h 1 o r i d e w h i c h w a s added t o an a q u e o u s suspension
of activated c a r b o n .
C o - p r e c i p i t a t i o n by treatment w i t h N a O H and f i n a l :reduction
leads t o t h e desired m e t a l l i c phase. The c a t a l y s t suspension
w a s f i l t e r e d and washed. T h e catalyst w a s u s e d in t h e g l u c o s e
o x i d a t i o n without any further treatment.
Re a-c-tAo n - con d i t i m so- f..a1.uc o s e ~ i.dxa t i o n
A l l reactions w e r e c a r r i e d out in a 150 m l stirred g l a s
v e s s e l i n batch t y p e at a constant pressure a n d t e m p e r a t u r e
including pH-control. A stirrer (type: BR1 from Buddeberg GmbHl
assured a t h o r o u g h gas m i x i n g during t h e reaction.
The reaction products w e r e n e u t r a l i z e d w i t h N a O H t o t h e
c o r r e s p o n d i n g sodium salts during t h e reaction.
After a c e r t a i n reaction t i m e samples w e r e taken and t h e
catalyst was separated f r o m t h e product c o n t a i n i n g solution.
T h e f i l t r a t e w a s analyzed by HPLC. ion c h r o m a t o g r a p h y and t h i n
l a v e r chromatography. T h e stability o f t h e catalyst was
d e t e r m i n e d by recycling t h e catalyst and m e a s u r i n g t h e l e v e l
in a c t i v i t y .
S.t a-n.d a .rd. x.ea c_tion__wn-d i t i sn I
16 g g l u c o s e ( - 1 7 , 6 g g l u c o s e m o n o h y d r a t e l . dissolved in
100 m l w a t e r
temperature:
55 OC
02-pressure:
10 mbar
pH:
10.0
NaOH-solution:
10 w e i g h t % for neutralization
stirrer r a t e :
1 8 0 0 Rpm
0 , 2 4 g ( 1 . 5 weightx based on glucose1
catalvst c o n c . :
162
R E S U L T S A N D DISCUSSION
.C*t - a l Y s t _ s x s L e m P . M i o n.r.kixat.tSd_r;a.r$x~
The results obtained w i t h different P d - B i on activated
carbon catalysts are listed i n Table 1 .
TABLE 1
5 X P d - 5 i! Bi on activated carbon catalyst for t h e g l u c o s e
oxidation under standard reaction conditions: Influence o f t h e
catalyst preparation method on g l u c o s e conversion, gluconic
acid selectivity and catalyst activity.
catalyst A
reaction
time ( m i n )
catalyst B
catalyst C
35
60
35
60
35
60
conversion of
glucose (moll)
69
100
79
100
100
100
selectivity t o
g l u c o n i c acid
( m o l %1
95
93
95
96
96
90
1700
1300
2000
1400
2400
1300
catalyst
activity
g[gluconic acid]/
g[palladiuml x h
catalyst A : Prepared from t h e corresponding Pd on activated
c a r b o n catalyst followed by Bi-impregnation.
catalyst 8: Prepared from t h e corresponding Bi impregnated
activated carbon followed by Pd-impregnation and
reduction.
catalyst C: Prepared by co-precipitation of t h e m e t a l phase
followed by reduction. Degussa catalyst CE 196 RA/W
5 i! P d . 5 z Bi
163
The use o f Bi-Pd on activated c a r b o n catalyst w h e r e first
t h e Bi-salt i s fixed on t h e activated carbon followed by t h e
palladium impregnation shows high selectivities o f gluconic
a c i d , Na-salt but t h e activity of t h e catalyst i s low.
The use o f P d - B i on activated carbon catalyst w h e r e t h e
fresh prepared Pd on activated carbon catalyst has been
impregnated w i t h t h e Bi-salt shows an activity less 0:r
c o m p a r a b l e t o t h e o n e described a b o v e (1500 g [glucon.ic a c i d /
gCpalladium1 x hl.
P d - B i on activated carbon catalysts prepared by
co-precipitation o f t h e Bi-salt and t h e palladium(I1)chloride
acid w i t h NaOH showed an increase o f t h e activity t o 2400 g
[gluconic acidl/g [palladium1 x h without affecting the
selectivity t o gluconic acid.
W
w
m Pt on act wAti&LuAQ.tL
Pd an Pt behave q u i t e different during t h e oxidation o f
glucose.
Under l o w catalyst concentration ( s t a n d a r d reaction
c o n d i t i o n s ) t h e oxidation o f g l u c o s e with Pt-containing
catalysts leads t o yields of gluconic acid obtained l e s s t h a n
70 1 .
At high concentration of Pt on activated carbon catalyst
[standard reaction conditions but w i t h catalyst t o glucose
ratio
20 w e i g h t % ) , t h e oxidation of glucose produces glucaric
acid at a longer reaction t i m e (refs. 5 - B l . High selectivity
values o f m o r e than 8 0 moll of glucaric acid c a n be obtained
( s e e Fig. 1 ) . T h e formation o f by-products results from t h e
o x i d a t i v e degradation o f t h e gluconic acid resp. t h e g l u c a r i c
acid formed.
-
164
9
8
7
~6
'e
x
+
-5
E4
0
A
a
v
3
-
glucose
gluconic acid
- g l u c o r i c acid
- t a r t a r i c acid
-
tartronic acid
oxolic acid
2
1
10
20
30 40
50
60
70
80 90 100 110 120
time (min )
Fig. 1 . Pt o n a c t i v a t e d c a r b o n c a t a l y s t : g l u c o s e o x i d a t i o n
u n d e r s t a n d a r d r e a c t i o n c o n d i t i o n s but under c a t a l y s t t o
s u b s t r a t e r a t i o 20 : 100 w i t h O e g u s s a c a t a l y s t F 196 B/W 5 Z
Pt.
Cat a 1 y s t- s Y S t.em! P t= Bi,-prl_ac tir?I.tad-._c_n,rho_rl!
Pt-Bi on activated carbon catalysts improve the yield o f
gluconic acid obtained under standard reaction conditions ( i n
c o m p a r i s o n t o Pt o n a c t i v a t e d c a r b o n c a t a l y s t s ) but t h e y i e l d
o f g l u c o s e i s l i m i t e d t o 6 0 - 90 m o l % .
At higher c a t a l y s t c o n c e n t r a t i o n ( c a t a l y s t t o g l u c o s e
r a t i o : 20 w e i g h t Z l m o r e c l e a v a g e p r o d u c t s and v e r y u n s e l e c t i v e
formation o f glucaric acid were obtained.
165
The oxidation o f the a-position of glucose is described in
t h e literature (refs. 9 - 1 0 ) . This oxidation of t h e a-position
w a s also reported for other reaction types (alcohol oxidation)
(refs. 1 1 - 1 3 ) . Under t h e reaction conditions used i n t h e
p r e s e n t s t u d y t h i s b e h a v i o u r c o u l d not b e o b s e r v e d .
!ht a . 1 ~
_s t-s ~2.t
em-.!? d 9 t - B 110r l a c t i v e t d - c a_rb m ~
Fig. 2 s h o w s t h e r a t e o f f o r m a t i o n o f g l u c o n i c a c i d a s a
f u n c t i o n o f t i m e u s i n g a t r i m e t a l l i c Pd-Pt-tli o n a c t i v a t e d
c a r b o n c a t a l y s t u n d e r s t a n d a r d r e a c t i o n c o n d i t i o n s . It i s
evident that t h e use o f Pt can boost t h e activity o f t h e Pd-Bi
on activated carbon catalyst without influence on t h e
selectivity. Activity values higher than 4 0 0 0 g Cgluconic
acidl/g [precious metal1 x h can be obtained.
T h e a b o v e m e n t i o n e d c a t a l y s t s s y s t e m s a r e p l o t t e d on
Fig. 2 for comparison.
100
- 80
5
.Z
60
u
0
.-u
5u 40
3
m
-J
a
-
20
x
10
20
30
40
50
time (min.)
:
c
60
F i g . 2. Y i e l d o f g l u c o n i c a c i d o b t a i n e d f o r t h e d i f f e r e n t
catalyst systems used in the oxidation of glucose under
standard reaction conditions.
166
In T a b l e 2 t h e formation o f a l l reaction products a s a
f u n c t i o n o f t i m e for t h e t r i m e t a l l i c catalyst i s shown.
The i n f l u e n c e o f t h e t e m p e r a t u r e and t h e p H o f t h e
reaction a s well a s t h e m e t a l c o n c e n t r a t i o n s used in t h e
t r i m e t a l l i c catalyst w a s investigated.
It w a s f o u n d out that a m e t a l content o f 4 'L P d , 1 Z Pt
and 5 Z Bi g i v e s t h e best r e s u l t s . T h e o p t i m i z e d r e a c t i o n
c o n d i t i o n s already given in t h i s studv w e r e u s e d as standard t o
c o m p a r e t h e different c a t a l y s t s y s t e m s .
T h e catalyst stability has been i n v e s t i g a t e d . T h e
recycling o f t h e catalyst m o r e t h a n 5 0 t i m e s is possible w i t h
r e g e n e r a t i o n o f t h e catalyst. Further i n v e s t i g a t i o n s a r e
planned.
TABLE 2
P d - P t - B i o n activated carbon catalyst: G l u c o s e o x i d a t i o n under
s t a n d a r d reaction c o n d i t i o n s w i t h Oegussa catalyst
CEF 196 R A / W 4 Z P d , 1 Z P t , 5 'L Bi.
reaction t i m e Cminl
18
30
25
20
amount o f substances
[mol x l o - ' ]
.
I.
qlucose
0,15
< 0,Ol
< 0,Ol
g l u c o n i c acid
8,50
8,513
13.44
8.13
fructose
0.08
0,13
0 , 13
0,13
g l u c a r i c acid
0,03
0,05
0.09
0.32
< 0.01
< 0.01
0,05
0,07
t a r t r o n i c acid
< 0,Ol
< 0,Ol
0.09
0,17
o x a l i c acid
< 0.01
0.01
0,06
0,14
--
- ---
conversion ( 2 0 ' ) :
selectivity (20'):
activity ( 2 0 ' ) :
-
-
< 0,oi
t a r t a r i c acid
__
.
--
100 1
98 z
4200 g C g l u c o n i c acidl/q[precious m e t a l ] x h
-
CATALYST CHARACTERIZATION OF DEGUSSA CATALYST C E F 196 RAIW
4 II P d , 1 II P t , 5 II Bi
T h e a n a l y s i s o f t h e m e t a l l i c phase by energy d i s p e r s i v e
a n a l y s i s X - r a y IEDX) shows that t h e c a t a l y s t particles a r e
totally i m p r e g n a t e d and t h e m e t a l very well homogeneously
d i s p e r s e d throughout t h e catalyst particles.
T h e m e t a l d i s p e r s i o n on t h e c a t a l y s t s u r f a c e i s l o w
( m e a s u r e d by C O - a d s o r p t i o n ) and c o m p a r a b l e w i t h valuer o f o t h e r
P d I P t bimetallic c a t a l y s t s w i t h o u t Bi.
The c r i s t a l l i t e s i z e w a s m e a s u r e d by TEM a n d revealed w e l l
c r i s t a l l i z e d B i i n rod shape besides P t - P d agglomerates o f
about 2 - 5 n m s i z e ( c o m p a r a b l e t o c r i s t a l l i t e s i z e of Pd/Pt
bimetallic c a t a l y s t s w i t h o u t B i ) .
E S C A f S I M S i n v e s t i g a t i o n s d e m o n s t r a t e that under optimized
preparation c o n d i t i o n s t h e P d - p h a s e is s t i l l m a i n l y o x i d i z e d ,
w h e r e a s t h e P t - p h a s e i s m a i n l y r e d u c e d . The B i - p h a s e w a s found
t o be in t h e o x i d e f o r m a s B i 2 0 1 .and B i 2 O 2 C O 3 . T h e l a s t
compound c o u l d b e interpreted as a n interaction o f Bi w i t h t h e
support l e a d i n g t o t h e c a r b o n a t e f o r m a t i o n .
T h e predominant r o l e o f Bi in t h e very s e l e c t i v e o x i d a t i o n
o f g l u c o s e t o g l u c o n i c acid still remains undisclosed. N o
interaction o f Bi w i t h t h e precious m e t a l s Pd and P t c o u l d b e
d e t e c t e d . N o alloy f o r m a t i o n c o u l d be seen i n E S C A .
P u r e B i o n activated carbon catalyst ( w i t h o u t precious
m e t a l ) i s t o t a l l y i n a c t i v e in this reaction. T h e presence o f
precious m e t a l IPd o r P t ) is necessary.
SUMMARY
T h e u s e o f a t r i m e t a l l i c c a t a l y s t Pt-Pd-Bi on activated
carbon proved t o be superior in activity. selectivity and
stability i n c o m p a r i s o n t o other bimetallic P d - B i o n activated
c a r b o n o r P t - B i o n activated carbon c a t a l y s t systems for t h e
g l u c o n i c a c i d f o r m a t i o n f r o m glucose. T h e enhancement in
activity by t h e addition o f P t t o P d - B i o n activated carbon
c a t a l y s t i s surprising and c o u l d not be explained by t h e
e x p e c t e d behaviour o f both Pd ( s e l e c t i v e o x i d a t i o n o f t h e
a l d e h y d e f u n c t i o n o f t h e g l u c o s e ) and Pt Iselective o x i d a t i o n
o f t h e position 6 resp. t h e position 2 o f t h e g l u c o s e c h a i n )
alone.
A l s o t h e preponderant role o f Bi as a selectivity booster
in t h e g l u c o n i c acid formation r e m a i n s u n d i s c l o s e d and c o u l d so
far not be c l a r i f i e d by u s u a l physical c h a r a c t e r i z a t i o n
methods. Only t h e formation o f B i 2 0 2 C 0 3 c o u l d b e o b s e r v e d
s h o w i n g a c h e m i c a l interaction between t h e B i - p h a s e and t h e
support.
168
REFERENCES
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H. Hoffmann (Univ. o f Erlangen, West-germany): Can you i n d i c a t e how t h e pH
value changed d u r i n g t h e r e a c t i o n ?
How d i d you s t a b i l i z e an a l k a l i n e pH ?
B.M. Despeyroux (Degussa AG, West-Germany): The pH was maintained constant
d u r i n g t h e r e a c t i o n by t h e use o f a pH-regler and adding NaOH. A pH value o f
10 +
- 0.1 c o u l d be achieved.
R. Chunk (Lonza AG, Switzerland): The r o l e o f t h e bismuth promotor i n improving
t h e s e l e c t i v i t y remains unclear. Since t h i s observation i s n o t r e s t r i c t e d t o
t h i s r e a c t i o n , b u t i s an o f t e n observed phenomenon i n heterogeneous c a t a l y s i s ,
i t seems t o me important t o understand t h e r o l e o f promoters i n o x i d a t i o n c a t a l y s i s . An understanding o f t h e mechanismlstructures i n v o l v e d would a l l o w us t o
" t a i l o r make" c a t a l y s t s f o r s p e c i f i c o x i d a t i o n r e a c t i o n s .
G.Centi and F.Trifiro' (Editom), New Developments in Selective Oxidatinn
0 1990 Elsevier Science Publiehers B.V.,Amstardam - Printed in The Netherlands
LIQUID-PHASE
OF
OXIDATION
AND
HYDROCARBONS
169
ALCOHOLS
CATALYZED BY HETEROGENEOUS PALLADIUM AND PLATINUM CATALYSTS
M. HRONEC, Z. CVENGROSOVA, J. TULEJA and J. ILAVSKY
Faculty of Chemistry, Slovak Technical University
812 37 Bratislava (Czechoslovakia)
SUMMARY
Activity and selectivity of supported Po' and Pt
catalysts have been studied in the liquid-phase oxidation of
hydrocarbons and alcohols to ketones and carboxylic acids.
It was found that the rate of these reactions is mostly
controlled by mass transfer effects. At higher partial
pressure of oxygen the catalysts are reversibly deactivated
by oxygen. Higher resistance against deactivation and higher
catalytic activity of Pd and Pt catalysts is achieved b y
doping them with some metals.
INTRODUCTION
Palladium and platinum supported on charcoal are known
as selective catalysts
alcohols
metal
and
other organic
centers
oxygen
for the oxidation of hydrocarbons,
and
of
C-H
bonds,
r1-41.
compounds
these catalysts
but
are
The
capable
during
reactive
to activate
the
processes
a
deactivation by oxygen often occurs. An important influence
on the oxidation reaction catalyzed by these catalysts has
the
nature
of
a
solvent.
Thus,
in
n-heptane
solution,
primary alcohols are oxidized to aldehydes, but in water at
alkaline pH, the corresponding acids are produced.
Much still needs to be done to explore the effect to
other metals on the activity and selectivity of Pd and Pt
catalysts.
In
the
literature
only
a
few
such
data
are
available. There is also a lack of data in the in€luence of
the
structure
of
the
oxidized
substrate
on
the
catalyst
activity and the deactivation process.
METHODS
Materials
a-Pinene,
of
1-methoxy-2-propanol
phenoxyethanol
were
(MPOL)
purified
2,3;4,6-Di-isopropylidene-a-L-sorbose
by
and
derivatives
distillation.
( D I S ) was purified
by
170
double crystallization from methanol. Other reagents were of
analytical purity.
Apparatus
Oxidation experiments were performed in two types of
reactors. A 1 5 0 ml stainless steel reactor was fitted with a
magnetic stirrer (3 1000 rpm), air inlet at the bottom and
outlet through a condenser. The second reactor was a 8 0 cm
high bubble column (i. d. 3.1 cm) equipped with an air
introduction through a porous sparger (mean pore size less
than 0.2 mm). During the reaction the outlet gases from the
reactors were monitored continuously for oxygen.
Catalysts
The palladium and platinum catalysts were prepared by
impregnation of charcoal (surface area 1265 m 2 . g - l , particle
size < 0.12 mm) and CaC03 (2.9 m2 .g-', particle size (0.08
mm) with PdCIZ or HZPtClg ( 6 0 OC; 8 h), followed by a
reduction with formaldehyde [51. Some part of each catalyst
was re-impregnated (80 O C ; 5 h) with Co, Bi, Cd, Zn, Mn
water soluble salts (nitrates, chlorides, sulfates) which
were subsequently transformed to hydroxides, adding a
solution of NaOH. The catalysts thus obtained were
thoroughly washed with water and stored moist under
nitrogen. The metal content of the catalysts was determined
by polarography (after their transformation to soluble
salts).
Analysis
Samples of the reaction mixtures from MPOL and a-pinene
oxidation were analyzed by GC (Hewlett Packard 5830) after
separation of the catalyst and doping them with internal
standards.
The
reaction
mixtures
from
DIS
and
phenoxyethanols oxidation were neutralized with HC1 to pH y
3 after separation of the catalyst, and the formed acids
extracted (3x), esterified and analyzed by GC (using an
internal standard). The products were confirmed by GC-MS
and NMR spectroscopy.
171
RESULTS AND DISCUSSION
A series of Pt and Pd catalysts were tested during the
oxidation of following compounds:
(i) 1-methoxy-2-propanol to 1-methoxy-2-propanon
CH3-0-CH2-CH-CH3 t 1/2 O2
CH3-O-CHZ-C-CH 3
I
4
OH
0
(ii) a-pinene to verbenol and verbenon
+
H2°
(iii) DIS to 2,3;4,6-diisopropylidene-2-keto-L-guloiiic acid
0 0
I/
CH3-C
66 A
A
- CH3
CH3-C
(iv)
derivatives
of
phenoxyacetic acids
C'@O-CH$H~OH
CH3
I/
- CH3
phenoxyethanol
+
to
02-c'@-O-CH~COOH
corresponding
+
H,O
CH3
These compounds are used in the preparation of pesticides,
pheromones and Vitamine C.
The results in Table 1 show effect of the support and
its surface area on the activity of palladium catalyst
during the oxidation of 2-methoxyphenoxyethanol in an
aqueous solution of NaOH.
172
TABLE 1
Effect of the support on the activity of palladium catalyst
for the oxidation of 2-MPE.
Support
Surface
area
m2 . g - '
Reaction
time
min
1 265
970
443
2.9
200
Active
carbon
CaC03
Conversion
210
200
250
Yield, mol
%
%
2-MPA
2-MP
97.8
97.2
95.4
98.3
95.2
89.4
94.6
94.6
0.90
0.70
0.75
0.60
2-MPE = 2-methylphenoxyethanol; 2-MP = 2-methylphenol; 2-MPA
= 2-methylphenoxyacetic acid
4.5 g 2-MPE; 6 6 g H20; 1 . 3 g NaOH; 0.71 g catalyst (5 %
Pd/support); 99 OC; 0.2 MPa; gas flow (67 vol % O2 in N 2-0 2
mixture) = 20 cm3 min-l; batch reactor
From the results it is seen that the nature of the support
and its surface area affect the catalytic properties of
supported palladium. However, a very high yield of 8-MPA is
obtained with Pd/CaC03 catalyst having a veru low surface
area. Moreover, this catalyst remains active and selective
upon reuse (see Table 2).
TABLE 2
Change of activity and Pd content of the catalyst upon reuse
for the oxidation of 2-MPE.
Number of
runs
1
2
12
16
Reaction
time
min
270
440
450
450
Conversion
%
98.1
98.2
97.9
98.4
Yield, mol %
2-MPA
2-MP
94.7
95.2
95.0
94.8
0.50
0.60
%
Pd/CaC03
wt %
4.31
-
0.45
1.56
0.70
1.11
Catalyst: 4.31 X Pd/CaC03 ( 1 . 3 g ) ; after each osidation 0.13
g fresh catalyst was added to compensate losses during the
filtration
The drop of the oxidation rates is observed only after the
173
first run, and then it remains unchanged. Surprising is that
despite more than 73 X loss of palladium from the catalyst,
the activity remains unchanged.
Kinetic measurements in the bubble column and the
stirred reactor show that the reaction is always controlled
by mass transfer phenomena (see Table 3).
TABLE 3
Kinetic data of oxidation of phenoxyethanol derivatives in
two types of reactors.
Reactor
Substrate
Mass transfer resistance;
l/k,,a
l/ksas + l/kr
Batch
reactor
2-MPE
3,I-MCPE
2,4-MCPE
2-MPE
0.39
Bubble
column
0.42
0.59
3.51
s
2.71
2.09
1.86
1.72
3,4-MCPE = 3-methyl-4-chlorophenoxyethanol;
2,4-MCPE =
2-methyl-4-chlorophenoxyethanol; kLa = volumetric gas-liquid
mass transfer coefficient; ksas = liquid-solid mass transfer
coefficient; kr = reaction rate constant
The kinetic regime of the oxidation cannot be reached
at any conditions. The problem is that at higher partial
pressures of oxygen (above 0.26
MPa), the catalyst
reversibly deactivates. However, after decreasing the oxygen
pressure, the original activity of the catalyst is reached
again.
Hydrocarbons having secondary or tertiary C-H bond are
in the presence of Pd and Pt catalysts oxidized to alcohols
and ketones. A s it is evident from Table 4, a strong
influence on the activity of these catalysts have some
metals deposited on the catalyst surface in the form of
hydroxides and oxides.
Their presence on the catalyst surface does not
influence the distribution of formed alcohols and ketone
(ratio 2 : 1) and the ratio of cis/trans isomers of
verbenols. The highest promoting effect on both, Pd and Pt
catalysts has a mixture of cobalt and cadmium.
174
TABLE 4
Effect of the catalyst composition on a-pinene oxidation
Catalyst
wt % metal
Conversion
Selectivity, %
verbenon verbenol
%
cis/trans
verbenol
5% Pd/C
18.1
26.9
54.8
5%Pd-7.1%Bi-0.4%Zn/C
21.3
20.1
27.1
57.9
1.2
26.1
55.5
1.3
29.7
58.5
1.3
16.6
28.7
33.7
58.5
1.2
1 .o
33.9
49.7
1.1
5%Pd-7.1%Bi-0.4%Cd/C
5%Pd-2%C0-0.7%Cd/C
5%Pd-2%Mn-0.7%Cd/C
5%CO/C
5%Pt/C
5%Pt-2.09%C0-0.7%Cd/C
44.4a
25.4
6.8
1
35.6
1.2
a 9.6 wt % hydroperoxides in the reaction mixture
80 OC; 0.2 MPa; oxygen flow 300 cm3 min-1 ; catalyst 1.5 g;
a-pinene 250 ml; reaction time: 5 h; bubble column reactor
It is expectable that the activity and selectivity of
palladium and platinum catalysts will be different during
the oxidation of various organic compounds. However, as we
have found, the oxidized substrate plays also an important
role during
deactivated
the
by
reactivation of
molecular
oxygen.
the
catalyst which
Thus,
the
was
monometallic
catalyst, 5 % Pd/C stored moist, is highly active for the
oxidation of DIS, but when this catalyst is dried and
exposed to air before the reaction, its activity sharply
decreases (see Table 5).
On the other hand, the same Pd/C catalyst deactivated
in this manner has the same activity during the oxidation of
phenoxyethanol derivates, MPOL and another alcohols. A s it
is shown in Table 5 , the resistance of Pd/C catalyst against
irreversible deactivation by oxygen is achieved by doping it
with some metals, e.g. Co and Cd.
The palladium and platinum
catalyzed
oxidation
of
alcohols in aqueous solution proceeds via a dehydrogenation
mechanism. This reaction proceeds on the catalyst surface
and obbeys the Langmuir-Hinshelwood kinetics, It means that
Pt and Pd surface can be covered with oxygen, oxidized
175
substrate, hydrogen atoms and products formed in the
dehydrogenation process. The fraction of the surface covered
by each component depends on experimental conditions and the
type of organic substrates.
TABLE 5
Effect of the catalyst history on its activity for the
oxidation of DIS
Catalyst
4.9%Pd-2XC0-0.7XCd/C
4.9XPd-2XC0-0.7%Cd/C
5% Pd/C
5% Pd/Ca
5% IJd/Cb
Reaction
time
h
5.5
5.5
6
7
7
Conversion
x
100
100
96.5
52.3
1.3
Selectivity
x
99.8
99.7
99.5
99.1
99.4
a 1 week exposed to air;
1 month exposed to air
130 OC; 0.35 MPa; 15 g DIS; 150 ml HZO; 3.02 g NaOH; air
batch reactor
flow 20 cm 3 lain-';
According to the L-H mechanism, an alcohol adsorption
and its rate of oxidation are influenced by oxygen
concentration and adsorptive properties of alcohol. Thus,
during the oxidation of phenoxyethanols at partial pressure
above 0.25 MPa, oxygen completely covers the catalyst
surface and totaly deactivates it. When the pressure
decreases, the catalyst is again active. However, in the
oxidation of DIS, a higher pressure of oxygen (above 0.4
MPa) is needed to deactivate only partly the catalyst [S].
The dehydrogenation of alcohol is a reversible reaction
and the hydrogen on the catalyst surface is continuously
oxidized. In some cases it can also hydrogenate the formed
of
product.
This
is
suggested
by
the
results
1-methoxy-2-propanol oxidation to ketone MPON which proceeds
only to a ca. 50 % conversion with various catalysts at any
experimental conditions. The added MWN retards the rate of
oxidation and supresses the conversion of alcohol.
On the basis of the above mentioned results and the
literature data [6-8], we assume that the equilibrium
concentrations of reactants adsorbed on Pd and Pt surfaces
176
are responsible
these
catalysts.
for
the activity
The
observed
and
the deactivation of
promoting
effect
of
some
metals deposited on the catalyst is probably connected with
their ability to change the fractional concentration of the
surface oxygen and oxidized substrate. In order to prove
this assumption, additional physicochemical investigat on is
continued.
REFERENCES
1 R.A. Sheldon and J.K. Kochi, Metal Catalyzed Oxidat on of
Organic Compounds, Academic Press, 1 9 8 1
2 U S Patent 4 5 9 9 4 4 6 ; C.A. 1 0 5 , 2 0 9 3 4 5
3 German Offen 3 135 9 4 6 ; C.A. 9 9 , 7 0 2 1 7
4 US Patent 4 5 7 9 6 8 9 ; C.A.105, 1 1 6 9 9 1
5 Belg. Patent 8 5 1 8 0 4 ; C.A. 8 8 , 1 7 0 4 7 1
6 M. Hronec, Z. Cvengrosova and M. Stolcova, React. Kinet.
Catal. Lett., 20 ( 1 9 8 2 ) 2 0 7
7
H.E. von Dam, P. Duijverman, A.P.G. Kieboom and H. van
Bekkum, Appl. Catal. 3 3 ( 1 9 8 7 ) 3 7 3
8
P.J.M. Dijkgraaf, H.A.M. Duisters, B.F.M. Kuster and K.
van Wiele, J. Catal. 1 1 2 ( 1 9 8 8 ) 3 3 7
H.Mimoun C l n s t .
F r a n c a i s du P e t r o l e ,
F r a n c e > : I n t h e case
o x i d a t i o n of c i s - p i n e n e . i s y o u r r e a c t i o n a r a d i c a l c h a i n one'?
of
M.Hronec: O x i d a t i o n o f c i s - p i n e n e p r o c e e d s v s a a f r e e r a d i c a l
mechanism. P a l l a d i u m a n d p l a t i n u m c a t a l y s t s p r o b a b l y a c t i v a t e t h e
C-H
bond i n t h e h y d r o c a r b o n .
S i n c e t h e i n f l u e n c e of
these
catalysts
on
the
hydroperoxide
decomposition
is
very
low,
h y d r o p e r o x i d e s formed as t h e p r i m a r y p r o d u c t s a r e a c c u m u l a t e d i n
t h e r e a c t i o n m i x t u r e C s e e T a b l e 41.
S. C o l u c c i a C D i p a r t i m e n t o d i Chimica. Torino3:
You show t h a t t h e
a c t i v i t y does n o t c h a n g e s i g n i f i c a n t l y d u r i n g several r u n s , i n
s p i t e of a s u b s t a n t i a l decrease of t h e m e t a l c o n c e n t r a t i o n . Does
t h i s o b s e r v a t i o n s u g g e s t a n y h y p o t h e s i s o n t h e a c t u a l e x t e n t of
a c t i v e sites a n d p o s s i b l y o n t h e i r s t r u c t u r e ' ?
W e s u g g e s t t h a t o n l y a p a r t of t h e m e t a l l o a d e d on t h e
i s a c t u a l l y c a t a l y t i c a l l y a c t i v e . I t i s b a s e d on t h e
m e a s u r e m e n t s of the a c t i v i t y o f t h e p a l l a d i u m c a t a l y s t s h a v i n g a
d i f f e r e n t amount of t h e l o a d e d m e t a l .
For e x a m p l e . t h e P d K
c a t a l y s t c o n t a i n i n g on1 y 1.11 X Pd a f t e r s i x t e e n r e u s e s Csee T a b l e
21 w a s still several t i m e s m o r e a c t i v e t-han t h e f r e s h l y p r e p a r e d
Pd/C c a t a l y s t s w i t h a 1 . 2 - 2 . 5 % c o n t e n t of p a l l a d i u m .
The p a l l a d i u m c a t a l y s t s b e f o r e a n d at-ter r e a c t i o n h a v e b e e n
methods.
From t h e ESCA
s t u d i e d by ESCA a n d e l e c t r o c h e m i c a l
measurement f o l l o w e d t h a t t h e s u r f a c e of
t h e c a t a l y s t always
c o n t a i n s t h e P d - p h a s e a n d PdO. N o c o r r e l a t i o n w a s f o u n d b e t w e e n
c a t a l y s t s d i f f e r i n g i n composition and t h e i r r e d o x p r o p e r t i e s
m e a s u r e d b y e l e c t r o c h e m i c a l method.
M.Hronec:
carrier
0.Centi and F.Tnfiro' (Editom),New Developments in Sekctive Oxidation
0 1990 Elsevier SciencePublishere B.V.,Amsterdam -Printed in The Netherlands
177
CATALYTIC OXIDATION OF 1 -ALKENES WITH MOLECULAR OXYGEN AND PALLADIUM NITRO
COMPLEXES
N.H. KIERSl, B.L. FERINGA*' and P.W.N.M. van LEEUWEN'
'University of Groningen, Department of Organic Chemistry, Nyenborgh 16, 9747 AG
Groningen (The Netherlands)
2Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research B.V.), Badhuisweg 3,
1031 CM Amsteraam (The Netherlands)
SUMMARY
(CH3CN)2PdClN02 is capable of catalysing the oxidation of 1-alkenes to methyl
ketones, epoxides (refs. 1-9) and aldehydes (ref. 6) using molecular oxygen. In
this paper we report the influence of solvent, co-catalyst and additional
ligands on the reactivity and selectivity in the oxidation of 1-alkenes to
a 1dehydes by (CH3CN)2PdC 1NO2.
INTRODUCTION
Selective catalytic oxidations of alkenes with molecular oxygen are commercially important and synthetically useful processes (ref. 10). It is well-known
that 1-alkenes can be selectively oxidized to methyl ketones (ref. 11). Based on
this oxidation reaction alkenes can be regarded as masked ketones.
Oxidation reactions of alkenes with molecular oxygen mediated by (CH3CN)2PdClNO, have been described (refs. 1-9). Alkenes are generally oxidized to the
corresponding ketones (refs. 1,2,4.6,7). With specific alkenes epoxides were
formed (refs. 3,4,7,8,9). However, we observed aldehyde formation in a good
yield using (CH3CNI2PdClNO2 as catalyst with t-butyl alcohol as solvent and
CuC12 as co-catalyst (ref. 6). It is assumed that the oxidation of alkenes to
ketones goes by an intramolecular nucleophilic attack of the nitro group on the
palladium bonded alkene followea by a hydride shift (refs. 1-9,12,13). We assume
that formation of aldehydes goes by a comparable mechanism (scheme 1).
We now report the influence of solvent, co-catalyst and additional ligands on
the catalytic oxidation reaction.
RESULTS AND DISCUSSION
In a typical oxidation reaction 1-octene was converted using a catalyst
(5 mol %) prepared from (CH CN) PdC1NO2, CuC12. in an oxygen-saturated solution
3 2
of t-butyl alcohol. After a reaction time of 16 hours under an oxygen atmosphere
octanal and 2-octanone (ratio 60:40) were obtained in a 970 % combined yield
based on (CH3CN)2PdClN02. A low isomerization activity was observed resulting in
178
the formation of 80 7, (based on Pd) octene isomers. The proposed mechanism for
the catalytic oxidation of 1-alkenes to aldehydes is given in scheme 1.
0
It
N
CI
‘Pd/
L/
CUCI,
I0
‘L
1
0
IIII
R
L
cL\ / N
Pd
I0
CH,CN
R
0.5 0 ,
0
I1
L = CHJCN
R C H 2 C t l0
I
R
Scheme 1. The proposed mechanism for the catalytic oxidation of 1-alkenes to
aldehydes by molecular oxygen mediated by (CH3CN)2PdC1N02.
We propose that the regioselectivity in the cycloaddition o f the alkene coordinated to the pallaaium nitro catalyst, determines the aldehyde to ketone ratio.
It may be anticipated that the constitution of the palladium nitro complex and
the nature of the ligands strongly influence the stereoselectivity in the
cyclisation step. Subtle effects on the stereochemical results of 1,3-dipolar
cycloaddition to alkenes are well preceaented and the mechanistic pathways described above certainly show similarities with 1,3-dipolar cycloaddition reactions (ref. 14).
Preliminary experiments showed that several factors like metal salts, stjlvent
and ligands influence the reactivity and selectivity of the oxidation reaction.
In order to assess these factors we have undertaken a systematic investigation.
The results on variation in solvent, co-catalyst and ligands are described herewith.
The influence of solvent on the oxidation reaction is summarized in table 1.
Entry 2,3,4 and 7 show the strong tendency of Pd(I1) complexes to catalyse the
selective oxidation of 1-alkenes to methyl ketones. Coordinating solvents almost
completely inhibits the oxidation reaction and results in isomerization of the
starting 1-alkene. Possibly this effect is due to blocking of the necessary
coordination places at palladium or is the result of a fast substitution of the
179
coordinated a1kene.
TABLE 1
Oxidation of I-octene. 0.2 nun01 (CH3CN)2PdC1N02 t 0.8 mmol CuC12 .t 4 mmol
I-octene, 25 ml solvent, 50°C. (Product determination (GC) after 16 hours,
amounts in % based on Pd).
Entry
1
2*
3
4
5
6
7
8
9
10
11
Solvent
Octanal
2-Octanone
t-butyl alcohol
t-amyl alcohol
isopropyl alcohol
hydroxyacetone
2-hydroxypropionitrile
acetonitrile
toluene
nitromethane
acetone
HMPA
DMF
580
170
390
1490
500
1000
80
460
130
<30
<30
40
300
100
50
-
<30
c30
t30
-
-
-
-
30
500
500
<30
<30
Octene isomers
<I00
<30
*Reaction time of 8 hours.
In apolar solvents we only observed a very slow reaction, partially due to the
low solubility o f the catalyst in these solvents. So far t-butyl alcohol i s the
only solvent in which aldehyde than ketone can be obtained.
The role of CuC12, the co-catalyst, in the classical Wacker oxidation is to
oxidize Pd(0) to Pd(I1). The effect o f co-catalyst on the oxidation of I-octene
with (CH3CN)2PdC1N02 in t-butyl alcohol is summarized in table 2.
TABLE 2
The influence of co-catalyst on the oxidation of 1-alkene. Reacticn in t-butyl
alcohol, at 30°C with 20 equivalents (based on Pd) of I-octene. The amount of
co-catalyst and of products (GC) are based on Pd.
Entry
I*
IL
13
14
15
16
17
Lo-catalyst
(eq)
Reaction
time (h)
Octanal
(eq)
2-Octanone
(eq)
4 CUCl2
4 CUCl2
1 CUCl2
10 CuC12
15 CuC12
4 CUCl2
4 CuCN
16
4
19
5
5
0.5
6
-
4
3
4
0.5
0.5
1
0.2
0.7
2
6
1
2
2
Octene isomers
(eq)
1
1
9
1
1
19
10
180
TABLE 2 (continued)
Entry
18**
19
20
21
22
23
24
Co-catalyst
(eq)
4
4
4
4
2
2
2
Reaction
time (h)
C ~ ( C 1 0 ~ ) ~ . 6 H ~ 01
CU(CO~)CU(OH)~ 2
CuC12 t 1 LiCl
7
CuC12 t 1 LiF
4
CuC12 t 2 SnC12 0.5
CUCl2 + 2 C0Cl2 1
CuC12 t 2 NiC12 1
Octanal
(eq)
2-Octanone
(eq1
Octene isomers
(eq)
5
19
56
8
5
6
3
3
2
4
1
6
15
15
14
*Reaction temperature of 5OOC.
**lo0 Equivalents of 1-octene.
Increasing the amount of CuC12 shows a maximum in the reactivity and the selectivity to aldehyde formation using four equivalents of CuC12. Decreasing the
amount of CuC12 gives an increase in isomerization rate and a decrease in oxidation rate and selectivity. Increasing the amount of CuC12 beyond four equivalents shows only a small influence on the oxidation reaction, but increases
the isomerization rate. Substituting two equivalents of CuC12 for other metal
salts like SnC12, FeC13, ZnC12, NiC12, HgC12, CoC12 or PdC12 results in the
acceleration of isomerization reaction and gives only small amounts o f ketones
and aldehydes. The enhanced isomerization using other metal halides than CuC12
might be attributed to a lewis acid effect on the palladium catalysed reactions.
The role of CuC12 in the Wacker oxidation is well established. The role of
CuC12 in the oxidation of 1-alkenes by (CH3CN)2PdC1N02 is however not cornpletely clear. In table 2 it is shown that not only the oxidation rate but also the
selectivity of the oxidation reaction strongly depends on the amount of CuC12
used. We therefore assume that the active species in the formation of aldehydes
is not just a palladium nitro complex, but a species that also contains CuC12,
presumably a chloride bridged binuclear complex. Unfortunately we were not yet
able to isolate such a species to prove our assumption.
In table 1 it is shown that the oxidation reaction was almost completely inhibited using coordinating solvents (entry 5,6,10,11). The effects of additional
1 igands on the oxidation reaction of 1-octene by (CH3CN)2PdClN02-4C~C12 in t-butyl alcohol under an oxygen atmosphere is summarized in table 3. It was shown
(ref. 12) that NO; easily dissociates from palladium. However, we found that
increasing the amount of NO; in solution by adding KN02 (entry 26,27) leads to
an increase in the isomerization rate and in a decrease of the oxidation reaction rate. This effect was even much stronger using an NO2 atmosphere above the
181
reaction medium.
TABLE 3
Ligand effect on the oxidation of 1-octene. Reaction of (CH3CN)2PdC1N02-4C~C12
in t-butyl alcohol at 5OoC with 20 equivalents (based on Pd) of 1-octene,
amount of ligand and product (GC) based on Pd.
Entry
12
25
26
27
28
29
30
31
32
ligand
NO2 (atmosphere)
KN02
KN02
acetonitrile
2-hydroxypropionitrile
trichloroacetonitri le
HMPA
(CH3CN)2PdBrN02
Amount
(eq)
2
4
4
2
2
2
-
Reaction Octanal
time (h) ( 9 6 )
2-Octa- Octene
none ( 9 6 ) isom.(%)
4
5
4
4
8
3
0.1
3
19
300
180
125
50
250
50
<50
600
<20
125
50
200
80
<50
t50
390
<50
690
100
1350
375
300
850
<lo
>500
700
540
Addition o f nitriles increases the isomerization rate and decreases the oxidation rate, probably by blocking the necessary coordination places on palladium
or by a fast substitution of the coordinated alkene by the additimed ligands.
CONCLUSIONS
The reactivity and selectivity o f the oxidation reaction of 1-octene to
octanal using molecular oxygen and (CH3CN)2PdC1N02 strongly depends on the
reaction conditions. Reasonable amounts o f aldehyde are only observed in t-butyl
alcohol using four equivalents of CuC12 as the co-catalyst. The oxidation of
other alkenes is now under investigation and will be reported later.
REFERENCES
1 B.S. Tovrog, S.E. Diamond and F. Mares, J. Am. Chem. SOC., 1979. 101, 270.
2 M.A. Andrews and K.P. Kelly, J. Am. Chem. SOC., 1981, 103, 2894.
3 A. Heumann, F. Chauvet and B. Waegell, Tetrahedron Lett., 1982, 23, 2767.
4 M.A. Andrews, T.C.-T. Chang, C.-W.F. Cheng and K.P. Kelly, Organometallics,
1984, 12, 1777.
5 M.A. Andrews, T.C.-T. Chang, C.-W.F. Cheng and K.P. Kelly, J. h. Chem. SOC.,
1984, 106, 5913.
6 B.L. Feringa, J. Chem. SOC., Chem. Comnun., 1986, 909.
7 J.P. Solar, F. Mares and S.E. Diamond, Catal. Rev.-Sci. Eng., 1985, 27(1), 1.
182
8 M.A. Andrews and C.-W.F. Cheng, J. Am. Chem. SOC., 1982, 104, 4268.
9 P.K. Wong, M.K. Dickson and L.L. Sterna, J. Chem. SOC., Chem. Commun., 1985,
1565.
10 G.W. Parshall, Homogeneous Catalysis, Wiley, New York, 1980.
11 P. Henri, Palladium-Catalysed Oxidation of Hydrocarbons, Riedel, Dordrecht,
1980.
12 M.A. Andrews, T.C.-T. Chang and C.-W.F. Cheng, Organometallics, 1985, 4,
268.
13 B.S. Tovrog, F. Mares and S.E. Diamond, J. Am. Chem. SOC., 1984, 102, 6618.
14 A. Padwa ( E d . ) , 1,3-Dipolar Cycloaddition Chemistry, Vol. 1, 2, Wiley, New
York, 1984.
ACKNOWLEDGEMENTS
This investigation was supported by the Netherlands Foundation for Chemical
Research (SON) with financial aid from the Netherlands Organization for
Scientific Research (NWO).
183
H . MIN10IiN ( L a b . d ’ 0 x y d a t i o n I n s t . F r a n c . P e t r o l e , F r a n c e ) : D i d
you o b s e r v e P d ( 0 ) p r e c i p i t . a t i o n ? H o w c a n you e x p l a i n t h e
a n t i m a r k o v n i k o v o x i d a t i o n i n d u c e d by t.-BuOH? Did you t r y t.-BuO1 i g a n d s on Prl?
N . H . KIERS ( I J n i v e r s i t . y o f G r o n i n g e n , The N e t h e r l a n d s ) : I n some
cases w e o b s e r v e d a smal 1 amount o f P d ( 0 ) p r e r , i p i t a t i o n a f t e r
a b o u t 4 t u r n o v e r s , r e s u l t i n g i n a n i n c r e a s e of t h e i s o m e r i s a t i o n
r a t e a n d a decrease i n t h e s e l e c t i v i t y tciwards a l d e h y d e
e were n o t a b l e t o make a complex c o n t . a i n i n g b o t h NO2format i o n . W
a n d t-BuO- l i g a n d s . O x i d a t i o n r e a c t i o n s u s i n g Cu(0‘Bu) or Cu(OtBu)2
i n s t e a d o f CuClz were n o t s u c c e s f u l .
G . FRANZ ( F a . H u l s A G , B R D ) : Comment on Wacker P r o c e s s :
S c . i e n t i s t s o f some i n s t i t u t e s of S i b i r i a n Branch of Academie of
S c . i e n c e s o f U S S R h a v e some y e a r s a g o s u c c e e d e d i n r e p l a c i n g CuCl2
1)y h e t e r n p o l i c a c i d s i n t h e Wacker Process. T h e r e f o r e Cu is n o t
e s s e r i t i a l i n t h i s syst.em. Pd i s a v e r y e f f e c t i v e d e c a r b o n y l a t i o n
c a t a l y s t o f a l t l e h y r l e s . Did yciu ever. pay a t t e n t i o n t o
d e c a r b o n y l a t i o n p r o d u c t s d u r i n g your e x p e r i m e n t a l s t u d i e s ?
N . H . K l E R S C l J r i i v e r s i t y o f G r o n j n g e n , The N e t h e r l a n d s ) : The main
d i f f e r e n c e between t h e Wacker o x i d a t i o n a n d o u r o x i d a t i o n p r o c e s s
is the u s e of a P t l ( l l ) - N 0 2 ,’ P d ( I l ) - N O instead o f a Pd(:1I) / P d ( 0 )
c o u p l e . I n our s y s t e m P d ( 1 I ) is riot r e d u c e d . W e f o l l o w e d t h e
o x i d a t . i o i i react. i i m s w i t h GC/MS a n d w e d i d n o t o b s e r v e t.he
f o r m a t iori o f dec.at~boriylation p r o d u c t s .
.J.M. BREGEAULT ( l i n i v . P et M . C u r i e , F r a n c e ) : Did you icibserve t h e
for.mat ioii of c . h I c i r i n a t e d p r o d u c t s when you work w i t h a n e x c e s s of
CuClz? What. about. t h e d e c a y p e r i o d uf y o u r s y s t e m ?
N . H . KIERS ( U n i v e r s i t y o f G r o n i n g e n , T h e N e t h e r l a n d s ) : W
e did not
o b s e r v e c h l o r i n a t e d p r o d u c t s . l’he d e c a y p e r i o d s t o n g l y d e p e n d s
upon t.he c.oriclit i o n s u n d e r which t . h e c a t a l y t i c syst.em i s p r e p r e p a r e d . Whit h o u t p r e - p r e p a r e t . i o r i we o b s e r v e d a s l o w react i o n
arid a decxiy of t h e react i o n a f t e r a b o u t 4 h o u r s . With p r e p r e p a r e - ti o n o f t h e c a t . a l y s t by s t i r r i n g t h e mixt,ure o f
(CH$N)$dClNU?
a n d CuCI2 u n d e r a n oxygen a t m o s p h e r e f u r a b o u t 5
t r o u r s w e o b s e r v e d a f a s t o x i d a t i o n r e a c t . i o n b u t also a r a p i d
d e c a y of t h e r e a c t i o n a f t e r a b o u t 20 m i n u t e s .
G . STRUKUL ( D i p . d i Chimica U n i v . V e n e z i a , I t a l y ) : The f o r n i a t i o r i
of alrlc?hyrtes i n y o u r s y s t e m r e q u i r e s oxygen t r a n s f e r at. the C:1
i n s t e a d o f 1:2 a s n o r m a l l y h a p p e n s . A l s o , i t i s c l e a r t h a t t h i s
u n u s u a l b e h a v i o r . d e p e n d s on t-BuOH a n d t h e Cu c o - c a t a l y s t . D o you
have any s u g g e s t i o n s f o r t h i s unusual b e h a v i o r ?
N . H , KIERS ( U n i v e r s i t y of Groriirigeri, The N e t h e r l a n d s ) : What w e
p r o p o s e a s t h e c a t a l y s t is a d i n u c l e a r c o m p l e x , c o n t a i n i n g b o t h
Pd a n d Cu i n which t-BuOH i s a c t i n g a s a b r i d g i n g l i g a r t d . Steric
e f f e c 1 . s are p r o b a b l y t h e main r e a s o n s f o r t h e c o o r d i u a t i o n o f t h e
a l k e r i e i n s u c h a way t h a t oxygen t r a n s f e r t o C1 becomes more
€acile.
184
X . J . CHALK ( G i v a u d a n C o r p . , U . S . A . ) : Is i t p o s s i b l e t h a t t . e r m i r i a 1
e p o x i c l e s are i n t e r m e d i a t e s i n t h i s react i o n a n d t h a t t h e y
r . e a r r a n g e t o t h e m i x t u r e o f k e t o n e a n d a l d e h y d e ? H a v e you t r i e d
a d d i i i g t e r m i n a l e p o x i d e s t.o t h e r e a c t . i o n m i x t u r e t o see i f t h e y
i s u m e t - i s e t o test. t h i s p o s s i h i l i t y ?
N . H . KIEKS ( U n i v e r s i t y o f G r o n i n g e n , The N e t h e r l a n d s ) : E p o x i d e s
clo react under- t h e r e a c t i o n c o n d i t i o n s u s e d , b u t t h e y g i v e a
m i x t u r e of t - b l l t y l e t h e r s w h i c h are not. o h s e r v e d i n the o x i d a t i o n
r e a c . t i o n o f 1-a1 kenes. W
e d i d n u t o b s e r v e t h e correspond i n g
a l d e h y d e s or k e t i m e s i n t h e react i o n w i t t i t e r m i n a l e p o x i d e s .
JAMES ( l k i i t . o f C h e m . U n i v . of B r i t i s h C o l u m b i a , C a n a d a ) :
Y o i r s t a t e t h a t " t h e r x i l c ! of C u C l ~i n t h e Wac.ker o x i d a t i o n i s w e l l
e s t a b l i s h e d " . 1 a m n o t ccrnvinc.ed t h a t t h i s is so. I t i e l i e v e t . h a t
mi.ued, m e t a l s p e c - i e s ( P ~ ~ C U ?b) u, t i 1 1 - c h a r a c t e r i s e d , h a v e b e e n
i s o l a t e d f r o m Wac.ker s y s t e m s . In y o u r s y s t e m y o u i m p l y t h a t CuCIZ
i s a I i g a r t d , a n d so t h i s p a r t . o f t h e c a t a l y t i c c y c l e may n o t be
so d i f f e r e n t t.o t h a t . i n the Wacker c y c l e . I t w o u l d b e of v a l u e t o
cortLirtue t h e a t t e m p t t o i s o l a t e s u c h b i m e t a l l i c s p e c i e s .
H.K.
N . H . KIERS ( U n i v e r s i t y of G r c r n i n g e n , The N e t h e r l a n d s ) : T h e Wacker
r:rxiciat i o n p r - c . i r : e s s d o e s n o t work w i t h o u t a c o - c a t a l y s t l i k e CuCl2
t o n s i d i z e P d ( 0 ) t o P d ( 1 1 ) . H o w e v e r , CuC12 i s n o t e s s e n t i a l f o r
t he r e o x i d a t i o n of Pd-NO t . o Pd-NO2, b u t h a s a n e n o r m e o u s i n f 1u e n c e
CIKI the s e l e c t i v i t y o f t h e o x i c l a t i c r n r e a c t i o n . T h i s i m p l i e s a
d i f f e r e n t . . I-UIF! f o r CuC12 i i t our s y s t e m . W
e will continue our
e f f o r t s t.o i s o l a t e a ( t ) i m e t . a I I i c ) c o m p l e x whic.h i s a c . t i v e i n t h e
o x i d a t i o n of l - a l I , e n c s t o a l d r h y d e s .
C.Centi and F. Wi'
(Editors), New Developments in Selective Oxidutwn
0 1990 Elsevier Science Publiehem B.V., Amsterdam-Printed in The Netherlands
185
SELECTIVE CYCLOHEXANE OXIDATION CATALYZED BY THE GIP SYSTEM
ULF SCHUCHARDT and VALDIR MAN0
Instituto de Qufmica, Universidade Estadual de Campinas, Caixa Postal 6154,
13081 Campinas, SP (Brasil)
-
SUMMARY
The turnover number and selectivity of the cyclohexane oxidation by the Gif
system were studied as functions of the quantities of cyclohexane and catalyst
and of the reaction temperature and time. It was found that the cyclohexanoneproducing catalytic species is only formed after the reaction has started and
that the turnover number decreases after the first 60 min. Under an atmosphere
of pure oxygen turnover numbers as high as 100 h-l were observed, but the Gif
system loses its selectivity and reacts with cyclohexanone to produce further
compounds.
INTRODUCTION
In a series of publications, Barton et al. (refs. 1-6) describedanew system
for the selective oxidation of saturated hydrocarbons at ambient conditions.
This Gif system (ref. 4 ) consists of an iron catalyst in the presence of a
reducing agent (normally metallic zinc) and pyridine with a carboxylic acid
(normally acetic acid) as solvent and proton source, plus molecular oxygen. The
main features of this system are its high selectivity for the oxidation of
secondary carbon atoms, forming ketones as the major products, and the high
yields obtained compared with those of analogous systems (ref. 7). The Gif
system was successfully used for the selective oxidation of steroids (ref. 8)
and other natural products (ref. 9 ) . but most of the zinc is losc in an useless
side reaction with acetic acid to form zinc acetate (ref. 6 ) . This prompted
Barton to develop in collaboration with Balavoine et al. the Gif-Orsay system
(refs. 10,ll) in which the oxygen is reduced electrochemically. This system
oxidizes saturated hydrocarbons with the same specificity as the original Gif
system, but with a much better efficiency, giving coulombic yields of up to 59%
(ref. 12).
Barton et al. studied mostly adamantane in these oxidation reictions,
obtaining adamantanone as the principal product with a selectivicy of up to 88%
(ref. 12). The turnover number (nrmolof oxidized products per mmol of catalyst
per hour) is normally small, but for very low catalyst concentracion it is
reported to be higher than 100 h-' (ref. 5). The selectivity for the products
depends very much on the flow rate of air and the stirring velocity (ref. 5).
In the oxidation of cyclohexane. Barton found a selectivity for cyclo-
186
hexanone (one/ol) as high as 22.1 with a turnover number of 30.3 h-l (ref. 5).
We have reexaminated the oxidation of cyclohexane with the Gif system in order
to determine which quantities of catalyst and cyclohexane permit the highest
turnover number. We have, furthermore, studied the influence of the reaction
temperature and time on the selectivity and turnover number. Kinetic studies
have been performed on the oxidation under an atmosphere of pure oxygen,
EXPERIMENTAL
All reagents and solvents used were analytical grade. Cyclohexane was purified by washing with conc. sulfuric acid, water, 5% sodium hydroxide solution
and water and then distilled. The iron catalysts Fe11Fe2T110(CH3C0,)6(C5H5N)3
(ref. 13) and Fe(bipy) C1 (ref. 1 4 ) were prepared according to the literature.
3 2
The oxidation reactions in open air were performed in a 125 ml erlenmeyer,
using 28 ml of pyridine, 2.3 ml of acetic acid, 1.8 ml of water, 1.31 g (20
mmol) of finely powdered zinc, normally 1.1 ml (10 m o l ) of cyclohexane and 8
pmol of the catalyst. The reaction temperature was adjusted with a thermostated
water bath at the value indicated. The reaction mixture was magnetically stirred
for 4 h at 1000 rpm, which proved to be appropriate for having all the zinc in
suspension. In the kinetic measurements, 1 ml of solution was taken out of the
reaction mixture every 30 min. In the other experiments, the reaction mixture
was filtered after 4 h. The liquid products were analyzed with a CG 37 gas
chromatograph equipped with a 4 m packed column of 5% Carbowax 20 M on
Chromosorb WHP coupled to a flame ionization detector and temperature programmed
at 8OC min-'
from 80 to 17OoC. Cyclooctane was added as an internal standard and
the observed retention times were: cyclohexane (1.5 min), cyclooctane (3.5 min),
cyclohexanone (7.8 min) and cyclohexanol (9.3 min).
The reactions carried out under an atmosphere of pure oxygen (99.5%) were
performed in a 125 ml round bottom Schlenk flask. After introduction of the same
amounts of solvents and cyclohexane as used in the previous experiments, pure
oxygen was passed through the flask for approximately 5 min. The catalyst and
1.31 g (20 mmol) of zinc were thenadded and the flask was sealed with a septum
already connected to silicon tubing whose other end was inserted into a 500 ml
graduated cylinder filled with oxygen and immersed up-side-down in a water
reservoir. The reaction mixture was magnetically stirred at 1000 rpm. The
oxygen consumption was measured every 3 min after equalizing the water level
inside the cylinder with the level of the water reservoir. Every 30 min, 1 ml of
the solution was taken out of the reaction mixture with a syringe, inserted into
the flask through the septum. The reaction products were analyzed as described
before.
RESULTS AND DISCUSSION
Reactions i n open a i r
The c a t a l y s t Fe(bipy)3C12 shows e x a c t l y t h e same r e a c t i v i t y as
Fe11Fe21110(CH3C02)
6(C5H5N)
i n t h e o x i d a t i o n of cyclohexane. This confirms t h e
r e s u l t of Barton e t a l . . who b e l i e v e that F e ( b i p y ) F is t h e a c t i v e s p e c i e s i n
t h e oxidation r e a c t i o n ( r e f . 6). As t h e f i r s t complex is much e a s i e r t o prepare,
i t was used i n a l l experiments described i n t h i s paper.
The values obtained f o r t h e turnover number and t h e s e l e c t i v i t y f o r cyclohexanone (one/ol) depend s t r o n g l y on t h e s t i r r i n g v e l o c i t y . If i t is too low,
some of t h e z i n c adheres t o t h e w a l l of t h e r e a c t i o n f l a s k and t h e turnover
number lowers considerably. I f i t is t o o high, t h e s e l e c t i v i t y is s t r o n g l y
reduced. These e f f e c t s were a l r e a d y observed by Barton e t a l . ( r e f . 6 ) . In our
experiments t h e s t i r r i n g v e l o c i t y of 1000 rpm w a s a good compromise and was
c a r e f u l l y maintained i n a l l experiments i n o r d e r t o make t h e r e s u l t s comparable.
The i n f l u e n c e of t h e q u a n t i t y of cyclohexane i n t h e r e a c t i o n mixture on t h e
turnover number and t h e s e l e c t i v i t y is shown i n Fig. 1. Both values i n c r e a s e up
t o a q u a n t i t y of 10 mnol of cyclohexane and then s t a y approximately c o n s t a n t ,
showing that t h e cyclohexane c o n c e n t r a t i o n is s u f f i c i e n t l y high and does not
c o n t r o l t h e k i n e t i c s of t h e r e a c t i o n anymore. Under t h e s e c o n d i t i o n s , 0.969 m o l
of cyclohexanoneand0.075mol of c y c l o h e x a n o l a r e o b t a i n e d a f t e r 4 h o f r e a c t i o n .
The q u a n t i t y of t h e c a t a l y s t e x h i b i t s a s t r o n g i n f l u e n c e on t h e turnover
number and s e l e c t i v i t y (Pig. 2). As observed by Barton e t a l . i n the oxidation
w
rn
E
a
w
>
0
z
a
3
t-
2
6
14
10
CYCLOHEXANE
[mmol]
18
Fig. 1. Turnover number and s e l e c t i v i t y as a f u n c t i o n of t h e q u a n t i t y of cyclohexane in t h e r e a c t i o n mixture (8 pmol of Fe(bipy)3C1z9 20°C, 4 h).
188
5ol
1
$ 1
-7
c
Y
W
m
r
n
-
c
- 20 20
t
40
0
Y
\
I
/i
- 15
30
>
a
3
-10
l
>
I>
F
W
-I
W
-5
4
7
10
13
16
v)
20
C A T A L Y S T [pmol]
Fig. 2. Turnover number and selectivity as a function of the quantity of
catalyst in the reaction mixture (10 mmol of cyclohexane, 20°C, 4 h).
of adamantane (ref. 5 ) , the turnover number increases strongly with the reduction of the catalyst quantity. With 4 p o l of catalyst, 0.831 mmol of oxidized
products are formed after 4 h, which only increases to 0.966 mmol if five times
the quantity of catalyst (20 pmol) is used. On the other hand, the selectivity
is much better for the higher catalyst concentrations, reaching a value of 22
for 20 pmol of catalyst.
-I
The best turnover number of 3 5 . 6 h
i s obtained at 20°C.
At higher tempera-
tures, the turnover number decreases but the selectivity increases slightly
(Fig. 3 ) . This was also observed by Barton et al. for the oxidation of adamantane (ref. 6 ) . The reduction of the turnover number at 10°C, we believe, cannot
be attributed to a loss of activity of the catalyst, but to the slower reaction
of the zinc with molecular oxygen, which now controls the reaction rate.
As can be seen in Fig. 4 , the turnover number is not constant during the
reaction course. In the begining of the reaction it increases to a value close
to 50 h-' after approximately 60 min andthendecreases steadily. On the other
hand, more than 90% of cyclohexanol i s formed in the first 30 min. After this
the catalyst becomes very selective, producing nearly exclusively cyclohexanone,
which makes the overall selectivity increase from 1.4 to 10.6. This shows
clearly, that the mechanism of the oxidation in the first minutes is different.
The cyclohexanone-producing catalytic species is formed only after the reaction
has started. The decrease in the turnover number at longer reaction times
-k
Y
-
40
CK
w
*2
30
3
z
20
rI
W
>
0
z
5
10
-T
I-
I
I
I
I
I
I
I
10
20
30
40
50
60
70
REACTION T E M P E R A T U R E
I
80
["C 1
Fig. 3. Turnover number and selectivity as a function of the reaction
temperature (7.4 Umol of Fe(bipy)jC12, 10 mmol of cyclohexane, 4 h).
2 4
>
0 3
30
60
90
120
REACTION
150
180
TIME
[min]
210
240
Fig. 4 . Turnover number and selectivity as a function of the reaction time
(7.9 pmol of Fe(bipy)jClz, 10 mmol of cyclohexane, 20°C).
cannot beattributed to the reduction of the cyclohexane concentration, as
approximately 6 mmol of cyclohexane and 7 mmol of zinc can be recovered after
the reaction. Presently we believe that the catalytically active species
decomposes slowly under the reaction conditions.
Under the same reaction conditions cobalt bis(dimethylglyoximate), manganese
190
diacetate and manganese diacetylacetonate show very poor activity for cycio-1
hexane oxidation (turnover number ( 1 h
and produce only cyclohexanol. Cobalt
-1
tetraphenylporphyrin gives a better turnover number (4.2 h ) but the selectivity for cyclohexanone (0.5) is poor. Zinc can be replaced by ascorbic acid
-1
(turnover number 7 h , selectivity 2.11, powdered iron (turnover number 5.2
-1
h-', selectivity 13.8) or powdered copper (turnover number 9.6 h , selectivity
8.5), but the quantity of oxidized products formed is smaller. Substitution of
the acetic acid by oxalic. malonic or adipic acid reduces both the catalytic
activity and the selectivity of the catalyst. Substitution of half of the
pyridine by acetone reduces the turnover number (24.6 h-l) and the selectivity
(7.8).
In pure acetone nearly no oxidation of cyclohexane is observed (turnover
number 0.8 h-').
We have also varied the pH of the reaction mixture by substituting the 1.8 ml
of water by the same amount of 1 M solutions of perchloric acid, sulfuric acid
or potassium hidroxide. A reduction of the turnover number from 35.6 h-l to
11.2, 20.7 and 22.5 h-l, respectively, is observed and only for sulfuric acid
does the selectivity increase slightly from 12.9 to 14.2, while it is reduced
to values between 5 and 6 for the other systems. Addition of 1 mmol of the
electron transfer reagents hydroquinone. antraquinone or phenantroquinone does
not improve the oxidation reaction of cyclohexane. Under the conditions used,
the turnover number drops to values between 13 and 16 h-l and the selectivity
to values between 5 and 7.
Reactions under an atmosphere of pure oxygen
The reactions performed under an atmosphere of pure oxygen show a very
steady consumption of oxygen, which stops after all the zinc is used up. The
total amount of oxygen consumed (15.8 mmol) is not much smaller than the
quantityofzinc used in the reactions (20 mmol). The rate of oxygen consumption
cannot be used for a kinetic analysis of the cyclohexane oxidation. Therefore,
we have determined the turnover number and selectivity after every 30 min of
reaction. The results are shown in Fig. 5. In the begining of the reaction the
turnover number increases slightly, reaching a maximum of more than 100 h
-1
at
60 min. Then it decreases linearly until 210 min, when all the zinc has reacted.
After this, the decrease is even more accentuated. The selectivity increases up
to 120 min, reaching a moderate value of 8.6 and then decreases. The total
quantity of cyclohexanone and cyclohexanol produced is 0.79 mmol after 60 min
and 1.35 mmol after 120 min, when more than 5 mmol of cyclohexane is still
present in the reaction mixture. Although the consumption of cyclohexane
continues, the total quantity of the products only increases slightly to 1.42
m o l after 180 min and 1.44 mmol after 210 min. After this, when all zinc has
already reacted, the total quantity of products drops to 1.02 mmol after 240
-
c
100
r
U
K
80
E
3
z
a 60
W
>
0
40
3
+
,
1
I
I
30
60
I
90
I
120
I
150
REACTION TIME
I
180
I
210
I
240
[min]
Fig. 5. Turnover number and selectivity as a function of the reaction time under
an atmosphere of a pure oxygen (7.85 ~ m o lof Pe(bipy)jClz, 10 m ~ ofl cyclohexane, 2OoC)
.
min, when approximately 3 mmol of cyclohexane is still left in the reaction
mixture. These results show clearly that the oxidation products suffer further
reactions under an atawsphere of pure oxygen.
Similar to the results in open air, the selectivity increases sharply during
the first 120 mfn. Approximately half of the cyclohexanol is formed during the
initial 30 min, then the catalyst becomes more selective for the cyclohexanone
production. From 120 min on, the quantity of cyclohexanol formed continues to
increase slightly, while the quantity of cyclohexanone stays approximately
constant. This, we believe, is due to the fact that cyclohexanone is further
oxidized at the same rate as it is produced. In agreement, the selectivity
decreases during this part of the reaction. After 210 min, when the cyclohexanone is only consumed, the turnover number decreases sharply and the
selectivity drops to 5.8.
When 1 m o l of antraquinone is added to the reaction mixture, the initial
turnover number is only 69.3 h-' and reduces during the reaction to 22.0 h-'.
The selectivity is poor with values between 3.6 and 5.5.
This s h o w that
antraquinone is also uneffective as an electron transfer reagent in a pure
oxygen atmosphere under the reaction condition8 employed.
CONCLUSIONS
The results show that the reaction system used by Barton et al. is the best
192
and that any substitutions result in lower turnover numbers and/or selectivities in the cyclohexane oxidation reaction. On the other hand, the
catalytically active species i s not known. The selectivity change in the
beginning of the reaction and the reduction of the turnover number after 60 min
of reaction time show that this species suffers modifications during the
reaction course, which are at present not explained. Under an atmosphere of
pure oxygen the Gif system loses its selectivity, resulting in further
reactions of the oxidation products. More research is required to identify the
catalytically active species, to explain the loss of selectivity and identify
the products formed under an atmosphere of pure oxygen.
ACKNOWLEDGEMENTS
This work was financed by Nitrocarbono S.A.. Fellowships from the Conselho
Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) are acknowledged.
The authors thank Prof. Keith U. Ingold, National Research Council of Canada,
for- helpful discussions and Sir Derek H.R. Barton, Texas ALM University, for
his interest in our work.
REFERENCES
1 D.H.R. Barton, M.J. Gastiger and W.B. Motherwell, J. Chem. SOC. Chem.
Commun., (1983) 41-43 and 731-733.
2 D.H.R. Barton, R.S. Hay-Motherwell and W.B. Motherwell, Tetrahedron Lett.,
24 (1983) 1979-1982.
3 D.H.R. Barton, J. Boivin, N. Ozbalik and K.M. Schwartzentruber, Tetrahedron
Lett., 25 (1984) 4129-4222.
4 D.H.R. Barton, J . Boivin, N. Ozbalik, K.M. Schwartzentruberand K. Jankowski,
Tetrahedron Lett., 26 (1985) 447-450.
5 D.H.R. Barton, J. Boivin, M. Gastiger, J . Morzycki, R.S. Hay-Motherwell, W.B.
Motherwell, N. Ozbalik and K.M. Schwartzentruber, J. Chem. SOC. Perkin Trans.
6
I, (1986) 947-955.
D.H.R. Barton, J. Boivin, W.B. Motherwell, N. Ozbalik and K.M. Schwartzentruber, Nouv. J. Chim., 10 (1986) 387-398.
7 R.A. Sheldon and J.K. Kochi, Metal-Catalyzed Oxidations of Organic Compounds,
Academic Press, New York, 1981, Ch. 8, p. 215.
8 D.H.R. Barton, J. Boivin and C.H. Hill, J. Chem. SOC. Perkin Trans. I, (1986)
1797-1804.
D.H.R. Barton, J.-C. Beloeil, A. Billion, J. Boivin, J.-Y. Lallemand and S.
Mergui, Helv. Chim. Acta. 70 (1987) 273-280.
10 G. Balavoine, D.H.R. Barton, J. Boivin, A. Gref, N. Ozbalik and H. Riviere,
Tetrahedron Lett., 27 (1986) 2849-2852.
11 G. Balavoine, D.H.R. Barton, J. Boivin, A. Gref, N. Ozbalik and H. Riviere,
J. Chem. SOC. Chem. Commun., (1986) 1727-1729.
12 G. Balavoine, D.H.R. Barton, J . Boivin. A. Gref, P. Le Coupanec, N. Ozbalik,
J.A.X. Pestana and H. Riviere, Tetrahedron, 44 (1988) 1091-1106.
13 C.T. Dziobkowski, J.T. Wrobleski and D.B. Brown, Inorg. Chem., 20 (1981)
9
679-684.
14 J.E. Fergusson and G.M. Harris, J. Chem. SOC. (A), (1966) 1293-1296.
193
B.R. James (Dept. o f Chemistry, Univ. o f B r i t i s h Coulumbia, Vancouver, Canada):
You observe a maximum t u r n o v e r a t about 20°C and a t t r i b u t e t h i s el s l o w r e a c t i o n
o f Zn w i t h 0 a t t h e lower t e m p e r a t u r e (10°C). You a l s o d e s c r i b e t h e t e m p e r a t u r e
r e a c t i o n p r o f i l e as "enzyme-like". C o n s i d e r i n g t h a t y o u r r e a c t i o r i s a r e d i f f u s i o n
c o n t r o l l e d , I t h i n k t h e analogy t o enzymas tenuous a t b e s t . Have t h e e f f e c t s o f
s t i r r i n g r a t e been measured a t a l l temperatures, and do you have d i r e c t evidence
f o r a s l o w e r Zn + 0 r e a c t i o n a t 10°C ? It i s n o t o b v i o u s how t h i s c o u l d g i v e
2
r i s e t o a maximum v a l u e w i t h i n t h e g i v e n t e m p e r a t u r e range (what i s t h e Zn +
O2 r e a c t i o n ? ) .
U. Shuchardt ( S t a t e Univ. o f Campinas, B r a z i l ) : Z i n c r e a c t s w i t h oxygen t o f o r m
t h e s u p e r o x i d e anion, which i s t h e r e a l o x i d a n t . The e f f e c t s o f s t i r r i n g r a t e
have o n l y been measured a t 2O"C, whose t h e b e s t t u r n o v e r numbers a r e o b t a i n e d .
We have no d i r e c t evidence f o r a slower Zn + O2 r e a c t i o n a t 10°C. Turnover
numbers a r e s l i g h t l y b e t t e r w i t h a h i g h e r s t i r r i n g r a t e a t t h i s temperature,
w h i l e t h e y decrease w i t h i n c r e a s e o f t h e s t i r r i n g r a t e a t h i g h e r temperatures.
On t h e o t h e r hand, t h e s t i r r i n g f o u n d b e s t i s q u i t e v i g o r o u s and t h e r e d u c t i o n
o f t h e t u r n o v e r numbers a t h i g h e r t e m p e r a t u r e s cannot be e x p l a i n e d by d i f f u s i o n
phenomena.
H. Mimoun, IFP, Rue1 Malmaison, France): What i s t h e e l e c t r o n i c y i e l d o f t h e
r e a c t i o n ? i . e . how much Zn i s used f o r e v e r y mole o f oxygenated p r o d u c t
produced ?
U. Shuchardt:Considering t h a t more o r l e s s 10 mnol o f z i n c a r e used up i n t h e
r e a c t i o n s , t h a t two atoms o f z i n c a r e needed t o produce one m o l e c u l e o f c y c l o hexanone and t h a t s l i g h t l y more t h a n 1 mmol o f o x i d i z e d p r o d u c t s i s formed, t h e
e l e c t r o n i c y i e l d i s o n l y around 2036. The o t h e r z i n c r e a c t s w i t h t h e a c e t i c
a c i d t o form zinc acetate.
M. Baerns (Ruhr U n i v . Bochum, BRD): You have mentioned t h a t s e l e c t i v i t y depends
on s t i r r i n g speed w i t h o u t g i v i n g any e x p l a n a t i o n f o r t h i s o b s e r v a t i o n . Could
you i m a g i n e t h a t d i f f u s i o n a l e f f e c t s i n t h e l i q u i d p l a y a r o l e ; t h i s would be
t h e case i f any s e g r e g a t i o n e f f e c t s p r e v a i l ?
U. Shuchardt: A t l o w s t i r r i n g v e l o c i t i e s t h e s e l e c t i v i t y i s good b u t t h e t u r n over numbers a r e s m a l l . A t v e r y h i g h s t i r r i n g v e l o c i t i e s t h e s e l e c t i v i t y i s
s t r o n g l y reduced, as t h e amount o f oxygen d i s s o l v e d i n t h e r e a c t i o n m i x t u r e i s
l a r g e , f a v o r i n g r a d i c a l r e a c t i o n s which produce c y c l o h e x a n o l . As t h e s t i r r i n g
used i s v e r y e f f e c t i v e , I do n o t b e l i e v e t h a t d i f f u s i o n a l e f f e c t s i n t h e l i q u i d
p a l y a r o l e n o r t h a t any s e g r e g a t i o n e f f e c t s a r e encountered.
J . K i w i (EPFL, IPC 11, Lousanne, S w i t z e r l a n d ) : ( a ) Could you a t t r i b u t e t h e
decrease i n t u r n o v e r and s e l e c t i v i t y observed i n y o u r r e a c t i o n t o t h e disappear e n c e o f some species produced b y F e ( b p y I 3 C l 2 added a t t h e b e g i n n i n g o f t h e
reaction ?
( b ) Could you f i n d o t h e r m e t a l ( b p y 1
compound , e.g. Co, Ru-complexes t o
a v o i d many problems you have encountered when you use Fe(BPY) system2+?
3
U . Shuchardt: ( a ) As evidenced by UV-visable spectroscopy t h e Fe(BPYI3 c a t i o n
d i s s o c i a t e s i m m e d i a t e l y on d i s s o l u t i o n i n t h e r e a c t i o n m i x t u r e . The s p e c i e s
produced by t h e d i s s o c i a t i o n s u f f e r s f u r t h e r m o d i f i c a t i o n s d u r i n g t h e r e a c t i o n
course, p r o b a b l y by o x i d a t i o n , which reduce t h e t u r n o v e r number and under an
atmosphere o f p u r e oxygen a l s o t h e s e l e c t i v i t y . The t r u e n a t u r e o f t h e c a t a l y t i c a l l y a c t i v e species i s n o t y e t known.
194
( b ) Co(bpy):+
and Mn(bpyIn+ cations show only very poor c a t a l y t i c a c t i v i t y
3
under t h e conditions of t h e G i f system. Rutbpy)
complexes have not been
t r i e d y e t , but we hope t h a t t h e i r use w i l l avoia some o f t h e problems encount e r e d w i t h t h e Fe(bpy)3 system.
G. Centi and F.Trifm'(Editors),New Deuelupments in Selective Oxidatwn
Q 1990 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands
195
METALLOPORPHYRIN-CATALYZED OXIDATION OF CYCLOHEXANE WITH DIOXYGEN
R. IWANEJKO, T. HLODNICKA and J . POLTOWICZ
Institute of Catalysis and Surface Chemistry,
Sciences, 30-239 Krakow,Poland
Polish
Academy
of
SUMMARY
Oxidation of cyclohexane with dioxygen in the presence of
propionaldehyde and some metalloporphyrins as catalysts have been
investigated. The main products of the hydrocarbon oxidation are
of
cyclohexanol
and
cyclohexanone.
Also,
the
products
propionaldehyde oxidation such as peroxypropionic acid and carbon
dioxide are present. The yields and the product distribution
appeared to be dependent on the character of the metal centre.
These differences in the catalytic behaviour shed light on the
character of the active forms of the catalyst and on Ithe mechanism
of the investigated reaction.
INTRODUCTION
Oxygen containing complexes of
recently become
the
subject
investigations.
This
of
interest
important
functions.
great
was
biological oxidation processes in
effects
some
metalloporphyrins
interest
raised
which
This
by
iron
and
have
intensive
efficiency
of
protoporphyrin
species
consl'itutes
IX
the
essential part of cytochrome P-450 which mediates a bi-oad range of
biological oxygenations. The modelling of this system has
the
development
in
the
of
field
hydrocarbons i.e. epoxidation of
alkanes.
Chemical
metalloporphyrin,
Cref.11,
models
dioxygen
borohydride
metalloporphyrin
of
and
single
reducing
atom
oxidation
hydroxylation
P-450
consist
agent
such
2, 3>, ascorbate
oxygen
donor
Cref.
7>,
hydrogenpersulphate
(ref.
9>
of
as
<ref.
acid
iodosylbenzene Crefs. 5,6>, peroxy
(ref. 81, potassium
phase
and
cytochrome
and
Crefs.
liquid
olefins
led
to
of
of
a
HZ/Pt
4>
or
such
as
hypochlorite
and
hydrogen
peroxide (refs. 10, 11).
Our investigations focused first on
epoxidation
of
olefins
196
with dioxygen in the presence of aldehyde as
some
metalloporphyrins
as
reducing
<ref. 12)
catalysts
agent
and
then
on
and
oxidation of cyclohexane under similar conditions.
EXPERIMENTAL
Tetra-p-tolylporphyrins CTTP> with metal cntres such as Cr3+,
Mn3+, Fe3+, Go2+, Ni2+, Cu2+ and Zn2+ have been prepared according
to the procedure described in (ref. 13)
chromatography
on
alumina.
and
purified
by
column
Chloro-tetrakis<2,6-dichlorophenyl>
porphyrinatomanganese<III> Mn<TDCPP>Cl has been prepared according
to the procedure described in <ref. 14>. Propionaldehyde
produced
by Fluka was redistilled before each series of measurements
argon. Benzene used as solvent produced
by
purity grade. Cyclohexane of purity grade
POCH-Gliwice
was
also
under
was
of
produced
by
POCH-Gliwice.
Reactions were carried out in a
thermostated
glass
reactor
equipped with magnetic stirrer <ref. 15> at 30°C.
In
a
standard
experiment
the
reactor
was
filled
with
molecular oxygen under noriaal pressure and catalyst was introduced
into it. Then 10 ml of cyclohexane and propionaldehyde solution in
benzene was added. The amounts of
correspond to this volume.
The
products
reagent
stirred and the reaction was
carried
progress of the reaction was
followed
quoted
mixture
out
by
for
in
was
the
text
vigorously
90 minutes.
measuring
the
The
oxygen
uptake.
The yields of cyclohexanol and cyclohexanone were
determined
using GC Chrom 5 with columns filled with Tenax GC. The
carbon dioxide was determined using TCD with columns
yield
filled
of
with
Porapak QS. The amount of peroxy acid was determined by iodometric
titration.
RESULTS
Cr<III>, Mn<III>, Fe<III>, Co<II>, Ni<II>,
Cu<II>-tetra-p-tolylporphyrins <TTP> have been used
Zn<II>
as
in the reaction of oxidation of cyclohexane with dioxygen
presence
of
propionaldehyde
as
reducing
agent
in
and
catalysts
in
the
benzene
197
solutions.Some kinetic experiments have also been car]-ied out with
chloro
-
tetrakis <2,6-dichlorophenyl> porphyrinatomangaiiese (III>
<Mn<TDCPP>Cl>. The
main
reaction
products
of
the
hydrocarbon
oxidation were cyclohexanol and cyclohexanone. Also, Lhe
derived from aldehyde
oxidation
such
carbon dioxide and propionic acid
as
were
products
peroxypropionic
found.
No
acid,
oxidation
of
cyclohexane takes place when one of the reagents is missing.
The induction time , oxygen consumption as well as the yields
of products and their distribution appeared to be deptmdent on the
character of the
metal centre
of
the
purphyrin
molecule.
The
experimental data are listed in Table I.
Fig. 1 shows the
oxygen
uptake
curves
for
carried out in the presence of the investigated
the
reactions
metalloporphyrins
while Fig. 2 illustrates the changes in the product concentrations
during the reaction course. The latter reaction was carried out in
the
presence
of
MnCTDCPP>Cl
which
in
comparison
with
othei.
metalloporphyrins is more stable in oxidizing media. As it follows
from Fig. 2 the amounts of peroxy acid as well as cyclohexanol and
cyclohexanone grow parallelly. When the conversion of the aldehyde
reaches 100% the concentration of peroxy acid
starts to
diminish
while the concentrations of cyclohexanol and cyclohexanone
remain
practically at the same level. C02 evolution is still observed but
at much lower rate.
TABLE 1
Oxidation of cyclohexane.
catalyst
induction
time
Cmin. >
CrCTTP>J
52
MnCTTP>Cl 10
Fe<TTP>Cl 0
10
Co<TTP>
33
Ni<TTP>
oxygen
yield of products Cmmoles> turnover
consump- alcohol ketone peroxy
frequency
acid C02 t:cycles/min>
t ion
1
2
Cmmoles>
8.4
0.01
0.03
0.03
0.075
6.9
0.05
6.6
5.8
7.8
0.02
4.0
0.05
0.1
5.7
0.6
6.5
0.06
0.07
0.03
2.6
0.7
8.7
2.2
2.4 68
16
4.0
0.3
8
0.02
0.3
0.7
0.5
0.1
~~~
~cyclohexanel=l
.8xlO-'M,
[propionaldehydel=l M, [cata~lystl=2x10-3M
of products, 2 - to the
amount of cyclohexanol and cyclohexanone, t=30 C.
I- corresponds to the total amounts
198
5
45
25
85
65
105
t i m e Cmin.1
Fig.1 Oxygen absorption during the course of the reactions.
-IE
.-I
E
E
d
70
50
30
10
20 40 60
20 40 60 timeImin.3
Fig.2 The distribution of products during the course of the
reaction catalyzed by MnCTDCPP>Cl. l-cyclohexanol,2-cyclohexanone,
3-total~i+2>,L-conversionof aldehyde,S-peracid.
DISCUSSION
It was
reduction
shown
step
in
plays
-
our
an
earlier
paper
essential
role
Cref.
in
121
the
that
the
investigated
process. The metalloporphyrins are transferred to lower
oxidation
states and simultaneously acyl radicals are generated:
M”TTP+ + RCHO
M”-~TTP+ RCO-+ H+
Acyl radicals are then free to
react
biradical character and this process
uptake :
Cl>
with
gives
dioxygen
rise
to
which
the
has
oxygen
199
RCO' + O2
'5RCOOOH
+ RCO'
<2>
Hence the induction time may be related to the ease
of the metalloporphyrins
used
as
catalysts.
of
Thus
reduction
Fe<III>
Mn<III>-porphyrins which have low reduction potentials give
induction times while for NiCII>
more difficult to
reduce
the
and
short
and Cr<III>-porphyrins which are
induction
time
is
imuch
Fe<II>TTP
Reduced forms of some metalloporphyrins as for instance
of
and Pln<II>TTP are reoxidized in the process
but the oxygenated adducts have
different
binding
forms,
longer.
dioxygen
character
and
ability to react with organic molecules <refs. 16, 17>.At variance
with FlnCIII) and FeCIII>-porphyrins,CoO- porphyrin is difficult
to reduce, however,this latter binds readily molecular oxygen
the odd electron is transferred from
antibonding oxygen
orbitals.The
aldehyde molecule in the
rate
the
cobalt
oxygenated
determining
centre
adduct
step
and
to
the
reacts
with
which
acyl
in
radicals are released Cref. 17):
C~<III>TTPO;
!S%
C~CIII>TTPO;HCR -.+ C~<III>TTP-O~;H
+ RCO.
(3)
li
0
It might also be
expected
that
under
reducing
dominating at the beginning of the reaction
so
oxygen activation may happen,
yet
however,
as
conditions
called
no
reductive
experimental
support has been found in favour of this assumption.
Zn<II> and Cu<II>-porphyrins are difficult to reduce and
may
undergo only ring reduction nhich means that the accepted electron
resides
on
the
porphyrin
ligand.
Such
an
anion
radical
difficult to oxidize by oxygen since the latter can oi?ly be
bound
orbital
overlap
exist and thus the reaction cannot be recycled. For these
reasons
to the metal centre when the conditions
for the
is
these porphyrins do not show any catalytic activity.
Peroxy acid is a more efficient oxidant than molecular oxygen
and on one hand may oxidize metalloporphyrins to higher
oxidation
states and on the other hand interacts with the porphyrin
species
to form intermediate complexes responsible for the insertion of an
oxygen atom to the hydrocarbon molecule.
Peroxy acid may react with the metalloporphyrin molecule in a
200
homolytic Cone electron> and heterolytic
The homolytic process is accompanied by
<two electron>
decomposition
pathway.
peroxy
of
acid resulting in formation of carbon dioxide CEqn. 4 ) Cref. 19).
M ~ T T P+ RCOOOH + M”+~TTP++ OH-+ R . + co2
M”TTP + RCOOOH + M”+~CTTP>= o + RCOOH
The heterolytic pathway
results
in
<4>
C5>
formation
high-valent
of
metal-oxo species CEqn. S>.The latter are believed to
incorporate an oxygen atotn to the hydrocarbon
be
molecule.
able to
However,
it is reported that the efficiency of the process depends
on
the
character of the oxidizing agent used to produce these active
0x0
species <ref. 9>. In such a case the structure and character of an
intermediate
complex
composed
oxidant and hydrocarbon
a metalloporphyrin
of
molecule,
molecule would play the decisive role
in
the catalytic step.
A s seen from the data in the Table and oxygen uptake diagrdni,
Fe<TTP>Cl shows the highest activity which is
highest total yield of products and
However,
the
absorbed
absence
manifested
of
the
induction
is predominantly
oxygen
by
Lime.
involved
formation of peroxy acid. The explanation of this phenomenon
in the fact
initial
that
FeCII>TTP,
porphyrin at the
resulting
first
stage
from
of
reduction
the
in
lies
the
of
reaction
reacts
rapidly with dioxygen in the following sequence of reactions <ref.
19>:
-
Fe<II>TTP +
O2 Fe<II>TTPO2 m T T P F e C I I I > - 0 - 0 - C I I I > F e T T P Fe<IV>CTTP>=O
TTPFeC I1I>-O-FeC III>TTP
Fast
reaction
with
dioxygen
five
coordinate p-0x0 dimer which
gives priority to the formation of
metalloporphyrin. The access of
molecules
C6>
is an
both
inactive
aldehyde
and
form
peroxy
to the iron centres involved
in
dimeric
difficult and the compound is resistant
to
reduction
potential = -0.9 V> as
well
as
to
of
the
acid
structure is
Creduction
oxidation.
Therefore
large
amount of peroxy acid is found at the end of the
reaction.
Under
the described conditions the active forms
O=FeCIV>TTP
i.e.
and
201
O=FeCV)TTP+
which could effect oxidation of hydrocarbcin cannot
generated in significant
concentrations
in
the
course
of
be
the
reaction.
Longer than for Fe<TTP>Cl induction
consumption and
more
significant
time
amounts
at
of
higher
oxygen
cyclohexanol
cyclohexanone are observed for CoTTP. Simultaneously,
large
of peroxy acid is decomposed to C02, These results are
and
part
consistent
with our previous investigations on epoxidation of propylene under
similar conditions which showed that
Co-porphyrin
exhibited
the
highest activity in both oxygen consumption and epoxide production
Crefs. 12, 20). It has been shown that
the
active
form
cobalt porphyrin is its Il cation radical obtained in
of
the
the
process
of homolytic oxidation of the initial porphyrin with the generated
peroxy acid which is
simultaneous
C02
with
evolution
starting
before the epoxide is detected. The induction time is necessary to
generate perosy acid and
another
peracid
Il cation
inolecule
forms
intermediate complex capable to
radical
a
which
of
precursor
incorporate
together
an
an
oxygen
with
active
atom
to
the hydrocarbon molecule :
tCoCIII>CTTP)X+:. .RCOOOH3 + RH
An
alternative
proposal
.--t
given
ROH + RCOOH
in
(ref. 21)
(7)
is
formation
of
a cobalt(V>- 0x0 species.
Still less active appeared Mn<TTP>Cl which shows
the
lowest
oxygen uptake and smallest yield of products. However, the
amount
uf cyclohexanol and cyclohexanone consists more import.ant part
the
total
yield
of
products
than
in
the
case
of
of
other
metalloporphyrins. It means that the system is more selective with
respect to these
products. Also,
the
amount
of
comparatively higher which indicates that homolytic
C02
found
is
decomposition
of peroxy acid is more important here. This is justified
by
high
number of oxidation states (11-V> accessible for Mn-pcrrphyrins. In
a heterolytic reaction with peroxy acid such catalytically
active
species as O=Mn<V>TTF+ and O=MnCIV>TTP are likely to be generated.
The former has already been recognized as responsible
for
oxygen
202
atom transfer to hydrocarbon
molecules
<refs.
6,
Recently
8).
Groves et al. have shown that TMPMnCIV>=O species are also
active
in epoxidation of olefins (ref. 22>.
NiTTP and Cr<TTP>Cl show the lowest activity and
yields predominantly
peroxy
acid
and
negligible
cyclohexanol and cyclohexanone. The former porphyrin
large amounts of peroxy acid but the amounts
latter
of
amounts
also
yields
of cyclohexanol
cyclohexanone are comparable to those found for
means that the complex is more effective in
the
and
Mn-porphyrin.
heterolytic
It
reaction
with peroxy acid than in its homolytic decomposition.According
Kochi et al. a putative 0x0-nickel<IV> and/or
to
p-0x0-nickel<III>
intermediates are engaged in oxidation of hydrocarbons (ref. 23>.
The
investigated
system
employs
two
oxidizing
molecular oxygen and peroxy acid. Both oxidants
are
agents:
involved
in
generation of oxygen containing metalloporphyrin species. However,
the structure and activity of the oxygenated forms depend
on
the
character of the metal centre and on kinetics of their formation.
ACKNOWLEDGMENT
The authors wish to express their gratitude to
ffniversit.6 Ren6 Descartes Paris VI for
Dr P.Battioni from
chemicals
and
scientific
guidance in preparation of Mn<TDCPP>Cl.
REFERENCES
I. I.Tabushi, A. Yazaki, J.Am.Chem. SOC., 103 <1981> 7371-7375.
2. I.Tabushi, N. Koga, J.Am. Chem.Soc. , 101 <1979> 6456-6458.
3. M. Perree-Fauvet, A. Gaudemer , J.Chem.Soc. Chem. Commun. ,C 1981>
874.
4. M.Fontecave. D.Mansuy, Tetrahedron, 40 <1984> 4297-5311.
5. C.L. Hill, 8.C.Schardt. J.Am.Chem.SOC.,102 (1980) 6374-6375.
6. J. T.Groves, W. J. Kruper and R.C.Haushalter, J. Am. Chem. SOC.,
102 (1980) 6375-6377.
7. J.T.Qroves,Y.Watanabe,T.J. McMurry, J.Am. Chem. SOC., 105 <1983>
4489-6490.
8. B. Meunier. Bull.Soc.Chim.Fr., (1986) 578-594.
9. A.Robert, 8.Meunier, New.J.Chem. , 12 (1988) 885-896.
10. J. P.Renaud, P.Battioni. J . F.Bartoli, D.Mansuy. J. Chem. SOC.
Chem.Commun., (1985) 888-889.
11. P.Battioni, J. P.Renaud. J. F.Bartoli, D.Mansuy, J . Chem.SOC.
Chem.Commun. , <1986> 341-343.
12. J. Haber, T.Mdodnicka, J. Pobtowicz, J.Mol.Catal. in press.
203
13. A. D. Adler, F. R. Longo, F. Kampas, J. Kim, J. Inorg.Nucl.. Chem. ,
32 (1970) 2443-2416.
14. A. W. van der Made, E. J. H. Hoppenbrouwer, R . J. M. Nolte, W. Drenth,
Rec. Trav.Chim. Pays Bas. , 107 C1988> 15-16.
15. J. Haber, A. Marchut, T.Mlodnicka, J. Poltowicz, J. J. Ziolkowski,
React. Kinet.Catal. Lett. , 8 (1977) 281-286.
16. R. D. Jones, D. A. Summerville. F . Basolo, Chem. Rev. ,70 (1979)
139-179.
17. T. Mlodnicka, J. Mol. Catal., 36 (1986) 205-242.
18. R. A. Sheldon, J. K. Kochi, Metal-Catalyzed Oxidation of Organic
Compounds, Academic Press, New York, 1981, pp. 43-45.
19. A. B. Hoffman, D. M.Collins, W. V. Day, E. 8 . Fleischer,
T. S.Srivastava, J. L. Hoard, J - Am. Chem. SOC. , 94 C197;!>
3620-3626.
20. J . Haber, T. Mlodnicka, M. Witko, J. Mol. Catal. , 52 Ci089) 85-97.
21. W. A. Lee, T.C.Bruice, Inorg.Chem., 23 (1986) 131-135.
22. J. T.groves, M. K. Stern, J. Am. Chem.Soc., 109 C1987> 3812-3815.
23. J. D. Koola, J. K. Kochi, Inorg.Chem., 26 C1987> 908-9l6.
B.R. James (Dept. o f Chemistry, Vancouver, Canada): I n p r i n c i p l e , you are using
an aldehyde as a co-reductant i n r e d u c t i v e - a c t i v a t i o n o f 0 , and i n one step
invoke r e d u c t i o n o f the m e t a l l o p o r p h i r i n by RCHO t o generage RCO'. Such acyl
r a d i c a l s may lose CD r a p i d l y and lead t o decarboxylation o f aldehydes (which
we have demonstrated w i t h Ru and Fe prphyrins, Can. J . Chem. (1988)). Do you
see r e a c t i o n o f M(TTP) w i t h aldehydes ( i n t h e absence o f 0 ) t o g i v e any decarb o x y l a t i o n products ? For example, C H from propionaldehyge ?
6
T. Mlodnicka: The r e a c t i o n was t e s t e g i n t h e absence o f 0 and no products a t
approciable concentrations were detected. However, i t wou?d be i n t e r e s t i n g t o
check i t once more paying special a t t e n t i o n t o t h i s problem.
J. K i w i ( E P F L , Lousanne, Switzerland): How s t a b l e are t h e peracids you postulat e formed i n your system ? Ifthey are s t a b l e have you measured b,y a s p e c i f i c
method o r a general t i t r a t i o n method ?
