Pbce.diags oJlnkmtio8l
CobJeftnceOn Cheaical ann BioprucessEagiwennB
2T - 2f Auqun 2N3 , Unive6iti Malarsia Sabah,Kota Kitubalu
st Screeningfor COz Oxidative Coupling of Methane to C2Hydrocarbons
Istadi
Nor AishahSaidinaAmin
Chenical Rea.tion EngineefingGroup (CREGI Faculty of Chemilnl ann Naturul ResourcesEneineeinS,
U iversiti TeknologiMal'eysia81310Skudai,Johor Bahna MalLysia
E-flnil : noraishah@ftkksa.utu ny
Abstract
Basicnztal oides arc suitabLecatalystsfor the selective
oxidfutivecoupling of nethane rcaction using CO, as an
ondantknown as CO, OCM hercarter. Catal)sts scrceninS
far CO, OCM waspedomed usins CeOr and Mso'based
catabsts.TheperJormanceof the catalysts v',astetted in a
ftred-bedquaflz rcactor. The CeO, suppott h'as combined
eith alkaline eafth metal oride (CaO) and trMsition nvtal
oxide(WOt or MnO) toform a tenary etal oaidecataltst
'Thaother cataust testedwas MgO doped v,)ithLirO. The
additionof WOr ta the CeOzcaralystis abk to enhtnce the
O.hr(lrocarbon selectiviry, whik the additinn of MnO
ltopin. was able to increasethe C, hydrccaftotl, yield and
CHaconvcrsion. TheLirO/M80 cataltst is a morepromising
calaDslfor COzOCM ruther than Ceoz'batedcataltst The
C, hydrccarbonyield of 5.7 %a selectivitt of92.7 %were
obtainedorer LizO/M|O catalyst.RaMn spectroscorywas
enalojed to charactenaehe catalrsts
Thermodynamiccalculationsfor the overall rcactionof CO2
OCM showedthat the equilibrium yields of CrH6and CrH.l
formed by the reactionof CHa and CO, were 15 and 25 %,
respectivoly,at lempemture>800'C for COy'CHaratio of 2
[71.ln anotherstudy, the equilibrium convenion of CH{ to
C2hydrocarbonscouldreach12 % (6 % for C2H6a\d 6% for
CrH{ formation)at lemperatureof 600"Cwith COr/CItr ratio
of 2 [4]. Concemingthe reactionpathwayof the co? ocM
to C, hydrccarbons,two main reaction schem€shave been
pmposod[2]: oneis themethane-carbon
dioxide reactionfor
pmducing CrH6(Eq. l) and the oth€r is the methane-carbon
dioxide reaction for producing CrHa (Eq. 2). Carbon
monoxideand water arealsoproducedin ihis reaction.
zCHa+ CO?e CrH6+ CO + HrO
AH'res= +17 kYmol
(l)
zCHa + 2COz <., C2ll{ + 2CO +2HrO
AIflqS = +1?6 kymol
(2')
Keywords:
co, oxidative coupling of Methane,CHa-CO?Utilizatioo,
catalysts scleening, MgO- and ceor-based catalysts,
RamanSpectroscopy
Introduction
It is knownLhalnaruralgas in many are€s,ltke Nalunas
natural gas, contains CO, in large quantity other than
methaneand lower alkanes nl. The direct s]'nthesis of
ethane, ethylene and acerylene (C? hydrocarbons)ftom
methaneis one of the most attactive methods fol the
ifficient useof nabrralgasas a chemicalrcsouce. Recently,
muchattentionhasbeenfocusedon ihe conversionofnatural
gasusingCO, asanoxidant[2,3,4,5,6,7,8].
It will be highly
desirableto utilize such low value natu-dl gas without
emitting CO, andconvertboth methaneand carbondioxide
ftom naturalgasinto higher value-addedchemicals.Based
on ihe fact that oxygeocan inducegas-phas€mdicals in the
gasphase,it is necessary
!o find an altemativeoxidantwhicb
suppressed
the radical induction [2]. Carbon dioxide could
avoid suchproblembasedon the fact that it will rct induce
gas-phaseradical reactions [2,3]. More impotant, the
reacdonof CIl.| andCO, to produceC, hydrocarbonsshould
catalyst.Thereforc,it
mainly be controll€dby heterogeneous
is highly desirableto develop catalysis which are active
towards achieving high selectivity and high yield of Cr
hydrocarbons.
