. GHARDA1 and C. M. SLIEPCEVICH2
Department of Chemical and Metallurgical Engineering, University of Michigan, Ann Arbor, Mich.
Copper Catalysts in
Hydrogenating Nitrobenzene to Aniline
Ind. Eng. Chem. 1960.52:417-420.
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Up to 400°
F.,
stable and produce nearly
Industrial application is enhanced by
the catalysts
are
theoretical yields of aniline.
• availability of raw
materials from petrochemical operations
•
possibilities for controlling catalyst poisons in the feed
•
ease
IVÍanufacturing
of maintaining the required temperature range in fluidized bed reactors
aniline by cataly-
tically hydrogenating nitrobenzene is
attractive as raw materials are available,
high, and the process incurs
no by-product disposal problem. Nitrobenzene can be hydrogenated over a
variety of catalysts in both liquid and
vapor phases. In this study, the vapor
phase
hydrogenation ovfer copper
catalysts was investigated.
It was found that under optimum conditions copper catalysts are stable and
produce nearly theoretical yields of
aniline from nitrobenzene. However,
sulfur compounds and dinitrobenzenes in
the feed rapidly deactivated the catalyst
at all operating temperatures. Furthermore, as the highly active catalysts are
stable only in a very narrow
temperature
range, the high heat of reaction must be
removed. A fluidized-bed reactor would
be ideal for this purpose.
The mole ratio of nitrobenzene to
hydrogen and the particle size of the
catalyst are important inasmuch as
they affect the actual temperature of
the catalyst particle. Catalyst deactivation is essentially irreversible—i.e., catalyst activity could not be restored by
oxidation followed by reduction.
The rate of hydrogenation of nitrobenzene in gram moles/hour/gram of
barium-promoted copper chromite catalyst at 400° F. was correlated by the
following equation:
yields
are
r
=
adsorbed hydrogen and nitrobenzene
dissociated on two active centers.
Experimental
Equipment and Materials. The experimental equipment is shown. The
reactor
and all connecting lines were of
catalytic
glass to eliminate extraneous
effects. Further details have been reported by Gharda (2).
Hydrogen and nitrogen, 99.9% pure
with traces of oxygen and water, were
obtained from commercial cylinders.
The various catalysts used are designated below:
Catalysts
No.
Composition
A
CuCrOz"
B
CuCrOz
Preparation
Vs-in. unsupported
pellets
l/s-in. pellets
impregnated
on
kieselguhr
(CuO,
5%;
Cr203,
2%)°
C
Ba-promoted
Vs-in. pellets
D
CuCrOz6
CuO
Impregnated
silica gelc
on
No.
Composition
E
CuO
F
CuO -f- CrzOz
G
CuO
“
“
Preparation
Supported on silica
gel prepared by
coprecipitation
Supported on silica
gel prepared by
coprecipitation
on
Impregnated
activated AlzOa1*
Harshaw Chemical Co. 6 Girdler Co.
Davison Chemical Co. d Alcoa F-10.
0.475 V-PN„2 pn
The rate controlling step appears to be
the surface reaction between molecularly
1
Present
address,
145
Bandra, Bombay 20, India.
2
E.
Hill
Rd.,
Present address, University of Okla-
homa, Norman, Okla.
Reactor and connecting lines were of glass, as a steel reactor introduced
traneous catalytic effects and the studies were made at atmospheric pressure
VOL. 52, NO. 5
·
MAY 1960
ex-
417
O
1.0
2.0
3.0
TIME
Figure 1. Nitrobenzene prepared in the laboratory from
sulfur-free benzene gave maximum conversions and least
catalyst deactivation
Four grades of nitrobenzene
as designated below:
were
used
Nitrobenzenes
No.
Preparation
A As-received analytical reagent grade®
B
Prepared from reagent grade benzene,
HNOs, and HiSOj; free of dinitrobenzene
C
petroleum-derived
Prepared from
benzene and technical HNOs and
H2SO4; about 0.1% dinitrobenzene
D
Nitrobenzene B + 1% (wt.) pure indinitrobenzene
“
Fisher Scientific Co.
was
first
Procedure. The reactor
with catalyst. To maintain
isothermal conditions to within 10° F.,
the catalyst was diluted with either
Ve-inch glass balls or Ottawa sand.