T. Mlodnicka: A t t h e given concentrations o f t h e s o l u t i o n components and t h e
temperature o f t h e r e a c t i o n mixture, peroxy a c i d i s a comparatively s t a b l e
species and i t s concentrations were determined by iodometric t i t r a t i o n .
U. Shuchard(Brazi1): Could you say anything about t h e mass balance w i t h respect
t o t h e cyclohexane ?
T. Mlodnicka: We worked a t a s i g n i f i c a n t excess o f cyclohexane. I n such a case
t h e determination and evaluation o f mass balance i s a d i f f i c u l t task w i t h a
l a r g e experimental e r r o r . To my knowledge many i n v e s t i g a t o r s have t h e same
problem.
G. Centi and F.T r i f i r o ' (Editors), New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printedin T h e Netherlands
OXOMETALATES AND DIOXYGEN
J.-M.
205
IN CATALYTIC OXIDATION
BRiGEAULTl, B. EL ALI1,
J . M E R C l E R 2 , J . MARTIN1, C . MARTIN' and
0. MOHAMMEDI'
' I E p a r t e i r i e n t de Chiriiie U n i v e r s i t C P . e t M. C u r i e , C a t a l y s e e t C h i i i i i e des
S u r f a c e s ; T o u r 44 - Ze ; 4, P l a c e J u s s i e u ; 75252 P a r i s Cedex 05 ( F r a n c e )
n
L
D e p a r t e m e n t de Chiinie U n i v e r s i t e P. e t M. C u r i e , L a b o r a t o i r e de C h i m i e
O r g a n i q u e S t r u c t u r a l e , B l t . 74
SUMMARY
The a p p l i c a t i o n o f oxovanadium ( I V ) o r ( V ) complexes I VO(acac) 1,
I V O i OCH( CH3)21, I a n d h e t e r o p o l y a c i d s
IPMo12-nVn0401 "HPA-n", as e a t a l y s t
p r e c u r s o r s i s examined f o r t h e o x i d a t i v e cleavage o f ketones. I n t h e presence
o f d i o x y g e n , a t room t e m p e r a t u r e o r a t 60"C, t h e y r e a c t w i t h b e n z y l i c k e t o n e s ,
Ar-CH2-C(0)R, t o p r o d u c e t h e c o r r e s p o n d i n g c a r b o x y l i c a c i d s , R-COOH, a n d
benzaldehyde and/or benzoic a c i d i n h i g h y i e l d . S u b s t i t u t e d cycloalkanones,
u - k e t o l , u - d i k e t o n e s and 1 - p h e n y l a l k a n o n e s a r e a l s o o x i d a t i v e l y c l e a v e d b y
HPA-2 and d i o x y g e n u n d e r v e r y m i l d c o n d i t i o n s . The e x p e r i m e n t s show t h a t t h e
e f f i c i e n c y o f HPA-2 i s r e l a t e d t o t h e key r o l e o f Va(V) and t o t h e l a r g e
s o l u b i l i t y o f t h i s " a c i d i c c o m p l e x " i n o r g a n i c media.
INTRODUCTION
As p a r t o f o u r c o n t i n u i n g i n v e s t i g a t i o n i n t o o x i d a t i o n p r o c e s s e s , we h a v e
been i n t e r e s t e d i n t h e u s e o f vanadium ( V ) p r e c u r s o r s i n homogenems c a t a l y s i s
( r e f . 1). The o x i d a t i o n o f o r g a n i c compounds by q u i n q u e v a l e n t van3diutii has,
b e s i d e s i t s own i n t e r e s t , i m p l i c a t i o n s c o n c e r n i n g t h e b e h a v i o r o f vanadium
o x i d e c a t a l y s t s ( r e f . 2 ) . The s u b j e c t was f i r s t examined b y M o r e t t e ct a l .
e s s e n t i a l l y f r o m a n a n a l y t i c a l v i e w p o i n t ( r e f . 3 ) and l a t e r b y L i t t l e r e t < X I .
( r e f . 4 ) ; molecular oxtlgen h a s no s i y n ~ f l c a n tc f f e c t o n t h e r a t e s o f o x i d a t i o n
and a l l t h e s e o x i d a t i o n p r o c e s s e s a r e s t o i c h i o m e t r i c . I n t h i s c o n t e x t , i t was
i n t e r e s t i n g t o t e s t M a t v e e v ' s systems as o x i d i z i n g a g e n t s : p a l l a d i u i i i ( 1 1 )
complexes a n d s a l t s o f h e t e r o p o l y a c i d s a b b r e v i a t e d t o "Pd( I I ) / H P A - n " ( r e f . 5 ) ,
HPA-n
+
Red
+ iHt
+
H1 I HPA-riI
+
(1)
OX
Thus, HPA-11 i n eqn. 1 i s a h e t e r o p o l y a c i d w i t h t h e K e g g i n s t r u c t u r e :
H3+,,1PM12-nVnOq0(
; M = Mo, W; Red i s a r e d u c i n g a g e n t i n v o l v i n g . [ e l e c t r o n s :
a r e d u c e d f o r m o f a c a t a l y s t o r a s u b s t r a t e . The r e d u c e d fortii,
v1
v I V 040) I o r t h e " h e t e r o p o l y - b l u e " c<,n be
HI I H P A - n l z H1 IH3+n{ PM12-nVan-l
Vl
r r o x i d i z e d by dioxygen under v e r y mi I d conditions
:
206
Hi IHPA-n/ + (i/4)02
+
+ (i/2)H20
HPA-n
(2)
S i n c e p o l y o x o i n e t a l a t e s have no s o p h i s t i c a t e d o x i d i z a b l e a n c i l l a r y l i g a n d s ,
t h e y have c o n s i d e r a b l e p o t e n t i a l as l o n g - l i v e d c a t a l y s t s . A c c o r d i n g t o eqn.
( 1 ) and (2), HPA-II can be used as c o c a t a l y s t s , f o r example, i n a s s o c i a t i o n
w i t h p r e c i o u s m e t a l complexes ( r e f . 5-6) o r as d i r e c t c a t a l y s t s ( r e f . 7 ) .
While t e s t i n g r e c e n t l y - d e s c r i b e d systems : "Pd( 11) o r Rh( 111) complexes
w i t h o r w i t h o u t a c o c a t a l y s t and i n t h e presence o f dioxygen" ( r e f . 5-6,
8)
f o r t h e o x i d a t i o n o f s e v e r a l o l e f i n i c s u b s t r a t e s , we o b t a i n e d c l e a r e v i d e n c e
f o r secondary c a t a l y t i c processes; one of t h e s e i s t h e o x i d a t i o n of ketoiies.
These r e s u l t s on t h e vanadium(V) c a t a l y t i c o x i d a t i v e cleavage o f ketones w i t h
dioxygen a r e discussed.
RESULTS AND D I S C U S S I O N
L
O x i d a t i o n o f 1-phenylpropan-2-one,
; evidence f o r a p r e c u r s o r e f f e c t
,
I n o r d e r t o compare s e v e r a l p r e c u r s o r s we t o o k l-phenylpropan-2-one,
as a model s u b s t r a t e .
i s o x i d i z e d w i t h o u t an o r g a n i c s o l v e n t . R e s u l t s a r e surnitiarized i n
Ketone
Table 1.
TABLE 1
O x i d a t i o n o f 1-phenyl-2-propanone,
2
,
by h e t e r o p o l y a c i d s o r vanadium oxo-
complexes and dioxygena
~~
T
Precursors
Run
Products
Coriver-
$ionb
)
(0.20 mol 1-l
time
("C)
(h)
.
II
X
6.5
1.5
x
(% Yieldsc)
5
6.5
X
L
1
X i '
-~
1
H31PMo120401. 30 H20
20
6
-
5
2
ti4 IPMollV10401
20
6
70
37
20
62
3.5
9.5
3
H51PMo10V20401. 36 H20
20
6
99
78
10
88
10.5
0.5
4
t i 6 I P M O ~ V ~ O34~ ~H20
~.
20
6
99
77
10
8R
10.5
20
6
3
20
6
46
. 33 H20
. 29
5
ti3 IPW120401
6
H4 IPWllV1040/.
7
IVO{OCH(Me)213
8
IV0(acac)21
H20
30 H20
I
a Reaction c o n d i t i o n s
. runs
traces
-
traces traces
-
28
7
38
4.5
78
3
66
6
60
24
82
10
Tra
60
24
75
10
56
0.5
traces
7
-
_______
1-6 : s u b s t r a t e = 3 cm3 (22.4 m m o l ) ; runs 7 - 8 : s u b s t r a t e = 1.5
p (02)= l o 5 Pa; w i t h o u t o r g a n i c s o l v e n t ;
% o f s u b s t r a t e consumed;
coupled CC-MS (OV 17 and OV 105 columns);
i n t e r n a l standard anisole.
~ 1 1 1 ~ ;
P r o d u c t s analysed by
I n a l l experiments ( r u n s 1-B), o x i d a t i o n l e a d s t o benzaldehyde,
I
Jand a c e t i c
a c i d , X I , as m a j o r p r o d u c t s , and t o b e n z o i c a c i d , J, r e s u l t i n g f r o m benzaldehyde
n d
207
c o o x i d a t i o n . Two m i n o r p r o d u c t s , 1-phenylpropan-1.2
dione,
Xc
I 1 and
tr;lns-
s t i l b e n e , X I I I , a r e formed. The l a t t e r was i d e n t i f i e d i n a r e a c t i o n between
N
benzaldehyde a n d 2 ( r e f . 9 ) . Comparison o f r u n s 1 and 5 w i t h r u n s 2-4,
6-0
shows t h a t vanadium (V) i s a key-element i n t h e c a t a l y t i c s y s t e m . Variadiiiiii
( V ) oxoal k o x i d e ,
IV010CH(Me)213/ o r variadiuiii ( I V ) a c e t y l a c e t o i i a t e can have a l s o
a c a t a l y t i c e f f e c t w i t h dioxygen,
but the substrate/vanadiuin r a t i o , the
c o n v e r s i o n and t h e r e a c t i o n times i n d i c a t e t h a t t h e y a r e l e s s e f f i c i e n t t h a n
HPA-1 w i t h Mo ( r u n 2 ) .
These r e s u l t s and t h e easy s y n t h e s i s ( r e f .
.
"H5
10) of t h e a c i d
30-36 H20", o r o f t h e a c i d s a l t s " H 5 ~ x N a x ~ P M o 1 0 V ~ ~,0 4y0 ~H20"
l e d us t o choose these p r e c u r s o r s f o r t h e o x i d a t i v e cleavage of o-ther ketones.
O x i d a t i o n o f b e n z y l i c ketones
HPA-2 can be used i n t h e o x i d a t i o n o f b e n z y l i c ketones ( r e f . 11). A l l d a t a
a r e c o m p a t i b l e w i t h scheme 1 :
HPA-2. 0
X-C6H4-CH2-C(0)-R
20°C; MeCN
2X-c
H -CHO t R-COOH
major products
( y i e l d s : 70
90 b)
-
+
X-C6H4-COOH
+ X-C6H4-C(0)-C(0)R
minor products
( y i e l d s 4 10%)
X
:
H
R : Me; E t ; A r o r A r - C H 2
X
:
p-OMe
R : Me
Schenie
1
The o x i d a t i v e c l e a v a g e o f phenylacetones g i v e s h i g h y i e l d s o f a c e t i c a c i d
and o f benzaldehyde ( o r o f f u n c t i o n a l i z e d benzaldehyde). The r e a c t i o n can be
a p p l i e d t o d i f f e r e n t b e n z y l i c ketones.
Oxidation o f a-ketol
,
a - d i ketones and 2-methyl cyclohexanone; e v i d e n c e
f o r a solvent effect
I n MeCN t h e c o n v e r s i o n s and t h e s e l e c t i v i t i e s a r e u s u a l l y c l o s e t o t h e
values o b t a i n e d w i t h t h e o t h e r p u r e l i q u i d s u b s t r a t e s ( r e f . 1 2 ) . Fladical c h a i n
r e a c t i o n s can be i n v o l v e d i n t h e mechanism o f o x i d a t i v e cleavage of ketones
( v i d e i n f r a ) ; t h r s e r e a c t i o n s a r e l e s s s u b j e c t t o i n t e r f e r e n c e frclin p o l a r
e f f e c t s . N e v e r t h e l e s s , f o r some s u b s t r a t e s we observed a d r a m a t i c change on
g o i n g f r o m MeCN t o e t h a n o l . For an (1-ketol, 2-hydroxy-2-phenylacetl~phenone,
I,I, t h e
conversion i s low i n a c e t o n i t r i l e ( r u n 9
-
Table 2 ) b u t reaches 95%
i n e t h a n o l ( r u n 1 0 ) even a t room temperature; o t h e r n o n - a l c o h o l i c s o l v e n t s
cause i n h i b i t i o n .
208
TABLE 2
O x i d a t i o n o f a k e t o l and t u - d i k e t o n e s b y HPA-2 and 02a
- - -
...icliil'
(11)
(4.8)
0.05
McCN
24
10
2
2
(4.8)
0.05
ELOII
11
ILI
(7.4)
0.03
EtOll
12
IV
(.2 . 4 5 ),
0.05
ECOll
9
-
a R e a c t i o n conditions
p (02;<
b,
x
IX
( "6 )
____
0 1
XI
X'
u
__~___.~
. HPA-2
=
25
24
6
95
93
3
24
=loo
-
~
~
91
c
"100
102
52
ti ~ M O , ~ V ~ O ,., 30-36
~ ~
H20 ; solvent : 6
5
-
211
100
-
cm3 ;
.
T : 20°C ;
Pa
see T a b l e 1
Benzaldehyde,
A ' , are the o n l y i s o l a b l e products.
ILI ( o r XJI) and Ph-C(0)-C(0)-Ph, I V ,
Q ,a n d e t h y l b e n z o a t e ,
a - D i k e t o n e s s u c h as Ph-C(0)-C(0)-Me
-
a r e n o t c l e a v e d i n MeCN, w h i l e t h e y r e a c t w i t h h i g h s e l e c t i v i t y i n e t h a n o l a t
room t e m p e r a t u r e ( r u n s 1 1 - 1 2 ) . These s u r p r i s i n g r e s u l t s l e d us t o exainirie t h e
solvent e f f e c t
on t h e o x i d a t i v e c l e a v a g e o f 2 - m e t h y l c y c l o h e x a n o n e b y HPA-2
( T a b l e 3 ) . Some s o l v e n t s (benzene, d i c h l o r o - 1 , 2
e t h a n e , ...)i n h i b i t t h e r e a c t i o n
o r reduce t h e r a t e o f o x i d a t i o n o f 2-methylcyclohexanone,
1. The
novel c a t a l y s t
s y s t e m s s t u d i e d c a n o p e r a t e i n a l c o h o l i c media ( m e t h a n o l , e t h a n o l , t e r t butanol,
...) b u t
t h e c o n v e r s i o n i s l o w e r t h a n w i t h MeCN o r a c e t i c a c i d a n d
n i t r o m e t h a n e . The m a i n p r o d u c t i s t h e 6 - o x o h e p t a n o i c m e t h y l e s t e r XLV' ( r u n 1 5 )
o r t h e corresponding keto-acid
-
XLV
( r u n 16), w i t h a r i n g - c o n t r a c t i o n product
(cyclopentanone, XV) corresponding t o e t h y l group e l i m i n a t i o n . Nitroinethane
f a v o r s t h e f o r m a t i o n o f t h i s minor product, although t h e conversion i s equal
t o t h a t o b t a i n e d w i t h MeCN ( r u n 1 4 ) . M i x e d s o l v e n t s ( r u n s 17-18) s u c h as
-
MeCN/MeOH g i v e a l s o X I V ' ,
t h e methyl ester. With ethylene g l y c o l dimethyl
e t h e r ( r u n 19), moderate y i e l d s o f t h e e s t e r s , X I V ' ,
p a r t ia 1 m e t hy 1a t ion o f t h e ke t oac id
.
a r e o b t a i n e d , due t o t h e
O x i d a t i o n o f c y c l o a l kanones b y HPA-2 a n d O2
The r e s u l t s f o r t h e r e a c t i o n o f some c y c l o a l k a n o n e s a r e p r e s e n t e d i n
T a b l e 4. Each k e t o n e was s u b j e c t e d t o n e a r l y i d e n t i c a l o x i d a i o n c o n d i t i o n s
w i t h HPA-2. T r e a t m e n t o f 2 - m e t h y l c y c l o p e n t a n o n e ,
5 - o x o h e x a n o i c a c i d , XJI,
6-oxoheptanoic acid,
XLV,
c,
f o r 2h
r u n 20) g i v e s
i n h i g h y i e l d (94%). 2-methylcyclohexanone,
( r u n 2 1 ) ; 2,6-dimethylcyclohexanone, VI-I,
m a i n l y 6-0x0-2-methylheptanoic
acid,
Xgl,
1,g i v e s
produces
a l s o i n good y i e l d ( 8 9 % ) ( r u n 2 2 ) .
O t h e r c y c l i c k e t o n e s c a n be c l e a v e d , b u t t h e s e l e c t i v i t y depends o n t h e
s u b s t r a t e and on t h e c a t a l y t i c system; f o r example, u n d e r t h e s e c o n d i t i o n s ,
209
TABLE 3
O x i d a t i o n o f 2-methylcyclohexanone,
1 , by
HPA-2 and O2 : solvent. effect'
~
Solvent
Convers iomb
(cm3)
($)
Riiii
T iine
( 11
x IV
XIV'
-
-
9
.-u
13
MeCN
(B
Products
xv
98
4
85
2
98
4
81.5
1.5
54
6
0
49
2.5
81
6
69
1
10.5
96
6
4
86
3
96
6
6.5
81.5
5
96
24
16
11
(6)
14
MeN02
14.5
(6)
15
MeOH
(6)
t.iccoon
16
(6)
17
MeCN/McOH
(5)
18
(1)
MeN02/MeOH
(5)
(1)
MeO- ( CH2) 20Me
19
74
.5
(6)
a Reaction c o n d i t i o n s
b9
. HPA-2
: 0.075 minol ; s u b s t r a t e : 12.4 nimol ; I : GO'C
; p ( 0 2 ) = 105Pa ;
see Table 1 ; i n t e r n a l standard : l i c p t a n o i c a c i d
TABLE 4
Oxidation o f cycloalkanones by HPA-2 and 02a
Substrate
HPA-2
Solvent
Time
Conver-
P r o d u c t s :% Y i e l d s c )
s ionb
Run
20
(nun01 1
(mmol)
0.025
(4.4)
(cm3)
MeCN
(h)
(9)
2
96
Xi1
1914)
4
98
XLV
185)
6
91
XZI
189)
24
100
(6)
21
0.075
(12.4)
MeCN
(6)
22
0.075
VLI ( 1 1 . 0 )
MeCN
(6)
23
a Reaction c o n d i t i o n s
b3
0.05
V L I I (4.7)
. HPA-2
MeCN/MeOH
(5) ( 1 )
= H5[PlIo10V20,,,-j.
XVLII (40)
30-36 H20 ; T : 60°C ; p ( 0 2 ) Y 105Pa
see Table 1 ; i n t e r n a l s t a n d a r d : h e p t a n o i c a c i d
XLX
____-_____
(17)
210
cyclohexa-1,3 dione, which e x i s t s m a i n l y i n t h e nionoenolic form, can be c l e a v e d
w i t h l o s s o f one o r two carbons; i t g i v e s two m a j o r p r o d u c t s i n MeCN-MeOtl :
0
0
COOMe
____)
COOMe
MeCN/MeOH
OH
+
C OCOOMe
O M e
X
L
X
XVLII
The s p e c i e s u n d e r g o i n g cleavage has n o t been i s o l a t e d , b u t i t c o u l d be
d i f f e r e n t f r o m t h e t r i k e t o n e formed w i t h t h e sodium p e r i o d a t e system ( r e f . 13)
which g i v e s m a i n l y g l u t a r i c a c i d .
Mechanism o f o x i d a t i o n and search f o r i n t e r m e d i a t e s
EPR r e s u l t s g i v e c l e a r evidence o n l y of i s o l a t e d V I " s p e c i e s . To d e t e r m i n e
whether one mechanism i s more p l a u s i b l e t h a n another, we c a r r i e d o u t a l a b e l i n g
experiment u s i n g 1802 b u t w i t h o u t c l e a r - c u t r e s u l t s . C o n s i d e r i n g t h e e x p e r i mental f a c t s , we can propose a p l a u s i b l e mechanism as shown i n scheme 2 f o r
c y c l o a l k a n o n e s i n which t h e f o r m a t i o n o f a vanadium e n o l a t e i s a key s t e p ; i t
"O\,+)
,.I/
=0
(>0'
-
-0,
b
0
R
Scheme 2
II
~
C
O
O
H
211
c o u l d g e n e r a t e s h o r t - l i v e d r a d i c a l s p e c i e s which c o u l d i n t e r a c t w i t h O2 i n a
vanadium-assisted pathway. The i n t e r m e d i a t e p e r o x i d e c o u l d undergo d i r e c t o r
vanadium-assisted decomposition t o y i e l d t h e k e t o - a c i d . The most i n t r i g u i n g
s t e p i s t h e h o m o l y t i c c l e a v a g e o f V-0 bond t o g i v e t h e s h o r t - l i v c b d r a d i c a l
s p e c i e s . A more thorough s t u d y o f t h e system i s now i n p r o g r e s s .
CONCLUSION
A new c a t a l y t i c method f o r t h e o x i d a t i v e cleavage o f soiiie open-chain
ketones o r o f s u b s t i t u t e d c y c l o a l k a n o n e s has been found. I t emplciys a r a t h e r
i n e x p e n s i v e "PMoV" a s s o c i a t i o n as t h e c a t a l y s t i n a homogeneous phase i n
c o m b i n a t i o n w i t h dioxygen as t h e p r i m a r y o x i d a n t . Some o f t h e r e a c t i o n s
r e p o r t e d h e r e may have s y n t h e t i c p o t e n t i a l : some k e t o - a c i d s have been u t i l i z e d
i n t h e s y n t h e s i s o f m a c r o c y c l i c l a c t o n e s . Other a p p l i c a t i o n s i n c l u d e t h e
p r e p a r a t i o n o f c a t e c h o l a m i n e c o n j u g a t e s and n a t u r a l - p r o d u c t t o t a l s y n t h e s i s .
The r e a c t i o n can be extended t o o t h e r carbon-carbon bond cleavages u s i n g
dioxygen; f o r example a - d i o l s have been smoothly c l e a v e d ( r e f . 14) by a
c a t a l y t i c amount o f H5 [ P M O ~ ~ V ~ O30-36
~ ~ ] . H20 o r o f [VO(OCH(CH3)213] under
m o l e c u l a r oxygen and v e r y m i l d c o n d i t i o n s .
REFERENCES
1 J.-M. B r e g e a u l t , F. Derdar, J. M a r t i n , C. M a r t i n e t J . M e r c i e r , Proc. 6 t h
I n t . Symp. Homogeneous C a t a l y s i s , Vancouver, August 21-26, 1988, p. 34;
J.-M. B r e g e a u l t , B. E l A l i , J. M e r c i e r , J. M a r t i n and C. M a r t i n , C.R. Acad.
S c i . P a r i s , 307 (1988) s C r i e 1 1 , 2011-2014.
2 G. C e n t i , J. Lopez N i e t o , C. I a p a l u c c i , K. Brickman and E.M. Serwicka, Appl.
Catal., 46 (1989) 197-212; J.G. H i g h f i e l d and J.B. M o f f a t , J. C a t a l . , 98
(1986) 245-258; M. Misono, C a t a l . Rev.-Sci. Eng., 29 (1987) 269-321
3 A. M o r e t t e e t G. Gaudefroy, B u l l . SOC. Chim. France, (1954) 956-964.
4 J.S. L i t t l e r , J . Chem. SOC., (1962) 832-837; J.S. L i t t l e r and W.A. Waters,
J. Chem. SOC., (1959) 3014-3019.
5 I . V . Kozhevnikov and K . I . Matveev, Russ. Chem. Rev., 51 (1982) 1075-1088;
Appl C a t a l . , 5 (1983) 135-150; I . V . Kozhevni kov, Uspeckhi K h i r n i i , 56( 1987)
1417-1443; E.G. Z h i z h i n a , L . I . Kuznetsova and K . I . Matveev, React. K i n e t .
C a t a l . L e t t . , 3 1 (1986) 113-120.
6 B. E l A l i , J.-M. B r e g e a u l t and J . M a r t i n , J. Organoinetal. Cheiri., 327 (1987)
C9-Cl4.
7 I . V . Kozhevnikov, V . I . Siniagina, G.V. Varnakova and K . I . Matveev, K i n e t . i
K a t a l . , 20 (1979) 506-510.
8 0. Mohammedi, Ph. D., U n i v e r s i t e P. e t M. Curie, may 11, 1987.
9 W.V. M i l l e r .and G. Rohde, B e r i c h t e , 23 (1890) 1070-1079.
10 G. Canneri, Gazz. Chim. I t a l . , 56 (1926) 871-889; P. C o u r t i n , Rev. Chim.
Min., 8 (1971) 75-85; G.A.T. T s i g d i n o s and C.J. H a l l a d a , I n o r g . Chem., 7
(1968) 437-441; J.-M. B r e g e a u l t e t a i . , u n p u b l i s h e d r e s u l t s .
11 8. E l A l i , J.-M. B r e g e a u l t , J. M a r t i n , C. M a r t i n and J. M e r c i e r , New J .
Chem., 13 (1989) 173-175.
12 B. E l A l i , Ph. D., U n i v e r s i t e P. e t M. C u r i e , j u n e 21, 1989.
13 M.L. Wolfrom and J.M. B o b b i t , J . Amer. Chem. SOC., 78 (1956) 2489-2493.
1 4 J.-M. B r e g e a u l t , B. E l A l i , J. M e r c i e r , J . M a r t i n e t C. M a r t i n ,
C.R. Acad. S c i . P a r i s , 309 (1989) s e r i e 11, 459-462.
.
212
DISCUSSION CONTRIBUTION
B.R. JAMES [ U n i v e r s i t y of B r i t i s h Columbia, Vancouver. Canada) : I am confused
r e g a r d i n g y o u r i m p l i c a t i o n s f o r t h e mechanism. You n o t e t h a t t h e reduced h e t e r o p o l y - b l u e H.[HPA-nl i s r e o x i d i z e d by O 2 t o g i v e t h e o x i d i z e d f o r m HPA-n,
y e t i n Scheme’2 you make a mechanism showing unchanged o x i d a t i o n s t a t e i n t h e
HPA m o i e t y ( V 0 2 + ) w i t h 0 a t t a c k i n g t h e c o o r d i n a t e d c y c l o a l k a n o n e t o g i v e a
2
peroxo r a d i c a l . Do you f a v o u r O2 p l a y i n g a r o l e w i t h i n t h e HPA m o i e t y o r
w i t h i n t h e organic moiety ?
BREGEAULT [ U n i v e r s i t g P . e t M. E u r i e , P a r i s , France) : The e q u a t i o n s o f
t h e i n t r o d u c t o r y p a r t a r e n o t t h o s e o f a mechanism [ i . e . elementary processes)
b u t o n l y t h o s e of t h e presumed o v e r a l l process. Scheme 2 shows t h e f o r m a t i o n
o f i n t e r m e d i a t e vanadium [ I V ) species, b u t o m i t t h e f o r m a t i o n o f an a l k y l p e r o x i d i c complex which would i n v o l v e f r e e radical-dioxygen-vanadium i n t e r a c t i o n . T h i s i n t e r m e d i a t e s p e c i e s c o u l d a s s i s t r e o x i d a t i o n o f vanadium [ I V l .
A t p r e s e n t , we have no e x p e r i m e n t a l r e s u l t s which show t h e p r e f e r r e d i n t e r a c t i o n o f d i o x y g e n w i t h t h e o r g a n i c r e s t . so t h e t h i r d and f o u r t h s t e p o f t h e
mechanism [Scheme 21 indeed. has t h e c h a r a c t e r o f a p r o p o s a l .
J.-M.
R.A. SHELDON [ANDEND, The N e t h e r l a n d s ) : Do any o f t h e oxometalate o r vanadium
c a t a l y z e d o x i d a t i v e cleavage o f 3 . 2 - d i o l s a l s o work under n e u t r a l o r b a s i c
conditions ?
J.-M. BREGEAULT [ U n i v e r s i t b P. e t PI. C u r i e . P a r i s , France) : V i c i n a l d i o l s c a r
a l s o be c l e a v e d by some HPA-salts, b u t t h e r a t e o f r e a c t i o n . t h e c o n v e r s i o n
and t h e y i e l d s a r e lower t h a n t h o s e o b t a i n e d w i t h HPA-2. I t s h o u l d be m e n t i o ned t h a t r e o x i d a t i o n o f t h e reduced f o r m [ s l i s c o n t r o l l e d by t h e a c i d i t y
f u n c t i o n of t h e medium.
H. MIMOUN ( I n s t i t u t FranCais du P e t r o l e , RueiZ-Malmaison, France) : What i s
t h e s t a b i l i t y of t h e HPA d u r i n g t h e cleavage r e a c t i o n ?
J.-M. BREGEAULT [ U n i v e r s i t Q P. e t M. C u r i e , P a r i s . France] : The s t a b i l i t y o f
PPA-n u n d e r our r e a c t i o n c o n d i t i o n s has n o t y e t been s t u d i e d in d e t a i l , b u t
t i l l now we have no e x p e r i m e n t a l e v i d e n c e o f i t s i n s t a b i l i t y . Work o n t h i s i s
i n progress.
G. Centi and F. Trifiro' (Editors), New Developments in Sekctive Oxidation
0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
213
'H NMR STUDY OF THE MECHANISM OF P T H Y L E ~ G L Y C O LMONOACETATE
FORMATION IN OXIDATIVE ACETOXYLATION OF ETHYLENE CATALYZED BY
Pd(I1) COMPLEXES
E.V.
K.I.
GUSEVSKAYA, 1.E.
ZAMARAEV
BECK, A.G.
STEPANOVj V.A.
LIKHOLOBOV a d
Institute of Catalysie, Novosibirsk 630090, USSR
SUMMARY
A detailed mechanism of ethylene oxidation by Pd(N0 )ClL complexes (nt2,3; LPCD CN) in a chloroform-acetic acid %ixt&e
is studied by 1H N d R spectroscopy. The end reaction products
are ethyleneglycol monoacetate (EGMA), aueteldehyde, nitroethylene and com ounds with the general formula CH3-CHXY (X,Y u OH,
OAc, C1, NO
whoee ratio depends upon the
solvent composition. Kinet?c and speotral data obtained indicate the formation
of a number of intermediates. The structure and route8 of decomposition of the intermediates to EGMA and other reaction products
are suggested.
p,
INTRODUCTION
Oxidation of olefins catalyzed by Pd(I1) complexes is a rapidly developing trend in selective synthesis of oxygen-containing
organic compounds. The main product of d -olefins oxidation in
acetic acid solutions containing ealts of nitric acid and palladium(I1) is glycol monoacetate, while in the absence of nitrate
ions carbonyl compounds and vinyl ethers are formed (ref. 1).
The mechanism of formation of glycol monoacetates from &-olefins has been studied for Pd(OAc)@iN03/HOAc
(ref. 2) and
Pd(N02)C1(CH3CP3)2/HOAc
(ref. 3) systems using lithium nitrate or
nitro ligands in Pd(I1) complexes labelled by heavy isotopes of
oxygen. It has been established that the resulting glycol monoacetate contains labelled oxygen in the carbonyl position of the
acetate group. Based on data obtained on distribution of labelled
oxygen in reaction products, the authors have suggested the mechanism of glycol monoacetate formation; however, the structure
of intermediates has not been confirmed by spectroscopy.
The objective of this work was to etudy the mechanism of ethylene oxidation by Pd(I?On)C1L2 complexes in chloroform-acetic acid
solution by 1 H IrJIy[R spectroscopy.
214
METHODS
Pd(NOn)C1L2 complexes were prepared as in (ref. 4 ) . ‘H lyMR
spectra were recorded using a Bruker CXP-300 spectrometer with a
magnetic field induction of 7 T. Chemical shifts of signals were
measured with respect to the internal reference hexamethyldisiloxane. The temperature of samples was continuously monitored with
a precision of l o by a W-1000 thermocouple. In all experiments,
the concentration of palladium in solution was 2 ~ 1 Ml”;
0 ~ ~5tlO
mol o f ethylene per palladium i o n being introduced into the solution of complex. CDC13 and CD3COOD(DOAc) were used as solvents.
RESULTS AND DISCUSSION
Addition of ethylene (I) to solution8 of Pd(HOn)C1L2 complexes in chloroform-acetic acid medium (content of DOAc is O-lo%)
gives rise to the appearance of several lines in the NMR spectra.
Analysis of the change8 In the line intensities with reaction
time permitted us to isolate groups of lines, whose inteneities
varied in the same manner and that could,for this reason,be attributed to the same compounds. For this purpose the parameters
of J(H-H) of multiplet lines were also used.
Reaction products
Seven groups of lines that do not disappear for a long period
of time can be assigned to end reaction products whose ratio depends upon concentration of DOAc in solution.
Acetaldehyde (11) ( 8 2.17 pprn (d),
= 9.73 ppm (qd)) is
the main product (95-97% per reacted olefin) of the reaction in
chloroform; its yield tends t o decrease with increasing concentration of DOAc in solution.
During ethylene oxidation in chloroform nitroethylene (111)
8 7.14 pprn (dd)) is
( 8 = 5.91 ppm (dd), s = 6.65 ppm (dd),
accumulated (up to 5%) with a long induction period; in the presence of DOAc nitroethylene is formed in trace amounts,
In solutions containing DOAc one of the products of ethylene
oxidation ie EGMA (IV) ( 6 P 3.77 ppm (m), 8 = 4.14 ppm (m)),
in glacial DOAc the yield of E G U ie 95-97%.
In the range of DOAc concentrations 2-20 ~01.4% ethylene oxidation gives rise to the formation of compounds V - V I I I (total
yield up to 45$), whose NMR spectra are similar in line structures and positions ( 6 m 1.35-1.71 ppm (d) and
a 6.46-6.97
ppm
(qd) with intensity ratio 3 : l ) . Analysis of M6R spectra of comP
-
215
pounds V-VIII and peculiarities of their accumulation in solution
permitted us to suggest the following compoeition for theee products: CH CH(OAc12 (V), CH3CH(OAc)(OH) (VI), CH3CH(OAc)(Cl) (VII)
3
and CH3CH(OA~)(N0,) (VIII).
Intermediates
Based on the initial increase and subsequent decrease of their
intensities with time the groups of linee IX-XVI (see table 1)
seem to belong to intermediatea formed during the reaction. As a
result of 'H NMR studies of the kinetics of ethylene oxidation by
Pd(NOn)C1L2 complexes at various concentrations of DOAc in chloroform, we have registered intermediatea that may be responsible
f o r the formation of observed reaction producte. The maximum observed line intensities of the intermediatee formed during ethylene oxidation in CDC13 solutions with different concentrations
of DOAc are shown in Fig. 1 f o r Pd(N03)C1L2 and in Fig. 2 for
Pd(N02)C1L2. An analysis of 1H IWdR spectra of intermediates and
kinetic curves of accumulation-decomposition o f the intermediates
and end products at various concentrations of DOAc allowed us t o
suggest the structures of compounds IX-XVI (table 1) as well ae
the possible routes of their formation and decomposition.
Mechanism o f ethylene oxidation
Palladium complexes with NO2 ligande in chloroform solutions
exist as two isomers: Pd(ONO)C1L2 (complex A) and Pd(N02)C1L2
(complex B) (ref. 5); in the presence of DOAc Pd(OAc)C1L2 oomplex C) may be aleo formed. Then it is reasonable to suggeet that
in the first step of ethylene oxidation displacement of the neutral liganda from complexee A,B,C and Pd(N03)C1L2 (complex D) and
formation o f the corresponding SE-olefin complexes of palladium
A, 2, C and 2 take place. Due to insertion of coordinated ethylene
into Pd-0 bonds in complexes A, 2 and 2 and into the Pd-N bond in
complex l3, organopalladium intermediates XII, XI, IX and XIII,
reepectively, are formed (table 1).
A wide variety of ethylene oxidation products is determined by
the step of decompoaition of organometallic compounds IX, XI-XI11
Pd-CH2 CH2Z. The transformation of these key intermediatea depend
on the nature of subetituent 2, ligands in the palladium complex
and solvent composition. Based on the results of IR and NMR spectroscopy studies on the mechanism of ethylene oxidation by
Pd(I1) complexes ( f o r chloroform solution8 the reeults have been
-
216
25
0
*
Fig. 1. D’isxlmum observed intensities of
lines of intermediates registered during ethylene interaction with Pd(N03)C1L2
VS. solvent compoeition at 295 K O
I, %
I,%
25
75
I
&
50
I
0
25
I
% CDCL3
125
XVI
xv
7
*
Fig, 2. hbximum observed intensities of NMR lines of intermediates registered during ethylene interaction with Pd(N02)C1L2
vs. solvent composition at 295 K.
the toLine intensities in spectra are given per one
ial quantity of reacted (during observation timeproton;
ethylene is
taken as 100%. The yield of products is also given per reacted
ethylene
.
217
TABLE 1
Characteristics o f 'H NMR spectra lines attributed to reaction
intermediates and their propoeed structuree
Corn- Line struc- 8(ppm) J(H-H)
(He)
pound ture
IX
X
a triplet
b triplet
a triplet
b triplet
XI
a triplet
b triplet
IntenProposed structure
sity rati0
8
b
I
1.61
7.3
4.30
7.3
I
1.56
3.72
6.2
6.2
I
I
1.67
3.92
a broad line 2.373.25
b triplet
4.054.3
XI11 a triplet
1.08
b triplet
4.22
XI1
6.8
6.8
a
'
d
P
'
/
I
I
-
I
6.3
I
/
/
Pd
I
7.2
I
/
,pd\
\
/
,pd\
XIV
a doublet
b triplet
XV*
2.34
9.56
3.5
3.5
2
I
a multiplet 4.29
I
4.89
b broad
multiplet
I
OAc
a
b
a
b
CH2-YH2
0n0
9-FH2
N02
a
H
,Pd\
\
b
CHrF%
'
\
7.2
OH
-\
a
\
b
CH2-FH2
CHz-C,
/
&0
H
b
8
-H3c
XVI
a triplet
b triplet
* The spectrum typical for a four-spin system with JAA, =
Jm JAB, = JAtB JAtB, 4.55 H z ~
1.4 Hz; JBB, = 3.1 HI;
0
3
31
218
published in (refs, 5 , 6 ) ) we propose the following possible mechanism of the formation of 1 , l - (i.e. containing an ethylidene
fragment) and 1,2-additlon products.
1.2-Addition Droducts. D u r i n g ethylene oxidation by
Pd(N02)C1L2 EGMA seems to form at least by three parallel routes
via key intermediate E:
5%'
CH -CHz
a 0 A c
L
2 0
\O
IV
Organometallic intermediate E may be formed: from a-nitritoethylpalladium complex XI1 via reesterification-byacetic acid
(route 1); by heterolysis of the Pd-C bond in J-nitroethylpalladium complex XI11 under the influence of DOAc resulting in 1nitro-2-acetoxyethane XY followed by oxidative addition of the
Pd(0) complex to the C-M bond in intermediate XV (route 2) and
by direct acetoxypalladation of ethylene in palladium complex
with nitro ligand (route 3 ) . Then intramolecular rearrangement
of intermediate E leads to the Pd(I1) complex with a hydroxyalkyl ligand and acetylnitrite XVI.
Decomposition of complex XVI to form EGMA and nitrosyl complex of Pd(I1) by heterolyeis of the Pd-C bond under the action
of the coordinated molecule of acetylnitrite.
It should be noted, that the mechanism proposed here is consistent with stereochemical data, labeling studies and the regiochemistry observed in (refs, 2,3,7).
It has been established that during ethylene oxidation by
Pd(N03)ClL2 EGW is formed directly from the ethylene nitrate
219
complex of Pd(I1). The mechaniem of interaction of the ethylene
nitratopalladation product IX with DOAc 8eme to be sfmllar to
r . 2 for the nitrite eyetern. Although in the nitrate eyetern the
EGMA formation intermediate analogoue to XV nae not found probably
due to it6 high reactivity, intermediate X (analogous to XVI),
which might contain aoetylnitrate a8 8 possible ligand, was registered. Heterolgeie of the Pd-C bond in the complex X under the
action of the coordinated acetylnitrate molecule yields EGIYUL. Unlike acetylnitrite, acetylnitrate can easily be dieplaced from
the palladium complex followed by decompoeition of the ethylene
oxypalladation product into 1,l-addition produats (mainly acetaldehyde) by 4 -hydrlde elimination.
1.1-addition products. me products of 1,l-addition (acetaldehyde and CH3CHXY) seem to form during the decomposition of or-nopalladium intermediatee IX-XI11 via the following eeheme: (a)
reversible 6 - 3 -rearrangement of complexee IX-XIII; (b) 8Z
6-transformation of hydridepalladiumolefin oomplexee via the attack of coolrliaated vinyl ether by the nualeophile 1 leading to
the formation of either regietered intermediate XIV (aoetaldehyde
preouraor) or complex 0 (preoureor of 0 5 C H X Y producte); (c) decomposition of hydride complexes XIV and 0 via reductive elimination producing acetaldehyde snd compounds Y-VIII:
-
I/
t
R =NO?(complex
H (complex
NO [ c o m p l e x
CI
X
= OAC.
X
CH, -CH
\
L/
Pd /
-
2-
?L
R= OH ( c o m p l e x
OAc(camp1ex
NO2 [ c o m p l e x
X = OAC, C I
L/
b
L'
h
L /
b
IX),
XI,
XII)
-L
-4
L
-
XI,
XI).
m)
CH3CHRX
V-Vm
+ PdoL2
XIV
220
During the formation of acetaldehyde the Pd(0) complexes are
oxidized by nitroxgl or nltroayl chloride (or by the corresponding acetates); in the other cases the palladium black is formed,
along with the products of 1,l-addition.
In chloroform, during the decomposition of intermediate XI11
nitroethylene is formed, as ha8 been deacribed by us in (ref. 5).
REFERENCES
P.M. Henry, Palladium-catalyzed oxidation of hydrocarbons,
Reidel, Dordrecht, 1980, p. 99.
V.A. Likholobov, N . I . Kuznetsova, M.A. Fedotov, Yu.A. Lokhov
and Yu.1. Y e m k o v , Interaction between oxidants and olefine
in solutions containing palladium complexes, in: 6th Nat.
Symp. Recent Advances in Catalyeie and Catalytic Reaction Engineering, Pune, India, 1983, pp. 217-228.
F. Maree, S.E, Diamond, F.J. Regina and J.P. Solar, Bomnation
of glycol monoacetates in the oxidation of olefine catalyzed
by metal nitro complexes: mono- VS. bimetallic system, J. Am.
Chem. SOC., 107 (1985) 3545-3552.
I,E. Beck, E.V. Gusevskaya, V.A.
Likholobov and Yu.1. Yermakov,
Synthesis of Pd(I1) nitro and nitrate complexes and studies of
their reactivity towards oxidation of olefins in organic solvents, React. Xinet. Catal. Lett., 33 (1987) 209-214.
E.V. Gusevskaya, I.B. Beck, A.C. Stepanov, V.A.
Likholobov,
V.M. Nekipelov, Yu.1. Yenaakov and K.I. Zamaraev, Study on the
meohanism of ethylene oxidation by a nitrite com lex of palladium in chloroform medium, J. Molec. Catal., 37 f1986) 177-188.
I.E. Beck, E.V. Gusevskaya, A.G. Stepanov, V.A. Likholobov,
V.M. Nekipelov, Yu.1.
Yermakov and K . I . Zamaraev, Study of the
mechanlem of ethylene oxidation by palladium(I1) complexes
containing nitro and/or nitrato ligande in chloroform, J. Molec. Catsl., 50 (1989) 167-179.
Jan-L. Backvsll and A. Henmnnn, A cromment on the recently proposed mechaniem f o r the oxidation of olefins with
PdCl(HO,)(CH-,CN),,
J. Am. Chem. Soc., 108 (1986) 7107-7108.
G. Centi and F. Trifiro' (Editors), New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
221
PERSPECTIVES IN IWROVEI~NT OF SELECTIVITY IN LIQUICD PHASE
OXIDATION BY DIOXYGEN. NEW MODELS OF ENZYM.rlATIC OXIDATION
I.P. SKIBIDA and A .&I. SAKHAROV
Institute of Chemical Physics , Academy of Sciences of the USSR,
117334 Moscow, Kosygin street, 4 (USSR)
SUKlvhiRY
JCuC I)(0-phen) J complexes effectively catalyze the oxidntion
of primary alcohsls to aldehydes by dioxygen in non aqueous solutions at pH>7 and moderate temperatures and can be considered
as an adequate model of galactose-oxidase. The anion coordine.tion
on Cu(1)-centers results in a strong increase in anion renctivitg towards 0 The use of D -labeled methanol confirms that
dioxygen reduc3ion in the coulse of oxidation occurs by a concerted two-electron mechanism,
+
.
INTRODUCTION
IiTany different catalytic systems involving metal complexes
and phase transfer catalysts were proposed recihtly ELS essentially good models of enzymatic oxidation El 1. Xost of them
uses hydrogen peroxide or other single-oyygen donors as the oxidants. As before the idea of using dioyygen as the cheepest and
most ecologically pure oxidant remains attractive. However
despite the important successes in this field, the problem is
still far from solving.
At the same time in the presence of enzymes dioxygen is
known to oxidize various substrates at high rates and with good
selectivity even at room temperature. In this connection the
interest in modelling enzymatic oxidations greatly increased
in recent years.
Of special interest is to mimic such enzymes as tyrosinase
catalyzing the oxidation of o-phenols to quinones I2],galactoseoxidase in which presence the -CH20H groups react with O2 to
yield aldehydes [ 3 , 4 1 , dioxyygenase catalizing oyygenation of
catechols [ 5 J. One specific mechanistic feature of the mechanism
of catalysis by these enzymes is the participation of the substra
te coordinated to enzyme active center in dioxygen activation.
Certain copper complexes having a N or 0 donors set cata-
222
lyze alcohols oxidation at pH > 7 and can be considered as
pus$ible chemical models of galactose-oxidase i6-8 1.
It was demonstrated in our previous reports [ 7,9 ,that
copper/o-phen complexes can be used as very active catalysts for
primary alcohols one-step oxidation in aprotic solvents and pH> 7
giving aldehydes
(cuprous catalysis ) o r acids ( cupric
catalysis) .
The present communicationsubmitsthe additional proves for
the mechanism of one-step oxidation of alcohols to aldehyd-esand
demonstrates that the proposed catalytic system represents an
adequate model of galactose-oxidase.
RESULTS AND DISCUSSION
Methanol was oxidized by dioxygen in the presence of copper
complexes and bases at 30-50°. Both cuprous and cupric complexes
with 1,lO-phenanthroline ( o-phen) ; 5,6-dirnethyl-l,lO-phenanthroline (CH3-phen) ; 5-nitro-1,lO-phenanthroline (N02-phen ) and
bipyridile (bipy) were used as catalysts. The complexes were preCuC12 or CuCl and the ligands in alcohol.
pared by dissolving
The reaction began after addition of alkali.
In neutral media methanol virtually is not oxidized at moderate temperature even in the presence of a catalyst. It seems that
just as in many enzymatic systems [ 7 1 , the formation of an
anionic form of the substrate is an obligatory condition for an
increase of its activity towards oxygen. The use of Ba(OH)2,
Ca(OHI2 or triethylamine as bases does not provoke effective
deprotonation of methanol in non agueous solutions , and the
oxidation rate in the presence of these bases and the cntalyst is still low. High rates of oxygen consumption are observed only in catalytic oxidation of methanol in the presence
of strohg bases-such as NaOH , KOH or NaOCH3.
The semiconvertion time of primary alcohols under these
conditions is about 4 0 min , n = [RCH20H]/[Catlo
= l o 3 , turnover
time -1 sec. Fig.1 shows the rate of oxygen consumption as a
function of alkali concentration in the oxidation of 20 % mol.
methanol solutions in acetonitrile in the presence of
[Cu(o-phen)2]+
at 30'. The increase in the oxidation rate with
base concentration seems to be connected with increasing of the
methylate anions concentration. Very high medium alkalinity
results in a decline of the oxidation rate caused by hydrolysis
of copper complexes as observed earlier for the catalytic oxidn-
223
tion of ketones in the presence of copper/o-phen copplexes at
pH > 7 [ l o ] .
The methanol oxidation rate is dependent not only on concentration but also on the donor capacity of alcoholate ions in so1u.tion. The dilution of methanol with aprotic solvents ( benzene ,
acetonitrile, D W A , etc.) increases essentially the rate of
CH OH oxidation. In the presence of small amounts of water the
3
reaction becames cornplitely passivated due to much lower electron
donating activity o f anions in water-containing solutions as compared t o that in aprotic solvents.
2
[base] x 10 , M
[C~(I)(o-phen)~] x l o 4 , M
Fia.1. The oxygen consumption rate as a function of KOI-; (1) ;
NaOE (2) ; NaOCH (3) concentration in methanol oxidation.
3
[CuCl2J= 2.5 x
M ; [C1130H] =20%
EJ ; lo-phenl = 1.0 x
vol., acetonitrile as solvent , 30'.
Fia.2. The rate of oxygen consumption as a function of Cu(I)L2
concentration in methanol oxYidation : L = o-phen ( 1 9 4 ) , bipy
( 2 ) , CH3-phen (3).
cUn+I = 1.0 x 10-3 M , OH J = 4.0 x
Y , [CH~OH]= 20% mol.
acetinftrile (1~3)and D W A (4) as solvents
c
Cu(1) complexes act as active catalytic species in the oxidation of alcoholate ions in acetonitrile or in bulk. Thus ,when
Cu(II)(o-phen)2 2+ are used as catalyst CH3OH oxidation occurs
with some induction period. During this period Cu(1) is accumulated in solution. No induction period is observed when using
C~(I)(o-phen)~ as catalyst.
The Cu(1) concentration and the rate of methanol oxidation after
induction period is finished are independent on the copper valenCy state in the initial catelyst and are a function of the experimental conditions ( solvent , alkali and ligand c:oncentration
etc. 1
+
224
The rate of methanol oxidation linearly increases with Cu(I)
concentration*. One of the most important factors responsible
for the catalysis by [ C~(I)(o-phen)~l+ complexes seems to be the
increasing reactivity of A- ions in LCU(I).A- .adducts toivards
dioyygen. This is supported by the fact that the complexes
catalytic activity increases with the electron donating activity
of ligand ( L ) . In the range L = o-phen bipy , CH3-phen the
highest oxidation rates were observed for [Cu(I)(CH
3-phen)2]+
(curve 3) and it is just for CH phen the donor activity is the
3highest in the above mentioned range.
LOW activity of Cu(II) complexes in the catalytic oxidation of
Primary alcohols in the presence of bases seems to be due to the
fcct that cupric complexes act usually as electron acceptors and
the coordination o f alcoholate ions to Cu(I1)-centers results in
lowering of their donor activity and thus in decreasing of the
ions reactivity towards dioxygen.
It would have been expected that
electron withdrawing
substituents (such as-N02) in o-phen molecule make the Cu(1)
complexes activity lower. It however appears that the activity of
copper/N02-phen complexes is rather high: the rate of methanol
oxidation in the presence of copper/N02-phen conplexes is twice
that in the presence of copper/o-phen complexes, other conditions
being equal.
When the [Cu(II)(N02-phen)2]2+
complex is used as catalyst,
I)(N02-phen)3]f
a short induction period is observed. However/&(
accumulation is not detected in the system in contrast with
methanol oxidation in the presence of' copper complexes with other
ligands.!Phe spectrum of cuprous complexes disappears completely
in the course of reaction also when [C~(I)(ItO~-phen)~]+ is used
as a catalyst. It seems that the introduction of a strong electron withdrawing N02-group into the ligand sharply increases the
rate of one-electron reaction of methylate ions with catalyst
yielding to free radicals. The ligand interaction with free r a d i cals results in fast irreversible consumption of N02-phen.Similar
ly when catalytic oxidation of ketones at pH > 7 occurs in the
presence of [C~(II)(o-phen)~]~+ and redox-active additives the
reaction results in irreversible consumption of the ligand and
increasing of the role of free radical reactions.
,
A
The value of Cu(1) was Varied by changing
concentration in solution.
o-phenmthroline
225
A s found in
[91 aldehydes are the main products of Cu(1)-cata-
lyzed oxidation of primary alcohols ( propanol , benzyl alcohol ,
etc. ) in aprotic solutions. With a low medium basicity ( [NaOH]/
/[Cat] = 10 f 20 ) the selectivity of aldehyde formation attains
90%. In accordance with these results formaldeh3de must be the
primary reaction product of methanol oxidation in the presence
of Cu(I)-complexes.
However at the first stages of methp,nol oxidation only very
small amounts of formaldehyde can be detected in solution due to
high rate of its condensation at high pH. The condensation products are further oxidized to form acids with very high rate.
When the medium basicity is lowered due to acids formation and
the concentration of Cu(1)-complexes is rather high CH20 is accumulated in considerable amounts.
The ratio of oxygen and alkali, consumed in methanol oxidation
( , ; i 0 2 ]/A[ITaOH])
in the presence of Cu(1)-complexes is always higher than 1, and in low alkaline solutions it can attain 5 + 10
because of formation of nonacidic products, such as formaldehyde
o r its pondensation products ; the rate o f oxygen consumption
depends to a great extent on O2 partial pressure. A decrease in
po f r o m 1 to 0.4 atm results in an almost ten-fold decrease in
thg oxygen consumption rate. Simultaneously the '_Cu(I)( o-phen)2]+
concentration drops.
) increases
The relative rate of oxygen uptake ( Wo /[Cu(I)]
linearly with po within the oxygen pressud variation from 0 to
1 atm. This seem$ to be an evidence of dioxygen participation in
the rate determing step of the oxidation process,
An very important specific feature of the catalytic system
under investigation is its nctivity only in the oxidation of primary alcohols. Secondary alcohols are not oxidized under our conditions and can be used as inert solvents in the oxidation of primary alcohols.
DISCUSSION
The Mechanism of Primary Alcohols Oxidation
(i) Cu(1) catalysis. It is apparent from the above discussion that
the oxidation of primary alcohols takes place due to oxygen initeraction with the Cu(1) .A- adducts whose reactivity towards dioxygen is higher than that of non-coordinated anions.
It should be suggested that the oxidation of alcoholate
226
ions to aldehydes in our system occurs by one-electronmechanism
according to reaction :
~
< o=o
RCH20-
...Cu(1)
-
O2
RCH20'
...Cu(1)
-
RCHO
+
CU(I)
+
HO;
(1)
However the one-electron reduction of dioxygen in reaction ( 1 )
the
can not explaine the chgmioselectivity ofioxidation reaction : as
mentioned above in the presence of [Cu(I)(o-phen)2]f
at pH
7
only primary alcohols can be oxidized.
The alternative two-electron mechanism of oqygen reduction has
been suggested by us in [ 91:
It shouldbe mentioned thet the HO; formation suggested in both
( 1 ) and (2) reactions does not lead to any change in the reaction
rate and does not favour the one-electron oxidation since under the given conditions hydrogen peroxide decomposes quiclclY
to form H20 and O2 and does not contribute to CH OH oxidation.
3
Oxygen protonation is the most important step in the proposed
two-electron oxygen reduction. The same was suggested also for O2
reduction over Cu(I)-centers of dopamine-D-monooxygenase [ 1 2 i .
Such an assumption perdits to explain why i-propanol and other
sec.alcohols are not oxidized under the reaction conditions:
hydrogen transfer from substrate to oxygen to form HO; ,by the reaction sinilar (2) is obviously impossible f o r sec. alcohols.
The kinetic regularities of deuterated methanol ( CD30D) oxidation =re studied in order to provide evidence for the possibility
of simultaneous transfer of two electrons and a proton (formally
corresponding t o transfer of a hydride ion to dioxygen)from the
anion coordinated to Cu(1) center to the dioxygen molecule.
Fig.4 shows the kinetic curves f o r oxygen consumption and Cu(1)
accumulation in the oddation of methanol ( curves 1,3) and
deuterated methanol (curves 2,4). It can be seen that the rate of
CD30D oxidation is almost byone order of magnitude lower than
that of CH30H oxidation. This is in part due t o the lesser concen
tration of [Cu(I)(~-phen)~']+. The variations in C~(I)(o-phen)~ +
concentration must necessarly be taken into account in celcula tion of the kinetic isotope effect. The ratio of the effective
Cu(1) , can be taken as
first oder rate constants, keff = Vf
221
nensureof the kinetic isotope effect.
M
4
8
rnin
5
10 rnin
Fig.4
Fig.3
Pi
The kinetic curves of 0 ( 1 ) and NaOCH (2) consumption
I)( o-phen)2]t( 3 ) and for&aldehyde (4) a&xmulation in
&?bu(
methanol oxidation.
[CuCl 1- 2.5 x 10-% ; [o-phen]= 1,0 x loe2 M ; acetonitrile as
0'.
solve&-, 3
Fi 4. The kinetic curves of oxygen consumption (1,2) and Cu(I)/
o p en complexes accumulation in oxidation of 20% v o l . solutions
of CH30H (1,3) and CD30D (2,4).
[CuCl 1 = 1.0 x
M; [o-phen] = 2.0 x
M ; [NaOH]= 0.05 A;
acetogitrile as solvent , 30'
*
.
For the methanol oxidation the kinetic isotope effect calculated from the data represented on Fig.4 is kH/kD = keff/keff
H
D
=
= 2.7. When both methanol and CD OD are oxidized using Cu(I)/o3
phen as catalyst in the presence of o-phen excess the concentrations of cuprous complexes during the oxidation coincide and are
about 80% of "&(I)],.
The ratio of the rates of oxygen consumption in this conditions is 2.6.
The obtained values of kinetic isotopeeffect are close to
that for some @ydride transfer reactions occuring via a non-linear activated complex [73].
Thus, the measured isotopeeffect values for methanol oxidation
catalysed by Cu(1) complexes provide convincing evidence for
the importance of hydri.de ion transfer by interaction of coordinated methylate ions with O2 , i . e . these values are in favour
of the two-electron mechanism of alcohol oxidation in the catalytic system under investigation.
(ii) Cu(I1)-catalysis.
It appeared that when DMFA is used as
a solvent not only cuprous but also cupric complexes are active
catalyst for methanol oxidation at pH> 7 4 fig.2, curve 4) that
228
seems to be accounted € o r by the higher DMFA donor activity compared to that of acetonitrile Ill]. The d o n o r capacity of methylate
ions coordinated to Cu(I1)-centers seems to be sufficient in this
case to ensure the high rate of their interaction with dioxygen.
Formic acid is the main product of methanol oxidation in the presence of CU(II) complexes.
The one-step oxidation of alcohols to acids catalysed by Cu(I1)
complexes occurs by two-electron mechanism C91:
0-
-1
HO'
I s expected,the rates of CH OH and CD OD oxidation in DYIA
3
3
(Cu(I1)-catalysis) virtually coincide in agreement with (3).
The participation of Cu(II1) ions in the mechanism of primary
alcohols oxidation to aldehydes in the presence of galactose-oxidase [2] or some Cu(1) complexes [14] in neutral media was proposed. However for the system under investigation the Cu(II1) ions
catalysis is not very probable becose this oxidant c ~ n n tact as
E chemioselective one ( the system is quite inactive in oxidation
of secondary alcohols) ;lo],
REFZREBCES
1 . B.Neunier, Bull.Soc.Chim.France, 1986 (4) 578-584.
2'. E.L.Solomon, in T.G.Spiro (ed.) Metal Ions in Biology, v.2,
'Jiley, N.-Y., 1981, 41-102.
3 . G.A.Hamilton, P.K.Adolf , J.de Jersey, G.S.Du Bois, J.Amer.
Chem.Soc., 1OC (1978) 1899-1901.
4. A.N.Klibanov, R.N.Alberty, ifl.A.Marletta, Biochim.Biophys.Res.
Commun., 108 (1982) 804.
5. L.Que, Jr., Coord.Chem.Rev., 50 (1983) 73-78.
6. W.Brackman, C.L.Gaasbeek. Rec.trav.chim.Pay-Bas , 85(2) (1966)
242-256.
7. I.P.Skibida, A.M.Sakharov, in: Itogi nauki i tekhniki, ser.
Kinetika i kataliz , v.15 (1986) 110-234
8. N.Kitajama, K.Wan
Y.TJoro-oka, A.Uchida, Y.Sasada, J.Chem.Soc.
Chem.Commun., 198fi2) 1504-1506.
9. k.M.Sakharov, 1.P.Skibida , Izv.AN SSSR , ser.khim., 1980 (2)
523-528.
10.A.N. Sakharov, I.P.Skibida,J .Molec.cat., 48( 2-3 1 ( 1988) 157-174
ll.V.Guttman, Coord.Chem.Rev., 18 (1976) 225-228.
lZ.S.d.Miller, L.R.Klinman, Biochemistry , 24 (1985) 2114-2116.
13.W.P.Jenks, Catalysis in Chemistry and Enzymology, Mc Graw Hill
1969.
14.P.Capdevielle, P.Audebert,X.PdInur, Tetr.Lett., 25 (1984) 4397.
G . Centi and F. Trifiro’ (Editors),Nelv Developments in Sekctiue Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
229
CATALYTIC NITROXIDATION OF ALIPEIATIC AND AROHATIC tiyDROCABBONS BY NO (review)
G.M.
PAJONK
UniversitE Claude Bernard Lyon I, ISM,
Laboratoire de Thermodynamique et
Cingtique Chimiques, 4 3 bd du 1 1 novembre 1918, 6 9 6 2 2 Villeurbanne Cedex,
France.
SUMMARY
Catalysts containing supported NiO o r PbO are very active and selective for
the transformation of paraffins, olefins, toluene and the three xylenes in
their corresponding nitriles when NO is reacted with them.
Paraffins and olefins give unsaturated nitriles while aromatics lead to
aromatic mono and/or di-nitriles.
The reaction mechanism disclosed is of the redox type for all the hydrocarbons studied.
INTRODUCTION
The industrial way of making nitriles, especially unsaturated ones uses the
ammoxidation process (refs. 1, 2) i.e.
the reaction between the hydrocarbon and
a mixture of ammonia and air or oxygen in the presence of mixed oxides (BiMoO,
SbSnO) as catalysts. This process suffers from several drawbacks such as : very
high exothermicity, necessity to neutralize the ammonia in excess by sulfuric
acid leading to commercial products of very low values, necessity to obey
severe safety conditions because of the production of considerable amounts of
HCN and also of the use mixture of O 2 and hydrocarbons as reactants.
In this laboratory a new process was developed to synthesize the nitriles
in order to escape these drawbacks by reacting NO instead of the conventional
ammoxidation mixture (refs. 3 , 4 ) . This type of making nitriles has been called
nitroxidation and it exhibits a much lower exothermicity than the ammoxidation,
it does not release HCN and operates at reaction temperatures lower by 100 K
than the industrial ones ( 6 8 3 up t o 7 2 3 K for nitroxidation reactions).
The
nitroxidation catalysts are very specific. they are based upon NiO and PbO
oxides supported by alumina, silica or magnesia (ref. 5 ) . For instance they are
able to nitroxidize aliphatic as well as aromatic hydrocarbons (which is not
the case for the ammoxidation process), they are also actlve and selective in
ammoxidation conditions which is not true in the reverse situation.
Schematically nitroxidation consists in substituting 3 H atoms bonded to
same carbon atom of the hydrocarbon by a N one as in equation ( 1 ) :
a
230
A-..
,CH
A”
3
+ - N O 3
2
A
: a l i p h a t i c group
A’
: a r o m a t i c group
A.
A’
,CN
3
+-H
2
1
0
+-N
4
2
2
EXPERIMENTAL AND RESULTS
t h e t e s t s d e s c r i b e d were performed
All
flowing
c o n d i t i o n s and
in d i f f e r e n t i a l
in U
conversion
pyrex m i c r o r e a c t o r s under
(from
1 to
10 % ) . The
r e a c t a n t f e e d was d i l u t e d by He and i t s t o t a l p r e s s u r e was t h e a t m o s p h e r i c one.
A l l d a t a were recorded a t s t e a d y s t a t e O E a c t i v i t y u n l e s s o t h e r w i s e s t a t e d and
in t h e chemical regime. The c a t a l y s t s were a c t i v a t e d i n s i t u a t 7 1 3 K i n O 2 f o r
24 h o u r s b e f o r e any run.
GC chromatography was used o n l i n e t o a n a l y z e t h e r e a c t i o n p a r t n e r s w i t h two
d e t e c t o r s : k a t a r o m e t e r and flame i o n i z a t i o n .
Beside t h e main p r o d u c t s ( n i t r i l e s , COz, H20, N2) i t was always d e t e c t e d
t r a c e s of
NH3
in
the outlet
stream.
After
c a t a l y s i s t h e a c t i v e o x i d e was
always, p a r t i a l l y , i n a reduced s t a t e .
P r e p a r a t i o n of t h e c a t a l y s t s
The c a t a l y s t s were prepared a c c o r d i n g t o t h e s o l - g e l procedure and d r i e d a s
xero- o r aero-gel
ratios
of
1 and
(refs.
0.5
corresponding xerogels
6, 7).
A e r o g e l s c o n t a i n i n g N i O on alumina w i t h Ni/41
were
labelled
respectively
were
noted
NA
X
VIII
and
NA
VIII
X NA
V.
and
NA
The
V,
the
catalysts
c o n t a i n i n g PbO on alumina were denominated r e s p e c t i v e l y PA VIII and PA V f o r
a e r o g e l s , and X PA VIII and X PA V f o r x e r o g e l s t h e r a t i o s Pb/A1 b e i n g a g a i n
r e s p e c t i v e l y 1 and 0.5.
The pure s u p p o r t s were t o t a l l y i n a c t i v e i n t h e r e a c t i o n c o n d i t i o n s w h i l e
unsupported N i O was v e r y a c t i v e and s e l e c t i v e towards n i t r o x i d a t i o n but v e r y
u n s t a b l e w i t h time on s t r e a m w h i l e pure PbO a c t i v i t y was c l o s e t o n i l .
P r e c u r s o r s o € t h e c a t a l y s t s were r e s p e c t i v e l y n i c k e l and l e a d a c e t a t e
c o n c e r n i n g t h e a c t i v e phase and r e s p e c t i v e l y t e t r a r n e t h o x i s i l a n e and aluminium
secondary b u t y l a t e . M i x t u r e s of adequate a c t i v e and s u p p o r t p r e c u r s o r s i n a l c o o l
were cohydrolysed and d r i e d a s a e r o g e l s ( i n an a u t o c l a v e ) o r a s x e r o g e l s ( r e f .
8 ) . X e r o g e l s were prepared i n w a t e r , and d r i e d i n an oven ( i n a i r ) .
S u r f a c e a r e a s were measured w i t h N 2 u s i n g t h e BET method w h i l e XRD a n a l y s i s
was performed t o d e t e r m i n e t h e s t r u c t u r e s of t h e c a t a l y s t s .
T a b l e 1 g i v e s t h e s e d a t a f o r t h e c a t a l y s t s d e s c r i b e d i n t h e forthcoming
nitroxidation reactions.
231
TABLE 1
P r o p e r t i e s of t h e c a t a l y s t s and s u p p o r t s
2
Catalyst
XNA
NA
XNA
NA
VIII
VIII
V
V
N i O (xerogel)
A 1 0 (aerogel)
2 3
X PA V I I I
PA V I I I
X PA V
PA V
Pb 0 ( x e r o g e l )
3 4
S i n m Jg
X RD a n a l y s i s
127
208
193
350
23
254
NiO
NiO
N i O , NiA1204
NiO, NiA1204
very well c r i s t a l l i z e d
amorphous
8
84
36
132
1
Pb(0H)
Pb 04.2Pb0
baaly c r i s h j i i z e d
Pb304, PbO
very w e l l k $ i t a l l i z e d
A f t e r e a c h r u n N i o and Pb2+ were d e t e c t e d i n a l l corresponding c a t a l y s t s .
N i t r o x i d a t i o n of p a r a f f i n s
Propane and i s o b u t a n e were s e l e c t i v e l y c o n v e r t e d by NiO based c a t a l y s t s
c o n t a i n i n g a l s o chrornia or Pe203 as shown i n T a b l e s 2 and 3 r e s p e c t i v e l y ( r e f .
4).
TABLE 2
N i t r o x y d a t i o n of propane i n a c r y l o n i t r i l e and a c e t o n i t r i l e a t 753 K.
Catalysts
x
NAC 2 5
X NAC30
25
30
in
cr3+
Selectivities i n X
C 3H 3N CZH3N
co2
Activities i n
C3H3N
20
30
17
14
27
25
52
45
moles/g/s.
C2H3N
23
11
TABLE 3
N i t r o x i d a t i o n of i s o b u t a n e in m e t e c r y l o n i t r i l e , a c r y l o n i t r i l e and a c e t o n i t r i l e
a t 753 K.
Catalysts
NA V
NA V I I I
NAFS
x NAC~O
X in
or Fe
0
0
5
50
Selectivities in X
C4H5N
39
32
45
43
C3H3N
8
6
11
9
C 2H 3N
20
20
14
20
A c t i v i t i e s in lo-'
moles/g/s.
C3H3N
C2H3N
3
5
5
3
8
11
7
C4H5N
16
17
21
18
5
232
One can easily remark that a double functionalisation of each paraffin has
been obtained (insertion o f the nitrile group and creation of a double bond)
even on a pure NiO-alumina aerogel.
Nitroxidation of olefins
For both catalysts, with isobutylene or with propylene the selectivities in
metacrylonitrile or acrylonitrile were comprised between 80 and 87 % as shown
respectively in Tables 4 and 5 (refs. 3, 4).
TABLE 4
Nitroxldation of isobutylene in metacrylonitrile and acetonitrile at 683 K
Catalysts
Selectivities in %
C4H5N
C2H3N
co2
NA V
NA VIII
X NA V I I I
82
85
82
8
8
1
7
5
13
The activity of formation of metacrylonltrlle was of the order of 30.10-8
moleslgls.
TABLE 5
Nitroxidation of propylene in acrylonltrlle and proplonitrile at 683 K.
Catalysts
Selectivities in X
C3H3N
C2H3N
c02
NA V I I I
X NA VIII
NA V
PA V
77
79
75
87
9
7
13
-
11
14
8
14
The activity of the NIO based catalysts was of 150.10-8 moles/g/s towards
acrylonltrile formation and it was even greater with the PbO based catalyst
260.10-8 moles/g/s.
All the feeds containing an aliphatic hydrocarbon were characterized by a
NO/hydrocarbon ratio 1:9 excepted in the case of lsobutylene where it was a s
low as 1:2.
Selectivities are very large and in the case of lsobutylene higher than
those reported in the literature (refs. 1, 2) concerning the ammoxldation
process.
233
N i t r o x i d a t i o n of t o l u e n e
For b o t h t y p e s of c a t a l y s t s t h e s e l e c t i v i t y towards b e n z o n i t r i l e f o r m a t i o n
was s u p e r i o r t o 87 % a s shown i n T a b l e 6 and a g a i n t h e PbO c a t a l y s t s e x h i b i t e d
h i g h e r a c t i v i t i e s t h a n t h e N i O c a t a l y s t s ( r e f . 7).
TABLE 6
N i t r o x i d a t i o n of t o l u e n e i n b e n z o n i t r i l e a t 723 K.
Catalysts
Selectivities in %
Activities in
i n C6H5N
'6"gN
co 2
NA V
X NA V
NA VIII ( a )
X NA VIII ( b )
91
91
86
86
7
8
11
8
20
12
PA V
X PA V
93
95
94
88
3
2
79
57
78
22
P A VIII
X PA VIII
moles/g/s.
16
5
-
-
( a ) no s t e a d y s t a t e achieved. Values r e c o r d e r a f t e r 5 h r s of r e a c t i o n
( b ) same a s i n ( a ) b u t v a l u e s recorded a f t e r 10 min of r e a c t i o n .
From t h e d a t a c o l l e c t e d i n Table 6 i t is c l e a r t h a t t h e n i c k e l o x i d e
c a t a l y s t s a r e much l e s s s t a b l e w i t h time on s t r e a m (when t h e i r r a t i o N i / A 1 is
e q u a l t o u n i t y ) t h a n t h e c a t a l y s t s c o n t a i n i n g l e a d oxide. Another d i f f e r e n c e i s
r e g i s t e r e d between PA V and NA V concerning t h e t r a n s i e n t s t a t e heEore r e a c h i n g
t h e s t e a d y one : PA V i s much more r a p i d l y a t s t e a d y s t a t e t h a n NA V.
These f a c t s emphasize once more t h a t l e a d o x i d e s u p p o r t e d by alumina
d e v e l o p s v e r y good c a t a l y t i c p r o p e r t i e s c o n t r a r y t o i t s u s u a l r e p u t a t i o n .
N i t r o x i d a t i o n of
0,
p and m xylene
Orthoxylene is transformed a t 673 K i n t o t h e m o n o n i t r i l e ( o r t h o t o l u n i t r i l e )
w i t h a s e l e c t i v i t y of more t h a n 90 % on N i O a s w e l l a s on PbO c a t a l y s t s .
But t h e d i n i t r i l e ( p h t a l o n i t r i l e ) i s o b t a i n e d s e l e c t i v e l y ( S
at
713 K and
o n l y w i t h t h e PbO t y p e c a t a l y s t s .
b e n z o n i t r i l e ( S = 20 %)
and o r t h o t o l u n i t r i l e
=
20 %)
only
The o t h e r p r o d u c t s b e i n g
(S = 44 %). Thus t h e t o t a l
s e l e c t i v i t y i n t o n i t r i l e s i s of t h e o r d e r of 84 % n e v e r t h e l e s s .
Again
the
metaxylene
is
selectively
(S
>
m e t a t o l u n i t r i l e a t 673 K on b o t h k i n d s of c a t a l y s t s .
catalysts
are
able
ophtalonitrile (S
=
to
convert
the
hydrocarbon
90
% ) converted
into
But once more o n l y PbO
into
the
,dinitrile
(is
13 %) a t 713 K. The o t h e r p r o d u c t s being m e t a t o l u n i t r i l e (S
= 68 X ) and b e n z o n i t r i l e
(S
-
11 %). The t o t a l s e l e c t i v i t y i n n i t r i l e s is of 92
% f o r t h e c o n v e r s i o n of metaxylene ( r e f . 9 ) .
234
Finally paraxylcne is equally well converted into paratolunitrile ( S
= 42
X ) on both types of catalysts at 6 7 3 K, but again only PbO is able to convert
this xylene
into
terephtalonitrile at
713
K with
in
this case a good
selectivity in the dinitrile ( S = 4 3 % ) which i s here the major product of the
reaction (benzonitrile S
= 25 %
and paratolunitrile S
= 18 % ) .
Therefore i t seems that PbO is even better than chromium oxide based
catalysts for the same conversion which give only a selectivity of 5 % in
terephtalonitrile, the major products being paratolunitrile (S = 6 5 X )
benzonitrile ( S = 16 %)(ref.
Nitroxidation of
0,
and
10).
m and p tolunitrile
It was of interest from a mechanistic point of view to convert the three
mononitriles in the corresponding dinitriles in order to assess if
two step(s)
a
one o r a
reaction mechanism (ref. 11) i e working during the catalysis of
xylenes conversions.
OKthOtOlUnitKile was not transformed into phtalonitrile but into benzonitrile, benzene and C02 whatever the catalysts or the reaction conditions.
However PAV catalyst was able to give isophtanonitrile from metatolunitrile with a selectivity of 16 % at 713 K but NA V was incapable to give the
dinitrile.
Finally paratolunitrile gave terephtalonitrile only in the presence of PA V
catalysts with a selectivity of 87 %, the other nitrile product as benzonitrile
(ref. 11).
In summary it can be said that the three xylenes can be selectively
converted in the mononitriles on both catalysts (NiO and PbO) but only lead
catalysts give selectively the dinitriles.
DISCUSSION
Proposed reaction mechanism
Independently of the nature of the hydrocarbons tested in this work, a
redox mechanism is able to explain the whole kinetic results as follows :
-
k
Hydrocarbon + Oxidized Cat -fj Reduced Cat + Adsorbed dehydrogenated
hydrocarbon adsorbed (releasing up to 3 hydrogen atoms).
-
NO
+
k
Reduced Cat 0
,Oxidized Cat
+
N(adsorbed).
The next step, a fast one, is the combination of the dehydrogenated
hydrocarbon species with the N atoms giving the corresponding nitrile.
When aliphatic hydrocarbons are involved the adsorbed species is of a
dehydrogenated
n-ally1 type and when aromatics are the reactants then the
adsorbed species is of a dehydrogenated benzylic type.
235
The f o r m a t i o n of t r a c e s
Of
N H 3 i n t h e e f f l u e n t g a s was c o n s i d e r e d a s an
i n d i r e c t proof of t h e d i s s o c i a t i v e a d s o r p t i o n of NO g i v i n g N a s adatoms (and
f i n a l l y t h e n i t r i l e ) and 0 adatoms (which o x i d i z e t h e reduced c a t a l y s t s ) . A
check of t h i s i d e a was performed by t r y i n g t o c o n v e r t NO by H2 i n t o NH3 on b o t h
t y p e s of
c a t a l y s t s which was indeed observed,
while
i t was
impossible t o
t r a n s f o r m t h e c l a s s i c a l ammonia syngas on t h e same c a t a l y s t s .
The a s c e r t a i n t h e p o s s i b i l i t y of t h e redox mechanism, v a l u e s of ko and k,
were measured f o r a s e r i e s of f o u r n i t r o x i d a t i o n s and c o l l e c t e d i n T a b l e 7.
TABLE 7
Values of ko, kr f o r n i t r o x i d a t i o n r e a c t i o n s .
Arbitrary
units
N i t r o x i d a t i o n of
Isobutane
Propylene
Isobutylene
Toluene
k
ko
Rgf e r e n c e s
0.66
0.19
3.33
2.44
4.7
1.4
(6)
7.5
3.5
(11)
(3)
(12)
The v a l u e s of ko, k r f o r e a c h converted hydrocarbon are v e r y c l o s e t o e a c h
o t h e r which is a good f i t of t h e model. Moreover i t is clear t h a t i n e v e r y c a s e
ko
>
kr which means t h a t r e o x i d a t i o n i s easier t h a n r e d u c t i o n of t h e c a t a l y s t s
and t h i s remark i s i n good agreement w i t h t h e composition of t h e r e a c t a n t f e e d s
always r i c h e r i n hydrocarbons ( w i t h r e s p e c t
t o NO) t h a n t h e s t o e c h i o m e t r i c
ones.
COMPARISON BETWEEN AMMOXIDATION AND NITROXIDATION
I t was checked t h a t t h e n i t r o x i d a t i o n c a t a l y s t s were a b l e t o g i v e n i t r i l e s
i n t h e ammoxidation c o n d i t i o n s (no NO). The s e l e c t i v i t i e s e x h i b i t e d i n n i t r i l e s
were of t h e o r d e r of 30-40 X only.
The
conventional
ammoxidation
catalysts
such a s
Sb-Sn-0.
Bi-Mo-0,
were u n a b l e t o g i v e n i t r i l e s i n t h e n i t r o x i d a t i o n c o n d i t i o n s . They
V205/A1203
were a l s o i n a c t i v e i n t h e conversion of NO by H 2 i n t o NH3. T h e r e f o r e i t is
possible
t o claim
that
a
necessary
(but
not
sufficient)
condition f o r
a
c a t a l y s t t o be s e l e c t i v e i n n i t r o x i d a t i o n i s i t s a b i l i t y t o d i s s o c i a t e NO i n t o
N and 0 s p e c i e s .
CONCLUSIONS
To c o n v e r t a l i p h a t i c s i n t o u n s a t u r a t e d n i t r i l e s is p o s s i b l e on N i O a s w e l l
a s on PbO based a e r o g e l s o r x e r o g e l s . G e n e r a l l y speaking t h e a e r o g e l s a r e more
a c t i v e t h a n t h e corresponding x e r o g e l s .
Aromatics l i k e t o l u e n e i a e a s i l y transformed i n t o b e n t o n i t r i l e by b o t h
t y p e s of c a t a l y s t s w h i l e PW c a t a l y s t s are more e f f i c i e n t and s t a b l e w i t h time
on s t r e a m i n o r d e r t o c o n v e r t s e l e c t i v e l y t h e x y l e n e s o r t h e m o n o t o l u n i t r i l e s .
236
It i s worth to mention the particuliar good catalytic nitroxidation
properties exhibited by catalysts containing PbO.
REFERENCES
1
T.
Dumas,
W.
Bulani,
Oxidation
of
Petrochemicals
Chemistry
:
and
Technology, Applied Science, Londres, 1974.
2
D.J.
Hucknall,
Selective
Oxidation
of
Hydrocarbons,
Academic
Press,
Londres, 1974.
3
F.
4
F. Zidan, G. Pajonk, J.E.
5
V.M.
Zidan, G. Pajonk, J.E.
(1978)
Germain and S.J.
Teichner, J .
Catalysis, 52
133-143.
Germain and S.J.
Teichner, 2. Phys. Chem.,
111
( 1 9 7 8 ) 91-103.
Belousov, V.V.
Korovina, M. Ya. Rubanik, Kataliz i Katalizatory, V o l .
6, Naukova Dumka, Kiev, 1970, 89-100.
Grinenko, V.M.
S.B.
Belousov, Kinetika i Kataliz, V o l .
15 ( 1 9 7 4 )
n o 2,
522-524.
V.M.
Grinenko, Kataliz i Katalizatory, Vol.
Belousov, S.B.
14,
Naukova
Dumka, Kiev, 1976, 27-31.
Teichner, Bull. Soc. Chim. France, 1976,
6
G.E.E.
7
S.
8
G.M. Pajonk in Proceed. 2nd Int. Symp. on Aerogels in press. Les Editions
9
S.
Gardes, G. Pajonk, S.J.
1321-1326.
Abouarnadasse,
G.M.
Pajonk, J . E .
Germain
and S . J .
Teichner, Appl.
Catal;., 9 ( 1 9 8 4 ) 119-128 ; J . Chem. Eng., 62 (1984) 521-525.
de Physique, Paris 1989.
Abouarnadasse, G.M.
237-247
1936-1943.
10
S.
Pajonk and S . J .
; Proceed. 9th ICC Calgary, M . J .
Teichner, Appl. Catal., 16 (1985)
Philips and M.
Ternan Eds, 4 , p .
The Chemical Institute of Canada, Ottawa, 1988.
Zine, A. Sayari and A. Ghorbel, Can. J . Chem. Eng. 65 (1987) 127.
11 S. Abouarnadasse, G.M.
Pajonk and S . J . Teichner in "Heterogeneous Catalysis
and F i n e Chemicals", M. Guisnet et al. Eds, Elsevier, Amsterdam, 1988, p.
371-378.
237
B. DELMON (Universite Catholiquede Louvain, Belgique). I have some reservation with respect to
your emphasis on a redox mechanism. Ni and Pb are extremely different with respect to oxidoreduction behaviour. On the other hand, both metals could interact with alumina, adjusting adequately
the acidity of the latter, thus explaining the similitude of the catalytic behaviour. I suggest the role of
acidity could be investigated.
G.M. PAJONK (UniversitC Claude Bernard, France). It has been shown as reported in (ref. 1) that
the acidity did not play a role at steady state at least in the case of the synthesis of methacrylonitrile
(from isobutene and NO) upon the selectivitiesinto the nitrile.
The acidity seemed to intervene only during the transient period before reaching the steady
state, the greater the acidity of the catalyst the shorter the transient period and the lower the
simultaneous degradation activity during this regime.
Moreover the presence of traces of NH3 in the outlet stream allows to assume that the acidity
is probably neutralized at steady state.
1 A. Sayari, A. Ghorbel, G.M.Pajonk and S.J. Teichner, Bull. SOC. Chim., 16, 1981 (see also
reply to G. Golodets).
R. CHUCK (Lonza A.G., Switzerland). 1. Is nitroxidation limited to -CH3 side-chains, or can
longer-chain akyl groups be oxidized ?
2. What is the % of NO in the exhaust gas ?
3. Is there a possible loss of Pb/Ni in the environment ?
COrnDare to ammoxidation :(with e.g. V/Ti catalysts)
No NO in atmosphere (recyclingof NH3 necessary)
No heavy metal problems
Is not limited to methyl side-chains.
G.M. PAJONK (Universitb Claude Bernard, France). No experiment was performed on other
aromatics than the xylenes. Ethylbenzene is under study at present,
No NO is detected in the exhaust gas, only N2, N 2 0 are present, probably resulting from the
disproportion of NO (which is observed with pure NO over the catalysts).
As the same steady state is observed at least for tens of days it is likely that no loss of Pb or
Ni occurs during catalysis.
Compared to ammoxidation I agree with the comments which can also be made for
nitroxidation with the exception of the last point (in progress now).
J. OTAMIRI (University of Lund, Sweden). In your paper you stated that a necessary condition for a
catalyst to be selectivein nitroxidation is the ability to dissociateNO into N and 0.
V2O5 is a known catalyst for NOx reduction and hence should meet your requirement,
however nitroxidation does not occur on it.
Will it not be more appropriate to suggest that the necessary condition will be for the catalyst
to be able to form NH3, or at least NH3-precursors at the surface ?
G.M. PAJONK (UniversitC Claude Bernard, France). The study presented here involved only NO
(and not the NOx as a whole). For example N 2 0 (instead of NO) resulted in a total oxidation of the
hydrocarbons and it was checked directly that the catalysts were unable to synthesize N H 3 from a N2
+ H2 mixture whereas NH3 was obtained quantitativelywith a NO + H2 feed.
No attempt at reacting NO + H2 on a V2O5/Al2O3 aerogel catalyst was carried out. This type
of catalyst was indeed not selective towards the nitroxidation reaction (ref. 1).
1 S.Abouamadasse, Ph.D. Doctoral Dissertation Lyon 1986, no 86-46 (France).
238
F. VAN DEN BRINK @SM Research BV, Netherlands). 1. Experimental results presented were
obtained in a differential reator, so presumably conversionsof the hydrocarbon were low (< 10 % ?).
Could you indicate the dependance of the selectivity upon conversion ?
2. What was the ratio Nohydrocarbon used and how does this influence selectivity and
yield ?
3. Comment : HCN is a valuable by product from the production of acrylonitrile ;toxicity of
acrylonitrile is also very high, although not as high as of HCN. The fact that HCN is not a by product
of the nitroxidation is therefore hardly an advantage.
G.M. PAJONK (UniversitC Claude Bernard, France). The conversion used in this work varied
between 1 and 10 - 15 96 (at most). No systematic study was performed at higher conversions.
The NO-hydrocarbon ratio was of the order of 1:8 for aliphatics and 1:3 for aromatics. Only
under conditions where the hydrocarbon was in a fairly large excess with respect to stoechiometry
were the selectivities as high as reported even in the case of conversions reaching a value of 10 %
(yields were of the order of 9 96). This is also true when one considers the stability with time on
stream exhibited by both types of catalysts.
Now considering ammoxidation, if HCN is produced only under the form of traces for
instance then the severe safety conditions necessitated by the process are very expensive for a poor
yield and on the contrary if HCN is obtained in large amounts then it is at the expense of the desired
product and therefore it competes with the well known Andrussow'sprocess.
G. GOLODETS (Institute of Physical Chemistry, URSS). 1. Have you any idea on the reasons why
PbO is a better catalyst for the nitroxidation ?
2. What are the experimental evidences in favour of the proposed mechanism ?
G.M.PAJONK (UniversitC Claude Bernard, France). The reasons why PbO based catalysts are
better than the NiO ones are not yet known.
The arguments of favoring a redox mechanism are based on the observation that during
catalysis Ni2+ is at least partially, reduced in NiD(ferromagnetic properties) and Pbde is also reduced
in Pb2+ cations as seen from XRD analysis.
By flowing the hydrocarbons (without NO) over our catalysts reduction was always recorded
and subsequently shifting to NO (without hydrocarbon) resulted in a reoxidation of the reduced form
of the catalyst, see ref. 1 for instance for chromia-alumina aerogel using EPR spectrometry.
1 H. Zarrouk, A. Ghorbel, G.M. Pajonk and S.J. Teichner, Procedings, IXth Ibero American
Symp. on Catalysis, Lisbon, 1984, 339.
G . Centi and F. Trifiro’ (Editors), New Deuelopments in Selective Oxidation
01990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
239
SGUCTIVE OXIDATION OF HYDROCARBONS BY UIETRIC OXIDS TO NITRILES
V.M.BELOUSOV a d S.B.GRINENK0
The L.V.Pisarzhevski I n s t i t u t e of Physical Chemistry, Academy
of sciences of Ukrainian SSR, 252028, Kiev (USSR)
SUMgdARY
The oxidation of 20 hydrocarbons, having d i f f e r e n t s t r u c t u r e ,
by n i t r i c oxide has been i n v e s t i g a t e d on composite l e a d oxide cat a l y s t s . The CH -group, conjugated with a double bond o r t h e a m matic ring, i s &tacked by NO t o form cyan0 group. N i t r i c oxide
is reduoed t o N 0 and N
Pb-Ti-0, Pb-Sn-0 *+Pb-Z+O
syetems
acts aa an a c t i v e
proved t o be th8 most e f f e c t i v e c a t a l y s t s . Pb
component.
.
INT RODUCT IOM
The Heterogeneous catalytic interaction between n i t r i c oxide
and hydrocarbons l e a d s t o the formation of the n i t r i l e s of carboxylic acids. For example, a c r y l o n i t r i l e ie formed from propene
aud b e n e o n i t r i l e from toluene t
C%nCH-CH3
+ 1.5NO I C%mCH-CN + 1.5H20 + 0.25N2
C6H5-CH3
+ 1.5NO I CGH5-CN + 1.5%0 + 0.25N2
The reaction of n i t r o x i d a t i o n of hydrocarbons i s more s e l e o t i ve in n i t r i l e s than the oxidative aormonolisie of hydrocarbons.
RGSULTS AND DISCUSSION
Catalyst 8
The r e a c t i o n i s catalysed by s i l v e r (refs. 1.2) end oxides of
s e v e r a l metals. In the n i t r o x i d a t i o n of propene we have i n v e s t i gated 17 metal oxide8 as cataly8t8, which, by t o t a l coacnrmption
of NO at 673 I, are arranged in the order (ref. 3):
CUO 7 b 0 2 > Co203 7 V205 7 C r 2 0 3 7 Fe203 > N i O > B i 2 0 3 7 ZnO =U 0 7 PbO 7 Sn02 7 T i 0 2 7 Z r 0 2 > W03 7 Sb20q 7 M O O ~ .
3 8
The y i e l d of a c r y l o n i t r i l e a t 673 K decreases i n the s e r i e s :
Co203 7 Bi203 7 V205 > Mn02 7 N i O 7 PbO 7 Fe203 7 CuO > Cr203>
ZnO 7 Sn02 > NO3 2 U308 > Sb204 > Ti02 7 Zr02.
However a t higher temperatures the lead, n i c k e l and zinc oxides
are the most a c t i v e in a c r y l o n i t r i l e formation. Our r e s u l t s correl a t e with correspondens d a t a on the oxidation of propene by oxy-
3r
v205
2 -
0
,
1
1
2
3
4
lPGC3H6
5
+
6
I0
Productivitg of propene consumption (GC .)
i n the oxidation of propene by O2
3 b
Fig. 1. The c o r r e l a t i o n between the c a t a l y t i c a c t i v i t y of dif-
f e r e n t oxides in propene n i t r o x i d a t i o n at 673 K and i n propene
oxidation by oxygen at 573 R.
gen (Fig. 1 ) . However, there a r e some differences:
1 The r a t e of propene n i t r o x i d a t i o n is lower, then of i t s oxidat i o n by oxygen.
2 The reduction of n i t r i c oxide proceeds by a parallel-consecut i v e scheme:
3 The most s e l e c t i v e c a t a l y s t s for the one reaction have poor
s e l e c t i v i t y i n the other and vice versa.
Since lead oxide proved t o be the most s e l e c t i v e among individual oxide8 its c a t a l y t i c properties were investigated i n some
detail. The dependence of the a c t i v i t y and s e l e c t i v i t y of propene n i t r o r i d a t i o n on the valent s t a t e of lead in oxides was invest i g a t e d by the nonstationary c a t a l y s i s method (ref. 4). It has
been shown, t h a t the s e l e c t i v i t y of propene conversion t o acrylon i t r i l e on P b O I s higher than that on Pb02. On the other hand,
Pb02 is more a c t i v e , than PbO by an order of magnitude. The pro-
241
TABU ?
Phase composition snd c a t a l y t i o a c t i v i t y of lead-titanium oxide
c a ta l y s t s
Composition
NO
Chemical
PbO
1
2
9Pb0.Ti02
3
3Pb0.Ti02
4 Pb0.1'i02
5 Pb0.3Ti02
6 Pb0.9Ti02
7
?I!
Ti02
Phase
-
Acrylonitrile
productivitp
Phase PbO yellow, rhombi0 modif i o a t i o n (I).
(I) m a i n phase. Admixed phase
PbTiO perovskite s t r u c t u r e (11)
3
(11) -in P ~ S Small
.
mount
of u n i d e n t i f i e d phase.
(11) clean
(11) maln phase. Ti02 i n a small o r quantity.
(If) and Ti02 i n comparable
amounts.
Ti02 r u t i l e e t r u c t u r e
- -
-
-
Seleotivity
calculated f o r
6.0
58
5-5
60
5.0
65
3.8
2.0
35
30
0.8
28
0.2
12
713 K, r e a c t i o n mixture: 30 Vole% c3H6, 10 Vole% NO, N2 is
the rest.
p e r t i e s of the i n i t i a l oxides 81% equalized a8 the number of rea c t i o n m i x t u r e pulses f e d t o c a t a l y e t s is increaeed. A f t e r 3
p u l s e s Pb02 markedly reduces t o PbO. In t h i s c a m the s e l e c t i v i t y
of a o r y l o n i t r i l e formation increases sharply and, hence, the con5 pulversion of propene deoreaees. On PbO, during the f i r s t 3
s e s , t h e s e l e c t i v i t y a l s o somewhat increases due t o the removal
of the chemisorbed o d d a n t . Moreover, the a c t i v i t y of PbO drops
becauee in the cour8e of c a t a l y s i s the a c t i v e surface area deorea s e s ( r e f . 3).
Composite lead-titanium, l e a d - t i n aud lead-zirconium oxide cat a l y s t s are more s t a b l e and as e f f i c i e n t aa PbO ( r e f s . 5 , 6 ) . Tabl e s 1 and 2 represent phase corqposition of these c a t a l y s t s . The
comparison of c a t a l y t i c p r o p e r t i e s and phase composition indicat e e that the c a t a l y t i c a c t i v i t y is i n a g r e e m n t with the amount
of PbO, PbTi03 and Pb2SnOq pharres. Thus, Pb2+ cationee are respon s i b l e f o r c a t a l y s i s , while the second component in the composite
c a t a l y s t s ensures the a t a b i l i s a t i o n of the lead c a t i o n i n the val e n t s t a t e of two. Moreover, the r e f r a c t o r y tin and titanium oxide phases prevent the o a t a l y s t s from s i n t e r i n g .
-
242
TABLE 2
Phase composition and c a t a l y t i c a c t i v i t y of lead-tin oxide
catalysts
Acrylonitrile
produativitJT
mol/m* * s
x108
Composition
No
Chemical
Phase
-
6 Pb0.3Sn02
7 PbO.9SnOi
Phase PbO yellow, rhombic
modification (I).
(I) main phase. Smal m o u n t
o r pb SnO i s o s t m c t u r a l t o
red d a d PIII).
(111) main phase. (I) admixed phase. There ie smal
amount of unidentified comound (IV).
main phase. Sn02 admixed phase
Sn02 = tetragonal, isoetructural t o r u t i l e , main phase. ddmixture of an unidentified compound.
The same
Solid s o l u t i o n based on phase
8
Phase sno2
1
PbO
2
9Pb0.Sn02
3 3Pb0.Sn02
5
T'
PbO.2SnO2
-
Sn02
-
-
-
-
-
SnO,
713 IC,reaction mixture: 30
the rest.