ISBN:983-2643-15-5
Only a few researchers
haveattemptedto studythe oxidative
coupling of methaneusing carbon dioxide as an oxidant.
Aika andco\torkers [4] reportedthat carbondioxide showed
a positive role in the formation of C, hydrocarbonsin the
oxidative coupling of methaneover a PbO-MgO cetalyst.
Among 30 metal oxides studied for the reactior of CO,
OCM to produce C, hy&oca$ons, pras€od)mium and
terbiumoxidesshowedpotentialcatalysts[2,3,4].A serie.sof
bioary metal oxides catalysts based on cao-ceo, [4],
CaO-ZnO [6] and La2Ot.znO were developed, but the
catalyticperfomlancewasnot satisfactory.The modification
of binaiy metal oxide system was also developed by
combining MnO, wi$ alkaline eaflh (CaO, SrO and BaO),
while CrrO3,CeO?andZnO were alsousedto replaceMnO,
Pl, but the catalytic perfomance was also not satisfactory.
Recentdevelopmentover Mn-SrcO3 catalystby Cai et al.
[8], C, hydmca$ons selertivity approached79.1 % with a
C2 hydrocarbons yield of 4.5 %. These rcsults closely
. r€semblethe resultsindicatedby Wanget al. t4.6,71.
It is rcported that the surface basicity of the catalysts,
€speciallyfor interm€diateandstrongbasicsites,contnbutes
to be(€r C2hydrocarbonselectivityby lhe synergisticeffeat
[6,9]. CeO, catalyst is known as having the Foperties of
high reducibility, high oxygenstorage€apacityandabiliry to
the formation of defect sites such as oxygen vacancies[6].
The oxidation activity, oxygen mobility and ionic
1069
PbceediBs aJlitenational Conf.Ence On Chenical anABioptuces Enqineerins
27' - 2q^ Ausu! 20A3, Uniyeriti Malatsia Saboh Kota knabalt
conductivity of Ceor-based catalyst can be increasedby
doping CeO, with transition metal oxide, while the C,
hydrocarbonsselectiviry can be increasedby doping CeO?
wirh alkaline earth metal oxide (CaO) [4,5,6]. The
magnesium
oxidecatalyst,LilMgO, is oneof
lithium-doped
the mostextensiveiystudiedcatalystsfor methaneoxidative
couplingby oxygenI9,10j.The activesitesfor the initial
abstractionof hydrogenfron melhanewere believedio be
formedby thesubstitution
ofl-i'at latticesites
Li'O species
normally occupiedby Mg'z* {11,121.The Li+-iors was
ions on
incorporaEdin the MgO latticeas substitutional
magnesium sites, lvilh oxygen vacancies for charye
This paper repons the development of a more effective
catalyst system for C, hydrocarbons formation fiom
methane
andcarbondioxidereaction.CeOr-andMgo-based
calalystswerescreenedin searcbof the bestcatalystfor CO?
OCM to produceC, hydrocarbons.It is expectedthat methyl
radicals are prdducedby a reaction betweenCHa and O'
cenlersthat formed by CO, adsorptionand activation io the
catalystsurface.The improvementin the catalytic aclivity
with incrcasingnumberof oxygenvacanciessuggeststhat in
the oxidative coupling of methaneis activated by oxygen
trappedin an anionvacancy.
Materials and Methods
CH4/CO!=1/2,F (rotalflow rate)= 100Inl min ', W (catatysr
loaded) = 2 g for Ceor-based catalyst and 0.5 g for
Mgo-based catalyst. Gas flow rates were measuredand
contolled by the volumerric flow controllers (Alicat
Scientific, Inc.). The Foducts and unreactedgaseswere
analyzedby an online gaschromatogaphy(AGILENT 6890
series)equippedwith a thermalconductivity (TCD) detecror
and PORAPAK N packed-column. The online gas
chromatogmphywas controlled by Hewlett-Packard(HP)
computerequippedwith Chemsradon
soirware.