After the catalyst was charged to the
reactor, the system was purged with
hydrogen for 15 minutes. The reactor
packed
was then adjusted to the desired
pretreatment temperature. Generally,
the reaction temperature was less than
the pretreatment temperature; therefore, following pretreatment the reactor
heater had to be adjusted for the reaction
heater
t.0
(
5.0
6.0
HOURS)
Figure 2. The higher rate of deactivation at low hydrogen
to nitrobenzene feed ratios was caused by localized temperatures and diffusion effects
temperature, and the nitrobenzene feed
was then introduced.
The products were sampled periodically and analyzed for aniline by titration in glacial acetic acid against a
standard solution of perchloric acid in
glacial acetic acid, using methyl violet
as an
indicator. In all experiments
where the catalyst stability was good the
condensed reaction products contained
only aniline, water, and unreacted
nitrobenzene. No intermediate reduction products such as azoxybenzene
or azobenzene could be detected.
Results
The experimental work involved finding first factors affecting catalyst stability
and activity before determining the
effect of space velocity on degree of
Blank runs
conversion.
at 600° F.
using only the catalyst diluents, glass
balls, and Ottawa sand proved that no
homogeneous reaction was taking place.
Factors Affecting Catalyst Stability
and Activity. Optimum conditions for
catalyst stability were affected by particle size of the catalyst, time and tem-
perature of pretreatment with hydrogen,
reaction temperature, quality of nitrobenzene, and ratio of nitrobenzene to
hydrogen in the feed. Under optimum
conditions, activity of the catalyst was
also reproducible with great precision.
Grade A nitrobenzene, even when
purified by washing with dilute sodium
hydroxide and redistilling, showed less
stability and activity than the grades
prepared in the laboratory (Figure 1).
Grade B showed some improvement over
petroleum-derived nitrobenzene, Grade
C, from which the olefins had been removed by sulfuric acid, but which contained about 0.1% dinitrobenzene.
An experiment was made in which
hydrogen gas was first passed over
catalyst C in separate reactor before
using to hydrogenate nitrobenzene. No
improvement in catalyst stability was
noted.
Figure 3. The larger
gradients in the large
temperature
catalyst particles accounted
for
decreased catalyst stability
Figure 4. Above 400° to 450°
catalyst decreased rapidly
418
INDUSTRIAL AND ENGINEERING CHEMISTRY
F.
HYDROGENATING
0
2.0
1.0
was
The mole ratio of nitrobenzene to
hydrogen was varied between 1 to 6 and
1 to
18—i.e., from twice to six times
the theoretical amount
(Figure 2).
The deactivation at the lower ratios of
hydrogen to nitrobenzene was presumably a temperature effect associated
with diffusion into the pores and heat
conductivity of the catalyst.
Four sizes of catalyst were studied
(Figure 3). The effect of particle size
was concluded to be primarily the effect
of localized temperature and diffusional
gradients inside the pellet.
Experimental runs were made at 50° F. reaction temperature intervals between
350° and 550° F. Activity increased
with temperature; stability decreased
above 400° to 450° F. (Figure 4).
The pretreatment consisted of passing
hydrogen over the catalyst for 2 hours.
As shown in Figure 5, catalysts pretreated
at 400° and 450° F. attained the same
steady state activity; those activated
at higher temperatures had a slightly
initial activity which they lost rapidly.
Apparently, the stable catalyst component is an unreduced complex between
copper oxide and chromium
oxide. At a higher temperature, copper
is apparently formed which loses its
activity rapidly.
Catalysts A through
G (see designations above) were investigated for their
activity and stability under optimum
reaction conditions. The results are
summarized in Figure 6.
Catalysts A, C, and F showed nearly
the same stability; catalyst F was nearly
twice as active as A or C. Catalysts
D and E had slightly lower initial activity and were less stable than catalysts
A or C. Catalysts B and G had rather
low initial activity and lost it rather
rapidly. Besides, catalyst G gave rise
to
a
tarry product containing inter-
attained by pre-
S.O
4.0
S.O
ad-
Figure 6. Copper catalysts supported on silica were
versely affected by impregnation with sodium silicate
mediate reduction products.
In the
case of catalysts E and F, neither initial activity nor stability was affected
by the following treatments:
Removal of sodium ions by ion exchange with ammonium ions, followed
by ignition at 900° F.
Removal of sulfate ions either by
starting with nitrates or by ion exchange
of catalyst with ammonium and nitrate
ions, followed by ignition at 900° F.
However, initial activity and stability
adversely affected by impregnation
with additional sodium silicate.