6.0
58
6.5
30
8.0
30
8.0
30
5.6
30
4.0
30
3.2
30
0.6
VOL%
C3H6, 10
Selectivity
calklated f o r
NO
s m pconted
%
VOL%
5
NO, N~ is
Reactivity of hydrocarbons
W
e have studied the i n t e r a c t i o n of n i t r i c oxide w i t h 20 hydrocarbone of d i f f e r e n t s t r u c t u r e on s i l v e r end composite lead
oxide c a t a l y s t s (refs. 7 , 8 ) . Some r e s u l t s are given i n Table 3
and Figure 2. The r e g u l a r i t i e s in the influence the s t r u c t u r e of
hydrocarbons on t h e i r r e a c t i v i t y i n n i t r o x i d a t i o n BPB e s s e n t i a l l y similar t o those observed in t h e i r heterogeneous c a t a l y t i c
oxidation by oxygen. In the both cases the two reactions were
found t o proceed: the complete oxidation t o C02 and H20 and the
s e l e c t i v e oxidation of CH3-group, conjugated with a double bond
o r the aromatic ring, t o e i t h e r a cyan0 o r carboxylic group.
The observed r e g u l a r i t i e s may be formulated as f o l l w s :
1 me unsaturated a l i p h a t i c hydrocarborn a r e oxidized f a s t e r
than paraffins; the r a t e of oxidation increases in the order:
paraffins < monoolef ines < aoe t i l e n e s .
243
TABLE 3
The products of the oxidation of hydrooarbone by nitric oxide on
silver and on compoeite lead oxide catalysts
Hydrocarbon
Product8
B thene
HCN, CO2, H20
acrylonitrile
acrylonitrile, aoetonitrile
the 6the same
the same
the same
H20
the 88me
the same
the same
bensonitrile
p--olunltrile, tere&talod-dtrile,
benzonitrile
m-Xylene
m-tolunitrile, ieophtalodinitrile,
beneonitrile
o-Xylene
o-toluitrile, phtalodinitrile,
beneonitrile
p-Chlorotoluene p-chlorobenzonitrile, benzonitrile
o-Chlorotoluene o-chlorobenzonitrile, benmnitrile
p-Tolunitrile
terephtalodinitrile, benzonitrile
m-Tolunltrlle
lsophtalodinitrlle, benzonitrile
o-Tolunitrile
phtalodinitrile, benzonitrile
Propene
i-Butene
n-Butene
Pentene 1
Hexene- 1
Isoprene
Hexane
Cyclohexane
Pentane
Benzene
Toluene
p-Xylene
-
9,
Selectivity
in the sum
of nitriles
mol %
5
80
50
30
20
20
10
0
0
0
0
98
89
94
85
20
30
80
80
70
In the aliphatic hydrocsrbon homologous row the rate of oxidation Increases with inoreasing of the number of carbon
atoms, for example: ethene < propene
butene c pentene-1.
The rate of oxidation of olefine increases on branching:
n-butene < i-butene.
The substitution of the hydrogen atom in the aromatic ring by
a chlorine atom, a C h or CH3-group increases the reactivity
of the molecule and the conversion of hydrooarbon Inoreases:
beneene < toluene < o-, m-, p-xylenes<o-, m-, p-toldtrlle6 i
244
;.:I/
,“I
4/
0.8 r
0.4
0
01
kp
100
1
700
740
1
780 T,K
“I/
20
700
,
?@
,
?60T,K
G
24
48
20
40
16
32
12
24
8
f6
4
8
0
0
200
12 0
40
9
700
740
I
780 T,K
F i g . 2. The n i t r o x i d f l t i o n
of
k toluene: 1 - benzonitriC O , 3 - N20,
le,2
4 benzeze;
B o-chlorotoluene: 1
T’J20,
2
t o l u e n e , 3 - benzonitrile, 4
o-chlorobenzonitrile, 5
coZ;
C p-chlorotoluene: 1
N2C,
2
toluene, 3
benzop-chloronitrile, 4
benzonitrile, 5
o-chlorobenzonitrile,
C a t a l y s t 2Pb0.Sn02 7 c m ,
[hydrocarbon] = 5 vo1.%,
~ N O ] = 35 v o ~ . s ,
space v e l o c i t y = 55 cm3 /min.
- -
-
-
-
-
-
-
-
245
toluene L=o-chlorotoluene <p-chlorotoluene.
The reactivity of hydrocarbon8 correlates with their ionisation potentials o r the $-electron charge in the reacting CH2-groups of the corresponding radicals (refs. 7,8).
Mechanisms and kinetics
The stage me&ani8me of the formation of the products in the
nitroxidation of propene (ref. 91, toluene (ref. 101, xylenes
(refs. 11,12) aad other hydrocarbons are described by parallelconsecutive schemes. However, the kinetic8 equations may be derived on the basis of a simplified schemes. For example, in the case of propene nitroxidation on the lead-titanium oxide catalyst
the parallel formation of acrylonitrile (AN) and C02 predominates. The experimental data are expressed by the equations:
d [AN]
f
dt
d fNOl
-L.
dt
On the aame catalyst the nitroxidation of toluene i8 described by the sy8tem of equations based on the consecutive scheme:
kl
k2
C H CH
CgHgCN
COZ + H20 i
6 5 3
-
at
at
dt
at
Thus, the application of nitrio oxide as a reactant is promis i n g , because it allow8 to obtain useful product8 and to develop
new technological processes.
REFBMNCES
1 US patent
2736729, RZhKhim, (19601, 108808P.
246
2
3
4
5
6
7
8
9
10
11
12
-
I.Ya.Mulik, M.Ya.Rubanik, V.M.BO~OUEOV,Kataliz i katalizatoVol. 3 , Naukova dumka, Kiev, 1967, pp. 121 128.
V.M.Beloueov, V.V.Korovina, M.Ya.Rubanik, Katalit i katalizatory, Vol. 6, Naukova dumka, Kiev, 1970, pp. 89-96.
A.S.Plachinda, V.Ed.Belousov, Ukr. Xhim. 5urn., 39, (1973),
975 -978.
V.M.Belowov, D.B.Dulin, A. I.Gelbschtein, S.S. Stroeva, V.V.
Korovina, V.S.Roginskaya, Kataliz i katalizatory, Vol. 10,
Naukova dumka, Kiev, 1973, pp. 37 42.
V.M.Beloueov, D.A.Dulin, A.I.Gelbechteia, S.S.Stroeva, V.V.
Korovina, V.S.Roginskaya, Kataliz i katalizatory, Vol. 11,
Naukova dumka, Kiev, 1974, pp. 123
128.
V.M.Belousov, ltl.Ya.Rubanik,, I.Ya.NhiLik, m e t . i Katal., 10,
(19691,a41 846.
V.I.Beloueov, S.B.Grinenko, Kataliz i katalisatory, Vol. 14,
Haukova dumka, Kiev, 1976, pp. 27 32.
I.Ya. ldulik, V. M. Belou~ov,V. V. Korovina, A. V. Gerechingorina,
RI.Ya.Rubanlk, gatalia i katalizatory, Vol. 5, Naukova dumka,
Kiev, 1969, pp. 46
51.
V.M.Belousov, Kataliz i katalizatory, Vol. 26, Naukova dumka,
Kiev, 1989, pp. 8
18.
S.B.Grinenko, V.M.Belousov, Dokl. AN Ukr. SSR, 882. B, (19731,
1028
1031.
V.P.Bodrov, V.M.Belousov, S.B.Grinenko, Dokl. AN Ulcr. SSR,
aer. B, (19751, 317
320.
ry,
-
-
-
-
-
-
-
G. Centi and F. Trifiro' (Editors), New Developments in Selective Oxidation
0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
N.T. Do, R. Kalthoff, J.
247
Laacks, S . Trautmann and M. Baerns
Ruhr-University Bochnn, POB 10 21 48, D-4630 Boch\Hn
s-
The oxidation of anthracene was studied on different unsupported V/Mo/P
oxides catalysts as w e l l as on almina- and silica-supported catalysts a t 673
and 723 K. Selectivity of the reaction t o anthraquincne and phthalic anhydride
was affected by catalyst c a p x i t i o n , support mterial and temperature. A kinet i c reacticn scheme was set up and the kinetic parameters were determined. Adsorbate structures of anthracene, anthraquincgle and phthalic anhydride an both
the supported catalysts and m the pure support materials were derived fran ins i t u FTIR transmission spectnsscopic measurermlts.
INTRca3ucTIoN
Polycyclic aranatic hydrwarbns such as anthraoene, phenanthrene and fluom e being formed in coal pyrolysis may be used as feedstocks f o r producing guinones and dicarboxylic anhwides. ccmversion of s a w of the mopounds by heterqenmus catalytic gas-phase oxidation or in the liquid phase with chrun.ic acid
are w e l l knckJn technologies (ref. 1).For the gas-phase reaction, mixtures of
vanadium oxide and the oxides of molykdenm, mganese (ref. 21, tungsten (ref.
3) , iron, alkali ( r e f . 4) and phosphorus are used. as oatalytic active canp e n t s ; al-,
silica and titania are often applied as support materials
(refs. 5-7). In the present work anthracene has been subjected t o catalytic gas-
phase oxidation t o study the effect of catalyst ccnq?ositian and of the s u p p r t
material on selectivity; furthemre a kinetic m c t i m scheme is proposed for
characterization of the catalysts by kinetic pameters. Finally, adsorbate
structures of anthrame, anthraquinone and phthalic anhydride were determined
fran in-situ I R spectroscopic measurements. The investigations a h a t a better
understandkg of the fundamentals of t h e anthracene oxidatian.
EwERlMENTAL
Preparation of catalysts
Vanadium oxide was med as base c a p n e n t for the catalysts; it was W i f i e d
by adding m0lyMenum oxide arid phosphoric acid; in sane instances silica and
almina were applied as support materials. Vanadium oxide was dissolved in concentrated hydrochloric acid as reducing agent a t 80'C for 2 h. Sukequently, m e
lybaenum oxide and phosphoric acid were added. When preparing fllpported catalysts the carrier material was dispersed in the afore mentioned solution. After
248
solvent evaprization t h e solid material was dried a t 1 2 0 ' ~and subsequently
calcined a t 500'C for 16 h. The canpositions and surface areas of the unsupported catalysts and the supported ones used in the oxidation of anthracene are
given in Table 1.
Apparatus
A schematic diagram of the apparatus used for catalytic testing is given in
Fig. 1. Anthracene was oxidized in an electrically heated fixed-bed quartz react o r (length 300 mn, I . D . 8 m n ) . Axial t a p e r a t m e profiles in the catalyst bed
were measured by a mvable thermxouple. Anthracene and s a w of t h e o w e n a t e s
were analyzed by on-line GC. A l l condensable products of the effluent fran the
reactor were collected a t room tmperature and analyzed by off-line GC and HPLc
(ref. 8). The carbon oxides a3 and CO, were determined by cn-line Gc.
'* '+
Heated cwriw oil
I
Fig. 1. Scheimtic diagram of the apparatus for catalytic testing (A: capillary
flaw meter, R: fixed hed reactor, F: separator, S: saturator for anthracene).
For measuring I R transmission spectra a FTIR s p e c t m t e r (Perkin-Elmer &el
1710) was used. T?m
. identical I R cells made of quartz were incorporated into the
spectrcmeter. A schematic diagram of the I R cell which could be used as a react o r when the catalyst was inserted is sham in Fig. 2.
The I R cell was c a p x e d of a cylindrical quartz tube (length 100 mn, E.D. 35
mn) which was sealed on both ends by NaCl wind-.
The catalyst sample, hold in
a guartz frame, was kept i n a fixed position by guide ledges containing heated
fihments for direct heating of the catalyst up t o 773 K.
The catalyst pm3er was pressed a t 32 bar t o a 10x30 mn specimen of about 1030 n q / a n 2 . A continuous gas stream of about 30 l/h loaded w i t h anthrame ( 0 . 1
vol.%) was passed through both cells one containing the catalyst sample. Both
cells =re alternativelymved into the I R beam; hereby the spectrum of the gas
249
phase surrounding the catalyst m u l d be eliminated. The adsorbate spectrum was
then obtained by dividing the transmittances obtained for the catalyst plus the
adsorbate by the respective transmittances of t h e clean catalyst, i.e. without
adsorbate as measured before adsorption (refs. 9, 10).
dn
10
1: quartz cylinder
2: sample holder
3: heated filament
I Ill I
4: guide 1
5: thermocouple
6: gas i n l e t s / o u t l e t s
7: NaCl windaw
8: graphite washer
9: v i a ring
10 : pole
11 : Al-ring
10,
Fig. 2. In-situ I R cell.
RESULTS AND DISCUSSI@I
Catalytic testing
Oxidation of anthracene (0.25 vol.3 in a i r ) was carried out using the catalysts listed in Table 1 (grain size: 0.5 t o 0.7 mn) a t 673 and 723 K. The
concentrations of anthracene and of t h e products were m u r e d as a function of
contact time (qarfi).
Therefran, the depdence of the s e l e c t i v i t i e s on anthracene conwrsion was derived. Moreover, a reaction scheme w a s set up. Assming a
LO
-
a:
20
5
10 0 '0
Anthracene
+ 9, 10-Anthraquinone
o 1, 4-Products
*Phthalic anhydride
20
LO
60
80
100
XI%
miV
+ 9, 10-Anthraquinone
XCO,
Fig. 3. Dependence of the partial
pressures on amtact th? at 723 K.
Catalyst: V:kb = 4.17; P:V = 0.11.
1, 4-Products
t Phthalic anhydride
x
cox
Fig. 4. Wpendence of the selectivities
on anthracene mversicn a t 723 K.
Catalyst: V:Mo = 4.17; P:V = 0.11.
250
first-order reaction with respect to the hydrocarban canpourdis the kinetic paramters were detennined characterizing catalyst p e r f o m c e .
For illustraticn, a typical dependence of the partial pressures on contact
the is given in Fig. 3 for a selected catalyst (synbols: mebsufed data, lines:
calculated according to the kinetic data, which are reprted further belaw); the
corresponding depenaenCe of t k selectivities on anthracene conversion is presented in Fig. 4.
The pattern of the relaticnships shc%-in
in Fig. 3 and 4 indicate that 9,lOand lf4-anthraquinoneas w d l as the carbon oxides can be considered as prirrary
prcducts. With increasing anthracene conversion 9,lO-anthraquinone is further
oxidized to phthalic anhydride under simultaneous fonnaticn of carbon oxides.
Reacticn scheme
The oxidation of anthracene can occur in the 9,10- and/or 1,4-pitim. An
attack of the oxygen in the 9,lO-pitions leads to 9,lO-anthraquinone while an
1,4-attack results in 1,rl-anthraquinOne.The 9,lO-anthmqumm
‘
ereactsfurther
to phthalic anhydride while the lf4-anthraquinoneis oxidized further to 2,3naphthalic anhydride arad finally to pyranellitic anhydride. A sinplified reaction scheme for the anthracene axidation as derived fran the kinetic relatimskips is presented in Fig. 5.
1: Anthracme
2: 9,lO-AnthraquI-
ncne
” \
3: Phthalic
anhydride
4: 1,4-An--
none
5: 2,3-Naphthalic
anhydride
6: Fyrawllitic
anhydride
Fig. 5. Readion scheme of the anthracene oxidation.
catalyst perfomlance
The effect of the V/t% ratio on the selectivity of different catalysts with a
ccnstant Pfl ratio of 0.11was studied at 673 arid 723 K; the selectivities are
canpared at an anthracene conversion of about 80%.An increase in temperature
fran 673 to 723 K results in a higher 9,lO-anthraquinone selectivity. The results presentd in Fig. 6 s h that the selectivity of 9,lO-anthraquinone decreases with increasing V/bb ratio.
25 1
sI %
S/%
70 r
LO
I20
*" 51%
-I
I
'
0833 167
286
L17
v . Mo
+ 9, 10-Anthraquinone
0 cox
556 6.90 833
* Phthalic anhydride
Fig. 6. Effect of the ratio V:b@
on the selectivities at 723 K at an
anthracene canversion of about 80%.
"
unsupported
catalyst
Si02-
A120,supported catalyst
I9.10-Anthroquincne
El Phthalic anhydride
COX
Fig. 7. Effect of support material
on the selectivities at 673 K at an
anthracene mversion of abxt 80%.
When using a support material for the catalytic CanpoUIlds, catalyst activity
increases for the oxidation of anthracene. The selectivity is differently affected depending on the support applied. The effect of a silica and an a l h support on the selectivity loaded with catalytic material (Vm= 0.83 and P/V =
0.11) at 673 K and at an anthracene conversion of abcplt 80% is sham in Fig. 7.
The unsupported and the Si02-supported catalyst show alnrxt the same selectivity
behaviour; 9,lO-anikmqunm
'
e selectivity decreased, haever, markedly when
using a l h as support material.
Kinetic chracterization of the catalysts
A statistical discrimination betdifferent kinetic models based on different reaction scfiemes shawed that the total oxidation of the oxygenates, i.e.,
9,1O-anthraquinone,phthalic anhyd.ride and the other 1,4-products could be
neglected as a first approximation up to anthracene conversions of about 90%;
for simplification all the prcducts formed by the 1,4-attack of anthracene were
1
as a pseudc-canpcprent (1,4-products; cp. Fig. 5). All the reactim steps
were assumed to be first-order with respect to anthracene and to the various
oxygenates; this
justified because of the l m ccncentration of these
cunpurh. For catalyst characterization various ratios of rate ccnstants were
defined. The dependence of these values on the V/Mo/P atanic ratio arid on the
support material used for the catalytic materials are sham in Tab. 1.
252
TABLE 1.
Ratio of t h e rate constants for the oxidation of anthraene
al P:V = 0 . 1 1
Temperature
673 K
723 K
V:W
0.83
1.67
2.86
4.17
5.56
6.90
8.33
1.4
1.2
0.8
0.5
0.1
1.4
0.6
0.2
0.3
0.4
0.4
0.6
0.5
0.8
0.5
0.5
0.5
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.5
0.5
0.3
0.3
0.2
0.2
0.4
0.2
0.2
0.6
0.7
0.7
0.7
0.6
0.5
0.6
0.3
0.2
0.2
0.2
0.3
0.4
0.3
0.5
0.5
0.2
0.6
0.8
0.4
0.2
0.4
0.6
0.5
0.3
0.4
b) P:V = 0.4
1.67
8.33
0.3
0.8
0.6
0.6
0.4
0.4
0.5
0.5
1110 mass-% of catalytic material, 90 mass-% support material
2)Alon-C/Degussa; SBET = 95 mz/g
3)~e~osil-200/De9ussa;
SBET = 140 nP/g
The ratio k2/kl is a measure for the consecutive oxidation of the primary
product 9,lO-anthracpinone t o phthalic anhydride while the ratios k, /(kl tk3+k, )
and k3/(kl+k3+k4) represent the extent of the selective reaction route t o 9,lOantluxpinone and of the total oxidation t o CO, respectively (cp. Fig. 5 ) . The
follawing results can be derived by a canparison of the numrical values:
i) The consecutive oxidation of 9,lO-anthracpinone t o phthalic anhydride
increases with an increase of the V/Mo ratio a t the lm reaction temp
rature of 673 K and a t the low P/V ratio of 0 , l l .
ii) Total oxidation is more marked a t 673 K than a t 723 K .
iii) Silica as support material results i n better selectivities than almina.
iv) No significant effect of the P/V ratio was observed i n the range fran
0 . 1 1 t o 0.40.
These results are i n agreement with the qualitative data described above.
I n -si t u I R spectroscopic identification of adsorbate structures
When oxidizing anthracene Cox f o m t i o n was increased by the use of the alumina support material for the V/fao/P oxides catalysts while by the use of silica
no significant change i n selectivity was oberved. To elucidate this behaviour
the follawing experiments were conducted. Anthracene was adsorbed on b t h the
supported catalysts between 573 and 723 K i n the presence of a i r . I R transnis-
253
623 K
62 5
723 K
693 K
673 K
673 K
823 K
673 K
Fig. 8. I R transmissicn spectra
of the anthracene adsorbates on
the SiO, -supported catalyst.
Fig. 9. IR transmission spectra
of the anthracene adsorbates on
the Al,o,-supported catalyst.
sicm spectra of anthrame adsorbates are shown in Figs. 8 and 9.
AnthraquinOne (vC=O: 1672 an-'), phthalic anhydride (vC=O: c. 1850 and 1780
at+ ) and carboxylate ccmplexes (v,,coO- : 1543 and v,O- : 1431 an-l) (ref. 11)
were observed as adsorbate structures on the surface of both supported catalysts.
The intensity ratios of the carboxylate bands t o those of anthraquinone and
phthalic anhydride bands respectively are larger on the Alp03-supportd catalyst
than an the SiO, -supported one. Ftxthemre, a strong product adsorption was obSenred on the A l , O 3 - s u p r t & catalyst up t o 723 K while on the Si0,-supported
catalyst no adsoption was ohserved any mre above 623 K. The adsorbate spectra
of anthracene on the Al,03-supported catalyst shmed additional strong negative
OH bands of Al,O, a t 3640 - 3740 an-1 ( r e f . 12) as w e l l as bridged OgI bands a t
3500 an-1 arid a s t m g CH band of adsorbates a t 3073 cn-1 while on the Si0,supported catalyst the negative OH band of SiO, a t 3741 an-1 and the CH band a t
3073 an-1 were very w d c ; the negative bands are ascribed t o an interaction of
OH groups w i t h the reactants. Fmn these results it can k derived that the
interacticn between the catalyst and the reactants, i.e. intennediates and
products was stronger on the Al,O, -supported catalyst than on the SiO, -supported
catalyst. This could be confirmed by desorption r n e a s u m ~ ~at
~ ~723
t s K: mnplete
desorption was &en& w i t k i n less than 1minute on the Si02-supported catalyst
while desorption on the Al,03-supported catalyst toak more than 30 minutes. The
IR spectroscapic results indicate that non-selective axf o m t i o n is favored on
the AL,03-supported catalyst due t o the formation of carboxylate structures
which are considered as precursors t o oxidative degradation of phthalic anhydride k i n g a ecxlsecutive oxidation product of anthraquinone. Rxthemre, it
254
ms s h m that anthracene adsorbed between 573 and 723 K only on pure A.l,O,; no
adsorpticn was observed on pure SiO, . When pure SiO, and the silica supported
catalyst were w e d t o gaseous anthraquinone and phthalic a n h w i d e no adsorpt i o n was okerved while on pure U , O , and on t h e alumina-supported catalyst
s t m g cdmxylate formation occured on the solid surface.
a
I
N
C
L
U
S
1
m
Catalysts canposed of V/MD/p oxides are suitable f o r the oxidation of anthracene t o anthraquinones. For 9,lO-anthraquinone a maximum s e l e c t i v i t y of 65%was
obtained ( T = 723 K, X = 85%); smming-up a l l t h e valuable prcducts, i . e . , 1,4anthraquinone, 2,3-naphthAic anhydride, p y r a w l l i t i c anhydride and phthalic anhydride a total selectivity of about 85%w a s achieved. The catalytic p e r f o m c e
of the various s o l i d s used as a catalyst could be quantitatively described by
first-order rate anstants. IR spectroscopic studies slm& that adsorbate
structures of d i f f e r e n t mnaentrations e r e f o d on the catalyst surface when
using alumina o r silica as support materials. The alumina support having higher
surface a c i d i t y when mnpared t o silica resulted in an extensive formation of
surface carboxylates which are considered t o be precursors t o oxidative degradation of the valuable oxygenates.
-
Financial support by Dsutsche Forschungsgerrreinschaft (grant SFB-O218/B3) is
gratefully acknowledged.
REFERENCES
1 Ullmanns ~CyclopSdieder Technischen chemie, Vol. 7 , 4th edn., Verlag
c3laanie, Weinheim-New York, 1974, pp.578.
2 J. Vymetal and J. Norek, Czech., CS Pat. 205981 (1983).
3 J . E . Gemah, Catalytic Cmversion of Hydrocarbons, Academic Press, New York,
1969, pp.256.
4 W. Wettstein and L. Valpiana, Swiss Pat. 407079 (1966).
5 U l l m a n n s hCyclo@die der Tedmischen M e , Vol. 17, 4 t h edn. , Verlag
Chgnie, Weinheim-New York, 1974, pp.510.
6 J. Vymetal, J. Norek and V. Ce&,
chem. P m . , 34(9) (1984) 467.
7 H. Y a s i and K. Ota, J p . Kokai Towry0 Koho JP, 75, 108254 (1975).
8 A. Zeh and M. Baerns, J. chranat. Science, 27 (1989) 249.
9 A. Ranstetter and M. Baems, J. C a t a l . , 109 (1988) 303.
10 N.T. Do a n d M . Baems, Appl. Catal., 45 (1988) 9.
11 L.J. E~llamy,The Infrared spectra of caoplex Molecules, 3rd edn. , C h a v
ard IW.1 Ltd. , Iondon, 1975.
12 A.V. Kiselev and V . I . Lyyin, Infrared Spectra of Surface CEmpoUnas,
John Wiley & Sons, New York-Torcmto, 1975.
255
B. Delmon (Universitg catholiqe de Louvah, Belgium): You have obtained a very
law selectivity when yaur V/Mo/P catalyst was supported an a1mi.m. It is k
n
m
that m, i n its oxide fonn, has such a strong affinity for A 1 2 0 3 that it fonns
strongly adherhg mrmohyers . A very likely reason for the low activity of the
Al,03-supported catalyst is that Moo3 segregates cut of the V b / P axqound for
reacting with A1203 (phcsphorous, t o a certain extent, could do the same). I naw
refer t o your I R - s p e c t r a of Fig. 9. Did you take, for a n p r i s o n , similar
spectra for Mo0,/Al,O3 (and P 2 0 5 ~ A 1 2 0 3 )The
? fomatim of a &HI3 m o l a y e r could
explain the presence of the species you detect.
M. Baerns (Ruhr-University Bochum, W.-Germany): W
e have no IR-spectra of
~ , / A l , O , or P,05/A1203 ht we studied the pure carrier materials under the
sarne reactim conditicns. The adsorbates on Al,O, shcmd similar IR-spedra as
the Al,03-supprted catalyst. Our mclusion was that the law selectivity of the
Al,03-supported catalyst is mainly a f f e c t 4 by the support material.
S. L. Kipennan (N. D. Zelinskii Institute of Organic Chemistry, A r x t d q of
Scienes, USSR): F i r s t question: I n this mrk the authors have proposed that a l l
the reaction s t e p are of f i r s t order with respect t o h y d r o c a r b carqxxlnds.
This was aSSuI[YXZ to be possible as concentrations of reagents and their prcdu&s
=re srrall; ht law gas phase concentrations do not mean that surface coverings
=re also small. Therefore, the f i r s t order of a l l the reactions was not proved.
what do the authors think about i t ?
Second question: Do you have a possibility t o measure the surface concentrations
of the reaction mnponents?
M. &ems (Ruhr-University Eochum, W.-Germany): (1)W
e have described
OUT kinet i c r e s u l t s w i t h i n the range of m d i t i o n s s t u d i e d by the first-order reactions;
this was possjble since this type of rate equation was applicable and described
our experimental data sufficiently. I f , a v e r , the reactant mcentratirms are
varied over a w i d e r range this skplification cannot any longer be wed; more
anplex kinetics of the Hougen-Watson type are required.
(2) W
e have no possibilityto measure the absolute surface concentrations of the
reaction ampcmds. We d y can estimate the relative surface concentratims of
the adsorbed species fmn the IR-spectra by carpcison of the areas k l a w the
bands.
G . Centi and F. Trifiro’ (Editors), New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
257
VAPOR-PHASE OXIDATION OF ALPHA-METHYLSTYRENE TO PHENYLACROLEIN
M. A1
Research L a b o r a t o r y o f Resources U t i l i z a t i o n , Tokyo I n s t i t u t e o f Technology,
Yokohama 227 (Japan)
4259 Nagatsuta, Midori-ku,
ABSTRACT
V a r i o u s Mo-Te-based t e r n a r y o x i d e s were t e s t e d as c a t a l y s t s f o r t h e vaporphase a i r o x i d a t i o n o f P t - m e t h y l s t y r e n e t o f o r m p h e n y l a c r o l e i n (atropoaldehyde).
The b e s t c a t a l y s t performances were found w i t h Mo/Te/Ti, Mo/Te/W, and Mo/Te/Fe
oxides: t h e one-pass y i e l d o f p h e n y l a c r o l e i n a t t a i n e d 63 mol% a t t h e ol-methyls t y r e n e c o n v e r s i o n o f 96.5 %. The c a t a l y t i c f u n c t i o n s o b t a i n e d w i t h t h e s e
c o m b i n a t i o n s o f o x i d e s were a l s o i n v e s t i g a t e d i n t h e l i g h t o f b o t h acid-base
and o x i d i z i n g f u n c t i o n s o b t a i n e d f r o m t h e c a t a l y t i c a c t i v i t y f o r d e h y d r a t i o n dehydrogenation o f 2-propanol and o x i d a t i o n o f 1-butene.
INTRODUCTION
Propylene i s o x i d i z e d t o a c r o l e i n w i t h a v e r y h i g h s e l e c t i v i t y o v e r Bi-Moand Sb-based mixed-oxide c a t a l y s t s .
i.e.,
F u r t h e r , m e t h y l - s u b s t i t u t e d propylene,
n-butene and isobutene, a r e o x i d i z e d t o b u t a d i e n e and m e t h a c r o l e i n ,
r e s p e c t i v e l y , o v e r s i m i l a r t y p e o f mixed-oxide c a t a l y s t s .
However, i n t h e
o x i d a t i o n o f a r o m a t i c compounds, t h e s e c a t a l y s t s cannot u s u a l l y e x h i b i t an
e x c e l l e n t performance.
Indeed, i n t h e o x i d a t i o n o f ethylbenzene t o s t y r e n e ,
t h e c a t a l y s t s proposed t o be e f f e c t i v e a r e d i f f e r e n t f r o m t h o s e used i n t h e
o x i d a t i o n o f p r o p y l e n e and butenes ( r e f . 1 ) .
benzaldehyde, V-P o x i d e s ( r e f . 2),
I n the o x i d a t i o n o f toluene t o
Mo-P o x i d e s ( r e f . 3 ) . Mo-P-based
o x i d e s ( r e f . 4), Mo-based o x i d e s ( r e f .
5).
and V-Ti o x i d e s ( r e f .
ternary
6) have been
proposed t o be e f f e c t i v e .
As f o r t h e o x i d a t i o n o f p h e n y l - s u b s t i t u t e d propylene, i.e.,
[2-phenylpropene]
t o f o r m d - p h e n y l a c r o l e i n [ atropoaldehyde,
d-methylstyrene
2-phenylpropenal ]
( a b b r e v i a t e d h e r e a f t e r as PhA), t h e r e have been v e r y few s t u d i e s .
Adams ( r e f .
7 ) r e p o r t e d t h a t Bi-Mo o x i d e i s n o t e f f e c t i v e : t h e y i e l d o f PhA i s about 3 mol%
a t t h e d - m e t h y l s t y r e n e c o n v e r s i o n o f 45 %. Recently, G r a s s e l l i e t a l . ( r e f . 8)
r e p o r t e d a 30 mol% y i e l d o f PhA a t t h e c o n v e r s i o n o f 50 % o v e r Nb-promoted
Sb-U oxides.
I n t h e p r e c e d i n g s t u d y ( r e f . 9).
i t was found t h a t t h e b e s t performance f o r
t h e p r o d u c t i o n o f PhA i s o b t a i n e d w i t h Moo3 among t h e v a r i o u s s i n g l e - o x i d e s
t e s t e d and t h a t Mo/Te atomic r a t i o = 10/4 o x i d e e x h i b i t s t h e b e s t performance
among t h e v a r i o u s Mo-based b i n a r y o x i d e s t e s t e d .
The y i e l d o f PhA a t t a i n e d
258
48.5 mol% a t t h e c o n v e r s i o n o f 96.3 %. I t was a l s o found t h a t d - m e t h y l s t y r e n e
i s much more r e a c t i v e t h a n p r o p y l e n e and butenes.
I n t h i s study, f o r purpose o f e x p l o r i n g more e f f e c t i v e c a t a l y s t s f o r t h i s
p a r t i a l o x i d a t i o n , v a r i o u s o x i d e s were combined w i t h t h e Mo/Te atomic r a t i o =
10/4 o x i d e and t h e i r c a t a l y t i c p r o p e r t i e s were t e s t e d .
Then, t h e f u n c t i o n s o f
o x i d e r e q u i r e d f o r c a t a l y z i n g t h i s o x i d a t i o n r e a c t i o n were i n v e s t i g a t e d .
EXPERIMENTAL
Catalysts
The c a t a l y s t s used i n t h i s s t u d y were Mo/Te/X ( X i s t h e t h i r d component)
a t o m i c r a t i o = 10/4/x ( x = 0 t o 16) t e r n a r y oxides.
They were supported on 8-
t o 20-mesh s i z e pumice o r i g i n a t i n g from v o l c a n i c stone c o n s i s t i n g o f macropores
2
( p a c k i n g d e n s i t y = ca. 0.4 g/ml and s p e c i f i c s u r f a c e area = 0.3 t o 0.6 m /g).
F o r example, t h e Mo/Te/W = 10/4/8 c a t a l y s t was prepared as f o l l o w s . (NH ) 4 6
Mo7024.4H20 (35.3 g ) was d i s s o l v e d i n h o t w a t e r and 41.8 g o f (NH4)10W,2041'
5H20 was a l s o d i s s o l v e d i n a n o t h e r h o t w a t e r u s i n g o x a l i c acid.
The two s o l u -
t i o n s were mixed and 18.0 g o f H6Te06 was d i s s o l v e d t o t h e s o l u t i o n .
Excess
w a t e r was evaporated o f f w i t h s t i r r i n g w i t h t h e a i d o f h o t a i r c u r r e n t , y i e l d i n g a s t i c k y syrup.
T h e r e a f t e r , 100 m l o f t h e pumice was added t o t h e s t i c k y
s y r u p and t h e m i x t u r e was evaporated t o dryness w i t h s t i r r i n g w i t h t h e a i d o f
h o t a i r current.
The o b t a i n e d s o l i d was evaporated a g a i n i n an oven a t 200°C
f o r 4 h and t h e n i t was c a l c i n e d a t 450°C f o r 6 h i n a stream o f a i r .
R e a c t i o n procedures
The vapor-phase c o n t a c t o x i d a t i o n o f d - m e t h y l s t y r e n e was conducted w i t h a
c o n v e n t i o n a l c o n t i n u o u s - f l o w system.
cm l o n g and 1.8 cm i.d.,
The r e a c t o r was made o f a s t e e l tube, 50
mounted v e r t i c a l l y and immersed i n a l e a d bath.
Air
was i n t r o d u c e d f r o m t h e t o p o f t h e r e a c t o r . w i t h d - m e t h y l s t y r e n e b e i n g i n j e c t e d
i n t o t h e p r e h e a t i n g s e c t i o n o f t h e r e a c t o r by means o f a s y r i n g e pump.
Unless o t h e r w i s e i n d i c a t e d , t h e f e e d r a t e s were f i x e d as f o l l o w s : a i r , 400
m l ( a t 20°C)/min (ca.
i n air).
1.0 mol/h);
d - m e t h y l s t y r e n e . 11.9 mmol/h (ca. 1.19 mol%
The e f f l u e n t gas f r o m t h e ' r e a c t o r was l e d s u c c e s s i v e l y i n t o f o u r
c h i l l e d scrubbers c o n t a i n i n g 2-propanol
pounds.
(120 m l ) .
A f e t r 1 h time-on-stream,
t o r e c o v e r t h e 2-propanol-soluble
com-
t h e c o n t e n t o f t h e scrubbers was c o l l e c t e d
The r e a c t i o n p r o d u c t s and unreactedd-methylstyrene were analysed by
gas chromatograph: a 1-m column o f M o l e c u l a r s i e v e 13X f o r CO; a 6-m column o f
p r o p y l e n e carbonate f o r C02; a 2-m column o f PEG 20M a t 160°C f o r d - m e t h y l s t y rene, benzaldehyde, and 2-propanol;
a 1-m column o f AT-1200 t H3P04 a t 160°C
f o r PhA, benzaldehyde. m a l e i c anhydride,
and benzoic acid.
The amount o f t o t a l
a c i d was a l s o checked by t i t r a t i o n w i t h 0.1 N NaOH u s i n g a pH meter.
The
259
amount was u s u a l l y i n accord w i t h t h e sum o f maleic anhydride and benzoic a c i d
measured by gas chromatograph.
Since PhA i s n o t a v a i l a b l e as a chemical agent, t h e i d e n t i f i c a t i o n was performed by means o f GC-MS ( H i t a c h i H-80) and t h e q u a n t i t y was determined on t h e
b a s i s o f t h e approximation t h a t t h e peak area o f PhA recorded i n gas chromatograph ( F I D ) i s equal t o t h a t o f cinnamaldehyde [p-phenylacrolein,
3-phenyl-
propenal 1.
The y i e l d and s e l e c t i v i t y o f a p a r t i c u l a r product were defined as mole percentage y i e l d and s e l e c t i v i t y on a carbon-account-fo;
o f carbon oxides [COX].
basis.
As f o r t h e y i e l d
t h e COX accompanied w i t h t h e formation o f benzoic acid,
benzaldehyde, and maleic anhydride was excluded.
RESULTS AND DISCUSSION
Performances o f t h e Mo-Te-based
t e r n a r y oxide c a t a l y s t s
The r e s u l t s obtained over 10 g-portions o f Mo-Te-based t e r n a r y oxide catal y s t s a t t h e o p t i m a l r e a c t i o n temperatures are l i s t e d i n Table 1, according t o
t h e c l a s s i f i c a t i o n o f oxide i n view o f both acid-base and o x i d i z i n g f u n c t i o n s
( r e f . 9,lO).
The r e s u l t s may be summarized as follows.
( 1 ) The a d d i t i o n o f W03, Ti02, and Fep03 t o t h e Mo/Te = 10/4 oxide enhances
markedly b o t h t h e o x i d a t i o n a c t i v i t y and s e l e c t i v i t y t o PhA.
The presence
o f an optimum amount was observed f o r each t h i r d component.
( 2 ) The b e s t r e s u l t s are obtained w i t h t h e Mo/Te/Ti = 10/4/4 oxide: t h e onepass y i e l d of PhA a t t a i n s 63.0 mol% a t t h e d - m e t h y l s t y r e n e conversion o f
96.5 %.
(3) The second best r e s u l t s a r e obtained w i t h t h e Mo/Te/W = 10/4/8 oxide: t h e
PhA y i e l d a t t a i n s 59.5 mol% a t t h e conversion o f 98.4
4.
(4) The t h i r d b e s t r e s u l t s are obtained w i t h t h e Mo/Te/Fe = 10/4/4 oxide: t h e
PhA y i e l d a t t a i n s 58.0 mol% a t t h e conversion o f 96.7 %.
(5) The a d d i t i o n o f Zr02,
Bi20g, and Co304 enhances t h e o x i d a t i o n a c t i v i t y , b u t
i t enhances t h e s e l e c t i v i t y o n l y a l i t t l e .
(6) The e f f e c t o f V205 i s small.
( 7 ) The a d d i t i o n of U308. SnO2. ZnO. NiO, and Mn02 enhances t h e o x i d a t i o n
a c t i v i t y , b u t i t decreases t h e s e l e c t i v i t y .
(8) The a d d i t i o n o f an a c i d i c oxide such as P205, B203, and Sb205 decreases
markedly t h e o x i d a t i o n a c t i v i t y and i t does n o t improve t h e s e l e c t i v i t y .
Performances o f t h e b i n a r y oxide c a t a l y s t s
For understanding t h e f u n c t i o n o f each component i n t h e Mo/Te/W, Mo/Te/Ti.
and Mo/Te/Fe t e r n a r y oxides, t h e c a t a l y s t performance f o r t h e o x i d a t i o n o f
d-methylstyrene obtained over each b i n a r y oxide c o n s i s t i n g o f t h e t e r n a r y
260
TABLE 1
Performances o f Mo-Te-based t e r n a r y o x i d e c a t a l y s t s *
Catalyst
atomic r a t i o
Mo
T
("C)
Conv
(2)
PhA
Baci
Y i e l d (mol%)
Bald MA
COX
450
460
81.5
90.0
18.5
20.5
12.5
13.8
15.9
17.1
5.6
7.0
4.1
6.4
24.9
15.2
23.0
23.0
other
'PhA
(mol%)
Mo/Te
1014
430
440
92.5
96.3
45.0
48.5
11.3
13.7
2.5
2.0
2.1
2.0
4.4
5.0
27.2
25.1
48.5
50.5
Mo/Te/P
Mo/Te/B
Mo/Te/Sb
Mo/Te/Zr
101414
101414
101414
101414
468
500
490
415
95.7
58.5
78.0
91.4
44.0
26.5
42.0
51.5
12.4
3.9
8.2
11.5
4.3
2.5
7.7
5.0
4.5
2.0
3.0
4.5
8.1
3.3
4.8
7.0
22.4
20.3
12.7
11.9
46.0
45.0
54.0
56.0
Mo/Te/W
101412
395
400
88.8
95.9
52.5
55.5
11.9
14.6
3.0
5.2
3.4
3.3
4.7
3.7
13.3
13.6
59.1
57.9
101414
390
410
54.0
93.5
37.0
53.5
6.0
11.3
2.4
4.5
1.5
1.6
1.8
5.0
5.3
17.6
69.0
57.0
101418
385
390
395
89.0
96.2
98.4
58.5
58.5
59.5
12.3
13.2
16.9
3.0
3.6
4.2
2.6
2.5
2.9
3.8
4.4
4.9
8.8
14.0
10.0
66.0
61 .O
60.5
1014116
375
38 5
51 .O
85.5
21.8
28.2
8.7
16.3
3.6
8.4
2.9
5.0
3.0
7.8
11.0
19.8
43.0
33.0
Mo/Te/V
101414
435
440
88.5
95.0
46.5
45.5
15.8
14.1
6.7
7.1
2.5
2.5
2.9
6.0
14.1
19.8
52.5
48.0
Mo/Te/U
101414
400
94.7
44.1
14.1
4.8
4.7
4.8
22.2
46.5
Mo/Te/Ti 101412
440
4 50
83.7
88.3
47.8
49.3
8.1
9.3
3.6
4.8
3.2
2.8
2.6
4.1
18.4
18.0
57.0
56.0
101414
390
400
83.5
96.5
59.3
63.0
13.7
15.0
2.4
3.6
3.5
3.2
2.3
3.8
2.3
7.4
71 .O
65.0
101418
400
41 0
90.6
96.7
50.5
56.8
12.8
15.7
2.4
4.8
3.2
3.2
5.5
5.6
16.2
10.6
56.0
59.0
Mo/Te/Sn 101414
390
90.0
27.0
12.3
3.6
7.2
24.7
14.7
30.3
Mo/Te/Fe 101414
420
430
375
385
89.0
96.7
86.5
90.0
56.8
58.0
40.7
44.1
11.4
13.3
22.0
18.5
4.0
3.6
9.6
8.4
3.4
3.5
4.5
3.0
0.6
0.6
1.4
3.7
12.8
17.2
8.3
12.3
64.0
60.0
47.0
49.0
Mo/Te/Bi 101414
Mo/TelZn 101414
Mo/Te/Ni 101414
395
41 0
390
83.2
96.2
96.4
47.6
47.2
40.5
9.6
16.0
15.8
3.0
4.8
4.2
2.1
5.0
4.6
7.5
4.7
10.5
13.3
18.5
20.8
57.2
49.0
41.8
Mo/Te/Co 101414
390
400
82.0
94.8
46.1
49.3
14.7
14.7
2.4
3.6
0.7
3.6
6.0
7.7
12.1
15.9
56.2
52.0
Mo/Te/Cr 10/4/4
Mo/Te/Mn 101414
405
41 0
89.6
92.7
40.2
36.3
17.4
15.3
4.8
5.4
5.0
4.1
7.0
10.7
15.0
20.9
44.6
39.1
101418
*
T = temperature, PhA = phenylacrolein, Baci = benzoic
hyde, MA = maleic anhydride, COX = carbon oxides, o t h e r
of d-methylstyrene) - ( sum o f t h e y i e l d s of PhA t Baci
SphA = s e l e c t i v i t y t o PhA, amount o f c a t a l y s t used = 10
acid, Bald = benzalde= [ ( o v e r a l l conversion
t Bald t COX)],
g.
261
TABLE 2
Comparison o f t h e performances of t h e t e r n a r y oxides w i t h those of b i n a r y oxides
Cata 1y s t
T
Conv
atomic r a t i o
("C)
(X)
PhA
Baci
Bald
MA
1014
1014
1014
101418
440
400
410
395
96.3
88.0
95.8
98.4
48.5
32.5
46.2
59.5
13.7
11.5
18.6
16.9
2.0
4.8
9.6
4.2
2.0
4.1
4.0
2.9
5.0
12.9
4.0
4.9
25.1
22.2
13.4
10.0
50.5
37.0
48.3
60.5
Mo/Te
Ti/Te
Mo/Ti
Mo/Te/Ti
1014
1014
1014
101414
440
410
350
400
96.3
52.0
89.0
96.5
48.5
23.3
6.4
63.0
13.7
4.9
9.4
15.0
2.0
2.4
5,4
3.6
2.0
2.1
15.3
3.2
5.0
14.7
31.4
3.8
25.1
4.6
21.1
7.4
50.5
45.0
7.2
65.0
Mo/Te
Fe/Te
Mo/Fe
Mo/Te/Fe
1014
1014
1014
10/4/4
440
345
370
430
96.3
33.0
79.4
96.7
48.5
1.7
18.1
58.0
13.7
0.
14.7
13.3
2.0
4.8
7.8
3.6
2.0
0.
6.5
3.5
5.0
17.5
4.9
0.6
25.1
9.0
27.4
17.2
50.5
5.1
22.8
60.0
Mo/Te
W/Te
Mo/W
Mo/Te/W
Y i e l d (mol%)
COX
other
'PhA
(molX)
~
~~
Abbreviations a r e t h e same as f o r Table 1.
The amount o f c a t a l y s t used i s 10 g.
oxides were compared w i t h those obtained over t h e t e r n a r y oxides. The t e s t s
The r e s u l t s a r e shown i n
were performed u s i n g 10 g-portion o f t h e c a t a l y s t s .
Table 2.
The r e s u l t s may be summarized as follows.
W03 by i t s e l f i s n o t e f f e c t i v e as a c a t a l y s t f o r t h i s o x i d a t i o n ( r e f . 9)
and Te02 has no o x i d a t i o n a c t i v i t y .
However, t h e combination o f t h e two
oxides generates a h i g h o x i d a t i o n a c t i v i t y , b u t t h e s e l e c t i v i t y t o PhA i s
lower and t h e formation of COX i s much g r e a t e r than those obtained w i t h t h e
MoITe oxide.
On t h e o t h e r hand, t h e a d d i t i o n o f W03 t o Moog enhances
markedly both t h e o x i d a t i o n a c t i v i t y and s e l e c t i v i t y :
t h e performance o f
t h e Mo/W = 1014 oxide i s comparable w i t h t h a t o f t h e Mo/Te = 1014 oxide.
Therefore. i t i s considered t h a t t h e a d d i t i o n o f Te02 improves t h e Mo/W
oxide much as i t improves t h e Moo3 alone c a t a l y s t .
The performances o f t h e T i I T e and Fe/Te oxides are much lower than t h a t
o f t h e W/Te oxide.
Further. t h e a d d i t i o n o f Ti02 and Fe203 t o Moo3
increases t h e o x i d a t i o n a c t i v i t y , b u t i t decreases t h e s e l e c t i v i t y t o PhA.
However, an e x c e l l e n t c a t a l y t i c performance i s obtained by t h e a d d i t i o n o f
Te02 t o t h e MoITi and Mo/Fe oxides, suggesting t h a t t h e presence o f Moo3
and Te02 i s e s s e n t i a l f o r a c a t a l y s t t o be e f f e c t i v e f o r t h i s o x i d a t i o n .
Acid-base p r o p e r t i e s of t h e c a t a l y s t s
The acid-base p r o p e r t i e s of t h e t e r n a r y and b i n a r y oxide c a t a l y s t s were
studied.
Simce t h e c a t a l y s t s are colored, t h e i n d i c a t e r / t i t r a t i o n method i s
262
TABLE 3
C a t a l y t i c a c t i v i t y f o r d e h y d r a t i o n and dehydrogenation o f 2-propanolQ
S
Catalyst
( X 10
mol/h
2
m )
(m2/g)
r
P
ra
ralr
P
0.61
2.4
1.1
1.05
7.0
2.3
15.4
8.6
15.7
2.0
26.5
20.0
2.3
0.9
1.7
2.4
Ti/Te
1014
Mo/Ti
1014
Mo/Te/Ti 10/4/4
3.7
8.6
0.9
0.18
28.7
13.3
1 .o
7.6
18.6
5.7
0.26
1.4
1014
Fe/Te
MoIFe
1014
Mo/Te/Fe 101414
24.6
1.6
0.45
0.11
15.7
4.6
0.96
15.6
8.7
8.6
1.0
1.9
atomic r a t i o
Mo/Te
W/Te
Mo/W
MoITelW
*
1014
1014
1014
101414
S, s u r f a c e area:
n o t applicable.
r
P'
r a t e o f dehydration:
Therefore,
ra, r a t e o f dehydrogenation.
t h e p r o p e r t i e s were e s t i m a t e d i n d i r e c t l y from t h e
c a t a l y t i c a c t i v i t i e s f o r a c i d - and base-catalyzed t e s t - r e a c t i o n s .
As a measure
of t h e a c i d i c p r o p e r t y , t h e a c t i v i t y f o r d e h y d r a t i o n o f 2-propanol
t o propylene,
and as a measure o f t h e b a s i c p r o p e r t y , t h e ( a c t i v i t y f o r o x i d a t i v e dehydrog e n a t i o n o f 2-propanol t o a c e t o n e ) / ( a c t i v i t y
r a t i o , were employed (refs.11-14).
f o r d e h y d r a t i o n o f 2-propanol)
The a c t i v i t i e s were measured under t h e
f o l l o w i n g c o n d i t i o n s : temperature, 220°C; 2-propanol c o n c e n t r a t i o n ,
1.3 mol%
i n a i r : f e e d r a t e o f a i r , 400 ml/min.
The r e s u l t s a r e l i s t e d t o g e t h e r w i t h t h e s p e c i f i c s u r f a c e area i n T a b l e 3.
They may be summarized as f o l l o w s .
( 1 ) The o x i d e s which a r e poor i n t h e a c i d i c p r o p e r t y a r e n o t e f f e c t i v e as c a t a l y s t s f o r t h e f o r m a t i o n o f PhA; f o r example, t h e T i / T e and Fe/Te o x i d e s .
( 2 ) The o x i d e s which a r e poor i n t h e b a s i c p r o p e r t y a r e n o t e f f e c t i v e i n t h e
o x i d a t i o n : f o r example, t h e Mo/Ti.
W/Te.
and Mo/Fe oxides.
( 3 ) The a d d i t i o n o f Te02 suppresses t h e a c i d i c p r o p e r t y and enhances t h e b a s i c
property, t o a c e r t a i n extent.
( 4 ) The possession o f a c e r t a i n l e v e l i n b o t h t h e a c i d i c and b a s i c p r o p e r t i e s
seems t o be r e q u i r e d t o achieve a good performance i n t h e o x i d a t i o n .
Performances i n t h e o x i d a t i o n o f 1-butene.
To know t h e c h a r a c t e r i s t i c f e a t u r e s o f t h e t e r n a r y o x i d e s which show a good
performance i n t h e o x i d a t i o n o f & - m e t h y l s t y r e n e . t h e performances o f t h e s e
o x i d e s i n t h e o x i d a t i o n o f 1-butene were s t u d i e d .
The r e a c t i o n was conducted
under t h e f o l l o w i n g c o n d i t i o n s ; 1-butene c o n c e n t r a t i o n , 2.03 molz i n a i r : f e e d
263
TABLE 4
Performances i n t h e o x i d a t i o n o f 1-butene"
Cata 1ys t
Atomic r a t i o
T
Conv
("C)
Y i e l d (mol%)
'qH6
'C H
(moWj
Acid
Mo/W
1014
440
460
27.7
39.0
20.2
22.6
74
58
Mo/Te/W
10/4/4
440
460
480
66.3
79.4
88.8
60.3
65.9
64.7
14.4
91.
83
73.
72.0
5.4
38.0
60.0
75.4
92.1
94.5
55.0
65.5
66.5
55.0
Mo/Ti
1014
360
Mo/Te/Ti
101414
420
440
460
480
~~
',
~
7.5
92.
87.
72.
58.
25.5
~
Mo/Fe
1014
440
460
47.5
57.0
11.3
12.0
24.
21.
Mo/Te/Fe
10/4/4
460
480
34.0
41 .O
33.0
39.5
97.
96.5
*ScqH6,
s e l e c t i v i t y t o butadiene;
amount o f c a t a l y s t used, 20 g.
Acid was measured by t h e t i t r a t i o n and t h e amount was c a l c u l a t e d as a c e t i c
a c i d o r maleic anhydride.
r a t e o f a i r , 280 ml/min:
amount o f c a t a l y s t used, 20 g.
The y i e l d s o f
butadiene and a c i d (mainly a c e t i c a c i d and maleic anhydride) and t h e select i v i t y t o butadiene are l i s t e d i n Table 4.
The r e s u l t s may be summarized as follows.
(1) The t e r n a r y oxides which show a h i g h s e l e c t i v i t y i n t h e o x i d a t i o n o f
pr-methylstyrene t o PhA, show a very h i g h s e l e c t i v i t y i n t h e o x i d a t i o n o f
1-butene t o butadiene, too.
A t a h i g h conversion, a f a i r amount o f
a c e t i c a c i d and maleic anhydride i s formed.
Possibly, they may be formed
by t h e consecutive o x i d a t i o n o f butadiene.
(2) The Mo/Ti and Mo/Fe oxides are n o t e f f e c t i v e i n t h e o x i d a t i o n o f 1-butene
t o butadiene much as i n t h e o x i d a t i o n o f ac-methylstyrene t o PhA.
s e l e c t i v i t y t o butadiene decreases i n t h e o r d e r o f Mo/W)
Mo/Fe)
The
Mo/Ti.
This order i s i n c o n f o r m i t y w i t h t h a t o f t h e s e l e c t i v i t y t o PhA.
(3) The a d d i t i o n o f Te02 t o t h e Mo/W oxide enhances t h e c a t a l y t i c a c t i v i t y i n
o x i d a t i o n o f both d-methylstyrene and 1-butene.
Whereas, t h e a d d i t i o n o f
Te02 t o t h e Mo/Ti and Mo/Fe oxides s t r o n g l y decreases t h e a c t i v i t y i n t h e
both o x i d a t i o n reactions.
The Te02 enhances t h e basic p r o p e r t y o f t h e
Mo/W oxide, whereas i t suppresses t h e a c i d i c p r o p e r t y o f t h e Mo/Ti and
Mo/Fe oxides (Table 3).
264
Discussion
The a d d i t i o n o f Te02 t o t h e Mo/Ti and Mo/Fe o x i d e s decreases markedly t h e
o x i d a t i o n a c t i v i t y . T h i s may be a s c r i b e d t o t h e decrease i n t h e s u r f a c e area.
Since b o t h a-methylstyrene and PhA a r e b a s i c compounds, t h e o x i d a t i o n o f
d - m e t h y l s t y r e n e t o PhA i s a "base
+ base
t y p e r e a c t i o n " ( r e f . 10).
Therefore,
t h e possession o f b o t h a c i d i c and b a s i c p r o p e r t i e s i n a p r o p e r l e v e l i s
r e q u i r e d as a c a t a l y s t f o r t h i s t y p e o f p a r t i a l o x i d a t i o n ( r e f s . 10.11.13).
The Mo/Te/Ti,
Mo/Te/W,
and Mo/Te/Fe t e r n a r y o x i d e s may b e s t f i t t h e r e q u i r e d
balance and/or l e v e l o f t h e two o p p o s i t e p r o p e r t i e s .
The presence of Moo3 i n t h e c a t a l y s t may be e s s e n t i a l t o have a c i d i c and
redox p r o p e r t i e s .
The W03. Ti02, and Fe203 p l a y a r o l e i n enhancing t h e a c i d i c
p r o p e r t y , b u t t h e p r o p e r t y may be t o o s t r o n g t o suppress t h e s i d e - r e a c t i o n s ;
f o r example, c o n s e c u t i v e o x i d a t i o n o f b a s i c p r o d u c t s and C-C bond f i s s i o n .
The a d d i t i o n o f Te02 t o t h e Mo-based b i n a r y o x i d e s suppresses t h e a c i d i c
p r o p e r t y t o a p r o p e r l e v e l and a l s o enhances t h e b a s i c p r o p e r t y .
I t s h o u l d be noted t h a t t h e o x i d e s which show a good performance i n t h e
o x i d a t i o n o f 1-butene t o butadiene.
do n o t always show a good performance a l s o
i n t h e o x i d a t i o n o f d - m e t h y l s t y r e n e t o PhA.
F o r example, 8i-Mo-
and Sb-based
o x i d e s a r e e f f e c t i v e f o r o x i d a t i o n t o butadiene, b u t a r e n o t e f f e c t i v e f o r t h e
o x i d a t i o n o f +methylstyrene
A t present.
t o PhA.
i t i s s t i l l hard t o e x p l a i n t h e reason.
We f e e l t h a t a more
s t r i c t l e v e l o f acid-base p r o p e r t i e s , which we c a n n o t measure now, i s r e q u i r e d
t o a c h i e v e a good performance i n t h e o x i d a t i o n o f d - m e t h y l s t y r e n e .
REFERENCES
1
2
3
4
5
6
7
8
9
10
11
12
13
14
G. Emig and H. Hofman. J. Catal.. 84 (1983) 15-26.
M. A i , Kogyo Kagaku Zasshi. 73 (1970) 946-950: Chem. Abst., 73 (1970)
76790k.
M. A i , Kogyo Kagaku Zasshi. 74 (1971) 1636-1639: Chem. Abst. 75 (1971)
109992~.
M. A i , Nippon Kagaku K a i s h i , (1972) 1151-1156: Chem. Abst., 77 (1972)
66559k.
N.K. Nag, T. Frasen and P. Mars, J. Catal., 68 (1981) 77-85.
A.J. Van Hengstum, J.E. Ommen. H. Bosch and P.J. G e l l i n g s , Appl. Catal..
8 (1983) 369-382.
C.R. Adams, J. Catal.. 10 (1968) 355-361.
R.K. G r a s s e l l i , J.D. B u r r i n g t o n , D.D. Suresh, M.S. F r i e d r i c h and M.A.S.
Hazle. J. Catal.. 68 (1981) 109-120.
M. A i , J. Catal., ( i n press).
M. Ai. i n T. Seiyama and K. Tanabe (Eds.), Proc. 7 t h I n t . Congr. Catal..
Tokyo, June 30 - J u l y 4, 1980. Kodansha. Tokyo/Elsevier, Amsterdam. 1981,
pp. 1060-1 069.
M. A i , J. Catal., 40 (1975) 318-326 and 327-333.
M. A i , B u l l . Japan P e t r o l . I n s t . . 18 (1976) 50-54.
M. Ai. B u l l . Chem. SOC. Japan, 49 (1976) 1328-1334.
M. A i . J. Catal., 52 (1978) 16-24.
265
(1) which have
V. CORTlS CORBERAN ( I n s t . C a t a l i s i s y Petroquimica. Spain):
been y o u r i n i t i a l c r i t e r i a f o r t h e s e l e c t i o n o f m e t a l l i c c a t i o n s , and a t o m i c
r a t i o s between them, f o r t h e c a t a l y s t c o m p o s i t i o n s you have used. (2) The
systems you have used a r e w e l l known by t h e i r p r o p e r t i e s i n t h e c a t a l y t i c
s e l e c t i v e o x i d a t i o n o f o l e f i n s and t h e o v e r a l l t e n d e n c i e s f i n a l l y found f o r
t h i s r e a c t i o n ( f o r example, a d d i t i o n o f t e l l u r i u m ) p a r a l l e l t h o s e p r e v i o u s l y
known f o r s e l e c t i v e o x i d a t i o n s o f o l e f i n s . Would you have expect, a p r i o r i ,
d i f f e r e n t tendencies, and i f so, why?
M. A 1 (Tokyo I n s t . Tech., Japan): (1) I had no i n i t i a l c r i t e r i o n : we t e s t e d
v a r i o u s k i n d s o f s i n g l e and b i n a r y o x i d e systems and, then, we s e l e c t e d some
t e r n a r y systems b a s i n g on t h e i n f o r m a t i o n o b t a i n e d f r o m t h e t e s t s o f b i n a r y
( 2 ) Many k i n d s o f mixed o x i d e systems have been known t o be e f f e c t i v e
oxides.
as c a t a l y s t f o r o x i d a t i o n of o l e f i n s . T h i s s t u d y i n d i c a t e s a t l e a s t t h a t a l l
I expected, a
o f them a r e n o t e f f e c t i v e f o r o x i d a t i o n o f P(-methylstyrene.
p r i o r i , d i f f e r e n t tendencies, because Bi-Mo-type c a t a l y s t s a r e n o t e f f e c t i v e
f o r t h i s oxidation.
R.K. GRASSELLI (Mobil Research and Develop., USA): You s t a t e i n y o u r conclus i o n s t h a t Sb-based c a t a l y s t w h i l e e f f e c t i v e f o r t h e o x i d a t i o n o f I - b u t e n e t o
butadiene are n o t e f f e c t i v e f o r t h e o x i d a t i o n o f d-methylstyrene t o atropoaldehyde, a p p a r e n t l y i n c o n t r a s t t o Mo-Te-Ti. Mo-Te-W, and Mo-Te-Fe based c a t a l y s t s
I should l i k e t o remind you t h a t o u r work which you k i n d l y
whichyou s t u d i e d ,
quoted as r e f e r e n c e 8, c l e a r l y showed t h a t d - m e t h y l s t y r e n e i s e f f e c t i v e l y conv e r t e d t o atropoaldehyde w i t h Nb-U-Sb based c a t a l y s t s , i n f a c t t h e s e l e c t i v i t i e s
which we r e p o r t e d w i t h o u r antimony based c a t a l y s t s r i v a l y o u r b e s t systems,
w h i l e I agree t h a t c a t a l y t i c systems must be o p t i m i z e d f o r each g i v e n r e a c t i o n .
There i s no a p r i o r i reason t o e x c l u d e antimony f o r t h e o x i d a t i o n o f d - m e t h y l styrene.
M.AI (Tokyo I n s t . Tech.,
Japan): I c o u l d n o t g e t a good performance w i t h Sb,
Mo-Sb, and Mo-Te-Sb oxides, b u t I d i d n o t t r y t o t e s t w i t h U-Sb o x i d e s because
you had a l r e a d y t e s t e d w i t h them. Therefore, I t h i n k t h a t y o u r a r e reason.
What parameter i s f o r you a measure
J. KIJENSKI (Warsaw P o l i t e c h n i k a , Poland):
o f a c i d i t y o r b a s i c i t y of m o l e c u l e and a l l o w you t o c o n s i d e r a p a r t i c u l a r
r e a c t i o n as e.g.,
"acid-base'' process?
M. A 1 (Tokyo I n s t . Tech., Japan): I have n o t s p e c i a l o p i n i o n about t h e d e f i n i t i o n o f acid-base.
Indeed, o r d i n a r y i n d i c a t o r / t i t r a t i o n method i s n o t a p p l i c a b l e f o r o x i d a t i o n c a t a l y s t s because o f t h e i r d a r k c o l o r . On t h e o t h e r hand,
t h e gas phase a d s o r p t i o n method c o n t a i n s some problems. Therefore, a t p r e s e n t
t h e measurement o f c a t a l y t i c a c t i v i t y f o r acid-base c a t a l y z e d model r e a c t i o n s
seems t o be t h e most combinient. though t h e r e remains arguments about t h e
d e f i n i t i o n and s t r e n g t h o f acid-base.
B. GRZYBOWSKA ( I n s t . Catal. S u r f a c e Chem.. Poland): ( 1 ) I n t h e Me-Mo-Te t e r n a r y
o x i d e systems t h e r e e x i s t w e l l d e f i n e d compounds (e.g.,
telluromolybdates o f
Co. Mn, N i , Cd) s y n t h e s i z e d and c h a r a c t e r i z e d by S l o c z y n s k i ( r e f . 1) and by
F o r z a t t i . T r i f i r 6 . and V i l l a ( r e f . 2 ) . They have shown t o be a c t i v e a l s o i n
a r o m a t i c hydrocarbon o x i d a t i o n ( r e f , 3). There e x i s t a l s o w e l l d e f i n e d phases
i n Mo-Te-0 system. Both acid-base and o x i d i z i n g p r o p e r t i e s w i l l depend
s t r o n g l y on phase c o m p o s i t i o n o f y o u r c a t a l y s t s and mode o f mutual arrengement
o f t h e phases and n o t o n l y on t h e presence o f p a r t i c u l a r i o n s i n p r e d e f i n e d
environment. I b e l i e v e t h a t t h e s e f a c t s should be t a k e n i n t o account when
(2) Adding some comments t o t h e q u e s t i o n
c o n s i d e r i n g t h e o x i d a t i o n mechanism.
l e t me remind t h a t some a u t h o r s c o n s i d e r t h e i o n i z a t i o n
of Prof. K i j e n s k i :
p o t e n t i a l o f a molecule as a measure o f a c i d i t y , b a s i c i t y [see r e v i e w s by
Ruckenstein e t a1 ( r e f . 4)]. ( 3 ) The d i s c u s s i o n on r e l a t i o n between oneI n t h e case o f
e l e c t r o n and t w o - e l e c t r o n (acid-base) p r o p e r t i e s i s s t i l l open.
266
s o l i d f o r i n s t a n c e t h e r e a r e some a t t e m p t s t o r e c o n c i l e t h e b o t h p r o p e r t i e s
( r e f . 5) - showing t h a t s u r f a c e o n e - e l e c t r o n a c c e p t o r s t a t e s can be i d e n t i c a l
and i n v o l v e t h e same o r b i t a l s as Lewis a c i d i c s i t e s , p r o v i d e d t h e a c c e p t o r
l e v e l l i e s below t h e Fermi energy l e v e l .
1 J. S l o c z y n s k i , Z. Anorg. A l l g . Chem., 438 (1978) 287.
2 P. F o r z a t t i , F. T r i f i r 6 , P.L. V i l l a , J. Catal., 55 (1978) 52.
3 B. Grzybowska, M. Czerwenka, J. S l o c z y n s k i , Catal., Today, 1 (1987) 157.
4 D.B. Dadyburjor, S.S. Jewur, E. Ruckenstein, Catal. Rev., 19 (1979) 293.
5 S.R. Morrison, Surf. Sci., 50 (1975) 329.
M.AI
(Tokyo I n s t . Tech.,
Japan):
Thank You f o r y o u r comments.
J. K I J E N S K I (Warsaw P o l i t e c h n i k a . Poland):
Comment t o t h e remark o f P r o f .
Grzybowska: I n o n i z a t i o n energy c a n n o t be c o n s i d e r e d as a measure o f a c i d i t y o r
b a s i c i t y w h i c h a r e i o n i c , i.e., two e l e c t r o n p r o p e r t i e s . There i s no g e n e r a l
p a r a l l e l i s m between t h e b a s i c i t y and e l e c t r o n donor p r o p e r t i e s .
J. HABER ( I n s t . C a t a l . S u r f a c e Chem., Poland): (1) What i s t h e r e p r o d u c t i v i t y
o f y o u r r e s u l t s . The d a t a seem t o - b e c o n s i d e r a b l y spread which may i n d i c a t e
( 2 ) One o f t h e i m p o r t a n t s i d e
t h e i r dependence on u n c o n t r o l l e d f a c t o r s .
r e a c t i o n s o f m e t h y l s t y r e n e i s c e r t a i n l y cracking which w i l l o c c u r on more a c i d i c
c a t a l y s t s and w i l l r i v a l u a t e t h e m e c h a n i s t i c c o n c l u s i o n s . D i d you t r y t o
d e t e r m i n e t h e c o n t r i b u t i o n f r o m c r a c k i n g by c a r r i n g t e s t experiments i n t h e
absence o f oxygen.
M. A1 (Tokyo I n s t . Tech., Japan): ( 1 ) I d i d n o t f i n d t h a t t h e r e p r o d u c t i v i t y o f
t h i s o x i d a t i o n i s s p e c i a l l y low. However, p h e n y l a c r o l e i n i s n o t s t a b l e , i.e.,
i t t e n d s t o p o l y m e r i z e t o dimer and t r i m e r .
Indeed, I cannot measure t h e
amount o f t h e s e polymers. Therefore, " o t h e r " may c o n s i t m a i n l y o f polymers.
(2) I n t h e p r e d e c i n g work ( r e f . 9 i n t h e t e x t ) , t h e e f f e c t o f oxygen concentrat i o n was s t u d i e d . The consumption o f d - m e t h y l s t y r e n e i n c r e a s e s almost i n
p r o p o r t i o n a l t o t h e oxygen c o n c e n t r a t i o n . Therefore, t h e cracking may be s m a l l
a t l e a s t o v e r t h e Mo-Te-based c a t a l y s t s .
G . Centi and F. Trifiro' (Editors), New Developments in Selectiue Oxidation
1990 Elsevier Science Publishers B.V.,Amsterdam - Printed in The Netherlands
267
Partial Oxidation of 0-Xylene to Phthalic Anhydride in a
Structured Fixed Bed Containing a Sequence of Catalysts
M. Kotter, D.X. Li, L. Riekert
Institut fur Chemillche Verfahrenstechnik, Univcrsitat Karlsruhe,
Kaiserstr.12, Postfach 6980, 7500 Karlsruhe 1, FRG
Abstract
It is shown that an elevated yield of phthalic anhydride (PAA) can be obtained in a fmed bed
consisting of a sequence of different catalysts. A suitable sequence of catalyits can be determined by a
computehed search, following the strategy of biological evolution on the basis of experimentally
obtained kinetic data.
Introduction
Overall selectivity is the most important objective in the design of catalytic processes,
the desired product in general being unstable relative to other possible species which can
be generated from the starting material. Selectivity will depend on the relative extent of
several parallel and consecutive reactions. The catalyst can be considered as a guide
leading the material through composition space on a path which avoids everywhere
descent to thermodynamically stable but undesired products. Composition and possibly
also temperature change along the length of a fixed bed, using a sequence of different
catalysts might therefore be more suitable to obtain high selectivity than using any
single catalyst from the set in the sequence alone. This proposition was investigated
experimentally. Partial oxidation of o-xylene to phthalic anhydride (PAA) in a fixed
bed of catalysts containing oxides of V and Ti as active components was chosen as a
typical example. The literature on this subject has been reviewed by Wainwright at al
[I] and Saleh at al [2]. We found that the network of parallel and consecutive reactions
taking place in this system can be described by the simplified scheme shown in Fig.1.
0-tolualdehyde, o-toluylic acid and phthalide are generated in only small amounts and
can be lumped into one pseudospecies ("intermediates"). The effect of different reactions
in this scheme on overall integral selectivity will depend on the local composition of the
gas phase (conversion of o-xylene), which means on the relative length into the bed. For
example it will be expedient to prevent reaction 1 -+ 4 near the inlet where the concentration of o-xylene is high, whereas reaction 3 44 is not of much concern where almost
268
I
o-Xylene (1)
kl2
k13
Intermediates ( 2 )
k23
1
PAA (3)
Fig. 1 Simplified scheme of reaction in catalytic oxidation of o-xylene
no PAA is present. The opposite must be true towards the outlet where almost no
o-xylene is present and total oxidation of PAA has to be prevented. It appears unlikely
that both requirements can be met by the same single catalyst in an optimal way,
whereas it seems feasible that a structured bed consisting of several different catalysts in
series (Fig. 2) can possibly fulfill both conditions and analogous requirements concerning
suppression or allowance of remaining reaction pathways at different locations.
-
Product gas
Fig. 2 Structured fmed bed reactor (SFBR)
Physical implementation of this concept and experimental verification of its merits
proceeded through the following four steps in sequence:
Preparation of a set of catalysts with different properties, containing the same metals
(Ti,V,Cs,Li) as oxides in the active component in different relative amounts. The experimental
investigation wan thus restricted to a set ("family") of similar but different catalysts.
Mathematical modelling of the catalytic properties for all catalysts: quantitative determination
of the kinetics of all reactions in the scheme shown in Fig. 1 for each single catalyst prepared.
Computer calculation of the expected selectivity behaviors of various sequences consisting of
several catalysts of different amounts from the set prepared, based on the data from (2);
search for an optimal sequence. The number S of possible sequences of N given catalysts is
N
S(N)=
we have already SZlO7 for N=lO. It is therefore impracticable to compute
i=l
and compare the behavior of all poesible sequences. The search for an optimal sequence w a s
based on the strategy of biological evolution as a known shortcut t o a relative optimum in a
multi-parameter system [3,4].
(r)
Si!;
Experimental verification of the selectivity of the structured bed consisting of the sequence of
catalysts found in step (3).
269
Experimental
Catalvst DreDaration
Two groups of supported catalysts were prepared and investigated. The first group
comprises catalysts in the form of monoliths, the support being cordierite of low porosity
with parallel channels of 1x1 mma cross section. This carrier was impregnated with solutions of Ti(OC3H7)4 and VO(OC3H7)3 in isopropanol with some water added [5]. After
impregnation the monoliths were exposed to air saturated with water vapor for 14 h at
7OoC, then calcined at 45OoC for 4 h.
The second group of catalysts was prepared by coating nonporous spheres of steatite of
2-3 mm diameter with a porous layer consisting of Ti02 (anatase), VzO5 and small
amounts of CSZOor Liz0 as modifiers. The carrier was immersed in a suspension of
anatase in a solution of VO(OC3H7)3 in isopropanol and water, Li or Cs being added in
the form of nitrates to the solution. After immersion for 10 min the spheres are removed
and dried at 6OoC for 20 h, then calcined at 45OoC for 3 h. The procedure was repeated
to increase the thickness of the porous layer.
Waste g a s
S V Sample v s l v s
Fig. 3 Apparatus for the catalytic oxidation of o-xylcne
270
Kinetic measurements
Kinetic measurements were performed by monitoring composition along the length of a
fixed bed of catalyst 1500 mm long and 15 mm in diameter. The reactor was made of
stainless steel, consisting of 5 segments with individual temperature control in each
segment (Fig. 3). Heated capillaries were located at the inlet, outlet and between
segments, leading to a multiposition valve which fed gas samples to the analysis train,
consisting of nondispersive JR-analyzers for CO and COz and a chromatograph with
FID for separation and determination of organics. The feed was prepared by saturating
an air stream with c-xylene at controlled temperature. Unattended continuous operation
of the unit was possible as all functions were actuated, controlled and recorded by
computer. The reactor with its high length/diameter-ratio was treated as an isothermal
plug-flow system. The following range of reaction conditions was investigated:
Temperature:
37OoC to 41OoC
Total pressure:
1.6 bar
Mole fraction of 0-xylene in feed
0.3.10-2 t o 0.8-10-2
Mole fraction of 02 in feed:
0.15 to 0.3
Volumetric flow rate measured:
20 to 240 ml/s (OOC, 1.013 bar)
Results and Discussion
The rate of individual reactions in the system shown in Fig. 1 can be represented by
the rate equation
where r.. is in mol-g-1.s-1, c1 and c representing the local and initial concentration of
11
180
o-xylene, respectively. In order to obtain a set of coefficients k.. and b for a given cata4
lyst the experimental results were first represented by polynomial series. The coefficients
in the set of 6 simultaneous differential equations of type (1) were then found by linear
regression. Fig. 4 shows as an example composition as function of conversion of o-xylene
and of space time as represented by the mathematical model in this way together with
experimentally observed points. It is clearly evident from Fig. 4 that parallel and consecutive reactions are occurring simultaneously in this system. The temperature coefficients (activation energies) of different reactions in the network were found to be
different, reaction path and selectivity are therefore sensitive to temperature. An
271
increase in temperature can be beneficial with respect to initial differential selectivity of
certain catalysts. Addition of Li or Cs to the active component reduces the activity of
the catalysts and leads to an increase in differential selectivity at low conversion.
LI
1.0
0
+0
.L
m
;0.8
I
u
b
Run-No:
LI58-192
-- -
C a t : L19
0.880
V/TI
T
P
0.6
0.4
663 K
1.6 bar
Xox,O
8.885
Xo2,0
8.21
Symbo I s :
o-Xylsns
0.2
0.0
0.0
0.3
0.6
1.2
0.9
A
Intermedlatss
0
PRR
0
CO+COZ
1.5
Space t i me/s. g. cm-3
1.0
.9
.>
.-
Run-No:
Al
0.8
V/TI
al
-
LI58-192
-
0.080
T * 663 K
Ln
P
0.6
1.6 bar
0.4
0.2
0.01
0.0
.
’
0.2
.
‘
0.4
‘
’
0.6
.
’
0.8
A
Intcrmedl ates
0
co+coz
.-d
1.0
Conversion
Fig. 4 Product distribution
272
Evolution results from endless repetition of change by various principles, such as mutation, substitution, selection etc. As an example the principle of mutation in the
computerized search-procedure for an optimal sequence of different catalysts on the
basis of mathematical models describing the behavior of individual catalysts is depicted
in Fig. 5. From a set of different sequences of 9 catalysts (numbered 1 to 9) one
sequence is chosen at random. In this sequence the position of two catalysts chosen at
random is interchanged, thereby a new member in the set is generated which now
contains p+l members. For all members the attainable maximum yield is computed
numerically by Runge-Kutta method and considered to be a criterion of quality. The
member with the lowest quality is then dropped, so that a new set of p sequences
results. This procedure is repeated until no improvement in the quality of the best
member of the set results when the cycle is repeated a certain number of times. Yield of
PAA (integral selectivity times conversion of o-xylene) is thus the objective function in
the optimization procedure.
Set of sequences
with known quality
Picking a sequence of
catalysts at random
1
1
1415121613[8/7[911]
Addition
to original set
415121619/8171311]
\
Choosing 2
catalysts in
the sequence
at random,
interchange
of their
posit ion
Sequence with lowest quality
in the set of p+l sequences
is dropped, new set of p
sequences results.
-
I4]512161918/71311]
Computation
of quality
Fig. 5 Mutation
New sequence
273
Table 1 shows results obtained in this way for 3 sets of individual catalysts containing
each 9 members. The resulting optimal sequence, arrived at after a few hundred cycles of
evolution contains only between 2 and 4 different catalysts. In all these optimized
sequences the ratio of V/Ti in the catalyst increases from the inlet towards the outlet of
the reactor. The computed maximum yield of PAA which can be achieved in a structured
Table 1 Results of the optimization
0.709
0.692
0.780
0.724
0.794
SFBE, exp
Table 2 Composition of the catalysts
Active
component
Cat V/Ti
Cat V / T i
A01 0.20
A04 0.40
B10 0.115
BOB 0.13
B03 0.20
B05 0.50
Cat V / T i
Prom
L21 0.02 cs
L18 0.06 L23 0.06 Cs
I Support, Form I Cordierite, Ponolith I S t e a t i t e , Sphere
I Prep. method I
1
I
2
~
~~
274
fixed bed containing several catalysts lies between 1.4 and 6.6 percentage points above
the yield which could be achieved with a single catalyst from the set under consideration.
Two such optimal sequences resulting from evolution in the computer were filled into the
reactor in order to verify the result experimentally. The observed yields corresponded to
expectation, as shown in the last line of table 1.
References
[l] M.5’. Waznevright and N.R. Furster, Catal. Rev.-Sci. Eng., 19 (1979)211-292
[2] R. Y.Saleh and I. E. Wachs, Appl. Catal., 31 (1987)87-98
[3] I. Rechenberg, Evolutionsstrategie - Optimierung technischer Systeme nach
Prinzip der biologischen Evolution, F. Fromman Verlag, Stuttgart (1973)
141 G.L. Stebbiw, Evolutionsprozesse, Fischer Verlag, Stuttgart (1968)
[5] M.Kotter and L. Riekert, Chem.-1ng.-Tech., 59 (1987)733-734
Keywords
Fixed bed reactor, oxidation of o-xylene, phthalic anhydride, V/Ti-catalyst
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
275
YBa2Cu3Q - A SELEClWE AMMOXIDATION CATALYST
J.C. OTAMIRII, A. ANDERSSON', S. HANSEN2 and J.-0. BOV&
bepamnent of Chemical Technology,Chemical Center, University of Lund, P.O. Box 124,
S-22100 Lund (Sweden)
2Department of Inorganic Chemistry 2, Chemical Center, University of Lund, P.O. Box 124,
S-22100 Lund (Sweden)
SUMMARY
YBa2Cu30~,, OSxSl, was used as a catalyst for the oxidation of toluene in presence of oxygen
and ammonia, The partial pressures of reactants were varied. It was observed that when x is above
zero,the material is active for total combustion. The activity is highly dependent on the value of x.
After reductive treatment of sample having x>O in reactant stream without molecular oxygen
producing yBa2cu306, the partial pressure of oxygen was increased from low to high. At low
oxygen pressure, the material was active and selective for formation of benzonitrile. A dramatic
transition from selective to non-selective region was observed to occur at a distinct pressure, which
according to X-ray diffraction analysis is due to incorporation of oxygen species into the lattice.
Under selective condirions, Yh2Cu306 is more active for nitrile formation in comparison with
vanadium oxide catalysts. Catalytic behaviours are discussed considering possible surface structures.
INTRODUCI'ION
Earlier it was discovered that CuO can be used as a catalyst for oxidation of propene to acrolein.
However, its usefulness is limited due to the difficulty of maintaining the surface coverage of oxygen
in a suitable range. It was demonstrated that when clusters of more than 5 adjacent oxygens are
present at the surface, total combustion is predominant. A hypothesis was advanced that for selective
reaction to occur, it is necessary to have structurally isolated sites of appropriate metal-oxygen bond
strength [l-31. Considering the recently discovered superconductormaterial YBa2Cuj07 [4], it is of
potential interest for use in catalytic oxidation since it has structurally isolated Cu-layers containing
mobile oxygen species, which can easily be abstracted [5]. When they are all removed, YBa2Cu306
is formed having characteristic layers of Cul+ [6], which hypothetically can adsorb oxygen giving
Cu3+ and nucleophilic oxygen species. The latter are believed to be involved in selective oxidation
and ammoxidation mechanisms [7-91. Therefore, in order to gain some insight into the catalytic
behaviour of these new materials, they are in the present investigation used as catalysts for the
amoxidation of toluene to produce benzonitrile, which is used e.g. in the synthesis of
benzoguanamine [lo].
METHODS
YBa~Cu306+~.
with x equal to 1 and 0, Y2BaCuOg and SrnBa2Cu307 were prepared from
stoichiometric mixtures of appropriate and pure, >99 %, chemicals of Y2O3, Sm2O3, BaC03 and
CuO according to the procedure described elsewhere [111.
276
Catalytic activity investigations of prepared phases were carried out in a differential and
isothermal plug flow reactor made of Pyrex glass and maintained at 400 OC. Reactants, oxygen,
ammonia, and toluene, mixed with inert nitrogen, were introduced and controlled by HI-TEC mass
flow controllers. Products, benzonimle, benzaldehyde, C02, and CO, were analyzed on a Varian
Vista 6OOO gas chromatograph. Reactor and analysis setup has earlier been described in detail [9].
Freshly prepared YBa2Cu306+,, with x=l, and reduced sample, with x=O, were studied at a
constant high and low partial pressure of oxygen, respectively. while varying the partial pressures of
ammonia and toluene, and also at constant pressures of ammonia and toluene, while varying pressure
of oxygen.
Powder X-ray data were recorded at room temperature using a Guinier-Haggcamera with quartz
monochromator, CuKal radiation and Si as internal standard. Accurate lattice constants were
obtained by least-squares refinement. The smooth variation of lattice constants with lattice oxygen
content observed for YBa2CU306+, phases [12] was used to estimate the value of x.
RESULTS
Catalysison YBa&&Q7
Figure 1 shows the dependency of rates over freshly charged YBa2Cu307 as partial pressure of
oxygen (Po) is varied from high region towards low at constant pressures of ammonia (PA)and
toluene (PT). The rates show partial order dependency, however, it is worthy to note that the xates
hardly increase at high pressures of oxygen. At zero pressure of oxygen the rate for nitrile formation
decreases slowly with time, whereas other rates rapidly go to zero. The x-value is dependent on the
partial pressure of oxygen. After use at PO = 17.30 H a , the composition is YBa2Cu306.4. When the
partial pressure of oxygen is set to zero, the x-value gradually approaches zero.
4'o
I
___
0
10
20
30
v,
v
Fig. 1. Rates for formation of nimle 0 , aldehyde 0 , C02
and CO
over
YBa~Cu306+,,x>O, as a function of partial pressure of oxygen. PA = 2.58 kPa and PT = 0.77 kPa.
277
In Fig. 2 is given the dependency of rates on partial pressure of ammonia with pressures of oxygen
(high) and toluene maintained constant. The rates for formation of benzaldehyde, C02, and CO
decline with increase in pressure of ammonia while that of nitrile increases and remains constant at
higher pressures.
2.0
0
4.0
a0
8.0
Fh3 (kPa)
Fig. 2. Rates for formation of products over Y B a 2 C ~ 3 0 6 +x>o,
~ , versus partial pressure of
ammonia. PO = 17.30 kPa and PT = 0.77 kPa. Notations: cf. Fig. 1.
The variation of rates with partial pressure of toluene, Fig. 3, shows also partial order
dependency. In this figure, it could be seen, that the dependencies for benzaldehyde and C02 are
strong, whereas for nimle and CO, the rates are virtually constant at high pressures of toluene.
"
-
0.5
n
n
Y
0
@
0
1.0
1.5
20
2.5
F O L (kPd
Fig. 3. Influence of partial pressure of toluene on rates for formation of products over Y B ~ ~ C U ~ O ~ + ~ ,
x>O. Po = 17.30 kPa and PA = 2.58 H a . Notations cf. Fig. 1.
278
Reductive treatment
Fresh samples, YBa2Cu307, SmBa2Cu307 and YzBaCuOg which is a wellknown contaminant in
superconductor materials [13] were heated to reaction temperature (400 OC) in presence of oxygen.
Reduction of samples was canied out by performing the experiments in the absence of molecular
oxygen for one hour at fixed conditions of temperature (400 OC) and pressures of toluene (0.77 H a )
and ammonia (2.58 kPa). Then, the pressure of oxygen was increased to the level of selective
conditions for nimle formation, and activities were measured as a function of time. The results are
given in Table 1. For comparison data are also included for a sample freshly prepared as YBa2Cu306
and heated to reaction temperature in nitrogen.
TABU 1
Reaction ratesa at 400 as a function of the-on-stream for various
samples after reductive treatment.
Sample
Rate x I@ (moles m-2 min-1)
Ti
Niaiie
CO,
co
YBa2Cu306
1.97
2.05
2.14
0.36
0.27
0.23
0.02
0.02
0.01
10
25
40
YBa2cu3@
1.65
1.61
1.59
0.33
0.32
0.32
0.02
0.02
0.02
10
25
40
smBaZcu3%
1.77
2.08
2.10
0.33
0.34
0.33
0.02
0.03
0.03
10
25
40
Y2BaCuOg
0.27
0.42
0.5 1
0.89
0.63
0.52
0.03
0.03
0.03
10
25
~~
40
aPo = 2.16 kPa, PA = 2.58 Wa, and P, = 0.77 kPa.
From the table it could be observed, that the behaviour of reduced YBa2Cu307, and
SITIB~~CU~O,
is similar to that of YBa2Cu306, which is active and selective for toluene ammoxidation
under the conditions used in the experiments. The Y2BaCuOg compound is found to be less active
and less selective.
Catalvsis on Y B a D &
Reduced YBa2Cu307 sample, with a composition close to YBazCug06, was then used for
experiments in which the partial pressures of reactants were varied. The results are given in Figs. 4-6.
In series where the partial pressure of ammonia or toluene was varied, the partial pressure of oxygen
was kept low.
279
In Fig. 4 are the rates obtained when the partial pressure of oxygen was varied gradually from
low region towards high. This figure shows some features worth noting: i) There is a clear region of
sharp transition in selectivity, ii) At low partial pressure of oxygen, the catalyst is selective for nitrile
formation, iii) At higher pressures, the activity towards total combustion dramatically increases and is
about ten times higher than before reduction, and iv) The passing of the rate for CO formation
through a maximum.
40 r
Fig. 4. Effect of partial pressure of oxygen on rates for formation of products over YBa2Cu306x,
x = 0. PA = 2.58 kPa and PT = 0.77 kPa. Notations: cf. Fig. 1.
..
h H g @pa)
Fig. 5. Rates for formation of products on YBa2Cu30bx, x = 0, versus partial pressure of ammonia.
PO = 2.16 kPa and PT = 0.77 Ha. Notations: cf. Fig. 1.
280
Figure 5 (above) shows how the rates vary as the partial pressure of ammonia is varied from high
region towards low at fixed partial pressures of oxygen and toluene. The rate for nitrile formation
passes through a maximum and is higher than before reduction, cf. Fig. 2. Benzaldehydeis formed at
low partial pressure of ammonia but not in its absence. A sharp increase in the rate of C02 formation
occurs as low partial pressures are approached.The rate for CO formation also increases but declines
at zero pressure of ammonia.
The dependency of rates on partial pressure of toluene is given in Fig. 6. There is an almost first
order dependency of rates on pressure of toluene. Comparison with Fig. 3 shows that the rates for
formation of nitrile and C02 have reversed places. The rate for formation of C02 before reduction
was higher than after reduction, whereas for nitrile formation the opposite is the case. Another feature
is the fact that after reduction aldehyde is not formed when the partial pressure of oxygen is
maintained low.
Fig. 6. Influence of partial pressure of toluene on rates for formation of products over YBa2Cu306tx,
x = 0. PO = 2.16 P a and PA = 2.58 P a . Notations: cf. Fig. 1.
In Fig. 7 are reaction rates plotted as a function of reaction time for a YBa2CU306 sample which
before use had been stored under ambient conditions for 10 days. Initially, the material though active
was non-selective. After use for few hours, the rate for C02 formation dropped to a very low value,
while that for nitrile formation increased more than twice. This behaviour was always observed when
using YBa~Cu306samples which had been stored in an air atmosphere for several days. It is
probably due to removal of some oxygen species which have been incorporated into the lattice during
storage.
281
Time on stream
(mln)
Fig. 7. Reaction rates over YBa2Cu306 as a function of time. PO = 2.16 kPa, PA = 2.58 kPa, and
PT = 0.77 kPa. Notations: cf Fig. 1.
TABLE 2
Lattice constants (A) and oxygen content (x) of catalysts.
Fmh sample
orthorhombic
a=3.8203(8)
b=3.8853(7)
~=11.679(2)
x= 1
tetragonal
a=b=3.8582(2)
~=11.830(1)
X=O
At high Po,
before reductive
eeatmentg
tetragonal
a=b=3.858l(5)
c=l1.764(1)
x=0.4
At IOW Po,
after reductive
treatmentb
tetragonal
tetragonal
a=b=3.8572(4) a=b=3.8569(2)
~=11.830(1)
x=o
At high Po,
after reducuve
treatmen@
~~
~=11.834(1)
X=O
orthorhombic
a=3.855(1)
b=3.892(1)
c=l1.712(2)
orthorhombic
a=12.177(2)
b=5.6571(9)
c=7.130(1)
tetragonal
a=b=3.8841(5)
orthorhombic
a=12.172(1)
b=5.6590(5)
c=7.1294(7)
x=l
c=l1.829(2)
x=o
tetragonal
tetragonal
a=b=3.8572(4) a=b=3.857l(4)
c=l 1.818(2)
~=11.819(2)
x=o
X=O
~~
aP0 = 17.30 kPa, PA = 2.58 kPa, and PT = 0.77 kPa
bPo = 2.16 kPa, PA = 2.58 kPa, and PT = 0.77 kPa.
~
-~
282
Lattice constants determined by X-ray diffraction are given in Table 2 for various samples. Also
included are x-values as estimated using the published relationship between lattice constants and
oxygen content of YBa2C~306+~
phases [12]. The composition of Sm-substituted samples was
estimated by comparing cell parameters and catalytic activity with corresponding values for
Y B ~ ~ C U ~phases.
O , ~ +From
~ the table, it can be concluded that the x-value of used YBa2CugOhX
sample not being subjected to reductive treatment is well above zero.After reductive treatment and
further use at low and high partial pressure of oxygen, respectively, the oxygen content of catalysts is
close to 6 oxygen atomdunit cell. However, the c axis repeat of catalysts used at high oxygen
pressure, non-selective conditions. was always found to be slightly shorter than that measured after
use at low oxygen pressure, selective conditions. This implies that the x-value for catalysts run under
non-selective conditions is slightly above zero.The lattice constants determined for Y2BaCuOg are
identical for freshly prepared and used samples, and they also agree with those reported in the original
structure determination [141.
DISCUSSION
A drawing of the YBa2Cu307 structure is shown in Fig. 8. There are two structurallydifferent Cu
positions, noted Cu(1) and Cu(2) [15]. The formers are connected via 0(4), thus, forming chains in
the [OlO]direction between Ba-layers. Cu(1)-chains are connected to Cu(2)-layers by O(1). In the
Cu-layers, Cu(2) is coordinated to five oxygen species, 2 x 0(2), 2 x O(3) and 1 x O(1). It has been
shown that there are no distinct Cu2+ and Cu3+ sites. The valence of Cu in both sites is intermediate
between +2 and +3 [6].Oxygen O(4) in the chains have been found to be mobile and can be totally
abstracted [5].When this occurs, the structure changes from orthorhombic YBa2Cu307 to tetragonal
YBa2Cu306. The latter structure can simply be derived from the former by removal of O(4) so that the
coordination of Cu(1) is changed from square. planar to linear twofold [16]. As aresult, distinct Cu*+
at Cu(1) sites, and Cu2+ at Cu(2) sites are formed [6].
Fig. 8. Drawing of the YBa2Cu307 structure.
283
From the fact that the main difference between the structures of the orthorhombic and tetragonal
phases is connected to the coordination of Cu( l), it follows that it is reasonable to compare their
catalytic behaviom in terms of possible surface coordinationsof Cu( 1).
At the surface of YBa~Cu307.undercoordinatedCu(1) and Cu(2) can exist, serving as possible
adsorption sites for toluene and ammonia. The number of undercooniinated species depends on the
partial pressure of oxygen. Molecular oxygen can adsorb in the form of diatomic species. As a
consecutive step, when dissociation is possible, monoatomic oxygen species can also be formed.
However, dissociation is probably not facile due to lack of oxygen vacancies in the bulk. A common
feature of oxygen species pmjecting from the surface is that they are undercoordinated,which renders
them electrophilic in character. It has been established that electrophilic oxygen participates in the
degradation of hydrocarbons leading to total combustion [7,9,17]. Indeed, YBa2Cu306tx. with x
well above zero, was found to be non-selective in catalytic (ammhxidation, cf. Figs. 1-3.
In YBa2Cu306, Cu(1) is two-coordinated due to that O(4) positions are vacant. After adsorption
of molecular oxygen, two options are possible depending on the partial pressure of oxygen. At low
pressure of oxygen, adsorbed diatomic oxygen can react with co-adsorbed ammonia to give water
under simultaneous oxidation of low valent Cu( 1) to Cu3+ and formation of nucleophilic Cu=Oand
Cu=NH species. Substantial evidence exist for nucleophilic oxygen species and imido species to be
involved in selective oxidation and ammoxidation mechanisms, respectively [7-9], which is vexified
by the present investigation. Figures 4-6 show that YBa2Cu306 is selective for nitrile formation at
low partial pressure of oxygen. Furthermore, the finding that the rate for formation of benzaldehyde is
zero in absence of ammonia, and passes through a maximum as the partial pressure of ammonia is
increased suggests that co-adsorption of ammonia is a prerequisite for formation of nucleophilic
oxygen species. On the contrary, when the partial pressure of oxygen is high, the catalyst is nonselective, cf. Fig. 4. This can be seen as a result of the facile dissociation of adsorbed diatomic
oxygen at YBa2Cu306 One of the oxygen species can migrate into a neighbowing oxygen vacancy
situated between two Cu(1) sites. Consequently, the remaining monoatomic surface species will have
electrophilic character due to that Cu has to share its availablevalence electrons between both oxygen
species.