Accordingly, the dataprocessingconcerningrhe calculation
of conversion.selectivity and yield calculation was
conductedbasedon the work reporledby Asami and Wang
Gaschrcmatographic
datawereprocessed
[2,3,4,5,6,7].
on
the following assumptions:the carbonin carbondioxide was
only convened into carbon monoxide and rhe cafton or
hydrogenin methanewas convertedinro ethane,erhytene,
acetyleneand carbon monoxide, or hydrogen and water,
respectively. Conversion, selectivity and yield of C,
hydrocarbonareexFessedoo the basisofcarbon in merhane,
which are defined in Equations(3, 5 and 6), resp€ctivety
[2,3,5]. The selectivity of carbonmonoxideiScalculaEd or
the basisof ca$on in methaneand carbondioxide which is
definedin Equation(4).
cao"on".=toltofcHn ttr-
Catalytic Reaction
Each run was peformed using a conventional fixed-bed
quartz reactor.The schematicdiagmm of the experimental
set-up is shown in Figure 1. Befor€ rcaction, the catalyst
loadedinto the reactorwas recalcinedat 850'C in air flow
(100 ml nin ! for I h and flushed with high puruy muogen
(>99.999%, 100 ml min'') at 8500Cfor snother I h. The
reaction wrs initiated by introducing the feed gas of CHa
(>99,999%) and CO1 e99,995%) !o tbe rcactor. The
following inlet conditionswere usually usedfor the reaction
at catalyst screening st€p: T (temperature) = 850'C,
1070.
(3't
moleof CHa fin)
Catalyst Preparation
All binary and temary metal oxide catalysts used in rhis
study were preparedby lhe conventionalwet-impregnation
method.CeO, OmRCK) or MgO (97%, MERCK) powder
are used as catalyst supports, while LiNOr (99.995%,
(NIL4)6W,rO4..xHrO (85%,
MERCK).
FLUKA),
Mn(NOr)r.4HrO (98.5%. FLUKA) and,/orCa(NO3r.4HrO
(99%, ALDRICH) are used as precursors.Powdefy CeO,
and MgO were first immersedinto an aqueoussolution of
M(NOtn.xH,O(M=Ca,Li, Mn,) or (NHa)6WrrOa0.xHrO
for
6 h al ambienttemperatura.The water was then evaporaied
in an oven (MEMMERT) at 120'C ovemight. The powder
was lhen calcined at 850"C in a muffle fumace
(CARBOLITE) for 4 h andthencnishedinto the d€siredsize
(42-62mesh).
tol"ofcHo t"r
*rr*
CO sl6t, =
mole of C aton in C? produced
noleof C atom@Ied Lon Ctt4 &CO2
xtooEo @)
m9le9fcatomincr
producen
c, serecr.=
xl00%
moleof C alom@cted&on CH.
(5)
c2 yield=cHacorverionr c) selecrivi;x100%
i6r
Catalyst Characterization
The vibrational and molecular chamcteristi; of catalysrs
before and after reaction was characrerizedusing Raman
specrroscopyto detectthe presenceof crysrallinephaseand
molecularstructureof the meraloxidescatalysts.TheRaman
spectroscopyis predominandyapplicabteto the qualirativ€
and quantitative analysis of covalently bond€d molecules
ratfierthan ionic sFucrures.
The Rarnansp€cnoscopy
can
infomation
about
the
lanice
stuctute
of
ionic
Fovide
molecul€sin the crystalline state and the intemal coval€nt
sttucnfe of complexions as well as the ligand srrucrureof
coordination compoundin the solid staies, Laser Raman
spectroscopywasconductedusinga Perkin Elmer SpecEum
GX NIR FT"Ramanappafttus equipped wirh a Nd:YAG
laser source,The powder catalystsamplewas pressedinto
the sanple holder.The excitation line was at 514.4nm. The
excitaiion power is constanrat 350 mW for each sample,
exceptthe power for Mn-basedsamplesis conductedin the
range of 25-500 mW, The Raman spectrawere measured
with 2 cm'' spectralresolurionwiih an absoluteaccuracyof
ISBN:983-2643-r5-5
PbceedingsoJlaktutioBl
ConferenceOt Ch.mi.aL Mn Bioprcces Engireennq
27' -2q ALEL! 20A3, Universili Malarsia Sabah,Kota KinabalL
Tabk 2 - Performancesof Ceoz-basedCatalysts
I0.I cm '. TheRamanshiftbandof* s sampjewasinitially
perfomed in $e raflge of 4000 cm' up to 100 cmr,
on themetaloxidecatalyst.