Catalyst Reactivation.
Upon the
completion of a run, the nitrobenzene
flow was stopped and the hydrogen was
kept on for another 15 minutes. The
reactor was then purged with nitrogen.
The temperature of the reactor was
raised to 600° F., and air was passed
gradually over the catalyst so that the
temperature did not exceed 650° F.
After initial reactivation had subsided,
air was passed for another hour at 600° F.
The catalyst was allowed to cool in
the reactor in the current of air. It
was
removed from the reactor
and
ignited with free access to air in an
electric muffle furnace at 900° F. for
2 hours.
After cooling, the catalyst
was
returned to the reactor, pretreated
with hydrogen in the standard way at
450° F., and the reaction was carried
out as usual at 400° F.
The activity
of the regenerated catalyst was the
were
in
tivity was restored by treatment
hydrogen for 2 hours at 450° F.
The regeneration procedure was carried out on fresh catalyst pretreated in
hydrogen at 450° F. but which had not
been deactivated by the reaction of nitroThe regenerated catalyst
benzene.
showed no change in activity. This
experiment proved that there was
nothing fundamentally wrong in the
method of reactivation.
Effect of Space Velocity on Degree
of Conversion. This variable was investigated for catalyst C at only one
°
temperature, 400 F., where the catalyst
was
known to be stable. The other
conditions and results are summarized in
Figure 7. Space velocity was varied by
using different amounts of catalyst at a
constant feed rate to avoid a high temperature rise and at the same time
minimize mass transfer effects. For each
experiment, a fresh charge of catalyst
was
used.
Correlation of the Reaction Rate
Data. The raw experimental data on
conversion were
graphically differentiated to obtain the rate of reaction, r.
The rate
was
then correlated against the
partial pressures of nitrobenzene,
+
/·
/>no2
+
r
is a
7Ch2 />h2
as when the reaction was
interrupted to start the regeneration. This
behavior is in contradiction with German
practice (3).: When deactivation
was
caused by dinitrobenzene, the ac-
/>NOi,
and hydrogen, ^. These results
summarized in Figure 8.
The rate equation
=
0.475
V/’noj Ph,
+
are
(1)
simplified form of the equation
¿V/’NOj ¿Ha
(1
same
6.0
( HOURS I
TIME
Figure 5. Maximum catalyst stability
treatment at 400° to 450° F.
NITROBENZENE
TCa
Pa
+ · 2 / 2 )2
(2)
where the K’s represent the adsorption
equilibrium constants for nitrobenzene,
The
hydrogen, aniline, and water.
denominator in the above equation reBecause
presents the resistance term.
VOL. 52, NO. 5
·
MAY 1960
4 9
1
the data could be fitted by the simplified
form, the resistance was essentially con-
the range.
2 holds when the rate controlling step is the surface reaction between
molecularly adsorbed hydrogen
and nitrobenzene dissociated on two
active centers.
From purely theoretical
considerations, such dissociation is difficult to visualize.
stant
over
Equation
Discussion
As was expected, sulfur compounds in
the feed deactivated the catalyst at all
The sulfur
operating temperatures.
compounds, probably thiophene derivatives, could not be removed from
the nitrobenzene; it was necessary to
make nitrobenzene from a sulfur-free
benzene.
The deactivation of the
catalyst by dinitrobenzene was unexpected; it is not of great technical
significance because the nitrobenzene
as usually prepared does not
contain
enough dinitrobenzene to affect catalyst
stability appreciably. The deactivation
was
apparently due to strong adsorption of the product phenylenediamines,
as the activity was restored by treatment
with hydrogen at 450° F.
Next to impurities in the feed, the
most important variable affecting catalyst
stability appeared to be the actual
catalyst particle temperature. The mole
ratio of nitrobenzene to hydrogen affected both the rate of reaction and the
heat transfer characteristics of the bed.
At lower mole ratios of nitrobenzene to
hydrogen, not only was the amount ot
heat release per unit weight of catalyst
less but the heat transfer between the
flowing gases and the particles was also
Flence, stable operation
improved.
should be realized at a higher bed temperature. Similarly, at a higher mole
ratio of nitrobenzene to hydrogen,
catalyst stability should be improved
by operating at a lower bed temperature.
Both of these predictions were verified
experimentally.
The easier deactivation of larger
catalyst particles could be explained by
the larger temperature gradients present
between the reaction site and the bulk
of the catalyst in the larger particles.