The rate for formation of CO2 over YBa2Cu306tx at high pressure of oxygen depends on the
value of x. When the value is small, the rate is much higher compared to when x is high, cf. Figs. 1
and 4. Several explanations are possible for this behaviour, of which a few will be mentioned briefly.
One is that the electronic properties of surfaces must be influenced by the occupancy frequency of
exterior O(4) positions, cf. Fig. 7, consequently affecting adsorption and reactivity properties.
Another factor of importance is that the number of active sites increase when the value of x decrease.
In case of YBa2Cu307, if extending the bulk structure to the surface, Cu(1) at (100) faces cannot
adsorb pmjecting single coordinated oxygen species. When the composition is close to YBa2Cu306,
such an adsorption is possible producing electrophilic oxygen species on the condition that
neighbowing O(4) positions are only partly filled.
284
At low pressure of oxygen, 2.5-5 kPa, the rates for formation of nitrile and C02 over
yBa2cu306 at 400 % are 16-19 and 2-4 pnole m2min-l, respectively. Over V205, under the same
conditions. the corresponding rates are 2-4 and 0.3-0.5 pmole m-2 min-1, respectively [18,19]. In
conclusion, it has been shown that YBa2Cu306 is an active and selective catalyst for ammoxidation of
toluene at low partial pressures of oxygen.
ACKNOWLEDGMENT
Financial support from the National Swedish Board for Technical Development (STU) and the
Swedish Natural Science Research Council (NFR) is gratefully acknowledged.
REFERENCES
1
2
3
4
5
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285
B. DELMON (Univ. Catholique de Louvain, Belgium): Due to the fact
that catalyst surfaces are usually reduced in their steady state
during catalytic oxidation it might seem doubtful that copper
remains in the Cult o r Cus+ oxidation state, with no Cuo and,
consequently, Cu crystallites being formed. The absence of new
lines in X-ray diffraction cannot be a fully convincing proof
since small crystallites might not be detectable.
Did you find a change in the intensity ratio of Cu/Y or Cu/Ba
XPS lines, or changeso of ISS signals after use of the catalyst?
If really no Cu were formed, this would indicate a really
exceptional strength of the chemical bonds involving Cu. In
ammoxidation, ammonia is a very strong reducing agent, even in
the presence of 0,. If this is so, this could give a clue to the
very special electronic structure of superconductors.
A. ANDERSSON (University of Lund, Sweden): For both freshly
prepared samples and used samples only X-ray diffraction lines
belonging to YBa,Cu,O,+x could be detected. Use in catalytic
reaction did not cause any change in the intensity of the X-ray
lines that cannot be explained as due to change in oxygen content. Also, XPS analysis did not show formation of Cuo. However,
the ratio of Cu/Y and Cu/Ba XPS lines showed some dependence on
reaction conditions. In this regard, it should be noted that
YBa2Cu306+xfaces can expose both Cu-, Y-, and Ba-layers and that
their distribution possibly depends on the composition of the
reactant stream.
O.V. KRYLOV (Acad. of Sciences, MOSCOW, USSR): In connection with
an interesting observation of Dr. Andersson and his collaborators
I should like to comment about many similarities between high
temperature semiconductors and oxide catalysts of partial oxidation. Both of them have oxygen-deficient lattice. In the case of
high temperature semiconductors, oxygen vacancies in the lattice
must be stable and only motion of electron pairs must be observed. On the contrary, in oxide catalysts of partial oxidation
such vacancies must move. It is very possible now to search new
high temperature semiconductors from oxidative catalysis.
A. ANDERSSON: Thank you for your comment, we believe that such an
approach may yield fruitful results.
M. MISONO (The University of Tokyo, Japan): Very interesting
results. I would like to know more about the chemical reactivity
and the composition of the surface of YBa,Cu,O,+x.
Is it stable at high temperatures against CO,, H,O, etc.? Is
the surface composition the same as in the bulk? Segregation of
certain elements (Ba, etc.) has often been indicated in the
reported papers of electric conductivity.
A. ANDERSSON: Our XPS results, that will be published elsewhere,
clearly show the existence of carbonate species both in freshly
prepared samples and in used samples. In fresh samples, the
amount is highly dependent on the preparation method used. After
use in catalytic reaction, only a minor variation of the amount
of carbonate species in comparison with fresh samples was observed.
Examination of the catalyst before and after use in the
reactor by high-resolution transmission electron microscopy,
286
r e v e a l s a n i n c r e a s e i n t h e number o f c r y s t a l s t r u c t u r e d e f e c t s on
t h e ( 0 0 1 ) p l a n e ( r e f . 1 ) . Such d e f e c t s a r e p o s s i b l y formed under
t h e i n f l u e n c e o f H 0 ( r e f . 2 ) . However, once a s t e a d y s t a t e h a s
been r e a c h e d , no change w i t h t i m e i n t h e f o r m a t i o n o f p r o d u c t s
was detected f o r t h e p e r i o d it was used, which was up t o 3 d a y s .
1
2
S . Hansen, J . O t a m i r i , J.-0. Bovin and A .
3 3 4 (1988) 1 4 3 .
B . G . Hyde e t a l . , N a t u r e , 327 (1987) 4 0 2 .
Andersson, N a t u r e ,
PAJONK (Univ. Claude B e r n a r d Lyon I , F r a n c e ) : I would l i k e
t o know i f your c a t a l y s t i s s t a b l e w i t h t i m e on s t r e a m .
Due t o t h e m o b i l i t y of oxygen i n s i d e t h e s t r u c t u r e o f your
h i g h Tc s u p e r c o n d u c t o r , why d i d you n o t t r y t o o x i d i z e , e . g . ,
p r o p y l e n e which c o u l d have been l e s s complex t o i n t e r p r e t w i t h
r e s p e c t t o t h e r e a c t i o n mechanism?
G.M.
A . ANDERSSON: R e f e r r i n g t o t h e a n s w e r s g i v e n t o p r o f e s s o r s Delmon
and Misono, some s t r u c t u r a l changes were o b s e r v e d a s a r e s u l t of
c a t a l y t i c r e a c t i o n . Once a s t e a d y s t a t e was r e a c h e d , t h e
p e r f o r m a n c e o f t h e c a t a l y s t was s t a b l e a s l o n g a s i t was u s e d ( u p
t o 3 days).
I n comparison w i t h t o l u e n e o x i d a t i o n , w e do n o t t h i n k t h a t
t h e mechanism o f p r o p y l e n e o x i d a t i o n i s less complex.
the
G. Centi and F. Trifiro' (Editors),New Developments in SelectiveOxidation
0 1990 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands
287
CATALYTIC PROPERTIES OF THE HIGH -TEMPERATURE SUPERCONDUCTOR
Y-Ba-Cu-Ag-0 TOWARDS THE OXIDATION OF METHANOL
D. KLISSURSKIl, J. PESHEVAl, Y. DIMITRIEV2, N. ABADJIEVA' and
L. MINCHEV3
'Institute of General and Inorganic Chemistry, Bulgarian Academy
of Sciences, Sofia 1040 (Bulgaria)
'Higher Institute of Chemical Technology, Sofia 1756 (Bulgaria)
31nstitute of Kinetic and Catalysis, Bulgarian Academy of Scien ces, Sofia 1040 (Bulgaria)
SUMMARY
The behaviour of oxygen in superconducting ceramic materials of
the systems Y-Ba-Cu-0 and Y-Ba-Cu-Ag-0 and their catalytic activity
and selectivity with respect to the oxidation of methanol have been
studied simultaneously. The catalytic properties of the high-temperature superconductors are compared with their structure and phase
composition and the reactivity of surface and bulk oxygen. It has
been shown that at least two different forms of oxygen are presented
in Y-Ba-Cu-Ag-0 catalysts. It is found that this class of compounds
catalyzesmainly complete oxidation. On the contrary the Y-Ba-Cu-0
catalyst is selective towards the oxidation of methanol to formaldehyde. Comparative studies of the two classes of compounds have
shown that the selectivity with respect to mild oxidation of methanol depends strongly on the structureandphase purity of the super
conducting materials.
-
INTRODUCTION
Extensive studies of the physico-chemical properties of various
compositions of the Y-Ba-Cu-0 system have been carried out(refs.l,
2 ) in associationwith the high-temperature superconductivity (above
90K) established for the YBa2C~307,~(1:2:3)phase. Since the oxy gen content in this phase can vary within definite limits ( 6 < x < 7 )
and depends strongly on the additional treatment (ref. 3), it can be
assumed that materials of this kind would be o f both practical and
scientific interest.
The investigations carried out up to now show that the structure and the electric properties of the compound YBa2Cu307,xstrongly
depend on the values of "x" (ref. 2 ) . The changes in the transition
temperature Tc are ascribed to the transformation of the crystalline phase from an ortho-rhombic to a tetragonal structure. It is
shown that Tcdecreases monotonically when the values of "x" change
in the range 7-6,4,which is accompanied by destruction of the o r -
288
tho-rhombic s t r u c t u r e ( r e f . 3 ) . I t i s e v i d e n t t h a t t h e c h a n g e s i n oxyg e n c o n t e n t o f t h e Y B a 2 C u 3 0 7 - x p h a s e a n d t h e t r e a t m e n t of t h e m a t e r i a l s
a r e t h e main f a c t o r s a f f e c t i n g t h e s u p e r c o n d u c t i n g p r o p e r t i e s . 0 n t h e
o t h e r h a n d , i t i s known t h a t a number of p h y s i c o c h e m i c a l p r o p e r t i e s
of t h e complex o x i d e systems and t h e i r c a t a l y t i c a c t i v i t y d u r i n g
o x i d a t i o n p r o c e s s e s depend s t r o n g l y on t h e i r s t o i c h i o m e t r y . T h i s
provoked a p a r a l l e l s t u d y of t h e b e h a v i o u r of oxygen i n s u p e r c o n
d u c t i n g ceramic m a t e r i a l s of t h e s y s t e m s Y-Ba-Cu-0 and Y-Ba-Cu-
-
Ag-0 ( r e f s . 4-6) and the c a t a l y t i c a c t i v i t y and s e l e c t i v i t y of
t h e s e s y s t e m s . T h e t e s t r e a c t i o n u s e d was o x i d a t i o n of m e t h a n o l .
METHODS
were s y n t h e s i z e d b y a c l a s s i c a l c e ramic t e c h n o l o g y . T h e m i x t u r e c o n s i s t i n g o f Y 2 O 3 , B a C O 3 a n d C u 0 w i t h a
Y:Ba:Cu r a t i o of 1 : 2 : 3 was baked u p t o 93OoC f o r 1 2 h . Samples of t h e s y s temY-Ba-Cu-Ag-Owereprepared f r o m t h e s a m e i n i t i a l m a t e r i a l s w i t h l - 1 0
w t % A g 2 0 ( r e f . 7). X-ray p h a s e a n a l y s i s w a s p e r f o r m e d w i t h a D R O N 2M
d i f f r a c t o m e t e r . The s p e c i f i c s u r f a c e a r e a s were d e t e r m i n e d b y t h e BET
m e t h o d u s i n g k r y p t o n a d s o r p t i o n . T h e m o b i l i t y o f oxygen i n t h e s a m p l e s
w i t h t h e c o m p o s i t i o n Y - B a - C u - O a n d Y - B a - C u - A g - 0 ( 5 w t % A g 2 0 ) w a se s t i m a t e d by q u a n t i t a t i v e d e t e r m i n a t i o n o f u n s t a b l e oxygen. For t h a t p u r p o s e ,
adirectthermaldesorptionmethoddevelopedbyoneof theauthors (ref.8)
was u s e d f o r s t u d y i n g t h e s t o i c h i o m e t r i c d e v i a t i o n s i n o x i d e c a t a l y s t s
and o t h e r o x i d e materials.
Temperature-programmed d e s o r p t i o n of oxygen from t h e same compos i t i o n s was c a r r i e d o u t a t 25-800°C and a h e a t i n g r a t e of 25'C/min.
A
f l o w a p p a r a t u s and a He f l o w ( 9 9 , 9 % ) w i t h a r a t e of 60ml/min were
utilized.
The p h o t o e l e c t r o n s p e c t r a of t h e s a m p l e s Y-Ba-Cu-0 and Y-Ba-CuAg-0 ( 5 w t % A g 2 0 ) w e r e r e g i s t r a t e d withanESCALAB-1 a p p a r a t u s a t room
t e m p e r a t u r e . The s p e c t r a were c a l i b r a t e d a l o n g t h e C l s ( 2 8 4 , 8 e V ) l i n e .
The main k i n e t i c p a r a m e t e r s of m e t h a n o l o x i d a t i o n w e r e i n v e s t i g a t e d on the c o m p o s i t i o n s Y-Ba-Cu-0 and Y-Ba-Cu-Ag-0 ( 5 w t Z A g 2 0 ) a n d a
f l o w - c i r c u l a t i o n a p p a r a t u s was u s e d ( r e f s . 9 - 1 1 ) . The measurments
were made w i t h i n t h e t e m p e r a t u r e r a n g e 300-425°C a t methanol con c e n t r a t i o n s of 3 , 3 6 - 3 , 4 0 v o l % and c a t a l y s t g r a i n s of 0 , 3 - 0 , 6 mm.
The r a t e of the i n i t i a l m e t h a n o l - a i r m i x t u r e v a r i e d between 6,O 8,O m l / m 2 s .
Samples of the s y s t e m Y-Ba-Cu-0
RESULTS AND DISCUSSION
The X-ray a n a l y s i s of sample Y-Ba-Cu-0
shows t h a t the p r o d u c t ob-
289
t a i n e d h a s an ortho-rhombic s t r u c t u r e w i t h c h a r a c t e r i s t i c "d"va1ues
( 8 ) r a n g i n g from 20-60' ( d = 1 , 5 8 ; 1 , 9 4 ; 2 , 3 3 ; 2 , 7 2 ; 2 , 7 4 ; 3 , 8 9 8 ). I t
was establishedthatthesampley-Ba-Cu-Ag-0 ( 5 w t % A g 2 0 ) i s p o l y p h a s e
andcontains, inadditiontothemain1:2:3phase, acertainamountofCu0
s t u d i e s on t h e m i c r o s t r u c t u r e shoa n d a 2:l:lphase(seeFig.I).Earlier
wed s i l v e r t o a p p e a r m a i n l y a t t h e c r y s t a l l i t e i n t e r f a c e s a s m e t a l i n c l u s i o n s o r a c o p p e r - c o n t a i n i n g a l l o y ( r e f . 7 ) . I t was s h o w n t h a t p a r t o f t h e
s i l v e r might be d i s s o l v e d i n t h e s u p e r c o n d u c t i n g ( 1 : 2 : 3 ) phase ( 0 , 2 3
1 , 1 5 w t % )( r e f . 7 ) . T h e c r y s t a l s i z e o f A g v a r i e s i n a r e l a t i v e l y l a r g e
r a n g e : 5 - 2 0 , w ( r e f . 7 ) . Nochange i n t h e s i l v e r c r y s t a l s i z e i n t h e sample
a f t e r t h e c a t a l y t i c t e s t w a s foundby t h e e l e c t r o n - m i c r o a n a l y s i s (Phil i p s SEM505 EDEX). The f o l l o w i n g superconductivityparameters werefound
f o r t h e same composition: Tc(0,5Rn)=90,7 K ; Tc end= 87,O K ; ATc=3 , O K.
T h e m a i n r e s u l t s f r o m a s t u d y o n t h e c a t a l y t i c p r o p e r t i e s o f thecompos i t i o n Y - B a - C u - O a r e p r e s e n t e d i n F i g . 2. Formaldehydewasthemainreact i o n p r o d u c t i n t h e t e m p e r a t u r e r a n g e 31O-36O0C.
-
s
Y
C
r
I
9
20
10
u: 0
10
e
F i g . 1. X-ray ' D i f f r a c t i o n of
Y-Ba-Cu-Ag-0 (5 w t % Ag20).
25
-
I
I
t
I
300
340
380
420
T (OCI
F i g . 2. Temperaturedependence
of t h e c o n v e r s i o n o f m e t h a n o l t o
formaldehydeandC02 onaY-BaCu-Ocatalyst.
The r e s u l t s on t h e c a t a l y t i c p r o p e r t i e s of a sampleoftheY-Ba-Cu-Ag-0
S y s t e m a r e p r e s e n t e d i n F i g . 3 . I t was e s t a b l i s h e d t h a t f o r t h e A g - c o n t a i n i n g c o m p o s i t i o n themain r e a c t i o n p r o d u c t was c02. Carbon monoxide andhydrogenwerenot found under the e x p e r i m e n t a l c o n d i t i o n s used. Ihe y i e l d o f the
formaldehyde o v e r the whole t e m p e r a t u r e r a n g e was r e l a t i v e l y low (6,4-7,9%).
T h i s wasalso observed w i t h i n v e s t i g a t i o n s of the c a t a l y t i c p r o p e r t i e s of
290
some o x i d e s - Co30, N i O , Mn203duringtheoxidationof m e t h a n o l ( r e f . 1 2 ) . T h e
selectivitytowardsthecomplete o x i d a t i o n depending o n t e m p e r a t u r e r a n ged from 41-91%, whereas the t o t a l c o n v e r s i o n d e g r e e of methanol reached 86%
.
- 8(3
-s 6C
s
30
I
hl
-J20
0
0
V
V
N
N
40
10
2c
300 340 380 420
T
1
I
24
40
Time ( h )
8
(OC)
F i g . 3. T e m p e r a t u r e d e p e n d e n c e of
t h e c o n v e r s i o n o f m e t h a n o l t o C 0 2 on
aY-Ba-Cu-Ag-Ocatalyst.
I ,
Fig. 4.Dependenceontimeof
theconversiondegreeofmethan o l t o C 0 2 onaY-Ba-Cu-Ag-Ocat a l y s t a t 300 and 35OoC.
The p a r a m e t e r s of t h e c a t a l y t i c p r o c e s s (conversiondegrees of m e t h a n o l
t o f o r m a l d e h y d e and c a r b o n dioxide,%) were d e t e r m i n e d a f t e r e s t a b l i s h i n g
aregime c o r r e s p o n d i n g t o thesteady s t a t e o f the c a t a l y s t . F i g . 4 shows thedependence of the conversion degree of methanol t o carbon dioxide on t h e time of cont a c t of the samplewith themethanol-air mixture a t two different temperatures. A t 30O0C
the amount of formed d i o x i d e d e c r e a s e s two times f o r t h e f i r s t e i g h t h o u r s .
With a rise of temperature,up to 40O0C and subsequent cooling t o 300°C, a f t e r the 40th
hour the amount of carbon dioxide in the reaction mixture remains constant. The same
trend t o a decrease i n a c t i v i t y of the sample was observed a t 35OoC. These r e s u l t s permit
theass~tionthatduringthecatalyticprocessasteadystatecanpositionofthesamp l e is attained, which significantly d i f f e r s from the canposition of the fresh ( i n i t i a l )
samples.Thedecreaseofthecatalyticactivitycouldbeattributedtothegradualevo1ution of unstable (weakly bound) oxygen.The s p e c i f i c surface areas of the fresh and used
Y-Ba-Cu-Ag-0 catalyst are i n the range 0,42 - 0,5 m2/g.The similarity of thesevalues
shows that there is not a process of sintering during the c a t a l y t i c test.
Fig. 5 shows the t e m p e r a t u r e d e p e n d e n c e of t h e amounts of e v o l v e d a n d
u p t a k e n oxygen f o r f r e s h and u s e d A g - c o n t a i n i n g c a t a l y s t s . M e a s u r a b l e
amounts of oxygen a r e e v o l v e d from t h e f r e s h sample a l r e a d y a t 3 8 O o C ,
7OO0C i s a b o u t
w h i l e t h e t o t a l amount o f e v o l v e d oxygen a t 20
-
t
291
4,6ml/g(curvea).Obviously,the
t o t a l amount of oxygen c a n n o t be
uptaken f o r t h e time of t h e e x 5
p e r i m e n t s . ( c u r v e b ) . These res u l t s a r e i n agreement w i t h t h e
s t u d i e s of T r i p a t h i e t a 1 p e r formed by o t h e r methods ( r e f . 1 3 ) .
Curves c and d s h o w t h e t h e r m a l
desorptionand subsequentadsorp t i o n o f oxygen f o r t h e u s e d c a t a l y s t . O x y g e n d e s o r p t i o n i s observed
o n l y a t 5OO0C, t h e t o t a l amount of
e v o l v e d oxygen being c o n s i d e r a b l y s m a l l e r . T h e d r o p of temperat u r e r e s u l t s i n a d s o r p t i o n of
practicallythewholearnountofdeT("C1
s o r b e d oxygen ( c u r v e d ) .
Fig. 6 p r e s e n t s theTPDcurves
F i g . 5. Temperature dependence
of t h e a m o u n t s o f e v o l v e d ( a , c )
of t h e samplesY-Ba-Cu-OandY-Baand u p t a k e n ( b , d ) o x y en f o r
Cu-Ag-0. E v i d e n t l y , t h e oxygen
f r e s h ( a , b ) and used & , d )
Y-Ba-Cu-Ag-0 c a t a l y s t s .
chemisorbedonthecatalyst s u r f a c e o f t h e sampleY-Ba-Cu-Ag-0
e x i s t s i n a t l e a s t two forms, t o which d e s o r p t i o n maxima a t 530 and 74OoC
c o r r e s p o n d . TheTPDcurveof t h e u s e d c a t a l y s t i n d i c a t e s p r a c t i c a l l y no
oxygen d e s o r p t i o n a t 400-600°C, and t h e e v o l v e d amount of oxygen i s
two times 1ower.The TPD c u r v e of a freshY-Ba-Cu-Osample h a s a d i f f e r e n t s h a p e . Only one d e s o r p t i o n peak a t 6OO0C i s observed.
The chemical a n a l y s i s of t h e s u r f a c e of theinvestigatedcatalysts
c o n f i r m s t h e T P D r e s u l t s . The p h o t o e l e c t r o n s p e c t r a o f a Y-Ba-Cu-0 samp l e c o n t a i n s i n g l e peaks c o r r e s p o n d i n g t o t h e b i n d i n g e n e r g i e s of t h e
3d e l e c t r o n s of Y ( 1 5 6 , 4 e V ) a n d B a ( 7 7 9 , 8 eV) and 2p e l e c t r o n s of Cu
( 9 3 3 , 9 e V ) . .The s p e c t r a of a Y-Ba-Cu-Ag-O(5wt%Ag20) s a m p l e e x h i b i t
d o u b l e peaks with t h e f o l l o w i n g 3d e l e c t r o n b i n d i n g e n e r g i e s :
Y - E B = 1 5 5 , 7 ; 1 5 7 , 4 e V ; B a - E B = 7 7 9 , 7 ; 782,beV; A g - E B = 3 6 7 , 8 ;
369,9eV. I t can be assumed t h a t on t h e s u r f a c e of t h e sample w i t h
5 w t % Ag20 t h e oxygen i s bonded t o t h e s e p a r a t e e l e m e n t s i n two d i f f e r e n t ways which probably c o r r e s p o n d t o two a d s o r p t i o n forms of
oxygen.
The p r e s e n c e of two a d s o r p t i o n forms of oxygen i s u s u a l l y o b s e r v e d
w i t h s i m p l e a n d complexoxides (Cr203,Mn02, C0304) i n t h e p r e s e n c e of
which d e e p o x i d a t i o n of o r g a n i c s u b s t a n c e s i s a c h i e v e d ( r e f . 1 4 ) .
100
200
300 400
500 600 700
T I°Cl
800
F i g . 6 . TPD c u r v e s of oxygen from Y-Ba-Cu-0 ( a - f r e s h , b - u s e d )
and Y-Ba-Cu-Ag-0 ( c - f r e s h , d - u s e d ) c a t a l y s t s .
The l o w - t e m p e r a t u r e form o f oxygen a d s o r p t i o n i s d e f i n e d a s "weak
bonded". I t h a s v a l u e s c l o s e t o t h e s e of t h e bond e n e r g y of c h e m i s o r b e d s u r f a c e oxygen p o s s e s s i n g a h i g h r e a c t i v i t y .
I t c a n be assumed t h a t d u r i n g t h e c a t a l y t i c p r o c e s s , w e a k l y b o u n d
oxygen forms from t h e Y-Ba-Cu-Ag-0
s u r f a c e a r e t h e f i r s t t o react
with substance being oxidized with r i s i n g temperature. This determin e s a h i g h i n i t i a l c a t a l y t i c a c t i v i t y of t h e f r e s h c a t a l y s t w i t h resp e c t t o t h e d e e p o x i d a t i o n of m e t h a n o l a t r e l a t i v e l y low t e m p e r a t u r e s . According t o c u r r e n t c o n c e p t s ( r e f . 15) c o m p l e t e , i . e . d e s t r u c t i -
ve o x i d a t i o n of methanol t o CO and C 0 2 p r e v a i l s a t lowbond e n e r g i e s
of s u r f a c e oxygen.
The s e l e c t i v i t y w i t h r e s p e c t t o t h e p a r t i a l o x i d a t i o n o f m e t h a n o l
u n d o u b t e d l y d e p e n d s s t r o n g l y on t h e p h a s e p u r i t y of t h e s u p e r c o n d u c t i n g m a t e r i a l s . I n t h e c a s e u n d e r c o n s i d e r a t i o n t h e p r e s e n c e of f r e e
C u ( 1 I ) o x i d e and Ag f a v o u r s t h e d e s t r u c t i v e o x i d a t i o n of m e t h a n o l .
The h i g h - t e m p e r a t u r e s u p e r c o n d u c t o r s a r e of g r e a t i n t e r e s t a s c a t a l y s t s f o r o x i d a t i o n p r o c e s s e s . E x t e n s i v e s t u d i e s on t h e i r s t r u c t u r e
and p h y s i c a l p r o p e r t i e s a l l o w l o o k i n g f o r new c o r r e l a t i o n s between
t h e s e p a r a m e t e r s a n d t h e c a t a l y t i c a c t i v i t y and s e l e c t i v i t y . T h e con c l u s i o n s on the c a t a l y s t s s e l e c t i v i t y t o w a r d s p a r t i a l o x i d a t i o n p r e s u p p o s e a very p r e c i s e s t u d y of t h e i r p h a s e c o m p o s i t i o n , s t r u c t u r e
293
and the behaviour of the oxygen in them.
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294
E-MICHELI (Snamprogetti, Milano, Italy): Do you correlate the
different selectivity of the two classes of compounds with their
structure 7
J.PESHEVA (Institute of General and Inorganic Chemistry, Bulg.
Acad.Sci., Sofia, Bulgaria): Obviously, the different chemical
and phase composition as well as the surface properties determine
the different catalytic behaviours of these types of compounds.
The both initial X-ray diffraction patterns show the presence
of an orthorhombic structure and a transition to a tetragonal
structure appears after the catalytic test. On the other hand,
the X-ray analysis show that Y-Ba-Cu-Ag-0 samples are rnultiphase and contain in addition to the main 1 : 2 : 3 phase, someamount
of CuO and a 2:l:l phase. This can be related with the lower selectivity of these materials with respect to the partial oxidation of methanol.
E.MICHEL1 : How have you determined the amounts of evolved and uptaken oxygen by increasing and subsequantly drop of the temperature ?
J.PESHEVA : This is a new direct and sensitive thermodesorption
method for determination of non- stoichiometric oxygen in oxide
catalysts. The method is developed by Klissurski D. and is reported in the references of the paper (ref. 8).
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SELECTIVE
CATALYSTS
J.M.
OXIDATION
OF
PROPYLENE
OVER
RARE
295
EARTH-MOLYBDATE
LOPEZ NIETO, R. BIELSA*, G. KREMENIC'l and J.L.G. FIERRO
Instituto de Catalisis y Petroleoquimica, C.S.I.C., Serrano 119,
2 8 0 0 6 Madrid (Spain)
*On leave from INTEC-CONICET, 3000 Santa Fe (Argentina)
ABSTRACT
Catalytic activity for the selective oxidation of propylene
over Mo-RE-0 (RE=Pr,Sm,Tb,Yb) catalyst series, with Mo/(Mo+RE)
atomic ratios ranging between 0 and 1, has been studied. For all
catalyst series, both activity and selectivity to partial
oxidation products exhibited a relative maximum in the Mo-rich
compositions region. These data are interpreted in terms of
surface and bulk characteristic of the catalysts as revealed by
X-ray diffraction, temperature-programmed reduction, laser-Raman
and X-ray photoelectron spectroscopic methods.
INTRODUCTION
Molybdenum-based catalysts are commonly used in many industrial processes which involve selective oxidation of olefins [I].
Rare earth (RE) oxides catalyse a great variety of reactions and
promote the partial oxidation of light hydrocarbons [ 2 ] . With the
only exception of Ce-containing catalysts [3], the role of rare
earth oxide on the selective oxidation process is not well understood [ 4 ] . Recent studies carried out in our laboratory [ S - 8 1
revealed that catalytic behaviour markedly depends on the composition and type of phases present in the catalysts. This work is
part of a broad study to investigate the effects of the rare
earth promoters on the structure and reactivity of Mo-based
catalysts. For this purpose, the information revealed by several
bulk and surface sensitive techniques is compared with activity
and selectivity of the binary Mo-RE-0 preparations.
EXPERIMENTAL
The catalysts were prepared by mixing
ammonium heptamolybdate and/or RE nitrate solutions of selected
Catalyst preparation.
Deceased
296
concentration and volume to obtain fixed Mo/ (Mo+RE) ratios. The
solutions were evaporated until dryness and then the remaining
solids calcined in a forced flow of air at 823 K for 14 h [ 5 ] .
Catalyst testing. Details of the experimental technique used for
catalytic activity experiments have been given elsewhere [5-81.
In short, 1 . 0 g-samples (particle size between 0.42 and 0.59 mm)
were mixed with Sic (in a volume ratio, catalyst:SiC= 1:4). The
molar ratio of the components in the reactant mixture was
C3H6 : O 2 :He:H20 = 20:30:30:20 and the contact time W/F= 30-90
g.h (no1 C3H6).
'- Experiments were carried out over the temperature range 623-723 K, at atmospheric pressure. The efluents of the
reactor were analyzed by gas chromatography.
Catalyst characterization. Specific surface areas of catalysts
were calculated by the BET method from the Kr adsorption isotherms at 77 K. X-ray diffraction (XRD) patterns were obtained
using a Phillips PW-1100 diffractometer using Ni-filtered CuKa
radiation ( A = 0.15406 nm). Raman spectra (LRS) were recorded
using a Jarrell-Ash 25-300 spectrometer equipped with halographic
gratings. x-ray photoelectron spectra (XPS) were acquired with a
Leybold Heraeus LHSlO electron spectrometer equipped with a
magnesium anode (MgK, = 1253.6 eV) and a hemispherical electron
analyzer. The binding energies were referenced to the Cls line at
284.6 eV. Details of all these techniques are given elsewhere [581. TPR experiments were made in a Cahn microbalance.
I
1
Mo/(Mo+RE)
Figure 1. Reaction rate for C3H6 at 673 K over Mo-RE-0 (RE= Pr,
Tb, Sm, Yb) catalyst series. Contact time W/F= 30 g.h.mo1-l.
297
+
U
W
1
p.1AA
Mo -T b
d
QI
ul
"0
0.5
Mo-Yb
I
1.0
Mo/(Mo + RE1
Figure 2. Selectivity to acrolein ( 0 ) and acetaldehyde + acetic
acid ( A ) at 673 K for a propylene conversion = 5 mole %.
Samples (0.2-0.3 mg) were first heated to 773 K in helium flow
(7.2 dm3 h-l) , and the cooled to ambient temperature. After this,
they were contacted with hydrogen (7.2 dm3 h")
and heated at a
rate of 240 K h-l to a final temperature of 793 K. This temperature was maintained about 0.5 h.
RESULTS AND DISCUSSION
The selective oxidation of propylene over Mo-RE-0 (RE= Pr, Sm,
Tb, Yb) catalysts has been shown to depend strongly on the
catalyst composition. As Fig.1 shows, all four catalyst series
display a clear maximun for the rate of propylene conversion in
the Mo-rich composition region. However, the compositions (expressed by the Mo/(Mo+RE) atomic ratios) at which the maximum appears, varies according to sequence Mo-Yb-0(0.89), Mo-Pr-0(0.89),
Mo-Tb-0 (0.70) and Mo-Sm-0 (0.60) With the only exception of the
Mo-Pr-0 catalyst series, a further decrease of the Mo/(Mo+RE)
ratios, e.g. increasing the fraction of RE oxide added, induces a
marked decrease of the specific catalytic activity. Beside that,
from the data of Fig.1 the activity sequence for the pure RE
oxides ( (Mo/(Mo+RE)= 0.0) follows the order, Pr6011 > Tb4O7 >
.
298
Sm203 > Yb203, which agrees with the one reported by Minachev et
al. 191 for the same reaction.
Selectivity values to acrolein and acetyl (acetic acid +
acetaldehyde) (Fig. 2) also show a similar maximun to the one
found on the activity profiles in the Mo-rich composition region
(Mo/ (Mo+RE) between 0.60 and 0.89), while carbon oxides are
almost the unique C-containing molecules.
As already shown by the XRD patterns, formation of quite
disimilar crystalline phases occurs as catalyst composition is
varied (Table 1). In agreement with literature findings [ i O , i i ] ,
the Mo-rich composition range exhibits the Moog phase as the
major crystalline entity, in parallel with small amounts of
stoichiometric molybdates, and probably some type of tetra- and
hexamolybdates [ i l l , whose abundance decreases for the less Morich preparations. One important point to be considered is that
catalysts with maximun in activity profiles are those having the
largest proportion of molybdates among the overall crystalline
phases. Of course, the Mo-Pr-0 is the exception as no crystalline phases were detected along all compositions range.
TABLE 1
Crystalline Phases as Identified from X-Ray Difrattion Patterns.
Mo
(Mo+RE)
Pr
0
PrsOll
Pr6011
Pr6011
Mo03(e)
Mo03(e)
Moo3
<0.30
0.57
0.70
0.80
0.89
Sm
Tb
Tb407
Sm203( a )
TbsMoO12(c)
Sm203(b)
Sm2M020g(el Tb2 (Moo4) ( f l
Tb2Mo4Ol5(g)
ns
Moo3 (h)
Moo3 (hl
Moo3 (k)
Moo3 (k)
Yb
Yb203 (a)
Yb203(dl
Yb2 (Moo4) (e)
ns
Yb2Mo4ol5(h)
Moog (h)
a= cubic; ns= not studied.
Minor phases: b= 9Sm203.4Mo03: c= Tb4O7; d=Yb2(Mo04)3; e= RE20g:
f=Tb2Mo209; g= Moog; h’RE203.4Mo03
(likely): k= RE203.6Mo03
(likely).
Laser Raman spectra of the Mo/(Mo+RE)= 0 . 8 (RE= Pr, Sm, Tb, Yb)
catalyst samples were also recorded to monitor the presence of
molybdate structures. A s shown in Fig. 3 , all spectra show the
bands at 998 and 820 cm-l characteristic of Mo=O stretch and
antisymmetric Mo-0-Mo stretching, respectively in Moo3 isolate
299
>
c
.I/
C
a
4C
Figure 3 . Laser Raman Spectra of
Mo-RE-0 catalysts (atomic ratio
Moj(MO+RE)= 0 . 8 ) : a) Mo-Yb-0:
b) Mo-Tb-0; C) Mo-SHI-0; a) MoPr-0 catalysts.
*
I
1000
I
900
I
I
800 700
Ag (cm-11
phase. Other bands in the region 800-960 cm'l,
very intense for
Mo-Yb-0, moderately intense for Mo-Tb-0 and very low for Mo-Sm-0
catalysts have been assigned, in agreement with XRD patterns, to
that vibrations in RE molybdates 1121 as its intensity increased
with decreasing Mo-loading. The exception is Mo-Pr-0 catalyst in
which small bands in the same region seem to be due to polymolybdates in a separate phase 161.
To obtain an estimate of the metal-oxygen strength as well as
to explain activity and selectivity changes as a function of
catalyst composition. TPR profiles were obtained for all preparations. Table 2 summarizes the reduction degree of catalysts obtained at 793 K. One important point to be considered is the strong
dependence of TPR profiles upon catalyst composition. For example
in the RE-rich preparations, mostly Mo-Pr-0 [S] and Mo-Tb-0 catalysts series, the reduction degree is larger than in Mo-rich preparations, and also the kinetics of reduction decreases continuously with time indicating that this process takes place
300
Mo
Mo+RE
0.00
0.20
0,57
0,80
0,89
0)
Mo-Pr-0
1.04
0.80
0.70
0.40
0.13
(b)
Mo-Tb-0
1.20
0.70
0.75
1.00
0.20
Mo-Sm-0
0.00
1.04
0.68
0.75
0.31
Mo-Yb-0
0.00
1.27
1.27
1.28
0.91
(a) Calculated by the ratio between the experimental weight loss
and the theoretical one espected for the quantitative reduction
of Moog to MOO? ( a = l ) .(b) Reducible oxides such as Pr6O11 and
Tb4O7 present in the catalysts were considered to be reduced to
Pr203 and Tb2O3, respectively.
according to the contracting sphere model. However, Mo-rich catalysts begin to reduce at higher temperatures and present S-shaped
TPR profiles, i.e., they reduce according to a nucleation model.
Photoelectron spectroscopy (XPS) has also been used from a
quantitative point of view to reveal the surface composition of
catalysts. The dependence between the Mo/(Mo+RE) XPS ratios and
those corresponding to the chemical analysis are given in Fig.4.
As can be observed, for the Mo-RE-0 (RE= Pr, Sm, Yb) catalyst
series there is, in general, a good correlation between surface
XPS and chemical compositions, while for Mo-Tb-0 series an important RE surface enrichment is clearly observed throughout the
explored compositions. In this latter case a Tb molybdate-phase a
few layers thick seems to be formed over Moo3 nuclei, as also
suggested by the well resolved LRS spectra of Tb-molybdates
(Fig.3).
When comparing activity and selectivity data for oxidation of
propylene with those of catalyst characterization it results that
partial oxidation products are more likely to occur on catalysts with lattice oxygen of a lower reactivity, viz., more difficult to be reduced. Moro-oka et al.[lS] found the more active
oxides for total oxidation of hydrocarbons to be those with lower
heat of formation of the oxide ( A H M - O ) . Pr6011 and Tb407 have
low A%-o
values and an important part of unstable lattice oxygen of a high mobility, thus explaining their tendency to form
deep oxidation products when present as separate phases in RE
rich Mo-RE-0 (RE= Pr, Tb) preparations (Figs.1 and 2 ) . A s already
301
-s
1.C
I0
L
;Of
c
w
lx
+0
0
5 0.f
0
r:
0
0.4
0
0.;
I
10 0
0
7'
0
#
0
A
0
0.2
O
I
01,
0.6
I
1
R E)chem
0.8
MOl(MO+
Figure 4 . Dependence between the surface XPS and chemical Mo/
(Mo+RE) atomic ratios: RE= Pr (V);Sm ( 0 ) ;
Tb(0); Yb ( A ) . In
this calculation, the integrated Mo3d and RE4d intensities and
published sensitivity factors [ 1 4 ] were considered.
shown by TPR, AHM-o tends to be larger for catalyst which are
more difficult to reduce. The reduction degree ( a ) at 793 K in
the region Mo/ (Mo+RE)= 0.7-0.8 is the lowest but simultaneously
selectivity to partial oxidation products is the highest (Fig.2).
A similar correlation among catalyst reduction and conversion and
selectivity were found by Sachtler and de Boer [lS] in the propylene oxidation over metallic molybdate catalysts.
These results are closely related to those reported by Trifiro'
et al. [l?], who found that the most selective catalysts (within
a series of molybdates) for the same reaction are those
exhibiting the lowest diffusion rate of lattice oxygen. Oxygen
may be removed by diffusion of lattice oxygen to the interface
reduced phase in all the ternary catalyst systems employed in
this study. Thus, the diffusion rate of oxygen ions will be lower
and the selectivity will be higher for catalysts with lower
reducibility, as it effectively occurs. The fact that maximun
selectivity to partial oxidation products occurs for Mo/(Mo+RE)
ratios in the region 0.7-0.8, where XRD patterns and LRS spectra
302
revealed excess of Moo3 and several kinds of molybdates, indicates that nucleophilic oxygen species, which then would lead to
allylic oxidation, are optimized.
AKNOWLEDGEMENTS
The authors are indebted to CSIC and CAICYT for sponsorship of
this work (Project No. 120).
REFERENCES
a) R.K. Grasselli, J.D. Burrington, A d v . C a t a l . ,
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30 (1981)
133.
r21
131
141
b) C.F. Cullis, D.J. Hucknall, in G. Bond & G. Webb
(Eds.), ggCatalysisgl,
Vol. 5, Specialist Periodical Reports
The Chemical Society, London, (1982) ch. 7, p. 273.
a) M.P. Rosynek, C a t a l . R e v . - B c i . Eng., 16 (1977) 111.
b) P. Pomonis, R e a c t . Kinet. C a t a l . R e v . , 18 (1981) 247.
a) J.C.J. Bart, N. Giordano, J. C a t a l . , 75 (1982) 134.
b) J.F. Brazdil, R.K. Graselli, J. C a t a l . , 79 (1983) 1 0 4 .
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b) V.M. Khiteeva, Sh.M. Rzakulieva, RUBS. J. Phys. C h e m . ,
55 (1981) 1202.
r91
J.M. Lopez Nieto, J.L.G. Fierro, L. Gonzalez Tejuca, G.
Kremenic', J. C a t a l . , l 0 7 (1987) 325.
J.M. Lopez Nieto, G-Kremenic', A. Martinez Alonso, J.M.D.
Tascbn, J. Mater. S c i . , (in press).
G. Kremenic',J.M. Lopez Nieto, J. Soria, J. Marti, Proc.
Inter. C o n f . R a r e E a r t h D e V . L A p p l . , Beijing, China,
September 1985, Vol. 1, p. 614.
G. Kremenic', J.M. Lopez Nieto, J.L.G. Fierro, L.G.
Tejuca, J. L e s s - C o m m o n Met., 136 (1987) 95.
K.M. Minachev, D.A. Kontratev, G.N. Antoshin, K i n e t .
I101
a) K. Nassau, J.W. Shiever, E.T. Keve, J. S o l i d State
151
161
171
I81
Kata.,
8 (1967) 131.
Chem.,
3 (1971) 411.
b) L.H. Brixner, P.E. Biersted, A.W. Sleight, M.S. Lisic,
I111
Mat. Res. B u l l . ,
6 (1971) 545.
a) F.P. Alekseev, E.I. Get'man, G.G. Koshchoev, M.V.
Mokhosoev, R u s s . J. Inorg. C h e m . , 14 (1969) 1558.
b) E. Ya Rode, G.V. Lysanova, L.Z. Gokhman, Inorg. Mater.,
7 (1971) 1875.
1123
H. Jeziorowski, H. Knozinger, J. Phyo. Chem.,
1131
J.M. Lopez Nieto, A.G. Valdenebro, J.L.G. Fierro, in
preparation.
C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.H.
Raymond, L.H. Gale, Surf. Interface A n a l . , 3 (1981) 211.
Y. Moro-oka, Y. Morikawa, A. Ozaki, J. c a t a l . , 7 (1967)
1141
t 151
I161
1171
1166.
23.
83
(1979)
W.M.H. Sachtler and N.H. de Boer, Proc. 3rd. I n t . C o n g r .
C a t a l . , Amsterdam, 1964 (W.M.H. Sachtler, G.C.A. Schuit
and P. Zwietering, Eds), Wiley, New York, 1965, vol.1,
p.252.
F. Trifiro', P. Centola, I. Pasquon and P. Jiru, P r o c .
4 t h . I n t . C O n g r . C a t a l . , MOSCOW, 1968 (B.A. Kazansky,
Ed.), Adler, New York, 1968. Vol.1, p.252.
303
J.C. VEDRINE (I. de Recherche sur la Catalyse, Villeurbanne,
France): I was surprised that you concluded that selective
molybdate catal st exhibit lower diffusion rate of lattice
oxygen. Using l20 labelled C02 as a probe we have observed that
lattice 0 of bismuth molybdates ( a or B phases, kown to be very
selective in propene oxidation to acrolein) are exceptionally
labile involving both surface and bulk lattice oxygen. How did
you determine the lattice oxygen lability of your samples?
J.M. M P E Z NIETO (I. Catdlisis y Petroleoquimica, Madrid, Spain):
The term diffusion rate of oxygen in the rare earth molybdates
refers here to the relative ease with which oxygen can be
released from the catalyst. We found that the catalyst whose
Mo/(Mo+RE) ratio is 0 and 1 are poorly selective to partial
oxidation products, viz. carbon oxides and water were the major
oxidation products.To explain this behavior, it was assumed that
catalysts with these extreme compositions have highly reactive
oxygen species, such as oxygen adsorbed. On the contrary, in the
region of intermediate Mo/ (Ho+RE) ratios , where molybdates were
found to ocour, the bulk lattice oxygen seems to be involved in
the selective oxidation of adsorbed hydrocarbon. The mobility of
the latter oxygen species must be high as confirmed by the
observation that the surface prereduction of the different
molybdates at temperatures close to 600 K is faster than the
subsequent oxygen adsorption on the partially reduced surface.
This particular behaviour has been explained as due to partial
restoration of the original surface, upon surface reduction, by
diffusion of bulk lattice oxygen to the surface which then
adsorbs oxygen slowly until initial state recovery.
J.C. VEDRINE (Ins. de Recherche sur la Catalyse, Villeurbanne,
France): You also found high selectivity in acetic acid and
acetaldehyde which was interpreted as electrophilic attack of
propene rather than allylic. The last is giving acrolein. In a
recent paper by us on MoO3/SiO2 (ref.1) much allylic attack was
detected at low Mo coverage but yielded propanal. Did not you
observed any propanal in your products? Acetic acid results from
a more complex reaction mechanism with C-C cleavage as for
acetaldehyde.
J.M. M P E Z NIETO (I. Cathlisis y Petroleoquimica, Madrid, Spain):
For the Moo3 and MoOj/Si02 systems, Vedrine et al. (ref.1) found
high selectivity toward propanal at conversions levels below 1%.
For the MoOj/Si02 catalysts studied early in our laboratory, we
did not detect propanal at conversion levels as high as 15-20%
(ref.2). In this study working at conversion levels around 10% on
Mo-RE-0 systems, no propanal was detected in any case. Only
acrolein, acetic acid and acetaldehyde were observed.
Acetaldehyde is mainly a primary product
(from propene
degradation), but it also forms by decomposition of acrolein
(ref.3). However, acetic acid is formed by oxidation of C2- and
c3-oxigenated products.
R. LARSSON (I. Inorganic Chemistry, Lund, Sweden):
determined the activation energies of these -action?
Have you
J.M. M P E Z NIETO (I. Catalisis y Petroleoquimida, Madrid, Spain):
Temperature coefficients for propene oxidation on the various
rare earth molybdates have been calculated. They have been not
summarized for practical reasons. In general, the values obtained
304
summarized for practical reasons. In general, the values obtained
do not vary significantly along the explored compositions with
the exception of the Mo/(Mo+RE) ratios with maxima in activity
and selectivity which led to values substantially higher. To
illustrate this, the temperature coefficients for the Mo-Pr-0
catalyst series were 106-119 kJ/mole for compositions Mo/ (Mo+RE)
< 0.88, while a value of 143 kJ/mole was obtained for the most
active no/ (Mo+RE) = 0.91 catalyst.
R. LARSSON (I. Inorganic Chemistry, Lund, Sweden): Have you IR
spectra of the catalysts?
J.M. LOPEZ NIETO (I. Catdlisis y Petroleoquimica, Madrid, Spain):
Exploratory experiments using IR technique revealed the
appearance of several M-0-M lattice vibrations, however the
unambiguous assignment of that bands to specific compounds was
not straighforward. Very recently, an in-depth analysis of these
molybdate series was carried out by Laser Raman Spectroscopy in
our laboratory. This study will constitute the next step of the
research of the bulk and surface properties of the rare earth
molybdates.
Liu, M. Forissier, G. Coudurier, J. C. Vedrine, J. Chem.
Faraday Trans. 1, 85 (1989) 1607.
2) J. M. M p e z Nieto, G. Kremenic', A. Martinez-Alonso, J. M. D.
Tascbn, J. Mater. S c i . , 24 (1989) (in press).
3) J. M. M p e z Nieto, J. M. D. Tascbn, G. Kremenic', Bull. Chem.
SOC. Jpn., 61 (1988) 1383.
1) T.
SOC.,
G. Centi and F. Trifiro’ (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
OXYESTERIFICATION OF KETHANOL TO ME-RMATR
305
OVER V-Ti OXIDE
CATALYSTS
A.S. EL MI^, G. BUSCA~,c. CRISTIANI~,P. FORZATTI~
and E. TRONCONIl
Dipartimento di Chimica Industriale e Ingegneria Chimica del
Politecnico, P.zza L. Da Vinci 32, 20133 Milano (Italy)
Dipartimento di Chimica, Facolth di Ingegneria dell ’ Universith, 16129 Genova (Italy)
SUMMARY
Based on previous work and on new data for various V-Ti oxide
systems, generalized results are presented concerning their
physico-chemical characterization, their catalytic behavior in the
oxyesterification of methanol to methyl formate, and the related
reaction mechanism. The
feasibility of
industrial process
configurations for the the production of methyl formate, possibly
combined with formaldehyde, are discussed.
INTRODUCTION
Methyl formate is regarded as a convenient intermediate in the
synthesis of several chemicals. The current technology for its
production involves carbonylation of methanol in the liquid phase
in the presence of basic catalysts, typically sodium methylate, at
low temperatures and under moderate CO pressures (ref.11,
CH30H + CO --> HCOOCH3
A route using the gas-phase dehydrogenation of methanol over Cubased catalysts has been recently proposed (ref.21,
2 CH30H --> HCOOCH3 + 2 H2
for which attractive yields in methylformate have been claimed. An
alternative oxidative route, based on the reaction
2 CH30H + 02 --> HCOOCH3 + 2 H20
was studied by Ai (ref.3) over various Mo- and W- based catalysts.
We have reported that this reaction occurs also over V-Ti oxide
catalysts prepared either by coprecipitation (refs.4-5) or by
impregnation (ref.6) techniques. Encouraging preliminary results
concerning global selectivities and yields of methyl formate
(ref.4) prompted us to perform a complete process variable study
(ref.5), and to address the mechanistic features by an FT-IR study
306
on the interaction of methanol and its oxidation products with the
V-Ti02 surface (ref.6). The characterization of the catalyst
samples was also fully investigated (refs.4-9). Recently, our
understanding of the reaction mechanism has been completed and
refined by the results of a series of flow reactor experiments
where reaction products and intermediates were used as reagents,
which have confirmed the oxidative nature of the reaction step
leading to methyl formate ("oxyesterification") as compared to the
disproportionation mechanism previously suggested (ref.7).
Based on our previous work as well as on new data for various
coprecipitated V-Ti systems, in this paper we present generalized
results concerning their physico-chemical characteristics, their
catalytic behavior and the related mechanistic features. The
effects of the catalyst preparation parameters (V/Ti a.r. and
calcination temperature) and of the operating conditions is
discussed in connection with the selection and the feasibility of
alternative process configurations (production of methyl formate
only versus coproduction of formaldehyde and methyl formatel.
EXPERIMENTAL
V-Ti oxide samples with V/Ti atomic ratios (a.r.1 0 - 0.5 were
prepared by coprecipitation from VOCl3 and Tic14 at r. t.,
followed by drying
and calcination. Different samples were
obtained varying the calcination temperatures between 500 and 700
"C. The procedures
and the
equipment used
in
catalyst
characterization and in
flow reactor experiments have been
described elsewhere (refs. 4-9).
RESULTS AND DISCUSSION
Catalyst Characterization
Coprecipitated V-Ti oxide catalysts have been characterized with
respect to the influence of both calcination temperature and V/Ti
a.r. Samples with low V/Ti a.r. and low activation temperature are
constituted by the anatase phase only. XRD, W-visible diffuse
reflectance, ESR, FT-IR and chemical analysis provide evidence for
the presence of a solid state solution characterized by the
incorporation of V4+ in the bulk (ref.8). For the samples with
V/Ti a.r.=0.0375, on increasing the calcination temperature the
rutile phase becomes predominant (Tc= 6 5 0 "C), and a sudden drop
in surface area is observed. On the other hand, on increasing the
307
V/Ti a.r. at a calcination temperature of 600 "C, V2O5 appears in
the samples with VITi a.r.2 0.0625, again causing a drop in
surface area. Appearance of a rutile phase of Ti02 is detected in
the sample with V/Ti a.r.= 0.5. For a calcination temperature of
700 O C , the rutile phase is first detected at V/Ti a.r. 20.0125,
while V2O5 is observed at higher V loadings. Table 1 presents the
specific surface area of the coprecipitated V-Ti02 catalysts and
the detected phases other than the anatase phase as functions of
the sample calcination temperature and of the V/Ti a.r..
TABLE 1
Effects of calcination temperature and V/Ti a.r. on the surface
areas (m2/g) and on the phase composition of V-Ti oxide samples.
V = V2O5 detected. R = rutile phase detected.
CALCINATION TEMPERATURE ("C)
V/Ti (a.r.) 500"
0
0.01
0.0125
0.025
0.0375
0.05
0.0625
0.125
0.25
0.50
-
70
82
83
78
-
80
30
550"
-
600"
(ref.5)
--
53
-
59
54
64
62
-
-
48
44
37
28
5
4
6
7
625"
39
34
v
v
V
R+V
--
-
650"
675"
700"
(ref.4)
20
16
9 R
9 R
6 R
-
-1 R
3 R+V
-
The boundary between the samples constituted by the anatase
phase only, and those where also the rutile phase and/or V2O5 are
detected is seen to correspond to a dramatic reduction of the
surface area. The results of elemental chemical analysis further
indicate that V interacts with the support in the form of V4+ and
that it is also present at the surface as V5'
(ref.7). The
characterization by adsorption of probe molecules and a combined
FT-IR and Laser Raman microscopy study demonstrate that both V and
Ti centers, and specifically mono-oxo vanadyl centers with a
coordinative unsaturation, are present at the catalyst surface
(refs. 5,9).
308
Catalytic behavior of V-Ti02 samples: effects of V/Ti a.r. and of
calcination temperature
During the flow reactor experiments, the observed reaction
products included HCHO, methyl formate, water, dimethylformal
(DMFL), CO, C02 and formic acid (refs. 4 - 5 ) . Varying the V content
of the catalysts was found to affect significantly both the
conversion of methanol and the distribution of products. For the
samples calcined at 600 OC, the overall conversion is seen to go
through
a maximum
(V/Ti a.r.= 0.0625-0.1251, which can be
attributed to the contrasting effects resulting from increasing
the V loading: while this enhances the oxidizing capacity of the
6
rL
0
I
2
0
0
0.025
005
0075
01
0125
V l T i atomic ratio
Fig.1 - Effects of the calcination temperature Tc and of the V/Ti
a.r. on the HCOOCH3/HCHO molar ratio in the oxidation of methanol
over V/Ti oxide catalysts.
catalyst, it also reduces its surface area (see Table 1). For the
same calcination temperature, Fig. 1 shows that a maximum is
evident also in the HCOOCH3/HCHO molar ratio. As discussed in a
later
section, this ratio is of specific interest for the
implementation of an industrial process for the production of
methyl formate: depending on its value, different process designs
have to be considered. As compared to the optimal V content in
Fig. 1, the low selectivities to HCOOCH3 corresponding to low and
high V contents appear to be associated with poorly active systems
due to deficiency of oxidizing capacity, and to deficiency of
surface area and excess of V, respectively. This interpretation is
consistent with a reaction mechanism where formation of HCOOCH3
occurs consecutively to the formation of HCHO, requiring V-related
309
catalytic centers and adequate surface areas.
In this work
the influence of changes in the catalyst
calcination temperature has also been studied, as shown in Fig. 1.
Higher calcination temperatures correspond to lower surface areas
for the same V/Ti a.r., as indicated in Table 1, and also to
greater amounts of V at the surface: accordingly, the HCOOCH3/HCHO
ratio is seen only to decrease in the case of the catalysts
calcined at 700 OC, where the optimal HCOOCHJ/HCHO ratio is
shifted to lower V loadings; on the other hand, for the catalysts
calcined at 550 OC, associated with higher surface areas and lower
V contents at the surface, only the rising branch of the curve is
apparent, the maximum being shifted to greater V/Ti a.r..
Catalytic behavior of V-Ti02
- samples: effects of the process
variables
The effects of the main process variables, including methanol
and water feed concentrations, space velocity, temperature and
pressure have been investigated over various catalysts. The 02
feed molar concentration was fixed at = 10% in all runs in order
to
remain below
the flammability limits of methanol/oxygen
mixtures.
30
15
$y
,
'\
10
I
V
I
'p5
0
I
/
/
/A
I
t
I
I
20
30
.
I
I
I
10
00
I
ifeed
Fig.2 - Effect of methanol feed content on % HCOOCH3 and on the
HCOOCH3/HCHO molar ratio. Catalyst: sample with V/Ti a.r.= 0.0625
calcined at 55OOC. Reaction conditions: T= 165 OC, 10% 0 2 feed,
F/Wc= 10 cc/g min.
310
For a catalyst with V/Ti a.r. = 0.0625, Tc= 550 'C, Fig. 2
illustrates the effects of the methanol feed level on the
concentration of methylformate
in the products and on the
HCOOCH3/HCHO molar ratio. Distinct optimal values of the methanol
feed content exist for the output concentration of methylformate
and for the HCOOCH3/HCHO ratio. Selectivities to valuable products
(HCOOCH3+ HCHO+ DMFL) in excess of 90% were achieved with methanol
concentrations greater than 15%. An excess of methanol enhanced
DMFL with respect to HCHO, and almost suppressed the formation of
CO and CO2.
The effect of reaction temperature at two space velocities on
the HCOOCH3/HCHO is presented in Fig. 3 for a catalyst with V/Ti
a.r.= 0.0375 calcined at 600 OC. Both high temperatures and long
contact times are seen to favor methyl formate with respect to
formaldehyde, in line with the consecutive nature of the reaction
scheme. At temperatures higher than 180 OC, however, a sudden drop
in the selectivity to methyl formate has been observed for
prolonged contact times. The addition of H20 to the feed was found
to depress the overall conversion of methanol, and also reduced
the ratio HCOOCH3/HCHO.
Fig.3
-
T ('C
1
Effect of reaction temperature and of contact time on the
HCOOCH3/HCHO molar ratio. Catalyst: sample with V/Ti a.r.= 0.0375
calcined at 600 OC. Reaction conditions: 10% CH30H, 10% 0 2 feed;
F/Wc = 12 cc/g min (curve A ) and 2 4 cc/g min (curve BI.
311
Similar effects of the process variables had
observed with other V-Ti catalysts (refs. 4,5),
to be representative of the general catalytic
systems. They are interpreted in the following
light of our findings on the reaction mechanism.
been previously
so that they seem
behavior of such
section in the
Mechanism of the oxidation of methanol over V/TI oxide catalysts
The mechanistic features of the oxidative route to methyl
formate over V-Ti oxide catalysts have been studied by FT-IR
techniques, investigating the interaction of methanol and its
oxidation products with the catalyst surface (ref.61, and by
running a series of flow reactor experiments where intermediates
and reaction products were used as reactants (ref.7). The results
are supportive of the reaction scheme presented in Fig. 4 ,
consisting essentially of successive oxidation steps. Each of
these steps has received experimental validation by FT-IR and/or
specifically designed flow reactor runs. Thus, in the case of the
route leading from formaldehyde to methyl formate, IR spectroscopy
has provided evidence for a Cannizzaro-type disproportionation of
dioxymethylene (step 111, and the occurrence of this reaction has
been confirmed by flow reactor experiments with a HCHO + He feed,
where HCOOCH3 was produced. However, the results of similar
experiments with a HCHO + 02 + He feed show that the oxidation
route (step 6) is considerably faster under typical, oxidizing
reaction conditions.
HCOOCH3v
HCHO,
3tl
$I4
H- H
Y .H
FH3
0 2
-L-
-
-
/"\
1
- 9 9 L
ox v
Fig.4
Reaction mechanism for the oxidation of methanol over V/Ti
oxide catalysts.
312
Flow reactor runs with HCOOH in the feed have proved that CO and
C02
originate through
decomposition of formate groups. The
esterification of formate groups to methyl formate appears however
to be faster than their decomposition, provided that methanol is
available in the reaction mixture.
All of the effects of the operating variables can be interpreted
on the basis of the scheme in Fig.4. Thus, step 1 is consistent
with the observed inhibiting effect of water on the conversion of
methanol. The effect of the CH30H feed concentration can be
rationalized by observing that, for CH30H < lo%, the excess of
oxygen favors the oxidation steps, leading preferentially to the
terminal
products HCOOCH3, CO and HCOOH. On increasing the
methanol feed content, the steps involving gaseous methanol are
beneficially affected, resulting first in a decreased selectivity
to CO and HCOOH, corresponding to an increased selectivity to
HCOOCH3, and finally in enhanced selectivities to HCHO and
particularly to DMFL. The data in Fig. 3 are explained considering
that an increase in temperature and contact time results in
enhanced methanol conversions, and reduces the concentration of
gaseous methanol. Accordingly, first the selectivity to HCOOCH3
grows at the expense of HCHO + DMFL, then the selectivity to CO is
favored at the expense of methyl formate.
Catalytic tests for the oxidation of methanol over pure Ti02
(ref. 5) have confirmed the fundamental role of Vanadium in the
oxidative steps of the mechanism (steps 2 and 6 in Fig. 4).
Process considerations
The general results, of the flow reactor experiments indicate
that the ratio HCOOCH3/HCHO in the products can be adjusted within
a wide range of values by appropriate choices of both the catalyst
preparation parameters and of the reaction conditions, depending
on the desired features of the reaction product. One possible goal
is to design a process aimed at the production of methyl formate
only. Fig. 1, 2 and 3 illustrate a few examples where the
production of methyl formate can be optimized by a suitable
selection of either V/Ti a.r. and calcination temperature, or of
the methanol feed concentration or of the reaction temperature.
Along these lines we have achieved values of HCOOCH3/HCHO as high
as 20, corresponding to weight ratios =40/1, with productivities
to HCOOCH3 exceeding 200 g/Lh.
313
Alternatively, the V-TiOz catalysts appear suitable also for the
industrial coproduction of HCOOCH3 and HCHO by the mild gas-phase
oxidation of methanol. In this case, the ratio HCOOCH3/HCHO is
expected to have a strong impact on the design of the separation
section of such a process, for which a tentative schematic diagram
is given in Fig.5.
ASES
ri
C H30H
'"
YHC
HO
Fig.5 - Tentative process scheme for the coproduction of methyl
formate and formaldehyde by mild gas-phase oxidation of methanol.
Units 1 and 2 are devoted to the separation of HCHO, which is
dissolved in water, and to the concentration of the resulting
aqueous solution. The remaining separation units effect removal
from the gaseous stream of the inert gases (unit 3 1 , which may be
in part recycled to dilute the oxygen in the air feed stream, of
methyl formate (unit 4 ) , and eventually of unreacted methanol
(unit 5 ) , which is recirculated to the synthesis reactor. In this
scheme, the trickiest section is that effecting separation of
formaldehyde, its target being the production of a commercial
aqueous solution of HCHO. If concentration of the solution is
required, a lower bound exists on the acceptable content of
formaldehyde in the reaction products. This implies that it may be
desirable to design operation of the reactor without necessarily
maximizing the HCOOCHJ/HCHO ratio.
Preliminary calculations of
the separation section were
performed assuming a reactor outlet stream containing 10.5% H20
314
and 1.5% HCHO. Results demonstrate that concentrations of HCHO of
20% w/w and more in the final solution are feasible by autothermal
operation of units 1 and 2 under slight pressure. Polymerization
of HCHO can be prevented by allowing a small concentration of
residual methanol in the final solution. The final choice of the
reactor working conditions, however, is controlled by a balance
between the increase in revenues expected from maximization of the
HCOOCH3
production and
the increased
costs resulting from
concentration of more dilute aqueous solutions of HCHO, for which
a detailed economic analysis is required.
ACKNOWLEDGEMENTS
This work was supported by Minister0 Pubblica Istruzione (Roma).
REFERENCES
1 The Leonard Process Co. - Kemira OY, Formic Acid, Hyd. Process.,
November (1983).
2 S.P. Tonner, D.L. Trimm, M.S. Wainwright and N.W. Cant,
Dehydrogenation of Methanol to Methyl Formate over Copper
Catalysts, I&EC Prod.Res.Dev., 23 (1984) 384.
3 M. Ai, The Production of Methyl Formate by the Vapor Phase
Oxidation of Methanol, J. Cat., 77 (1982) 279.
4 P. Forzatti, E. Tronconi, G. Busca and P. Tittarelli, Oxidation
of Methanol to Methyl Formate over V-Ti Oxide Catalysts, Cat.
Today, 1 (1987) 209.
5 E. Tronconi, A.S. Elmi, N. Ferlazzo, P. Forzatti, G. Busca and
P. Tittarelli, Methyl Formate from Methanol Oxidation over
Coprecipitated V-Ti-0 Catalysts, I&EC Res., 26 (1987) 1269.
6 G. Busca, A.S. Elmi and P. Forzatti, Mechanism of Selective
Methanol Oxidation over Vanadium Oxide - Titanium Oxide
Catalysts: A FT-IR and Flow reactor Study, J. Phys. Chem., 91
(1987) 5263.
7 A.S. Elmi, E. Tronconi, C. Cristiani, J.P. Gomez Martin, P.
Forzatti and G. Busca, Mechanism and Active Sites for Methanol
Oxidation to Methyl Formate over Coprecipitated VanadiumTitanium Oxide Catalysts, I&EC Res., 28 (1989) 387.
8 G. Busca, P. Tittarelli, E. Tronconi and P. Forzatti, Evidence
for the Formation of an Anatase-Type V-Ti Oxide Solid State
Solution, J. Solid State Chem., 67 (1987) 91.
9 C. Cristiani, P. Forzatti and G. Busca, On the Surface Structure
of Vanadia-Titania Catalysts: Combined Laser-Raman and FT-IR
Investigation, J. Cat., 116 (1989) 586.
315
V. CORTES CORBERAN (Instituto de Catalisis y Petroleoquimica,
CSIC, Madrid, Spain): Concerning the composition of the different
V-Ti-0 catalysts, have the authors experimental evidence of the
surface composition ? And, if s o , does the bulk composition
represent the actual surface composition of samples having
different V/Ti atomic ratios ?
G. BUSCA (University of Genova, Italy): A qualitative analysis of
the catalyst surface composition has been performed for one of
the most active catalysts (V/Ti a.r. 0.0375, calcined at 6 0 0 OC
( 1 ) ) using FT-IR spectroscopy. It has been shown that vanadyl
species (resaonsible for a well evident surface-sensitive IR band
at 2050 cm , first overtone
the V=O stretching) and of
coordinatively unsaturated Ti
ions (responsiblf for the
formation of carbonyl species absorbing at 2195 cm
when the
catalyst is put into contact with CO gas) are both present on the
surface. By measuring the intensities of these bands and by
compearing them with those are measured on pure Ti0 and on VTi02 catalysts prepared by impregnation with measure2 amounts of
vanadium compounds, a quantitative evaluation can be obtained
( 2 ) . In this case, a surface enrichment of vanadium seems
evident.
af
1. E. Tronconi, A.S. Elmi, N. Ferlazzo, P. Forzatti, G. Busca and
P. Tittarelli, Ind. Eng. C h e m . Res., 2 6 (1987) 1269.
2. G. Ramis, G. Busca and V. Lorenzelli, 2.
Folge, 153 (1987) 189.
Phys. C h e m . ,
Neue
G. Centi and F. Trifiro' (Editors), New Developments in Selective Oridation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
317
OXIDATIVE TRANSFORMATION OF METHANOL M HIGHER ATDEHY!3FS OVER
ZEOLITE - OXIDE CATALYSIS
P. J h , Z. Tvaruzkova and K. Habersberger
J. Heyrovsky I n s t i t u t e of Physical Chanistry and E l e c t r o c h m i s t q ,
Dolejskova 3, 182 23 Prague 8, Czechoslovakia
SUMMARY
Methanol may be i n the tenperatwe range 35@5OO0C catalytically oxidized in
one step to acetaldehyde and benzaldehyde over bifunctional catalysts containing
the redox active ccmponent Bi-k-0 together with the H Z W S zeolite. In ccmparison with HZSM-5 zeolite alone (without the redox active ccmponent Bi-Ib-0) a
16 times higher selectivity to C2+ aldehydes was attained. The analysis of the
infrared spectra of adsorbed d -acetonitrile and of the surface canplexes of
methanol formed i n its interackon with the proton d m r sites of the
bifunctional catalyst a t 4OO0C led to the suggestion of the probable mechanism
of the oxidative transfonnation of m e t h a r o l to higher aldehydes.
INTRODUCTION
Together with the developnent of C1 c h d s t r y , rethan01 becomes a possible
source for C2+ oxygen derivatives ( a m l e h e , acetaldehyde, benzaldehyde etc.).
Accordhj to the present state of art such a prcductbn muld be performed in
two steps, nmely the transformation of methylalcol-ol to C2+ hydrocarbons, foll m d by the selective oxidation of hydrocarbons to the desires p r d u c t s . The
present study investigates the possibility of a single step process of oxidative
methanol transfarmation over a bifunctional catalyst ad represents a continuation of the previously published short catmumication (ref. l ) .
EXPERmmrAL
The following types of bifunctional catalysts w e prepared: i) C a t a l y s t 5
- HZ-5,
-
con3wt% of Bi-Mo-O canpnent, prepared by hydrotermal s y n t h e
sis. Tetrapropylammniun ions were used, as structure directirq agent. The start-
an autoclave for 6 days
ing mixture f o r the hydrothermal synthesis (performed
a t 15OoC) contained 6.67% Si02 (as silica sol), 0.41% AlC13.6H20, 1.86% NaOH,
3.15% (C33)4NBsr 0.39% Bi(N33)3.5H20 arid 0.10% (NH4)$~7024.4H20. ii) Catalyst
-B - a mechanical mixture of HZSK-5 ad 5wt% of Bi203.PW3 was p r w d according
to (ref. 2 ) . iii) Catalyst C - B i 2 0 3 . M 3 (23wt%) supported on HZSM-5, bras prepared by impregnation, f i r s t with a solution of (NH4)6~7024.4H20and, a f t e r
calcination a t 5OO0C in a flow of dry oxygen, with a solution of Bi(N03)3. The
Si/Al r a t i o of the original HZN-5 was 19. A l l samples were before the catalytic
test activated in a flcw of dry oxygen for 2 hours a t 35OoC.
318
Both the activity and the selectivity of the catalysts were investigated i n
an integral f l m microreactor, the products were determined ky gas chranatcgraphy
ard (after adsorption in mter) by p l a r ~ The
~ measurments
.
w e r e performed
i n the reaction tanperatwe raqe 350-500°C, with T.JHsv = 1 to 2 h-l ard the
m o l a r r a t i o CH30H/02 = 3-12 i n the feed. The grain s i z e of the catalysts was
0.3-0.6 mm.
P I R spect.t-osccpy w a s used both for the determination of the c r y s t a l l i n i t y
of the catalysts (which was found to correspond i n a l l cases to HZSEI-5 structure and was preserved also a f t e r the catalytic test) in the skeletal vibration
rarge and for the investigation of the structure ard reactivity of the proton
donor sites of the catalysts after their interaction w i t h d3-acetonitrile and
m e t h a r d , performed in vacuum cuvettes on selfsupporting catalyst p e l l e t s of
2
1 0 mg/m thickness. The experimental details have been published previously
(refs. 3,4)
RE,RILTS AND DISCUSSION
Catalytic activity and selectivity
In the investigated range of reaction conditions ( b w , tgnperature, feed,
r a t i o W / 0 2 ) higher hydr~arbons,carbon dioxide, formic acid, formaldehyde,
acetaldehyde and benzaldehyde were fourad as reaction products of the oxidative
transfonnation of methanol over all investigated catalysts. Practically no acroleine (yield<0.02%)was present i n the products. The relative selectivity
SF
to C2+ oxygen c o n t a i n i q products (here acetaldehyde and benzaldehyde) was
de!fas the percentage represented by the sm of the respective s e l e c t i v i t i e s
to acetdldehyde and benzaldehyde in the total sm of the respective s e l e c t i v i t i e s
to a l l oxygen containing products,
+Em A survey of the catalytic
c2+
tests (reaction mnditions and s e l e c t i v i t i e s to the individual reaction products)
with the respective highest relative s e l e c t i v i t i e s Sox obtained over the inc2+
dividual catalysts A,B and C is given in Table 1. According to this criterion,
the highest relative selectivity S y was obtained over the catalyst A, catalyst
c being the s e c ~ n din order. over & catalyst g (consisting of a mechanical
mixture of both mpnents, z e d i t e and oxide) the l w s t value of this c r i t e r i o n
was attained. TIE 1-r
value of S?
(see T&. 1) attained over the catalysts g
ard is connected with the f a c t
these t m catalysts c o n v a t me-1
into
formaldehyde to a nnxh higher extent than catalyst 5: the respective conversion
d
d
= 4.2%, whereas
= 0.16% only.
= 19.6%,
degrees to f o d d e h y d e are
The catalysts B,C may therefore k e used in a sin$e-step
preparation of a
mixture of f o d d e h y d e with higher aldehydes (acetaldehyde, knzaldehyde). It
the reaction r a t e of the methanol transmay be assme3 t h a t over the catalyst
formation to higher aldehydes is either approximately the sane or higher t h a n the
r a t e of the methanol d d a t i o h to formaldehyde. Although the differences i n the
deep oxidation of metharm1 to C02 ov- the individual catalysts are not so
i.e.zox
c
.
dt
$
319
TABLE 1
Catalytic oxidation of CH30H to higher aldehydes
&OH
-
Cat. OC
conv.
02
co2 HCCOH
A
450
1.3
12
99
2.07
B
450
2
3
70
4.39
C
456
2
6
82
7.91 0.00
Fd
selectivity
Fd
ICY Ac
0.05
0.17
0.00 28.13
5.16
- fonnaldehyde, Ac - acetaldelyde,
Bz
f cz.100
OK
Bz :C
E
rccix+zc~
:2C'
2.29
0.35
0.4
0.76
32.59
1.53
0.21
1.74
5.08
13.07
2.15
0.20
2.35
15.24
24.9
- benzaldehyde
significant as i n the case of the formaldehyde formation, a higher formation of
c02 contributes also to a decrease in the relative selectivity SQx
c2+
O u r further discussion of these results i n t h i s paper w i l l concentrate on the
catalyst
ard its function a t reaction conditions leading to the h i e s t values
i n methanol oxidation t o both formaldehyde and C02. By this choice we obtain the
reaction mnditions and the type of catalyst w h e r e most probably both the transformation r a t e of methanol to higher hydroczrbons (the total methanol conversiori
a t t a i n s 99%) on the acidohsic sites of the zeolitic m p n e n t of the catalyst
and the subsequent oxidative t r a n s f o m t i o n rate of these higher hydnxarbons to
higher aldehydes on the redox sites of the Bi-Mc-0 ccnpnent (Sox
c2+
canparable.
= 24.9%) are
?he total conversion of methanol and the respective s e l e c t i v i t i e s for its
transformation to the individual products over catalyst
11 a s
function of the
reaction t9nperature a t constant values of WHSV a d MeoH/02 are given i n Figs.
1 and 2. In Fig. l a also the total conversions obtained under the same c o d tions over the hybridic catalyst
(mechanicalmixture of zeolitic and oxidic
m p n e n t ) are given for canparison. Similarly in Fig. 2c ccanparative selectivities obtained over the original HZSM-5 zeolite (Si/Al = 19) without B i - h b - 0
redox camponent are given.
Fran the data sham both in Figs. 1 ard 2 a d in Table 1 the f o l l m i q conclusions about the action of the investigated catalysts m y be drawn:
1) The respective values of the total conversion of m e t h a n o l i n the t m p r a t u r e
range of 400-500°C over the original HZSM-5 arid over the hybridic catalyst
-(
'cal mixture) are very near to each other whereas they d i f f e r fran the
values obtained over the catalyst A, where the redax ccmponent has been added i n
~ f a c t it follows t h a t on the catalyst A
the hydrothml synthesis. F K this
methanol is t r a n s f m d on a different type of active sites.
2) Both the hybride catalyst B ard the original zeolite HZSM-5 exihibit in the
reaction tenperatwe range of 300-500°C i n the methanol tramsfonnation significantly higher s e l e c t i ~ t i e s
to both formaldehyde ard c02 than athe catalyst
(Figs. lb, lc). On the different type of active sites, specific for catalyst
&,
320
Fig. 1. Depenaence of the total
conversion of methanol 5
(a),
of the selectivity of me&
trans- 30.
(b) and of
formation to C02
the selectivity of
M3011 transfornation to formaldehyde S
(c) 20
on the tarperatwe of the ca&!ytic
reaction over the catalysts AO,
Board the original EIZS4-5 (without B i - b O Ccmwnent)
,at
MISV = 1.33 anrl ?WH/O2 = 12.
-
-
,
I
I
I
-
0.
C
0.4 i-
Fig. 2. W d e n c e of the selectivity of
methanol transfornation to acetaldehyde (a), of the selectivity of netha n k t r a n s f o r r a t i o n to benzaldehyde - S
(b) and of the relative s e l e c t i a t y to
oxygen containiw p r d u c t s - Sc
(c) on
the reaction tmperature over 2 the
catalysts A 0 ,B a and the original
~2m-5 (without Ei-Noi) m p n e n t ) O , a t
iIIsV = 1 . 3 3 an3 MeoH/02 = 12.
-
y+
321
the axidation of methanol to formaldehyde and c02 is reduced. Both these reactions are therefore probably connected with the active sites on the original
zeolite
mm-5
and not with the presence of the B i - b b - 0
3) With the specific type of active sites on catalyst
A
mpent.
a higher relative selec-
tivity S F may be achieved, resulting fran the formation of acetddehy3e and
benzaldeh$e
(Figs. 2a, b, c ) . This type of sites is not a t disposal on the
g, h e r e therefore substantidly 1values of : S
are obtained.
4) The r e s u l t s i n Table 1 (the values of
and. SFd) show &t the catalytic
properties and therefore also the active sigs of catalyst c (prepared
impregnation with the Bi-Mo-0 cunpnent) are mre similar to those of catalyst
(hydrothermal addition of Bi--0)
than t o those of catalyst g ( r n M c a l mixture).
catalyst
SF
Surface properties
I n Fig. 3 the I R spectra of the catalysts
A,C in the
wavenuthr range of the
structural OH groups are presented. W i t h regard to the analogy w i t h the respecof the original HZSM-5 zeolite, the bard at 3610 an-'
tive bard i n the specmay be ascribed to the proton-danor centres. The band a t 3740 an-' corresponds
to the terminal OH groups w h i c h do not exhibit acidobasic properties. The lawer
values of absorbancies of the bard a t 3610 a
t
'
,
dicate that a part of the Bi3'
found with both catalysts, in-
ions excharged into the cationic sites of the
zeolite, and an interaction of the Bi--0
redox camponent w i t h the OH groups
(their "neutralization") takes place. Such a neutralizing effect has been Observed already before (ref. 5 ) . me concentration of OH groups is therefore
reduced, when carrpared to the original m a - 5 zeolite.
fig. 3
I R spectra of the structural OH groups
after the evacuation a t 35OoC overnight:
1- catalyst C, 2- catalyst
without Bi-6-0 ccmponent.
A, 3-
HZSM-5
322
"he presence of acidobasic sites i n the catalysts &and
c is indicated also
by the Il? spectra of adsorbed d3-acetonitrile. In order to give a m r e precise
characterization of the acidobasic properties of catalyst
A,
Fig. 4 presents the
I R spectra of adsorbed d3-acetonitrile a t the equilibrim. adsorption pressure of
2 mn Hg, adsorption tanperatwe of 2OoC and subsequent desorptions for 30 minutes
a t 20 and a t 100°C. For canparison, also the I R spectra of d3-acetonitrile
sorption on the original HZSI-5 z e o l i t e , registrated under the sane conditions,
are given. The spectnm exkibits three bands which are characteristic for dj-acetonitrile adsorbed on different centres of the zeolitic catalysts, which are
characterized by their different thermal stabiliw i n the course of the desorp
tion (see the spectrum after desorption a t 100°C). With regard to the data given
i n the literature (refs. 6, 7)
I
the band a t 2320
-
2330 an'-' may be ascribed to
the interaction of d3-acetonitrile w i t h the electron-acceptor sites of the catal y s t 5, whereas the band a t 2294 - 2275 an-' t o the interaction with either the
proton-donor centres or the electrostatic field of the cations present (an interaction with the Bi3+ cations cannot be excluded). The third band a t 2266 an-'
has w i t h both samples the sane intensity and is thermally unstable. This band
corresponds probably to a weak unspecific physical sorption of d3-acetonitrile
on the zeolite. The shift of the band a t 2321 can-'
(HZSM-5) to 2300 an-'
(catalyst &) indicates that the electron-acceptor sites of the catalyst & are less
Fig. 4. I R spectra of adsorbed d3-acebnitrile on catalyst A and % original
HZSM-5 without Bi--0
canpnent after precedirg evacuation a t 350 C for 4 hours:
sorption a t 25OC
I subsequent desrption a t 25OC for 30 minutes
desorption a t 100°C for 30 minutes ----
-
.
.....
323
acidic than those of the original HZSM-5. This conclusion is sustained by the
lmer values of both absorbance and thermal stability of this band in the case
of catalyst 9; t h i s irdicates also a 1
concentration of electrowacceptor
centres. The interpretation of the I R spectra of adsorbed d3-acebnitrile on
catalyst
represents thus another irdependent information on the acidobasic
properties of this catalyst whi& is i n agreement w i t h the views given above.
Fig. 5a presents the I R spectra of the catalyst after its interaction w i t h
methanol in the tgnpesature range of 20
-4
0 0 in
~ ~the wamm+xr range of the
vibrations of structural OH g r ~ Fig.
,
5b shcm the IR spectra of t h i s catal y s t i n the range of the V (c-H) vibrations in the .sane temperature range.
Fig. 5
IR spectra of catalyst A
after its interaction w i t h
CH30H after Feeding evacuabon at 350 C fog 4 hours:
inWaction a t 25 C (1), a t
100 c ~ 2 1 ,a t 200"~ (3) and
a t 400 C (4).
CA'
~ t e the
r adsorption of methanol a t ~ o O C ,follmed by a short desorption of
the gaseous phase a t the same temperature, the spectrum in the region of vibrations of structural OH group exihibits a broad band a t 3530 a
n
'
,
shifted w i t h
respect to the bard 3610 an-' to a 1
wavenmkr indicating thus the strorq
interaction of M30H w i t h proton-donar centres of the catalyst. In the region
of the f (C-H) vibrations two strory bands a t 2950 an-' and 2850 ut-l respec-1
tively, are formed, acmnpanied by a shoulder a t 2985 cm
when the tmperature of the interaction is raised to 100, 200 and 400°C,
respectively, both strang bands appear in the spectrum again, but w i t h a lmer
intensity when wrrpared to the interaction a t 2OoC (see Fig. 5b). These s b p i s e
changes in intensity with the increase of the tenperature of interaction indicate
that a part of methanol has either desorbed or reacted to its transformation
p r d u c t s ; i n the course of this process a p a r t i a l regeneration of the original
.
324
pmton-donor sites took place. Similar, although substantially less significant
changes *re
observed i n the spectrum of the cakdyst
(prepared by inprqna-
tion).
The interpretation of the I R spectra in this region, indicating the formation
of reactive methoxy group, has been described previously (refs. 3, 8 ) .
?he catalyst A represents a typical bifunctional catalyst with ha types of
active centres, namely: i) acidohsic sites represented by proton-donor structural OH groups as w e l l as (as it follows fran the interpretation of the I R
spectra of d3-acetmitrile) electron-acceptor centres, ii) redox sites -of- the
Bi-Pb-0 mnponent.
Presently there is no indication whether the l a t t e r mnponent is finely dis? r s d on the HzBG5 z e o l i t e or a t least partially inserted
i n the zeolite net-
work.
The methoxy groups formed in the interaction of CH30H w i t h an OH group sean
to act as precursors i n the formation of ethylene and higher hydrocarbons by
to a mechanisn described previously (8). mese produds can subsecpmtly interact with the surface
oxygen of the Bi-m-0 redox mnponent of the bifunctional catalyst under forma-
participation of both types of acidohsic sites accord*
tion of the respective aldehydes. 'Ihe participation of the surface oxygen i n t h i s
process s e a s to be indicated by the charqe of the colour of the catalyst after
i t s interaction with CH30H in the IR cuvette a t higher tgnperatures frun yellow
t o bluishgreen. Such a cycle of interactions (alternatively w i t h methanol and
with oxygen) may be perfonred repeatedly.
"he priority in the f o m t i o n of acetaldehyde and benzaldehyde over these
catalysts is probably caused i) by the leer reactivity of these aldehydes i n
canparison with e.g. acroleine, ii) by the low reactivity of ethylene and toluene
when cunpared to the olefins C3-C4, the latter being mre readily oliganerized
to surface polyenes than oxidized so that the p s s i b i l i t y of the formation of
other aldehydes is reduced.
REFERENCES
1 P. Jim, 2. Tvaruzkova and Habersbeqer, React. Kinet. Catal. L e t t . in press.
2 A. Batist, J.F.H. Bowens and G.C.A. Schuit, J. Catal., 25 (1972) 1.
3 L. KubelkoM, J. Nwakova ard P. Jim, Structure and Reactivity of Wdified
Zeolites, (P.A. Jaccbs e t al., Eds.) msevier, Iknsterdam 1984, S t d . surf.
Sci. Catal. Vol. 18 p. 217.
4 Z. Tvaruzkova, M. m a , P. Jim, A. Nastro, G. Giordano and F. Trifirb,
Zeolites as Catalysts, Sorkents and Deteryent B u i l d e r s , (H.G. Kaqe and
J. Weitkmp Eds.), ELsevie.r, AmSte.rdam 1989, Stud. Surf. Sci. Catal. in press.
2. Tvmzkova, G. Centi, P. Jim and F. Wifirb, pppl. Catal. 1 9 (1985) 307.
C.L. Angel1 and M.V. H o w e l l , J. Phys. Chen. 73 (1969) 2551.
H. K d z i r q e r and H. Krietenbrinck, J. C h m . Soc. Faraday I, 71 (1976) 184.
E.R. Jkrouane, J.B. Nagy, P. Kkjaifve, J.H.C. van Hoff, B.P. Speahan,
J.C. V e d r i n e and C. Naccache, J. Catal., 53, (1978) 40.
325
J. IrADER (Institute of Catalysis
and Surface chanistry, EOlish Academy of
sciences, Wakcw, Poland): In the Mil prccess methanol i s passed over the
ZsEl catalyst in the absence of cocygen, and hydrogen transfer reactions are
important steps in the chain leading fran methanol to higher olefins and aromatics. One may worder whether the failure to obtain aldehydes in your experiments was not due to the f a c t that the presence of oxygen i n the feed has
strorgly perturbed the hydrcgen transfer steps?
P. JI& (J. Heyrovsky mtitute of Physical chemistry and ~ l e c t r o ~ h m i s t r y ,
Czechoslovak Acadmy of Sciences, Frague, Czecbslovakia) : I I ~the oxidative
transformation of CH OH over the investigated catalysts A,B and C the total
mnvezsion of CH30H ?see Tab. 1) w a s in the rarge of 70-90%, w h e r e a s the
conversion of CH30H to cxygemted products was in the range of 2-24%, the
ranainirq methanol transfarmation products be*
higher hydr-hns.
Fran this
f a c t it follows that the presence of oxygen i n the feed has no significant
effect on the hydrosen transfer step.
P. JACOBS (Katholieke U n i v a s i t e i t Leuven,
Laboratory of Surface Chenistry,
&wen, Belgiun) : When a ZSM-5 zeolite is synthesized i n presence of Mo and B i ,
a very cmplex m a t e r i a l may result. Mo andlor B i may be occludfd in the zeolite
crystal o r even i n mimr munts be substituted i n the lattice. Furtherrrore,
Bi-Pb-0 phases external to the zeolite may also be present. Your I R spectra
seem to suggest that a t least part of the negative framework charge is neutralized by B i or Mo (oxide). Yy question, therefore, is hcrw you do visualize in
terms of chemical wnpositian and.morphology your catalyst A?
JIa
P.
(J. Heyrovsky Institute of Physical &anistry and ElectroCh&stry,
Czecbslovak Acadeny of Sciences, Prague, Czechoslovakia) : The synthesized cata l y s t represents in effect a very ccmplex systan. By electrme microsco~it
was found that the B i - W O ccmponent in this case most probably forms a thin
layer cweriq the surface of the 2524-5 zeolite. Our experiments give rn evidence whether a part of B i and/or l&~ has entered the f r m m k of the zeolite
or not. The f a c t that, i n canparism with the original zeolite, the i n t m s i t y
of the absorpticm bards of the OH groups of the 2%-5 zeolite w i t h the Bi-M-0
c m p n e n t is decreased (see Fig. 3) irdicates either t h a t the hydrogen ions are
W t l y substituted by B i ions o r that the Bi-k-0 redox oomponent interacts with
the CSI g r o q x of the ZSM-5 zeolite. These results indicate that a part of the
redm ccenpanent is really located in the pares of the zeolite.
G.Centi and F.Trifiro' (Editors),New Developments in Selective Oxidation
327
1990 Elsevier Science Publishers B.V.,Amsterdam - Printed in The Netherlands
TIN-GERMANIUM PHOSPHATES
AS
SELECTIVE
FOR
CATALYSTS
THE
OXIDATIVE
DEHYDROGENATION OF ETHYLBENZENE TO STYRENE
1
Turco M . , Bagnasco G.',
3
1
La Ginestra A. , Russo G.
Ciambelli P.',
'Dipartirnento
di Ingegneria Chimica, Universith di Napoli, Italy
'Dipartirnento
di Chimica, Universith di Napoli, Italy
3Dipartimento di Chimica, Universith "La Sapienza" , Roma, Italy
ABSTRACT
Tin-Germanium phosphates, with formula Sn Ge
(HPO ) .H 0 (OSxSl) were
synthesized, characterized and tested as catal&tk'for
tke' o?ydehydrogenation
of ethylbenzene to styrene. X-ray analysis showed that mixed compounds form
solid solutions, in agreement with thermal analysis. Surface acid sites
concentration increased with Ge content. Mixed compounds were more active than
pure phosphates and were highly selective to ST (up to 9 7 % ) , giving mainly CO
X
as byproducts. The role of acidity is discussed.
INTRODUCTION
Styrene (ST) is produced on
of ethylbenzene (EB).
industrial scale by
Such a process
and high amounts of steam giving rise
catalytic dehydrogenation
requires high temperatures
(600-700%)
to conversions of abt. 508 with
tivity to ST higher than 90%. ST production could be
selec-
performed through EB
oxidative dehydrogenation at lower temperatures with ST yields not limited by
thermodynamics.
Among the catalysts described
in literature for
this reaction metal
phos-
phates showed high selectivity and activity (1,Z).
Recent works have
indicated that mixed Zr-Ti, Zr-Sn phosphates present
im-
proved catalytic properties in respect to pure compounds (3, 4), although no
correlation between catalytic activity and structural properties was found. On
the base of
these results we
formula Sn Ge
(HPO ) ' H 0 (OsHl), as
x 1-x
of EB to ST.
have studied the
4 2 2
Sn-Ce mixed phosphates, with
catalysts for the
oxydehydrogenation
EXPERIHENTAL
Pure Sn and Ge
phosphates were synthesized as
Sn-Ge phosphates were
described in (5). The
obtained by coprecipitation
from
the
mixed
corresponding
chlorides in different molar ratios (3:1, 1:1, 1:3) by adding H3P04 and HN03.
328
The mixtures were refluxed
for 100 h and
the white precipitates were
fil-
tered and washed with ethanol.
X-ray analysis was carried out by a
CuK, radiation. Simultaneous DTA
Philips diffractometer using
filtered
and TG thermal analysis was performed by
Stanton mod. 781 thermoanalyzer (Pt-Pt/Rh thermocouples) at 2-5'C/min
heating
rate. S M micrographs were obtained by Hitachi 2300 apparatus. Surface
were measured by N2 adsorption at -196'C
Acidity measurements were
a
areas
using a Carlo Erba Sorptomatic 1800.
effected by NH3
TPD as described
in (6). The
NH3
adsorption was effected at room temperature on samples pretreated at 450
and
600'C.
The measurements of catalytic
reactor (i. d.-12
pretreated
in a
mm) loaded
activity were performed in
with 0.7-4.0 g
10% 02/N2 mixture
of catalyst.
flow
The samples
were
temperature of the
reaction
overnight. EB was fed by a metering pump. The reaction products were
analyzed
by gaschromatography. The
at the same
a fixed bed
catalytic tests were
effected in following
condi-
gaseous flow rate of 10 N1 h
tions: reaction temperature from 450 to SSO'C,
-1
,
contact time from 0.16 to 0.9 g cat/g EB/h, EB molar fraction-0.1.
RESULTS AND DISCUSSION
Table 1
lists the
chemical composition of the
materials. In Fig. 1
reported the simultaneous TG and DTA curves of the five samples. The
tion process occurs for SnP between 40 and 200'C
higher temperatures
(200-35O'C).
intermediate behaviour. Moreover
transition at 400'C,
For the
GeP shows
while
mixed
for GeP it occurs
phases we
a well
are
dehydra-
can observe
evident reversible
are condensed to pyrophosphates between 400 and 650'C.
All the
an
phase
absent in pure SnP. Also the mixed phases with high
content show the same phase transition between 350 and 400.C.
at
Ce
samples
Between 900 and
1OOO'C
the mixed phases show an exothermic effect due to the transformation to
cubic
pyrophosphates.
In Table 1
the dOo2
values obtained from X-ray analysis
on the hydrogen
phosphates phases are reported. They indicate that the mixed Sn-Ge phopsphates
give rise to solid solutions in all the range of compositions examined.
treating at 650.C
After
for 12 h pure GeP and SnP samples are completely transformed
to layered pyrophosphates phases while the condensation process of mixed
compounds is not complete. After treatment at 450'C
for all the samples. After treatment at 1OOO'C
such process is incomplete
the samples show the signals of
329
cubic pyrophosphates:
their gradual shift vith composition
indicates
that
solid solutions are present.
TABLE 1. Chemical composition and d
SAMPLE
002
values of Sn-Ge phosphates.
CHFXICAL FORMUU
d002
A
GeP
Ge(HP04)2.H20
SnGel3
SnGell
20Ge0.80(HP04)2 *H20
Sn0.41Ge0.59(HP04)2'H20
7.82
SnGe31
Sno.S2Ge0~48(HP04)2.1.1H20
7.85
SnP
Sn(HP04)2.1.5H20
7.89
In Fig.
7.70
7.78
2 SEM micrographs are reported. GeP
sample shows
aggregates of
laminar shaped particles, 1 to 2 ~ mlarge and 0.1pm thick. In micrograph of SnF'
sample we can observe complex
structure formed by
(diameter-2pm, thickness-O.O5pm),
SnGel3 and SnGe31
lamellar shaped
linked together as a
show aggregates of particles with
regular
elements
succession.
dimension smaller
those of pure compounds (0.5pm); the morphology of particles varies
than
gradually
with composition; moreover the tendency to give aggregates increases with
tin
content.
After treatment at 450 and 600'C surface areas of mixed compounds are higher
than those of pure compounds (Table 2 ) . achieving a maximum for SnGell sample.