depending
C2
Hydrocarbons
CII4
co
Conv. SelecL
(Eot
(Eo) Select. Yield
Cstslysts
L:xpennenratSet-Up
set-up
Diagran of
ol Experinental
FiEurc I - SchematicDiagrun
Results
Catalyst Performance Testing
The performancesof the selecledbinary and temary metal
oxide calalystsfor CO, OCM are given in Tables I and 2,
while the perfonna ces of all catalysls tested are given
graphicallyin Figures2-7. Accordingto Table I , the orderof
methane conversion among Mgo-based catalysts are
MnO/MgO > UrO-wOy'MgO > LiiO-MnO/MgO >
LiOlMgO > MgO, while the orderof C, hydmca$ons yield
are LirOlMgO > MgO > Li?O-MnO/MgO = MnO/MgO >
LirO-WOy'MgO. The order of C2 hydrocarbonsselectivity
of Ceor-based catalysts are CaO-WO3/CeO, >
CaO-MnO/CeO,> CaO/CeO,> CeO2> MnO/CeOr. The
resulrin Table2 showedtheorderofCHaconversion
among
Ceo,-basedcatalystsareMnO/CeO, > CeO, > CaO/CeO,>
CaO-MnO/CeO,> CaO-WOy'CeOr,while lhe order of C,
hydrocarbons yield are CaO-MnO/CeO, > CeO, >
CaO-WOr/CeO, CaO/CeO?> MrO/CeO,.
Table l:Perlormancesof MEo-basedCatalysts
Catalysts
cIr4
co
Conv.
Select.
(s.)
(c.)
C2
Hydrocarbons
Select.
(Eo)
Yi€ld
(Eo)
Mgo
6.1
47.0
53.0
zqaLi2oMEO
8.2
7.3
92.7
5%Li,2O/MgO
'7.1
6.8
98.2
3.8
5%MnO,MgO
20.1
82.6
22.4
r.7
5qoLizo-5%Mn
OA4CO
9.8
29.r
75.9
l;7
l%Li,O-3EoWOj
/MCO
15.0
30.6
69.4
1.1
(E")
CeO,
13.5
94.8
t2.3
r.7
S MnOlCeO2
13.9
98.3
l;7
o.2
15%CaO/CeO,
10.3
41.6
52.4
1.3
l5%CaO-SqaM
nO/CeO,
9.2
42.4
62.6
2.5
'74.6
1.3
15%CaO-3%w
oy'ceo,
4.8
Table3 - Cataltst PerformanceResuhs
Iron Prcrious
cIr{
Catalysts
co
Conv. Select.
(Eo)
(8")
c,
Hydrocarbons
Ref,
Select. Yield
(E")
(E")
CeO,
T2
0.5
0.1
t5l
CaO-CeO,
(Ca./Ce=o.I)
5.1
46
2.3
t5l
CaO-CeO,
(CalCE=A.2\
4.9
55
2;7
t5l
CaO-CeO,
(CalCF0.5)
5
CaO-Ceo,
(CalCFl)
3.4
CaO
0.3
CaO-ZnO
(Cann4.5\
CaO-MnO
(Ca,eln=1)
SIo-MnO
(sr,Mn=D
ISBN:983-2643-i5-5
(vo)
15l
t8
3.9
3.9
l5
36
0.1
tsl
82
2.8
t6l
68
2;l
t7l
85
3.3
t1)
1071
Prc.adkst
BaO-MnO
(Ba,/Mn{.5)
3.8
33
BaO-MnO
(8a,4\4n=1)
2.3
15
85
1.9
t71
MnO-Srcor
(Mn/sr=o.2) 5.',l
79.1
20.9
4.5
t8l
MnO-Srco3
(Mn/SF0.l)
87.9
l2.L
4.9
I7)
I8l
The catalystscreeningresultspertainingto CO, OCM is also
d€picled in Figures 2-4 and Figures 5-7 for MgO- and
Ceor-basedcatalysts,respectively.The screeningresultsare
alsocomparedto the r€suttsreportedby previousresearcheN
Iisred in Table 3 t5,6,7,81.Majority of the catalysrsgave
lower CHaconversionandsmallerC, yield if carbondioxide
is the oxidant for oxidative coupling of merhaneinsteadof
oxygen. Li-doped MgO catalysr showed potenrial in
catalyzingCO, OCM, alfiough previouslymosr researcners
used it for OCM by oxygen. In brief, rhese MgO- and
Ceorbased catalystsshowed unique performancesfor C,
hydrocarbonsfomation from methaneand ca$on dioxide.