The stability of large catalyst particles
was improved by operating at lower bed
temperatures.
Even when sulfur-containing impurities were
absent catalyst deactivation
was
irreversible—i.e., catalyst activity
could not be restored by oxidation followed by reduction. This behavior led
to the conclusion that catalyst deactivation was not due to deposition of tars
but was caused by slow, irreversible,
thermal transformation of the catalyst
structure.
In this connection, it might
be noted that copper chromite loses its
activity in liquid phase hydrogenations
if used at too high a temperature be-
420
0
Figure 7. Adverse effects of high
temperature rise and mass transfer resistance were overcome
by operating
at sufficiently high space velocities
W, weight of catalyst, grams
feed rate, gram moles/hour
F,
0.02
0.08
0.10
0.i2
· gm. mole/ hr. gm.
Figure 8. Agreement between data
and simplified rate equation indicates
that resistance to reaction remained
constant over
the conditions investi-
gated
p,
partial pressure,
r, reaction
of reduction to metallic copper.
Hence, it is believed that the stable
active catalyst is essentially unreduced
copper chromite and that metallic copper rapidly loses its activity. This
hypothesis was verified when it was
found that catalysts pretreated at temperatures over 450° F. had both a lower
initial activity and a lower stability.
The only extensive published work on
the catalytic hydrogenation of nitrobenzene using copper catalysts is that
of Brown and Henke (7). They used
copper oxide prepared either by ignition
of the nitrate or by precipitation from
the nitrate by hot dilute sodium hydroxide. The catalyst prepared by the
latter method was superior to that prepared by the former.
Their conclusions are not directly
comparable with the present study.
Their catalysts were not promoted or
supported and hence were almost certainly reduced to metallic copper.
Their catalysts were also very finely
divided and yet of low activity—i.e.,
the amount of nitrobenzene reacted per
unit weight of catalyst was low. Moreconcerned only with
over, they were
complete reaction, and hence their
catalyst deactivation studies were not
sensitive enough to detect small changes
in catalyst activity. Furthermore, as
they used a simple heated tube and
measured only the exit temperature of
the gases, no great reliance can be placed
on their temperature values.
The Germans have operated a process
for the reduction of nitrobenzene to
a
aniline over
copper catalyst (3).
Their catalyst was made by impregnating
lumps of pumice stone with a slurry of
basic copper carbonate in sodium silicate
solution. Their operating temperatures
ranged from 200° C. at the entrance of
the reactor to 350° C. at the exit; they
also used 60 moles of hydrogen per mole
of nitrobenzene feed—i.e., 20 times the
theoretical requirement. They revivified
their catalyst by controlled oxidation
with air followed by reduction with
0.06
0.04
atm.
rate, gram moles/hour/gram
cause
INDUSTRIAL AND ENGINEERING CHEMISTRY
hydrogen. However, the present authors
have found that initial activity and
stability of copper catalysts supported
on
silica were
adversely affected by
impregnation with sodium silicate.
The only published work on the
kinetics of the reaction in the vapor
phase is a brief mention by Wilson in
connection with the design of a reactor
(4). The rate of the reaction is given
by the equation:
5.79 X 104 C”·57^-2·958-'7,
r
where
r
nitrobenzene reacting, gram
rules/cc. hour expressed in
terms of void volume in the
=
=
reactor
C
T
=
=
concentration of nitrobenzene,
gram moles/cc.
°
K.
No mention was made about the type
of catalyst used; very high ratios of
hydrogen to nitrobenzene were used.
At these high ratios the partial pressure
of hydrogen is essentially constant, and
so the equation is similar to the one
proposed by the present authors.
Acknowledgment
A portion of this work was supported
by grants from the Michigan Gas Association and Monsanto Chemical Co.
Literature Cited
(1) Brown, O. W., Henke, C. O., J. Phys.
Chem. 26, 161, 715 (1922).
(2) Gharda, K. H., Ph.D. thesis, University of Michigan, Ann Arbor, Mich.,
1958.
(3) Groggins, P. H., “Unit Processes in
Organic Synthesis.” 4th ed., McGrawHill, New York, 1952.
(4) Wilson, K. B., Trans. Inst. Chem.
Engrs. (London) 24, 77 (1946).
Received for review May 4, 1959
Accepted January 11, 1960
Part of the Ipatieff Award Presentation,
Division of Industrial and Engineering
Chemistry, 135th Meeting, ACS, Boston,
Mass., April 1959.