However the treatment at 600'C
leads to a marked
increas of surface area
of
pure SnP and GeP samples.
The surface concentration of acid sites, evaluated as in
ture of
peak maxima
in TPD
observe an increase of acid
spectra are
also reported
(a),
and
in Table
sites concentration with Ge
2. We
content. The
600*C, particularly
strength increases with thermal treatment at
temperacan
acid
for pure
compounds. The large acid site concentration of GeP treated at 600% could be
releated to the hydrolysis of Ge-0-P bonds
leading to formation of Ge-0-H
groups.
In Fig. 3 the EB total conversion,
selectivity to ST and BA, and
area as function of chemical composition are
shown
.
surface
Mixed compounds
higher EB conversion in respect to pure phosphates. Moreover these values
give
are
330
Fig. 1. TG and DTA curves of Sn-Ge phosphates
Fig. 2. SEM micrographs of a)SnP, b)GeP, c)SnGel3, d)SnGe31.
331
.2
0
.4
.6
.E
1
Sa/~a.sal
Flg. 3. ( 0 ) EB conversion, (m) selectlvity to ST, (A) selectivity
to BA, (*) surface area as a function of chemical
composition of Sn-Ce phosphates. Contact time-0.1 g cat./EB/h;
T-4SO'C;
ocher conditlonr i n the text.
Fig. 4. ( 0 ) EB conversion, conversions to (1)S? and to (A) COX
(IS
a
functton of time on stream for SnGell sample.
Contact time-0.42 g cat/g EB/h, T450.C;
in the text.
other conditions
332
TABLE 2. Surface areas and surface acidity of Sn-Ge phosphates.
SAMPLE
GeP
SnGel3
SnGell
SnGe31
SnP
SURFACE AREA ACID SITES CONCENTRATION
2 -1
(cm-2x1~-14
(m g )
5.2
18.7
29.7
20.0
10.0
15.3
18.4
31.0
20.0
19.7
5.5
4.6
5.0
4.2
2.2
7.0
Tmax
('C)
126
140
142
140
140
4.1
4.6
3.6
2.2
154
140
154
148
170
a) after treatment at 450'C, b) after treatment at 600'C.
TABLE 3. Catalytic activity of Sn-Ge phosphates.
T-450°C, other conditions as reported in the text.
SAMPLE
Ge P
CONTACT TIME
h
EB CONVERSION
a
SELECTIVITY
ST
a
BA
COX
0.16
1.4
80
0
20
0.42
0.90
4.0
12.0
90
96
0
0
10
4
SnGel3
0.16
0.42
0.90
4.2
9.5
16.8
93
93
93
0
1
1
7
6
6
SnGell
0.16
0.42
0.90
7.7
14.5
26.0
97
96
95
0
1
1
3
3
4
SnGe31
0.16
0.42
0.90
3.7
7.0
16.0
96
96
97
0
0
1
4
4
2
SnP
0.16
0.42
0.90
3.4
5.6
7.5
56
75
92
35
16
0
9
9
8
333
higher than those found over different mixed systems such as Sn-Zr phosphates
( 4 ) because of the neglectable conversion to BA.
As shown in Table 3 EB conversion is strongly dependent on contact time
g cat./g
the range 0.16-0.9
maintained at higher
ST over
EB/h. Selectivity to
in
Sn-Ge systems
is
EB conversion, whereas it is enhanced over pure com-
pounds. It must be remarked that SnP improved selectivity to ST as a result of
the progressive decreasing of conversion to BA.
The effect of temperature on conversion and selectivity over Sn-Ge catalysts
is shown in Table 4. For all the systems the large increase of EB conversion
at 500 and 540'C
oxidation to CO
X'
resulted In a decrease of selectivity to ST due to
Therefore the performance
improved
of Sn-Ge catalysts at 450'C
comparable to that reported for Zr phosphate (11, but at higher
is
temperature
selectivity to ST is lower than that reported in (2) for different phosphates.
TABLE 4. Catalytic activity of Sn-Ge phosphates at a) T-500'C
b) T-54O'C.
Other conditions in the text.
EB CONVERSION
SAMPLE
SELECTIVITY
%
SnGel3
SnGell
SnGe31
In all
%
a)
b)
21.0
31.7
20.0
32.6
54.0
28.1
ST
a) b)
BA
a) b)
0
1
1
85 69
88 64
84 67
experimental conditions
during the first
and
2-3 hours
15
11
15
31
36
33
investigated catalytic
of the reaction up to
shown for SnGell sample in Fig. 4.
and mixed compounds without
0
0
0
co
5)
a)
activity
increased
constant conversions,
Coke formation was observed for both
loss of activity after
as
pure
10 hours on stream. This
behaviour is typical of many different acid catalysts reported in the
litera-
ture for EB oxydehydrogenation (1, 2, 7, 8). Tagawa et al. (7) correlated
the
oxydehydrogenation activity of various acidic catalysts with their moderate
acid strength, whereas Fiedorow (8) suggested that coke formed on the sites of
moderate acid strength was the effective catalyst for ST formation on alumina.
More recently Hattori et
pared by chemical vapor
over SnO /Si02 catalysts pre2
deposition, which were very selective for ST formaal. (10) showed that
tion, deposition of coke was not observed. With reference to metal phosphates
Schraut et
el. (11) proposed
phosphate during
that an
the first hours of
active coke deposited on
reaction is
zirconium
the actual catalyst, but
334
surface acidity
does not
play any
role, as
it disappears
at the
reaction
temperature. Vrieland (2) found very high conversion and selectivity over many
different metal
pyrophosphates and
formed on pyrophosphate groups of
catalyst surface, The results
proposed
that a carbonaceous overlayer
moderately acid
obtained by us
strength is
the
can be interpreted
actual
by
this
"active coke model", but we think that the role of acidity should be carefully
taken into account. In fact, we have shown by NH3 TPD (6) that after treatment
at 600.C
surface
acidity of Zr phosphate is
confirmed by IR measurements (12).
Sn-Ge mixed
systems. For
all
The same
These m e d i m
eventual role
of metal
the surface sites
should be
are
responsible for
surface layer, as proposed by
(2). On the other hand high selectivity
Therefore the
been
to
not
medium strength with lower amount of
strength sites
formation of a selective carbonaceous
it has
conclusion can be extended
these phosphates
pyrophosphate groups, but P-OH sites of
stronger sites.
still present and
Vrieland
to BA seems to be specific of
ion on its
the
formation should be
SnP.
not
excluded.
REFERENCES.
1) Emig, G., and Hofmann, H., J. Catal. E, 15 (1983).
2) Vrieland, G . E., J. Catal. IlJ, 1 (1988).
3) Frianeza, T. N., and Clearfield, A., J . Catal. 85, 398 (1984).
4) Galli, P., La Ginestra, A., Patrono, P., Massucci, M.
A.,Ferragina, C., Ciambelli, P., and Bagnasco, G., Italian
Patent 21587 A/86.
5) La Ginestra, A . , Patrono, P., Berardelli, M. L., Galli, P.,
Ferragina, C., and Massucci. X. A., J . Catal. 103, 346 (1987).
6) Turco, M., Ciambelli, P., Bagnasco, G., La Ginestra, A., Galli, P..
and Ferragina. C., J. Catal. 117, 355 (1989).
7) Tagawa, T., Hattori, T., Murakarni, Y., J. Catal. B , 56 (1982)
8) Alkhazov, T. G., Lisovskij, A . E., Safarov, X. G., and Dadasheva, A. M.,
Kinet. Catal. 13. 509 (1972).
9) Fiedorow, R., Przystajko, W., Sopa, M., and Dalla Lana, I. G., J . Catal.
68, 33 (1981).
10)Hattori, T., Itoh, S., Tagawa, T., and Murakami. Y., Studies
Surf. Sci. Catal. 31, 113 (1987).
11)Schraut, A., Emig, G., and Hofmann, H., J. Catal. 112, 221
(1988).
12)Ramis, G., Busca, G., Lorenzelli, V., La Ginestra, A.. Galli,
P., and Massucci, H. A., J. Chem. SOC. Dalton Trans., 881
(1988).
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
1990 Elsevier Science PublishersB.V.,
Amsterdam - Printed in The Netherlands
335
SELECTIVE PHOTOCHEMICAL CONVERSION OF METHANE INTO WORTHIER
COMPOUNDS
K. OGURA*, C. T. MIGITA, T. YAMADA and S. CHAKI
Department of Applied Chemistry, Yamaguchi University, Tokiwadai
Ube 755 (Japan)
SUMMARY
Methane and ammonia has been photochemically converted with no
catalyst to methylamine and ethylenediamine. Methylamine is
produced by the reaction of CH and NH radicals which are provided by the hydrogen-abstraction from2methane and ammonia I respecwith NH2* competed with the formatively. The reaction of CH
tion reactions of methanol 2nd ethane, but the former reaction
became predominant in the presence of higher concentration of
and CH=NH were
ammonia. Three radical species, CH 0, CH NH
detected as the reaction intermediazes by2thg spin trap-ESR
method, and it was suggested that the generation of ethylenediamine is attributed to the coupling of CH2NH2 radicals.
INTRODUCTION
Methane is the main constituent of natural gas which is not so
unevenly distributed as crude oil. Prospects of future carbon
resource after the depletion of petroleum are good, provided that
natural gas can be used as raw material for the chemical industry.
However, direct chemical utilization of methane is very difficult,
and methane "activation" by some means is essential. For instance, Solymosi et al. (ref. 1 ) have reported the conversion of
methane to formaldehyde with N20 over a Bi203-Sn02 catalyst at 5 5 0
'C. Kitayama and Schwartz (ref. 2 ) employ silica-supported
rhodium complexes in the catalytic conversion of methane to
methylchloride and hydrogen chloride with lesser amounts of other
chlorinated methane. In our previous works (refs. 3 , 4 ) , methane
was activated with hydroxyl radicals which are formed by the
photolysis of water. In the process, the initial activation of
methane is caused by hydrogen-abstraction from methane. The
reaction of methyl radical with OH* and CH3* leads to methanol and
ethane, respectively.
hv F
*H
+
*OH
H2O
CH4
+
*OH
+ *CHJ
+
H2°
336
3
+
*OH
*CH3
+
*CH3
CH
__f
__f
CH30H
(3)
'ZH6
(4)
In the present work, methane was converted to nitrogen- and
oxygen-containing compounds in the photolysis of the gas mixtures
of methane, ammonia, and water. The major products were methylamine and ethylenediamine except methanol, ethane, and hydrogen,
and the mechanism for the formation of these products was
investigated.
EXPERIMENTAL
The photochemical reaction apparatus consists of the reaction
chamber, water-cooled condenser, solution reservoir, and pressureadjusting tank. A 50 W low-pressure mercury lamp made of synthetic quartz, which was used as the light source, was put in the
reaction chamber (2.5 dm 3 1.
The radiation was polychromatic, and
the wave lengths of 185 and 254 nm were main. A flask (1 dm 3 )
containing 0.4 dm3 of water was connected to the reaction chamber,
and the total volume of the whole system was about 6.6 dm3 .
Various initial concentrations of methane were adjusted by changing the volume ratio of methane to nitrogen. After the whole
system was filled with methane, a given amount of aqueous ammonia
w a s admitted.
The reaction products were found in the condensed solution and
gas phase for the most part. The quantitative analyses of the
products were performed with a gas chromatograph (Shimadzu GC-8A,
JEOL JGC-1100), a steam chromatograph (Ohkura Model SSC-11, and a
high-performance liquid chromatograph (HPLC, Hitachi 655A). The
gas chromatograph was used at 100 OC with a flame ionization
detector (FID) and a Porapak Q column or at 30 OC with a thermal
conductivity detector (TCD) and a molecular sieve 5 A column. The
steam chromatograph was employed at 130 OC with a FID and a
Porapak R column, and the high-performance liquid chromatograph
was employed at 6 0 OC with an UV monitor and a GL-C610H column.
Formaldehyde was determined by a colorimetric analysis using
chromotropic acid. The absorption spectrum of the solution was
obtained with a Hitachi 100-50 type double-beam spectrometer.
Reaction intermediates were detected by the spin trap-ESR
method. The spin trap was the silica gel-PBN (a-phenyl N-tert-
butylnitrone) mixture.
The sample for the purpose of ESR measure-
337
ments was prepared by flowing the photolyzed gases onto the spin
trap and dissolving the PBN-adducts in benzene. ESR spectra were
recorded on a JES-ME-1X spectrometer with 100 kHz field modulation
and 1 mW x-band microwave power.
RESULTS AND DISCUSSION
The photolysis of the CH -NH -H 0 mixture led to the formation
4
3 2
of methylamine and ethylenediamine with small amounts of acetonitril, monoethanolamine, diethanolamine, and nitromethane in
addition to oxygen-containing species, ethane, and hydrogen found
in the photochemical reaction of CH4 with H20. The results
obtained are shown in Table 1 where aqueous ammonia was added by
two different methods: (A) before beginning of the photolysis and
(B) every 30 min during the photolysis. It is seen that the yield
of ethane is smaller in method B than in method A, but methylamine
TABLE 1
Products (umol) obtained in the photolysis of the CH4-NH3-H20
mixture. a
Ammonia (mmol)
28.8
CH3NH2
CH3N02
CH3CN
NH2C2H4NH2
NH2C2H40H
NH(C2H40H)2
CH30H
C2H50H
HCHO
‘ZH6
H2
57.6
Ab
B
A
1809
4
62
730
2247
-
231 3
5
103
195
9
3
1067
115
86
4620
18940
1
1686
87
98
3080
191 60
-
B
-
75
651
4
1578
59
72
2920
19040
2761
9
67
651
15
9
1658
58
163
1810
19540
aAmount of methane, 275 mmol; reaction time, 5h; temperature,
OC.
DA, ammonia was added before beginning of the electrolysis;
B, 5 ml of aqueous ammonia was added every 30 min during the
photolysis.
.loo
338
gives the opposite results.
The hydrogen atom of NH3 is abstracted by OH radical formed in
reaction 1:
+
NH3
+
*OH
*NH2
+
(5)
H2°
Methylamine is produced by the reaction of NH2* with CH3*:
+
*CH3
'NH2
CH3NH2
__f
Reactions 3 , 4 and 6 are competitive, and high ammonia
concentration during the photolysis may expedite reaction 6 ,
leading to higher yield of methylamine.
As shown in Table 1 , considerable amount of hydrogen is formed
regardless of the presence or absence of ammonia. Two possible
routes for the formation of hydrogen are considered:
OH
CH4
+
*H
+
*H
+
+
H2
H2
+
*CH3
However, the formation rate of hydrogen via reaction 8 is
probably much smaller than that via reaction 7, which bases on the
Fig. 1 . Products versus initial volume 3 of methane.
time, 5h; temperature, 1 0 0 'C; [NH31, 432 mmol.
Reaction
339
large activation energy (11.9 kcal mol-’ ) for reaction 8 (ref. 5).
Moreover, the major source of CH3 radicals may be ascribed not to
reaction 8 but to reaction 2, because the activation energy for
this reaction is about one-half (4.8 kcal mol-’ ) of that for
reaction 8 (ref. 5).
The four products, which were most abundant, are displayed
versus the volume percentage of methane in Fig. 1. In this photochemical system, 100 volume % of methane was equivalent to 275
mmol. The added quantity of ammonia was kept to 432 mmol, and
hence there was an excess of ammonia. The yield of methylamine is
approximately proportional to the initial concentration of methane.
The yields of ethylenediamine and methanol are both inclined to be
saturated at higher concentration of reaction gas. On the other
hand, the yield of ethane shows the exponential increase from
about 40 volume % of methane. These results indicate that in the
presence of the excess ammonia, CH3* first reacts with NH2* and
OH* to form methylamine and methanol, ethylenediamine is the secondary product from methylamine as described below, and the formation
TABLE 2
Products (pmol) obtained in the photolysis of CH3NH2-H20 and C2H6a
NH -H 0 mixtures.
3 2
CH3NH2b
CH3NH2
CH3N02
CH3CN
NH2C2H4NH2
NH2C2H40H
NH(C2H40H12
CH30H
C2H50H
HCHO
‘ZH6
CH4
H2
c 2H6 + N
H
-
1081
0
22
13
21 1
26
5
138
3
35
31 0
1810
17250
322
:Reaction time, 5h; temperature, 100 ‘C.
3.86 mmol.
C
C2H6, 41.6 mmol; NH3, 144 mmol.
57
1
86
1434
840
113
-
3890
19920
~
~
340
of ethane becomes conspicuous at higher concentration of methane.
Methylamine and ethane plus ammonia both could be a candidate
as the starting material for the formation of ethylenediamine. 'To
examine this, the photolysis of methylamine and ethane plus
ammonia was made in the presence of water, and the product
distribution is shown in Table 2. As seen from this table,
methylamine gives about four times as much ethylenediamine as
ethane does though the initial concentration of ethane is 10 times
that of methylamine. The photolysis of ethane plus ammonia leads
to large amounts of methanol and methane, and in this photolysis
the splitting of the C-C bond rather than the hydrogen-abstraction
from ethane seems to be predominant. Hence, the reaction route
via methylamine is reasonable for the formation of ethylenediainine, and the diamine observed in the photolysis of C2H6 + NH3
(see Table 2) is probably originated from the methylamine produced
in the photochemical reaction of ethane with ammonia.
Hyperfine coupling constants of the PBN-adducts obtained from
CH4-NH3-H20 system are shown in Table 3 along with the data which
have been reported previously by the liquid-phase trapping method.
From these values, three radicals were assigned: CH30*, *CH2NH2,
and -CH=NH. The assignment for methylene iminoyl radical bases on
the data reported by Janzen et al. (fer. 7). The finding of
CH NH radical leads to following mechanism for the formation of
2 2
ethylenediamine.
TABLE 3
Hyperfine coupling constants of the PBN-adducts obtained from
CH -NH -H 0 system.
4
3 2
Rad ica1s
a ( N ) /mT
a (8-H1 /mT
CH30
1.35
1.36
1.48
1.47
0.20
0.20
0.35
0.35
0.64
CH NH
CH=NH
1.49
a
bSolid-phase
trapping (this work).
Liquid-phase trapping (ref. 6).
341
CH3NH2
+
*OH
+
*CH2NH2
+
*CH2NH2
(9)
*CH2NH2
+
(10)
NH2C2H4NH2
CH 0 and CH=NH radicals are formed in the photolysis of
3
methanol (ref. 8 ) and by hydrogen-abstraction from CH2NH2 radicals
(ref. 9 ) , respectively.
The formation rate (R,) of methylamine was 39.2 and 63.0 umol
dm-3 h-' at 70 and 90 OC, respectively, and the Arrhenius plot of
Rm is shown in Fig. 2a. The corresponding activation energies
were 3.1 and 6.9 kcal mol-' at the NH3 concentrations of 86.4 and
28.9 mmol, respectively. The NH3 concentration-dependency of the
activation energy indicates apparently the existence of the
secondary reaction of methylamine. As described above, ethylenediamine is formed through the coupling of CH2NH2 radicals, and the
total formation rate of methylamine may be considered as the sum
of the formation rates of methylamine and ethylenediamine:
+
Rm+e = R(CH 3NH 2 )
(11)
R(NH2C2H4NH2)
In Fig. 2b, Rm+e is plotted against reciprocal temperature, and
the activation energy was 4.8 kcal mol-I indipendent of the added
~
a
-
2.c
7
I
c
I
1.8
E
a
rl
0
5
\
l.E
a,
+
e:E
tE
2
2.1
2.9
03T-1 /K-I
3.1
1.4
2.7
2.9
1 03+
3.1
/K-'
Fig. 2. Arrhenius plot of the formation rates on the presence of
ammonia: ( 0 ) 86.4 and ( 0 )28.8 mmol; [CH4], 80%. (a) Methyamine
( b ) Methylamine plus ethylenediamine (Rm+e).
(R,).
342
concentration of ammonia. Accordingly, this result also supports
the formation route of ethylenediamine via methylamine.
Thus, the gas mixtures of methane, ammonia, and water can be
converted to nitrogen-containing compounds such as methylamine and
ethylenediamine by the photochemical reactions, and the
dimerization of *CH2NH2 leads to the formation of ethylenediamine.
This process is noble as a scientific idea, but further works to
improve the yields of the products are required for practical
application.
REFERENCIES
1
5
6
7
8
9
F. Solymosi, I. Tombgcz and G. Kutsan, Partial oxidation of
methane by nitrous oxide over Bi203-Sn02, J. Chem. SOC., Chem.
Commun. , (1 985) 1455-1 456.
N. Kitajima and J. Schwartz, Activation of methane by supported
rhodium complexes, J. Am. Chem. SOC., 106 (1984) 2220-2222.
K. Ogura and M. Kataoka, Photochemical conversion of methane,
J. Mol. Catal., 43 (1988) 371-379.
K. Ogura, C. T. Migita and M. Fujita, Conversion of methane to
oxygen-containing compounds by the photochemical reaction,
Ind. Eng. Chem. Res., 27 (8) (1988) 1387-1390.
M. Imoto, Free radicals (in Japanese), Kagaku Dojin, Kyoto,
1975, pp. 111-116.
K. Ogura, C. T. Migita and T. Yamada, Photochemical formation
of methylamine and ethylenediamine from gas mixtures of
methane, ammonia and water, Chem. Lett., (1988) 1563-1566.
E. G. Janzen and H. J. Stronks, Assignment of the ESR spectrum
of the cyanyl radical spin adduct of phenyl tert-butyl nitrone,
J. Phys. Chem., 85 (1981) 3952-3954.
C. T. Migita, S. Chaki and K. Ogura, ESR spectroscopic detection of methoxyl radicals formed in the photochemical gas-phase
reaction of methane and water, J. Phys. Chem., in press.
C. T. Migita, S. Chaki and K. Ogura, Application of the spintrap-ESR method f o r the detection of radical intermediates
produced in the photochemical reaction gases, Nippon Kagaku
Kaishi , (1989) 1233-1 239.
G . Centi and F. Trifiro' (Editors),New Deuelopments in Selective Oxidation
0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
343
WORKING PRINCIPLE OF Li DOPED MgO APPLIED FOR THE OXIDATIVE COUPLING
OF METHANE
J.M.N. van Kasteren, J.W.M.H. Geerts and K. van der Wiele
Department of Chemical Technology, University of Technology,
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
SUMMARY
The nature of the active compound in Li doped MgO was investigated
by comparing the activity and deactivation of Li/MgO catalysts with
that of Li2C03 supported on an inert carrier (Zr02). The conclusion is
that Li,CQ3 itself is a very active catalyst ( o r a catalyst
precursor). Also the role of the catalyst in the oxidative coupling
of methane was determined: The selectivity of the active catalyst is
mainly due to a very high production rate of methyl radicals.
INTRODUCTION
According to Driscoll et al. (ref. l), the catalytic activity of
Li/MgO is due to the presence of Li'O-centres, stabilized in the MgO
matrix. He has shown that the formation of methyl radicals is
proportional to the amount of the centres as detected by EPR.
However, the reported amount of lithium dopant needed for an optimal
activity is extremely high. On the other hand, deactivation occurs
due to loss of lithium, in contrast with the assumed stability of Lit
centres in the MgO matrix.
Therefore, the lithium content and catalytic activity of Li/MgO
catalysts during deactivation were examined in more detail. Moreover,
the performance of Li/MgO was compared to that of LizC03 on ZrO,, a
carrier which does not interact with lithium compounds at the
temperatures used.
EXPERIMENTAL
Lithium doped magnesia was prepared by wet impregnation of MgO
with Li,C03. The lithium content was varied from 1 to 7wt% Li/Li+MgO.
After drying of the paste at 120°C for 16 hours the catalyst was
calcined for 4 hours at 900°C. The catalyst was ground and sieved to
the desired particle size of 0.3
0.5 nun.
The methane oxidation experiments were carried out in a micro
fixed bed flow set-up described elsewhere (ref. 2). The process
conditions used were: Temperature 8OO"C, atmospheric pressure,
-
344
CH4/02=5, CHJHe=l. 25, pseudo contact time (W/F) varied between 0.30.6 g.s/cm3(s.t.p.).
RESULTS AND DISCUSSION
Figure 1 shows the normal behaviour of our "home made" 7 wt%
Li/MgO catalyst during the first 50 hours on stream. The oxygen
conversion increases during the first 16 hours to a maximum which can
only be maintained for a few hours. After this, deactivation sets in
and the oxygen conversion decreases continuously. Also in the same
figure, the total lithium content of the catalyst is plotted as
function of the time on stream showing a continuous loss of lithium.
Although the lithium content decreases from the beginning of the
reaction, the activity for oxidative coupling initially increases.
This means that only part of the total lithium present in the
catalyst is responsible for the activity.
0.2 w t % Li,COJMgO
7wt% Li,COJMgO
m"
T=BOOC.CH,/O,=S.W/F=O.3
-s
loo
Y
T=800C.CH,OZ=5.W/F=O.6
&ml
4
50
*
5
Qs/rnl
0.50
C
0
0.25
Y
C
U
'
0
0
90
ao
i
Runtime [ks]
Fig. 1. CH,, 0, conversion and lithium content of a 7
wt% Li/MgO catalyst versus runtime.
0.00
0
70
35
Runtime [ks]
Fig. 2. CH,, 0, conversion and lithium content of a
0.2 wt% Li/MgO catalyst versus runtime.
Only very small amounts of lithium are responsible for an active
and selective catalyst. This is proven by Figure 2 in which the
oxygen conversion is plotted against time on stream for a 0.2wt%
Li/MgO catalyst. At pseudo contact times of 0.6 g.s/cm3( s t p . ) almost
complete oxygen conversion can be reached when starting with a fresh
catalyst. Because of the low lithium content the catalyst deactivates
from the beginning of the reaction and the deactivation rate is
proportional to the rate of lithium loss.
The reason for the lithium loss is twofold: 1) Lithium is lost by
..
345
evaporation of LioH which is formed by reaction of water and lithium
carbonate. 2) The lithium carbonate, which is a liquid at reaction
conditions, reacts with quartz (reactor wall material) to form
lithium silicates which are almost inert for the oxidative coupling
of methane. Dilution of the catalyst bed with quartz particles
accelerates the loss of lithium (ref. 2 ) .
Driscoll et al. (ref. 1) stated that Li'Ocentres stabilized in
the MgO matrix were the active centres for the generation of methyl
radicals from methane. Especially the role of MgO is essential in his
theory because of the substitution of M$'
ions (r,,2+=0.66 A ) in the
MgO lattice by Lit ions (ru+=0.68A) from the LizC03 phase. Korf et
al. (ref. 3 ) have shown that carbon dioxide, continuously added to
the gas phase, reduces the activity of the Li/MgO catalyst, while a
short treatment of a deactivated Li/MgO catalyst with carbon dioxide
restores the initial activity for some time. From these experiments
it appears that the presence of Li,CO, is essential for an active
catalyst.
To prove that Li,CO, can generate an active catalyst, Li,C03 was
impregnated on an inert carrier: ZrO,. Figure 3 shows the activity of
LizCOJZr02 as function of time on stream. Clearly the oxygen
conversion increases to a maximum followed by a decrease to almost
no activity.
Li,CO,/ZrO,
Li,C03/Li,Zr0,
T=800C.CH,I02=5. W/F=0.3 g.s/Nnl
-s
100
100
Y
c
0
g
T=800C.CHJ02=5.WIF=0.3
-s
gs/Nml
100
x
50
50
c
2
50
0
5
0
0
2
u
-
cn
cr,
0
0
70
g
-
0
0
-s
140
Runtime [ksl
Fig. 3. CH,, 0, conversion and product selectivity
versus runtime for oxidative coupling over
Li,CO,/ZrO,.
0
35
0
70
Runtime [ks]
Fig. 4. CH,, 0, conversion and product selectivity
versus runtime. for the oxidative coupling over
Li,CO,/Li&O,.
346
Indeed the performance of this catalyst is identical to Li/MgO,
except for a lower activity due to a lower surface area. The activity
lasts as long as lithium carbonate is present. The interaction of
ZrO, (r,4+=0.79
A) with Li,CO, (rU+=0.68A) at 800'C is far less than
that of Li2C0, with MgO. Only at very high temperatures (>lOOO"C)
detectable amounts of lithium zirconate (Li2zr03) are formed. This
lithium zirconate is itself a catalyst for the oxidative coupling of
methane with a reasonable activity and a high C,, selectivity. However
Li,ZrO, is not the active phase in the Li,CO,/ZrO, catalyst, because
also the activity of Li,ZrO, can be increased temporarily by doping
it with Li,CO, (Figure 4 ) .
Also this catalyst loses its activity
Due
to the interaction of Li,CO, with MgO
more rapidly than Li/MgO.
the loss of the lithium phase is retarded. In that respect the
carrier plays an essential role: stabilization of the lithium phase.
These results clearly show that Li2C0, is essential for an active
lithium catalyst. Combining of all this leads to a possible working
principle of the Li/MgO catalyst shown in Figure 5.
Li2C03decomposes in the presence of oxygen to an active centre and
CO,. This active centre reacts with methane to form a methyl radical.
Deactivation of the catalyst occurs due to reaction of Li,CO, with
water to LiOH which evaporates or with quartz to lithium silicates
which are almost inert.
Knowing the working principle
of the catalyst, the role of the
catalyst
in
the
oxidative
to2
coupling of methane can be
-CO,
understood as well. As shown by
.I&o, ,
"
Active Site
Geerts
et al. (ref. 4 )
the
(LI,O,?)
catalyst plays an important role
in the generation of methyl
radicals and in the oxidation of
CO to Cot. Ethane is formed by
ig. 5. Working principle of the Li/MgO catalyst.
coupling of methyl radicals in
the gas phase. Ethylene is formed
by dehydrogenation of ethane and in turn is oxidized to CO. Figure
6 shows a comparison of selectivities for the oxidative coupling of
methane between a totally deactivated catalyst and no catalyst.
I
347
CATALYTIC vs NON CATALYTIC
T=800C, CH,/O,=5, Li/MgO
I
100
-11
Awed oat
no cat
75
1
r
2
c
50
0
(s;
25
0
0
25
50
75
Oxygen conversion
100
Pi]
Fig. 6. Comparison between catalytic (Li/MgO) (solid
symbols) and non catalytic (open symbols) oxidative
coupling.
II
Fig. 7. Simplified reaction scheme for the
oxidative coupling of methane.
Remarkably the selectivity of the deactivated catalyst is
identical to the selectivity of the homogeneous gas phase reaction,
at the same oxygen conversion. However, the reaction rates are much
higher with the aged catalyst than in the empty tube. This means that
the surface acts as a radical initiator, which releases radicals into
the gas phase, without further interfering with the course of the
reaction. A fresh catalyst obviously releases (methyl) radicals at
a much higher rate. This fact alone might be responsible for the
selectivity of a fresh catalyst: high methyl radical concentrations
may cause relatively high reaction rates to ethane, as the coupling
is second order in methyl radicals, while the oxidation is probably
first order. Thus a catalyst may improve selectivity, if it produces
methyl radicals at (locally) high concentrations. This reasoning is
visualised by the simplified reaction scheme in Figure 7. Reaction
1 is the abstraction of a hydrogen atom from methane. Reaction 2 is
the coupling reaction of the methyl radicals to Cz+ components.
Reaction 3 and 4 are the total oxidation reactions in which C-0 bonds
are formed irreversibly. Thus the methyl radical is the key to
selectivity. This hypothesis is supported by calculations with a
computer model that simulates the gas phase oxidation by taking
account of practically all elementary radical reactions (ref. 5). The
effect of the catalyst is simulated by adding an extra equation that
increases the rate of formation of methyl radicals. The results are
shown in Table 1. This table shows a comparison between two
simulations: with and without increased methyl radical production.
348
TABLE 1.
Computer simulations of gas phase oxidative coupling of methane with increased methyl
radical production.
Methyl radical production
Normal
Increased
(s)
Contact time
CH4 conversion ( % )
0, conversion ( % )
c,+ selectivity ( % )
CO, selectivity ( % )
2.7
2.7
0.13
0.06
3.4
0.09
0.1
8.2
0.1
79
70
97
21
30
3
At the same contact time, the higher methyl radical production rate
increases both the methane and oxygen conversion, as expected, while
the Ctt selectivity is somewhat lower. However, when results at the
same conversion level are compared (first and last column of Table
1) it is clear that much higher selectivities are achieved at
increased methyl radical production rate, in accordance with the
hypothesis proposed.
CONCLUSIONS
The presence of the lithium carbonate phase in the Li/MgO catalyst
is essential for the activity. Lithium carbonate itself can generate
an active catalyst if supported on an inert carrier like ZrO,. Very
small amounts of lithium are sufficient to create an active and
selective Li/MgO catalyst. The main function of the Li-catalyst in
the oxidative coupling of methane is the activation of methane. This
results in high local methyl radical concentrations which favour the
coupling reaction to ethane.
ACKNOWLEDGEMENT
The financial support for this research, which was provided by the
European Communities under contract number EN3C-0038-NL and the
Netherlands Organization for Scientific Research (NWO), is gratefully
acknowledged.
349
REFERENCES
l D . J . Driscoll, W. Martir, J-X. Wang, J.H. Lunsford,
J.Am.Chem.Soc. , 107 (1985) 58-63.
2 J.M.N. van Kasteren, J.W.M.H. Geerts and K.van der Wiele,
Proceedings ''9 ICC Calgary, Alberta, Canada, Vol 2 (1988) 930936.
3 S.J. Korf J.A.
RoOs, N.A. de Bruijn, J.G. van O m e n , J.R.H. ROSS,
J.Chem.Soc.,Chem.Comn.
(1987) 1433-34.
4 J.W.M.H. Geerts, J.M.N. van Kasteren and K. van der Wiele, Catal.
Today, 4 (1989) 453-461.
5J.W.M.H. Geerts, Q. Chen, J.M.N. van Kasteren and K. van der
Wiele, Catal. Today, submitted for publication.
350
WORKING PRINCIPLE OF Li DOPED MgO APPLIED FOR THE OXIDATIVE COUPLING
OF METHANE
J.M.N. van Kasteren, J.W.M.H. Geerts and K. van der Wiele
Department of Chemical Technology, University of Technology,
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
1.
(Institut de Catalyse, Vileurbanne, France) :
1) What is the loss of Li in the case of your Li,CO,/Li,ZrO,
J.C. Volt.
catalyst?
2) Is there some isomorphity between Li,CO, and Li2Zr03?
J.M.N.
van Kasteren (University of Technology, Eindhoven, The
Netherlands): We did not measure the lithium loss for our
Li2C0,/Li,Zr03 catalyst but it is very probable that the lithium
carbonate which we impregnated on this catalyst is almost
completely lost during reaction. The performance of the
catalyst after a few hours is exact that of pure Li2Zr03,which
we tested separately. The cristal structure of the two phases
is not similar.
2.
Ross (University of Twente,The Netherlands) : I am glad
to see that your conclusions are similar to those which we have
reached (S.J. Korf, N.A. de Bruijn,J.G. van O m e n and J.R.H.
ROSS, Catalysis Today 2(1988) 535) in relation to the nature of
the active sites. However, we differ with you about the
importance of surface reactions in steps such as the reaction
C2H6 + 0.502 ----- C,H, + H20, C2Hx + O2 ---- COX, etc. ( j . A .
R O O ~ ,S. J. Korf , R.H. J. Veehof , J.G. van O m e n and J.R.H. ROSS,
Catal. Today, 4(1988) 471; Appl. Catal. , 52 (1989) 147). Do you
think that your conclusions are still applicable at lower
temperatures?
J.R.H.
van Kasteren (University of Technology, Eindhoven, The
Netherlands): We agree with you that at lower temperatures gas
phase reactions lose importance compared to surface reactions.
However, the best results with respect to C,, yield have been
reached at conditions where gas phase reactions play a very
important role. Our conclusions are in this way applicable at
lower temperatures that the lower C2+ yield can be explained by
the occurrence of more total oxidation reactions which occur
mainly at the catalyst surface. A low temperature means a lower
activity and thus lowering of methyl radical concentrations
especially in the gas phase. This will favour the total
oxidation reactions thus a lowering of the C,, selectivity. Also
at low temperatures gas phase reactions can not be excluded
totally because radical coupling reactions have no activation
energy and are possible even at room temperature ( 6 ) .
J.M.N.
3.
J.P. Brasdil (BP Research, Ohio, U SA ) : My question relates to
your computer simulation of the methane coupling reaction.
351
Increasing local concentration ot methyl radicals should
increase the yield of C,, products. Have you looked for
conditions with the model that will give a maximum yield of C,,?
If so, what is the maximum yield predicted by your model?
van Itasteren (University of Technology, Eindhoven, The
Netherlands) : We did not look for process conditions which give
the optimum C,+ yield, but this is an objective for the future.
J.M.N.
4.
J . K i w i (EPFL, Lausanne, Switserland) : You report CO and CO,
formation in your processes leading to C2+ at 800°C on Li/MgO
catalysts. How does in time scale the CO and COP evolve? Does
the CO,, with time form at the expense of CO? How do you account
for this in your model?
J.W.H. van Kaateren (University of Technplogy, Eindhoven, The
Netherlands): Indeed does the CO, form at the expense of CO. it
can be shown that a fresh catalyst converts almost all CO to
CO,, while a deactivated catalyst produces mostly CO (3). Also
the gas phase oxidative coupling of methane produces only CO
and this can be well described with our computer model (4). We
are at the beginning of our catalytic modelling and our object
is to add a dummy reaction set analogous to the methane
activation steps to simulate the CO to CO, catalytic reaction.
5.
Cortes Corberaa (Institute of catalysis, CsIc, Madrid,
Spain) :
1)Have you experimental evidence of the presence of Li2C03
phase in the working catalyst?
2) In the initial stage of catalytic test of Li/MgO catalyst
(Fig. 1) activity increases while Li content is decreasing:
I wonder if the important factor for activity is not the Li
content but the nature of the Li containing phase, taking
also into account that carbonate could be decomposed at lower
temperatures with a reducing atmosphere ( such as reaction
conditions) than with an oxidant atmosphere ( such as in the
calcination step). Then, possibly the active phase could be
a lithium oxide (or peroxide) instead of carbonate, being the
latter a precursor of the active phase.
3)Have you tried the catalytic activity of pure Li2C03?
V.
J.M.N. van Itasteren (University of Technology, Eindhoven, The
Netherlands) : We know that the decomposition of the Li,CoJ phase
plays an important role during the oxidative coupling of
methane over Li/MgO. Addition of CO, to the feed gas lowers
immediately the activity of the catalyst. Also the initial
activity of a deactivated Li/MgO catalyst can be temporarily
restored by a treatment with COe as shown by Korf et al. (2).
At 800°C in a stream of oxygen the Li,CO, is decomposing: the
loss of CO, can be measured. However, when the reaction is
started CO, is formed immediately and this reacts with the
active site and with the Li,O to form Li2C03 again. Under
reaction conditions an equilibrium exists between Li,CO, , Li,O
and Cot. We have tested pure Li2c03 itself although this is not
easy at 800"C, because it melts at 723°C. We constructed a
352
bubble reactor with which it is possible to bubble CH, and 0,
through the liquid Li2C03 (5). The C,, selectivity is much lower
compared to the Li/MgO catalyst. The addition of MgO to the
melt of Li,CO,
leads to improvement of the catalytic
performance. The conclusion from this work is that MgO is
needed to give a good coupling catalyst.
REFERENCES
l J . M . N . van Kasteren, J.W.M.H.
Geerts and K.van der Wiele,
Proceedings gth ICC Calgary, Alberta, Canada, Vol 2 ( 1 9 8 8 )
936.
930-
S.J. Korf J.A. ROOS, N.A. de Bruijn, J.G. van Ommen, J.R.H. ROSS,
J.Chem.Soc.,Chem.Comun. ( 1 9 8 7 ) 1 4 3 3 - 3 4 .
3 J.W.M.H. Geerts, J.M.N. van Kasteren and K. van der Wiele, Catal.
Today, 4 ( 1 9 8 9 ) 4 5 3 - 4 6 1 .
4 J.W.M.H. Geerts, Q. Chen, J.M.N. van Kasteren and K. van der
Wiele, Catal. Today, submitted for publication.
5 J.W.M.H. Geerts, J.M.N. van Kasteren and K. van der Wiele, to be
published.
6 J.M.N. van Kasteren, P. de Been, J.G.A. Holscher, I X t h European
Sectional Conference on Atomic and Molecular Physics of Ionized
Gases, Lissbon, Portugal, 3 5 1 - 3 5 2 ( 1 9 8 8 ) .
2
G . Centi and F. Trifiio’ (Editors),New Deuelopments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
353
INVESTIGATIONS ON THE HETEROGENEOUSLY CATALYZED OXIDATIVE
COUPLING OF METHANE OVER ALKALI DOPED METAL OXIDES
S. BARTSCH, H. HOFMANN
Instltut fur Technlsche Chemle I der Unlversitat Erlangen-NUrnberg
Egerlandstr. 3, 8520 Erlangen
ABSTRACT
The heterogeneously catalyzed oxidative coupling of methane into
ethane and ethene using alkali doped cerium oxide has been investigated. Among all promotors tested the activity towards C2 products
decreased in the order Li>Na>Cs>K. An optimal lithium loading of
5 wt% Li2C03 in Ce02 was found. The effect of the BET surface area
for thls catalyst was negligible. For a cerium doped LiAgO catalyst
the influence of the operating conditions has been studied to find
conditions which produce an optimal yield of C2 hydrocarbons.
INTRODUCTION
Because of its large deposits natural gas wlll become the most
important primary energy source in the future (ref.1). Consequently
there is great lnterest in using methane, whlch is the predominant
component of natural gas, as a chemical feedstock. Different approaches have been proposed to provide new routes for produclng
useful chemlcals by partial oxldatlon of methane into methanol,
formaldehyde or Cp-hydrocarbons (ref.2). Although a great deal of
work has been done in this area the economlc barrier has not been
overcome. This circumstance leads to the sltuation that at present
7-109 Nm3/a of natural gas are burned off in the oil fields or at
remote natural gas deposits (ref.3).
Since the pioneerlng work of Keller and Bhasin in 1982 (ref.4)
the heterogeneously catalyzed oxidative coupling of methane into
ethane and ethene by means of metal oxides has attracted increasing
attention. In some recent publications on the use of alkali doped
metal oxides as catalysts good chances f o r the above reaction have
been opened up (refs.5-17).
It is reported (refs.5,18) that an 0- specles on the catalyst
surface is an active site which is able to abstract a hydrogen atom
from methane. This process is consldered as an initiating step In
the methane coupling reaction. As to the Li/MgO system, It was
354
shown that such 0- species can be produced by solid diffusion of
A similar creation
of active sites can be assumed for other alkali doped metal oxides.
Figure 1 illustrates schematically how the reaction is thought
to occur: If effects of adsorp2CH,
tion and desorption are neglected, there are two principal steps involved: on the one
hand, methane molecules react
with lattice oxygen forming hy2CH3. + ZOHdroxy groups and methyl radicals which can produce ethane
0
2
4
by reconblnation. On the other
112 0,
CH
, , t H,O
hand, the regeneration of the
active sites has to take place
Fig.1:
with oxygen from the gas phase.
Radical generating mechanism
Water is formed as a by-product.
In a previous paper (ref.20) we reported that the addition of
cerium oxide as a third compound to a Li/MgO catalyst improves the
C2 yield. The Ce/Li/MgO system was proposed for the first time by
Y. Bi et a1 (ref.15). We have investigated the role of cerium in
this system by means of pulse technique (ref.21). It could be demonstrated that the Ce/LiMgO catalyst produces ethane as well as
ethylene from methane even if no gas phase oxygen is available.
The amount of C2 products decreases with respect to the number of
pulses (see Flgure 2). After reduction with methane, the catalyst was
reoxidized with oxygen pulx c q '3
ses. This treatment enabled the
l % l '2
catalyst to show its initial
10
behaviour. As these results
VMgO : x XcH4
l a
could not be achieved by the
use of pure L i M g O in place of
6
the cerium doped catalyst, it
L
is assumed that lattice oxygen
acts as a highly selective
2
Lit cations into the MgO lattice (refs.5,19).
Y-)
P
I
oxidizing agent and cerium
changes its oxidation state during the methane coupling reaction. We therefore concluded
that the rate-controlling step
of the coupling reaction is
not the formation of methyl
0
1
2
3
4
-
5
6
pulse number
Fig.2: CH, conversion ( X I and
C, yield ( Y ) with respect to
the number of CH4 pulses over
L i A g O and Ce/Li/MgO.
355
radicals but the regeZCH,
neration of the active
I
sites on the catalyst
surface. This reaction
step might be enhanced
‘O\
nby the use of a multivalent metal oxide as
a charge carrier as depicted in Figure 3. A
number of other metal
f
Hz*
oxides have been tested
as substitutes for ceFig.3:
rium but without sucSpeculative radical generating mechanism
cess (ref. 21).
This paper presents results of our investigations into the effect of alkali dopands on the activity and selectivlty of Ce02.
The influence of the operating conditions on the C2 yield over a
Ce/Li/MgO catalyst is discussed.
EXPERIMENTAL
The catalyst testing experlments were carried out with a standard
flow system. In Figure 4 the catalytic section of the reactor tube
as well as the operating conditions are shown. The axial temperature
OPERATING CONDITIONS
-
800
-
25
= 650
= 1.2 P
= 1.6
W a T = 1.0
& ~ R T = 16.3
X(N2) = 0 . 9
x(CH4) = 0.06 x ( 0 2 1 = 0.01
TO
PO
dP
= 0.8
&
=
0.39
-
OC
7.0 bar
Nml/a
g
9
0.1
0.03
1.0 mm
Flg.4: Catalytic section of the reactor tube and operating conditions. ~ ( 1 ) :mol fraction of component i. dp: particle diameter.
E: void fraction of the fixed-bed. The temperature profile was measured under reaction conditions.
356
profile is measured by means of a thermocouple which can be moved
inside a capillary tube made of a-alumina. In order to avoid undesired temperature gradients in the catalytic fixed-bed, the feed gas
consists of 90 mol% N2 and 10 mol% of the desired CH4/02 ratio.
Thus, a typical temperature profile with respect to the reactor
length as depicted in Figure 4 was achieved under reactlon condit ions.
For the examination of the selective oxidation of hydrocarbons at
temperatures above 500 OC, the construction material of the reactor
becomes important. Stainless steel was shown to enhance the unselective oxidation of light hydrocarbons (ref.221. In order to eliminate
any catalytic influence of the reactor itself, a special fixed-bed
reactor constructed from a-alumina was developed. The tube has a
length of 110 cm and is f illed with ceramic particles. The catalyst
particles are spread over a length of 10 cm in the middle of the tube
(catalytic section). It can be heated electrically up to 800 OC.
Catalyst preparation has been reported on previously (ref.20)
and was carried out according to prescriptions given in the literature (ref.5). Table l lists all catalysts which were studied in
the investigations covered in this paper.
Using Ce02 as the main compound, the amount of lithium loading
(Catalysts 1-5) as well as the effect of alkali dopands (Catalysts
3.6.7.81 were investigated. These catalysts were prepared in such
a way that the molar ratio of alkali/CeOz was constant.
TABLE 1
1s t
compound
Ce02
CeO2
CeO2
CeO2
CeO2
Ce02
CeO2
Ce02
2nd compound
L12CO3
Li2CO3
L12CO3
L i2CO3
L i2CO3
Na2C03
K2C03
cs2co3
wt%
Catalyst
2
3.5
5
7.5
10
7.1
9
18.9
In order to find conditions which produce an optimal yield of C2
hydrocarbons, the influence of the operating conditions on conversion and selectivity over the Ce/Li/MgO catalyst Iref.20) were
studied in detail. The values of T,p,W/F and p O ~ ~ ~ /were
p 0 varied
~ ~
corresponding to those given in Figure 4 . For an interpretation of
357
these results i t was necessary to elumlnate the reaction scheme.
Therefore mixtures of C2H6/02/N2 and C2H4/02/N2 were also used as
feed gases. The operating conditions in these runs were the same
as stated above, but temperature (750 OC) and pressure (atmospheric)
were kept constant. In order to distinguish between homogeneous
and heterogeneously catalyzed reaction steps each run was carried
out with and without catalyst.
Blank runs with CH4/02/N2 (molar ratio 0.67/0.33/9) showed that
homogeneous oxidation is negligible up to a temperature of 770OC.
RESULTS AND DISCUSSION
Alkali doped Cerium oxide
Alkali doped Ce02 was found to be a selective catalyst f o r the
methane coupling reaction. This must be due to the presence of
dopands, since pure Ce02 yields only total oxidation products, a
fact that is observed by several researchers (refs.23,24). Figure 5
presents a comparison of the C2 yield obtained over CeO2 doped
with different alkali metals, such as lithium,
sodium, potassium and cesium. The influence of
such dopands on the catalytic behaviour of MgO
(ref.25) as well as of 2n0
(ref.26) was studied by
Matsuura and co-workers.
They found that Lithium
wasthemostattractivealFlg.5: Effect of different alkali
kali dopand for both sysmetals as dopands in Ce02.
tems, ZnO and MgO. In
agreement with these results it is shown that the amount of C2 hydrocarbons decreases in
the order Li>Na>Cs>K. Li/Ce02 is an effective catalyst yielding
12% C2 hydrocarbons under the following operating conditions:
T = 750 OC, W/p= 0.16 g.s/ml,
2, atmospheric pressure.
In literature it is suggested that the simllarlty in the ionic
radii of the main catalyst compound and the respective dopand
might have a decisive influence on the formation of the active
sites (ref.5). The alkali cation should fit into the cation vacancies of a higher valent metal oxide matrix in order to create 0centres. The catalytic properties of Li/MgO, Li/ZnO and Na/CaO can
be explained in this way.
358
The radii of some interesting cationes are summarized in Table 2.
As to Ce02 one would expect the Net cation as the most attractive
alkali dopand but the obtained C2 yields over the alkali/Ce02
system are not coherent with this theorie. There is only a marginal
difference in the activities of Na/Ce02, K/Ce02 and Cs/Ce02, but
LI/Ce02 is a much more active and selective catalyst for the methane coupling reaction.
TABLE 2
~
Cation
Radius
~
~~
Lit
Na'
Kt
0.68 0.97 1.33
-
~-
Cst Mg2' Ca2' Zn2' Ce3' Ce4'
1.67 0.66 0.99 0.74 1.07 0.94
radii values in 10-lom (ref.2 8 ) .
Another important fact that governs the catalytic properties is
the amount of alkali loadlng. Figure 6 shows the C2 yield obtained
at different reaction temperatures with respect to the wt% Li2C0,
used in catalyst preparation. At each temperature the yield of C2
hydrocarbons goes through
a maximum at a value of
5wt%Li2C03. Similar behaviour is reported by Matsuura et a1 (ref.25) for
Li/MgO, by Iwamatsu et a1
(ref.13) for Na/MgO and
Rb/MgO and by Otsuka et
a1 (ref.6) for Li/Sm203.
There has been a discussion in the literature on
Flg.6: Effect of the amount of
the fact, that alkall doLi2CO3 as dopand in Ce02
ping causes the formation
of active sites and therefore an increase In activity, but a simultaneous reduction in surface area. Since these two properties have opposite effects on the
C2 yield, there should exist an optimal alkali loading with respect
to the Cz yield (ref .27). We cannot confirm this consideration in
the case of lithium doped Ce02, because the BET surface area of
pure cerium oxide is slightly reduced by the lithium loading and
no correlation with respect to the C2 yield is observed as depicted
in Figure 6.
359
Effect of the operatinq conditions
In Figure 7 the influences of temperature (650- 8OOOC) as well
as of the CH4/02 ratio at the reactor inlet (0.2-10) over Ce/Li/MgO
are shown. With
increasing temperature, the con&Iro
version and the
~ 3 0
selectivity also
increase. Alarge x
excess of oxygen
increases the meMx) 653 700 750 &oo 85
thane conve r s ion,
T[OC
but the selectivitydeclines siFig. 7: Effect of temperature and po,-H4/poo20n
mu 1t aneous ly so
CH4 conversion ( 0 1 , Cz selectivity ( x ) and
that an optimal
C2 yield ( A )over a Ce/Li/MgO catalyst.
yield exists at
pabs = 1.2 bar, w / F = 0.16 g*S/Nml.
P"CH4/P"o2 = 2
A temperature of 750 OC and a CH4/02 ratio of 2 were used for the
examination of W/F and the total pressure on the C2 yield. Because
of the dilution of the reactants (see Figure 4 ) a total pressure
of e.g. 6bar is equivalent to partial
pressures of 400mbar CH4 and 200mbar
O2 respectively. With increasing residence time the C2 yield also increases. At each value of W/F the C2
yield moves through a maximum with
respect to pressure. At lower values
of W/F this maximum is shifted to
higher values of pressure.
'-
-
6
In order to achieve a high yield
of hydrocarbons over this cata-
0.
I
-
I
I
I
I
I
I
1 2 3 4 5 6 7
P Ibarl
Fig.8: Influence of W/F and
lyst, one must operate at a high value of W/F and at a low pressure.
total pressure on the C2
These results must be due to the
yield over a Ce/LiAgO
complexity of the reaction system
catalyst at T = 750 OC and
consisting of homogeneous as well as
= 2.
heterogeneously catalyzed reactions,
some of which are parallel or consecutive steps.
Figure 9 summarizes the main reaction pathways that were found in
the catalytic (left hand scheme) andnon-catalytic (right hand scheme)
360
experiments. Methane is oxidized into C02 in both cases, but only in
the presence of the catalyst ethane is produced. Ethane is oxidatively dehydrogenated into ethylene with and without catalyst. However
dehydrogenation of ethane also takes place to some extent in the
absence of oxygen. Ethylene is homogeneously converted into CO
which undergoes further oxidation into C02 by a catalytic reaction
step.
Fig.9:
Catalytic (left hand) and non-catalytic (right hand) reactions
In order to gain a clear understanding of the results shown in
Figures 7 and 8 extensive kinetic studies on each reaction step pointed out in Figure 9 have to be done. This is the main prospect which
our future work will encompass.
REFERENCES
1 W. HOfele, W. Terhost, Chem. Ind., 37 (1985) 10
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4 G.E. Keller, M . M . Bhasin, J. Catal., 73 (1982) 9
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J. Am. Chem. SOC.,107 (1985) 5062
6 K. Otsuka, Q. Liu, M . Hatano, A . Morikawa, Chem. Lett., (1986) 467
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10 C.H. Lin, J . X . Wang, J.H. Lunsford, J. Catal, 111 (19881 302
11 M . Y . Lo, S.K. Agarwal, G . Marcelin, J. Catal, 112 (1988) 168
12 H.S. Zhang, J.X. Wang. D.J. Discroll, J.H. Lunsford,
J . Catal., 112 (1988) 366
13 E. Iwamatsu, T. Moriyama, N. Takasaki, K. Aika,
J. Catal., 113 (1988) 25
14 G.J. Hutchings, M.S. Scurell, J.R. Woodhouse,
J . Chem. SOC., Chem. Commun., (1987) 1862
15 Y. Bi, K. Zhen, Y. Jiang, C. Teng, X. Yang,
Appl. Catal., 39 (1988) 185
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361
19 Y . Chen, H.T. Tohver, J. Narayan, M.M. Abraham,
Phys. Rev., 16 (1977) 5535
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Ind. Eng. Chem,, Proc. Des. Dev., 16 (1977) 271
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25 I. Matsuura, Y. Utsuml, T. Dol, Y. Yoshida,
Appl. Catal., 47 (1988) 299
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362
R.K. Graselli (Mob11 Res.L Dev. Corp.,
USA): Just a brief comment
on your interesting paper. You conclude from your work that Li is
the most effective alkali dopant for your CeO, system, while the
ionic radii of Li+ (0.68) and Ce3+ (1.031, Ce4+ (0.92) are rather
poorly matched; other other alkalies e.g. Na+ (0.97) [or K+ (1.3311
would be a better size match for Ce3+/Ce4+. I should line to offer
the following possible explanation for your finding that the effectiveness of alkali dopants dereases in the order Li>Na>Cs%K. Not
only should an ionic fit between dopant and base catalyst elements
be considered, but also the elektronic factors of them. Considering
the electronegativlties of the alkali series, Cs is the most basic
while Li is the least basic in this series. Thus Cs wlll force Ce
more readlly into the Ce4+ state than Li. Thls results in a stronger
Ce-0 bond and thus a more difficult release of lattice oxygen to
the hydrocarbon (CH4) in the case of Cs and/or K doping, while
doping with Li will allow a more facile yet selectlve release of
lattice oxygen; resulting in an optimum for the system Li/Ce among
alkali dopants.
J.G. van Ommen (University of Twente, The Netherlands): In your
reaction scheme of heterogeneously catalyzed reactions, you exclude
the oxidation of C2 (C2H6+C2H4) products to C02. Do you have experimental evidence to support this hypothesis?
S . Bartsch (University of Erlangen, Germany): We have carried out
experiments under catalytic and non-catalytic conditions using
CH4/02/N2 or C2H6/02/N2 or C2H4/02/N2 as feed gas. Up to now we
did not use mixtures of those hydrocarbons in our investigations.
In each case we obtained profiles of partial pressures of the reactants with respect to the reactor length. All these data were obtained at atmospheric pressure, T = 750 OC, F = 6 Nml/s, W = 0.1
gcatalyat/Cmrcactor length Phydrocarbon/Poxygen = *.*
Comparing these profiles we found the main reaction pathways as
depicted in Figure 9: Ethylene is mainly homogeneously oxidized
into CO which undergoes further oxidation into COz in the presence
of a catalyst (ref.1). Ethane reacts into CO and C02 to some extent
with and without catalyst, but the main product is ethylene. The
catalyst enhances ethylene production, while the COX formation
remains almost unchanged. The small amounts of COX are due to the
consecutive oxidation of ethylene as already mentioned. Only in
the case of catalytic CH4 oxidation remarkable amounts of C02 were
found. The amount of COz cannot be explained by consecutive oxidation of C2 hydrocarbons, indicating that methane is directly converted into C02,
1 S. Bartsch,H.Hofmann, submitted for publication in Catalysis Today
J. Kiwi (EPFL Lausanne, Switzerland): 1) You have not elaborated
on the stability in time and reuse of your cerlc catalyst doped
with Li at 780 OC; What is the situation as shown by your experimental results?
2 ) Ce02 as support for Li is used in your work. What is the loss
of Li at 700 - 800 OC during experiments?
363
S. Bartsch (University of Erlangen, Germany): 1) We have investigated the conversion of methane, the C2 selectivity and the C2
yield as a function of time on stream for a Li/Ce02 catalyst. The
results are shown in the following Flgure:
---
0.16 p / m i
7
1023 K
p
1.2 bar
LWn2 2.0
Y/T
Catalyst:
L Mc02
0
5
10
15
tlme on stream
20
[
h 1
All of the experimental results presented in our paper have been
obtained wlth fresh catalyst in order to avoid the influence of deactivation effects.
2 ) It is known from the literature that lithium loss is the main
reason for the deactivation of lithium containing catalysts (ref. 1 ) .
We have not studied the effect of lithlum loss of a Li/Ce02 catalyst, but we assume similar behaviour as we found for a Ce/Ll/MgO
catalyst which has been investigated in our laboratory (ref.2). In
summarizing these results it was found, that lithium loss is mainly
caused by the influence of temperature, time on stream and turnover
rate. The lithlum loading decreases nearly to zero under severe
reaction conditions.
1 S.J. Korf, J.A. Roos, N.A. de Bruijn, J.G. van Ommen, J.H.R. Ross,
Catalysis Today, 2 (1988) 535
2 S.Bartsch,H.Hofmann, submitted for publication incatalysis Today
G.I. Golodets (Ukralnian Academy of Science, USSR): What is the
degree of oxidation of lattice oxygen which is, as you told us,
“the oxidizing agent in the oxidative coupling of methane” 7
S. Bartsch (University of Erlangen, Germany): Up to now we have no
detailed information about the nature of the active oxygen species
(e.g. oxidation state) in our catalyst. This will be one of the
main aspects of our future work on cerlum containing catalysts.
W.J. Vermeiren (K.U. Leuven, Belgium): There is now enough evldence that gas phase reactions between methane and oxygen are pos-
364
sible, especially at high partlal pressure of oxygen in the feed.
I think that the optimum, you obtained in Figure 8 is due to a
combination of gas phase and catalytic reactlons. In these conditions of CH4/02 = 2 the gas phase reactions produce low amounts of
C 2 products. This is the reason for the decrease of C 2 yleld at
higher partlal pessures of methane and oxygen.
Did you perform experiments with the same conditions as shown
in Figure 8, but wlth an empty reactor to investigate the contribution of gas phase reactions?
S. Bartsch (University of Erlangen, Germany): We dld not perform
the same set of experimental runs as depicted in Figure 8 wlthout
catalyst, but we checked the influence of gas phase reactions under
the following operating conditions: po~-4/p002 = 2.0. F = 6 Nml/s,
no catalyst. The results are summarized in the following Table:
p (bar) T ( OC
QH4
(%)
1.13
1.2
708
755
1.85
1.2
1.2
780
2.17
1.2
800
3.11
10.0
750
11.10
n.e. not evaluated
YCO
( %)
n.e.
0.0
n.e.
n.e.
3.27
yco2 ( %
n.e.
1 * 49
n.e.
n.e.
6.46
YC2H6 ( %
n.e.
0.18
n.e.
n.e.
0.80
YC,H,
(%
n.e.
0.18
n.e.
n.e.
0.57
From these data It Is clear, that our results (see Figure 8)
cannot be simply explained by the influence of homogeneous gas phase
reactions, because of the very low converslon of methane. However,
we agree that the interaction between homogeneous and heterogeneously catalysed reactlons, especially the consecutive reactions of
ethane and ethylene, must be clarlfied to gain a clear understanding of our results.
G. Centi and F. Trifiro' (Editors), New Developments in Selective Oxidation
0 1990 Elsevier Science PublishersB.V.. Amsterdam - Printed in The Netherlands
OXIDATIVE COUPLING OF METHANE OVER LnLi02 CATALYSTS.
(Ln = Sm,
365
Nd,
La).
PROMOTING EFFECT OF MgO AND CaO.
A. KIENNEMANN, R. KIEFFER, A. KADDOURI
Laboratoire Chimie Organique Appliquee U.A. CNRS 469
E.H.I.C.S.
1, Rue B l a i s e Pascal 67008 Strasbourg (France)
P. P O I X , J.L. REHSPRINGER
Groupe de Chimie des Materiaux Inorganiques I.P.C.M.S.
E.H.I.C.S.
1 , Rue B l a i s e Pascal 67008 Strasbourg (France)
SUMMARY
The c a t a l y t i c a c t i v i t y i n methane coupling o x i d a t i o n on d e f i n i t e LnLiO
(Ln = Sm, Nd, La) compounds where the a l k a l i i s enclosed i n t h e c r y s & l
l a t t i c e i s reported. LnLiO s t r u c t u r e s have the advantage t o be a host
s t r u c t u r e f o r promoting gations (Mg, Ca). The doped systems provide
s i g n i f i c a n t increases i n a c t i v i t y and s e l e c t i v i t y towards C2 hydrocarbons.
INTRODUCTION
The proved world reserve o f methane i s widely superior t o t h a t o f higher
hydrocarbons. Furthermore, the d i f f e r e n t methane production f i e l d s are o f t e n
located wide away from i t s use spot ( r e f . 1 ) . That's why many c o u n t r i e s have
undertaken studies on t h e v a l o r i z a t i o n o f n a t u r a l gas. Today, t h e most common
transformation way (except t h e manufacturing o f halogenated d e r i v a t i v e s , o f
CS2
or of
acetylene)
passes through
synthesis
gas
(CO,
C02
, H2) as
intermediate. An important energy l o s s takes place upon conversion o f n a t u r a l
gas t o a CO, C02, H2 mixture. Therefore, any d i r e c t transformation o f methane
i s i n t e r e s t i n g . I n heterogeneous c a t a l y s i s t h e d i r e c t production o f aromatics,
o f methanol and t h e o x i d a t i v e coupling o f methane t o ethylene and ethane are
worth mentioning ( r e f .
2-51.
The f i r s t
catalysts,
i n methane o x i d a t i v e
coupling o f methane worked i n an sequential way (e.g. w i t h a l t e r n a t e a d d i t i o n
o f N 0 and methane t o t h e gas f l o w ) , b u t now t h e simultaneous a d d i t i o n o f t h e
2
o x i d a t i n g agent and o f methane i s possible.
The c a t a l y s t s working on a
sequential mode are r a t h e r e a s i l y r e d u c i b l e m e t a l l i c oxides (e.9.
Sb
...
Pb, Mn, B i ,
) whereas t h e c a t a l y s t s operating on a simultaneous mode are non - o r
hardly r e d u c i b l e oxides l i k e MgO, CaO o r a l k a l i ( L i , Na, K) doped r a r e e a r t h
oxides
.
The operating conditions o f these c a t a l y s t s (temperature,
constituents
p a r t i a l pressure, CH4/02 r a t i o s ) vary i n a l a r g e range. The v o l a t i l i z a t i o n o f
366
e i t h e r t h e a c t i v e m e t a l (Pb) o r o f t h e promoter ( L i ) i s t h e main cause o f
c a t a l y s t d e a c t i v a t i o n which i s v e r y o f t e n r e p o r t e d i n t h e l i t e r a t u r e .
Thus,
l i t h i u m when d e p o s i t e d on t h e c a t a l y t i c s u r f a c e m i g r a t e s and r e a c t s w i t h t h e
quartz reactor i n the reaction conditions.
Magnesium, samarium o r lanthanum o x i d e c a t a l y s t s doped by a l k a l i s ( L i , Na) o r
a l k a l i n e e a r t h (BaO, CaO, SrO) o x i d e s ( r e f s 6 - 8 ) a r e mentioned t o l e a d t o t h e
h i g h e s t C 2 hydrocarbons y i e l d s . The amount o f l i t h i u m added t o s u r f a c e v a r i e s
s t r o n g l y depending on t h e a u t h o r s : ( f r o m a few % up t o 25% o r even more)
( r e f s 7,111.
Our c a t a l y t i c systems a r e based on r a r e e a r t h o x i d e s and a l k a l i s . They were
p r e p a r e d w i t h t h e f o l l o w i n g aims :
- good d i s t r i b u t i o n and c o n t r o l l e d l o c a l i z a t i o n o f a l k a l i atoms
- r a i s e d amount o f l i t h i u m i n t h e c a t a l y t i c f o r m u l a t i o n ( 5 0 mole %)
-
decreasing o f the l i t h i u m l o s s during t h e reaction.
I n o r d e r t o achieve t h i s goal d e f i n i t e compound o f g e n e r a l f o r m u l a
L n L i 0 2 (Ln
Sm,Nd,La)
were p r e p a r e d by r e a c t i o n o f t h e r a r e e a r t h o x i d e w i t h
t h e a1 a l i s i n s t e a d s i m p l y d e p o s i t i n g t h e l a t t e r on t h e r a r e e a r t h o x i d e s .
T a k i n g i n t o account t h e i o n i c r a d i u s o f Ln3'
partia
i n a s i x f o l d coordination, the
If the
s u b s t i t u t i o n o f Ln and L i by s u i t a b l e c a t i o n s i s p o s s i b l e .
o x i d e o f t h e s e c a t i o n s i s a c t i v e i n o x i d a t i v e c o u p l i n g o f methane,
one can
e x p e c t t h e o b t e n t i o n o f a c t i v e , s e l e c t i v e and l o n g l i v e d c a t a l y s t s . D e f i n i t e
compounds
were
prepared
(LnLi02)(l-x)(Mg0,CaO)
by
this
method.
They
correspond
to
o r (LnLiO
(Mg0,SrO)
compounds i n w h i c h
xt
2
( 1S-r3 k f o r Ln 3 t x (0.1 5 x g 0.33).
Mg2' s u b s t i t u t e s f o r L i and Ca2' o r
EXPERIMENTAL PART
A c t i v i t y and s e l e c t i v i t y o f t h e d i f f e r e n t samples were determined i n a
f i x e d bed q u a r t z r e a c t o r (6.6 mm I D ) i n t h e f o l l o w i n g c o n d i t i o n s
t e m p e r a t u r e : 600-750°C;
atm.
f e e d gas p a r t i a l p r e s s u r e s : 0.133 atm. CH4,
O2 and 0.8 atm. He; gas f l o w : 4.5
1.h-'
w e i g h t : 0.67 g; CH4/02 r a t i o : 2 (2CH4 t O2 --)C2H4
g.cat-l
t
(N.T.P.);
: unlet
0.0665
catalyst
2H20).
Methane c o n v e r s i o n i s expressed as : moles of transformed CH4 X 100/moles o f
i n t r o d u c e d CH4.
S e l e c t i v i t y i n p r o d u c t ( i ) i s d e f i n e d as
: moles o f CH4 t r a n s f o r m e d i n t o
p r o d u c t ( i ) X 1 0 0 h o l e s of t r a n s f o r m e d CH4.
Y i e l d i n p r o d u c t ( i ) i s g i v e n as c o n v e r s i o n X s e l e c t i v i t y X 100.
The c a t a l y s t s were prepared f r o m aqueous s o l u t i o n s o f t h e g i v e n r a r e e a r t h
n i t r a t e and t h e l i t h i u m carbonate o r h y d r o x i d e . The s o l i d s a r e o b t a i n e d by
e v a p o r a t i o n t o dryness a t 100-120°C o f t h e s o l u t i o n o r o f t h e suspension i n
367
which t h e r a r e e a r t h has been p r e c i p i t a t e d as oxalate by o x a l i c a c i d (pH =
2.2). These s o l i d s were then c a l c i n e d a t 750°C during 24 hours. The f o l l o w i n g
d e f i n i t e LnLi02 compounds characterized by then XRO mesh parameters ( r e f . 12)
were obtained by t h i s method : SmLi02, NdLi02 and LaLi02. Compounds having a
(LnLi02)1-x (MgO,CaO),
o r (LnLi02)l-x
(MgO,SrO),
s t r u c t u r e were prepared as
follows :
i ) an ethanol s o l u t i o n o f lanthanide (Sm, Nd o r La), magnesium, calcium o r
strontium n i t r a t e s was p r e c i p i t a t e d by o x a l i c a c i d (pH = 2.2).
The s o l i d s were
obtained by evaporation o f the suspension t o dryness a t 110-120°C and then
heated between 550 and 650°C under argon during 24 hours t o decompose t h e
oxal ates.
i i ) t h e obtained s o l i d was then suspended
i n an e t h a n o l i c s o l u t i o n containing
l i t h i u m hydroxide o r carbonate. The mixture was s t i r r e d during one hour and
t h e solvent was eliminated by evaporation a t 110-120°C.
The s o l i d residue was
heated under argon (24 h.) a t 850°C. The obtained c a t a l y s t was o n l y taken o u t
o f the furnace a f t e r c o o l i n g t o room temperature under argon.
RESULTS
Although the obtained LnLi02 compounds have a d e f i n i t e s t r u c t u r e ,
precursors
used
in
the
preparation
play
an
important
role.
Thus
the
the
s e l e c t i v i t y r e s u l t s are markedly a l t e r e d when t h e s t a r t i n g r a r e e a r t h compound
changes from hydroxide t o oxalate o r n i t r a t e and l i t h i u m hydroxide t o l i t h i u m
carbonate.
The r e s u l t s obtained f o r NdLi02 a r e given i n t a b l e I .
TABLE 1
Precursors e f f e c t s on NdLi02 a c t i v i t y .
NdL102
A
B
C
Conversion
'2
CH4
26.3
30.4
31.2
51.6
56.1
46.7
C2H4
5.9
14.0
9.6
Sel e c t iv i ty
C2
C2H6
8.3
24.0
17.4
14.2
38.0
27.1
COP
CO
81.6
60.0
70.9
4.2
2.0
2.0
ratio
C 2 sat./
C 2 unsat.
Yield
1.4
1.7
1.8
C2
3.7
11.6
8.5
A : neodynium oxalate and l i t h i u m carbonate
B : neodymium oxalate and l i t h i u m hydroxide
C : neodymium n i t r a t e and l i t h i u m carbonate
(T = 700"C, r a t i o CH4/02 = 2, gas f l o w 4.5 1.h-' g - l c a t a l y s t ; weight
c a t a l y s t : 0.7 g, P = 1 atm : 0.133 atm CH4; 0.0665 atm. 02; 0.8 atm He).
As f o r SmLi02 ( r e f . 13) one can n o t i c e t h a t t h e CH4 conversion remains more o r
l e s s constant w i t h a s l i g h t increase f o r t h e preparation based on neodymium
368
n i t r a t e and l i t h i u m carbonate. The s e l e c t i v i t y i n t o C2 hydrocarbons i s most
favoured
for
the
catalyst
obtained
from
neodymium
oxalate
and
lithium
h y d r o x i d e ( B ) . I n a p r e v i o u s work ( r e f . 13) an a t t e m p t of e x p l a n a t i o n based on
samari um o x a l a t e and n i t r a t e c a l c i n a t i o n t e m p e r a t u r e and on compared b a s i c i t y
o f LiC03 and LiOH was g i v e n . No d i f f e r e n c e f o r t h e t h r e e p r e p a r a t i o n s (A,B,C)
i s apparent i n t h e XRD s p e c t r a o f t h e NdLi02 samples. The BET s p e c i f i c area o f
5.75 and 4.00 m2 / g f o r A,B
t h e c a t a l y s t s a f t e r c a l c i n a t i o n a t 750°C a r e : 0.6,
and C
preparations
respectively.
The
surface
independant
CH4
conversion
suggests t h a t o t h e r s t r u c t u r a l o r homogeneity f a c t o r s may p l a y an i m p o r t a n t
role.
I n s e r t i o n o f o t h e r oxides i n t o LnLi02 structures.
The s u b s t i t u t i o n o f samarium and l i t h i u m atoms f r o m a L n L i 0 2 s t r u c t u r e can
be achieved by c a t i o n s
h a v i n g c l o s e metal-oxygen
number : s i x , m o n o c l i n i c s t r u c t u r e ) .
distances
(coordination
Our c h o i c e went t o Mg2+ and Ca2+ f o r
which t h e l i t e r a t u r e r e p o r t s e x c e l l e n t p r o p e r t i e s i n o x i d a t i v e c o u p l i n g o f
methane. F o r e l e c t r i c b a l a n c e reasons samarium and 1 i t h i u m s u b s t i t u t i o n must
t a k e p l a c e s i m u l t a n e o u s l y . However t h e f o l l o w i n g s u b s t i t u t i o n schemes :
Sm3+
must,
a priori,
+
Lit
4
2Ca2+
or
Sm3+ + L i + -
2Mg2+
be d i s c u s s e d because o f t h e i n c o m p a t i b i l i t y o f dimensional
f a c t o r s between c a l c i u m and l i t h i u m , magnesium and samarium. The metal-oxygen
d i s t a n c e s computed by t h e i n v a r i a n t method ( r e f .
2.405A (Ca");
(
2.135A
F i g . 1 : X.R.D.
Ptheta Y : 1596. Linear
d a t a o f SmLi02 and s u b s t i t u e d SmLi02
a : SmLiOp
are
respectively
( L i + ) ; 2.473A (Sm3+) and 2.106A (Mg").
x :
1e.m
14-16)
b : SmLi02 l-x(MgO,CaO)x
199.899)
.
369
F i g . 2 : X.R.O.
.
d a t a o f L a L i 0 2 and s u b s t i t u e d L a L i 0 2
c : LaLi02~1-x)(Mg0-Sr0)x
d : LaLi02(l-x)n(Mg0-Ca0)x
Thus dimensional f a c t o r s a r e c o n s i s t e n t between Ca2' and Sm3'
L i t . a n d Mg2',
Sm3'.
b u t t h e d i f f e r e n c e i s t o o l a r g e between Ca2'
Therefore
the
substitution
by
MgO
and
CaO
and between
and L i t o r Mg2' and
must
be
undertaken
s i m u l t a n e o u s l y . The s u b s t i t u t i o n b y Mg2' and Ca2' t a k e n i n equal amounts f i t s
f a i r l y w e l l s i n c e t h e c o m p a t i b i l i t y i s r e a l i z e d between Sm3'(2.473A)
Ca2'(2.405A)
and between Lit(2.135A)
(SmLi021,-x(Mg0,Ca0)x
and Mg2'(2.106A).
and
D e f i n i t e systems o f
c o m p o s i t i o n can be o b t a i n e d as c o n f i r m e d b y XRD a n a l y s i s
( F i g . 1 and 21.
Table 2 summarizes t h e r e s u l t s o b t a i n e d a t 700°C a f t e r s u b s t i t u t i o n by MgO and
CaO, x b e i n g equal t o 0.33.
An i n c r e a s e d C 2 hydrocarbon ( 60%) s e l e c t i v i t y as w e l l as a changed
C 2 s a t u r a t e d / C 2 u n s a t u r a t e d hydrocarbon r a t i o when compared t o L n L i 0 2 o r
Ln203 a r e observed. Conversion i s s l i g h t l y l o w e r . When MgO i s used a l o n e i n
the substitution,
t h e XRD s p e c t r a shows t h e presence o f f r e e MgO and t h e
c a t a l y t i c system works as i f MgO was d e p o s i t e d on t h e SmLi02 s u r f a c e ( h i g h e r
a c t i v i t y but s i m i l a r s e l e c t i v i t y ) .
The v a l u e o f x can v a r y i n a l a r g e range.
F o r 0.1
Q
x <
0.33,
fig,
3
r e p r e s e n t s t h e e v o l u t i o n of s e l e c t i v i t y and c o n v e r s i o n . F o r SmLi02 an enhanced
s e l e c t i v i t y i s reached f o r a x v a l u e as l o w as 0.1.
A decreased c o n v e r s i o n
appears t o o f o r a low s u b s t i t u t i o n by MgO and CaO. A d d i t i o n a l amounts o f MgO
and CaO seem t o i n f l u e n c e n e i t h e r s e l e c t i v i t y n o r a c t i v i t y f u r t h e r .
370
Table 2 : A c t i v i t y and s e l e c t i v i t y of pure Sm and Nd oxides, SmLi02 and NdLi02
and s u b s t i t u e d by Mg and Ca o f SmLi02 and NdLi02.
catalysts
S e l e c t iv i t y
Conversion
CH4
2'
'ZH4
'2"6
C02
C2
CO
Ratio
Yield
C2 sat./
C2
C2 unsat.
Sm203
25.5
45.7
14.7
10.4
25.1
67.9
7.0
0.7
6.4
Nd203
SmLi02
28.6
65.7
14.5
16.5
31.0
63.5
5.5
1.1
8.9
31.9
50.3
4.6
24.1
28.7
65.5
5.8
5.2
9.2
NdLi O2
30.4
56.1
14.0
24.0
38.0
60.0
2.0
1.7
11.6
SmLi02(1-x) 26.5
*
51.3
30.8
28.2
59.0
38.6
2.4
0.9
18.2
NdLi02 ( 1 - ~ 1 2 1 . 9
47.7
28.4
21.8
50.2
46.8
2.9
0.8
11.0
xMgO-CaO
SmLi02 (1-x135.0
63.1
26.2
15.8
41.9
54.8
3.3
0.6
14.7
xMgO-CaO
*
xMgO
*
*
x = 0.33.
Same c o n d i t i o n s as i n Table 1.
The e v o l u t i o n i s s l i g h t l y d i f f e r e n t f o r (NdLi0211-x (MgO,CaO),.
The f a c t t h a t
t h e Ca2+ s u b s t i t u t i o n f o r Nd3+ i s l e s s f a v o u r a b l e (Nd3+ = 2.513A;
Ca2+ =
2.405A) than f o r Sm3+ must be u n d e r l i n e d here. The s u b s t i t u t i o n i n LaLi02 i s
even
less
likely
haphazardous.
(La3+
=
2.596A)
and
the
obtained
results
are
more
Except i n one case, t h e s e l e c t i v i t y i s c l o s e t o t h a t obtained
w i t h LaLi02 alone b u t t h e conversion i s increased.
T h a t ' s why f o r lanthanum oxide, t h e chosen a l k a l i n e e a r t h i s s t r o n t i u m ( S r 2 + =
2.580A;
La3+ = 2.596A.
Results f o r x = 0.33
r e a c t i v i t y o f (LaLi02)1-x(Mg0, SrO),
a r e given i n t a b l e 3.
The
versus x i s g i v e n i n F i g . 3. Here too,
t h e a d d i t i o n o f even weak amounts o f MgO and S r O i s s u f f i c i e n t t o i n c r e a s e
s i gn if icant l y t h e s e l e c t i v i t y
.
CONCLUSION
The present work shows t h e p o s s i b i l i t y t o work w i t h compounds o f d e f i n i t e
s t r u c t u r e i n t h e o x i d a t i v e c o u p l i n g o f methane i n s t e a d w i t h c a t a l y s t s obtained
by impregnation.
S t r u c t u r e s such as LnLi02 can be used alone o r as h o s t
s t r u c t u r e f o r o t h e r c a t i o n s (Mg,Ca,Sr
... 1
which a r e a c t i v e i n t h e o x i d a t i v e
c o u p l i n g . I n t r o d u c t i o n o f MgO and CaO i n t o t h e c r y s t a l frame o f SmLi02
increases t h e s e l e c t i v i t y up t o 60% i n C2 hydrocarbons compare t o 25% and
371
t
4
1
NdLiOl
9
A
C
D
F i g . 3 : E v o l u t i o n o f C2 s e l e c t i v i t y w i t h c a t i o n substitution.(Mg and Ca)
1 : SmLi02
2 : LaLi02 3 : NdLi02 4 : LaLi02 : s u b s t i t u t i o n by Mg and S r
A : x = 0.10 ; B : x = 0.16 ; C : x = 0.22 ; 0 : x = 0.33
Table 3 : A c t i v i t y and s e l e c t i v i t y o f La203, LaLi02 and s u b s t i t u e d by Mg and
Ca, Mg and S r o f LaLi02 ( x = 0.33). Same c o n d i t i o n s as i n Table 1
Catalyst
Conversion
CHI
02
C2H4
Selectivity
C2H6
C2
C02
CO
Ratio
C2 sat./
Yield
C2
C 2 unsat.
La203
LaLi02
LaLi02(1-x)
xCaO-MgO
26.9
46.9
12.3
11.2
23.5
61.2
15.3
0.9
6.2
17.7
44.8
39.0
84.3
13.1
23.0
29.8
16.8
42.9
39.8
56.0
48.5
1.1
11.7
2.3
0.7
7.6
17.8
LaLi02(1-x)
xSrO-MgO
14.9
51.9
18.6
37.1
55.7
40.3
3.9
2.0
8.3
372
29% f o r Sm203 and SmLi02 r e s p e c t i v e l y . Methane c o n v e r s i o n , a l t h o u g h s l i g h t l y
d i m i n i s h e d remains h i g h e r t h a n 25%.
LITERATURE
1 H. Mimoun, New J o u r n a l Chem. 11 (1987) 513-525
2 "Kirk-Othmer Encyclopedia o f Chemical Technology" Wiley, New-York Vol. 1,
p. 193 (2nd E d i t i o n )
3 B r i t i s h Petroleum European P a t e n t 93 543 (1983)
4 M. I t o and J.H. Lundsford, N a t u r e 314 (1985) 721-722
5 W. Hinsen, W . B y t y n and M. Baerns, Proc. 8 t h I n t . Congr. C a t a l . , B e r l i n ,
2-6 J u l y , 1984, S p r i n g e r V e r l a g , 1984, V o l . 111, pp. 581-592
6 T. I t o , J.X. Wang, C.H. L i n and J.H. Lundsford, J. Am. Chem. SOC. 107
(1985) 5062-5068
7 K. Otsuka, Q. L i u , M. Hatano and A. Morikawa, Chem. L e t t . (1986) 467-468
8 T. Moriyama, N. Takasaki, E. Iwamatsu and K. Aika, Chem. L e t t . (1986)
1165-1 168
9 N. Yamagata, K. Tanaka, S. Sasaki and S. Okazoki, Chem. L e t t . (1987) 81-82
10 J.M. De Boy and R.F. H i c k s , I n d . Eng. Chem. Res. 27 (1988) 1577-1582
11 S.J. K o r f , J.A. Ross, N.A. de B r u i j n , J.G. Van Ommen and J.R.H. Ross, Chem.
Comm. (19871, 1433-1434
12 M. Gondrand, B u l l . SOC. F r . M i n e r a l . C r i s t a l l o g . (1967) XC 107-108
13 A. Kaddouri, R. K i e f f e r , A. Kiennemann, P. POIX and J.L. Rehspringer, Appl.
C a t a l . 51 L l - L 6 (1989)
physiques des composes
14 P. Poix, " L i a i s o n I n t e r a t o m i q u e e t p r o p r i e t e s
mineraux " 1 . SEDES (1966) 82-120
15 P. Poix, C.R. Acad. S c i . P a r i s C 270 (1970) 1852-1853
16 P. Poix, C.R. Acad. S c i . P a r i s C 268 (1969) 1139-1140
G. Centi and F. Trifiro’ (Editors), New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
373
BEHAVIOR OF METALLIC OXIDES SUPORTED ON Li/MgO IN THE METHANE OLIGOMERIZATION
G.T. BARONETTI, E.J. LAZZARI, A.A. CASTRO and O.A. SCELZA
Instituto de Investigaciones en Catll i s i s y Petroqufmica -1NCAPESantiago del Estero 2654 - 3000 Santa Fe, Argentina
SUMMARY
The e f f e c t o f L i addition t o MgO on the performance in the methane oligomerization was determined. Besides, Li/MgO doped w i t h different lanthanide oxides
(Pr, La, Ce and Sm oxides) were also studied i n the same reaction. I t was found
a beneficial e f f e c t of ti addition t o MgO. Moreover, the addition of Pr o r Ce
oxide enhances the performance of Li/MgO catalyst.
INTRODUCTION
Methane selective oxidation i n t o C2 hydrocarbons i s a very interesting
process since t h a t more valuable chemicals could be obtained from natural gas.
However, methane conversion i n t o higher hydrocarbons presents a thermodynamic
barrier. In f a c t , homogeneous oligomerization in gas phase i s only feasible a t
temperature higher than 1000°C b u t w i t h low C2 yield ( r e f . 1 ) . This thermodynamic
barrier could be eliminated by u s i n g a substance able t o provide the necessary
oxygen t o react w i t h the hydrogen producing water. T h u s , reducible metal oxides
could be used as oxygen source, as Keller and Bhasin showed in t h e i r pioneer
paper ( r e f . 2 ) . Since then, a great e f f o r t has been made i n order t o find active
and selective catalysts t o produce higher hydrocarbons from methane ( r e f s . 3-6).
Several recent papers reported a good performance o f basic solids promoted
with alkaline-metal ions ( r e f s . 7-9). T h u s , i t has been reported t h a t the
a c t i v i t y of MgO i s due t o i t s capacity t o produce methyl radicals, since that
t h i s material posses i n t r i n s i c cationic vacancies which can react with O2 to
produce 0- centers (active centers for the methane activation) ( r e f s . 10, 11).
Alkaline-metal addition t o MgO enhances the capacity t o abstract He from methane
producing CH3- which could dimerize i n gas phase. Other catalysts based on MgO
have been reported i n the l i t e r a t u r e ( r e f s . 12-14).
The influence of the Li content in Li/MgO catalysts on the a c t i v i t y and
s e l e c t i v i t y in methane oligomerization i s studied in t h i s paper. Likewise, the
behavior of different lanthanide oxide/Li/MgO catalysts (Pr, Sm, Ce or La oxide)
i s reported.
Besides, in order t o elucidate the e f f e c t of Pr and Sm addition t o Li/MgO, a
more detailed study on these catalysts was carried o u t .
374
EXPERIMENTAL
MgO-based c a t a l y s t s (MgO, Li/MgO, 1a n t h a n i d e oxide/MgO,
lanthanide oxide/Li/
MgO) were o b t a i n e d by u s i n g t h e s l u r r y method proposed by I t o e t a l . ( r e f . 1 5 ) .
Magnesium o x i d e p.a.
f r o m k r c k was used f o r t h e c a t a l y s t s p r e p a r a t i o n .
Li/MgO c a t a l y s t s w i t h d i f f e r e n t L i c o n t e n t (0.5,
1, 4, 7 and 15wt%) were
o b t a i n e d by u s i n g Li2C03 as l i t h i u m p r e c u r s o r .
PrsOll/Li(lwt%)/MgO
c a t a l y s t s w i t h d i f f e r e n t P r l o a d i n g (2, 5 and 10 mol
l a n t h a n i d e p e r 100 mol L i ) were p r e p a r e d b y simultaneous a d d i t i o n o f Pr(N03)3.
6H20 and Li2C03 t o t h e s l u r r y which c o n t a i n e d MgO and water. Besides, L i ( l w t % ) /
MgO doped w i t h Ce, La and Sm o x i d e s were prepared i n t h e same way by u s i n g
n i t r a t e s as l a n t h a n i d e p r e c u r s o r s . I n t h e s e c a t a l y s t s t h e l a n t h a n i d e c o n t e n t was
2 mol l a n t h a n i d e p e r 100 mol L i .
A f t e r o b t a i n i n g t h e MgO s l u r r y w i t h t h e d i f f e r e n t components, t h e r e s i d u a l
w a t e r was evaporated, t h e n t h e c a t a l y s t s were d r i e d a t 120°C and f i n a l l y c a l c i n e d
i n a f l o w i n g a i r s t r e a m a t 750°C d u r i n g 5 h.
Several i n t i m a t e mechanical m i x t u r e s (MM) between b u l k samarium o r praseodymium o x i d e and MgO o r L i ( l w t % ) / M g O were a l s o t e s t e d i n t h e methane o l i g o m e r i z a t i o n . For t h e s e cases t h e c a t a l y t i c bed c o n t a i n e d t h e same l a n t h a n i d e o x i d e
amount as c a t a l y s t s prepared by t h e s l u r r y method. One sample o f b u l k samarium
o x i d e was impregnated w i t h Li2C03 and c a l c i n e d i n a i r a t 750°C.
Praseodymium and samarium b u l k o x i d e s were prepared by decomposition o f t h e
c o r r e s p o n d i n g n i t r a t e a t 750°C.
Methane o l i g o r n e r i z a t i o n r e a c t i o n was c a r r i e d o u t a t atmospheric p r e s s u r e i n a
f i x e d bed q u a r t z r e a c t o r by u s i n g a CH4-air m i x t u r e a s a feed. The r e s i d e n c e t i m e
was 3.35 g c a t , s/ml CH4 STP, t h e CH4/02 m o l a r r a t i o i n t h e f e e d was e i t h e r 2 o r
5 and t h e r e a c t i o n temperature ranged between 650 and 800'C.
were a n a l y z e d b y G.C.
Reaction products
The s e l e c t i v i t y t o C2 was d e f i n e d as t h e percentage o f t h e
methane c o n v e r t e d i n t o ethane and e t h y l e n e .
RESULTS AND DISCUSSION
Table 1 shows t h e values o f t h e CH4 c o n v e r s i o n ( X ) and s e l e c t i v i t i e s t o C2
(Sc2) and C02 (Sco2) a t 750°C f o r MgO promoted w i t h d i f f e r e n t l i t h i u m l o a d i n g s
and f o r d i f f e r e n t CH4/02 m o l a r r a t i o s i n t h e feed. I t can be observed t h a t
unpromoted MgO i s h i g h l y s e l e c t i v e t o deep o x i d a t i o n p r o d u c t s , b u t t h e a d d i t i o n
o f a s m a l l q u a n t i t y o f L i (0.5wt%) enhances t h e CH4 c o n v e r s i o n and produces a
d r a s t i c change i n t h e s e l e c t i v i t y , i n c r e a s i n g t h e C2 hydrocarbons f o r m a t i o n . The
e f f e c t o f t h e l i t h i u m a d d i t i o n on t h e a c t i v i t y and s e l e c t i v i t y i s more n o t i c e a b l e
when h i g h e r CH4/02 i n t h e f e e d i s used. Besides, Table 1 shows t h a t b o t h t h e
s e l e c t i v i t y t o C2 and methane c o n v e r s i o n p r e s e n t a broad maximum between 0.5 and
4wt% L i .
The r o l e o f t h e l i t h i u m i n MgO doped c a t a l y s t has been d i s c u s s e d by L u n s f o r d
375
TABLE 1
Values o f methane conversion ( X ) , s e l e c t i v i t y t o ethane + ethylene
(Sc-)
and
s e l e c t i v i t y t o C02 (Sco2) f o r MgO c a t a l y s t s doped w i t h d i f f e r e n t contents o f
l i t h i u m . T = 750"C, CH4/02 molar r a t i o ( R ) = 2 and 5
Catal v s t
MgO
0.5wt% LiIMgO
l . O w t % Li/MgO
4.0wt% Li/MgO
7.0wt% Li/MgO
15.0wt% LiIMgO
R = 5
R = 2
26.8
32.1
32.9
22.7
30.9
24.8
7.8
30.9
29.9
38.3
23.2
33.7
92.2
69.1
70.1
61.7
76.8
66.3
10.2
18.9
18.5
16.0
9.5
12.1
6.1
54.8
56.3
59.4
56.8
48.8
93.9
45.2
43.7
40.6
43.2
51.2
e t a l . ( r e f . 15), who show a r e l a t i o n s h i p between the production o f CH3- r a d i c a l s
and the d e n s i t y o f paramagnetic 0- species on the Li/MgO surface. I t i s accepted
t h a t the a c t i v e s i t e s o f the
Ha
a b s t r a c t i o n from CH4 would be (Li'O-)
species. These a c t i v e s i t e s would be produced from a Mg"
surface
s u b s t i t u t i o n by a
l i t h i u m c a t i o n . Moreover, i t has been reported t h a t t h e a d d i t i o n o f higher
amounts o f a l k a l i n e metals t o MgO leads t o a great diminution o f t h e surface
area ( r e f . 16).
Considering the f o l l o w i n g r e a c t i o n scheme ( r e f . 17):
2
2
C2 hydrocarbons
the steps 1, 2, 3 and 5 take place on the c a t a l y s t surface meanwhile step 4
occurs i n gas phase. With a h i g h surface area o f the c a t a l y s t , steps 1, 2, 3 and
5 are favored. When a small q u a n t i t y o f l i t h i u m i s added t o MgO n o t o n l y the
a c t i v e s i t e s concentration increases b u t a l s o t h e surface area decreases. Hence,
the step 2 i s enhanced, meanwhile the steps which l e a d t o t h e hydrocarbon
o x i d a t i o n products are n e g a t i v e l y affected. For h i g h l i t h i u m content the a c t i v e
s i t e s (Li'O-) concentration i s d r a s t i c a l l y reduced ( r e f . 11) and the formation
o f the o x i d a t i o n products are favored i n such conditions. Hence a maximum value
o f the s e l e c t i v i t y t o C2 hydrocarbons must be expected f o r a given L i content
such as the r e s u l t s o f Table 1 d i s p l a y .
I t must be i n d i c a t e d t h a t a d r a s t i c drop i n the MgO surface area a f t e r
l i t h i u m a d d i t i o n was observed i n our experiments. I n f a c t , areas values o f 59.7,
2
3.4 and 1 m /g f o r 0, 4 and 15wt% Li/MgO were obtained.
The e f f e c t o f doping Li(lwt%)/MgO w i t h P r , Ce, Sm and La oxides ( 2 mol%
lanthanide w i t h respect t o the L i content) was a l s o analyzed. X , Sc2 and the
376
y i e l d t o C2
(Yc2
=
X S c 2 ) values f o r CH4/02 molar r a t i o ( R ) = 2 and d i f f e r e n t
r e a c t i o n temperatures a r e shown i n Table 2 . It can be seen t h a t t h e methane
conversion increases when t h e r e a c t i o n temperature increases, meanwhile Sc2 and
Yc2
Yc2
present maxima values a t 750°C f o r a l l t h e c a t a l y s t s . By comparing t h e
values a t 750°C,
i t can be concluded t h a t t h e a d d i t i o n o f P r and Ce oxides t o
Li/MgO leads t o a b e t t e r performance o f t h e c a t a l y s t , meanwhile t h e a d d i t i o n o f
Sm and La oxides appears t o have a n e g a t i v e e f f e c t .