The other advantageis the process atso contributed to a
reductionin $eenhousegasemission.
)
(%)
CH4conversion
Figure 3 - MBO-BasedCatatystScrceningwith Respectto
CHaConversionvs C, yieA
)
r
fi.
c
i
I
c
lhptutioaat coaIezn.. O^ Cheni.al o"d Diop6c,s Eheinep--^I
27' - 29' Aueu,t 2A0J, Univer"iti Motqsia Sdbah.Kohki@boti
cr.l4 conv€Eion(%)
-l
'l
0L
-r
01234
FiSure 4 - M8O-BasedCatabst Screeningwith Respectto
CHaConyersionvs C, Selectivitt
a0
)
--.
--:
yiold(%)
C, Hydrcca,tons
Fiqure 2 - Mgo-Based CatatystScreehing,lith Respectto
C, yield vr CzSelectilit)l
20
c
"6
05
11.5
C2Hydrocabons
V€td(%)
2
2 ,5
Fi|ure 5 . :Ceor-BasedCatatystScre?ningwilhRespect
to
C2yield Conrersionrs C, Setecti|ity
1072
ISBN:983-264315-5
Prureedirys of Intematidal Co,fewe On Chm;tuI ant Bioptuca, Ensin*rk|
27 - 2E" Aus^t 2OO3, UaiversitiMataysia Sdbat, Kota KkobLtu
Ramanspectraof the ftesh and usedLirOlMqO catalystas
well as pure MgO in the band region of 200-1200cmr are
given in Figure 10. Thesespectraarc also comparcdto the
resultof ftesh and usedtungslenoxide-doped
LirOlMgO
catalystas presentedin Figurc l t in order to investieaterhe
effectandrheroleof WOl dopingon Lhecaraly.rstricLure.
,'5[_
)
,rl
g{
c
E '11'1I9Y:
'o
2
4
I
6
e
io
(ot)
cH4convelsion
r,
-ll
Fieure 6 - Ceo2-BasedCataltst Screeninewith Respectto
CHaConvenionvsC, yied
Fiqure 8 - :RamanSpectraofFresh CeOz CaO/CeO,and
Cao-WOy'CeO,
Catarysts
)
T
I
'0f
60i
::l
30.
I
,^l
._
lnl
i^
06!
s*@q
srso€tq
E ,*s6*Mre!
< $46\6aq
i
!
c
46 3t 0
l
J
(%)
cHaconveEion
Figurc 7 - Ceor-BasedCatalystScrceningwith Respectto
CHaCotueryion vs C2Selectiyit!
Catslyst Characterization
Fieurc 9 - Ranan SpectraofFresh CeO, MnO/CeO,and
CaO-MnO/CeO, Catatr sts
The Ramanspectraof CeOr- and MgO- basedcatalyst are
presentedin Figures 8-9 and Figurcs 10:11, respectively.
The Ramanspecta of the CeOr, CaO and WO3crystallin€
are presentedin Figure 8 for purc ceor, cao-ceo, and
CaO-WOy'CeO,catalysts.
The Ramancharacterizationsof purc CeOr,MnO/CeO, and
CaO-MnO/CeO, catalysrs aft depicied in Figure 9.
However, it is difficulr to chamcterize the Mn-based
calalysBby Ramanscailering.
asLhese
calalysts
samples
are
dark: believedto be Inanganese
oxide, physically.Tle
photon energyiiom laserpower may be either absorbedor
scatteredelastically, but cannot be scatteredinelastically
fRaman),depending
on LheMno-basedsamples.
However,
FIJR spectroscopy
whichis moresensirivemay be ableto
chamcterizethem andcomplimenb the Ramanscanering.