The e f f e c t o f t h e d i f f e r e n t P r l o a d i n g on Li(lwt%)/MgO was a l s o s t u d i e d and
t h e r e s u l t s a r e shown i n Table 3. It can be observed a maximum i n t h e methane
conversion and i n t h e s e l e c t i v i t y t o C2 f o r a P r l o a d i n g o f 2% (mol Pr/mol L i ) .
In o r d e r t o e x p l a i n t h e b e n e f i c i a l e f f e c t o f t h e
Pr6011/Mg0,
Pr6Ol1/Li
P r a d d i t i o n t o Li(lwt%)/MgO,
(Iwt%)/MgO, and t h e i n t i m a t e mechanical m i x t u r e s (MM)
between praseodymium o x i d e and MgO o r L i (lwt%)/MgO were t e s t e d i n methane
o l i g o m e r i z a t i o n a t 750°C.
F i g u r e 1 shows Sc2 and
Yc2
values f o r t h e above
mentioned c a t a l y s t s , By comparing t h e r e s u l t s o f Pr6011/Mg0 w i t h those o f t h e MM
TABLE 2
Values o f methane conversion ( X ) ,
to
C2 hydrocarbons (Yc,)
s e l e c t i v i t y t o C2 hydrocarbons (Sc2) and y i e l d
a t d i f f e r e n t r e a c t i o n temperatures f o r L i (lwt%)/MgO
c a t a l y s t s doped w i t h P r , Ce, Sm and La oxides ( 2 mol% l a n t h a n i d e respect
t o the
L i c o n t e n t ) and f o r R = 2
Temperature, " C
650
750
800
Catalyst
x, t
Sc23 %
Yc2,
Pr601 1/ L i /MgO
8.4
14.2
1.2
Ce02/Li /MgO
6.9
13.2
0.9
Sm203/Li/Mg0
6.7
12.7
0.8
La203/Li/Mg0
6.5
26.9
1.7
Li/MgO
8.0
18.9
1.5
Pr6011/Li/Mg0
34.8
33.7
11.7
Ce02/L i/ MgO
36.0
35.6
12.8
Sm203/Li /MgO
29.8
22.7
6.8
La203/Li /MgO
31.4
27.4
8.6
Li/MgO
32.9
29.9
9.8
Pr6011/Li /MgO
35.7
18.4
6.5
Ce02/Li/Mg0
36.8
13.2
4.8
Sm203/Li /MgO
35.0
15.3
5.3
La203/Li /MgO
35.0
20.4
7.1
L i/MgD
33.2
16.6
5.5
%
R =5
60
40
-
20 -
-
c
20
c
0
0
-n 1
0
r"+
c
12
YC2,%
0'
2
a
r
r
12
1:
0
r"
\
c
6
2
a
8
4
0
Fig. 1. S e l e c t i v i t y t o C2 and y i e l d t o Cp a t 750°C f o r d i f f e r e n t c a t a l y s t s .
MM: mechanical mixture. The l i t h i u m content i n Pr6011/Li/MgO was l w t % . P r
loading i n Pr6011/MgO was the same t h a t i n P r Oli/Li/MgO ( 2 mol Pr/100 mol L i ) .
The p r o p o r t i o n o f t h e components i n mechanica7 mixtures was the same t h a t i n
c a t a l y s t s prepared by the s l u r r y method
378
TABLE 3
Performance o f Pr6011/Li (lwt%)/MgO c a t a l y s t s w i t h d i f f e r e n t P r l o a d i n g i n t h e
methane o l i g o m e r i z a t i o n a t 750°C and d i f f e r e n t CH4/02 molar r a t i o s ( A )
Cata 1y s t
P r / L i molar r a t i o , %
R = 2
x,
%
R = 5
SC2’ %
x,
29.9
18.5
%
SC2’ %
L i /MgO
0
Pr6Ol1/Li/Mg0
2
34.8
33.7
19.6
64.3
Pr6Ol1/Li/Mg0
5
31.9
22.8
17.2
51.7
10
33.6
28.9
14.4
50.7
Pr6011/Li/Mg0
32.9
56.3
(Pr6011 + MgO), i t can be observed t h a t t h e f i r s t c a t a l y s t i s more s e l e c t i v e t o
C2 than t h e mechanical m i x t u r e . Hence, these r e s u l t s i n d i c a t e t h a t t h e c a t a l y s t s
o b t a i n e d by t h e s l u r r y method can n o t be considered as t h e sum o f t h e i s o l a t e d
components. There appears t o t a k e place a c e r t a i n i n t e r a c t i o n between t h e
l a n t h a n i d e o x i d e and MgO.
Considering t h e r e s u l t s o f t h e MM composed by Pr6011
+
L i (lwt%)/MgO and those
o f t h e Pr6011/Li(lwt%)/Mg0 c a t a l y s t ( F i g u r e l ) , i t must be noted again t h a t when
t h i s c a t a l y s t i s o b t a i n e d by t h e s l u r r y method t h e r e i s a c e r t a i n i n t e r a c t i o n
between t h e dopants and t h e support. I n f a c t , t h e c h a r a c t e r i s t i c praseodymium
o x i d e l i n e s were n o t found i n t h e XRD p a t t e r n s o f t h e Pr6011/Li(lwt%)/Mg0
c a t a l y s t . However, when t h e Pr6011/Li (lwt%)/MgO c a t a l y s t i s prepared by
impregnation o f L i (lwt%)/MgO w i t h Pr(N03)3.6H20
t h e XRD p a t t e r n s c l e a r l y show
t h e c h a r a c t e r i s t i c peaks o f Pr6011.
I t c o u l d be e x p l a i n e d t h e b e n e f i c i a l e f f e c t o f P r a d d i t i o n t o Li/MgO
c o n s i d e r i n g t h a t t h e praseodymium o x i d e c o u l d a c t as a charge c a r r i e r f o r t h e
r e g e n e r a t i o n o f t h e a c t i v e s i t e s o f l i t h i u m - d o p e d MgO c a t a l y s t s . S i m i l a r e f f e c t s
f o r a1 k a l i promoted-Pr6011
and Ce02/Li/Mg0 were r e p o r t e d i n t h e 1 it e r a t u r e ( r e f s .
18, 19).
P r e v i o u s l y , i t has been shown t h a t Li(lwt%)/MgO c a t a l y s t s promoted w i t h Sm
d i s p l a y s lower Sc2 and
Yc2
values than those o f Li(lwt%)/MgO c a t a l y s t s (Table 2 ) .
I n o r d e r t o study t h i s system, b u l k samarium o x i d e (Sm203), Sm203/Li( lwt%)/MgO,
and a mechanical m i x t u r e (MM) between b u l k Sm2O3 and Li(lwt%)/MgO were t e s t e d a t
750°C and R = 2 (Table 4 ) .
The r e s u l t s show t h a t t h e y i e l d t o C2 o f t h e c a t a l y s t prepared by simultaneous
d e p o s i t i o n o f Sm, L i . and MgO i s lower than t h a t o b t a i n e d f o r t h e i s o l a t e d
components and f o r t h e mechanical m i x t u r e . Hence, when Sm, L i and MgO a r e i n an
i n t i m a t e contact, an unfavorable e f f e c t would take place.
One a d d i t i o n a l experiment was c a r r i e d o u t by u s i n g b u l k samarium o x i d e doped
379
TABLE 4
X, Sc2 and Yc2 values a t 750°C and R = 2 f o r Sm c a t a l y s t s
Catalyst
Sm/Li molar
ratio, %
x,
%
SC2’ %
YC2’ %
Sm203
Sm2O3 + L i (lwt%)/MgO
(mechanical mixture)
-
12.8
61.2
7.8
2
27.3
27.4
7.5
Sm203/Li (lwt%)/MgO
2
29.8
22.7
6.8
32.9
29.9
9.8
L i (lwt%)/MgO
-
w i t h Li2C03 (Sm/Li molar r a t i o = 2%). I t showed Sc2 values 10% lower than t h a t
o f Sm203, meanwhile the methane conversion was s i m i l a r t o t h a t o f Sm203. These
r e s u l t s i n d i c a t e a negative e f f e c t o f L i on the performance o f b u l k samarium
oxide. K o r f e t a l . ( r e f . 20) reported t h a t the a d d i t i o n o f l i t h i u m t o Sm203
produces a s t r u c t u r a l m o d i f i c a t i o n from cubic t o monoclinic s t r u c t u r e w i t h a
simultaneous diminution i n the s e l e c t i v i t y t o C2. On the other hand, t h e r e s u l t s
reported by Otsuka e t a l . ( r e f . 21) were opposite t o those o f K o r f e t a l . ( r e f .
20) and t o our r e s u l t s . I n f a c t , Otsuka found t h a t t h e Li-doped Sm203 c a t a l y s t
was more a c t i v e and s e l e c t i v e f o r C2 hydrocarbons production than Sm2O3. However,
the negative e f f e c t o f samarium oxide a d d i t i o n t o Li/MgO can n o t be c l e a r l y
explained and much e f f o r t w i l l be needed i n order t o understand t h i s behavior.
REFERENCES
Ardiles, O.A. Scelza and A.A. Castro, Rev. Fac. Ing. Qufm. Santa Fe, 46
(1984) 7-16.
2 G.E. K e l l e r and M.M. Bhasin, J. Catal., 73 (1982) 9-19.
3 W. Hinsen, W. Bytyn and M. Baerns, Proc. 8th. I n t . Congr. Catal., 3 (1984)
581-92.
4 K. Otsuka, K. Jinno and A. Morikawa, Chem. L e t t . , (1985) 499-500.
5 J.A. Sofranko, J.J. Leonard and C.A. Jones, J. Catal., 103 (1987) 302-10.
6 K. Asami, 5. Hashimoto, K. Fujimoto and H. Tominaga, i n D.M. Bibby, C.D.
Chang, R.F. Howe and S. Yurchak (Eds.), Methane Conversion, (1988) 403-07.
7 F.P. Larkins and M.R. Nordin, i n D.M. Bibby, C.D. Chang, R.F. Howe and S.
Yurchak (Eds.), &thane Conversion, (1987) 127-31.
8 J.A. Roos, A.G. Bakker, H. Bosch, J.G. van Ommen and J.R.H. ROSS, C a t a l y s i s
Today, 1 (1987) 133-45.
9 Y.L. B i , K.J. Zhen, Y.T. Jiang, C.W. Teng and X.G. Yang, Appl. Catal., 39
(1-2) (1988) 185-90.
10 D.J. D r i s c o l l and J.H. Lunsford, J. Phys. Chem., 89 (1985) 4415-18.
11 0. J. D r i s c o l l , W. M a r t i r , J.-X. Wang and J.H. Lunsford, J. Am. Chem. SOC.,
107 (1985) 58-63.
12 I . T . A l i Emesh and Y. Amenomiya, J. Phys. Chem., 90 (1986) 4785-89.
13 E. Iwamatsu, T. Moriyama, N. Takasaki and K. Aika, J. Chem. SOC. Chem. Comm.,
(1987) 19-20.
14 E.J. Ereckson and A.L. Lee, Preprints, Div. P e t r o l . Chem., ACS, 33 (3) (1988)
443-44.
15 T. I t o , J.-X. Wang, C. L i u and J.H. Lunsford, J. Am. Chem. SOC., 107 (1985)
1 D.R.
380
5 062 -68.
16 T . Moriyama, N. Takasaki, E. Iwamatsu and K. Aika, Chem. L e t t . , (1986) 116568.
17 E. Iwamatsu, T. Moriyama, N. Takasaki and K. Aika, J. Catal., 113 (1988) 2535.
18 A.M. Gaffney, C.A. Jones, J.J. Leonard and J.A. Sofranko, P r e p r i n t s , Div.
P e t r o l . Chem., ACS, 33 ( 3 ) (1988) 445-52.
19 S. Bartsch, J. Falkowski and H. Hofmann, C a t a l y s i s Today, 4 (1989) 421-31.
20 S.J. Korf, J.A. Roos, J.M. Diphoorn, R.H.J. Veehof, J.G. van Ommen and J.R.H.
Ross, European Workshop on Methane Conversion, May 1988, FRG.
21 K. Otsuka, Q. L i u and A. Morikawa, 3. Chem. SOC. Commun., (1986) 586-7.
J.M. VAN KASTEREN ( U n i v e r s i t y o f Technology, The Netherlands): Did you observe
any d e a c t i v a t i o n o r l i t h i u m l o s s d u r i n g y o u r experiment? Did you measure y o u r
L i c o n t e n t a f t e r c a l c i n a t i o n and a f t e r r e a c t i o n ?
O.A. SCELZA (INCAPE, Argentina): It was n o t observed d e a c t i v a t i o n d u r i n g t h e
r e a c t i o n . The l i t h i u m contents on t h e c a t a l y s t s a f t e r r e a c t i o n were t h e same
as
those o f t h e f r e s h c a t a l y s t s ( c a l c i n e d c a t a l y s t s ) .
J. K I W I (EPFL Chimie Physique, Switzerland): You use Pr6O11 on MgO a t 750°C
when you prepare y o u r c a t a l y s t . I t i s p o s s i b l e t h a t P r MgO, (Li-doped)
e p i t a x i a l growth i s produced d u r i n g t h e p r e p a r a t i o n ofYthe c a t a l y s t o r d u r i n g
t h e r e a c t i o n a t t h i s r e l a t i v e h i g h temperature. Do you have any evidence f o r
this?
O.A. SCELZA (INCAPE,
and by impregnation
p a t t e r n s showed t h e
type. For t h e f i r s t
detected
.
A r g e n t i n a ) : Pr6O /Li/MgO prepared by t h e " s l u r r y " method
were examined by b;kD a f t e r t h e c a l c i n a t i o n step. The XRD
c h a r a c t e r i s t i c peak o f Pr6011 o n l y f o r t h e second c a t a l y s t
c a t a l y s t type, no P r compound ( i n c l u d i n g P r oxides) was
J.R.H. ROSS ( U n i v e r s i t y o f Twente, The Netherlands): How r e p r o d u c i b l e a r e y o u r
r e s u l t s i f you prepare more than one sample o f t h e same composition? Our
experience i s t h a t t h e behavior over t h e f i r s t few hours o f t h e r e a c t i o n depends
v e r y much on t h e p r e - h i s t o r y o f t h e m a t e r i a l even though t h e f i n a l behavior
a f t e r some hours i s t h e same o r very s i m i l a r .
O.A. SCELZA (INCAPE, Argentina): Several samples o f d i f f e r e n t c a t a l y s t s were
prepared and t e s t e d i n t h e methane o l i g o m e r i z a t i o n ( t h e r e a c t i o n t i m e was about
1 h ) . Results showed a good r e p r o d u c i b i l i t y . However, I b e l i e v e t h a t t h e
c a t a l y t i c behavior s t r o n g l y depends on t h e p r e p a r a t i o n and c a l c i n a t i o n c o n d i t i o n s .
G. Centi and F. Trifiro' (Editors), New Developments in Selective Oxidation
1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
381
Oxidative Coupling of Methane,
the Effect of Gas Composition and Process Conditions.
J.A. Roos, S.J. Korf, J.J.P. Biermann, J.G. van Ommen and J.R.H. Ross
Faculty of Chemical Technology, University of Twente, PO Box 217,
7500AE,Enschede, The Netherlands
ABSTRACT
The effect of process conditions on the oxidative coupling of methane has been studied; factors
examined included the effect on the product composition of the oxygen concentration in the
reactor feed and of backmixing of the gas in the reactor. It appears that the desired C2 products
are susceptible to degradation reactions and that this has consequences for the method of reactor
operation.
INTRODUCTION
A topic which has recently attracted interest is that of the direct oxidative conversion of
methane into products such as methanol, ethane and ethylene. A large number of publications (see
for example [l-141) have appeared over the last few years which show that several types of catalyst
can be used to give reasonably high conversions and selectivities for the formation of the latter
two products according to the all-over reactions (1) and (2):
2 CH, t v2 0 2 ---> q H 6 t 3 0
2 CH4 t
0, --->
CZH4 t 2 H20
In all cases, CO and CO, are also formed:
CH4 t 1%0, ---> CO + 2 H2O
CH, t 2 0 2 ---> C02 t 2 H20
(3)
(4)
Ito et al. [1,2] have shown that lithium-doped magnesium oxide is an active and selective catalyst
for the oxidative coupling reaction; they suggested that the lithium ions just fit into
cation-vacancies of the MgO matrix, the ionic radii of the Li' and M$+ ions being almost the
same. Using EPR spectroscopy they showed that [Li'O-] centres are formed when Li' is added
to MgO. Based on investigations of the formation of methyl radicals from methane over the same
382
type of material, they proposed a radical mechanism in which methyl radicals are produced on the
[Li'O-] sites; they assumed that the methyl radicals then recombine to yield ethane or react with
oxygen to form methoxy radicals which ultimately give rise to the formation of CO and CO,.
Other successful catalyst systems which have been reported are PbO supported on -pAI2O3,
various lanthanide oxides (i.e. Sm203 [4]) or promoted transition metal oxides e.g. NiO or MnO,
[5, 61. It appears that doping the lanthanides or transition metal oxides with alkali metal ions
results in an enhancement of the activities of the metal oxides for the formation of Cpompounds.
When alkali chlorides were used, a high C2H4/C2Ha ratio was found. This is thought to be a
result of the dehydrogenation of C,H, by Clr-radicals liberated by the catalyst [5, 61. According
to Otsuka and his coworkers, the methane activation step takes place on a peroxide anion species
(O,%)present at the surface of the catalyst [q.Alkaline earth metal oxides (or carbonates), other
than MgO, such as CaO, SrO or BaO, have also been found to be active for methane coupling,
especially if they are promoted with alkali metal ions [8]. Most authors appear to accept that the
oxidative coupling reaction occurs by the radical mechanism proposed by Lunsford et al. [1,2];
however, some authors favour a redox mechanism [3,9].
The aim of our work on selective methane oxidation has been to gain a greater understanding
of the factors which govern the activity and selectivity in the coupling reaction with a view to
developing more active, selective and stable catalysts. We therefore fKst compared the supported
lead oxide system with the Li/MgO catalyst under the same circumstances [It]. We next studied
the effects of pretreatment of Li/MgO catalysts [12,13]and the addition of promoters to Sm203
catalysts [14]. In the course of these investigations, we also found that process conditions, such as
linear gas velocity, gas composition, reactor geometry and temperature can have a profound
influence on the formation of C,products. We have therefore studied in some detailjn the work
reported in this paper, the effects of these variables using a standard Li/MgO catalyst. The
influence on the composition of the product mixture of the oxygen partial pressure in the reactor
feed under normal process conditions is presented first. This is followed by a discussion of
experiments showing the influence on selectivity of back-mixing in the reactor. In most of our
work published to date, these effects have been taken fully into account.
EXPERIMENTAL
The catalysts used for these experiments were samples of Lithium-doped Magnesium oxide [1,2]
prepared by wet impregnation of MgO powder by an aqueous solution of LiOH (both chemicals
obtained from B.D.H. Ltd.). Two types of Li/MgO catalysts were prepared: in the case of type B,
CO, was passed through the evaporating solution during impregnation [13], while in the case of
type A the CO, treatment was omitted.(Passing CO, through the evaporating solution during
impregnation results in a more active catalyst [13]$ After impregnation, the samples were dried
in air overnight at 140°C and then calcined in air in quartz or fused alumina crucibles at 850°C
for 6 h. After calcination, the catalysts were crushed and sieved to a grain size of 0.3-0.6 mm or
383
0.1-0.3 mm. The particular catalyst used for any specific experiment is given in the appropriate
Figures and the Table.
The catalytic experiments were carried out in vertically-placed quartz fixed-bed reactors (5 mm
id., 40 cm heated length) operated at a pressure of one atmosphere. The temperature of the
reactor, heated in an electric oven, was measured by a thermocouple (shielded by a quartz
capillary), placed on top of the catalyst bed. Unless otherwise stated, the bed consisted of a
mixture of catalyst particles with the same weight of quartz particles of the same size. The gases
were analysed by gas chromatography using a Carbosieve B column (2.25 m length; 2 mm id.).
The reactor could be used in two modes: single-pass or recycle. In the first case, the gas passed
through the reactor and was directly fed to the gas chromatograph; in the second case, the greater
part of the gas flow was recycled. The recycle ratio (R) and the gas flow are given in the
appropriate table; R is defined as the ratio of the recycle flow to the net flow through the system.
The conversion of the reactants
(aCH4and ao2)
is defined as the number of moles of the
reactant that have been converted, divided by the number of moles of the reactant in the feed. The
selectivity of a product is defined by the number of moles of CH, that have reacted to give this
product, divided by the total mumber of moles of CH4 that have been converted. The yield of a
product is given by the product of CH, conversion and the selectivity to this specific product.
In the experiments carried out to examine the effect of thevariation of the oxygen concentration
in the reactor feed, 93 mg of Li/MgO catalyst was used (0.3-0.6 mm). The catalyst patricles were
diluted with the same weight of quartz paticles of the same size. The gas feed contained 50 kPa
CH, and a variable amount of 0, (the balance being He), with a total pressure of 1 atm (101
H a ) . The gas flow rate was 0.42 cm3s-l (STP).
660
7CO
740
780
870
ZTC
660
jW
740
780
820
860
Figure 1. Methane and oxygen conversions and product concentrations as a function of reactor
temperature, T, ; 93 mg of Li/MgO catalyst (type A, 7 wt% Li, 0.3-0.6 mm.) with quartz dilution
and a gas feed containing 50 kPa CH,, 2 H a 0, and 49 kPa He, with a flow rate of 0.42 cm3s-l
(STP).
384
660
703
740
780
820
860
-Tr/'C
-Trl'C
Figure 2. Methane and oxygen conversions and product concentrations as a function of reactor
temperature, T, ; 93 mg of Li/MgO catalyst (type A, 7 wt% Li, 0.3-0.6 mm.) with quartz dilution
and a gas feed containing 50 kPa CH,, 21 kPa 0, and 30 kPa He, with a flow rate of 0.42 cm3s-'
(SP)*
RESULTS and DISCUSSION
The Effect of the Concentration of Oxveen in the Gas Feed.
A number of experiments were carried out, using a Li/MgO catalyst, to show the effect of the
partial pressure of oxygen in the feed on the product composition. Figures 1 and 2 give the results
(as a function of temperature) obtained at the extremes of the range of oxygen partial pressure
studied: 2.0 kPa in the experiment of Figure 1 and 20.7 kPa in that of Figure 2. In both cases, the
partial pressure of CH, in the feed was kept at 50 kPa; He was used to keep the total pressure
at 101 kPa. When a low oxygen concentration was present in the feed (Figure l), ethane and
ethylene were the major products, with the consequence that the selectivity for the formation of
C, products was high; a value of 88% was found at a reaction temperature of 800°C. When a high
concentration of oxygen was present in the feed (Figure 2), CO and CO, were the the major
products, with the consequence that the
C, selectivity was much lower (32% at SOO'C).
In the
latter case, the ratio of C2H4 to C,H, was higher than in the former, indicating that there was a
higher rate of oxidative dehydrogenation of C2H6 to q H 4 at higher oxygen concentrations. The
dip in the partial pressure of CO as function of temperature observed in both Figures 1 and 2
at temperatures around 830°C was probably caused by a competition for the available oxygen
between catalytic reactions (producing CO,) and gas-phase reactions (producing CO) [ 161; at
temperatures around 790°C and higher, the available oxygen was (almost) completely consumed
in both experiments. The catalytic reactions have relatively higher rates at
temperatures
of ca. 790-820°C than do the gas-phase reactions [16]; this has the consequence that the amount
of oxygen available for CO formation in the gas-phase is reduced. At temperatures well above
800"C, however, the rates of the gas-phase reactions are much increased [16] and gas-phase
385
reactions compete successfully with catalytic
reactions for the available oxygen, resulting in
the higher CO production observed in the
experiments shown in Figures 1 and 2.
Figure 3 shows the partial pressures of the
C, products and total oxidation products (at
the exit of the reactor) as a function of the
partial pressure of oxygen in the feed at
reactor temperatures of 700, 750 and 800°C;
the results of Figures 1 and 2 are included,
together with those from experiments with
partial pressures in the feed of 8.5 and 14.5
kPa. The following general conclusions can be
drawn from these results:
(i) the c2H4/qH6 ratio increases with
increasing temperature and Po, ;
(ii) the production of CO is low compared to
the CO, production at lower temperatures and
low values of Po, ;
(iii) the partial pressures of CO and CO, are
more affected by the partial pressure of
oxygen in the reactor feed than are the partial
pressures of the C,products, the sum of which
remains almost constant with increasing Po2
It is eenerallv acceDted in the literature
concerning the
network for this
reaction that the q H 4 is formed from the
Y
C2H6 [2,3,4,11]. The
increase
in
~i~~~~ 3, product concentrations as a
function of the partial pressure of oxygen in
the reactor feed containing 50 k P a CH,
(balance He, total uressure 101 @a’). with
a flow rate of 0.42 ‘Cm3i1.
the
C2H4/qH6 ratio may thus be explained by an
increase in the (oxidative) dehydrogenation of C2H6with increase in oxygen concentration in the
reactor and with increase in temperature.
The low values of the partial pressure of CO in the reactor effluent at lower Po, and lower
temperatures may be explained in the same way as done above in relation to the interplay of gasphase oxidation reactions (producing CO) and catalytic oxidation reactions (producing COj)[ 161.
The CO production will be relatively low compared to the production of CO, at lower
temperatures (gas-phase reactions are favoured at higher reaction temperatures) and at lower
partial pressure of oxygen in the empty volume of,the reactor between the catalyst bed and the
386
-
Po2 (kPa)
Figure 4. Product concentrations and oxygen conversion as a function of the partial pressure of
oxygen, Po,, in the reactor feed containing 50 kPa CH, (balance He, total pressure 101 H a , flow
rate = 0.42 cm3s-'), using 1.5 g of the Li/MgO catalyst (type A, 7 wt% Li, 0.3-0.6 mm.).
reactor exit (the "post-catalytic volume"). The oxygen partial pressure in the postcatalytic volume
will be low if the oxygen conversion by the catalyst bed is high, which will be the case at low
oxygen partial pressures in the reactor feed; compare Figures 1 and 2. We can thus explain the
relatively high rate of CO production (compared to CO,)at a reaction temperature of 750°C when
there is a partial pressure of oxygen of 20.7 Wa in the feed; see Figure 3. At this temperature,
the conversion of oxygen is relatively low (58%, see Figure 2), and this means that there is much
oxygen available for gas-phase oxidation to produce the CO. At a reaction temperature of 700"C,
the gas-phase reactions are probably too slow to produce much CO. At 800°C,the oxygen
consumption in the catalyst bed is much higher: relatively more CO, is produced on the catalyst
and less oxygen remains available for gas-phase oxidation (oxygen conversion: 95%). This
explanation of the observed ratio of CO and CO, was also used to explain the dip in the CO
production as a function of temperature as shown in Figures 1 and 2, see above.
The results of the experiments shown in Figure 4 further illustrate the importance of the
interplay between gas-phase and catalytic reactions. In this experiment, 1.5 g of catalyst was used
instead of the 0.093 g used for the experiments of Figures 1-3.Because of the high catalyst weight,
the oxygen conversion in the catalyst bed is high and it is thus to be expected that little gas-phase
oxidation, resulting in CO production, will take place after the catalyst bed. Figure 4 shows that
this is indeed the case. Only if the oxygen conversion is not complete does CO production start
to increase.
The data of both Figure 3 and 4 show that the sum of the partial pressures of the C2 products
(and hence the rate of their production) increases less with an increase in the partial pressure of
oxygen than do the partial pressures of the total oxidation products. We conclude elsewhere [16,
387
211 that this increase in the degree of total oxidation (in the presence of a Li/MgO catalyst) is due
to a high rate of further oxidation of the
compounds.
If the possibility of the sequential total oxidation of the %products is taken into account, some
observations which have been reported in the literature can be explained. Otsuka et al. [5] showed
in their experiments with a LiCI-Sm,O, catalyst (T=75O0C)that when they increased the sum of
the partial pressures of the reactants in the reactor feed (CH4/0, constant at 5/2) by lowering
the partial pressure of the helium diluent (at a constant total pressure of 101 H a ) , there was a
decrease in the C, selectivity and yield. Calculations based on their data show that the sum of the
C2H6 and C,H4 partial pressures levelled off at a value of approximately 1.9 kPa when the sum
of the partial pressure of the reactants was 43 kPa; the maximum of the sum of the partial
pressures of the C, products achieved was 2.7 kPa when the sum of the partial pressures of the
reactant was 99 F a . The corresponding partial pressures of C2H4for the two situations were 1.7
and 2.1 kPa respectively. An increase in the absolute C, concentration brought about by increasing
the absolute concentration of the reactants thus leads to a decrease of the (relative) C, selectivity
and
C,yield and this is probably caused by an increased oxidation of the C,
products at higher
partial pressures of these products.
A rather general observation which may be explained in the light of our conclusions is that
approximately the same absolute optimum C, partial pressure has been reported by different
authors using different types of catalyst. For example, Otsuka et al. [6] obtained a total partial
pressure of C, products of 5 kPa over a lithium doped NiO catalyst; the same total C,partial
pressure has been reported by Matsuura et al. [17] for a lithium-doped ZnO catalyst and by
ourselves [18] for a lithium doped MgO catalyst (under conditions which were better optimised
than those shown above). We have presented direct evidence for the occurrence of total oxidation
of the desired C, products in the case of a Li/MgO and a Ca/Sm,O, catalyst [16,21]; this limits
the maximum partial pressures of C, products that may be. reached with these catalysts. These
total oxidation reactions of the C,hydrocarbons may take place not only in the gas-phase, but also
on the catalyst: experiments with a Li/MgO catalyst at 720°C (when the rates of gas-phase
oxidation reactions of ethane and ethylene are negligible) show that all CO and CO, are formed
from C2H4 by a sequential reaction scheme [21]:
CH4 ---> C2H6 ---> CZH4 ---> COX
We have no direct evidence for the occurrence of total oxidation of C, products on the Li/NiO
and Li/ZnO catalysts mentioned above; however, we believe that the the striking similarity in the
partial pressures of C, products which can be achieved with these systems shows that the total
partial pressure of C, products is limited by sequential oxidation reactions. The same conclusions
were reached by Labinger and Ott for a Na-MnO,/MgO catalyst, used in the cyclic mode of
operation [22].
388
The Effect of Back-Mixing
The importance of the occurrence of sequential oxidation reactions of the
hydrocarbons is
demonstrated by the effect on the product distribution of back-mixing in the reactor. From a
reactor engineering standpoint, it is to be expected that the selectivity of a reaction system in
which there is the possibility of sequential reactions will be highest in a plug-flow reactor (In an
ideal plug-flow reactor, back-mixing is absent [19]). In the oxidative coupling of methane, backmixing of the (relatively unstable) intermediate C2 products wiU lead to transport of these
products to regions (nearer to the entrance) of the reactor where a higher partial pressure of
oxygen exists, and this will lead to an increased degree of sequential oxidation of the C2 products
and consequently a lower C2 selectivity [18].In order to investigate this effect, two experiments
involving different conditions of back-mixing (but with the same amount of catalyst and the same
residence time in the reactor) were performed and the results of these are shown Table 1. In the
first experiment of Table 1, the reactor was used as above in the single-pass mode (approaching
plug flow); in the second experiment, it was used in the recycle mode, with a recycle ratio of 10
(under conditions approaching those of an ideally mixed reaction system). As the other process
conditions were exactly the same in both experiments, any differences between the two
experiments can only be caused by the different degree of back-mixing. Table 1 shows that there
are distinct differences between the results of the two experiments. For the single-pass experiment,
both the conversion and selectivity are superior. The lower conversion obtained by the well-mixed
reactor is normal for this type of reactor: for reactions with an order greater than zero, a well
mixed reactor always gives lower conversions than does a plug-flow reactor (if the residence time
and the amount of catalyst is the same), due to the effect of back-mixing [19]. The difference in
selectivities in the two reactors provides further evidence for the suggestion that sequential
oxidation reactions take place.
Table 1 The effect of backmixing: a gas mixture was contacted with 500 mg of Li/MgO catalyst
=
(group B, 2.8 wt% Li, 0.1-0.3 mm., no quartz dilution) with a recycle ratio R. Reactor feed: P,
50 kPa, Po, = 10 Wa, flow rate: 0.83 cm3s-', T, = 800°C
R
0
10
CH4 conversion
1%
25
17
0, conversion
1%
95
82
C, selectivity
1%
67
54
C yield
7%
16.4
9.0
With respect to the effect of back mixing on selectivity, it is also important to know the degree
of back-mixing in the experiments of Figures 1, 2 and 3 (showing the effect on product
composition of variation of the oxygen concentration in the reactor feed). Both measurements of
389
the residence time distribution in the reactor (using a N2pulse in a He flow [IS]) and calculations
based on the method of Hsiung and Thodos [20] showed that the flow pattern in the reactor in
these experiments lay in between that of plug flow and of ideal mixing. Experiments carried out
with higher flow rates in the same reactor with the same value of W/F of 0.22 g.s.cmJ (i.e., at the
same residence time in the catalyst bed), in order to improve the plug-flow character of the gas
flow through the reactor (and also to decrease the residence time in the post-catalytic volume)
resulted in an increase in the selectivity with increasing flow rate [18]. With a flow rate of 3.36
cm3s”, the selectivity reached a maximum and further increase gave no improvement. We
attempted to repeat the experiments of Figures $ 2 and 3 with these optimal process conditions.
However, due apparently to the higher throughput of gas under these conditions, temperature
instabilities (i.e., the occurrence of severe hot-spots) occurred if the oxygen concentration in the
reactor feed was increased above 10 kPa, even in the narrow-bore reactor used. It thus follows
from our results that the use in the reactor feed of partial pressures of oxygen above ca 10 kPa
results in unnecessary loss of selectivity and also to temperature-instabilities when higher flow
rates are used.
CONCLUSIONS
1. An increase of the oxygen concentration in the reactor feed gives an adverse effect on the
selectivity. The total C, concentration levels off at higher values of the oxygen inlet concentration,
this being caused by total oxidation of the C, products.
2. As ethane and ethylene are susceptible to further oxidation, optimum selectivity is reached
under plug-flow conditions.
3. To obtain optimum
C, yields,
non-selective gas-phase reactions must be minimised by
minimising the residence time (and oxygen concentration) in the post-catalytic volume.
REFERENCES
1. T. Ito and J.H. Lunsford, Nature vol. 314 (1985) 721.
2. T. Ito, J.-X. Wang, C.-H. Lin and J.H. Lunsford, J. Am. Chem. SOC.,107 (1985) 5062.
3. W. Hinsen, W. B y t y and M. Baerns, Proc. 8th Int. Congr. Catal. Berlin (1984) 111, 581.
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9. I.T. Ali Emesh and Y. Amenomiya, J. Phys. Chem., 90 (1986) 4785.
10. G.-A. Martin and C. Miradotos, J. Chem. SOC.,Chern. Commun., (1987) 1393.
11. J.A. Roos, A.G. Bakker, H. Bosch, J.G. van Ommen and J.R.H. Ross, Catalysis Today, 1
(1987) 133.
12. S.J. Korf, J A . Roos, NA. de Bruijn, J.G. van Ommen and J.R.H. Ross, J. Chem. SOC.,Chem.
Commun., (1987) 1433.
13. S.J. Korf, J.A. Roos, N.A. de Bruijn, J.G. van Ommen and J.R.H. Ross, Catalysis Today, 2
(1988) 535.
14. S.J. Korf, J A . Roos, J.M. Diphoorn, R.H.J. Veehof, J.G. van Ommen and J.R.H. Ross,
Catal. Today, 4 (1988) 279.
390
15. H.M.N. van Kasteren, J.W.M.H. Geerts and K van der Wiele, Proc. 9th Int. Congr.
- Catal.
Calgary, (1988) KI 930.
16. J.A. Roos, S.J. Korf, R.H.J. Veehof, J.G. van Ommen and J.R.H. Ross, Catal. Today, 4 (1988)
A7 1
17. i: Matsuura, Y. Utsumi, M. Nakai and T. Doi, Chem Lett. (1986) 1981.
18. J.A. Roos, S.J. Korf, A.G. Bakker, NA. de Bruijn, J.G. van Ommen and J.R.H. Ross,
"Methane Conversion", Eds. D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak, Elsevier Sci.
Publ., Amsterdam, (1988) 427.
19. K.R. Westerterp, W.P.M. van Swaaij and A.A.C.M. Beenackers, Chemical Reactor Design and
Operation, John Wiley & Sons, Chichester (1984).
20. T.H. Hsiung and G. Thodos, Int. J. Heat Mass Transfer, 20 (1977) 331.
21. J.A. Roos,S.J. Korf, R.H.J. Veehof, J.G. van Ommen and J.R.H. Ross, Paper presented at the
196th National Meeting of the American Chemical Society, Division Colloid & Surface Science,
Los Angela, 1988; Appl. Catal. 52 (1989) 147.
22. J.A. Labinger and K.C. Ott, I. Phys. Chem. 91 (1987) 2682.
M. BAERNS (University of Bochum, FRG): Considering homogeneous gas-phase reactions
in oxidative methane coupling; the postcatalytic gas volumes of the plug-flow and the recycle
reactor should be accounted for when comparing the results obtained in these two types of
reactors (When high recycle ratios are applied the reactants pass frequently through the
postcatalytic volume).
J.R.H. ROSS (University of Twente, The Netherlands): In principle. you are correct.
However, in practice, the postcatalytic volume was expressly made as small as possible so
that such effects would be negligible in the types of experiments which we have reported
here. We have shown that the rate of oxidation of the
products in the gas phase is
negligible for residence times and temperatures used here.
K. VAN DER WIELE (Technical University of Eindhoven, The Netherlands): Your
suggestion to improve the selectivity of the methane oxidative coupling is to develop more
active "methane activation" catalysts and to work at lower temperatures, thus avoiding
consecutive homogeneous reactions (oxidation of C, 's to COX).In my opinion possibilities
are very limited because any methane activation catalyst will presumably activate ethane and
ethylene as well, the latter requiring a lower activation energy. So lower temperatures will
soon favour catalytic oxidation of C, 's. Moreover the product composition unfavourably
shifts from ethene to ethane at lower temperatures. Do you agree with my opinion ?
J.R.H. ROSS (University of Twente, The Netherlands): Your comment is correct if the preexponential factor is the same for both reactions. Although the same (number of) active sites
are likely to be involved, the entropy of activation is not necessarily the same as it will
depend on factors such as the geometries of the reacting molecules. Hence, it should be
possible to fiid catalysts favouring the oxidation of methane over that of ethane even if the
activation energy for the latter reaction is lower. Experience has indeed shown that the
selectivity to ethane increases at lower operation temperatures. However, this need not be a
problem if the process in which the C, hydrocarbons are used either includes a
dehydrogenation step or allows reaction of C2H4 and q H C simultaneously.
J.G. MC CARTY (SRI International, USA): I appreciate your comments about the need to
avoid post-reactor homogeneous reactions (with 03 and internal mixing. However, even
with plug flow reactors, product ethane and ethene must rise (for higher yields) to levels
where product oxidation takes place. Does this not place limits on the %+yields that can be
achieved with oxidative coupling ?
J.R.H. ROSS (University of Twente, The Netherlands): We agree. Our aim was to show
under which circumstances the various factors which we have described are of importance.
Our point is that there is no sense in talking about chemical limits if limits placed by method
are not fully recognized.
391
W.J. VERMEIREN (University of Leuven, Belgium): I am not convinced of your conclusion
that the main part of the COXis coming from C, oxidation. I have two comments:
1. The adverse effect on the C, selectivity upon increasing the oxygen concentration in the
feed, is caused by an increasing contribution of gas phase reactions. The selectivity for CO
with increasing oxygen concentration in the feed, is typical for gas phase reaction in
methane-oxygen mixture. I believe that the oxidation of methyl radicals in the gas phase has
a greater probability than the oxidation of C,products.
2. You compare a single-pass operation with an operation, approaching ideally mixing.
However, the decrease of C selectivity is not that high in the ideal mixed reactor and results
still in a C, selectivity of 546. According to me, this is an indication that the oxidation of 5
products is not so important as you think.
J.R.H. ROSS (University of Twente, The Netherlands):
1. Our conclusions are based not only on the results given in our paper but also on other
results presented elsewhere [1,2,3]. We have shown that the gas-phase reaction is not of
significant importance if the residence time in the reactor is short and the temperature is
relatively low. We quite agree that high temperatures and residence times favour the gasphase process, in which undoubtedly methane can react with oxygen. When we see CO as
product, we accept that it comes mainly from gas-phase oxidation but we think that gasphase oxidation of C2H6 and q H 4 also contribute.
2. The drop in selectivity reported here is well outside experimental error so we cannot
accept your remarks. The results reported here are only supportive of our arguments,
reported fully elsewhere, that C2 oxidation is the predominant route to COX[1,2,3].
1. J.A. Roos, S.J. Korf, A.G. Bakker, N.A. de Bruijn, J.G. van Ommen and J.R.H. Ross,
"Methane Conversion", Eds. D.M. Bibby, C.D. Chang R.F.Howe and S. Yurchak, Elsevier
Sci. Publ., Amsterdam, (1988) 427.
2. J.A. Roos, S.J. Korf, R.H.J. Veehof, J.G. van Ommen and J.R.H. Ross, Catal. Today, 4
(1988) 471.
3. J.A. Roos, S.J. Korf, R.H.J. Veehof, J.G. van Ommen and J.R.H. Ross, Applied Catalysis,
52 (1989) 147.
ANONYMOUS: From the results you presented on the competitive reaction of ethane or
ethylene with methane, can you quantify the relative rates for methane versus ethane versus
ethylene oxidation ?
J.R.H. ROSS (University of Twente, The Netherlands): In order to give a semi-quantitative
answer to your question, we have to refer to results given in [I]. It is possible to calculate
the data of the following Figure from the results given in reference [l]. The Figure shows the
is the rate of formation of CO + CO and
is the
ratio Z = R c b / k H 4 (where
rate of consumption of m e t h a 3 p I o t t e d against the partial pressures o?C2H4 and q H 6
added to the feed of a plug-flow reactor.
kH
P
C2HL
/kPa
'C2H6 'k Pa
Figure Ratio Z = RF,JRCH4 as a function of the partial pressures of C2H4 and C,H6
added to the gas feed o a plug-flow reactor.
392
A value of Z > 1 means that at least some of the COX most be formed from the C
1
hydrocarbon; the higher the value, the more that the C2 must be involved. A value of Z <
could imply that all the COXcomes from the methane, but this is not essential. It is thus
clear that the Z values are upper limits to the relative rates of reaction of the C
hydrocarbons to those of methane. It can be seen from the Figure that ethylene is
more reactive than methane and that the rate of reaction of ethylene at least at high
ethylene concentrations, is much higher than that of methane. For example, with a gas-phase
concentration of PCH4 = 67 kPa, PC2H4 = 10.5 kPa and Po, = 7.0 kPa (balance He), the
value of Z is 8.3.
mutt
1. J.A. Roos, S.J. Korf, R.H.J. Veehof, J.G. van Ommen and J.R.H. Ross, Catal. Today 4
(1988) 471.
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V.,Amsterdam Printed in The Netherlands
-
393
ROLE OF SOME EXPERIMENTAL PARAMETERS I N THE CATALYTIC CONVERSION OF METHANE BY
OX IOAT1VE COUPLING
.
Il.SPINICC1
Dipartimento d i Energetica, U n i v e r s i t a d i Firenze, Via S.Marta 3, Firenze
{Italy).
!SUMMARY
A study on t h e o x i d a t i v e coupling o f methane has been c a r r i e d out over CaO
m d over K20/CaO i n order t o d e f i n e t h e e f f e c t o f t h e d i l u t i o n o f the reactants
i n the c a r r i e r gas, e x p e c i a l l y on t h e s e l e c t i v i t y ; subsequently t h e a t t e n t i o n
lias been addressed t o t h e study o f the e f f e c t caused by the presence o f the
main products i n t h e reactant mixture, w i t h t h e aim t o obtain informations from
the presence o f species which can favour o r hinder the reaction. Moreover a
l t i n e t i c study o f the r e a c t i o n has been performed i n d i f f e r e n t i a l conditions i n
order t o gain a deeper i n s i g h t i n t h e development o f the reaction: the whole o f
t h e experimental r e s u l t s have been employed t o propose a r e a c t i o n mechanism,
which takes i n t o account the d i f f e r e n c e s between t h e two types o f c a t a l y s t s .
INTRODUCTION
The increasing i n t e r e s t i n t h e a c t i v a t i o n o f methane i s born by t h e attempts
t o e x p l o i t a t t h e best t h e l a r g e reserves of t h i s substance and i n t h i s context,
the i n t e r e s t i n the o x i d a t i v e coupling o f methane i s s u r e l y due t o t h e i n t e r e s t
i n the synthesis o f higher hydrocarbons and e x p e c i a l l y ethylene. The r e a c t i o n
occurs a t high temperatures on a number o f c a t a l y s t s (1,2) b u t i t has been
undoubtedly established (3,4) t h a t the r e a c t i o n develops homogeneously also, i n
the gas phase. I n any case the presence o f a proper c a t a l y s t appears t o be
determining, on the basis o f the experimental r e s u l t t h a t t h e r e i s no r e a c t i o n
w i t h no c a t a l y s t : t h e r e f o r e i t appears very i n t e r e s t i n g t o c l e a r out t h e r o l e o f
the c a t a l y s t s i n t h i s r e a c t i o n .
Following a previous work on t h e methane coupling over calcium oxide o r over
other oxides supported on calcium oxide (5), i t appeared i n t e r e s t i n g t o focuse
the a t t e n t i o n mainly on CaO and K20/CaO, t h a t i s on t y p i c a l l y basic oxides, i n
order t o t r y t o c l e a r out t h e r o l e o f the basic centers r e l a t e d t o t h e presence
394
of adsorbed oxygen species. Indeed the results previously obtained and many
1 iterature reports (6,7) lead to the conclusion that the observed selectivities
of many catalysts towards the formation of C2-hydrocarbons in methane coup1 ing
can be mainly attributed to the presence of proper surface oxygen species.
The investigation was therefore addressed towards the study of the influence
of the dilution of reactants, and to the investigation on the competitive
presence of the products in the reactant mixture. As a logical development, it
appeared necessary to get informations about the reaction mechanism by means o f
a kinetic analysis of the reaction, and therefore studies have been then carried
out, either about the overall oxidation or about the oxidative coupling, in
order to define the intervention of the catalysts in the reactions.
METHODS
Calcium oxide was obtained by the decomposition for 5 h at 950 'C of powdered
white marble MICROCAL FM-600, provided by Ingram: its surface area, measured by
means of a Perkin Elmer Sorptometer 512-0, was found to be 2.5 d9-l.
The X-ray analysis showed the typical cubic structure of calcium oxide, while
the semiquantitative analysis, performed in a scanning electron microscope,
revealed the presence of 0.5 - 1% SrO. The supported samples were prepared by
impregnating the calcium oxide with solutions of KNO3 in such a way to obtain
catalysts containing 7% KzO: after evaporation of the excess solution, the
samples were dried at 110 *C for 3 h and then calcined in air at 550 *C for 3 h.
The experiments were carried out in a quartz tubular reactor (1 cm 0.d.) at the
desired temperature, using 0.1 - 0.3 g, after a pretreatment in a f l o w of
helium plus oxygen (ratio 7.5/1) at 650 'C for 2 h. Methane and oxygen were fed
with a helium carrier over the catalysts and in some scheduled runs a chosen
amount of ethane, or ethylene, or carbon dioxide was added to the reactant
mixture. The composition of the effluent gases was analyzed by means of a C.
Erba 4200 gas-chromatograph, equipped with a hot wire detector and two 1/8" 0.d.
10 ft columns packed with Carbosieve S-11.
395
RESULTS AND DISCUSSION
Above all it must be specified that experiments have been carried out with
empty reactor in order to determine the activity of the reactor walls, which
cannot surely be neglected above 750 'C; indeed in the range 750 - 850 'C the
conversion of methane ranges from 0.5 to 0.9 moles% into carbon monoxide; from
1.4 to 2.2 moles% into carbon dioxide; from 1.1 to 1.9 moles% into ethylene and
from 0.1 to 2.2 moles% into ethane. The values reported in the following results
take always into account this reactivity o f methane.
4
-
conv.
10
15
He (ml/min)
26
Fig.1 Conversion of methane (mol %) into the main products at 780 'C over
samples o f 0.25 g, employing a mixture of methane and oxygen (10 and 2 ml/min)
with different amounts of carrier gas (He) and the two types of catalysts
In Fig. 1 the results are reported about the catalytic activity and the
related selectivity, when varying the dilution o f the reactant mixture. It can
be seen immediately that there is an appreciable amount of carbon monoxide only
on calcium oxide, while the presence of an active phase with monovalent ions
396
seems t o i n h i b i t the formation o f carbon monoxide: i t must be underlined a t t h i s
p o i n t t h a t the formation o f carbon monoxide i s accompanied by a p a r a l l e l
production of hydrogen, t e s t i f y i n g t h e common pathway o f the formation o f these
two products. ( I t must be said t h a t the amount o f hydrogen has n o t been included
i n Fig. 1 because, being l i t t l e , i t i s d i f f i c u l t t o be evaluated from t h e
chromatograms). Small amounts o f C2H2 have been also revealed.
But two other r e s u l t s can be seen immediately, t h a t i s : 1) the formation o f
COz a l s o decreases w i t h c a t a l y s t s containing potassium oxide, and 2) t h a t o f C2-
hydrocarbons, and e x p e c i a l l y C2H4, i s g e n e r a l l y favoured by d i l u t i n g reactants
i n the c a r r i e r gas, t h a t i s by decreasing t h e i r p a r t i a l pressures.
On the basis o f these r e s u l t s i t appears reasonable t o suppose t h a t t h e
formation o f hydrogen and the enhanced formation o f carbon monoxide on CaO can
be r e l a t e d t o the enhanced development o f t h e t o t a l and o f the p a r t i a l o x i d a t i o n
on t h i s c a t a l y s t : since among t h e products o f p a r t i a l o x i d a t i o n aldeydes can
a l s o be included it appeared l o g i c a l t o work about the hypothesis (8) t h a t HCHO
can be included among t h e products and t h a t i t can d i s s o c i a t e i n t o hydrogen and
carbon monoxide. Indeed T.P.D.
runs performed on CaO samples w i t h adsorbed HCHO
show a peak w i t h maximum a t 510 'C which i s due t o desorption o f formaldeyde
w i t h d i s s o c i a t i o n i n t o hydrogen and carbon monoxide: the experimental r e s u l t
t h a t the maximum of t h i s peak i s n e a r l y coincident w i t h t h a t o f carbon d i o x i d e
and w i t h those of methane and other C2-hydrocarbons ( 9 ) seems t o lead t o the
conclusion t h a t t h e r e i s a common surface intermediate which can g i v e oxygenated
compounds. This surface intermediate has been supposed (9) t o have i t s o r i g i n on
the l e s s basic centers o f t h e c a t a l y s t , w h i l e the more basic centers seem t o
develop the formation o f the C2-hydrocarbons.
Subsequently runs have been c a r r i e d out, over CaO and over 7% KZO/CaO,
introducing i n the reactant mixture a known amount (1.5 ml/min) o f ethylene o r
ethane o r carbon dioxide, w i t h the aim t o check i f t h e i r presence simply lowers
the formation o f the same product o r enhances t h e formation o f some others: t h e
r e s u l t s obtained are c o l l e c t e d i n Fig. 2.
397
7
conv.
(mol. %
5
3
1
CaO
C2H6
Fig. 2 Conversion o f methane (moles %) i n t o t h e main products a t 800 'C over
samples of CaO and 7% KZO/CaO (0.25 g), employing a mixture o f methane (10
ml/min) and oxygen ( 2 ml/min), w i t h the a d d i t i o n o f ethylene or ethane o r carbon
d i o x i d e ( 1 . 5 ml/min)
.
Some evidences can be immediately underlined, since t h e presence o f ethane i n
the gas phase stimulates undoubtedly the formation o f ethylene, e x p e c i a l l y over
K20/CaO. The hypothesis which seems more probable i s t h a t n o t o n l y t h e C-H bond,
b u t the C-C bond a l s o can be activated, g i v i n g intermediate species which behave
s i m i l a r l y t o those formed by methane, and t h e r e f o r e ( i n a d d i t i o n t o methane as a
cracking product) can produce ethylene.
P a r t i c u l a r i n t e r e s t can be now a t t r i b u t e d t o some p a r a l l e l runs, concerning
t h e execution o f runs based on t h e o x i d a t i o n o f ethane, by means o f oxygen: i n
these runs, where methane have been s u b s t i t u t e d i n the same r a t i o s by ethane, no
d i m e r i z a t i o n products have been formed, b u t o n l y cracking products, and above
a l l ethylene. It must be stressed, however, t h a t the c o n t r i b u t i o n of t h e
homogeneous r e a c t i o n i n the gas phase cannot be neglected and i s important,
398
e i t h e r i n the formation o f methane o r i n the formation o f ethy1ene:indeed the
conversion o f methane reach t h e value o f about 0.70 a t 800 'C.
The absolute amounts o f C2-hydrocarbons formed, on the contrary, seems n o t t o
be affected by t h e presence o f the carbon d i o x i d e i n the feed: i n these
experiments t h e i r percentages appear s l i g h t l y increased (expecial l y on K20/CaO)
b u t t h i s effect seems t o be due t o the coincidence o f t h e adsorption s i t e s f o r
carbon dioxide and f o r methane, as shown i n (9), which dcreases the absolute
conversions o f methane, as i t i s possible t o check experimentally.
I t was then
assumed t h a t a k i n e t i c analysis o f the r e a c t i o n could g i v e
d e c i s i v e informations about i t s mechanism and t h e features o f the c a t a l y s t s
investigated: a k i n e t i c analysis was t h e r e f o r e performed by means o f isothermal
experiments, which were c a r r i e d out i n such conditions t h a t t h e p a r t i a l pressure
o f t h e products can be neglected; these experiences were indeed performed
e i t h e r a t constant methane pressure (11.7 kPa), o r a t constant oxygen pressure
(2.8 kPa) i n order t o determine the e f f e c t o f P(02) and r e s p e c t i v e l y o f P(CH4)
on t h e r a t e s o f formation o f the
i n t h e range 740
-
products. The temperatures i n v e s t i g a t e d were
780 'C.
It was presumed t h a t the data obtained i n the experiments f o r examining
separately e i t h e r t h e pressure e f f e c t s o f methane o r those o f oxygen could be
t h e r e f o r e described by one o f t h e f o l l o w i n g basic k i n e t i c equation, t h a t i s :
I t appeared indeed i n t e r e s t i n g t o gain a deeper d e t a i l , by studying
separately f o r t h e two c a t a l y s t s t h e progression o f the r a t e o f conversion o f
methane i n t o C2-hydrocarbons, and r e s p e c t i v e l y i n t o carbon dioxide e i t h e r i n
399
function of the methane pressure (at constant oxygen pressure), or in function
of the oxygen pressure (at constant methane pressure). This follows the
hypothesis that the formation of these two types of products proceeds on
different types of active centers. In Fig. 3 these diagrams are reported for CaO
and in Fig. 4 the corresponding plots are reported for 7% KpO/CaO.
By examining these two figures it can be seen that the progress of the
reaction rates in function of the partial pressures i s similar to that of a
Langmuir isotherm, and this suggest that the rate can described by the relation:
r
=
k
*
e(CH4)
*
e(0p)
where e(CH4) and 8(02) are the surface coverages of methane and respectively of
oxygen: the application of this general relationship is based obviously on the
hypothesis that either the rate of formation of Cp-hydrocarbons or that of
carbon oxides is determined by the formation o f methyl radical CH3' during the
1
2
3
~ ( 0 ~ kPa
).
Fig. 3 Variation of the rate o f Cp-hydrocarbons and o f Cop production vs. the
oxygen pressure (at constant methane pressure), (a), and versus methane pressure
(at const. oxygen pressure), (b), at 780 'C, over CaO.
400
40
A
10
5
15
20
~
c,
4
V
0,
30-
V
01
VI
\
B
20-
--
10-
7
Y
m
0
d
I
V
Y
L
1
Fig. 4
2
4
3
~ ( 0 2 1 , kPa
V a r i a t i o n o f the r a t e o f C2-hydrocarbons and o f C02 formation versus
the oxygen pressure ( a t constant methane pressure), (a), and versus t h e methane
over 7% K20/CaO.
pressure ( a t constant oxygen pressure), (b), a t 780 'C,
surface r e a c t i o n o f methane and an oxygen surface species.
An i n t e r e s t i n g feature, which emerges from the examination o f Figg. 3 and 4,
is
t h a t f o r CaO there i s a wide range o f the p a r t i a l pressure o f oxygen, and t o
a l e s s extent o f methane, where the r a t e of production o f carbon d i o x i d e and o f
C2-hydrocarbons varies 1i n e a r l y i n f u n c t i o n o f ~ ( 0 2 )and r e s p e c t i v e l y o f p(CH4).
This suggests immediately t h a t t h e r e a c t i o n f o l l o w s a f i r s t order k i n e t i c s ,
and t h a t the adsorption constants could be t h e r e f o r e s u f f i c i e n t l y small and
could be neglected i n the k i n e t i c equation o f type iii).This confirms the
r e s u l t s obtained by the T.P.D.
runs ( 9 ) w i t h adsorbed oxygen o r w i t h adsorbed
methane, which demostrate t h a t a t the r e a c t i o n temperatures t h e r e i s no more any
s t a b l e adsorbed species on t h e c a t a l y s t surface.
The k i n e t i c analysis has been performed by l i n e a r i z i n g the equations i)- i v )
and checking i f t h e experimental data could agree w i t h one o f these l i n e a r i z e d
equations: i n t e r c e p t and slope allow us t o c a l c u l a t e
k
and K.
Fig. 5 shows
40 1
the fitting of the experimental data with some of these equations.
AS a result of this kinetic analysis it can be said that the formation of
carbon oxides and of C2-hydrocarbons at constant methane pressure follow a rate
equation of type i i i ) over 7% K20/CaO, because of the good linear correlation
observed by plotting l/r values versus l/p(Op) values ; indeed, in the case of
CaO, either for the production of Cp-hydrocarbons or for the production of
carbon dioxide a nearly satisfactory correlation is obtained by plotting r
versus p , testifying that in its formation the adsorptive term K
*
p for
oxygen could be neglected.
As far as the formation of C2-hydrocarbons and carbon dioxide at constant
oxygen pressure is concerned, the good 1 inear correlation observed by plotting
l/r versus l/p(CH4) for K20/CaO, shows that either the production of ethylene
and ethane or that of carbon dioxide follows again a rate equation of the type
J
L
\
c
10
.25
.5
.75
l/p
1
(kPa-’)
fig. 5 Linearization of eq. i i i ) for the production of CO2 (a), and for the
production of C2-hydrocarbons (b), on KpO/CaO.
402
Table I
Kinetic parameters related to CaO (a) and 7% K20/CaO ( b ) , as calculated from the
formation of ethylene, ethane and carbon dioxide.
740
760
780
C2
0.011 0.15
0.018
0.012
0.015 0.18
0.022
0.011
CO2
0.008 0.09
0.1
0.09
0.022 0.092
0.11
0.12
C2
0.005 0.11
0.025
0.02
0.009 0.15
0.029
0.018
COP
0.007 0.05
0.17
0.14
0.013 0.08
0.19
0.16
C2
0.001 0.06
0.041
0.035
0.003 0.11
0.045
0.031
CO2
0.003 0.02
0.23
0.18
0.006 0.06
0.25
0.19
iii); on CaO indeed it is more difficult to discriminate between the model i i )
and the model i i i ) and this confirms the weak adsorption of methane on this
catalyst, which however is not as weak as oxygen and therefore does not provide
a sure criterion, for establishing the kinetic equation.
The value of the kinetic constants, as determined from the intercepts o f the
diagrams l/r versus l/p (or from the slope of the diagrams r versus p), and
the values of the adsorption constants, as determined from the slopes of these
diagrams are reported in Table I.
Therefore, in the hypothesis o f a non-competitive adsorption of methane and
oxygen, the overall rate equations for the formation of carbon dioxide and
respectively of C2-hydrocarbons on CaO should take the form:
403
b)
for C2-hydr.
r
=
k.Ko*po* - - - Km*pm
-------1+
G‘Pm
or
r
=
k.Ko*po*Km*pm
while those for the formation of Cz-hydrocarbons and of carbon dioxide on
1(20/Ca0 should take the form:
In the hypothesis that the activation energy is determined by the step
concerning the activation of CH4 to give CH3’ radicals it appears reasonable to
consider the values of k for the reactions on 7% K20/CaO and on CaO, and to
consider the possibility of determining the corresponding activation energy: by
means of the appropriate Arrhenius plot it has been possible to find an average
value of the activation energy of 152 kJ/mol for the formation of COz and an
average value of 164 kJ/mol for the formation of C2-hydrocarbons on both
catalysts.
CONCLUSIONS
From the whole of the experiments it is possible to check that at the
reaction temperature oxygen and methane are more strongly adsorbed on 7% K20/CaO
than on CaO and this confirms the results reported in (9) and indeed obtained
with 7% K20/CaO also, from the TPD experiments with oxygen or methane adsorbed.
Indeed the kinetic results seem to indicate that on 7% K20/Ca0 a greater
number of surface centers for oxygen and methane adsorption i s occupied than on
CaO: a wide pressure range, indeed, is found with this catalyst where the
reaction rate depends linearly on the pressures of oxygen and, respectively, o f
methane.
404
The temperature dependence o f t h e r e a c t i o n r a t e s i s n e a r l y equal i n the two
c a t a l y s t s and t h e r e f o r e it i s possible t o suppose t h a t on both c a t a l y s t s the
CH3' r a d i c a l s ( formed d i s s o c i a t i v e l y on the c a t a l y s t s surface and recognized as
the intermediate species,
which lead probably t o C2-hydrocarbons, and t o CO and
COP, through d i s t i n c t pathways according t o the b a s i c i t y o f the surface s i t e s )
can dimerize on the surface o r i n the gas phase. I n t h i s perspective t h e r e s u l t
reported i n Fig. 2, concerning the increased formation o f ethylene, when ethane
i s present i n the feed seems t o support the hypothesis t h a t ethane i s an
intermediate step i n the production o f ethylene.
The d i f f e r e n c e s found w i t h t h e two c a t a l y s t s , as f a r as s e l e c t i v i t y i s
concerned, can be t h e r e f o r e ascribed t o the d i f f e r e n c e s i n the b a s i c i t y o f t h e
surface centers o f the two c a t a l y s t s : on 7% K20/CaO a l a r g e r amount of more
basic centers and t h e r e f o r e o f more charged surface oxygen species
addresses
t h e r e a c t i o n p r e f e r a b l y towards t h e formation o f Cp-hydrocarbons. Moreover t h e
f a c t t h a t on CaO the adsorption o f methane and oxygen proceeds t o a l e s s e r
extent allows us t o suppose t h a t w i t h t h i s c a t a l y s t the c o n t r i b u t i o n o f the
homogeneous gas phase r e a c t i o n i s greater.
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405
MODELS OF THE DIRECT CATALYTIC PARTIAL OXIDATION OF LIGHT ALKANES
J . G . McCARTY, A. B . McEWEN, and M. A. QUINLAN
S R I I n t e r n a t i o n a l , 333 Ravenswood Avenue, Menlo P a r k , C a l i f o r n i a ,
USA 94025
SUMMARY
A p p l i c a t i o n of homogeneous k i n e t i c models t o methane
a c t i v a t i o n i n d i c a t e s t h a t t h e h i g h e r hydrocarbon y i e l d may be
l i m i t e d by homogeneous o x i d a t i o n of methyl r a d i c a l i n t e r m e d i a t e s .
I n t h i s paper, w e d i s c u s s t h e development of a model t h a t
d q s c r i b e s t h e homogeneous and heterogeneous c h e m i s t r y i n v o l v e d i n
t h e s e l e c t i v e o x i d a t i o n of methane and l i g h t a l k a n e s and t h e
impact of t h i s c h e m i s t r y on a l k e n e and h i g h e r a l k a n e y i e l d s . We
a l s o p r e s e n t e x p e r i m e n t a l r e s u l t s f o r methane a c t i v a t i o n and
e t h a n e dehydrogenation u s i n g s t a b l e n o n - v o l a t i l e c a t a l y s t s
composed of a l k a l i n e and r a r e e a r t h c a r b o n a t e s s u p p o r t e d by
r e f r a c t o r y complex o x i d e s .
INTRODUCTION
There i s ample e v i d e n c e t h a t homogeneous r e a c t i o n s
substantially contribute t o the c a t a l y t i c oxidative dimerization
of methane and t h e c a t a l y t i c o x i d a t i v e dehydro-genation
t o ethene.
of e t h a n e
The p r o d u c t d i s t r i b u t i o n h a s o f t e n been
as
b e i n g c o n s i s t e n t w i t h homogeneous f r e e r a d i c a l c h e m i s t r y , b u t t h e
d e f i n i t i v e e x p e r i m e n t s a r e t h o s e of Lunsford, e t a 1 . , 4 - 6 who used
m a t r i x i s o l a t e d e l e c t r o n paramagnetic resonance ( M I E P R )
measurements t o d e t e r m i n e t h e d i s t r i b u t i o n of methyl r a d i c a l s
downstream of a Li/MgO c a t a l y s t bed.
T h e MIEPR r e s u l t s of
Campbell and Lunsford6 show t h a t product e t h a n e forms downstream of
t h e c a t a l y s t bed by homoaeneous methyl r a d i c a l recombination and
v e r i f y , w i t h i n a f a c t o r of two, t h e b i m o l e c u l a r recombination r a t e
constant.
I s o t o p i c exchange experiments
7-10
a l s o s u p p o r t t h e view
t h a t t h e methyl r a d i c a l i s t h e primary i n t e r m e d i a t e i n t h e
p r o d u c t i o n of e t h a n e .
Various r e c e n t l a b o r a t o r y r e s u l t s i n d i c a t e
t h a t d i r e c t c a t a l y t i c c o n v e r s i o n of premixed oxygen and methane
i n t o h i g h e r hydrocarbons approaches a s i n g l e - p a s s l i m i t of about
2 5 % y i e l d (on a C atom b a s i s ) r e g a r d l e s s of c a t a l y s t and r e a c t i o n
c o n d i t i o n s ( F i g u r e 1)
microreactors
11-14
.
F i n a l l y , o b s e r v a t i o n s t h a t empty
can s e l e c t i v e l y produce e t h a n e and e t h y l e n e
a f f i r m s t h e s i g n i f i c a n t r o l e of homogeneous k i n e t i c s i n a l l
a s p e c t s of t h e r e a c t i o n .
These f i n d i n g s i n d i c a t e t h a t homogeneous
o x i d a t i o n k i n e t i c s p l a y an important r o l e i n t h e s e l e c t i v i t y of
alkane p a r t i a l oxidation reactions
406
100
80
$
f
60
?
W
rn
+
u"
s
40
,a
20
osrco,
0
OCaO
0 MgO
CaO
0 CaO
I
0
1
I
20
0
I
I
I
I
60
40
I
80
I
I
100
Ye CHI CONVERSION
rn
This work
Aika al al. (Tokyo). J. Cham SOC. Cham. Commun. 1986. 1210.
Jones at al. (ARCO). Energy and Fuels 1,12 (1987).
A IIo 01 al. (Texas A 6 M), J. Am. Cham. Soc. ~ . 5 0 S Z(1985).
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V Hinsen ei al. (Berlin) 8th Int. Cang. Catal.. 1984.
X Otsuka 01 al. (Tokyo), J. Caial. 1pe 353 (1986).
Lunsford el al. (Tokyo). Texas A 6 M) (lo k publish& 1987).
Lin 01al. (Texas A 6 M). J. Phyr. Cham. 9Q.534 (1986).
0
0
*
+
0
KimMe and Kolls (Phillips Per.), Energy Prcgress 6.226 (1986).
Lin el PI. (Texas A 6 M).J. Am. Cham. SOC. 1p9.4808 (1987).
n Jones and Sofrank (ARCO), J. CaIal.
31 1 (1987).
u
Q
D
0
0
d
m,
lmai and Taaawa ITokvo). J. Cham. Soc. Cham. Commun. 1966.52.
Deboy and Hicks
Grace), J. Catal. U3,.
51 7 (1988).
Gaffnay a1 ill. (ARCO), J. Catal.
422 (1988)
Zhang at al. (Texas A 6 M). J. Catal.
366 (1988).
+.d.
m.
Fig.1. Laboratory fixed-bed catalytic oxidalive coupling of methane with premixed oxygen
RA-2614-9C
407
I n t h i s paper, we d e s c r i b e a model of c a t a l y t i c l i g h t alkane
p a r t i a l o x i d a t i o n used t o e v a l u a t e t h e r e l a t i v e importance of
i n d i v i d u a l homogeneous and heterogeneous r e a c t i o n s over a wide
range of r e a c t i o n c o n d i t i o n s . The model i n c o r p o r a t e s key
heterogeneous r e a c t i o n s t e p s i n t o a network of known g a s phase
alkane f r e e r a d i c a l o x i d a t i o n r e a c t i o n s .
We a l s o r e p o r t t h e
a c t i v i t y and s e l e c t i v i t y of s t a b l e n o n - v o l a t i l e strontium-based
complex oxide c a t a l y s t s for t h e d i r e c t o x i d a t i v e conversion of
methane i n t o h i g h e r hydrocarbons and t h e d i r e c t o x i d a t i v e
dehydrogenation of ethane t o e t h e n e . Comparison of t h e
experimental r e s u l t s and model c a l c u l a t i o n s shows t h a t t h e
c a t a l y s t s s e l e c t i v e l y o x i d i z e i n t e r m e d i a t e s such a s methanol and
carbon monoxide a t r a t e s h i g h e r t h a n expected f o r heterogeneous
hydrogen a b s t r a c t i o n r e a c t i o n s .
METHODS
The premise of our model i s t h a t most of t h e r e a c t i o n
chemistry i n c l u d i n g by product formation o c c u r s b y homogeneous
r e a c t i o n s i n t h e c a t a l y s t pores, c a t a l y s t bed void space, and
p o s t - r e a c t o r volume. Our complete modells c o n s i s t s of 1 4 4
r e a c t i o n s , 134 r e v e r s i b l e homogeneous r e a c t i o n s and 1 0 r e a c t i o n s
which i n v o l v e c a t a l y s t s u r f a c e s i t e s .
Most of t h e gas phase
r e a c t i o n parameters were o b t a i n e d from t h e review compilations of
Frenklach16, Warnatz17, or Tsang
18
.
The primary source of e t h a n e
i n o u r mechanism of methane co-oxidative coupling i s from t h e gas
phase recombination of methyl r a d i c a l s ,
.CH3
+ .CH3 ====> C2H6
(1)
while e t h e n e i s produced from e t h a n e by thermal ( 2 ) and o x i d a t i v e
( 3 ) dehydrogenation.
+ M ====> C2Hq
,C2H5
C2H4
+
*H
+
*OZH
O2 ====>
A fundamental q u e s t i o n is t o what degree deep o x i d a t i o n
r e s u l t s from g a s phase or s u r f a c e chemistry.19 The presence of
'2
H
5
+
premixed oxygen, although necessary t o provide a s i n k f o r hydrogen
and t h e thermodynamic d r i v i n g f o r c e f o r t h e coupling p r o c e s s ,
u n f o r t u n a t e l y l e a d s t o undesired oxygenated by-products,
C02,
e . g . CO,
and formaldehyde.
The f i r s t s t e p i n t h e c a t a l y t i c c y c l e i n v o l v e s t h e a c t i v a t i o n
of methane by a s u r f a c e oxygen atom.
The heterogeneous H
408
a b s t r a c t i o n s t e p can be g e n e r a l i z e d t o i n c l u d e s c i s s i o n of any C-H
bond by an Eley-Rideal
r e a c t i o n with s u r f a c e oxygen (0 ) t o form a
s
gas phase a l k y l r a d i c a l and a hydroxyl s u r f a c e s i t e ( H O S ) ,
RH
+
OS
>
====
*R
where RH = ( 4 a ) C H 4 ;
CH20.
+
HOs
(4b) C2H6;
( 4 c ) C2H4;
(4n)
( 4 d ) CH30H; and ( 4 e )
The a c t i v a t i o n e n e r g i e s used f o r t h e r e a c t i o n of 0
s
with
o t h e r C-H bonds ( e . g . C 2 H 6 ) r e f l e c t t h e i r bond s t r e n g t h s r e l a t i v e
t o methane.
The r a t e d e t e r m i n i n g s t e p i n t h e o x i d a t i v e c o u p l i n g
of methane over Li/MgO was shown by Cant e t a1.''
t o be methane C-H
bond s c i s s i o n ,
CH4 + Os ====> *CH3 +
(4a)
based on a l a r g e , p o s i t i v e ( 1 . 5 ) deuterium i s o t o p e e f f e c t .
Amorebieta and Colussi21 showed a t low p r e s s u r e
to
atm)
t h a t methane c o n v e r s i o n over Li/MgO i s h a l f o r d e r i n oxygen and
f i r s t o r d e r i n methane.
T h i s r e s u l t s u g g e s t s t h a t methane r e a c t s
w i t h atomic s u r f a c e oxygen.
Labinger e t a l . , 2 2 ' 2 3 r e p o r t t h a t w i t h
t h e Na/Mg/Mn c a t a l y s t , CZH6 c o n v e r t s 1 . 9 t i m e s f a s t e r t h a n CH4
.
We have a d j u s t e d t h e r a t e c o n s t a n t s f o r C 2 H 6 t o g i v e t h i s r a t i o
(4b/4a = 1 . 9 ) a t 1 0 0 0 K .
Rate c o n s t a n t s f o r t h e o t h e r r e a c t a n t s
(H-C2H3,
H-CH OH, and H-CHO) were determined by f i x i n g t h e i r
2
frequency f a c t o r s t o t h a t of e t h a n e and a d j u s t i n g t h e i r a c t i v a t i o n
e n e r g i e s r e l a t i v e t o methane i n p r o p o r t i o n t o t h e d i f f e r e n c e s i n
reaction enthalpy.
The r e a c t i o n of t h e methyl r a d i c a l with s u r f a c e oxygen can
s i g n i f i c a n t l y a l t e r t h e s e l e c t i v i t y p r e d i c t e d by t h e model.
Labinger and O t t 2 '
a n a l y z e d t h e i r r e s u l t s and concluded t h a t t h e
o x i d a t i o n of methyl r a d i c a l s w i t h t h e Na/Mg/Mn c a t a l y s t
(without
f e e d g a s oxygen) was 2 7 0 0 t i m e s t h e r a t e of methane a c t i v a t i o n .
The r a t i o of heterogeneous o x i d a t i o n t o homogeneous c o u p l i n g of
methyl r a d i c a l s i s t h e e s s e n t i a l f a c t o r governing t h e s e l e c t i v i t y
a t low c o n v e r s i o n .
T h e r e f o r e , t h e second key premise of o u r model
i s t h a t a l k y l r a d i c a l s i r r e v e r s i b l y react i n a n o n - a c t i v a t e d s t e p
with s u r f a c e oxygen t o form adsorbed complexes t h a t a r e p r e c u r s o r s
t o oxygenates.
.R
+ Os =====>
ROs
(5)
The heterogeneous r a t e p a r a m e t e r s i n v o l v i n g s u r f a c e s i t e s
were optimized t o f i t e x p e r i m e n t a l r e s u l t s f o r Na/CaO a t 1 0 7 3 K.
The v a r i a b l e , $s,
represents the i n i t i a l active s i t e density
( e s s e n t i a l l y t h e sum of 0
S
and U ) .
S
For a s p e c i f i c s i t e d e n s i t y ,
t h e a c t i v a t i o n energy for C-H bond a c t i v a t i o n was t h e n a d j u s t e d t o
409
o b t a i n e x p e r i m e n t a l l y observed conversion r a t e s . Reaction r a t e
and product r a t i o s were i n v e s t i g a t e d a s a f u n c t i o n of t h e a c t i v e
s i t e d e n s i t y a t 1073 K with a methane t o oxygen r a t i o of 1 0 ( F i g .
2
When t h e Os c o n c e n t r a t i o n and i s high ( i . e . Os = 10- t o
methane conversion i s high and o x i d a t i o n t o CO i s t h e dominant
p r o c e s s e s . A t lower s u r f a c e s i t e c o n c e n t r a t i o n s (4, <
the
conversion r a t e i s lower and t h e C2 s e l e c t i v i t y i s h i g h e r . The
2).
h i g h e s t C2 y i e l d was found t o b e @ s = lo-’.
These optimized
independent parameters t h a t a f f e c t t h e product s e l e c t i v i t y were
used for a l l subsequent c a l c u l a t i o n s
(OS
=
lo-’ and an a c t i v a t i o n
energy (E,) for t h e r a t e determining a b s t r a c t i o n s t e p ( r e a c t i o n
4a) of 6 3 . 6 k J mol-l) .
W
e used t h e Chemkin k i n e t i c modeling program t o s o l v e t h e s e t
of non-linear d i f f e r e n t i a l e q u a t i o n s . I n l i n k i n g t h e
heterogeneous r e a c t i o n s t o t h e homogeneous r e a c t i o n network, we
used a c o n s t a n t s u r f a c e t o volume r a t i o and c a l c u l a t e d t h e s u r f a c e
s i t e concentration.
The f r a c t i o n of a c t i v e c e n t e r s on t h e s u r f a c e
of s u r f a c e oxide c a t i o n s .
r e a c t i o n i s not s u r f a c e t r a n s p o r t l i m i t e d i n o u r model
calculations.
( @ s ) was normalized t o t h e amount
The
REACTIVE CENTER CONCENTRATION (ML)
Fig. 2. Effect of reactive oxygen center surface concentralion on melhane conversion and higher
hydrocarbon selectivity at 1073 K vrilh 0.3 atrn methane and 0.03 atm oxygen.
410
RESULTS
Once t h e heterogeneous parameters were e s t a b l i s h e d , t h e
temporal c o n c e n t r a t i o n s of co-oxidation products were determined
f o r various reaction conditions
.
The r e s u l t s o b t a i n e d a t 1 0 7 3 K
with CH4 and O2 c o n c e n t r a t i o n s of 0 . 3 and 0 . 0 3 atm., r e s p e c t i v e l y
( F i g 3 ) , show t h a t ethane i s t h e major carbon c o n t a i n i n g product,
followed by CO and e t h y l e n e . Other s i g n i f i c a n t p r o d u c t s a r e
methanol and formaldehyde, which d e c r e a s e i n r e l a t i v e importance
w i t h increased reaction t i m e .
The r e l a t i v e importance of s e v e r a l
gas phase r e a c t i o n s a t v a r i e s with i n i t i a l p a r t i a l p r e s s u r e s and
r e a c t i o n time (Table 1 ) . A t low p r e s s u r e t h e main source of
methyl r a d i c a l s i s t h e heterogeneous a c t i v a t i o n s t e p , while a t
high p r e s s u r e two a d d i t i o n a l gas phase s o u r c e s of methyl r a d i c a l s
a r e r e a c t i o n s i n v o l v i n g t h e hydrogen and hydroxyl r a d i c a l s .
Higher hydrocarbon s e l e c t i v i t y i n t h e methane coupling
p r o c e s s i s very dependent on oxygen p a r t i a l p r e s s u r e . T h e e f f e c t
oxygen p a r t i a l p r e s s u r e on methane conversion and product
s e l e c t i v i t y was s y s t e m a t i c a l l y examined ( F i g . 4 ) f o r f i x e d methane
9
.1 L
CONTACTTIME($1
Fig. 3. Calculated product distribution vs. contact time for methane coupling at 1073 K with 0.3 atrn
methane and 0.03 atm oxygen.
-
TABLE 1
Reaction Rates for T
1.Oe-5
-
1073 K, PCH4
CH3+02-CH302
CH302=CH3+02
CH4+MEOS-CH3+MEOHS
2.05E-06
MEOHS+MEOHS-H20+MES+MEOS
CH3+CH3-C2H6
MES+MES+OZ-MEOStMEOS
CH4+OH-CH3+H20
CH3+02-CH20+OH
CH3+H02-CH30+OH
9.6E-07
CH30+02-CH20+H02
CH30+CH4-CH3+CH30H
CH4+H-CH3+H2
CH3+MEOS-CH3MOS
CH3MOStMEOS-CHZO+MEOHS+MES
HCO+02-H02+CO
CH2O+MEOS-HCWMEOHS
CH4+H02-CH3+H202
CH3+H202-H02+CH4
C2H5-C2H4+H
CHZO+CH3=HCO+CH4
HCO+M=H+CO+M
CZH6+MEOS-C2H5+MEOHS
CH302+CH3-CH30+CH30
CH3O+M-CHZO+H+M
C2H6+CH3-CZHS+CH4
C2H5+02-C2H4+H02
MEOS+MEOS-MES+MES+02
-
411
- 0.3 atrn, PO2 - 0.03, Phi -
Eak
Bate
2.175E-05
CH4+H-CH3+H2
2.155E-05
CH4+MEOS-CH3+MEOHS
1.964E-05MEOHS+MEOHS-H20+MES+MEOS
3,~ O E - O ~
3.04E-06
1.04BE-05
C2H5-C2H4+H
9.74E-06
CH3+HZ-CH4+H
5.673-06
CH3+CH3-C2H6
2.84E-06
HCO+M-H+CO+M
1.47E-06
CZH6+H-C2H5+H2
1.46E-06MES+MES+02-MEOS+MEOS
2.01E-06
1.98E-06
1.94E-06
1.41E-06
1.13E-06
B.4E-07
7.6E-07
7.3E-07
6.73-07
6.7E-07
5.OE-0 7
3.9E-07
3.6E-07
3.2E-07
3. OE-07
2.9E-07
2.6E-07
2.2E-07
2.OE-07
1. CIE-O~
1.4E-07
1.2E-07
1.1E-07
5,BE-07
5.7E-07
5.5E-07
5.3E-07
4.4E-07
4.2E-07
4.OE-07
3.4E-07
3.1E-07
2.9E-07
2.7E-07
2.4E-07
1.9E-07
1.9E-07
1.9E-07
1.8E-07
1.7E-07
1.5E-07
Fig. 4. Methane coupling conversion and higher hydrocarbon selectivity vs. oxygen partial pressure at
1073 K with 0.3 atm methane.
412
p a r t i a l p r e s s u r e ( 0 . 3 atm) and f i x e d a c t i v e s i t e c o n c e n t r a t i o n ( $ s
=
The methane conversion i n c r e a s e d , t h e C2+ s e l e c t i v i t y
decreased, w h i l e t h e e t h y l e n e t o ethane r a t i o i n c r e a s e d t o a
c o n s t a n t l e v e l w i t h i n c r e a s i n g oxygen p a r t i a l p r e s s u r e . S e v e r a l
homogeneous methyl r a d i c a l o x i d a t i o n pathways a r e i m p o r t a n t .
Under high temperature and low p r e s s u r e c o n d i t i o n s t h e r e a c t i o n of
methyl r a d i c a l s w i t h hydrogen peroxy r a d i c a l s i s t h e prominent
o x i d a t i o n pathway,
*OCH3
+ *OH
'(6)
'02H ===='
w h i l e a t high p r e s s u r e and low temperature t h e r e a c t i o n of methyl
+
*CH3
r a d i c a l s w i t h methyl peroxy r a d i c a l s i s prominent. The major
s o u r c e s of -O2H a r e hydrogen a b s t r a c t i o n r e a c t i o n s of u n s t a b l e
r a d i c a l s such a s -CHO and *C2H5 with diatomic oxygen.
Conversion w i t h
Although a l k a l i promoted c a t a l y s t s have g r e a t e r s e l e c t i v i t y
t h a n unpromoted a l k a l i n e e a r t h and r a r e e a r t h oxides, t h e r e i s
some concern about s t a b i l i t y of t h e s e c a t a l y s t s given t h e high
vapor p r e s s u r e s of a l k a l i under r e a c t i o n c o n d i t i o n s . The
v o l a t i l i t y of a l k a l i under t y p i c a l a l k a n e a c t i v a t i o n c o n d i t i o n s i s
due t o t h e high vapor p r e s s u r e of t h e a l k a l i hydroxide molecules
i n t h e presence of steam and oxygen, although t h e s o l i d phase i s
l i k e l y t o be a l k a l i c a r b o n a t e . Thermochemically s t a b l e c a r b o n a t e
s a l t s w i t h t h e low v o l a t i l i t y i n steam are S r C 0 3 and BaC03.
Perovskite-supported a l k a l i n e e a r t h c a r b o n a t e s , Sr/SrZrOg and
Ba/SrZr03 are s e l e c t i v e and s t a b l e methane a c t i v a t i o n c a t a l y s t s
( F i g u r e 51, comparable or s u p e r i o r i n t h i s r e s p e c t t o Li-promoted
MgO and Na-promoted CaO.
Good ethene s e l e c t i v i t y was a l s o shown
f o r co-oxidative dehydrogenation of e t h a n e (Figure 6 ) . Unlike t h e
alkali-promoted a l k a l i n e e a r t h c a t a l y s t s , SrZrO o p e r a t e d 20 hours
3
a t 1 1 7 3 K with no evidence of evaporation o r c o r r o s i v e a t t a c k on
our q u a r t z r e a c t o r s . These c a t a l y s t s appear t o achieve
hydrocarbon s e l e c t i v i t y approaching t h e o r e t i c a l y i e l d s based on
l a b o r a t o r y r e a c t i o n c o n d i t i o n s and p r e d i c t e d homogeneous o x i d a t i o n
rates.
DISCUSSION
As a r e s u l t of o u r a n a l y s i s with t h e heterogeneoushomogeneous model, we conclude t h a t methane co-ox coupling
p r o c e s s e s with premixed oxygen and methane may be l i m i t e d t o a
413
Fig. 5. Methane conversion and Cp selectivity for SrZQ versus reaction temperature. Conditions: 0.29atm CH4, 0.029-atm Q,3.3 mL s-1 (NTP) flow at 1-atm pressure.
60
-
Elhene
Selectlvlty
/
40-
20
-
0,
L
800
900
TEMPERATURE (K)
Fig. 6. Ethane partial oxidation on SrZrO3 with 0.03 atm oxygen.
1000
414
maximum h i g h e r hydrocarbon y i e l d of about 30 mol% (carbon b a s i s )
and a maximum ethene/ethane r a t i o of about 2 . C a t a l y s t s t h a t can
f a v o r a b l y i n f l u e n c e t h e s e l e c t i v i t y by combining high t u r n o v e r
r a t e s f o r a l k y l r a d i c a l g e n e r a t i o n p e r r e a c t i v e s i t e with a very
low s u r f a c e c o n c e n t r a t i o n of r e a c t i v e c e n t e r s ( o p t i m a l l y one p a r t
p e r l o 5 s i t e s ) . Formation of a s u r f a c e o r bulk b a s e metal
carbonate l a y e r s may be one way of reducing t h e d e n s i t y of
r e a c t i v e oxygen c e n t e r s t o l e v e l s t h a t avoid t h e r a p i d o x i d a t i o n
of a l k y l r a d i c a l s a t t h e c a t a l y s t s u r f a c e , and indeed o u r
temperature programmed d e s o r p t i o n experiments show t h a t t h e
s u r f a c e s of t h o s e s o l i d - s t a t e b a s i c oxide c a t a l y s t s h i g h l y
s e l e c t i v e f o r l i g h t alkane dehydrogenation and methane coupling
a r e c o n s i s t e n t l y covered with a t l e a s t one monolayer of c a r b o n a t e .
Homogeneous r e a c t i o n s can f u l l y account f o r t h e p r o d u c t i o n of
hydrocarbon p r o d u c t s and t h e s e l e c t i v i t y between coupling p r o d u c t s
and COX, b u t p r e d i c t h i g h e r y i e l d s of CH30H, CH20, and H 2 , and
g r e a t e r CO/C02 product r a t i o s t h a n observed e x p e r i m e n t a l l y .
Heterogeneous o x i d a t i o n r e a c t i o n s a r e e v i d e n t l y r e s p o n s i b l e f o r
s i g n i f i c a n t o x i d a t i o n of oxygenate products, t h e s u b s t a n t i a l
conversion of product hydrocarbons o r r a d i c a l s , and p o s s i b l y
d i r e c t o x i d a t i o n of e t h y l e n e t o COP and H 2 0 i n co-oxidation
p r o c e s s e s by c a t a l y s t s with poor i n h e r e n t s e l e c t i v i t y .
ACKNOWLEDGEMENT
The a u t h o r s g r a t e f u l l y acknowledge t h e support of t h e Methane
Reaction Science Program d i r e c t e d by Dr. Daniel A . S c a r p i e l l o f o r
t h e Gas Research I n s t i t u t e and a s s o c i a t e d I n d u s t r i a l A f f i l i a t e cosponsors.
REFERENCES
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10.
C . , ( t o be p u b l i s h e d ) .
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( 1 9 8 8 ) 110, 5 2 2 6 .
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K . , Proceedinas of t h e
-,
2, p . 930, P h i l l i p s , M . J . , a n d T e r n a n , M . , e d s . ,
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a n d F u e l s (1988) 2, 574.
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e d . , Chpt. 5, p . 1 9 7 , S p r i n g e r - V e r l a g ,
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C a t a l . ( 1 9 8 7 ) 35, 1 3 9 .
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:
.
.
G. Centi and F. Trifiro’ (Editors), New Developments in Selective Oxidation
1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
General Mechanism for the oxidative couvlina oi methang
O.Foclani, U.Lupieri, V.Piccoli, S.Rossini* and D.Banfilippo
Snamprogetti, S.Donato Milanese, 20097 Italy
J.A.Dumesic
Dep.Chem.Engineering, University of Wisconsin, Madison WI
53706
L.A.Aparicio, J.A.Rekoske and A.A.Trevino
Shanahan Valley Associates, Madison, WI 53711 USA
SUMMARY
A detailed mechanism, composed of 27 reactions, for the
oxidative coupling of methane is described. The main
products may derive either from a surface route or from a
gas phase pathway. The kinetic parameters of the model,
handled by a computer program, have been calculated from
general chemical laws. The proposed mechanism has been
calibrated on the data of the Li/MgO catalyst, studied by
Lunsford and co-workers. The key steps are discussed in
details and the fair good agreement between calculated and
experimental data is given.
1. INTRODUCTION.
The huge availability and the low price of methane have
led researchers in recent years to look for a route more
direct than present technologies for the conversion of
methane to more valuable chemicals The promising advances,
mainly obtained with oxide catalysts, have been reviewed by
several authors (1)- ( 8 )
In general, the extensive
literature on the subject deals with two main processes:
a) Direct methanol synthesis, catalyzed by
oxides of
altervalent metals;
b) Oxidative coupling (C2 hydrocarbon synthesis), carried
out in one of the two ways:
1.) with methane and oxygen co-fed to the reactor,
catalysed by basic oxides(alka1ine earth usually
doped with alkali metals) and by rare earth oxides :
2.) with methane and oxygen alternatively fed to the
reactor, catalyzed by low melting metal oxides.
.
.
417
418
Recognising the importance of such studies, we have begun a
research program to investigate processes of type b.1).
We are using two complementary approaches in parallel:
i) The synthesis and testing of new catalysts:
ii) A semi-theoretical approach which combines experimental
catalytic behaviour with surface science and general
knowledge to guide new preparations.
This paper deals with point ii).
2.
AIM OF THE WORK
The aim of our effort is to describe a catalytic reaction
with a detailed kinetic mechanism that is consistent with
intermediates identified spectroscopically and other inhouse or literature observations. Every step is characterised by its rates, forward and reverse, given by the
general equation (2.1) :
x
=
Cj =
for, rev:
?r
=
product:
p
=
reaction order
gas phase concentration or surface coverage.
Starting from two basic equations, Arrhenius Law ( 2 . 2 ) ,
allowing the calculation of the rate constant k, and Polanyi
Law (2.3) , giving the activation energy DEatt, we have
evaluated *la priori In all the kinetic constants, except a
few experimentally available [(#1.14)- (#1.18)],
DEo
=
Constant term:
Q
=
Polanyi constant.
419
trough heats of formation of every fragment involved in the
mechanism, its bond strength with the surface and well known
theories for preexponential factors A, such as the collision
theory or the transition state theory. In this way we may
discuss the rate determining steps in terms of surface
chemistry and consequently tailor the catalyst that fulfills
the model suggestions at best. This is actually our ultimate
goal.
Naturally, the parameters cannot be calculated perfectly
as a first temptative value, but usually they require a
little adjustment (few Xcal/mol). So we have calibrated such
parameters with literature data measured on a single Li/MgO
catalyst over a wide range of conditions by Lunsford and
co-workers(9). From this point on, we are developing the
same approach to our own catalysts trying to correlate
intimate properties and reactivity.
The whole set of reactions is managed by a computer program.
3. MECHANISM
It is generally accepted that C2 hydrocarbons are formed
by a coupling of CH3(.) radicals; in particular C2H6 is obtained as primary product and C2H4(9-a) derives from a
dehydrogenative step of the saturated hydrocarbons.
The presence of CH3(.) radicals has been demonstrated with
Li/MgO catalyst by Lunsford et al.(g*b), while it is still
questionable if the coupling takes place in the gas phase or
on the surface.
We are convinced that a particular form of oxygen (Oact)
is responsible for the homolytic cleavage of the C-H bond:
following the suggestion of some authors, Oact may be
defined as O', in Li/MgO and similar catalysts due to the
formation of [M+O'] centers(9) as well as in other completely
different catalysts(lOtll), or 02- as proposed for lanthanum
oxide (12)
.
420
A bit more ambiguous is the genesis of carbon oxides:
three main paths can be envisioned(9.a):
> C2H6 > C2H4 > cox
a) 2 CH3(.)
b) CH3(.) + O(a-)Surf -> OCH3-> cox
~
C) CH3(.)
+
02
->
CH3O2(.)
> cox
(3.1)
(3.2)
(3.3)
We have tried to consider all these routes and we aim to
differentiate them on the basis of the reaction rate values.
We have set up the mechanism described in Scheme #1 from
this whole of considerations.
4.
RESULTS
The simulations of the data of Lunsford et al. (g-a) are
shown in Figures n.1-4. The agreement is satisfactory; the
model fits the behaviour of the catalyst in a fair good way,
except when considering the partial pressure of oxygen (see
figure n. 4). This will be discussed in the next point.
02
>
2 CH3(-)
CH3(.) +
CH302 ( *
2 MOact
> CH3MO
> C2H6
> CH3CHzOM
> C2H4
> MOCHzOM
> MOCH2OM
> MOCHO
> co
> CO2
> H20
> CH3(.)
> C2H6
> CH302 ( *
CHzO
+
+ 2 M
02
+ 2 M
CH4
+ MO +
2 CH3MO
C2H6 + MO +
CH3CH2OM
+
+
CH3CH2OM
CH30M + MO +
MOCHzOM +
+
MOCHO
MOCHO
+
2 MOH
CH3MO
2 MO
I
MOact
MOact
MO
2 MO
MOact
MO
MO
MO
02
+
+
+
+
+
+
+
+
+
+
+
MOH
2 MO
MOH
MO +
CH3MO
MOH
MOH +
MOH +
MOH +
MO +
MO
OH(.)
MOH
M
MO
M
M
421
CH2O
CHO(.)
CH3O2(.)
MO2CH3
CH2O
MOCH2
HO(.)
CHO(.)
HO2(.)
MO2H
co
+
+
OH(.)
02
+ M
+M
+ M
+ MO
+M
+ MO
+ M
+ M
+ MO
>
>
>
>
>
>
>
>
>
>
>
CHO(.)
+
co
+ HO2(.)
H20
MO2CH3
MO
+ CH3MO
MOCH2
MOCH2OM
MOH
co
+ MOH
MO2H
MO
+ MOH
C02
+ M
Scheme #1
5.
DISCUSSION AND COMMENT
The reaction mechanism may be divided into two main sect ions :
1) Surface Reactions; [Reactions from (#l.l) to (#1.12)]
2) Gas Phase Reactions; [Reactions from (#1.13) to (#1.27)]
The main reaction worth discussing is reaction (#1.2), in
which Oact species are formed. Molecular orbital calculation
have shown that the edge of the valence band in MgO is
composed by orbitals that are mainly 2p-oxygen in character(l3) .Hence, one can view the formation of 0- species as a
process in which holes are generated in the valence band of
MgO. The creation of a hole is an endothermic process, and
this would be in good agreement with the fact that 0species are usually observed experimentally only at high
temperature. The creation of holes in the valence band of
MgO can be envisioned in two ways:
1) through the transfer of an electron from the valence band
to an acceptor level within the band gap. The acceptor
level could be due to the presence of a doping agent or
it could be a Schottky defect (a cation vacancy);
2) through the generation of ionized Schottky defects by
422
C C2H6
0
C2h4
Model
Figure n.1 Methane conversion
YS.
Contact Time; T=620°CjInlet
Press (Torr): CH4=300 ,02=60,
He=4O.
Catalyst = lg
Figure n.3 Methane conversion
v s . Methane pressure3 T= 62OOC
Flow=.83 ml/sj Inlet Pressure:
(torr) 02=60, total=760
Catalyst = lg
(1)
-
(3)
Lines
Figure n.2 Methane conversion
vs. temperaturej Flow=.83ml/sj
Inlet P r e s s (Torr): CH4=300,
02=60, fle=4O. Catalyst= lg
Figure n.4 Methane conversion
v s . Oxygen Pressure; T=6Z0°C
Flow= .83ml/s; Inlet Pressure
(Torr): CH4=300, total=760
lg
Catalyst
:
423
oxygen from the gas phase.
A little of mathematics and some hypokheses allowed us to
predict the formations of Oact aacording to these ideas.
Steps (#1.3) and (Y1.5) arp similar hydrocarbon adsorption reactions that form surface OR species through the
cleavage of a C-H bond by Oact. Oact is involved also in
step (#1.8) where another C-H bond is broken to form the
surface COX precursor.
Ethane is produced by either the coupling of two surface
OCH3 species (#1.4) or by the coupling of two gas phase
CH3(.) radicals (#1.14). Ethylene derives only from a
dehydrogenation (#1.6) of a readsorbed ethane molecule, this
way being in competition with the cleavage of a C-C bond
leading to total combustion.
A gas phase CH3 (.) in the mechanism can combine with 02
to eventually yield CO [from shep (11.15) to (#1.18)] and to
C02 (#1.27) through some possibJe interaction with the solid
[Step (#1.19) to (#1.26)].
The discrepancies between experimental and calculated
data as function of oxygen partial pressure (v.Figure n.4)
are probably due to an overestimation of oxygen-surface bond
so that, at relatively high oxygen pressure, the model
predicts an high oxygen coverage. The OCH3 coverage is
forced to almost zero and the total activity declines
although the product distribution is preserved. We have been
able to better follow the trend at high oxygen pressure, but
doing so we were missing the Iquite characteristic and significant maximum in C2 products at about 5 0 torr of oxygen.
Under the low converbion conditions studied by Lunsford
and co-workers, the m o d d predjc€k that:
a) the coupling of gas phase CH3(.) radicals is negligible
if compared to the coupling of OCH3 species on the
surface:
b) the dehydrogenation of ethane is orders of magnitude
faster than the C-C bond cleavage i.e. the way (3.1) is
completely unsignificant at these levels of conversion;
424
c) the main route (ca.70%) to COX is the gas phase radical
cha$n via methylperoxide while the surface combustion
contributes for the remaining 30%.
The quite satisfactory agreement may be expressed in these
following points:
1) At 620-C and low conversion, the main products are C2H6
and C02; the selectivities and yields of C2 hydrocarbons
reach a maximum at low oxygen pressure; both the
production of C2 and COX have reaction orders with
respect to methane pressure less than one, the latter
being
the minor. This makes C2 selectivity always
increase with methane pressure although C2 yield reaches
a maximum.
2) At 720*C,themain C2 product becomes C2H4 instead of C2H6
3) In the temperature range 550-675-C and low conversion
conditions, the selectivity and yield of C2 products
increase with temperature.
4) Under none of the conditions studied by Lunsford et al.
formaldehyde is a significant product.
6. CONCLUSIONS
The mechanism set up to simulate the methane oxidative
coupling is quite satisfactory considering the many experimental values employed in the calculation. We believe that
our model, now that it has been calibrated with literature
data, will become a powerful tool in tailoring new catalyst
formulation. Preliminary results are confirming this feeling.
7. REFERENCES
(1)
(2)
(3)
Grigoryan E.A.; Russ.Chem.Rew.; 53(2) 210-220 (1984)
Foster N.R.; Appl.Catal., 19, 1-11 (1985)
Gesser H.D. and Hunter N.R.; Chem.Rew., 8 5 ( 4 ) 235-244
(4)
Pitchain R. and Klier K.; Catal.Rev.-Sci.Eng., 28(1),
13-88 (1986)
Mimoun H.; New Jour.Chem., 11(7), 513-525 (1987)
(5)
(1985)
425
(6)
(7)
Scurrell M.S.; Appl.Catal., 32, 1-22 (1987)
Lee J.S. and oyama T.S.; Catal.Rev.-Sci.Enq., 30(2),
(8)
Baerns M., van der wiele K. and Ross J . R ;
249-280 (1988)
4, 471-494 (1989)
Cat.Today,
'
(9) a. Ito T., Wang J., Lin C . and Lunsford J.H.; J.Am.
Chem.Soc., 107, 5062-5068 (1985)
b. Driscoll D.J., Martin W., Wang J. and Lunsford J.H.;
J.Am.Chem.Soc., 107, 58-63 (1.985)
c. Lin C., Ito T., Wang J., and Lunsford J.H.; J.Am.
Chem.Soc., 109, 4808-4810 (1987)
(10) a. Hill W., Shelimov B.N. and Kazansky V.B; J.Chem.Soc.
Faraday Trans., 1, 83, 2381-2389 (1987)
(11) b. Kaliaguine S.L., Shelimov B.N. and Kazansky V.B;
J.Catal., 55, 384-393 (1978)
(12)
Wang J. and Lunsford J.H.; J.Phys.Chem., 90, 3890(13)
3891 (1986)
Mehandru S.P., Anderson A.B. and Bradzil J.F.; J.Am.
Chem.Soc. , 110, 1715 (1988)
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
THE MECHANISM OF THE OXIDATIVE COUPLING OF METHANE:
ETHYLENE IS A PRIMARY PRODUCT
421
EVIDENCE THAT
GEORGE W. KEULKS and MIN YU
Laboratory for Surface Studies, University of Wisconsin-Milwaukee,
P.O. Box 340, Milwaukee, Wisconsin, 53201, USA
SUMMARY
The oxidative coupling of methane was studied over a MgO
supported Bi-P-K oxide catalyst. When CD was substituted for
CH4, the reaction exhibited a kinetic iso#ope effect as expected,
but the D-distribution in ethylene could not be explained by
assuming the sole pathway to ethylene was via ethane. The results
suggest that ethylene can be produced as a primary product as well
as a secondary product.
INTRODUCTION
The oxidative coupling of methane to ethylene and ethane has
generated considerable interest, following the published report by
Keller and Bhasin [ref. 1 1 in 1982. Most of the work has been
directed toward the identification of selective catalysts. A wide
variety of oxides now have been reported to be active and
selective for this reaction. Alkali, alkaline earth, and rare
earth oxides have shown good activity-selectivity behavior for the
formation of C2 hydrocarbons [ref. 21.
The mechanistic details of the reaction, on the other hand,
have received considerably less attention. Ito et.al. [ref. 31
detected CH3 radicals in the gas phase over lithium-promoted
magnesium oxide. They suggested that the formation of ethane
involves the coupling of 2 CH3 radicals in the gas phase.
ARC0 workers [ref. 4 , 5 ] also suggested that ethane is formed
via the gas phase coupling of CH3 radicals. In addition, they
suggested that ethylene is formed from ethane via an oxidative
dehydrogenation process on the surface of the catalyst.
The rate limiting step of the reaction appears to be hydrogen
atom abstraction from CH4 to form a CH3 radical. This is
supported by the observation of a kinetic isotope effect [ref. 61.
The use of CD4 also allows one to examine whether or not the
isotopic composition of ethane is consistent with the coupling of
two CH3 radicals. Nelson et.al. [ref. 7 1 reported that only C2H6,
CD3CH3, and C2D6 were formed when a mixture of CD4 and CH4 was
passed over lithium-promoted magnesium oxide. They also found
428
CD2CH2, a n d C 2 D 4 i n t h e e t h y l e n e p r o d u c t .
This is
c o n s i s t e n t w i t h t h e s u g g e s t i o n t h a t e t h y l e n e forms f r o m e t h a n e a n d
o n l y C2H4,
n o t as a p r i m a r y p r o d u c t o f t h e r e a c t i o n .
W e have been i n t r i q u e d a b o u t t h e p o s s i b i l i t y t h a t e t h y l e n e can
form as a p r i m a r y p r o d u c t i n t h e r e a c t i o n , n o t s o l e l y as a
secondary product.
D u r i n g t h e s u r f a c e i n i t i a t i o n of CH4 t o form
i t i s c o n c e i v a b l e t h a t some CH3 r a d i c a l s r e a c t
I n f a c t , workers [ r e f . 8,9] have
CH3 r a d i c a l s ,
f u r t h e r with t h e surface.
s u g g e s t e d t h a t s u c h a pathway may l e a d c o m p l e t e c o m b u s t i o n .
The
f o r m a t i o n of s u r f a c e s p e c i e s s u c h as C H 2 , CH, C , however, would
p r o v i d e t h e o p p o r t u n i t y f o r a d i r e c t pathway t o e t h y l e n e .
(:CH2)
Carbene
s p e c i e s have been proposed t o be i n v o l v e d i n t h e f o r m a t i o n
of h i g h e r h y d r o c a r b o n s [ r e f . 101 as w e l l a s t h e improvement i n
ethylene s e l e c t i v i t y a t higher temperatures [ r e f . 111.
I n t h i s work, w e h a v e f u r t h e r e x a m i n e d t h e p o s s i b i l i t y t h a t
e t h y l e n e is formed as a p r i m a r y p r o d u c t i n t h e o x i d a t i v e c o u p l i n g
of methane.
EXPERIMENTAL
Catalyst
W e p r e v i o u s l y r e p o r t e d [ r e f . 1 2 1 t h a t a MgO s u p p o r t e d Bi-P-K
o x i d e c a t a l y s t w a s a c t i v e and s e l e c t i v e f o r t h e o x i d a t i v e c o u p l i n g
of m e t h a n e .
The d e t a i l s o f t h e c a t a l y s t p r e p a r a t i o n were
described earlier.
Reaction Studies
A l l e x p e r i m e n t s were c o n d u c t e d i n a s i n g l e - p a s s
a t atmospheric pressure.
The f e e d g a s e s , methane
f l o w reactor
( M a t h e s o n , CP),
oxygen ( A m e r i g a s , 9 9 % ) , h e l i u m ( A m e r i g a s , 9 9 . 9 % ) , w e r e u s e d
without f u r t h e r purification.
The i n d i v i d u a l g a s f e e d rates w e r e
c o n t r o l l e d by T y l a n mass f l o w c o n t r o l l e r s (Model FC-260),
except
f o r C2D4 and C2D6 f e e d r a t e s , which w e r e c o n t r o l l e d by a
motor-driven,
s y r i n g e pump ( S a g e I n s t r u m e n t s , Model 3 4 1 ) , e q u i p p e d
w i t h a 10 c m 3 Hamilton g a s - t i g h t s y r i n g e .
The i s o t o p i c a l l y l a b e l l e d gases u s e d i n t h i s s t u d y w e r e :
(99.6 a t % D ) , C2D6
( 9 9 . 5 a t % D).
Canada L t d . ,
(99.5 a t % D ) , C2D4
( 9 9 . 2 a t % D ) , a n d D2
A l l were p u r c h a s e d from Merck,
CD4
S h a r p , a n d Dohme,
M o n t r e a l , Canada.
The r e a c t i o n g a s m i x t u r e of m e t h a n e a n d o x y g e n , d i l u t e d w i t h
helium t o a c h i e v e a t o t a l p r e s s u r e of 1 a t m . ,
was passed over t h e
c a t a l y s t while heating t o t h e desired temperature.
On-line
429
analysis of the effluent gas was achieved by gas chromatography,
using Porapak Q and molecular sieve 5A columns.
For analysis of the isotopic distribution in the products, the
effulent gas was first allowed to pass through a 5 cm3 sampling
trap for 2 min. The 5 cm3 sample was then injected into the gas
chromatograph. Separation of methane, ethane, and ethylene was
achieved by using a 4' x 0 . 2 5 " (O.D.) Porapak Q (80-100 mesh)
column at room temperature. The separated products were collected
in individual traps that could be used for subsequent mass
spectrometric analysis.
Before introducing the trapped product into the mass
spectrometer, each trap was evacuated at -196°C for 5 min.
Ethylene and methane were analyzed at 13 e.v. and 15 e.v.,
respectively, in order to minimize fragmentation. In the case of
ethane, however, the ionization potential of the molecular ion
does not differ significantly from the appearance potential of the
ethylene ion. Hence, elimination of fragment peaks was not
possible and mass spectra were obtained at 70 e-v., utilizing
calibrations with C2H6 and C2D6. The fragmentation patterns for
C2H5D, C2H4D2, C2H3D3, C2H2D4, and C2HD5 were calculated from the
calibration data using the method suggested by Amenomiya and
Pottie [ref. 131.
RESULTS
If the rate-determining step is the breaking of a C-H bond in
methane, then a kinetic isotope effect should be observed when CD4
is substituted €or CH4. The conversion of both CD4 and CH4 were
determined under the following conditions; T=650 'C and 700'C;
CH4/02 = 8/1; W/F= 0.06 gm-sec/ml. Under these conditions, the C2
selectivity was 39.1% (650'C, CH4), 56.8% (650 "C, CD4), 28.6%
(700"C, CD4), 50.6% (7OO0C, CD4).
Because the experimental conditions are the same for both
reactions, a kinetic isotope effect (kH/kD) can be calculated for
the formation of ethane, ethylene, and C02 by using the relative
product yields. The results are summarized in Table 1. The rate
of formation of ethane exhibits a kinetic isotope effect, but no
isotope effect is observed for the formation rates of ethylene and
C02. The observed isotope effect is in good agreement with
molecular data calculations [ref. 141.
430
TABLE 1.
Kinetic Isotope Effect
Temp. ,
k ~ / k ~
Ethane
Ethylene
"C
650
0.96
1.04
700
c02
1.00
0.95
1.75
1.22
The C2 products, ethane and ethylene, as well as the
recovered, unreacted methane, were analyzed by mass spectrometry.
The concentration for each isotopic component was obtained by
=
normalizing each compound to itself, e.g., % C2H4
- .
C2H4/(C2H4+C2H3D+C2H2D2+C2H3D+C2D4). The results are summarized
in Table 2 .
TABLE 2 .
Temp. ,
"C
650
700
Isotopic Distribution for Reaction of CD.+O4
Etffane
Methane
Ethylene
dO dl d2 d3 d4
dO dl d2 d3 d4
dO dl d 2 d 4 d5 d6
- - - 1 9 9 3 5 3 5 - 2 28 - - - - - - _9 5- - - 1 9 9 1 7 1 7 - 2 64 - - - - - - 95
~~
The surprisingly high amount of H in ethylene caused concern
that ethylene was undergoing an exchange reaction with a H-source.
We examined several possible exchange reactions. The reactions
studied are summarized in Table 3.
TABLE 3. *Possible Exchange Reactions
Mixture
Temp.
Ethylene
"C
dO dl d2 d3 d4
650
- - - 3 97
650
- - - 2 98
2
- - 3 95
650
- - - - - - _
- - - - - - 99
- -
-
-
-
-
-
96
C2D4 start ng material was 98% d4, 2% d3; total D
C2D6 start ng material was 99% d6.
=
650
*
Ethane
dO dl d m 4 d5 d6
4
-
-
2
94
992
-
-
-
99.5 at % .
We passed a C2D4/02 mixture over the catalyst at 650°C. Gas
chromatographic and mass spectrometric analyses indicated that the
products were C02, H20, and a trace of C2D3H. A mixture of
431
C2D6/02 produced traces of ethylene ( 9 8 % C2D4) and the unreacted
ethane showed no evidence of exchange.
We also studied the possibility of the exchange of C2D4 and
C2D6 with CH4. The ratio of CH4 to C2D4 or C2D6 was 11 to 1.
formation of C2H6 and C2H4 (experiments 3 and 4 in Table 3)
The
indicate that the CH4 reacted, as expected, but the C2D4 and C2D6
passed through the catalyst unchanged.
Having convinced ourselves that exchange reactions could not
explain the large amount of H-incorporation into ethylene, we
examined the D distribution obtained when a mixture of CH4 and CD4
was oxidized at 650°C.
Fig. 1.
The results are shown in Fig. 1.
The
Isotopic distribution of ethane, ethylene, and unreact d
methane for the reaction of CH4/CD4/02 at 65OOC.
detection of C2D6, CH3CD3, and C2H4 is consistent with the
suggested mechanism for the production of ethane via the coupling
of the methyl radicals.
The isotopically labelled ethylenes,
C2D4, C2H2D2, and C2H4, are the expected products resulting from
the consecutive oxidative dehydrogenation of the labelled ethane
species. However, the preponderance of C2H4 and C 2 H 3 D cannot be
explained by the consecutive reaction sequence, CH4 r C 2H6 *C2H4.
The most probable explanation is a parallel pathway involving the
reaction of a hydrogen-deficient, surface intermediate with a
hydrogen source on the catalyst.
Two additional experiments were conducted in an attempt to
432
g a i n a d d i t i o n a l e v i d e n c e f o r t h e f o r m a t i o n of e t h y l e n e as a
(1) a s t u d y of t h e H - d i s t r i b u t i o n i n e t h y l e n e a s
primary product:
a f u n c t i o n of m e t h a n e c o n v e r s i o n a n d ( 2 ) a s t u d y of H - d i s t r i b u t i o n
i n e t h y l e n e as a f u n c t i o n of s u r f a c e d e h y d r o x y l a t i o n .
The p r o d u c t i o n of e t h y l e n e v i a e t h a n e by a c o n s e c u t i v e
r e a c t i o n pathway i s o b v i o u s l y o p e r a t i v e ,
a s e v i d e n c e d by t h e
r e s u l t s o b t a i n e d from t h e r e a c t i o n of t h e m i x t u r e o f CD4 a n d C H 4 .
I f t h e c o n s e c u t i v e r e a c t i o n r a t e f o r e t h y l e n e p r o d u c t i o n is f a s t e r
than t h e parallel reaction rate f o r ethylene production, then t h e
D - d i s t r i b u t i o n i n e t h y l e n e s h o u l d be a f u n c t i o n of methane
c o n v e r s i o n . The r e a c t i o n of C D 4 / 0 2 a t 650 'C w a s s t u d i e d a t
v a r y i n g CD4 c o n v e r s i o n l e v e l s . The r e s u l t s are shown i n F i g . 2 .
R
-
a,
80-
V
c
a,
2 60.f
40-
I
E *O0
0
,
0
I
,
,
J
,
,
5
,
,
l
10
,
Conversion of Methane
,
,
(w)
,
I
Fig. 2 . A t o m 8 H i n e t h y l e n e f o r t h e r e a c t i o n o f CD4+02 a t 6 5 0 "C
a s a f u n c t i o n o f CD4 c o n v e r s i o n .
The a t 8 H i n e t h y l e n e d e c r e a s e s w i t h i n c r e a s i n g methane
conversion.
Thus, i f C2H4 and C2H3D are i n d i c a t o r s o f e t h y l e n e
p r o d u c t i o n v i a a p a r a l l e l p a t h w a y , and i f C 2 D 4 i s a n i n d i c a t o r of
e t h y l e n e p r o d u c t i o n v i a a c o n s e c u t i v e pathway, t h e n , a s shown i n
F i g . 2 , t h e a t % H i n e t h y l e n e s h o u l d decrease as methane
conversion increases.
Because p u r e CD4 i s u s e d a s t h e r e a c t a n t , t h e H-source must be
finite.
T h i s f i n i t e H-source
i s m o s t l i k e l y t o be s u r f a c e
h y d r o x y l g r o u p s on t h e c a t a l y s t .
I n an e f f o r t t o d e h y d r o x y l a t e
t h e c a t a l y s t s u r f a c e , we p r e t r e a t e d t h e c a t a l y s t a t i n c r e a s i n g l y
h i g h e r t e m p e r a t u r e s w i t h a f l o w of H e / 0 2 .
The r e s u l t s are
433
The decrease i n t h e H - c o n c e n t r a t i o n w i t h
p r e s e n t e d i n T a b l e 4.
TABLE 4 .
Pretreat
T ( C)
-
650
750
850
D - D i s t r i b u t i o n from CD4+02 a t 6 5 0 "C a s a F u n c t i o n of
Catalyst Pretreatment
Methane
Ethylene
Ethane
dO d l d2 d 3 d d
dO d l d2 d3 d4
dO d l d2 d 3 d4 d5 d6
- - - - - - - - - -
99
99
99
99
- - - -
4545 2 9
3 5 3 5 2 - 2 7
3 2 3 2 3 - 3 2
2 6 2 7 2 - 4 3
-
-
- - - -
-
-
-
- - - - - -
-
-
- - - - - -
95
95
95
95
higher c a t a l y s t pretreatment temperatures is consistent with t h e
a s s u m p t i o n t h a t t h e H-source i s t h e c a t a l y s t s u r f a c e h y d r o x y l
g r o u p s , which a r e removed a t e l e v a t e d t e m p e r a t u r e s .
W e have t r i e d r e h y d r o x y l a t i n g t h e s u r f a c e w i t h D 0 and
2
but l i t t l e D is detected i n the
subsequently r e a c t i n g CH4/02,
products.
A p p a r e n t l y , o n c e t h e s u r f a c e h y d r o x y l g r o u p s are
removed, t h e y a r e d i f f i c u l t t o r e p l a c e o r t h e r e a c t i o n of OD w i t h
t h e s u r f a c e i n t e r m e d i a t e i s c o n s i d e r a b l y slower t h a n t h e r e a c t i o n
w i t h OH.
DISCUSSION
The D - d i s t r i b u t i o n
i n e t h y l e n e i s b e s t e x p l a i n e d by a
mechanism t h a t allows f o r e t h y l e n e t o be p r o d u c e d by b o t h a
c o n s e c u t i v e pathway v i a e t h a n e and a p a r a l l e l pathway d i r e c t l y
from methane.
The p a r a l l e l pathway i n v o l v e s a H - d e f i c i e n t s u r f a c e
intermediate.
The c o n s e c u t i v e pathway i s t h e same as t h a t p r o p o s e d by o t h e r
w o r k e r s [ r e f . 3-51.
The r a t e - l i m i t i n g
s t e p i n the activation of
methane i s t h e h o m o l y t i c c l e a v a g e of a C-H
bond t o form C H 3
r a d i c a l s . While o u r e x p e r i m e n t s p r o v i d e no d i r e c t e v i d e n c e a s t o
t h e n a t u r e of t h e a c t i v e s i t e r e s p o n s i b l e f o r t h e a c t i v a t i o n of
m e t h a n e , a n i n c r e a s i n g body of e v i d e n c e [ r e f . 151 s u g g e s t s t h a t
s u r f a c e 0- s p e c i e s may b e t h e a c t i v e s i t e f o r methane a c t i v a t i o n .
Two CH3 r a d i c a l s combine i n t h e g a s p h a s e t o p r o d u c e e t h a n e as
a primary product.
The e t h a n e u n d e r g o e s a s u b s e q u e n t o x i d a t i v e
d e h y d r o g e n a t i o n by e i t h e r r e a c t i n g w i t h a c t i v e oxygen s p e c i e s on
t h e s u r f a c e o r i n t h e gas phase.
The p a r a l l e l pathway l e a d s t o e t h y l e n e as a p r i m a r y p r o d u c t ,
n o t a s e c o n d a r y p r o d u c t p r o d u c e d by f u r t h e r r e a c t i o n of e t h a n e .
s i t e o t h e r t h a n s u r f a c e 0- i s r e s p o n s i b l e f o r p r o d u c i n g a
A
434
H-deficient surface intermediate. The exact nature of the
H-deficient intermediate, responsible for the production of
ethylene, is still unclear. A carbene (:CH2) species would yield
C 2D4 when CD4 is used as the reactant, unless the carbene
intermediate undergoes rapid H-D exchange with the surface
hydroxyls. Other possibilities include CH and C that could react
with the surface hydroxyls to produce ethylene.
The results in Fig. 2 suggest that the formation of the
H-deficient surface intermediate is fast compared to the formation
of CH3 radicals. At low methane conversions, the ethylene
produced has a high at % of H and the ethylene/ethane ratio is
large. As the methane conversion increases, the ethylene/ethane
ratio and the at % of H in the ethylene decrease. This indicates
that a larger fraction of the ethylene is formed by the
consecutive pathway via ethane (C2D6).
CONCLUSIONS
The Bi-P system, which has been shown to be active and
selective for the oxidative dimerization of propylene, also is
active and selective for the oxidative coupling of methane. The
isotopic tracer results suggest that two sites exist on the
catalyst surface. One site is responsible for the formation of
methyl radicals from gas-phase methane. The methyl radicals
quickly dimerize to form ethane. Another site is responsible for
producing hydrogen deficient surface intermediates. These surface
intermediates produce ethylene as a primary product by reacting
with surface hydroxyl groups.
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2
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( 1 9 8 7 ) 1393.
11 C.H. L i n , K . D . C a m p b e l l , J . X . Wang, a n d J . H . L u n s f o r d , J. P h y s .
Chem., 90 ( 1 9 8 6 ) 534.
1 2 G.W. K e u l k s a n d M. Yu, R e a c t . K i n e t . C a t a l . L e t t . , 35, ( 1 9 8 7 )
361.
1 3 Y. Amenomiya a n d R . F . P o t t i e , Can. J . Chem., 46 ( 1 9 6 8 ) 1 7 4 1 .
1 4 2. M e l a n d e r , I s o t o p e E f f e c t s on R e a c t i o n R a t e s , R o n a l d P r e s s ,
N e w York, 1960, p p . 7-22.
1 5 J.H.
L u n s f o r d , Methane C o n v e r s i o n , E l s e v i e r , Amsterdam, 1 9 8 8 ,
p p . 359-371.
G. Centi and F. Trifiro‘ (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science PublishersL.V., Amsterdam - Printed in The Netherlands
437
HETXROLYTIC I ~ C U N I S MOF WTHANiS ACTIVATION IN OXIDATIVE
DSHYI)RODIIKERIZATION
V.D. SOKOLOVSKII, O.V. BUYEVSKAYA, S.M. ALIEV and A.A.
Institute of Catalysis, Novosibirsk 630090, USSR
DAVYL)OV
suPmA NY
The dependence of the rate of methane oxidative dehydrodimerization on oxides of alkaline earth metals on the concentration
of base sites has been studied. ‘The isotope CH4-CD4 exchange in
conditions of methane oxidative dimerization has been examined.
F I R spectroscopy data suggest the formation of metal-methyl
groups during methane adsorption on MgO. A heterolytic mechanism
of methane activation involving low-coordination surface sites
is proposed.
INT’HODUCTION
Oxidative dehydrodimerization of methane attracts attention
of many researchers as one of the most promising ways of production of ethylene from non-oil raw material. In recent years, considerable advances have been achieved in the development of catalysts f o r this reaction (refs. 1-3); however, the mechanism
and, primarily, the nature of methane activation on catalytic
surface remain open to discussion. Most popular is the hypothesis
of radical activation of methane on surface radical-ions of o x y gen put forward by Lansford and co-workers who studied the reaction with Li/hIIgO catalysts (ref. 4).
Recent studies of selective oxidative transformations of saturated and unsaturated hydrocarbons via the C-I1 bond on oxide casalysts have allowed us to propose an alternative mechanism implying a heterolytic activation of the C-H bond ( r e f . 5). Such
a mechanism does n o t require large concentrations of sites capable o f producing oxygen radical forms, which may lead to cotnplete oxidation (ref. 5).
The possibility of the heterolytic activation of methane in
oxidative dehydrodimerization has been mentioned in (refs. 6 , 7 ) .
In this work an attempt has been made to substantiate the
heterolytic mechanism of methane activation in oxidative dehydrodimerization with base catalysts.
438
EXPERIWNTAL
Samples of a l k a l i n e e a r t h metal oxides were obtained by c a l c i n a t i o n of n i t r a t e s (pure f o r a n a l y s i s grade) i n a i r a t 1173 K.
The c a t a l y t i c a c t i v i t y was measured i n a flow r e a c t o r , as described i n ( r e f . 6 ) . The r e a c t i o n mixture composition was 00% CH and
4
20% 02. The concentration of base s i t e s was determined by the ads o r p t i o n of benzoic a c i d ( r e f . 6 ) . I R s p e c t r a were r e g i s t e r e d a t
293 K on a Bruker-113 V PIIR spectrometer ( r e f . 8). The i s o t o p e
exchange was examined d i r e c t l y i n t h e course of o x i d a t i v e dehydrodimerization,the r e a c t i o n mixture composition being 45% CH
4’ 45%
CD and 10% 02.
4
USULTS AND DISCUSSION
A study o f t h e dependence of r a t e s of methane conversion and
C z products formation on concentration of base sites ( s e e Pig. 1)
has i n d i c a t e d t h a t both the t o t a l r e a c t i o n r a t e and t h e r a t e of
o x i d a t i v e dehydrodimerization o f methane tend t o i n c r e a s e with
increasing
concentration o f base s i t e s on c a t a l y s t surface.
Basicity, 105motes C,H~COOH/,Z
Total r a t e o f methane conversion ( a ) and r a t e of formaP+g. I.
t i o n of C$ hydrocarbons ( b ) vs. concentrations of base s i t e s on
t h e c a t a l y s t s u r f a c e ( T = 1153 K, GHSV = 18000 h-I), P r e t r e a t ment: 1173 K , 4 h, a i r .
However, f o r magnesium oxide the r a t e of oxidative conversion i s
found t o be lower than one might a n t i c i p a t e proceeding from the
concentration o f base s i t e s on t h i s c a t a l y s t .
A similar dependence has been found e a r l i e r f o r o x i d a t i v e ammonolysis of propane on c a t a l y s t s containing base s i t e s ( r e f . 9 ) .
This r e s u l t has made i t p o s s i b l e t o suggest a h e t e r o l y t i c dissoc i a t i o n of t h e C-H bond on base c a t a l y s t s . Support f o r t h i s con-
439
c l u s i o n comes a l s o from data on deuterium-hydrogen exchange i n
molecules of lower p a r a f f i n s on s o l i d bases ( r e f s . 1 0 , l l ) . The
l i t e r a t u r e r e p o r t s on some attempts t o d e t e c t methane a c t i v a t i o n
on s o l i d bases by d i r e c t p h y s i c a l methods. The authors of ( r e f .
12) have succeeded i n d e t e c t i n g t h e propyle h e t e r o l y t i c d i s s o c i a t i o n on MgO a t room temperature. However, they have f a i l e d t o observe methane chemisorption under the conditions employed ( r e f .
12).
To d e t e c t methane chemisorption on s o l i d bases, w e used magnesium oxide with a l a r g e s u r f a c e a r e a (200 m 2/g) and employed F I R
spectroscopy which allows a d r a s t i c i n c r e a s e i n s e n s i t i v i t y of
experiment. The oxidative dehydrodimerization r e a c t i o n i s carr i e d out a t high temperatures with an excess reductant which
should l e a d t o t h e appearance of low-coordination s i t e s on the
surface of the oxide. With t h i s i n mind, p r i o r t o experiments
magnesium oxide was outgassed a t 1000 K when, according t o UV
d i f f u s e r e f l e c t a n c e s p e c t r a ( r e f . 12), the s u r f a c e c o n t a i n s f i v e - ,
2+ 22+02four- and three-coordinated s i t e s (Mg2+ 025c Tc' Mg4c 04c and Mg3c 3c'
r e s p e c t i v e l y ) . Propylene adsorption on such a sample a t room temp e r a t u r e produced I R absorption bands (a.b.1 corresponding t o OH
groups (3650 cm'l) a n d 6-(1620 and 950 cm-') and T - a l l y 1 complexes (1550 and 1250 cm")
( s e e Fig. 2a). 7 - A l l y 1 complexes
of t h i s type were f i r s t found by Kokes ( r e f . 13) and a t t r i b u t e d
t o the anion type. Taking i n t o account d a t a on v a r i a t i o n s of UV
d i f f u s e r e f l e c t a n c e s p e c t r a a f t e r propylene adsorption on magnesium oxide ( r e f . 14) we may conclude that i n o u r case t h e adsorpt i o n occurs on low-coordination (4- and 3-coordinated) s i t e s p r o ducing OH groups and an organometallic s p e c i e s of magnesium.
We have f a i l e d t o observe d i s s o c i a t i v e adsorption of methane
on t h i s magnesium oxide sample a t room temperature, which seems
t o be due t o a s t r o n g e r and l e s s p o l a r C-H bond i n methane than
i n propylene. However, as the temperature of adsorption was r a i s ed t o 573 K a.b. corresponding t o OH groups (3600 cm-l) and new
a.b. i n the region of s t r e t c h i n g v i b r a t i o n s of the C-H bond
(2940 and 2980 cm-1 ) ( s e e Fig. 2b) appeared i n the I R spectrum.
Note t h a t beginning with these temperatures the H-D isotope exchange i n methane molecules on magnesium oxide i s t y p i c a l l y observed ( r e f . 10). A comparison of t h e spectrum obtained with
a.b. a s c r i b e d t o metal-methyl groups (2920, 2990 cm'l
f o r adsorpt i o n of (CH ) SnC12 on MgO) and oximethyl groups (2800, 2860 and
3 2
440
2920 em-’ f o r a d s o r p t i o n of CH OH on MgO) s u g g e s t s that i n o u r
3
c a s e t h e h e t e r o l y t i c d i s s o c i a t i v e a d s o r p t i o n of methane produci n g hydroxyl and magnesium-methyl groups t a k e s p l a c e ( r e f . 8).
0
P
6
3700
3600 3000
2900
a02
-
.Fig. 2. IK s p e c t r a of hydrocarbons a d s o r b e d on MgO. a
propyl e n e a d s o r p t i o n a t 300 K ( s u b t r a c t e d background o f MgO); b
methane a d s o r p t i o n a t 573 K ( s u b t r a c t e d background of MgO and
p a r t i a l l y compensated g a s p h a s e ) ; * t h e band c o r r e s p o n d i n g t o
t h e gas phase.
-
Probably, t h e a d s o r p t i o n o c c u r s on low-coordination
sites. A s
shown i n (ref. 1 5 ) , a f t e r methane a d s o r p t i o n on magnesium oxide
c o n t a i n i n g low-coordination s i t e s ( t r e a t e d under vacuum a t 1123
Kl
t h e oxygen a d s o r p t i o n l e a d s t o t h e f o r m a t i o n of r a d i c a l
i o n s 02.
The a u t h o r s of ( r e f . 15) ( l i k e t h o s e of r e f s . 12,14 who obs e r v e d s u c h an e f f e c t a f t e r p r o p y l e n e p r e a d s o r p t i o n ) have made
a c o n c l u s i o n a b o u t t h e p r e s e n c e on t h e s u r f a c e of a n a n i o n form
of a hydrocarbon r e s i d u e from which an e l e c t r o n is t r a n s f e r r e d
i n t o t h e oxygen molecule.
It s h o u l d be n o t e d that t h e exposure of t h e sample i n methane
a t 573 K g i v e s r i s e t o a 3085 cm-I band c h a r a c t e r i s t i c o f C-H
v i b r a t i o n s a t a double bond ( s e e F i g . 2b). This may be t a k e n as
evidence f o r the formation of dimerixation products (ethylene)
i n c o n d i t i o n s o f methane a d s o r p t i o n on t h i s sample.
441
Thus, w e have observed e x p e r i m e n t a l l y t h e h e t e r o l y t i c a c t i v a t i o n of methane on magnesium oxide c o n t a i n i n g low-coordination
s i t e s and s u g g e s t e d a p o s s i b l e r o l e of t h i s p r o c e s s i n o x i d a t i v e
d e h y d r o d i m e r i z a t i o n of methane.
As shown i n ( r e f . 101, due t o a h i g h t e m p e r a t u r e t r e a t m e n t of
magnesium o x i d e under vacuum, which results i n t h e appearance o f
l o w - c o o r d i n a t i o n s i t e s , t h e c a t a l y s t r e v e a l s a c t i v i t y toward H-U
i s o t o p e exchange o f methane. However, oxygen a d s o r p t i o n l e a d s t o
complete d e a c t i v a t i o n o f t h e sample, most p r o b a b l y , by d e s t r o y i n g low c o o r d i n a t i o n s i t e s . However, a s h a s a l r e a d y been mentione d , h e t e r o l y t i c a c t i v a t i o n of methane o c c u r s o n l y on l o w c o o r d i n a t i o n s i t e s . To v e r i f y whether low c o o r d i n a t i o n s i t e s which can
a c t i v a t e methane a r e r e t a i n e d d u r i n g t h e c o u r s e o f o x i d a t i v e deh y d r o d i m e r i z a t i o n i n a methane-oxygen m i x t u r e , we have s t u d i e d
t h e CH CD i s o t o p e exchange d i r e c t l y i n t h e p r o c e s s .
4- 4
The r e s u l t s o b t a i n e d a r e l i s t e d i n Table 1 . As can be s e e n i n
t h e t a b l e , on magnesium o x i d e a r a t h e r f a s t i s o t o p e exchange occ u r s , which may e v i d e n c e f o r h i g h c o n c e n t r a t i o n o f low coordinat i o n s i t e s on i t s s u r f a c e .
TABLE I
Comparison of r a t e s o f o x i d a t i v e d e h y d r o d i m e r i z a t i o n (Wd) tind
CH CD i s o t o p e exchange (W,) i n c a - t a l y s i s c o n d i t i o n s on
4- 4
a l k a l i n e e a r t h m e t a l o x i d e s ( T = 1073 K, GHSV = 22500 h-’ 1
MgO
CaO
1.9
12
0.09
6.5
1.99
18.5
2.14
7.94
0.045
0.35
As h a s a l r e a d y been mentioned, t h e i s o t o p e exchange i n methane on magnesium oxide b e g i n s a t f a i r l y low t e m p e r a t u r e s ( r e f . 10).
From t h i s f a c t i t f o l l o w s t h a t t h e primary h e t e r o l y t i c a c t i v a t i o n of methane is r e l a t i v e l y f a s t . The t o t a l r a t e of methane
a c t i v a t i o n ( d e f i n e d as a sum of observed r a t e s of exchange and
d i r n e r i z a t i o n ) i n c r e a s e s with i n c r e a s i n g b a s i c i t y of t h e o x i d e ,
which i s i n agreement w i t h o u r c o n c l u s i o n a b o u t h e t e r o l y t i c act i v a t i o n of methane a t c a t a l y s t base s i t e s . A t t h e same time,
442
t h e r a t e r a t i o of exchange and dimerization i s d i f f e r e n t f o r d i f f e r e n t oxides ( s e e Table 1).
A simultaneous occurrence of exchange and dimerization i n d i c a t e s t h a t the r a t e of t h e primary a c t i v a t i o n of methane i s high
enough t o provide the both r e a c t i o n s .
The general scheme of the a c t i v a t i o n process may be as f o l lows :
C H ~+
__
1
Me2+02-
2 3
M ~ ~ + - c H ~ +-
I
o~--H+
dimer
It can be supposed that p a r t o f metal-methyl groups t r a n s forms t o methyl r a d i c a l producing the dimer and t h e remainder
p a r t i s r e v e r s i b l y desorbed which l e a d s eventually t o the i s o tope exchange.
A slow s t e p o f the dehydrodimerization r e a c t i o n may be t h e
s t e p o f r u p t u r e of t h e metal-methyl bond. The e n e r g i e s of Mg-CH
3
binding f o r magnesium ions w i t h d i f f e r e n t coordination numbers
a b s t r a c t e d from ( r e f . 16) a r e l i s t e d i n Table 2. The e n e r g i e s o f
the metal-methyl bond on low-coordination ( 3 - and &coordinated)
magnesium i o n s a r e c l o s e t o a c t i v a t i o n e n e r g i e s of dimerization
on magnesium oxide-based c a t a l y s t s (ca. 200 kJ/rnol) ( r e f . 4).
TABLE 2
Energies of the bond rupture i n the 1VIg-CH group vs. coordina3
t i o n number for magnesium ( a b s t r a c t e d from r e f . 1 6 )
Coordination number
Binding energy, kJ/mol
3
4
5
288.0
192.7
153.4
The homolytic rupture of t h e metal-methyl bond is accompanied
by a n e l e c t r o n t r a n s f e r from the methyl group i n t o the c a t a l y s t .
This t r a n s f e r can be f a c i l i t a t e d by a c c e p t o r s i t e s of the catal y s t ( r e f s . 5,17). Calcium oxide is known t o possess h o l e cond u c t i v i t y even a t very low oxygen p r e s s u r e s (ca. lo-* T o r r ) ( r e f .
18). Due t o t h i s p r o p e r t y a l a r g e proportion o f metal-methyl
groups w i l l be consumed a t s t e p 3 , which may r e s u l t i n a higher
443
r a t i o of t h e d i m e r i z a t i o n r a t e t o t h e t o t a l r a t e o f methane a c t i v a t i o n on C a O i n comparison w i t h magnesium oxide ( s e e Table 1 ) .
CONCLUSIONS
The data o b t a i n e d a l l o w u s t o conclude that t h e h e t e r o l y t i c
mechanism of methane a c t i v a t i o n d u r i n g t h e o x i d a t i v e dehydrodim e r i z a t i o n p r o c e s s on base c a t a l y s t s i s more p r o b a b l e t h a n
a homolytic one i n v o l v i n g s u r f a c e r a d i c a l i o n s o f oxygen.
1. The mechanism of methane a c t i v a t i o n w i t h p a r t i c i p a t i o n of
r a d i c a l i o n s 0- s h o u l d l e a d t o a c o n s i d e r a b l e enhancement o f comp l e t e o x i d a t i o n p r o c e s s e s , which has been r e c e n t l y observed a t
temperatures of o x i d a t i v e d e h y d r o d i m e r i z a t i o n on a s e r i e s of
magnesium-containing c a t a l y s t s ( r e f . 1 9 ) .
2. The homolytic mechanism of a c t i v a t i o n i n v o l v i n g a r a d i c a l
a b s t r a c t i o n of t h e hydrogen atom from methane by 0- s h o u l d have
a l o w a c t i v a t i o n energy. The h i g h observed a c t i v a t i o n e n e r g y of
methane d i r n e r i z a t i o n on Li/MgO was e x p l a i n e d ( r e f . 4 ) by that
t h e r a t e - d e t e r m i n i n g s t e p i s t h a t of r e g e n e r a t i o n of r a d i c a l sit e s Li'O-.
However, as h a s been shown r e c e n t l y i n ( r e f . 201,
t h i s e x p l a n a t i o n i s i n c o n f l i c t w i t h k i n e t i c data on i s o t o p e e f f e c t f o r t h i s reaction.
3 . Most c a t a l y s t s o f o x i d a t i v e d e h y d r o d i m e r i z a t i o n of methane
a r e s o l i d b a s e s . On t h e o x i d e s c o n t a i n i n g low c o o r d i n a t i o n s i t e s
t h e h e t e r o l y t i c a c t i v a t i o n of methane o c c u r s v e r y e f f e c t i v e l y .
( I t has been r e p o r t e d r e c e n t l y ( r e f . 2 1 ) t h a t doping of magnesium o x i d e w i t h lithium i n c r e a s e s t h e number o f low c o o r d i n a t i o n
s i t e s ) . Thus, t h e r e i s a c o r r e l a t i o n between t h e number of base
s i t e s and t h e r a t e of d i r n e r i z a t i o n . By assuming that t h e r a t e d e t e r m i n i n g s t e p i s t h e decomposition of t h e metal-methyl spec i e s found i n t h i s work i t i s p o s s i b l e t o e x p l a i n t h e observed
h i & energy of a c t i v a t i o n of o x i d a t i v e d e h y d r o d i m e r i z a t i o n .
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1V.U. Zhanpeisov, A . G. Pelmentschikov, G.M. Zhidomirov, Clust e r quantum-chemical s t u d y of t h e i n t e r a c t i o n o f molecules
w i t h IgO s u r f a c e . D i s s o c i a t i v e chemisorption of H2, CH4,
C2H4, Kinet. Katal. ( i n p r e s s ) .
A . I . Suleimanov, A.G. Ismailov, S.N. Aliev, V.D. S o k o l o v s k i i ,
C o n t r i b u t i o n of one-electron a c c e p t o r c e n t e r s t o o x i d a t i v e
d i m e r i z a t i o n o f methane, React. Kinet. C a t a l . L e t t . , 34
(1987) 51.
K. Hauffe, Reaktionen i n und an Festen S t a f f e n , S p r i n g e r
Verlag, B e r l i n , 1955, v. 1.
G.J. Hutchings, I.S. S c u r r e l l , J.R. Woodhouse, The r o l e of
s u r f a c e 0 i n the s e l e c t i v e o x i d a t i o n of methane, J. Chem.
Soc., Chem. Commun., (19871 1388.
N.W.
Cant, C.A. Lukey, P.F. Nelson, R. J. Tyler, The r a t e cont r o l l i n g s t e p i n the o x i d a t i v e coupling o f methane over a lithium-promoted magnesim oxide c a t a l y s t , J. Chem. S O C . , Chem.
Comrnun., (1988) 766.
M. Anpo, M. Sunamoto, T. Doi, I. Matsuura, Oxidative coupling
of methane over u l t r a f i n e c r y s t a l l i n e MgO doped w i t h L i .
Role of lower c o o r d i n a t i v e s u r f a c e s i t e s produced by Li-dopi n g , Chem. L e t t . , (1988) 701.
445
CORTES CORBERAN, V. (Institut Catalisis y Petroleoquimica,
Spain): A s the basicity/acidity properties of metal oxides depend
on the temperature, does it make sense to compare catalytic activity measurements at high temperature with number of basic
sites as determined at room temperature? Would you expect that the
observed overall tendency (activity increases as basic sites number increases) can be extrapolated?
SOKOLOVSKII V.D. (Institute of Catalysis, USSR): The observed
concentration of basic centers depend not so much on the temperature of measurement (if adsorption of acid is quick and irreversible), as on the temperature of preparative treatment of specimens. The treatment of specimens during our experimmts before
measuring a catalytic activity and basicity was identical.
CORTES CORBERAN,V. (Institut Catalisis y Petroleoquimica, Spain):
You have used only one probe molecule to correlate activity with
basicity. Does it mean that centers with any basic strength are
equally active and must be taken into account or would you expect
that only very strong basic centers will be active?
SOKOLOVSKII V.D. (Institute of Catalysis, USSR): In these methodice we determined the basic centers concentration only, but not
their strength. Benzoic acid, used as a test molecule, is rather
= 4.2) and so strong and medium strong centers are deterweak (
mined with its help. We suppose that activation of such an inert
molecule as the methane, must be conducted on the strong basic
centers.
BUSCO GUIDO (Istituto di Chimica, Italy): The Mg-CH groups for
ciif must be responsible also for bands in the reg?on 1500-1300
cm , as well as for rocking modes at lower frequencies. However,
CH bands you detect are due to oxygen-containing speif the
cies, several characteristic bands would be observable in the
region below 1800 cm”. Have you also investigated the low-frequency region to confirm your argument?
SOKOLOVSKII V.D. (Institute of Catalysis, USSR): Absorption bands
were observed by us in the region of deformation bands of C-H
bonds
1800 cm-l. However, a6 these bands are not characteristic
(ref. 1) for identification of Me-CH3 groups, the region of
changes in valence.
1 D.C. McKeam, G. McQuillau, I. Torto, A.R.
Struct., 141 (1986) 457.
Morrison, J. Molec.
O W E N G.V. (University of Turent , The Netherlands): By a special
treatment of the MgO surface (more or less reduceit) you measure
Mg-CH bands at 3OOoC. These organometallic type
cannot be
preseat during methane coupling reactions at 7OOOC and in the
presence of oxygen. In the IR spectra of the same material treated under real coupling conditions we measure only oxygenates,
possible precursors of the total oxidation. These species are
more stable and can therefore be measured under these conditions.
My question is, do you suppose that the measured Mg-CH bands
play an important role during oxidative coupling of CH? at 7OOOC
in the presence of 02?
446
SOKOLOVSKII V.D. (Institute of Catalysis, USSR): We also think
that there is reason to believe that at 7OOOC under reaction conditions, the groups Me-CH must be unstable. If not, they could
not have acted as intermediates providing the reaction proceeding. Such intermediates must be formed and decay quickly in conditions of reaction. To fix their availability we have used a
lower temperature (pre-catalysis conditions), at which these forms
are stable enough to be recorded.
G . Centi and F. Trifiro’ (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
447
SERS-IN SITU STUDY OF THE SURFACE SPECIES FORMED IN METHANE
OXIDATIVE COUPLING
A.A.KADUSHIN, O.V.KRYLOV,
S.E.PLATE,
2
A.V.BOBROV and YA.M.KIMELFELD
YU.P.TULENIN, V.A.SELEZNEV’,
’Institute of Chemical Physics of the U.S.S.R. Academy of
Sciences, Kosygin str.4, 117334, Moscow, U.S.S.R.
21nstitute of Spectroscopy of the U.S.S.R.
142092, Troizk, Moscow Region
Academy of Sciences,
-
s m m y
The existence of stationary concentrations of CH and probably
CH -surface species bonded with carbon and noncaJbon surface siteg was shown on Sm 0 /MgO catalyst of methane oxidative coupling
using SERS-in situ 6e4hod in 670-970 K temperature interval. The
CH and CH -species could be intermediates in the formation of
C22hydrocar60ns (ethane and ethylene)
-
.
INTRODUCTION
Methane oxidative coupling to form higher hydrocarbons is gaining an increasing interest after pioneering work of Keller and
Bhasin [l]. Recently, a large number of catalysts manifesting a
high activity and selectivity in the reaction were developed.
However, a lack of information about the mechanism of the rcaction and the nature of intermediate species still exists in
the literature.
Lunsford et al. detected the formation o f C€13 radicals in the
gas phase over Li/MgO [2]and
La203 [3]using
ESR matrix isolation
method. Martin and Mirodatas 141have postulated carben (CH2-)
intermediate species formation over Li/MgO on the basis of chemical evidence of cyclopropane formation by introduction of C H
2 4
into the reaction mixture. Nelson et al. [5]studied the oxidation o f equimolar CH4+CD4 mixture over Li/MgO at 75OOC and did
not find any notable isotopic H-D exchange in methane molecules.
The main C2-products were c2H6, CH3-CD3 and C2D6. On the basis
of these data the authors [5] concluded, that the ethane formation takes place as a result o f CH3 and CD3-radicals recombination in the gas phase.
Surface enhanced Raman scattering (SERS) was used in o u r pre-
448
vious papers [6,71 for the investigation of CH4 and O2 interaction with Ni,Cd,Pb and Mg films at 77 K.
In the present work Raman spectra o f the surface species were
measured at 300-970 K during the interaction of CH4 and O2 on
MgO and Sm 0 / M g O (1 .O wt. % Sm2O3) which are active in the CH
2 3
4
oxidative coupling [
8
]
.
METHODS
Granulated catalysts samples of fraction 0.25-0.5 mm were
packed in a quartz reactor with small windows. Raman spectra
were excited by a beam of an argon-ion laser at 4880 A and 150
mw and registered by a double monochromator in photon counting
regime.
RESULTS
AND DISCUSSION
Spectroscopic measurements revealed that the interaction o f
the CH +O mixture with catalyst samples in the temperature range
4 2
670-970 K leads to appearance in the Raman spectra of several
bands at 1190, 1290, 1380 and 11.80 cm" (Pig.1, spectrum 1)
1480
1
1500
1380
1
1290
1190
I
1
i300
1090
lb80
900 850
990
' 960 t I
1100
900
Raman spectra of the surface species formed during mepig.1.
thane oxidative coupling over Sm203/Mg0 at 970° K. Feed mixtures:
CH4+ CD4+ O2 +He.
1
CH4+ O2 +He; 2
CD4+ 02+ He and 3
Contact time ('iT) 0.7 S.
-
-
-
449
This region is well characterised in the literature and these
bands can be related to bending vibrations of CH (1180 and 1290
2
cm” ) and CH (1380 and 1480 cm”
surface species. The quartet
3
of these bands appears in the spectrum at the temperatures higher
than 760 K when oxidative coupling begins to proceed with a noticeable rate. This was also confirmed by simultaneous analysis of
the reaction products ( Table 1):
TABLE 1
Catalytic properties of the 1% wt. Sm203/Mg0
Feed mixture (% vol): CH4-10.0; 02-2.9; He-87.1 ;(iT=0.7s
~~
Temperature
I<
770
870
970
~~
~~
Conversion, %
Selectivity, %
C2H6
CH4
O2
2.0
12.2
-
11.7
15.9
65.7
3.7
80.0
12.0
C2H4
-
6.7
C02
co
50.5
49.5
54.9
41 *4
48.2
33.2
It should be noted that these bands can exist only in the presence of reaction mixture and disappear in an argon flow. This
evidences in favour of an equilibrium between the gas phase and
hydrocarbon species on the catalyst surface. This means that the
bands observed in the Raman spectra during the reaction prove
the existence of surface species stationary concentration. In
connection with this it is interesting to note the work of Ekstrom and Lapszewicz 191 where unusually high adsorption of isotopically labelled methane molecules was observed on Sm203 in
the same conditions.
Similar measurements have been made with CD (spectrum 2). The
4
isotopic band shift ( =300-370cm-’ ) is in agreement with the
interpretation of the hydrocarbon spectrum.
Spectrum 3 was obtained when an equimolar mixture of CH4+CD
with O2 was passed through the catalyst. A new band at 960 cm-4
and an unresolved l o w frequency shoulder of 1080 cm” at 1090cm-’
band appears in the spectrum due to H-D exchange between the hydrocarbon surface species. The Raman-spectra of ethane-02 and
ethylene-02 mixtures, measured in the same conditions, differ
from the spectrum of methane-O2 mixture.
450
INTERPRETATION OF SPECTILA
The 1480 and 1380 cm" bands are ascribed to antisymmetric and
symmetric bending vibrations of CH3-groups, respectively. The positions of these bands indicate that the CH -groups are bonded
3
with carbon atoms of the surface.
A s to the interpretation of 1290 and 1190 l'mc
bands at least
three possibilities can be proposed:
1. These bands can be assigned to internal and external bending
vibrations o f CH2-groups in bridge structures
'
flu
u
I1
-
-M
,-N'2
where Id
is a noncarbon surface site.
The positions of these bands in the spectrum a r e considerably
lower, than those in hydrocarbon spectra. Similar spectra were
observed by C h a n g et al. [lo],
who studied FeCH2 and N2FeCH2 in
argon and nitrogen matrixes by FTIR matrix isolation spectroscopy.
2. These bands can be assigned to the same vibrations of CH2
groups (as in point 1) in ethylene-like structure.
3. These bands may be ascribed to antisymmetric and symmetric bending vibration of CHg-groups, bonded with noncarbon surface sites.
Similar IR-spectra were observed by Billups et al. [Illduring activation of methane with photoexcited atoms of some transition
metals (matrix isolation method).
Unfortunately, experiments with CH4+CD4 mixture do not exclude
any mentioned possibilities. We could not measure the C-H stretching vibrations due to large emission by the sample at high temperatures.
The 1080 and 960 cm" bands can be related to internal and
external bending vibrations in -CHD, -CH2D or -CD2H-groups bonded with noncarbon surface sites. For more detailed interpretation a further study is needed using other isotopiically labelled
methane molecules, 'Pherefore our data do not allow at the present
time t o conclude finally that CH2 species exist during the o x i dative coupling of methane.
Some band intensities in the "oxygen" region change considerably, however, their detailed interpretation needs further research. We compared Raman spectra of surface species measured
at 970 K for Sm203/Mg0 and MgO. Spectra for both of the samples
are similar but in the case of MgO after some hours of the reaction a new 1570 cm" band appeared typical f o r C-C double bond.
This .fact points to deeper surface transformations, for instance, to dimerization of CH2-fragments.
-
451
The S W S (enhance coefficient -100) is untrivial for nonconducting oxide systems. Usually the SERS having an enhance coefficient up to 105-10 6 is observed on metal surfaces [12], although
recently this effect waa registered for colloidal o l -Fe203 [13].
The above mentioned results are obtained for the first time
and future systematic work is needed for detailed interpretation
of the Raman spectra and their connection with the mechanism of
methane oxidative coupling.
However, even now it is possible to make some conclusions:
1. The possibility is shown o f Raman spectra measurements for
granulated oxide catalysts (MgO , sm203/Mg0) in conditions of catalysis process at temperatures up to 1000 K. The theory has to
explain the nature of the enhancement effect in the case of oxides and at high temperatures.
2. The existence is shown of stationary concentration of CH3fragments bonded with carbon atoms of catalyst surface and CH3or CH2-fragments bonded with noncarbon sites of catalyst' surface
in conditions of methane oxidative coucling at 970 K.
3. A notable isotopic H-D exchange between surface hydrocarbon
species is shown at 16% methane conversion.
4. The obtained results indicate a principally new level of Raman spectroscopy for the use in study of high temperature catalytic processes in situ.
REFERENCES
1 G.E.Keller and M.M. Bhasin, J.Catal., 73(1982) 9-19.
2 T. Ito, J.-X. W a n g , C.-H. Lin, J.H. Lunsford, J.Am.Chem.Soc.,
107(1985) 5062-68.
3 C.-H. Lin, K.D. Campbell, J.-X. Wang, J.H. Lunsford, J.Phys.
Chem., 90(1986) 534-537.
18
4 G.-A. Martin, C. Mirodatos, J.Chem.Soc.,Chern.Com.,
( 1987 1393-94.
5 P.F. Nelson, C.A. Lukey, N.1. Cant, J.Phys.Chem., 92 (1988)
6176-79.
. . . ~. - 6 S.E. Plate, A.V. Bobrov, A.A. Kadushin, Ya.M. Kimelfeld, Kinetika i Kataliz, XXVII (1986) 495-497 (RUss).
7 A.B.Bobrov, S.E. Plate, Ya.M. Kimelfeld, A.A.Kadushin,
XVIIIth European Congress on Molecular Spbtroscopy, Amsterdam, August 30-September 4, 1987, Abstracts, P.275.
8 V.H. Korchak, A.A. Kadushin, Yu.P. Tulenin, V.A. Seleznev,
Tezisy dokladov 6 konferenzii PO okislitelnomu geterogennomu
katalizu, Baku, November 15-17, 1988, pp.264-265 (Russ).
(1988)
9 A.Ekstrom, J.A.Lapszewicx, J.Chem.Soc.,Chem.Commun.,l2
747-749
10 S.C. Chang, R.H. Hauge, Z.H. Kafafi, J.L. Margrave, W.E. Billups, J.Am.Chem.Soc.,
llO(1988) 7975-80.
11 W.E. Billups, M.M. Kanarski, R.H. Hauge, J.L. Margrave, J.Am.
Chem.Soc., 102(1980) 7393-94.
452
M. Fleischmann, P.J.Hendra, A . J . McQuillan, Chem.Phys.Letters, 26 (1 974) 163-1 66.
13 P. Z h a n g , Y. Wang, T. He, B. Z h a n g , X. Wang, H. Xen, F Liu,
Chem.Phys.Letters, 153(1988), 215-218.
12
VAYENAS' (University of Patraa, Greece) : It is surprising
that SERS spectra have been obtained at temperatures up to 970 K
on an oxide surface. Did you study the temperature dependence of
the intensity of the SERS bands?
C.G.
A.A. KADUSHI" (Institute of Chemical Physics of the USSR Academy
of Sciences, Moscow, USSR): The SERS is really untrivial for nonconducting oxide aystems. Nevertheless we quite definitely established the presence of this effect in a wide temperature range.
It may be, for example, supposed that the microclusters of carbon
produced upon partial methane oxidation can be juat those species
which possess the metallic conductivity responsible for the appearance of bands in the spectra. These bands appear in the spectrum
at 670 K and their intensity increases up to 970 K. No special
investigation of the temperature dependence of these bands has
been carried out.
J.R.H. ROSS (University of Twente, The Netherlands) : Is there any
chance that gas-phase methane species can contribute to the spectra under your reaction conditions? Under similar conditions using
Li/MgO catalysts, we can find no evidence with FTIR for anything
other than oxygen-containing species on the catalyst surface (J.G.
van Ommer et al., unpublished reaults).
A.A.UDUSHM (Institute of Chemical Physics of the USSR Academy of
Sciences, Moscow, USSR) : We did not observe in the spectrum any
bands of gaseous methane, In the temperature interval 6_10-870K
we observed a series of bands in the range 700-1700 cm which
could be sssigned to oxygen-containing surface species. But at
higher temperatures these bands disappear.
G. Centi and F. Trifiro’ (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
In SItu Studies of
453
the Oxidative Coupling of Methane Over Li-Ni-0 Catalysts
I. J. Pickering. J. M. Thomas and P. J. Maddox
Davy Faraday Research Laboratory,
The Royal Institution of Great Britain,
21 Albemarle Street, London W1X 4BS, UK
Abstract
We describe a comparative in sftu X-ray diffraction study of the lithium nickel oxide
catalyst in the presence and absence of added gaseous oxygen. The results reveal several
interesting features and in particular pinpoint the involvement of crystallographic phases
implicated in the conversion of methane to various distinct gaseous products.
Introduction
Given that there is an abundance of naturally occurring methane, the question arises as
how best to convert it to liquid fuel or other feedstocks, such a s ethylene. that are more readily
usable in the chemical industry. Two well-known methods already exist: (I) partial oxidation to
synthesis gas [CO + H2) followed by FLscher-Tropsch conversion, and [ill steam-cracklng to yield
more reactive hydrocarbons and other useful by-products. These may well turn out to be the
most practical way forward industrially, but they may not be a s economically attractive as
other, more subtle conversions to which methane can be subjected. One such conversion, the
oxidative coupllng of methane, is the focus of our attention here.
We report below on laboratory in SUU X-ray diffraction studies of the conversion of
methane to C2 hydrocarbons over a lithium nickel oxide catalyst. both in the presence and
absence of added gaseous oxygen (catalytic and non-catalytic reactions respectfvely).The latter
proceeds by the extraction of the structural oxygen [of the Li-Ni-0 system) which is also known
[ref. 1) to be implicated in the catalysed oxidative coupling. In parallel with the gas
chromatographic analysis of reaction products we recorded, under fn sifu conditions. the X-ray
powder diffraction patterns of the solid oxide in both cases.
Experimental
The lithium nickel oxide catalyst used in the experfments was prepared by a solid-state
reaction of an intimate mixture of Li2CO3 and NiO at 80O0C in air. Atomic absorption
spectroscopy showed (ref. 2) the composition to be Lb.45Ni0.550. X-ray powder diffraction
patterns showed the structure to be rhombohedral. R-3m. with some orderlng of the lithium and
nickel ions on alternate close-packed layers.
The fn sftu catalysis was carried out in a specially constructed reaction cell (ref. 3)which
facilitates simultaneous monitoring of both diffraction patterns and gaseous products. The
454
catalyst was placed in a sintered quartz sample holder b i d e this cell. and the reactive gases
were passed Over the sample in a tubular fashion, the outlet gases being analysed by gas
chromatography. The cell was attached to a Siemens D500 diffractometer fitted with a rotating
anode source and scintillation counter, permitting rapid characterization by X-ray diffraction.
The conditions for the two experiments are shown in table 1:
Table 1. Experimental conditions for fn sftu catalytic and non-catalytic reactions.
Mass of
catalyst
(grams)
Catalyst
temperature
c a w
1.01
Gas-solid
reaction
1.04
reaction
Gas composition (96)
Flowrate
(ml/min)
Methane
Oxygen
Nitrogen
700
20
3
77
50
700
20
0
80
50
Results
The various regimes of catalysis determined during the catalytic run have already been
described (ref. 1). Briefly, there is an initial regime of near-constant conversion of methane,
with selectivity for C2 hydrocarbons decreasing. Conversions and selectivities during the
initial regime are surnmarised in table 2:
Table 2. Conversions and selectivities recorded during the initial regime of the catalytic
reaction.
Time after start of reaction
(min)
20
100
200
300
Percentage conversion of
methane
7.9
8.2
8.5
8.9
Percentage selectivity for C2
hydrocarbons
62
54
45
36
During the second and third regime there is rapid change as the catalyst breaks down. The
second regime yields C02 as the dominant product, and the major nickel-containing phase is
NiO. During the third regime CO is produced and the catalyst has been reduced to nickel metal.
We concentrate here on the initial catalytic regime and its comparison with the non-catalytic
results.
455
Ethene + ethane (catalytic)
+ Carbon dioxide (catalytic)
0
Ethene (non-cakdytlc)
0
lo0
Figure 1. Rate of appearance of products for the catalytic reaction (open symbols and crosses)
and the non-catalytic reaction (solid symbols). Carbon dioxIde is negligible in the latter case.
To compare the reactions in the presence and absence of gaseous oxygen. It is instructive
to examine the rate of appearance of products with time (figure 1).It I s apparent that, in the
presence of %, the rates of production of total C2 and of ethene in partfcular are fairly constant,
decreasing slowly over a period of some hours, whereas in the absence of added gaseous oxygen
the rate of production of Czs decays rapidly, dropping to less than 5 ~ 1 0mol.min-l
-~
during the
flrst 50 min. This decay is to be expected since, in this case, there is no replenishment of oxygen
in the system. It is. however, noteworthy that the initial rates of production are very similar for
both cases. Conversely the rate of production of C@ is very dmerent. being significant in the
case of the catalytic reaction, and essentially absent during the initial period of the gas-solid
reaction.
T h e X-ray Wraction patterns for the entirety of the experiments are shown in figure 2.
It is evident that, in the presence of q g e n . the initial phase remains essentially unchanged for
330 min: in the absence of oxygen the initial. ordered phase breaks down rapidly, a s can be seen
by the disappearance of the superlattice peaks at two-theta values of 18.5O [003)and 36.2O(101).
This phase is replaced by one in which the lithium and nickel cations are randomly distributed
in
a rock-salt structure (ref. 4). Other phases may be identified during the experiment: for
456
example. the peak at 25.7O two-theta is due to the strongest line of Li2Ni02 (ref. 5). and those
appearing at the end are due to an orientated form of Id2CO3.
400
wm
?Ime
(min)
300
x
*
*
-3
c)
m
w
C
A
A
1,
h
A
A A
.r
A
A .
h
A
n
A
h
A
A
A
A
A
n
A
n
loo
A
A
A
A
-
A .
A
A A
Figure 2. X-ray diffraction patterns for the duration of the experiments. Figure 2a shows those
for the catalytic run: they remain essentially unchanged from the initial diffraction pattern,
that of ordered Id-Ni-0. throughout the initial regime (about 330 minutes). There is subsequent
rapid change, to yield first NiO (a)and then nickel metal (*), together with LizC03. Figure 2b
depicts the ditfiaction patterns for the non-catalytic run: here the oxide decomposes much more
quickly. # is the strongest peak of Li2NiO2: are peaks due to a highly orientated form of
LizCO3.
+
Figure 3a shows how the unit-cell volume varies with time for both experiments. For the
catalytic reactton the unit-cell volume is essentially invariant throughout the duration of the
initial regime: in the gas-solid reaction it can be seen that the trend is to larger unit-cell
volumes as time progresses. This trend suggests (ref. 6) phases with smaller lithium to nickel
ratios. These unit-cell volumes reveal four Li-Ni-0 phases: the initial one ordered, the other
three random.
457
0
Catalyticreaction
Non-catdytic: A
Non-catalytic: B
.
73
Non-catalytic: C
Non-catalytic: D
71
" " " -- - -- -
70
69
0
Figure 3b.
I
I
100
U)O
Tune (min)
0
I
300
(003)catalyttc
(101)catalytic
(003)non-catalytic
(101)non-catalytic
Figure 3a. The variation of unit-cell volume with time. Values are obtained from a lattice
parameter refinement of the X-ray diffmction data.and are adjusted t o be equivalent to the
volume of4 lattice motifs, te. the volume of the f.c.c. unit cell ofthe random phases or 2/3 of the
volume of the rhombohedral unit cell (with hexagonal setti@.
Figure 3b.The variation with time of the intensity of two peaks of the initial phase of Li-Ni-0.
The intensity is calculated as the integral area of a Pseudo-Voigt profile fitted to the diffraction
data.
458
Figure 3b shows the intensity of two of the superlattice peaks as a function of time. Again
we see the now familiar pattern that in the presence of gaseous oxygen the intensities of these
peaks are largely unchanged, whereas in the absence of oxygen they decrease, this time in a
linear fashion. This can be qualitatively linked to the amount of phase A present at a given
time.
Discussion
From this comparison of catalytic and gas-solid reactions some interesting conclusions
may be drawn about the catalytic reaction itself. In the initial stages of the reaction the rates of
production of C2 products are very s m a r for both conditions. This suggests that structural
oxygen species is indeed responsible for the oxidation. as this is the only oxygen supply
available in the gas-solid reaction. By a similar argument, the COz which appears initially for
the catalysis reaction is absent for the gas-solid reaction, and so this suggests that the CO2 is due
to oxidation by gaseous or adsorbed oxygen. These observations are in agreement with those of
Otsuka (ref. 7-81. who also suggests that there are two distinct types of oxygen involved, just as
there are in other selective oxidations of hydrocarbons (ref. 9).
If the graphs of figures 1.3a and 3b and the diffraction patterns of figure 2 are examined.
an interesting trend can be observed. The values of Cp production, of unit cell volumes and of
peak intensities, and the appearance of the diffraction pattern observed at the beginning of the
two experlments are very similar, after which the values for the gas-solid reaction change
rapidly, the catalytic values changing much more slowly. Tc is observed that the conditions near
the end of the initial regime of the catalysis run correspond with those in the gas-solid run at
about 20-30 minutes, and thus the presence of oxygen is stabilising the initial high-lithium
content phase and, thereby prolonging the initial high rate of C z production.
References
1 I. J. Pickering, P. J. Maddox and J. M. Thomas, 'Probing changes in the structure and
performance of a lithium nickel oxlde catalyst during the high-temperature oxidative coupling
of methane by in situ X-ray diffraction', Angew. Chem., Adv. Mat., (1989)(in press).
2 L. D. Dyer, B. S. Borie, Jr. and G.P. Smith, 'Alkalimetal-nickel oxides of the type MNiOZ', J.
Am C h a . Soc., 76 (1954) 1499-1503.
3 P. J. Maddox, J. Stachurski and J. M. Thomas, 'Probing structural changes during the onset
ofcatalytlc activilyby h s i f ~ ~ x - r adtffractometry',
y
Cat. Lett., 1 11988) 191-4.
4 J. Deren and M. Rekas. 'Physico-chemical studies of NiO-Liz0 system', RoczniM Chemii.
Ann. Soc. Chim. Polonorum. 46 (1972) 1411-9.
5 V. H. Rieck and R Hoppe. 'Ein neuses Oxoniccolat: LiZNiOf, Z. Anorg. Allg. Chem.. 39213)
(19721 193-6.
6 J. B. Goodenough. D. G. Wickham and W.J. Croft, 'Some magnetic and crystallographic
properties of the system WXNi++l.~Ni++#. J. Phys. Chem. Solids, 5 (1958) 107-116.
7 M. Hatano and K. Otsuka, 'Alkali metal-doped transition metal oxides active for oxidative
coupling of methane', h o g . Chim.Acta. 146 (1988)243-7.
8 M. Hatano and K. Otsuka. 'The oxidative coupling of methane on lithium nickelate(i1I)'. J.
Chm. SOC..FaradayTrans. 1,85(2) (19891 199-206.
9 L. M. Kaliberdo, M. I. Tselyutuia. A. S.Vaabel, V. M. KalMman and B. N. Shvetsov. The role
of the catalyst lattice oxygen and the gas-phase oxygen in the oxidative dehydrodimerisationof
propene'. Russ. J. Phys. Chem.. 53[6)119791 843-5.
G.Centi and F.Trifiro' (Editors), New Developments in Selective Oxidation
0 1990 Elsevier Science PublishersB.V.,Amsterdam -Printed in The Netherlands
459
SELJEl'IVB
OHDATION OF lIETBAHB TO FORMALDEHYDE AT AIIBIm PRBSSURB: TBE ROLE
OF DOPANTS IN DETERHINIW OPTIMJH CARFUBR LOADING FOR TIE HOLYBDENA/ SILICA
SYSTEM
E. HacGiolla Coda and B.K. Eodnett
Department of Haterials Engineering h Industrial Chemistry,
University of Limerick, Limerick, Ireland.
ABSTRACP
Conversion of methane to formaldehyde from a 5:l mixture of methane and
nitrous oxide was investigated between 500 and 600C for a range of molybdena
catalysts supported on silica. This support was also modified by treatment
with a range of additives (sodium, phosphorus or copper) prior to impregnation
with ammonium heptamolybdate. An optimal molybdena loading could be defined
for each support; modification of the support generally increased the optimal
molybdena loading so that higher conversions could be achieved without loss of
selectivity. These changes are rationalised in terms of a modification of the
redox properties of the supported molybdena.
INTRODUCTION
Large reserves of natural gas have led in recent years to renewed interest
in C, chemistry (ref.1).
To date this subject has been dominated by
production processes involving synthesis gas. Since the mid-1980's a great
deal of interest has been aroused in methane coupling to ethane and ethylene,
and such has been the interest and progress made in this area that many
commercial plants are already being envisaged (refs 2,3).
Alternatives to these two processes are few in number but there has always
been an interest in direct oxidation of methane to methanol and/or
formaldehyde (ref.4).
Studies have appeared in the literature on this topic
in recent years and these include homogeneous and heterogeneous systems. In
general, the former (ref 4,5) have tended to be carried out at high methane
pressures (>30 bar), in the temperature range 200-4OOC; reasonable yields of
methanol have been attained in these systems with smaller amounts of
formaldehyde reported depending on the exact experimental conditions used; the
addition of heterogeneous catalysts have little effect and radical species are
commonly cited in proposed reaction mechanisms.
460
By contrast studies of heterogeneous systems (refs 6-12) are usually
carried out between 450 and 650C at ambient pressure with high CH,:Oxidant
ratios; in general formaldehyde is the usual selective oxidation product
observed, but selectivities usually decrease dramatically as conversion
exceeds ca. 1% of available CH,. This is often related to the decomposition
of formaldehyde within the catalytic reactor and for this reason most of these
studies have attempted to achieve kinetic isolation of the selective oxidation
product by operating with minimal contact times.
A primary indication of the stability of any hydrocarbon molecule in
oxidising conditions can be got from the strength of its weakest C-H bond.
These values are presented in table 1 (ref. 13) for a number of feed stocks
and products which feature in some commercial selective oxidation processes.
Table 1
Comparison
of
C-H
Bond
Strengths
Feed
Product
C-H bond strength
k~ mole-'
n-C,H,
405
0
c,=,03
412
C3B6
CE, CECEO
366
a20
393
366
CH30H
ca,
412
440
It is clear from this crude comparison that CH, is the most difficult
hydrocarbon to activate and CH,O is amoung the least stable of the selective
oxidation products which implies that the selective oxidation route would be
difficult to perfect.
It follows therefore that reasonable selectivities in formaldehyde by
selective oxidation of methane can only be achieved through very careful control
of all parameters involved in the process. Here we report efforts to optimise
the support - supported phase ratio for catalysts based on the molybdena/silica
system and used in the ambient pressure oxidation of methane to formaldehyde.
EXPERImAL
Catalyst Preparation
Three sources of silica were used in his study i.e, fumed silicas Cab-o-sil
M-5, and Aerosil supplied by the Cabot Corporation and Degussa, respectively,
and spherosil, a porous silica. Sodium, phosphorus, lead or copper were added
to these supports by impregnation in the way already described and molybdenum in
the form of (NH4)6 Mo,O,, was then added by further impregnation (ref 11,lZ).
461
Below catalysts will be cited as, for example 5Mo-2Na-Cabosil. This refers to
a Cab-o-sil support impregnated with NaCO, so as to achieve 2 wtX sodium,
followed by impregnation with (NH4)s Mo,O,, to achieve 5wt% MOO,.
Testing
Catalysts were tested by passing a 5:l ratio CH,:N,O mixture at 0.4 ml s-'
over 0.lg of catalyst held between 400 and 600C i n a lOmm i.d quartz reactor.
Analysis was by on time G . C . Full details have already been presented (ref.12).
Characterisation
Catalysts were analysed before and after use by X-ray diffraction with a
Philips diffractometer using Cu Ka radiation filtered through nickel.
In addition, samples were subjected to analysis by temperature programmed
reduction. This was carried out by placing the equivalent of ca.5 mg of MOO, in
a quartz reactor and passing a flow of 5% H, in N, over the catalyst at 20 ml
min-'.
The temperature was linearly increased from room temperature to 800C at
10 C min-' while hydrogen consumption was monitored using a thermal conductivity
detector.
RESULTS
Table 2 presents the conversion of methane and selectivity to formaldehyde
achieved at 500 and 600C over a range of supported molybdena catalysts in
standard
reaction conditions.
Essentially, formaldehyde decomposition was
small at 500C but appreciable at 600C for most systems studied (ref.12).
Good
selectivity was observed only over silica supported molybdena catalysts whereas
In
other combinations of support and supported phase were not selective.
addition, the porous silica used (spherosil) exhibited good performance at 500C,
but its selectivity diminished drastically at 600C, indicating that formaldehyde
could not survive within the pores of this support at the higher reaction
temperature.
Table 2
Conversion of Uethane and Selectivity to Formaldehyde Over a Range of Catalysts.
6OOC
500c
V/P
ConvX
SelX
ConvX
SelX
g s n1-I
0.25
0.25
0.25
0.25
Empty Reactor
Cabosil
2Na-Cabosil
2Uo-Cabosil
2Uo-Spherosil
2Uo-TiO,
ZUo-ZNa-TiO,
2no-ngO
0.25
0.25
0.25
Xu-Cabosil
O.1Pt-Cabosil
0.25
0.25
1.25
a,
CH, 0
0
0.01
0.01
0.01
0.03
0.57
0.54
0
0
72
58
0
-
0.13
0.09
1.23
4ooc
0
0
0
a*o
a,
0.01
0.03
0.02
0.05
0.90
2.20
0.12
0.67
0.31
-
0
5ooc
0
0
67
3
0
0
0
0
-
462
Table 3
Nature of the Support Additives for Fumed Silicas
5ooc
ConvX
SelX
Cow% SelX
m,o
co
CO,
0
60
0.05
0.05
71
85
72
15
9
0
0
2
5
85
31
100
34
27
10
0
0.05
67
11
28
22
a
5Ho-Cabosil
5Ho-ma-Cabosil
5H0-3Pb-Cabosil
5Ho-2Pb-Wa-Caboail
lOUo-Aerosil
IOKo-ma-Aerosil
lOH0-5Cu-Aerosil
10Ho-2P-Aerosil
6OOC
4
0.04
0.08
0.24
0.04
0.04
0
66
a 2 0
co
0.3
38
22
0.23
0.23
0.22
0.55
0.28
a
-
4
-
- -
-
co,
-
38
-
-
31
0
32
53
12
64
38
27
20
65
35
12
57
35
15
The influence of a number of support additives is presented in table 3
Sodium, phosphorus or copper, each impregnated onto the support prior to
addition of the molybdenum component enhanced the selectivity towards
formaldehyde particularly, during operating at 600C.
This effect is further elaborated upon in figures 1-3 which show the influence
of nominal MOO, loading on the conversion of C H I , the selectivity to
formaldehyde and the rate of formaldehyde formation at 500 and 600C for the
Cabosil, 2Na-Cabosil, 5Na-Cabosil series. For each temperature studied and for
each support material an optimal nominal MOO, loading in terms of selectivity
and rate of formaldehyde production can be identified. This optimal loading
depends upon the additive loading of the support, but generally allows catalysts
with vastly increased MOO, loadings to be made up without the severe loss in
selectivity observed without the additive.
A final point to note is the inhibiting effect of added sodium at low MOO,
contents. Essentially, the MOO, loading had to exceed a certain minimal value
(2-3 wt % MOO, in the case of 2Na-Cabosil) before any catalytic activity set in.
A further point of interest for all catalysts studied is the production of
large amounts of CO when formaldehyde selectivity diminished (Table 3). This
finding has been reported elsewhere and associated with formaldehyde
decomposition(ref.11).
Peaks due to Na,MoO, appeared during X-ray diffraction analysis of most of
the Na-Cabosil based catalysts used in this study before and after testing, with
smaller amounts of MOO, detected. For the 5Na-Cabosil series it was possible to
establish a correlation between the XRD phase composition and the rate of
formaldehyde formation (figure 4), which demonstrate a clear link between
formaldehyde production and the presence of crystalline Na,MoO,.
463
0.50 L
I
0.40 -
Figure 1: Influence
0.30 -
Moo,
loading
on
of
the
conversion of CE,.
(I)~Ho-Cabosil
( A)xHo-2Na-Cabos i 1
( 8Mo-5Na-Cabosil
0
15
10
5
20
Full symbols 6OOC.
MOO3 loadmg (%I
80
Open symbols 5OOC
I
Figure 2 Influence of Moo,
loading on the s e l e c t i v i t y
to formaldehyde.
(I)xHo-Cabosil
(A)r-o-ZNa-Cabosi 1
5
0
( 0No-5Na-Cabosil
10
15
20
Moo3 ioadirg (KI
s o l i d spkois 500c
-1s
60CC
Figure 3 Influence of HoO,
loading
on
the
rate
formaldehyde production.
(n)xno-cabosil
( A)xHo-ZNa-Cabosil
( 8)xJfo-SNa-Cabosil
F u l l symbols 5OOC
0
5
70
MOO3 'oading 1%)
15
20
Open s p b o l s 6OOC.
of
464
The T.P.R. patterns of
are shown in figure 5.
formaldehyde production
unsupported MOO, and the
a representative selection of the catalysts tested here
These demonstrate that the best catalysts in terms of
and selectivity are all more readily reduced than
sodium free molybdenaICabosi1 catalysts.
DISCUSSION
Methane conversions achieved in this work were low ((1%) and at first sight
the yields of formaldehyde are low when compared with other selective oxidation
reactions.
However, most selective oxidation processes operate with a
hydrocarbon: air ratio below the lower explosion limit. In practice the partial
pressure of hydrocarbon used can be as low as 0.015 atm in, for example the case
of n-butane oxidation and the partial pressure of product generated (ca 0.01 atm
of maleic anhydride) then compares with gas phase pressures of formaldehyde (ca
0.005 atm) achieved in this work (ref 14).
To date however attempts at
operating methane oxidation with low methane: oxidant ratios have failed due to
the poor methane activation properties of the molybdena catalysts in these
conditions.
We have already proposed that formaldehyde rather than methanol is the
predominant selective oxidation product observed at ambient pressure because the
following reactions occur either on the surface of the catalyst or in the vapour
phase (refs 11,12):
CH,.
+ 0 ---> CH,O.
111
---> CH,O + H.
[21
CH,O.
CH,O. + CH, ---> CH,OH + CH,.
131
In conditions of low methane partial pressure (ambient pressure) direct
decomposition of the CH,O radical (reaction 2 ) should be favoured. A t high
methane pressures collisions between radical species and methane molecules could
occur more readily, so that methanol would be the predominant selective
oxidation product in these conditions.
A recurring feature of the poor selectivities observed above particularly at
high temperatures is the appearance of CO in the reaction products, presumably
from the decomposition of CH,O (ref.11). Therefore, for the reaction conditions
used in this study i.e. high CH,:N,O ratios the molybdena silica catalysts can
be classified as sufficiently active but lacking the selectivity necessary to
permit the formaldehyde to exit the reactor without decomposition. In this
regard examination of Table 3 reveals that catalysts based on pure silica and
additive - silica achieved similar conversions of methane for a given nominal
MOO, loading.
However, improved selectivities were observed particularly at
600C when support additives were incorporated into these catalysts. It may be
concluded therefore that the support additives somehow reduce the amount of CH,O
decomposition which occurs, thereby increasing selectivity.
A general finding when additives, almost irrespective of their nature, are
465
50
75
u
0
0
Q
I
-E
I
F
E
-.
0
- 50
w
8
n
1
f
Y
.t
m
-z
W
- 25
0
5
@
I
m
!I
0
10
20
30
40
2
0
50
Moo3 loading (%I
Figure 4:
Rate of formaldehyde production over the 5Na-Cabosil series
(0) and the relative intensity of the X.R.D. peak at d-S.24A
for Na,HoO, (A).
-
$
.-
-;a
I)
Figure 5
T.P.R. profiles of a) no0
3 ;
6
b) 1Ho-Cabosil,
3
c) ZHo-ZNa-Cabosil,
b
U
d) 7Ho-ZNa-Cabosi1,
8
e) lOHo-5Na-Cabosi1,
f) ZOHo-5Na-Cabosil.
I
100
300
500
Temperatwe
700
(C)
900
466
incorporated into the molybdena/silica system is the very high molybdena
loadings achievable (ref. 8) without the corresponding losses in activity and
selectivity normally observed with the additive free systems. Several recent
studies of the molybdena/silica system have attempted to relate selectivity to
the presence of specific compounds on the silica surface.
These include
Based on the T.P.R. data
particularly heteropoly compounds (refs 8,lO).
presented in figure 5 and the correlation observed in figure 4 it is proposed
here that the additives bring about a change in the redox properties of the
catalyst surface, making it easier to extract lattice oxygen at the reaction
temperature. This in turn helps to establish an appropriate supply of lattice
oxygen at the surface so as to achieve a balance between the activation of
methane and the decomposition of formaldehyde.
Sodium has a beneficial effect in our test conditions provided the Mo:Na
ratio exceeds a minimal value.
It may be argued therefore that the sodium
modulates the redox properties of the molybdena through the formation of a
non-stoichiometric, hence defect rich, phase.
In operation this phase is
probably in a somewhat reduced state.
ACKNO-S
We gratefully acknowledge the support of the European Community non-nuclear
energy programme for this work (contract no: EN3C-0034-IRL)
REFERENCES
’I. N.R. Foster, Appl. Catal., 19 (1985) 1.
2 G.E. Keller and H.M. Bhasin. J. Catal.. 73 (1982) 9.
3 Methane Activation, Proc. 1st European Workshop, Bochum, May, 1988 Catal
Today, 4 (1989) nos 3-4.
4 H.D. Gesser and N. R. Hunter, Chem. Revs, 85 (1985) 235.
N.R. Hunter, H. D Gesser, J.A. Morton, P.S. Yarlagodda and D.P.C. Fung,
5
Symp. on Hydrocarbon Oxidation, New Orleans, Sept, (1987).
H . F . Lui, R.S. Lui, K.Y. Liew, R.E. Johnson and J.H. Lunsford, J. Amer.
6
Chem. So., 106 (1984) 4117.
M.M. Khan and G.A. Somorjai, J . Catal., 91(1985) 263.
7
8 S. Kasztelan and J.B. Moffat, J. Catal., 106 (1987) 512
9 N.D. Spencer, J. Catal, 109 (1988) 187.
10 Y. Barbaux, A.R. Elamrani, E. Payan, L. Gengembre, J.B. Bonnelle and B.
Grazbowska, Appl. Catal., 44 (1988) 117.
11 E. MacGiolla Coda. E. Mulhall. R. Van Hoek and B.K. Hodnett. Catal. Today,
-. 4
(1989) 383.
12 E. MacGiolla Coda, R. Van Hoek, E. Nulhall and B. K. Hodnett, Hydrocarbons,
Lyons, Sept 1988.
13 Handbook of Chemistry and Physics, 68th Edition 1987-88, CRC Press.
14 J.C. Burnett, R.A. Keppel and W.D. Robinson, Catal. Today, 1 (1987) 537.
467
Prof .E.Bordes (Vniversite de Technoloaie de ComDieanel : Have you
tried adding Eia, Cut P, or Pb after loading molybdenum on silica
to see how this influences the performance of your catalysts?
You mentioned that additives bring about a change in the redox
properties of the surface and I am in accordance with that. In
the case of sodium and copper you can form Na2Mo04 (which you
have seen)and CuMoO4, whereas with phosphorus or silicon you can
could form heteropolyanions. Do you see differences in reactivity
for these two kinds of additives?
Dr B.K.Hodnett (University of Limerick. Irelandl: We tried to
reverse the order of impregnation with the sodium system, i.e.,
adding the dopant after the molybdenum component. In such
conditions no appreciable beneficial effect was observed.
We have detected the presence of Na2MoO4 on our sodium treated
catalysts by XRD, but we have not detected any other complex
oxide by XRD when other additives were used. Generally we have
found that the presence of Na, Cu or P results in less combustion
of the selective oxidation products at elevated temperatures.
Prof 0. Krvlov [Institute of Chemical Phvsics. Moscow):
In
connection with the interesting results reported by Dr Hodnett I
would like to mention an interesting observation made by us. When
we used the reversed catalyst, i.e. silica supported on
molybdena, we have observed 100% selectivity in the oxidation of
methane to formaldehyde in similar experimental conditions.
Dr B.X.Hodnett
[Universitv of Limerick. Ireland): It is
gratifying to see a system which achieves activation of methane
and a reasonable conversion without combustion of the selective
oxidation product.
Dr Sinevmv Institute of Chemical Phvsics. Moscow): What can you
tell us about the efficiency of your catalysts in the presence of
02 as oxidant? If they do not produce formaldehyde in these
conditions does it mean that 02 cannot reoxidize the active sites
or that there are some other problems?
Dr B.K.Hodnett [Universitv of Limerick. Ireland):
We have
observed selective oxidation with this system using 02 as
oxidizing agent.
Prof. Baerns (Ruhr-Universitat Bochum): In your presentation you
defined the rate of formaldehyde production as yield which is
contradictory to its usual definition. To make comparison with
other data easier, please, indicate the degree of methane
conversion (X) besides the selectivity data (S) to calculate the
yield (Y) as commonly described:
Y% = ( S % * X%) / 100
468
Dr B.K.Hodnett (University of Limerick, Ireland) : We have
presented sufficient imformation in our paper which allows our
yields to be calculated. However, we find this single index of
catalytic performance to be misleading because it fails to take
into account the partial pressure of hydrocarbon in the feed
stream.
In many conventional selective oxidation processes a
hydrocarbon lean feed is used.
In these conditions high
conversion of hydrocarbon can be achieved. In our case we use a
hydrocarbon rich feed, so that a lower conversion can still
result in partial pressures of selective oxidation product being
produced which compare with those produced in many conventional
oxidation processes.
It is for this reason we expressed
formaldehyde production in terms of a reaction rate as this also
takes into account the reactor loading and feed gas flow rate.
Dr. Grzvbowska (Institute of Catalvsis. Krakow): Your TPR data
suggests that you have sever1 types of M-0, species dispersed on
silica which is in accord with the literature data on this
subject quoted by you (refs 8-10) in your paper. On the other
hand you show the correlation between the content of Na2Mo04
phase and the rate of aldehyde formation, stating that redox
properties may play a role in CH4 oxidation. Could you ascribe
any of your TPR peaks then to the reduction of Na2MoO47
Dr B.K.Hodnett funiversitv of Limerick, Ireland):
No.
G. Centi and F. Trifiro’ (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
469
OXIDATIVE DIMERIZATION OF IiETHANE IN HIGH TEMPERATURE ELECTROCHEMICAL DEVICES
V.D.
BELYAEV, O.V.
BAZHAN, V.A.
SOBYANIN and V.N.
PARMON
Institute of Catalysis, Novosibirsk 630090, USSR
SUMMARY
Studied is oxidative dimerization of methane in electrocatalytic and standard catalytic conditions on electrode-catalysts
made from Cu, Ag, Ni, Pt, Ag (20 at.%)
Pd (80 at.$) alloy and
on lanthanum chromite-based oxide systems in high temperature
electrochemical devices with solid oxides or molten carbonate
oxygen-conducting electrolytes. Similarities and differences in
reaction occurrence under the above conditions have been elucidated. As shown, the electrolytic regime may be more advantageous than the standard catalytic regime for the purpose of increasing the selectivity of production of C2-hydrocarbons.
-
INTRODUCTION
The development of efficient methods of CH4 converoion into
valuable chemicals, in particular %hydrocarbons (refs. 1-3) is
one of the most serious problems of applied interest faced by modern catalysis science (refs. 1-31. In this connection of great
interest are recent data on gas-phase electrocatalytic oxidative
dirnerization of CH4 in high temperature electrochemical devices
(fuel cells or oxygen pumps) (refs. 4-7). It ie essential that
when operated under definite conditions, such devices make it possible to produce simultaneously electricity and purpose product8
(operation with fuel cells). Also, these devices can be applied
for production of only purpose products with the aid of external
energy supply (operation with electrolizer or electrochemical
pump). These two regimes of operation will be called hereinbelow
as electrocatalytic.
The oxidation of CH4 in electrocatalytic regimes has been studied (refs. 4-6) on Ag, Ag-Bi203, Ag-Li/&lgO and LiNi02 electrodecatalysts which were in contact with a yttria-stabilized zirconia
electrolyte (YSZ) providing oxygen transport to the reaction zone.
The objective of this work was to atudy oxidative dimerization
of methane in high temperature fuel cells (FC) using a YSZ electrolyte (SOFC) on electrode-catalysts from Cu, Ni, Pt, Ag, Ag(80%)
470
Pd(20%) alloy and lanthanum chromite-based oxides as well as electrolyte in the form of molten carbonates of alkaline metals
(MC) on Ni and Ag electrode-catalysts (MCFC).
METHODS
SOFC were test-tubes made from a YSZ electrolyte with composition 0.9 Zr02+ 0.1 Y203 on the internal side of which a working
electrode (anode) was supported and on the external side a
counter electrode (cathode). The geometrical area of the electrode6 was 10 cm2
MCFC were prepared on the basis of a porous LiA102 disc matrix
impregnated with a mixture of molten carbonate of lithium and potassium, The geometrical area of electrodes which were placed on
the oppoaite s i d e s of the matrix was 4 cm2
The methane oxidation reaction was studied under atmospheric
pressure in electrocatalytic and, for comparison, in standard catalytic regimes at 650-680OC for MCFC and 700-890°C for SOFC. In
the both regimes a flow of methane or a helium-methane mixture
was fed into the anodic space of a fuel cell with a velocity of
1 cm3/s. Simultaneously, the cathode was blown by air for SOFC
and by an air-C02 mixture for MCPC.
In the electrocatalgtic regime oxygen was fed directly into
the reaction zone by pasaing the electric current through FC. The
initial methane or helium-methane mixture flow had no oxygen. The
operation of M: in this regime is shown in a schematic fashion in
Fig. 1, For S O X the passing of the electric current leads to the
-
.
.
CH4-
:,,2/ : ,:,
C0,C02 C H C H
+ 4e-
-
20
( 2C02+02+4e
O2
~
Anode
- YSZ (MC) electroly-
'Cathode
2CO;-)
Pig. 1. Schematic diagram of electrocatalytic performance of methane oxidative dimerization.
reduction of molecular oxygen to 02- ions
ions are then transferred through the YSZ
they can either discharge producing O2 or
ally, the similar situation occurs during
on the cathode. The
into the anode on which
oxidize methane. Actuthe electrocatalytic
471
oxidation of methane in MCFC. The only difference is that oxygen
anions.
is transferred from the cathode into the anode by C0:The relation between electric current (1) and the oxygen transfer flux ( Q ) from cathode to anode can be written as follows:
I = 4FQ,
where F is the F’araday constant.
In the standard catalytic regime oxygen was fed by portions
into f l o w s of methane o r a helium-methane mixture prior to their
feeding into the FC anodic space. The electric circuit of FC was
disconnected and the anode served as conventional heterogeneous
catalyst.
When the reaction was carried out in the electrocatalytic and
catalytic regimes the oxygen f l u x into the reaction zone was expressed in the same units (amperes, A).