ISBN:983-2643,15-5
Figure 10 - Ramanspectraof MeO, UrO/MgO
1073
Eneih?ennq
Pru.p.dinpsot Inktndit@l Conk?n" On Ch'ntcaLand sittP@c''s
"
24 AwA; 20AJ Uaite^iti Matdraa Sabah Kota Kindbatu
2/
l:il*ij:3itgS5lb,,-',
basicitv and led to increasethe C! hydroca$on selectivity
and yield t5l. The addirion of MnO to Lhe l5 wt%
CaO/ileO: caLalysrincreasedC? vield and sele{livitv
significantly,butdecreasedmethaneconversionslightly'
Calallst Chara€terization by Raman Spectroscopy
Fipnrc I1 - :Ranan Spectraof M?O, LirO/MgO ond
Lizo-woy'M8o
Discussion
Catalyst ScreeningSurtable for CO, OCM
The screeninaof the various calalysts over Mgo_based
is depicted
catalvsr
for C; hvdrocatbons
vieldandselecLivity
LjrOMgO
wt
%
the
2
thal
showed
Thi
results
in Fieure2.
oxidative
CO:
for
perlormance
best
gaue
lhe
.uulisr
in gamutof
increased
es liro conrenL
couoiineit mert'ane.
selecdvitvincreased'bttr
2 -5 w1.%,LheCi hvdrocarbons
unfortunalelv, their vield decreased.Based on the CI{a
asdepicledin Figures2
convenion,br yieldandsetectiviry
Li:O/MgO caLalyst
wl.%
2
and 3. iL is shown !ha!
for CO, OCM'
performance
catalyst
the b€st
demonstraieal
such
as tungsten
oxide'
metal
tBnsition
The addition of
increased
st
calab
WOrIvlgO
wL.%
Li!O'3
I
wt.%
o)ddein
methane conversion bui the C, setectivity and yield
alecreqsedas revealed in Figurc 4, I wi% Lt'2O-5' Lqo
MnO/MgOcalalystgave $e hiehestCtlr conversion'but
sloweainetoweirC: selecriviivForMgo-basedcatalysLil
is concluded that 2 wt.Ea Li2OA{gO catalyst $ more
Dotentialfor €arbondioxide oxidative coupling of methane
io produce Ci hyalrocarbonsin comparison with other
Mgo-modified catalyst.
For the Ceor-basedcatalyst' the screeningresults of
'he
various catalyst compositions with respect to C?
in
Figures5-7'
is
reveaied
hvdrocarbons
vietdandselectivity
CaO-swt.%MnO/CeOrgavethe
rire caolvslwirh l5 wL.7o
dioxideoxidalile couplingof
carbon
for
besLoerformance
of 9 2 %
gave
conversion
melhane
calalvsL
meth;e. This
The
%.
resPe.lively
and
62.6
of2.5
yield
selectiv;ry
and
Cr
WO, dopineon (he CaO/CeO,caLalystincreasedthe C,
hvdiocarions selecLivity | rcm 52.4qab 74 6 %' but did nol
The CeOr
yield significandy.
imoroveIhe C, hydrocarbons
and CeO.Aopea MnO caLalystgave high melhane
ani carbonmonoxideseleclivity.but low C,
"onu.."ion
selectivity. The MnO doPing to the CeO, pure catalyst
decreased C: hyalroca$ons yield and selectivity
conversionslightly The
meLhate
butincreased
sisnificantlv.
increasedlhe catalyst
earlh)
.iainnn oi cro raikaiine
1074
Ramanscaneringprovidesmuch more infotmaLionon a
materialthaniusl the molecularvibmtionslnlbrmationcan
also be obtained for about the electronic nature of the
materiat under siudy, e.g defect centers like reduc€d
can provlcle
fansition metals [13]. Ramanspectroscopy
ioformation on tattice vibrations as well a5 intemal
vibrationsl it canbe usedto determinethe krndsof Li phase
presentover MgO calalyslatler calcinaljonsand used Tn
areablero deleclthe
;ddirion,RamanandIR spectroscopies
grouPsof the
functioDal
of
specific
cha$cteristic vibrations
on the
gtoups
bonded
functional
speci€s,
surfac€metaloxide
their
into
insight
provide
direct
ind
of
catalyst
surface
molecularstsuclures,
TheRamanspeclrafromFigure8(a)showedthestrongpeak
at bandof466 cm '. which is assignedas a
CeO?Drcsence
flrorit" type sau.tute accordingto rhe liFralure [ll'l4l
Raman spectra of CeO, exhibited a most intense band
betweeni00-s00 crr' rangecenteredat 466 cmr, which is
ascribealthe characteristicof crystalline CeO, (Figure 8a)'
a most
cryslaliinevisualized
TheRamanspecraof Ca-based
bandsat 726'
inknseband;t 70G850cm_r'Thecharaclenstic
?32 and 803 cll|r are ascribedto the cryslalline CaO oxide'
The Ramanbanalat 912 and 333 cnr arc due to symmetric
andW-O bendingvibradonsl l4l while rhe
W-O strerching
to w-O-W deformation
slishl bandat t48 cmr conesponds
bandat 912 and
the
strerching
vibraliontl5l. Consequently.
to
characterize
used
[WO4]
are
cornmonly
333 crlr
while rhe slighl band al Z8 cm' is
sDecies,
Leuahedral
ascribedLocrlslallineWOr t l4l. ThePosilionof lhehighesl
Ranan bandis observedat a higher wavenumberin the case
of retmhedral twool Croups.(912 cm') lhan that of
octahodralWO6units (804 clrrr) In general,the position of
the W-O stretchingfrequencyvibration varied accordingto
the W-O bond length (W-O bond order) [14]. However, the
position of tie highest W-O streiching Ramanband could
;ot be consideredas a flogerpoint to speciry the nature of
polyheckalstsuctureespe{iallyin the caseof suPPoaed
madeup of lwoal
malerials.MonotungshlecomPounds,
tetrahedra.indicatedbands in the range of 910_1061cr['
(W-O symmetric sh€tching) and in the range of 300-830
cmr (antisymmetnc stretching and bending modes).
of wOb oclahedra
consisting
compounds,
IsoDolyLunqstale
'possin-g
t4O-980
cm' fw-O
ot
ut
Uuna"ifl $e range
antisymnetnc
symmetdc and W-O-W
. slretching
(W-O_W
cm'
200-550
tulge
of
frequencies)ard in the
of
The
absence
modes)
and
bending
syrnmetricsmlrhing
Lhat
revealed
cm'
at
80?
WOr
band
spectra
of
Rarnan
speciesmay exisLin letniedtal of [WOalgroups
rungsren
t161.
ISBN:983-2643-15-5
r
Pnuedksr ol!!!:tuti9ut
cadcr@ on aenical and Biapnca! EnsiaAtins
27 - 2{ Ausust 2003 , Unieesti MataNia Sabah,Kota Kinabai
t,
: Conceming
LirOA4gOcatalyst,it is believedthatth€active
on theLirO/MgOcatalystto be Ol-. SincerheO-O
species
'
stretching mode is Raman active, Ranan spectroscopyis
panicularlysuirableto dis.rypeof invesLigalion.
The s[ghL
Ramanband ar 1086cm' and snons band ar 761 cm'
Gigure loc) are attriburedro LirCO3.-TheRarnanband at
1086 clrrr is assignedto the symmetric stretching of ihe
carbonateions, while the band ai76l crl|r is attributed to
antisymmetricbending mode. The conespondingpeak ar
689 cm' in Figure lo(c) is assignedto the symmetric
stretchingmodeof the carbonateion, which is indicatedthat
the crystallineLi?COr may be presentolt this caralyst
(Figure toc) afier exposingthe catalystto reactantgas(CH1
andCO-r.
For LirO/MgO catalyst, the active speci€son this catalyst
was believedto b€ O_ceflters,but it was suggestedthat a
diabmic species,peroxide ion, Ol_, was also the active
specieson this catalyst u7l. It is therefoi€ interestedto
invesligateby Raman spectroscopywhether peroxide ions
areindeedpresenron ihe fresh andusedcatalyst.Tbe Raman
spectrumof LirO, is characterizedat bandsrangeof 794-860
cm_ and broad band at about 259 clrr' shown in Fisurc 10
lltl. The bandar 794-860cmr is arrribu|ed!o Lheinremal
O-O stretchingmodeof the peroxidespeciesOz2_
and that at
259 cm_' is aitdbuted ro an exiemal lattice vibrarion. The
Raman.pectra
in thebandsof500 and426cmifortle used
cahlyst (Figure 10c) are ascribedro the presenceof LirO
of rhe peaksat about 259 and 826
Il ll. The disappearance
cm'indicaredlhedeconposilionofLi,O7.Thenewpeaksar
500 and 42o cmr aff anribuledLo LirO formed by lhe
decompositionof Li2O2.