The gas mixture composition before and after the FC was analysed chromatographically. The experimental apparatus has been described in detail elsewhers (ref. 8 ) .
RESULTS AND DISCUSSION
It has been found f o r all systems studied that methane oxidation in the electrocatalytic regime yields ethane and ethylene,
along with CO, C02 and H20. Oxygen-containing compounds and higher hydrocarbons were in negligible amounts.
In the SOFC with Ni and Pt electrodes the rate of oxidative
conversion of methane sharply and irreversibly decreased with
time and current-voltage characteristics of working electrodes
worsened. These phenomena result from delamination of Ni and Pt
from the YSZ surface due to carbon which is formed during the reaction and coked at the metal electrolyte interface.
A s distinct from the SOFC with Ni and Pt working electrodes,
other FCs as a catalytic reactor were characterized by stable
operation for a long period of time (up to 70 h).
Consider now the data obtained for these FCa in more detail.
Figures 2 and 3 show dependences of steady-state rates of ethane
and ethylene formation, methane conversion and selectivity toward
C2 hydrocarbons on oxygen flow in methane oxidation on Ag and AgPd alloy-based electrode-catalysts supported on YSZ in electrocatalytic and catalytic regimes. As can be seen in Fig. 2a, when
the reaction is performed on the Ag electrode in both regimes
472
..
a,
Oxygen flow (A)
Oxygen flow ( A )
Fig. 2. Rates of ethane (la,2a) and ethylene (3a,4a) formation,
methane conversion (lb,2b) and selectivities toward C hydrocarbons (3b,4b) vs. oxygen flow rate in methane oxidatiog on A g contacting with YSZ in electrocatalytic (Ia,lb,3a,3b) and catalytic
(Za,Zb,4a,4b) regimes. Reaction conditio 8 : 795OC; CH4 concentration 100 vol.%; methane flow rate 1 cm9/a.
6
.a4
fi
\
rt
s"
$2
W
0
0.2
0.4 0.6
Oxygen flow (A)
0.8
"
0.2
0.4
0.6
0.8
Oxygen flow (A)
Fig. 3. Rates of ethane (la,2a) and ethylene (3a,4a) formation,
methane conversion (lb,2b) and selectivities toward C hydrocarbons (3b,4b) vs. oxygen flow rate in methane oxidatiog on the
Ag (80 at.%)-Pd(20
at.%) alloy contacting with YSZ in electrocatalytic (la,lb,3a,3b) and catalytic (2a,2b,4a,4b) regimes. Reaction conditions: 84O05; methane concentration 10 vol.%; methanehelium flow rate 1 cm /a.
473
the rates of formation of C2 hydrocarbons increase with increasing oxygen flow from 0.1 to 0.6 A (or the 02/CH flow rate ratio
4
from 8
to 4
According to data in Pig, 2b in the electrocatalytic regime the selectivity changed from 50 to 4% and
in'the catalytic regime remained almost unchanged (ca. 20%). In
both regimes oxygen conversion was close to 100% and the conversion of methane depended linearly on oxygen flow being no higher
than 3% (Fig. 2b).
A comparison of the reeults obtained indicates that in the
electrocatalytic regime the yield, rate and selectivity towards
C2 hydrocarbons are 3-4 times higher than those achieved in the
standard catalytic regime.
As is seen in Fig. 3a, when the reaction is carried out on the
Ag-Pd alloy electrode, in both regimes the rates of formation of
C2 hydrocarbons first increase and then reach a plateau with increasing oxygen flow from 0.1 to 0.75 A (or the 02/CH4 flow rate
ratio from 0.06 to 0.46). According to data in Fig. 3b the conversion of methane and the selectivity tend to change from 4-2496
and 58-43% and from 4-29% and 21-34%, respectively, for the electrocstalytic and catalytic regimes.
A comparison of results obtained for oxygen flows smaller than
0.4-0.5 A (02/CH4 u 0.25-0.31) evidences that in the electrocatalytic regime the yield of ethane is 3-5 times higher and of ethylene is 4-10 times higher than their yields in the standard catalytic regime. Note that under these conditions oxygen is completely converted in both regimes.
On increasing oxygen flow (> 0.55 A ) the rates of formation of
C2 hydrocarbons in both regimes are comparable (Fig. 3a) and the
selectivities are nearly the same (Fig. 3b). Interestingly, such
an effect has also been observed for the Ag electrode-catalyst
upon feeding methane diluted with helium into the anodic space.
This effect is due to that in the electrocatalytic regime athigh
rates of oxygen flow the reaction of oxygen evolution into the
gas phaae is predominant
and the oxidation of methane occurs as for standard catalysis.
Thus, the results obtained show that performance of oxidative
dimerization of methane on Ag and Ag-Pd alloy electrodes in the
electrocatalytic regime is, under definite conditions, more ad-
474
vantageous than in the standard catalytic regime. A s has been
found for Ag electrode-catalyst (Fig. 41, the electrocatalytic
regime becomes more advantageous than the catalytic one with increasing temperature. The differences observed between electrocatalytic and catalytic regimes seem to be due to an important
role of electrochemical steps in the former regime which affect
the state of oxygen on the electrode surface (ref. 9).
Results similar to those of Figs. 2 and 3 have been obtained
for other electrode-catalysts too. Below we summarize the main
results.
On the copper electrode which was in contact with YSZ at 80089OoC the steady-state rate of formation of C2 hydrocarbons in
the electrocatalytic regime of methane oxidation was more than
1.5 times higher than that in the catalytic regime. Different from
Ag and Ag-Pd, in the electrocatalytic regime on Cu electrodes the
rate of C2-hydrocarbons f ormation slowly achieved its steady siate. A s can be seen in Fig. 5, more than a two times decrease in
rate was observed. However, 6s current was switched off for only
one minute and then switched on, the rate tended to increase to
its initial value.
This observation suggests the possibility to enhance reaction
efficiency by applying the electrocatalytic method in unsteadystate conditions. In particular, it has been found that upon periodic switching on a switching off the current passing through the
cell (oxygen flow through the electrolyte) a period-average rate
of formation of C2 hydrocarbone is car 2 times higher than the
steady-state reaction rate. This fact seems to be due to partial
copper oxidation upon feeding oxygen through the electrolyte and
its reduction with methane at switching off the current.
In SOFC on the electrode based on lanthanum chromite the oxidative dimerization of methane was studied at 815OC. As found experimentally, in both regimes the selectivity toward C2 hydrocarbons did not exceed 446, and the main reaction product was C02.
However, addition of lithium chloride significantly enhanced the
selectivity toward C2 hydrocarbons. E.g., in the electrocatalytic
regime upon varying the 02/CH4 flow rate ratio from 0.06 to
0.24 the selectivity achieved 60-45% at methane conversions 8-2346.
In this instance the yield of ethylene was ca. 4 times higher
than the yield of.ethane being, under optimal conditions, 8,546.
475
I
800
850
750
800
8!
Temperature ( O C )
Temperature ( O C ;
Fig. 4. Rates (a) and selectivities (b) of formation C hydrocarb o n ~VS. temperature in methane oxidation on Ag contacfing with
YSZ in electrocatalytic (1,2) and catalytic (3,4) regimee. Reaction conditiono: methane concentration 10096, methane flow rate
1 cm3/e, oxygen f l o w 0.6 A.
750
-
-
30
60
90
120 Time (min)
VB. time in mePig. 5. Rate8 or formation C hydrocarbons (W/W*)
thane oxidation on Cu contachng with YSZ in electrocatalytic regime. +?- moment8 of switching on and off the current (I) through
the cell (oxygen flow through the electrolyte). W and 1* current
and steady-state rates. Reaction conditione: 860°C, methane concentration 1008,methane flow rate I cm3/e.
-
In the catalytic regime the yield and selectivity toward C2 hydrocarbone mere 3 time8 lower than those in the electrocatslytic regime. Thw, modification of the electrode by additives that inc r e m e the efficiency of the standard catalytic reaction improve
even to a larger extent the reaction parameters in the electrocatalytic regime.
476
In IVICPC the oxidative conversion of methane was studied on A g
and Ni electrode-catalysts at 650-68OOC. The rate of formation of
C2 hydrocarbons on these electrode-catalysts was found to be ca.
2.5 times higher in the electrocatalytic regime than in the catalytic one. The difference in selectivity toward C2 hydrocarbons
w a s the same; note that in any experiment the selectivity w a s low
and did not exceed 10% even at low conversions of methane (1-2%).
Such a l o w selectivity at methane oxidation in MCFC seems to be
due to that (i) electrode-catalysts had the composition far from
optimum and (ii) the temperature of operation of MCFC was by ca.
looo lower than the typical temperature of oxidative dimerization
of methane.
CONCLUSIONS
Thus, with some electrode-catalysts the electrocatalytic regime
for oxidative dimerization of methane is more advantageous t h m
the catalytio one. The nature of this phenomena is not quite clear and much work remains to be done in this direction. However,
w e believe that the electrocatalytic method opens up a new way
for oxidation reaction performance.
REFERENCES
1
2
3
4
5
6
T. Ito and J.H. Lunsford, Nature, 314 (1985) 721.
R. Pitcha and K. Klier, Catal. Rev., 28 (1986) 13.
Kh.M. Minachev, N.Ya. Usachev, V.N. Udut and Yu.S. Khodakov,
Usp. Khim., 57 (1988) 385 (in Russ.).
K. Otsuka, S. Yokoyama and A. Morikawa, Chem. Lett., (1985)
319.
S. Seimanides and M. Stoukides, Electrochem. SOC., 133 (1986)
1535.
K. Otsuka, K. Suga and I. Yamanaka, Chem. Lett., (1988) 317.
7 V.D. Belyaev, O.V. Bazhan, V.A. Sobyanin and V.N. Parmon,
Proc. 7th All-Union Conference on Electrochemistry, Chernovtsy,
USSR, 1988, vol. 3 , p. 134 (in Russ.).
8 V.D. Belyaev, V.A. Sobyanin, V.A. Arzhannikov and A.D. Neuimin,
Dokl. AN SSSR, 305 (1989) 1389 (in Russ.).
9 V.D. Belyaev, V.A. Sobyanin and O.A. Mar'ina, Izv. SO AN SSSR
Ser. Khim. Nauk (in Russ.) (accepted for publication).
G . Centi and F. Trifiro' (Editors), New Developments in Selective Oxidatwn
- Printed in The Netherlands
0 1990 Elsevier Science PublishersB.V., Amsterdam
411
SELECTIVE DEHYDROGENATION OF ETHANE BY CARBON D I O X I D J OVER Fe-?,In
OXIDE CATALYST. AX I N S I T U STUDY OF CATALYST PHASE COMPOSITION
AM) STRUCTURE.
22. K h m W E D O V , P.A. SHIRYAJN, D.
.
P. SHASIIICIN, 0. V. KRYLOV
I n s t i t u t e of Chemical Physics o f t h e USSR Academy o f Sciences,
117334 Moscow, Kosygin st 4, USSR.
SUMMARY
The r e s u l t s on a c t i v i t y , phase composition and c r y s t a l l i n e
s t r u c t u r e r e c o n s t r u c t i o n s t u d i e s concerning t h e Mn c a t a l y s t and
Fe-Mn c a t a l y s t i n t h e course of ethane dehydrogenation by carbon
dioxide a r e presented.
It has been shown t h a t manganese oxide systems modified by
s m a l l q u a n t i t i e s o f i r o n a r e e f f e c t i v e c a t a l y s t s f o r ethane dehydrogenation by carbon dioxide.
INTRODUCTION
Dehydrogenation o f methane and ethe.ie by t h e unconventional
oxidant-carbon dioxide i s an i n t e r e s t i n g process from both scient i f i c and p r a c t i c a l point o f view.
Carbon dioxide i n equimolecular mixtures w i t h methane c o n v e r t s
t h e l a t t e r i n t o syngas w i t h CO+H2 s t o i c h i o m e t r i c r e l a t i o n s h i p
( r e f s . 1-21:
C02 + CH2 -+2CO + 2H2
(11
Conversion o f ethane t a k e s place by t h e r e a c t i o n :
CzH6 + C02
C2H4 + CO + H20
(2)
The r e a c t i o n ( 2 ) assumed t o be s e l e c t i v e i s accompanied by t h e
by-reaction o f deep conversion:
C H + 2C02 -9 4CO + 3H2
(3)
2 6
Under t h e s e c o n d i t i o n s a t 78Oo-85O0C t h e following r e a c t i o n s
a r e proceeding :
2C2% -+C2H4 + 2CH4
(4)
-+
-+
iC02
CH4 + 2CO +H2
The composition o f t h e r e a c t i o n products depends on t h e
r e a c t i o n s ( 1 )-( 5 ) r a t e s r e l a t i o n s h i p .
C2H6
(5)
MPERIMFXL"L
The r e a c t i v i t y experiments were c a r r i e d out i n pulse and flow
r e a c t o r s w i t h t h e v i b r o l i q u i f i e d bed of t h e c a t a l y s t a t 780°-850?
The X-Bay s p e c t r a i n s i t u were taken using a d i f f r a c t o m e t e r DRON2.0 with FeKdradiation.
The r e a c t i o n mixture C2% + C02 w a s passed a t t h e r a t e o f 3
cc/min through t h e X-Ray chamber-reactor which allows t o analyse
simultaneously the changes i n t h e phase composition as w e l l as
t h e parameters of t h e c a t a l y t i c r e a c t i o n (ref.3). The s t u d i e s i n
s i t u were c a r r i e d out at 6OO0C, t h e c a t a l y s t volume w a s 1 cc,
The following systems have been i n v e s t i g a t e d i n t h e c a p a c i t y
of catalysts:
Mn
0 / SiO2
(11,
Fe
Mn
O / Si02 (II)
lChe c a t a l y s t s were prepared by impregnation of t h e s i l i c a g e l
KSX with nitrates of Mn and Fe. The content of Mn and Fe i n t h e
samples amounted t o 17% and 4% respectively.
-
-
RESULTS AMD DISCUSSION
The c a t a l y s t &fn-O/SiO, i s e f f e c t i v e i n t h e 50% C2H6 + 50% C 0 2
mixture. Under t h e steady-state c o n d i t i o n s the accumulation of
coke d e p o s i t s on t h e s u r f a c e i s not observed i n t h e presence o f
this c a t a l y s t .
The ethane conversion on t h e Mn-O/SiO, c a t a l y s t i n c r e a s e s when
it i s modified by Fe ( t a b l e 1 ).This c a t a l y s t when c a l c i n e d at
45OoC showed t h e presence o f o n l y one phase of manganese:p-MnO2.
The Si02 c a r r i e r i s h i g h l y dispersed. A f t e r t h e c a l c i n a t i o n a t
high temperatures of 600°-7000C p a r t i a l r e d u c t i o n o f p-Nn02 i n t o
the %03
phase i s observed.
TABLE 1
Data on dehydrogenation of ethane by carbon dioxide on Mn c a t a l y s t and Fe-Mn c a t a l y s t . C2H6/C02 = 1 :1.6-1.8
Catalyst
TOC
m-O/SiO2
770
800
Fe-Mn/Si02
770
810
Conversion, %
c2H6
49.8
73.1
65.9
81.0
"2
57.8
79.0
40.7
52.8
S, C
%
51.5
63.0
62.5
72.4
~
H
~Yield
C2H4, %
25.5
46.0
41.2
58r 7
I n Fe-Mn-Si02 c a t a l y s t another phase d-Fe 0 was detected.
2 3
With i n c r e a s e of the i r o n oxide content at t h e p r e p a r a t i o n stage,
479
only t h e change i n t h e Fe203-Ab02 phases r e l a t i o s h i p occurs.
When preparing t h e binary Fe-Mn c a t a l y t i c system p a r t i a l
screening o f t h e Mn surface by t h e oL-Fe203 phase i s supposed t o
occur. The dimensions o f t h e c r y s t a l l i t e s o f t h e &-Fe203 a r e l e s s
(-2OOA) t h a n t h e dimensiona o f t h e c r y s t a l l i t e s o f t h e pMn02
phase (h1000A). I n t h e course o f t h e r e a c t i o n t h e phase compo,ait i o n o f t h e Fe-Bbn c a t a l y s t changes: Mn oxide i s reduced t o t h e
f u l l and t h e d-Fe203 phase i s p a r t i a l l y reduced.
I n t h i s c a s e ethane comresion proceeds u n s t e a d i l y and t h e
r a t e s of t h e products formation depend on time. Soon a f t e r t h e
charge o f t h e i n i t i a l sample 11 (V = 1800 hr-’ c2H6/co2 = 1 ) t h e
s e l e c t i v i t y a t 780°C i s l o w (35%) as t h e r e s u l t o f C ~ pHa r ~t i a l
conversion i n t o C02 on t h e more oxidized phase o f Pn. A s t h e rea c t i o n proceeds and Mn oxide i s reduced t h e r a t e o f C02 formation
decreases; i n t h i s c a s e t h e s e l e c t i v i t y f o r C,H4 i n c r e a s e s up t o
60%.
Since t h e r e d u c t i o n o f t h e c a t a l y s t by t h e r e a c t i o n mixture
proceeds under steady-state c o n d i t i o n s , we have s t u d i e d t h e int e r a c t i o n o f ethane w i t h t h e c a t a l y s t and r e o x i d a t i o n o f t h e cat a l y s t with C 0 2 i n order t o model t h e processes accompanying t h e
catalysis.
Figure 1 demonstrates t h e curves presenting t h e i n t e n s i t i e s
changes of %03
and Fe20g while reducing t h e c a t a l y s t by ethane.
The Ebn203 i s t h e main Nn phase a v a i l a b l e bafore t h e feeding o f
C H a t 60OoC. A t first t h e polymorphous t r a n s i t i o n o f Mn203 t o
2 6
Mn 0 proceeds; t h e most i n t e n s i v e period o f i t i s a t t h e begin3 4
ning of t h e reduction. After t h e 70 minute exposure t o C2H6 N?O3
disappears completely, t h e i n t e n s i t y of t h e MnO phase i n c r e a s e s
and
0 passes through a maximum.
3 4
- M9O3
2 - MnO
3 - &Fe2O3
4 - m304
1
0
50
100
150
200
250
Time, min
Fig. 1. Change o f t h e c a t a l y s t phase composition as a function o f
time under t h e r e a c t i o n conditions.
480
The i n t e n s i t y of t h e Fe203
phase a t t h e i n i t i a l s t a g e o f
t h e r e d u c t i o n remains unchanged
and a f t e r t h e completion of t h e
phase t r a n s i t i o n f r o m Lin304 t o
MnO the i n t e n z i t y decreases.
Absence o f p r e c i s e d i f f r a c t i o 0
n
22
20
18 <- 8 nal maxima f o r t h e Pe 0 phase
3 4
i s l i k e l y t o be due t o t h e formation of t h i s phase i n h i g h l y
tr; 50
d i s p e r s e d state.
Figure 2 demonstrates parts
of t h e Pe-Idn c a t a l y s t d i f f r a c r ' o
tograms a t t h e preparRtion
20, 100
s t a g e , a f t e r h e a t i n g i n a He
stream and a f t e r r e d u c t i o n (reaction).
50
A f t e r feeding of ethane t h e
l e v e l o f i t s conversion decreas e s gradually. A t t h i s stage of
r e d u c t i o n the s e l e c t i v i t y f o r
C H i n c r e a s e s ( s e e Fig. 3).
2 4
Mainly, i t occurs a t t h e
Fig. 2. XRIU p a t t e r n s of t h e
i n i t i a l s t a g e of reduction.
c a t a l y s t Fe-Mn/Si02: ( a ) before
%hen t h e curve of s e l e c t i v i t y
heating; (b) a f t e r h e a t i n g a t
reaches the plateau, a maximum
60OoC; ( c ) a f t e r the r e a c t i o n
i s observed on t h e curve o f t h e
C2H6+C02*
IdnO phase change. I t s d i f f r a c kR 75
h
.rl
.P
L)
u
8)
U>
I
0
50
I
100
I
150
1
2m
250
Time, min
3. Change of ethane conversion and s e l e c t i v i t y for C 2H 4 a s
a f u n c t i o n of t h e c a t a l y s t r e d u c t i o n i n s i t u .
Fig.
481
t i o n l i n e s g r a d u a l l y widen i n t h e process of r e d u c t i o n i n d i c a t i n g
high d i s p e r s i t y of t h i s phase. Maximum s e l e c t i v i t y i s a t t a i n e d
when t h e c a t a l y s t composition becomes steady-state.
A s e r i e s of experiments on t h e dynamics o f t h e c a t a l y s t phase
composition change i n t h e course o f t h e r e g e n e r a t i o n with C 0 2 h a s
been c a r r i e d out.
I n t h i s c a s e t h e i n c r e a s e o f t h e Fe 0 content r e l a t i v e t o t h e
2 3
manganese-containing phase i s l i k e l y t o be due t o t h e o x i d a t i o n
of t h e Fe304 phase. The phase t r a n s i t i o n i s accompanied by CO
e v o l u t i o n i n t o t h e g a s phase involving 0 t r a n s f e r from C02 t o
2
t h e reduced Fe phase, Modification of t h e manganese system by
i r o n r e s u l t s i n t h e formation of t h e multiphase system which i s
l i k e l y t o promote f a c i l i t a t i o n of t h e exchange processes, involFe oxide i n t e r f a c e
v i n g oxygen removal from CO on t h e Mn oxide
2
Under t h e steady-state c o n d i t i o n s Mn i s a v a i l a b l e i n t h e MnO
phase and Fe i s a v a i l a b l e i n t h e Fe 0 and Fe 0 phases.
2 3
3 4
C2H6 conversion can be presented by t h e following scheme:
N(1) R(2) N(3)
-+C2H4
+ H2
1
0
0
2H6
1
1
1
2'10 + co2
-+ Z"C0 3
c2H6 + Z"CO3 -+C2H4 + H2O + Z"O + CO
O
1
I
C2H6 + 2 ' 0 -3C2H4 + H20 + 2 '
0
1
0
1
1
0
2' + Z"C03
- 3 Z ' O ' + Z"0 + co
1
0
0
2 ' 0 + H2
-+2'
+ H20
According t o t h e scheme C2H6 conversion by COP t a k e s place
mainly by two processes: o x i d a t i v e dehydrogenation and ethane pyr o l y s i s followed by t h e hydrogen oxidation.
Oxidative dehydrogenation of ethane t a k e s place w i t h the part i c i p a t i o n o f t h e Fe 0 ( 2 ' 0 ) phase and Pdn-containing fragment
2 3
( ZWO3 1.
A c t i v a t i o n o f t h e Mn-system by i r o n i s l i k e l y t o be due t o t h e
f a c t that t h e Mn-containing phase ( M n O ) i n t h e course of reoxid a t i o n p a r t i c i p a t e s i n t h e O2 t r a n s f e r from C02 c o n t r i b u t i n g t h u s
t o t h e phase conversion Fe304 -3 Fe203:
MnO.. .Fe3O4 % MnC03. ..2Fe304 --3 m0.. 3Fe 0 + CO
2 3
The MnO-Fe304 i n t e r f a c e i n t h i s c a s e makes t h e C 0 2 reduotion
easy. This scheme a g r e e s w i t h t h e data o f ( r e f . 3 ) where t h e
t r a n s f e r of O2 from Mn-oxide t o t h e reduced phase of Fe-oxide i s
shown. With i n c r e a s e o f Fe i n t h e c a t a l y s t up t o 10% t h e improvement of t h e t o t a l ethane conversion is accompanied by a dramatic
decrease o f s e l e c t i v i t y for ethylene. It seems t o be related to
-
.
482
the accumulation on the Fe-containing phase surface of carbon
fragments and t h e i r removal upon i n t e r a c t i o n with C02. I n t h i s
case the number of the s i t e s of CO a c t i v a t i o n decreases and the
2
number of the a c t i v e s i t e s f o r the rupture of C-H, C-C bonds increases involving 4 C ) - o r CO formation.
Therefore, dehydrogenation of ethane by carbon dioxide on t h e
Mn-catalyst, modified by small q u a n t i t i e s of Fe (2-3%), leads t o
the formation o f ethylene, CO and H2.
REFERENCES
1 Sh.A.IVurieV,
I.A.Guliev, Ak.H.Edarnedov, Proceedings 2 Republic
Conference of doung sclentists-chemists, Baku, 1986.
2 K.I.Aika,
!P.ITishiyama, Proceedings 9 International Congress
on c a t a l y s i s , 1988, v.2, p.907.
3 P.A.Shiryaev, D.P.Shaahkin, &Sh.Zurmuhtashvili, L.Ya.largolia,
O.V.Krylov,
Kinetika i U t a l i z , 1984, v.25, N5, p.1164.
G. Centi and F. Trifiro' (Editors), New Developments in Selective Oxidation
1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
483
ROLE OF ACTIVE OXYGEN FORMS AND ACIDITY I N OXIDATlVE
CONVERSION OF ETHANE ON ZEOLITES
S. N . VERESHCHAGIN, N . N . SHISHKINA, A. G. ANSHITS
I n s t i t u t e of C h e m i s t r y and Chemical Technology,
S i b e r i a n Branch
of t h e USSR Academy of S c i e n c e . K a r l M a r k s s t . , 42, K r a s n o y a r s k .
680049. USSR
SUMMARY
C a t a l y t i c o x i d a t i o n of e t h a n e w i t h n i t r o u s o x i d e a n d oxygen
ZSM-5 a n d m o r d e n i t e .
Selective
h a s been
i n v e s t i g a t e d over
c o n v e r s i o n of e t h a n e t o e t h y l e n e a n d p r o p y l e n e w a s believed
to
i n v o l v e s u r f a c e oxygen atomic s p e c i e s which w e r e formed by t h e
i n t e r a c t i o n of n i t r o u s o x i d e w i t h z e o l i t e s . C a t a l y t i c a c t i v i t y w a s
p r o p o r t i o n a l t o t h e number of N20-decomposition sites a n d d i d n o t
depend upon t h e t o t a l number of s t r o n g l y acidic sites measured by
ammonia c h e m i s o r p t i o n .
I NTRODUCTI ON
Zeolites
have
attracted
c o n s i d e r a b l e a t t e n t i o n as
catalysts
for m o r e t h a n t w o d e c a d e s a s a r e s u l t of t h e i r h i g h a c t i v i t y and
unusual
selectivity
for
acid-catalyzed
reactions.
The
lack
of
i n t e r e s t i n z e o l i t e s as c a t a l y s t s for s e l e c t i v e o x i d a t i o n may be
of poor s e l e c t i v i t i e s and h i g h r e a c t i o n t e m p e r a t u r e s .
a result
However
has
it
c o n v e r s i o n of
HZSM-5-type
Crefs.
recently
been
reported
methane t o h y d r o c a r b o n s ,
that
catalytic
the
i n c l u d i n g aromatics o v e r
z e o l i t e i n t h e p r e s e n c e of N 0 or O2 w a s o b s e r v e d
2
Differences
in
the
product
distribution
and
1,2>.
catalytic
activity
mechanism is n o t t h e
with
two
the
oxidants
indicate
that
the
s a m e for N 0 a n d 02. Although t h e mechanism
is unknown i t is s u g g e s t e d
2
that initial
r e a c t i o n is t h e p r o t o n a t i o n of
s t e p of
t h e catalytic
methane by s u p e r a c i d sites Cref.
3>. I t h a s been r e p o r t e d t h a t O2 a n d N 0 h a v e e x t r e m e l y d i f f e r e n t
2
a c t i v i t i e s and a l s o t h e o x i d e r a d i c a l i o n 0-. which
c a n be
generated
primarily
r e a c t i v i t y t h a n 02,
in
the
decomposition
2-,
03, 0 i o n s
Crefs.
s t u d i e d t h e o x i d a t i o n of e t h a n e b y N 2 0
of NgO.
4-63.
shows
higher
W e have t h e r e f o r e
a n d O2 over
HZSM-5 a n d
m o r d e n i t e which w i l l b e of s i g n i f i c a n t i n t e r e s t i n t h e c h e m i s t r y
of a c t i v e oxygen s p e c i e s and a c i d - b a s e p r o p e r t i e s of t h e s u r f a c e .
484
EXPERIMENTAL.
Materials.
High
purity
g r a d e C99.8+%3, e t h a n e ,
oxygen
n i t r o u s o x i d e w e r e used without f u r t h e r p u r i f i c a t i o n .
and
Helium w a s
p u r i f i e d by p a s s i n g t h r o u g h CaA-liquid n i t r o g e n t r a p .
The a c i d i c f o r m of
and mordenite w a s
t h e 234-5
e x c h a n g i n g t h e N a c a t i o n s w i t h NH C1 1 . 0 N a t QO'C
4
c a l c i n i n g i n a i r a t 550°C.
impregnation
w a s o b t a i n e d by w e t
1.5%Na-HZSM-SC413
HZSM-SC41>
of
with
aqueous
o b t a i n e d by
a n d f u r t h e r by
solution
NaOH.
of
Exper i ment s h a v e been p e r f or med on sampl es w i t h Si 0 2 / A l 203 r a t i 0s
equal
to
38.
41,
60.
90,
H a - 8 .
148 for
and
I1
HM.
for
Na
c o n t e n t w a s as l o w as 0.1%.
Zeol i tes
Equi pment a n d C a t a l y s t Eva1 u a t i o n .
60-80 mesh for u s e i n t h e c a t a l y t i c r u n .
were
A m i x t u r e of
si e v e d
C2H6.
to
NZO.
CO > w a s p a s s e d t h r o u g h a f i x e d bed i n a t u b u l a r f l o w reactor a t
2
atmospheric
p r e s s u r e . The c a t a l y t i c r u n w a s carried o u t under
the
following
conditions:
p r e s s u r e s of e t h a n e 370 kPa.
respectively,
catalyst
of NgO C 0 2 >
weight
0.3
g.
partial
37 kPa and of H e 606 kPa
r e a c t i o n t e m p e r a t u r e 387OC.
t h e e x t e n t of
ethane
c o n v e r s i o n w a s u p t o 5%. a n d t h a t of N 2 0 CO > u p t o 20%.
2
P r o d u c t s a n a l y s e s were c a r r i e d o u t b y o n - l i n e g a s
chromatography u s i n g
flame-ionization
detector
liquid
and catharometer
and t w o columns: Porapak Q a n d 5 A m o l e c u l a r s i e v e .
A c i d i t v measurements.
After
a c t i v a t i o n a t 5 5 O 0 C under h e l i u m
ammonia w a s a d s o r b e d on t o t h e c a t a l y s t a t 1 0 0 ° C .
r a t e 17°/minl
i n d i c a t e d t h e number
TF'D
Cheating
of a c i d sites. g i v e n as t h e
m i 11i mol es of ammonia chemi sor bed p e r gram of c a t a l y s t .
$0
the
decomposition.
reactor
under
He
Catalyst
C40 s c c m
s a m p l e s w e r e h e a t e d CEjSO°C> i n
at
1 atmosphere> for
3 hours.
P u l s e s of p u r e N 0 C 0 . 2 sccm3 i n H e w e r e i n t r o d u c e d a t 347-C.
2
The amount of oxygen h e l d by t h e s u r f a c e No w a s d e t e r m i n e d as
N o = N
- E N
N2
O2
where N
and N
were t h e amount of n i t r o g e n a n d oxygen r e l e a s e d
N2
O2
r e s p e cti v e l y .
RESULTS AND D I S C U S I O N
Upon p a s s i n g t h e r e a c t a n t s over
a c i d i c f o r m of z e o l i t e s t h e
C H a n d H 2 0 were d e t e c t e d .
2 4
The n a t u r e of t h e o x i d a n t u s e d had a s i g n i f i c a n t effect o n t h e
p r o d u c t s CO. CO,.
e t h a n e c o n v e r s i o n r a t e and p r o d u c t
f o r m a t i o n s e l e c t i v i t y Clable
l>.
With O2 as t h e o x i d a n t t h e main p a r t of e t h a n e u n d e r g o e s t h e
Table 1
C a t a l y t i c o x i d a t i o n of e t h a n e by n i t r o u s o x i d e over
ZSM-5 and rnordenite at 3 8 7 * C
Catalyst
O x i dant
R a t e of e t h a n e
conver si on,
1 0 a mol ec /c g . $33
HZSM-SC383
HZSM-SC601
O2
N2°
O2
N2°
HZSM-SCQO1
HZSM-SC1483
Oz
N2°
Oz
N2°
HM-1 I
O2
HZSM-SC413
N2°
N20
S e l e c t i v i t y to,%
C02
*h-peak
C3H6
0.5
48
58
-
6.3
2
88
8
0.3
so
50
-
4.4
2
88
10
0.2
60
40
7
3. 6
1
90
0.2
75
2s
-
0.4
3
8Q
6
total
h-peak
*
0.72
0.39
0.30
0.18
0.24
0.14
0.21
0.13
0. 86
0.34
0.3
59
39
-
2.8
13
85
I
9.7
1
91
7
0.74
0. 38
1
91
6
0.98
0. 08
I .5% N a -
HZSM-SC413 N20
C2H4
A c i d i t y , mmol /g
13.0
- t h e amount of ammonia which i s desorbed above 300*C.
486
deep o x i d a t i o n and no t r a c e of propylene and butenes w a s f o r m e d .
By changing Si02/A1203 r a t i o f r o m 38 t o 1 4 8 c a t a l y t i c a c t i v i t y of
z e o l i t e s w a s reduced by t h r e e t i m e s .
The
ethane
HZSM-SC383
conversion
rate
for
over
N20-C2H6
w a s by an o r d e r of magnitude g r e a t e r
Upon i n c r e a s i n g SiO /A1203
02-C2Hs.
reaction
than t h a t
for
ratio catalytic activity was
2
reduced by a f a c t o r of 10. Ethylene and propylene w e r e found t o
be t h e major p r o d u c t s with CO,.
methane and C -hydrocarbons
4
minor amounts.
The d i f f e r e n t
of c a t a l y t i c
level
a c t i v i t y for
02-C2H6
in
and
t h a t t h e r e a r e t w o r e a c t i o n mechanisms,
N20-C2H6
can i n d i c a t e .
caused
by t h e d i f f e r e n t
c o n t r i b u t i ons of
aci d i c and
oxi d a t i ve
pathways of e t h a n e conversion.
To e l u c i d a t e t h e r o l e of t h e c a t a l y s t
acid-base
a c t i v i t y w a s compared with a c i d i t y ,
catalytic
properties
e v a l u a t e d by TPD
s p e c t r a of ammoni a .
conversion for r e a c t i o n of
Upon comparing t h e r a t e s of C2H6
02-C2He
it is evident.
t h a t t h e d e c r e a s e of c a t a l y t i c a c t i v i t y
and t h e amount of s t r o n g a c i d i c sites t a k e s p l a c e c o n c u r r e n t l y .
The
0 -C
2
correlation
H
2 6
indicates
that
the
activity
of
zeolites
for
conversion can be caused by t h e a c i d i t y of t h e s u r f a c e .
With N 0 as t h e oxidant t h e v a r i a t i o n of z e o l i t e s a c t i v i t y is
2
r a n g i n g f r o m 0.2.10iB t o 13.0.10'*
m o 1 e c . C H / C g * s > and does n o t
2 6
c o r r e l a t e with t h e amount of ammonia adsorbed. The r a t e of
N 0-C H conversion over HZSM-SCQOI w a s 10 times g r e a t e r t h a n t h e
2
2 6
r a t e observed over HZSM-5C1483, t h e s e samples having t h e equal
amount of s t r o n g a c i d sites. The sodium f o r m of ZSM-SC413 was not
a c t i v e i n o x i d a t i v e conversion
of e t h a n e i n N20-C2H6
mixture.
D e c a t i o n i z a t i o n of samples l e d t o t h e appearence of s t r o n g a c i d
sites.
These
reaction.
samples
exhibited
also
high
activity
I n t r o d u c t i o n of 1.5%N a i n t o HZSM-SC413
temperature
ammonia
c o n v e r s i o n over
adsorption
form,
1.5% Na-HZSM-SC413
but
the
in
the
suppresed high
rate
of
w a s even 1 . 3 t i m e s
ethane
greater.
t h a n over a c i d i c form HZSM-5C413.
Therefore.
can n o t
be
t h e high l e v e l of c a t a l y t i c a c t i v i t y f o r N20-C
e x p l a i n e d by t h e
connected with s p e c i f i c NzO
a c i d i t y of
activation.
samples
and
could
H
2 6
be
T h i s is c o n s i s t e n t with
t h e e a r l i e r s t u d y concerning t h e decomposition of n i t r o u s o x i d e
a t 45O0C over H-mordenite Cref. 73.
To check
between
t h e p o s s i b i l i t y of
nitrous
oxide
and
N20
activation
zeolites
was
the interaction
studied
at
347-C.
487
E x p e r i m e n t s d e m o n s t r a t e d t h a t p u r e N 0 decomposed u n d e r s t u d i e d
2
c o n d i ti o n s t o g i v e g a s e o u s n i t r o g e n a n d n o a p p r e c i a b l e evol u t i on
of
The N 0 c o n v e r s i o n w a s h i g h e s t i n t h e
2
13, b u t a f t e r 10-19 p u l s e s d e c o m p o s i t i o n d i d
oxygen w a s o b s e r v e d .
f i r s t p u l s e CFig.
not occur.
E v o l u t i o n of
oxygen a l s o d i d n o t
a t 347OC i n
occur
f l o w i n g h e l i u m d u r i n g a n hour p e r i o d . Hence. t h e amount of oxygen
by
held
species
decomposed
calculation
z e o l i t es .
N
and
the
the
surface
nitrogen
to
equal
is
released.
the
This
amount
fact
of
N20
allows
the
of t h e number of N 20 d e c o m p o s i t i o n sites for each
I
F i g 1. The amount of n i t r o u s
o x i d e decomposed
-
I
on
HZSM-SC603.
2 - HZSM-8C1483
a s a f u n c t i o n of t h e
p u l c e number
n.
P u l c e volume 0 . 2 c c m ,
T=347-C.
Number of p u l c e s . n
The r a t e of C2H6 c o n v e r s i o n as a f u n c t i o n of N 2 0 d e c o m p o s i t i o n
sites is shown i n F i g .
samples l i n e a r
2.
I t is e v i d e n t t h a t for a l l
correlation exists,
, which h a s d i f f e r e n t s e l e c t i v i t i e s
HM-I1
examined
is t h e case e v e n f o r
that
and C3H6
t o C02, C2H4
f o r m a t i on.
The
determining
role
of
surface
oxygen
species
c o n v e r s i o n is a l s o c o n f i r m e d b y p u l s e e x p e r i m e n t s .
oxygen p r o d u c e d b y N 2 0
pretreatment
of
HZSM-SC38)
Upon i n c r e a s i n g t h e number of C2H6 p u l s e s e t h a n e
w a s s h a r p l y r e d u c e d t o z e r o a f t e r 5-6 p u l s e s .
i'
e t h a n e c o n v e r t e d w a s 5.0.10
m o 1 e c . C 2H6 / g .
C2H6
reacted with
e t h a n e t o f o r m e t h y l e n e as a major p r o d u c t w i t h minor
C02.
in
The s u r f a c e
amount of
conversion
The total amount of
Taking i n t o a c c o u n t
t h e s e l e c t i v i t y of e t h y l e n e and C02 f o r m a t i o n i t is p o s s i b l e t o
c a l c u l a t e t h e amount of
equal
t o 5.3.10''
s u r f a c e oxygen consumed.
a t o m O/g
that
T h i s v a l u e is
c o r r e s p o n d s t o 804 of
initial
488
5
10
-
6
5 -
/7
8
4
12
Number of s i t e s , No*lO-is
Fig. 2 .
R a t e of C2H6
decomposition sites N
conversion r
i n N20-CeH6
0
0
sites/g
surface N 0
2
r e a c t i o n a t 387OC on z e o l i t e s :
v e r s u s number of
I -NaZSM-SC 41 > , 2 - N a Z S M - S C 383 , 3-HZSM-SC 1483, 4-HZSM-SC QO> ,
5-HM-11,
6-HZSM-5C 603 , 7-HZSM-SC 38>, 8-HZSM-SC 41 3 ,
9-1.5% Na-HZSM-SC 41 3.
a t o m O/g for HZSM-SC38>>.
oxygen c o v e r a g e C 6 . 8 . 1 0 "
agreement between t h e amount of
the
consumed
composition
observed.
coverage
oxygen
, as well
dur i ng
o b t a i ned
These
results
determines
oxygen h e l d by t h e s u r f a c e and
as
the
pulse
and
c l e a r l y show
high
Hence, a good
degree
similarity
f 1o w
that
of
the
paramagnetic
resonance
studies
product
oxygen
catalytic
e x c e l l e n t s e l e c t i v i t y t o C2-C3 o l e f i n e s f o r N20-C2HE
Electron
of
exper i ments
have
was
surface
activity
and
reaction.
demonstrated
t h a t t h e r e are s t r o n g r e d o x sites on HZSM-5 and m o r d e n i t e c a p a b l e
to
ion-radical
form
cation-radicals
w i t h HZSM-5
of
organic
olefines
Cref.
species.
me
w a s observed d u r i n g t h e i n t e r a c t i o n of
appearence
8 3 , t h e anion-radical
w a s d e t e c t e d by EPR
90;
on HM d u r i n g a d s o r p t i o n of s u l f u r d i o x i d e C r e f . 9). Decomposition
of
nitrous
oxide
can
be
a
result
of
N 0
2
s t r u c t u r a l d e f e c t s on t h e z e o l i t e framework
3+
m e t a l i o n s . for example Fe .
interaction
Cref.
with
73 o r impure
As t o p o s s i b l e oxygen s p e c i e s r e s p o n s i b l e f o r t h e r e a c t i o n t h e
f o l l o w i n g o n e s may be mentioned:
t o work
Cref.
10>
O-..-O-type
0- and atomic oxygen.
sites
can
be
formed
According
by
a
high
489
temperature
being
t r e a t m e n t of
equal
to
lo"
CSi02/A1203=70-1403, t h e number
HZSM-5
spin/g
as
measured
by
EPR
spectroscopy.
W i t h i n a n o r d e r of magnitude i t c o r r e s p o n d s t o t h e number of NEO
decomposition
sites which
HZSM-SC148>.
The
was
surface
found
oxygen
to
be
5.10''
species
sites/g
formed
by
for
N20
d e c o m p o s i t i o n d o e s n o t e x h i b i t a n EPR s i g n a l . I t may b e c o n c l u d e d
that
they
are l i k e l y t o be uncharged
forms
having
an
atomic
c h a r a c t e r as p r o p o s e d for o x i d a t i v e d e h y d r o g e n a t i o n of e t h a n e by
n i t r o u s oxi d e o v e r c o b a l t -doped magnesi um oxi d e C r e f 11>.
REFERENCES
1.
2.
S. S. S h e p e l e v .
C19631 319.
S. S.
K . G. I o n e .
Shepelev.
K . G.
React.
Ione.
React.
Kinet.
Kinet.
Catal
Cata .
Lett. ,
23
L e t t . , 23
6.
Cl9833 323.
S. K o w a l a k , J . B. Moffat , A p p l i e d C a t a l y s i s , 3 6 C I
-23 C 19883 139.
K . A i k a . J . H . L u n s f o r d . J . Phys. Chem., 81 C19773 1393.
M. I w a m o t o . J . H. L u n s f o r d , J . Phys. Chem. 84 C19801 3078.
M. I w a m o t o , T. Taga. S.K a g a w a . Chem. L e t t . , ClQ823 1469.
8.
9
19C43. C19783 Q22.
S. J . S h i h , J . C a t a l . , 79 ClQ833 390.
A. A . S l i n k i n .
A. V. Kucherov, D. A. K o n d r a t j e v ,
3.
4.
5.
.
7. A . A . S l i n k i n .
T. K . Lavrovskaya,
I . V. Mishin.
Kinet.
Katal. ,
T. N. Bondarenko.
A.M.Rubinstein. Kh. M. Minachev. K i n e t . K a t a l . . 22 (1-3
156.
V. A. Poluboyarov. V. F. Anufrienko. N. G. K a l i n i n a . S. N. Vosel ,
10.
K i n e t . Katal . , 28 C19851 751.
11. K . A i k a . M.Isobe. K.Kido, T.Mariyama. T . O n i s h i . J . Chem. SOC.
F a r a d a y T r a n s . -1, 83 C18873 3139.
G. Centi and F. Trifiro' (Editors), New Developments in Selective Oxidation
1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
49 1
SELECTIVE OXIDATION OF PROPANE To ACROLEIN AND AMMOXIDATION TO ACRYLONITRILE
OVER Ag-DOPED BISMUTH VANADOMOLYBDATE CATALYSTS
Young-Chul KIM, Wataru UEDA and Yoshihiko MORO-OKA*
Research Laboratory of Resources Utilization, Tokyo Institute of Technology,
Nagatsuta-cho 4259, Midori-ku, Yokohama, 227 Japan
SUMMARY
Acrolein was formed as a major product in the partial oxidation of propane
using molecular oxygen over Ag-doped bismuth vanadomolybdate catalysts having
a scheelite structure. The catalyst system was also effective for the
selective amoxidation of propane to acrylonitrile. The reaction seems to
proceed via propylene involving a kind of autoxidation of propane at
temperatures about 5OOOC.
INTRODUCTION
The selective oxidations of lower alkanes to other chemicals are becoming
increasingly important both in fundamental and in industrial chemistry.
However, selective oxidations of alkanes to partially oxidized products are
accompanied by many difficulties due to their low reactivities compared to
alkenes and dienes.
Nevertheless, the selective oxidation of n-butane to
maleic anhydride has been successfully established using V-P-0 catalysts1*
)
.
Now, much attention is being denoted to the catalytic oxidative dimerization
of methane by many
investigator^.^^^)
On the other hand, little has been
reported for the partial oxidation of propane, Giordano et a1 reported the
oxidation of propane to acrolein over supported Te2Mo07 catalysts. ')
The
catalysts such as Ba5.55Bi2.3Te30186) and U-Sb-07) were clamed to be active for
the oxidation to form acrolein in some patents.
Direct formation of acrylic
acid from propane was also reported on V205-P205-Te02 catalysts . * )
However,
their catalytic performances are not yet sufficient for the practical
applications.
Compared to the simple oxidation of propane, more predominant
results have been obtained in the amoxidation to form acrylonitrile. A series
of patents have been published by using multicornponent metal oxide catalysts
containing Mo, Bi, Sb and many other elements.?)
The space time yield of the
target product has been improved remarkably but a long distance still remains
to the final goal.
Here, we report our discovery that Ag-doped bismuth
vanadomolybdates are promising catalysts for the conversions of proDane both
to acrolein and acrylonitrile.
EXPERIMENTAL METHODS
Ag-doped bismuth vanadomolybdate catalysts were prepared by an aqueous
slurry reaction of ammonium metavanadate, ammonium heptamolybdate, bismuth
nitrate and silver nitrate at pH = 10. After the evaporation of water, the
obtained precursors were dried at llO°C and calcined at 520°C for 6 hr in an
air stream.
All other mixed metal oxides used in the survey to find effective
catalyst were prepared according to the literature. All catalysts were used
in the form of powder with 100-200 mesh after diluted five times by quartz tips.
The reaction at atmospheric pressure was carried out in a conventional
flow system equipped with a quartz tube reactor of 18 mm inner diameter and a
tubular furnace.
Mixtures of propane, oxygen and nitrogen were fed in from
the top of the reactor.
The feed and products were analyzed by an on-line gas
chromatograph operating with two sequential columns {Gaskuropack 54, 3m and
Molecular sieve 5A. 2m).
RESULTS AND DISCUSSION
Survey of the effective catalyst
Various types of the catalysts reported for the selective partial oxidations
of lower alkanes and alkenes were examined for the oxidation of propane to
form acrolein.
Table 1 shows some typical results obtained in the oxidation
of propane over the various mixed oxide catalysts.
It is well-known that
V205-P205 systems give the best result for the selective formation of maleic
anhydride in the oxidation of Cq-alkane, n-butane.l)
However, the catalysts
were extreamly inactive for the acrolein formation in the oxidation of propane.
Although small amounts of acrylic acid were formed as reported,8) the major
products were carbon oxides.
Mixed oxides containing vanadium and magnesium
with or without phosphorous pentoxide were reported active for the oxidative
dehydrogenation of propane . l o ) Certainly, some amounts of propene were
produced under the present reaction conditions but acrolein was not yielded
at all.
On the other hand, mixed oxide catalysts containing both
and molybdenum showed better activity to form acrolein.
vanadium
Thus, catalysts
based on molybdenum seem to have an ability for the formation of acrolein.
Hence, various molybdate-based catalysts which are widely used for the
selective oxidations of propene and 2-methyl propene to the corresponding
unsaturated aldehydes were tested for the reaction. All these catalysts
showed improved results for the acrolein formation. Amoung them, the scheellte
type catalysts were found to show higher catalytic activity and selectivity
to acrolein.
The catalyst system having the following composition,
Bil-x/3V1-xMox0411) was found to give the highest catalytic activity and
selectivity to acrolein amoung the scheelite type catalysts tested.
493
TABLE 1
Conversion and selectivity to acrolein in the partial oxidation of propane
over various mixed oxide catalysts.a)
Catalyst
C3H8/02
ratio
v205-P205(P/V = 1/1)
Reaction
temperature
Propane
conversion
Acrolein
selectivity
("C)
(%)
(%)
0.8
469
30
tr
0.8
450
43
0
V-Mg-P-O(V/Mg/P = l / l . / o . l )
0.7
430
65
tr
V-Mg-O(V/Mg = 1/11
0.7
425
70
tr
1/1/4) 0.6
496
16
2
V205-P205(P/V
=
3/2)
V-Sb-Mo-O(V/Sb/MO
=
V ~ O ~ - M O O ~ ( V /=M O1/11
0.6
440
55
10
V205-Mo03 (V/Mo = 1/2 1
0.6
439
56
15
Bi2MoOg(Y-type)
1.0
475
36
18
Bi2M03012(0-type)
0.9
484
37
24
Bi2Mo209(8-type)
0.6
500
20
34
BilMo12Fe3CogOx
0.6
500
10
8
BiO. 77'0.
3M00.4'7
0.7
478
16
34
BiO. 8
5'0.
55M00. 4
5
'
0.8
488
35
31
Bi0.85V0.55Mo0.4504/purmice 0.7
Bi0.85V0. 35'0.
ZMoO.4
'
5
Bi0.97V0.91M00.0904
-9
0.8
473
19
34
486
40
18
476
13
31
a) Space velocity; 1800 cm3/g-cat-h,Feed gas; (C3H8 + 02)
=
91%, N2 = 9%.
Improvement of bismuth vanadomolybdate catalyst by the dopins of monovalent
metal cations
Tricomponent metal oxide catalysts having the scheelite structure,
Bil-x/3V1-xMox04 were first reported by Sleight et all1) as effective catalysts
for the selective partial oxidation of propene to acrolein.
Their catalytic
behaviours under reaction conditions were extensively investigated by Moro-oka
et a1 using an 1802 tracer.l2#l3) The product distribution in the propane
oxidation was examined changing the catalyst composition by changing x value
in the catalyst system.
The results are shown in Fig. 1.
The catalyst
including no molybdenum, BiV04, showed very low catalytic activity to form
acrolein.
Dehydrogenation of propane to propene was mainly observed on this
catalyst.
Increasing yield of acrolein was obtained by increasing concentration
of molybdenum in the catalyst system.
The highest yield of acrolein was
attained on the catalyst having the composition, Bi0.85Vo.55M00.450~.
494
Distribution of the products
BiV04
0
9
'
4
BiO .97"0 .91M00 Bi0.93V0.79M00.2104
Bi0.85V0.55M00.4504
Bio.77Vo.30MOo.7004
J?
Bi2M03012
c2
C ~ H
CH~=CHCHO
~
co
co2
Fig. 1 Catalytic activity and selectivity of
Bil-x/3Vl-xMox04 for the oxidation of propane.
a) Dependencies of the catalytic activity and selectivity
on the catalyst composition.
b) Products distribution at 10% conversion of propane.
Reaction temp. ; 5OO0C, Space velocity; 3000 cm3/g-cat-h,
C3H8/O2 ratio; 0.55.
In order to improve its catalytic performance, several kinds of monovalent
metal ion-doped bismuth vanadomolybdates were prepared and employed for the
oxidation of propane.
The results are summarized in Table 2.
It was found that
dopings of K, Rb, and T1 rather decreased catalytic activity to form acrolein
giving mainly propene by the dehydrogenation.
Some improvements were obtained
in the additions of Li and Na but the most prominent effect was observed in the
Ag doping, where both catalytic activity and selectivity to acrolein were
improved remarkably.
Fig. 2 shows the conversion of propane and selectivity
of products on AgxBi0.~~V0.55-xM00.5404 with variation of Ag amounts.14)
The selectivity was increased gradually by the silver addition at first, then
decreased by the further addition, showing a maximum at x = 0.01.
following composite metal oxide, Ago.01Bi0.85Vo.54M00.4504,
Thus, the
was obtained as the
most effective catalyst for the oxidation of propane to acrolein.
nuidatim nf propane over Aq-doped bismuth vanadomolvbdate catalyst
(i) Effect of the reaction temperature.
The oxidation of propane to acrolein
catalyst was examined changing the reaction
on the Ag~.01Bi0.85Vo.~~Mo0.450~
temperature from 380 to 540°C.15)
As shown in Fig. 3, observed changes in the
zonversion of propane and selectivity to acrolein with the reaction temperature
495
TABLE 2
Conversion and selectivity for the partial oxidation of propane over
monovalent metal ion-doped bismuth vanadomolybdate catalyst.a)
Monobalent
MI)^)
metal
(%)
CH~CHCHO
11.0
7.8
6.8
7.6
9.6
13.1
5.1
none
L1
Na
K
Rb
A9
T1
~
Selectivityc)
Conversion
38.5
41.5
38.3
16.2
4.7
63.5
10.7
co
co2
c2
31.5
28.0
29.7
18.2
11.1
15.6
17.9
19.5
21.6
23.5
11.4
9.5
10.5
11.8
12.9
c3n6
tr
0
8.8
8.5
0
10.0
8.2
9.1
14.2
44.2
66.5
0
44.2
~
a ) Reaction temp.; 500°C, Space velocity; 3000 cm3/g-cat-h, C3H8/02 ratio; 0.55.
bi Mr0.01Bi~.85V~.54M00.4504.
were quite unusual.
c ) Normalized by carbon number in each product.
Oxidation of
propane started suddenly at about
400°C and showed the highest conversion
at this temperature.
The conversion
60 -
of propane then decreased slightly
with increasing the reaction temperature.
40
tivity to acrolein increased markedly
20 -
with increasing the reaction temperature.
-
On the other hand, the selec-
The products besides acrolein
were propene at lower temperatures,
CO, C02 and C2-hydrocarbons.
The
-a ----40--Ol
fi
Selectivity
to Acmlein
Conversion
phenomena shown in Fig. 3 are quite
different from those observed in the
ordinary catalytic oxidations and
suggest strongly that the reaction
involves a kind of autoxidation in
the process and propene is an
intermediate to the main product.
(11)
Effect of the space velocity
Effect of the space velocity on the
reaction was examined at 500°C by
varying the flow rate of the feed gas
using a constant amount of the
I
0.0
Of
0.005
I
0.010
I
0.015
AgxBi0.85V0. 55-xM00. 45'4
(c!
Fig. 2. Conversion of proy;e:,c
and selectivlty of products on
A~xBi0.85V0.55-xM00.4504 with
variation of Ag amounts.
( A ) acrolein, ( A )C O ~ ,( 0 )CO,
( 0 )C2, Space velocity; 3000 cm3/g-h,
C;H8/0;
ratio; 0.55.
496
h
c,
.r
>
-
.r
$
80
aJ
60 -
01
c
V)
ca
t o acrolein
P
6
40-
'I-
VI
$
C
Conversion
o c n r l - 0
u
20
-
0
60 -
I
I
I
I
I
t
v
3
.r
>
.r
$
Q
40
20
7
$
0
0
420
380
460
500
600
1200 1800 2400 300(
Space v e l o c i t y
3
540
(cm /g-cat. h)
Temperature( " C )
Fig. 3. Conversion of propane and
selectivity of products on
Ag0.01Bi0,85v0.54M00.4504 catalyst with
variation of the reaction temperature.
Space velocity; 3000 cm3/g-cat'h.
A g 0 m ~ 1 B i 0 m ~ ~ V 0 ~ 5 4 M 0 0catalyst.
~4~04
Fig. 4 . Effect of the space velocity
on the partial oxidation of propane.
Reaction temp. ; 500°C.
( A ) CHZCHCHO, ( 0 )C2, ( 0 )C3Hg,
( 0 )CO, ( A )CO2.
Feed gas; C3H8 32%, 0 2 598, N2
The results are shown in Fig. 4.
9%.
The
general tendency on the space velocity of the catalytic oxidation of hydrocarbon
is also valid for this reaction.
The conversion of propane rose and the
selectivity to acrolein fell gradually with decreasing the space velocity and
increasing the contact time.
It is clear that some carbon oxides are formed
in the consecutive oxidation of acrolein.
Stable acrolein selectivity higher
than 60% was observed only in the range of the space velocity higher than
2000 cm3/g-cat'h.
(iii) Effect of the feed qas composition on the oxidation of propane
Dependency of the reaction on the reactant gas composition was also examined
by varying the C3H8/02 molar ratio in the feed gas.
All runs were carried out
under the steady reaction conditions (space velocity; 3000 cm3/g-catmh,
reaction temp.; 5OO0C) where total concentration of propane and oxygen in the
f e e d gas was fixed at 91%. As shown in Fig. 5, the conversion of propane
increases with increasing the C ~ H B / Oratio
~
in the feed gas keeping a stable
497
3
I%-t
80
.r
-
---
al
Ln
Sel e c t i v i ty
Selectivity
t o acrolein
t o acrolein
-
20
V
Conversion
2olL2!EzL
-0, n
0
0.2
0.4
C3Hs/O2
0.6
0.8
molar r a t i o
60
100
80
T o t a l concentration
o f C3H8 and
O2 (2)
Fig. 6. Effect of the total concentFig. 5. Dependency of the reaction on
the C3H8/02 molar ratio in the feed gas. lation of propane and oxygen on the
rate of the reaction.
( A ) CHzCHCHO, ( 0 )c2, ( 0 )C3H6,
( 0 )CO, ( A )CO2.
Feed gas; C3Hs/O2 molar ratio 0.55.
Feed gas; (C3H8 + 0 2 ) 91%, N2 9%.
N2 balance.
Conditions: Catalyst; Ago.olBio .85Vg.54Mo0.4504.
Reaction temp. ; 500'C.
Space velocity; 3000 cm /g-cat'h.
selectivity to acrolein.
It is noteworthy that the reaction stopped completely
under the lower C3H8/02 ratio than 0.14.
This does not mean that only the
C3H8/02 molar ratio is important factor to promote the reaction.
The rate of
the propane oxidation was further determined at various total concentrations of
propane and oxygen.
In the measurement, the C3H8/02 molar ratio was fixed at
0.55 by replacing the reactant gas by nitrogen to keep balance.
are shown in Fig. 6.
The results
The most striking is that the oxidation of propane does
not proceed at all when the total concentration of propane and oxygen is below
60% in the feed gas.
Thus, it is clear that fairly high concentration of
propane is required to promote the reaction. All these results suggest
strongly that the oxidation of propane involves a kind of autoxidation and
the catalyst does not participate seriously in the activation of propane.
Actually, considerable amounts of propene were formed without any catalyst
in the oxidation of propane under the reaction conditions employed in this
498
investigation. However, no acrolein was detected in the homogeneous gaseous
oxidation of propane using no catalysts.
It is concluded that propene is the
intermediate to acrolein and mixed oxide catalysts mainly promote the oxidation
of propene to acrolein in this reaction.
It should be referred that excellent
catalysts for the oxidation of propene to acrolein such as multicomponent bismuth
molybdates are susally used at lower temperatures below 40OoC. It was found
that these catalysts are not so effective for the oxidation of propane to
acrolein.
AS
shown in Table 1, one of the best catalyst for the propene
oxidation, BilMo12Fe3Co~Ox,showed very poor selectivity to acrolein in the
oxidation of propane.
Excellent catalysts for the propane oxidation are
required to act for the propene oxidation at more higher temperature where the
homogeneous oxidative dehydrogenation of propane to propene proceeds efficiently.
Ammoxidation of propane to acrylonitrile
It has been well known that selective catalysts to form acrolein from propene
are also active for the ammoxidation of propene to acrylonitrile. Most catalysts
employed in this investigation were also examined for the amoxidation of
propane.
It was found that considerable number of the catalysts showed very
high activity and selectivity to form acrylonitrile.
shown in Table 3.
The results are partly
The selectivities to acrolein obtained on the same catalysts
are shown in the last column of the table for comparison.
It is noteworthy that
most of the catalysts effective for the ammoxidation of propane are not so to
form acrolein.
TABLE
It seems that the difference mainly arises from the different
3
Conversion and selectivity for the ammoxidation of propane.a)
Catalyst
Ammoxidation to
acrylonitrile
Conv.
Bi3FelMo2012
(%)
Selec. ( % I
Oxidation to
acrolein
Conv. ( % I
14.3
Selec.
12.8
51.5
Bi3GalMo2012
10.1
65.3
11.7
34.5
Bi3Feo.3Gao.7M02012
11.3
60.0
10.2
12.4
17.0
Bi4NblMo200,
9.0
55.4
7.8
42.9
PbO. 8EBi0.0EMO04
8.4
52.5
9.7
40.1
Bi0.85Nb0 .55Mo0.4504
5.9
A~0.01B~0.85~0.54M00.4504 1 3 s 1
64.5
11.2
58.1
67 .O
13.4
63.0
a) Reaction temp. ; 500"C, Space velocity; 3000 cm3/g-cat.h,
Feed gas; NH3 20%, C3H8 SO%, 02 30%.
(%)
499
stabilities of two products at
the reaction conditions.
In fact,
a fair number of effective
catalysts have been insisted in
patents for the ammoxidation of
pr~pane.~)Amoung the catalysts
Acryl oni t r i 1e
tested, the best catalyst for
the acrolein formation,
01Bi0.85V0.54M00. 45O4r
gave the most excellent results
for the acrylonitrile formation.
n
The conversion of propane and
the selectivity to products with
variation of the reaction
temperature are shown in Fig. 7.
CONCLUSION
We have shown that propane
400
440
480
Temperature( O C)
520
Fig. 7. Conversion of propane and
selectivity to products in the ammoxidation
of propane on Ago. OlBiO. 85VO.54Moo. 4504
catalyst with variation of the reaction
temperature.
( A ) CHzCHCN, ( 0 )C2-hydrocarbon,
(e)
CO, ( A )
cO2.
CH3CNe ( 0 )C3H6r
Space velocity; 3000 cm3/g-cat*h,
Feed gas; C3H8 34%, NH3 20%, O2 46%.
can be converted selectively to
acrolein and acrylonitrile in
the oxidation and ammoxidation
over mixed oxide catalysts.
At this stage, the conversion of
propane is not satisfactory
even on the best catalyst.
Selectivities to the main
products still remain some room
to be improved.
However,
compaired to the oxidation or ammoxidation of propene, the concentration of
propane in the feed gas in this investigation is 5 to 10 times higher than that
of propene.
Therefore, the concentrations of the main products in the effluent
gas and space time yields reach almost the same values with those of the
propene reactions.
We think that this will stimulate further investigations
for the selective oxidation of propane in the near future.
REFERENCES
1 R. L. Varma and D. N. Saraf, Ind. Eng. Chem. Prod. Res. Dev., 18 (1979) 7.
2 F. Cavani, G. Centi, A. Riva, and F. Trifiro, Catal. Today, 1 (1987) 17.
3 T. Ito and J. H. Lunsford, Nature (London), 314 (1987) 721.
4 K. Otsuka and T. Nakajima, J. Chem. SOC. Faraday Trans. I, 83 (1987) 1315.
5 N. Giordano, J. C. J. Bart, P. Vitarelli, and S. Cavallaro, Oxid. C m u n . ,
7 (19841 99.
500
6
w.
C. Conner Jr., S. L. Soled, A. J. Signorelli, and B. A. DeRites,
U.S. Patent 4472314.
7 J. Dewing, C. Barnett, and J. J. Rooney, Ger. Offen, 1903617.
8 M. Ai, J. Catal., 101, (1986) 389.
9 U.S. Patents, 4609502, 4736054, 4746641, 4760159, 4767739, 4769355,
4783545, 4784979, 4788173, 4788317.
10 M. A . Chaar, D. Patel, and H. H. Kung, J. Catal., 109 (1988) 463.
11 A. w. Sleight, K. Aykan, and D. €3. Rogers, J. Solid. State Chem., 13 (1975)
231; A. W. Sleight, Advanced-Materials in Catalysis, Academic Press, New
York, 1977 p.181.
12 w. Ueda, K. Asakawa, C. L. Chen, Y. Moro-oka, and T. Ikawa, J. Catal.,
101 ((1986) 360.
13 w. Ueda, C. L. Chen, K. Asakawa, Y. Moro-oka, and T . Ikawa, J. Catal.,
101 (1986) 369.
14 Young-Chul Klm, W. Ueda, and Y. Moro-oka, J. Chem. SOC. Chem. Commun.,
in press.
P. F. Ruiz (Universite Catholique de Louvain, Belqium)
The figure 3 is typical of a decomposition of the catalysts as function of the
temperature giving a two phase catalysts. It is possible to explain the
increase of the selectivity by a cooperative effects between these phases
(Remote control mechanisum), namely the control by a donor phase (acceptor), via
oxygen spill over. I would like to know your opinion about these hypothesis.
Y. Moro-oka (Tokyo Institute of Technoloqy, JaDan)
I don't agree with your hypothesis that the results shown in figure 3 came from
the decomposition of the catalyst. Used catalyst gave the same results as
shown in figure 3 and the catalytic activity was quite stable for a long time
at any reaction temperature.
The catalyst gave the same XRD pattern before and after the reaction.
P. Courtine (Universit6 de Technoloqie de Complsqne, France)
Could you identify the reduced phase of the catalyst corresponding to the composition Bi0.85V0.55M00.4504?
Y. Moro-oka (Tokyo Institute of Technoloqy, Japan)
We reported the results on the reduction of Bi0.85V0.55Mo0.4504 catalyst previously (ref. 13). It is noteworthy that the reduction of this catalyst does not
take place in the vicinity of the surface layer of oxide because rapid migration
of lattice oxide ions prevents the local reduction of the catalyst. It was
found that the reduction spread over the whole oxide particles. Although
numbers of oxide ion vacancies were formed, the catalyst kept its original
scheelite type structure at least until the reduction to 6 % . Thus, we found no
new XRD peaks during the reduction of this catalyst.
Ashok Padia (Scientific Desiqn, USA)
Your research is very interesting. My comments are
1 ) to explore regions of commercial interest and
2 ) to explore i-C4methacrolein?
Y. Moro-oka (Tokyo Institute or 'I'ecnnoloqy,JapanJ
1 ) We have checked the possibility to develop the reaction to the commercial
scale by asking industrial specialists to evaluate its economical value. If
unreacted propane is effectively recycled, the process may be comparable to the
oxidation of propylene (SOHIG process). Several companies are now following
the reaction.
2) We have examined to extend the reaction to i-C4 oxidation to form methacrolein. Methacrolein was surely detected as one of the main products but
selectivity of i-C4 to it was fairly lower than that of propane to acrolein.
We still continue to improve the i-C4 oxidation.
501
Z. Osipova (Institute of Catalysis, USSR)
1) Because of different stability of acrolein and acrylonitrile in the reaction
conditions there is a different dependence of selectivity on conversion for
these compounds. Do you present the optimal yield of acrolein and acrylonitrile on your catalysts as you compare your results with those for oxidation
and ammoxidation of propylene?
2) Comment. The activity of molybdenum containing catalysts in ammonia oxidation to nitrogen and nitrogen oxide is rather high. In your conditions the
activity of these catalysts in ammonia oxidation may become higher than those
in propane activation. In this case the antimony containing catalysts may be
better than vanadium containing because of their low activity in ammonia
oxidation.
Y. Moro-oka (Tokyo Institute of Technoloqy, Japan)
1 ) We have tried to find optimum conditions to form acrolein and acrylonitrile
as far as possible. A s you know, the processes for acrolein and acrylonitrile
production from propylene have been established on the continuous modification
for 30 years. The reactions presented by us stay in far unpolished state
compared to those for propylene. Thus, we expect that the reactions have
great room for improvement in the future.
2 ) Thank you very much for your comment. I agree with your suggestion that
antimony is one of the best candidate for the catalyst for this reaction.
G. M. Pajonk (Univ. C. Bernard Lyon 1, France)
You explained the large differences of selectivities between ammoxidation and
partial oxidation by saying that acrolein is more unstable in your reaction
conditions. From mechanistic point of view it is generally accepted that both
reactions proceed through the same intermediates. So my question concerns the
conversion of acrolein in your amoxidation conditions, did you such an
experiment?
Second, assuming your hetero-homogeneous reactions what is the species initiating the ammoxidation in the gas phase?
Y. Moro-oka (Tokyo Institute of Technology, Japan)
1 ) No, we didn’t.
Recently, we obtained improved selectivities to acrolein using the same
catalysts listed in Table 111. We have written them in our revised manuscript.
However, they are still lower than those for acrylonitrile. At this stage, I
have no evidence to explain the difference.
2 ) I am sorry that I can not reply to your question about initiator of ammoxidation. Our estimation for the hetero-homo reaction mechanism is based on the
following experimental results.
i) Considerable amounts of propylene were formed without any catalyst under
the reaction conditions.
ii) Propylene was selectively converted to acrolein or acrylonitrile on
every catalyst adopted in this reaction.
We have no direct informations about unstable intermediates of the reaction
byond mentioned above.
502
0.
Watzenberger (Institut fL'r Technische Chemie I, Universitat ErlangenNcrnberq , BRD)
1) Does lattice-oxygen "migration" proceed only on the surface, or is there
oxygen ion transport in the bulk, too?
2)
How did you measure oxygen "migration" (or didn't you)?
3 ) How can you confirm that it is really oxygen transport?
4)
Do you have any values or estimation for the rate of oxygen migration?
Y. Moro-aka (Tokyo Institute of Technoloqy, Japan)
We have long studied lattice oxide ion migration under the working state
of the catalysts using 1802 tracer technique.
For example,
L s160
S (hydrocarbon) + 1802
%O-cat
The above reaction was clearly observed in the Bi0.85V0.55M00.4504 catalyst
system. Lattice oxide ions not only in the vicinity of the surface but also
in the bulk of the oxide particles were involved into the oxidation reaction.
Since the oxide ion incorporation to the reaction product was observed under
the steady state catalysis, it is clear that the migration proceeds not only
from the bulk to the surface but also from the surface to the bulk of the
catalyst (ref. 12). Migration of bulk oxide ions was also confirmed by the
XRD studies during the reduction of the catalyst (ref. 13). This involvement
of the lattice oxide ion into the reaction does not depend on the simple
exchange reaction between adsorbed oxygen species and the lattice oxide ion.
Indeed, migration of oxide ions is not so rapid under the completely oxidized
state of the catalysts. It takes place o n l y under the partially reduced state
in the presence of reductant such as hydrocarbon and this is the reason why
we could not determine the absolute rate of oxide ion migration. Thus, we
estimated the migration rate by measuring the degree of involvement of lattice
oxide ions in the reaction using 1802 tracer (ref. 12). On the basis of the
results, we proposed the following model of the catalyst.
Catalytic activity of the mixed oxide catalyst adopted in our investigation
(most of them have scheelite structures) was parallel with the estimated
value of the lattice oxide migration (ref. 12).
Water tank model of rapid
equalization of chemical
potential of active oxygen
through bulk diffusion of
W
oxide ions in the oxidation
0
1
U - h
of propylene to acrolein.
o 0
x
4
W e J
2 3
- u
- 0
V
zu z
c
u
W
J. Volta (Inst. de Catalvse, France)
1) Did you test your catalysts in the presence of water?
2)
What do you think about the role of silver in your catalysis of propane
503
oxidation?
Y. Moro-oka (Tokyo Institute of Technoloqy, Japan)
1) No, we didn't but I think that we should test it because I also know importance of the effect of water vapour in the oxidation of hydrocarbon.
2 ) At this stage, we have no direct evidence for the role of silver in this
reaction but I think that it may serve in the second step in the process
(oxidation of intermediary propylene to acrolein) by activation of molecular
oxygen.
G. Emig (University of Karlsruhe, BRD)
1) Diameter of your tube reactor is relatively large.
At the same time you
leave higher concentrations of propane and oxygen. Didn't you get problem in
keeping the bed temperature in radial and axial direction canstant?
2 ) How can you explain the different conversion vs temperature behavior for
acrolein (fig. 3 ) and acrylonitrile (fig. 7 ) formation?
Y. Moro-oka (Tokyo Institute of Technoloqy, Japan)
1) I should refer the effect of the reactor on the reaction. Since the reaction
involves homogeneous steps, the results depend seriously on the type of the
reactor. As you pointed out, temperature of the catalyst bed was no homogeneous under the reaction but we had no problem to control it.
2) We confirmed that t h e results shown in figures 3 and 7 are reproducible and
did not come from the decomposition of the catalysts or some experimental
faults. I think that conversion of propane to acrolein or acrylonitrile is
controlled by the homogeneous steps. It has been well known that the homogeneous oxidation including radical reaction does not obey to the usual conversion
vs temperature behavior and often shows a negative activation energy. Partial
difference between the results shown in figures 3 and 7 may come from the
presence and absence of ammonia in the reaction system.
M. Misono (The University of Tokyo, Japan)
1) Did you observe any XRD lines due to Ag for used catalysts?
2 ) The presence of Ag is necessary for oxidation to acrolein but it is not
necessary for ammoxidation. Is this correct?
Y. Moro-oka (Tokyo Institute of Technoloqy, Japan)
1) No, we didn't.
2 ) Addition of Ag component is effective for both oxidation and ammoxidation
of propane but is more effective for the acrolein formation than acrylonitrile.
8.
Delmon (Universite Catholique de Louvain, Belqium)
1) I accept your conclusion that homogeneous steps may be involved in your reaction. However, one could remark that Ag can produce electrophilic species
which after migration on the oxidic pox, could attack, or cooperate in the
attack of propane.
504
I notice that the selectivity increases when temperature increases. It is
known that oxide surfaces become progressively depleted in electrophilic
species, for the benefit of nucleophilic species, when temperature increases.
One could interpret your results by saying that attack of the propane molecule
needs a certain balance between electrophilic and nucleophilic oxygens.
Would this interpretation correspond to your conclusion?
2 ) Ag might be an effective producer of oxygen mobile species (spill over oxygen). In our experiments, we observe that spill-over oxygen protects oxide
catalysts from reductions. Did you compare the reduction state after catalytic
work of two catalysts of the same composition except for Ag, which would be
present in only one of them?
Y. Moro-oka (Tokyo Institute of Technoloqy, Japan)
1 ) I agree with that nature of active oxygen species based on their negative
charge is very important to determine the conversion and selectivity of the
oxidation reaction. However, I don't think that Ag component plays an important
role in the C-H activation of propane by controlling electrophilic or nucleophilic nature of active oxygen in our reaction. The reason is that oxidation
of propane gave considerable amounts of propylene under the reaction conditions
without any catalyst and propylene was easily converted to acrolein or acrylonitrile in the presence of catalysts adopted in this investigation.
2 ) I agree with your suggestion that Ag might be an effective producer of
active oxygen. Most important feature of the catalyst system used in this
reaction was demonstrated by the rapid migration of oxide ion through bulk
diffusion (refs. 1 2 and 13). We think that the total reaction rate is controlled by the rapid migration of oxide ion through bulk diffusion (refs. 12 and
13). We think that the total reaction rate is controlled by the homogeneous
steps but the positive effect of Ag is realized by its activation of oxygen
and distribution of active species through bulk diffusion in the step of
oxidation of intermediary propylene.
G. Centi and F. Trifiro’ (Editors),New Developments in Selective Oxidation
0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
505
DEHYDROGENATION OF ALKANES OVER ALKALI AND ALKALINE EARTH
OXIDATIVE
ORTHOV AN ADATES
K. SESHAN, H.M. SWAAN, R.H.H. SMITS, J.G. VAN OMMEN and J.R.H. ROSS,
Laboratory for Inorganic Chemistry, Materials Science and Catalysis, Faculty of Chemical
Technology, University of Twente, P.O.Box 217, 7500 AE, Enschede, The Netherlands.
SUMMARY
The orthovanadates of the alkaline and alkaline earth metals and the un-modified alkaline earth
oxides have been compared with vanadium pentoxide as catalysts for the oxidative dehydrogenation
of ethane and propane. The orthovanadates do not contain the V=O species known to cause deeper
oxidation; the presence of alkaline and alkaline earth metals also reduces the acidity of the vanadium
pentoxide component. It was found that pure vanadium pentoxide gave substantial combustion. The
orthovanadates gave an increase in selectivity towards olefin production in the order Ba<Sr<Ca<Mg
and this was coupled with an increase in activity in the same order. The unmodified alkaline earth
oxides also exhibited some activity and selectivity, the selectivity increasing in the same order;
however, the activity increased from BaO to MgO. For the orthovanadate series, there was a shift in
the V - 0 infra-red absorption band to higher frequencies. Selective oxidation over the orthovanadates
thus seems to depend mainly on the strength of the V - 0 bond.
INTRODUCTION
The global abundance of liquefied petroleum gas (LPG), consisting mainly of propane and
butanes, makes attractive the development of a new route for the production of olefins, important
building blocks of the chemical industry. Conventional dehydrogenation of LPG is besieged with
problems such as the equilibrium limitation of the reaction, and the consequent high temperatures
required to obtain reasonable alkane conversions, as well as rapid coking and hence short catalyst life.
A possible solution to these problems is to carry out oxidative dehydrogenation (ODEH), i.e.
dehydrogenation in the presence of oxygen. The possibility that total oxidation may occur is one of
the major disadvantages associated with ODEH. However, this can probably be overcome if a way
can be found to avoid strong interaction of the product olefins with the catalyst surface and to limit
the oxidation activity to the abstraction of hydrogen from the alkanes.
It has been reported by Chaar and his colleagues that the V,O,/MgO catalyst system fulfills these
conditions, showing appreciable activity and selectivity for the ODEH of propane (1) and butane (2).
Their choice of magnesia as a support was based on the idea that it was necessary to prevent a strong
interaction between the product olefins and the catalyst surface: a basic support such as MgO repels
the nucleophilic olefin. The formation of surface compounds between the V,O, and the MgO was
found to suppress the activity of the vanadia component for deep oxidation. Chaar et al. have
suggested that this is due to the absence of the V=O bonds found in pure V,O, which cause the
formation of oxygen containing products (2). Patent literature also cites vanadium and magnesium
506
oxides containing systems as useful catalysts for ODEH of lower hydrocarbons (3). Of the various
compounds which can be formed in the V-Mg-0 system, magnesium orthovanadate has been reported
to be the most effective as a catalyst, this compound being structurally most different from pure
V205 (4).
Preliminary work in our laboratory on the use of various other V205-containing materials as
catalysts for the ODEH reaction confirmed the importance of the absence of V=O bonds in the
catalyst surface. It thus appeared that further work on other orthovanadates of the alkaline earth and
alkaline metals for catalysis of the above reaction might provide greater insight into the requirements
of a good ODEH catalyst: the relationships between the activity and selectivity for the reaction and
parameters such as the nature of the V-0 bonding, oxidation-reduction behaviour, the oxidation state
and concentration of the components on the surface and the basicity of surface groupings. This paper
describes the behaviour of a number of alkaline and alkaline earth orthovanadates for the oxidative
dehydrogenation of propane and ethane and attempts to make a correlation between these results and
infrared spectra obtained with the orthovanadates. We show that the criteria for a good ODEH
catalyst put forward by Chaar et al. cannot be the only factors that govern the catalytic behaviour.
EXPERIMENTAL
The orthovanadates were prepared by coprecipitation using ammonia from solutions
containing stoichiometric mixtures of ammonium metavanadate and the nitrate of one of a series of
alkaline or aIkaline earth metals. The precipitates were dried at 300 K, and then calcined in air at
1025 K for 2 hours. This procedure yielded monophasic orthovanadates. The pure alkaline earth
oxides were prepared by decomposition of the corresponding hydroxides in air at 1025 K for 2 hours.
The phase structure of each of the orthovanadates was confirmed using x-ray powder diffraction.
All the materials were found by N, adsorption to have low surface areas (<0.5 m2/g). XPS
measurements were made using a KRATOS XSAM 800. Details of the equipment used for TPR
measurements are given elsewhere (5). The V4' and V5' contents of the catalysts after calcination
were obtained by wet analytical methods (6). The FTIR measurements of the samples, in the form
of thin discs of the powders mixed with KBr, were obtained with a Nicolet 20SXB spectrometer.
The catalysts were tested for the oxidative dehydrogenation of propane using a flow system with
a tubular quartz reactor (30 cm long, 0.5 cm internal diameter). Powdered samples of the catalysts
(normally 600 mg of 0.3 - 0.6 mm diameter particles) were supported in the reactor by quartz wool.
A total gas flow of 137 cm3 min-' of composition He:C3Hi0, = 1361. was used. The products were
analysed using a VARIAN 3700 gas chromatograph having a Heysep Q column (total product
analysis) and a MS 5A column (CO,O, analysis). A limited number of experiments were carried out
with ethane in place of propane, the same gas composition being used.
In the results which are given below, the activity (or conversion) is defined as the percentage of
the number of moles of propane (or ethane) fed which react; the selectivity is defined as the
percentage conversion to a particular product relative to the total products, taking into account the
number of carbon atoms in each product molecule.
507
RESULTS
vtic Results
ion of Prooanp. Table 1 gives typical results for the oxidative
Qxidative
dehydrogenation of propane at 873 K over the various oxides and mixed oxides tested. The choice
of the temperature used was based on the fact that above 900 K thermal cracking began to take place
and that a number of the catalysts showed very little activity below 873 K. The catalysts
Table 1 Results for the oxidative dehydrogenation of propane over various compounds
Catalyst
Carbon
balance
Conversion
C,H, 0,
Selectivity
COX CH,
(%I
(%I
(%)
MgO
CaO
SrO
BaO
99
101
100
100
12.6
2.1
3.2
0.9
45.6
13.0
8.5
8.4
44.1
49.1
8.5
25.8
8.5
8.3
18.9
18.1
24.1
19.4
25.1
0.0
23.2
23.2
47.0
56.1
v,o,
100
17.3
55.2
56.4
0.0
0.0
43.6
Mg,(VO&
101
17.1
22.0
13.7
5.6
30.8
59.9
47.4
100.0
10.6
6.3
10.4
0.0
0.0
14.0
17.7
0.0
58.6
20.0
24.5
0.0
18.4
3.8
40.0
100.0
7.9
0.0
17.9
0.0
28.9.
0.0
C,H,
C3H6
.....................................................................
Sr&VO&
Ba,(VO,),
99
100
7.6
4.7
3.2
0.3
Li,VO,
cs,vo,
101
101
3.0
0.2
c a - ~ ( V o , ) ~101
..
........................................................................
* rest C,
An empty reactor under these experimental conditions did not
show any reaction of propane below 900K
showed no change in activity or selectivity during typical experiments of four hours’ duration.
The carbon balance in all the experiments was 100 f 1 %.
With the pure alkaline earth oxides, the propane conversion decreased in the order
MgO>CaO>SrO>BaO. The selectivity to propylene was in the reverse order,
BaO>SrO>>CaO=MgO, and some cracking to methane and ethylene was also found; SrO gave a
remarkably low degree of total oxidation. Pure V20, gave a relatively high conversion but the
only products were now propylene and the oxides of carbon.
With the alkaline earth orthovanadates, the propane conversion decreased in the same order
as found for the corresponding pure alkaline earths although there were some small differences
in the absolute values; for example, the magnesium vanadate was less active than the MgO while
the calcium vanadate was more active than the CaO. The magnesium vanadate was now by far the
508
most selective vanadate, being more selective than MgO, while the other vanadates were less
selective than the corresponding oxides; the barium vanadate was totally unselective. The degree
of cracking exhibited by the orthovanadates was lower than for the pure oxides.
Results for the pure lithium and cesium orthovanadates are also shown in Table 1. The
former shows some selectivity; it also gives some cracking (compare pure vanadia). The cesium
compound gives only combustion.
Oxidative Dehvdrogenation of Ethane. The alkaline earth orthovanadates were found to have
very low activities (conversion <I%) for the oxidative dehydrogenation of ethane even around
950K.This was to be expected as the activation of the C-H bond becomes more difficult as the
chain length of the alkane decreases. Due to the low activities, a regular variation in selectivity
could not be observed. However, the overall behaviour seems to be similar to that for propane.
For example, magnesium orthovanadate gave a selectivity to ethylene of 97% and to carbon
oxides of 3% at a reaction temperature of 950 K; the corresponding figures for barium
orthovanadate were 40% and 60%, respectively. In both of these cases, the conversion was only
about 0.5%.
Catalvst Characterisation
XPS Measurements. Table 2 gives the binding energies of the vanadium electrons for the various
vanadium-containing compounds of Table I. The table also shows the V/M and O/V ratios
calculated for each sample from the appropriate peak intensities; the theoretical values are also
given for comparison purposes. From the positions of the peaks, it can be concluded that the
518.3
theoretical
524.9
517.3
524.4
516.8
524.2
516.6
526.0
524.0
516.0
theoretical
526.5
519.2
theoretical
531.5
--
530.0
529.7
529.5
0.8
0.4
0.4
4.1
2.5
4.9
5.5
5.1
529.5
0.4
0.7
0.3
0.3
4.0
6.5
4.0
531.9
5.6
vanadium was in the V5+ state in all the catalysts (7). However, there was a clearly
distinguishable chemical shift to lower binding energies for the electrons of both the vanadium
509
and oxygen ions of the alkaline earth orthovanadates; for lithium orthovanadate, there was a slight
increase in the binding energies for both V and 0. All the orthovanadate samples, with the
probable exception of magnesium orthovanadate, showed lower amounts of vanadium on the
surface than the stoichiometric requirement. All the catalysts had amounts of oxygen on the
surface which were above the stoichiometric requirements.
TemDerature Pronrammed Reduction. Temperature programmed reduction experiments carried
out using 5% H2 in argon did not show any reduction of the pure alkaline earth oxides at
temperatures up to about 1275 K. Pure V,O, underwent reduction in five stages to V,O,(via
1/3 V,013, 2 VO,, and V601,), as has previously been reported (5). As is shown in Figure 1, the
alkaline earth orthovanadates underwent reduction rather sluggishly; the first reduction maximum
occurred well above the temperature used for the catalyst testing (873 K).
Chemical Analvsis. The results for the analysis of the vanadium species in a number of the
samples are given in Table 3. It can be seen that the proportion of V4+in the different samples
is relatively low and that this proportion does not appear to change to any appreciable extent after
use.
Table 3. The amounts of V5+ and V4* (wt%) in the fresh and used catalysts
..............................................................
Catalyst
v4+
Fresh catal st
v5y
Used catal st
v4+
"!7+
0.9
58.0
-_
-_
Mg&VO4),
0.2
33.8
0.2
34.2
Ca3(V04),
0.1
28.3
--
--
Sr,(V0,)2
0.4
22.3
0.2
20.3
Ba&VO,),
0.3
15.5
0.3
15.3
____________________-----------------------------------------52'
...............................................................
IR Measurements. The IR spectrum of the pure V20, sample (Fig.2) was characterised by
absorption bands at 835 and 1020 ern-', corresponding, respectively, to vibrations of V-0-V
groups in the (010) plane and V=O projecting perpendicularly from this plane. The IR spectrum
of the orthovanadates, as expected, did not show the V=O bond but showed a gradual shift in the
V - 0 band (there are no V-0-V bands because in orthovanadates the (V04)3- units are isolated
by MgO, units) to lower frequencies in the order magnesium to barium orthovanadate. The
spectrum for strontium and barium orthovanadates also showed increased formation of carbonate
species (bands at 1700 cm-', not shown).
DISCUSSION
The basicity of alkaline earth hydroxides increase from Mg(OH)2 to Ba(OH),, the latter having
a basicity approaching that of alkali hydroxides. As the alkaline earth oxides easily form the
corresponding hydroxides it is probable that this is the form present under reaction conditions.
The results of Table I for the pure oxides showed an improved selectivity in the same order. It
510
300
500
700
1100
900
1300
T (KI
Fig. 1
TPR recordings of the catalysts
511
thus seems that an increase in the basicity had the expected effect on the selectivity, ie. yielding
more olefin as a result of a lower interaction between the catalyst and the olefin. However, when
the alkali and alkaline earth oxides were combined with vanadium to form the orthovanadates,
they showed a very different trend ie. while the activity decreased in the same order, the
selectivity to the formation of the olefin also decreased. These differences may be due to a change
in the nature of the V - 0 bonding of these compounds.
XPS results show that the oxidation state of the surface vanadium is similar in all the fresh
orthovanadates (Table 2). The chemical shift to lower binding energies from pure V,O, to
Ba,(VO,), probably reflects a gradual weakening of the V - 0 bonding in this series; if anything,
the bonding of the oxygen in the Li compound is slightly stronger than in the V,O,. The TPR
results show that the orthovanadates are not reduced to any appreciable extent at reaction
temperatures; any slight reduction of the catalyst under reaction conditions is probably offset by
fast reoxidation from the gas phase. This conclusion is confirmed by the analysis of the catalysts
which showed that the composition appeared to be almost the same before and after the reaction
(Table 3).
The presence of reactive surface V=O species in the case of pure vanadium pentoxide, shown
by the absorption band at about 1020 cm" in the FTIR spectrum (Fig.2), seems to be responsible
for the high selectivity to carbon oxides exhibited by this material. In contrast, there appear to
be no V=O groups in the magnesium orthovanadate or the other orthovanadataes; the magnesium
compound gave the best selectivity for the formation of propane and fewer combustion products
(Table 1). However, on going along the series from the magnesium to barium orthovanadate or
to the alkali orthovanadates, the amounts of carbon oxides formed increased in spite of the low
alkane conversions that were achieved. This is surprising in view of the fact that the surface
vanadium concentrations were lower in the Ca, Sr, and Ba orthovanadates than in the magnesium
compound and also of the fact that one would expect the basicity of the materials to increase in
the same order.
The FTIR spectra show that there are no V=O species in the orthovanadate catalysts but that
there is a gradual shift in the V-0 band to lower frequencies. Such shifts in V-0 frequencies in
orthovanadates have been attributed to changes in the effective nuclear charge on the cation in
the structure (8). Patel et al. (4) found that the magnesium orthovanadate (IR band for V - 0 at 859
a n - ' ) had superior catalytic properties to those of magnesium metavanadate (V-0-V band at 910
cm-') and magnesium pyrovanadate (V-0-V band at 975 cm"). They suggested that in the latter
two samples the V-0-V band had shifted nearer to that for the V=O (band at 1620 cm-') and
that these materials therefore has some of the character associated with that grouping. We might
thus have expected that a move of the V - 0 band to lower frequencies, as observed for the other
orthovanadates (Fig, 2), would lead to improved catalytic behaviour. However, a shift to lower
frequencies from magnesium orthovanadate to the other orthovanadates also seems to be
detrimental to the selectivity to olefin production (see Table I). This loss of selectivity may be
associated with a gradual weakening of the V - 0 bond, also evidenced by the chemical shifts in
the XPS spectra (Table 2); this weakening will have the consequence that oxygen from the lattice
is now available to give total oxidation.
512
It is difficult to explain why the ability to activate propane decreases for the alkaline earth
oxides from MgO to BaO and in the same order for the orthovanadates. This observation may
perhaps be associated with the increasing tendency of the higher molecular weight oxides to form
stable surface carbonates. The presence of the stable carbonate species on the surface may mean
that it is less easy to generate the active site under reaction conditions. The implication is that the
active site on these catalysts may have some similarity to that required for a good methane
coupling catalyst; results from our laboratory have shown that the active site on Li/MgO catalysts
used for this reaction may be created by the decomposition
of Li,CO, species (9). Further
work to attempt to clarify these ideas is in progress.
ACKNOWLEDGEMENTS
We wish to thank W. Lengton and G.L.van Assen for performing the V5+and V4+analyses
and Ing. H.J.M. Weierink for obtaining the IR spectra.
REFERENCES
I. M.A. Chaar, D. Patel, and H.H. Kung,
J. Catal.. 109. (1988) 463.
2. M.A. Chaar, D. Patel, M.C. Kung, and H.H. Kung,
J. Catal., 105, (1987) 483.
3. M. Lee Fu U.S.Patent 44607129. Philips Petroleum Co.. 1986.
4. D. Patel, M C . Kung and H.H. Ku-ng,
Eds, M.J. Phillips and M. Ternan, Proc. 9th Int. Cong.
Catal., Calgary, 4,(1988). 1554.
5. H. Bosch, B.J. Kip, J.G. van Ommen and P.J. Gellings,
J. Chem. Soc., Faraday Trans., 80, (1984) 2479
6. H.R. Grady, Treatise on Analytical Chemistry,
Part 11. (Eds) I.M. Kolthoff and P.J. Elving,
Interscience Publishers. New York Vo1.8, 1963, 224pp
7. Handbook of X-ray Photoelectron Spectroscopy,
(Ed) G.E. Muilenberg, Perkin Elmer Corporation,
Minnesota, 1979, p71
8. E.J. Baran and P.J. Aymonino,
2.Anorg. Allge. Chemie, 365, (1969) 21 1
9. S.J. Korf, J.A. Roos, N.A. de Bruijn, I.G. van Ommen
and J.R.H. Ross, J. Chem. Soc., Chem. Comm. (1987)
1433.
.
513
J.C. VEDRINE (C.N.R.S., France): You have observed a shift in the binding energy values of V
by XPS as a functionof the alkaline earth cations and in parallel some shift in the vibrational bands
in IR data. As you know all these parameters are related to the strength (ie. ionic character) of
V - 0 bonds. I suggest you to think more about such a condition. However this may be ruled out
if the samples are modified under catalytic conditions. Did you check that such modification did
not occur by analysing the catalysts after catalytic reaction ?
K. SESHAN (University of Twente, The Netherlands): The catalysts did not undergo any
modification during catalytic testing. The shift in the IR vibrational bands of V - 0 group in the
presence of different alkaline earth cations is indicative of change in the V - 0 bond strength and
hence the correlations drawn with catalytic activity takes care of your suggestion.
M. MICHMAN (Hebrew University of Jerusalem, Israel): You have shown the varying reactivities
of catalysts containing Mg,Ca,Sr and Ba. Are the selectivities shown by the cations consistent with
varying temperatures ?. Why have you chosen a specific temperature for the comparison of
selectivities ?.
K. SESHAN (University of Twente, The Netherlands): The catalytic measurements have been
carried out at 600'C only. This is because, above 625'C gas phase reactions dominate and below
590°C Ba3(V0& does not show any activityat all. This puts a restriction on the measurement of
catalytic activities at different temperatures.
G. BUSCA (University of Genova, Italy): The vibrational spectra of solids are very complex and
their interpretation is not straightforward. Have you Raman spectra that support the relation you
observe between IR frequencies and catalytic activity of orthovanadates ?.
K. SESHAN (University of Twente, The Netherlands): The Raman spectra of these catalysts have
not been recorded. The 1R spectra of alkaline earth orthovanadates have been studied extensively
(see references in the paper) and rather well understood. In my opinion, only the relationship
between IR frequencies for the V - 0 grouping and the catalytic activityneed further confirmation.
G . EMIG (University of Karlsruhe, FRG): In your table on product distribution, CH4 appears
always together with C2H4. Only in the case of Mg3(VO4)2. there is no C2H4 at all formed !. How
do you explain this ?. Is there a change in the mechanism on this type of catalyst ?.
K. SESHAN (University of Twente, The Netherlands): On the basis of the present set of data it
is difficultto say if there is a different mechanism of action on Mg3(VO4)2. However, the higher
activity of this compound may be causing total oxidation of any ethylene that is formed. This
probably may by be the reason for the absence of any C2H4.
J.C. VOLTA (C.N.R.S., France) : You compare the oxidative dehydrogenation ac