The Raman characterizationdepicted in Figure 1l was
conductedto investigatethe effect and properiiesof I wt,%
LirOlMgO catalystdue ro 3 wt.% WO3doping. The Raman
spectraof the ftesh I wt.7oLiro - 3 wt.% wo3,Mgo catalyst
werecomparedto both I wt.% LirolMgo andMgo catalyst.
In addition, the fresh and used I wt.% Li2O - 3 \tt.Eo
WOy'MgO catalysts were also compated. The Raman
spectrumof LirO? was charact€rizedat slight p€ak of 798
cm' aid strongerpeakat 816cm_'whichshowedtheactive
speciesof Or' of the cahlyst [11]. The presenceof LirO in
Lhecaralysr
is deLecred
in Ramanspecuaof422.509and520
cm'. Tle LirCOr may be presenlon rhe used calalysl
(Figure I ld) which is describedat Ramanspectum of aboul
751 cm_'dueto the rcaclionwith COr. The strongRaman
band ar 918 crrr is due to symmetricW-O srretchiflg
vibration ll4l rhat rcvealed the presence of [WOa]
tetrahedralspecies,while the slighr band at about 2?9 cl[r
showsW-O-W deformarionrhat atrributedto the qystalline
WO3.The Ramanbandat 553 cnrand shoulderat about
980 crn ' show W-O-W defo.marion and W=O stletchids.
respecrively.
rhatarrribuLed
ro ther*o dimensional
WO. un'ii
[14]. The weak peak a! 980 crr|r also anributed to W-O
suetchingrhaLmay be assignedro the crysrallineMgWOa.
The W-O bending\,ibrationderecledaLabout353 cmr is
probably due to the disrortion of the [WO4] structurc.h is
ISBN:983-2643-15-5
well known that a slight disrortion of the ideal terrahedron
fwoal unir led to a shift posilion which is ascribedto the
modificadonof bondordi" 1r+1.ri. Ramanspecrumar
aboutSl6cmris assi8ned
b w-O slrschingvibralionmode
auribured
lo knahedral[WOalunil.
CoDclusion
Basic meral o)ddesare suirable caralystsfor ihe selective
oxidativecouplingofmethanereactionusingcarbondioxide
asan oxidant known asCO, OCM. The temarymetaloxides
of Ceor-basedcatalyst,CaO-WOy'CeO?
or CaO-MrO/CeOr,
showedb€tterperformancefor C, hydrocarbonsproatuction
than that of their binary metal oxide Ceor_basedcaralysa.
The CIla conversion, C, yield and selecriviiy for
l5qaCAO-5%MnO/Cef, catalysr arc 9.2, 62.6 and2.5 qo,
respectivoly,while for l5%CaO-3%\,tOxlCeO,car^tystarc
4.8,74.6and 1.3%, respectively.
WO3doping.o rheCeO,
catalystenhances
rhe setectiviiyLoC? hydrocarbon,
while
MnO dopingenhances
lhe C, hydrocarbons
yield andCHa
convenion. The LirOlMgO catalyst shows a better
penormancero catalyze the CO, OCM than Ceo!_based
catalystin which the C, hydrocartJonsyield and sele.riviry
are 5.7 %
92.'7 %, re pe.tfuely. The catalysr
^ndby Raman specrroscopy
chamcrerizarjon
providesmore
lnrorrnaoon
aboutthepresence
of metaldop€din thesurface
as well as the stnrcturebasedon molecular vibmtion. The
Rarnanspcrroscopy of catalystcan give more informarion
aboutthe changingof molecularstructureor rhe surfaceof
ftesh and usedcalalyst.
Acknowledgments
One of the autho$ (Istadi) would like to thank the financial
suplort rcceiv€dfrom rhe Ministry of Science,Technology
and Environmenr under the project No 02-02-06-00i6
EA099. The authoNwould alsolike to thankDr. Asus Setvo
Budi and Mr. PuturMarwolo(PhysicsDepl. of in;versiLi
TeknologiMalaysia)for their help and discu.sionaboul
Ramanspectros€opy.The authorswould also acknowledse
LoAssoc.Prof. Dr. RosliHussinfor a owing Lheusage-of
RamanInstrumenrin the Laboratoriumof Material Analvsis.
PhysicsDepr..UniversiriTeknologiMalaysia.
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t0'76
ISBN:983-2643-15-5