Kuriacose, J. C.

Subrahmanyam, M.

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IndianJournalof ChemistryVol. 15A,July 1977,pp.589-599Nature& Role of Metal-OxygenBond in theActivity ofa Zn-Cr-Fe Oxide CatalystM. SUBRAHMANY AM & J. C. KURIACOSEDepartmentof Chemistry,Indian Institute of Technology,Madras600036Received 6 September 1976; accepted 11 March 1977Thetypeofmetal-oxy~enbondsandtheirrolein thereactionofpropan-2-o1ona Zn-Cr-Feoxidecatalystis studied.A chan~eofcharacterofthemetal-oxy~enbondneitherInfluencesthedecompositionor oxidationof propan-2-o1nor doesit affectheactivityof thecatalyst.INmetal.oxidesthe presenceof a band in theIR regIOn900-1000cm-l may be attributedto a metal-oxygenbondhavinga doublebondcharacteri•Such metal-oxygenbonds are presentin solidslike molybdates2whichareactiveoxidationcatalysts,whiletheyareabsentin theinactiveones.This band is alsoabsentin catalysts,suchas Fe,Ni and Co oxides3whichcauseprofoundoxidationof hydrocarbons.The catalyticbehaviourof molybdatesand therole of metal-oxygenbonds with double bondcharactercan be rationalizedOn the basisof anoxidative dehydrogenationmechanismwhich isthe samefor all themolybdates.In orderto checkwhether the metal-oxygenterminal bonds areinvolveddirectlyor indirectlyin the mechanismofthe reactionof propan-2-olover Zn-Cr-Feternaryoxidecatalyst,the IR spectraandcatalyticactivityof differentsampleshavebeenstudied. A catalystof composition1:1:1 on the basis of metal waschosenbecauseof its goodselectivity.MaterialsandMethodsAll the chemicalswereof analar (BDH) gradeand used as such.Preparation of catalysts- The Zn-Cr-Fecatalystwas preparedby the slurry methoddescribedbyBatist4• Fe(OH)aand Cr(OH)awere obtainedbyprecipitationfrom nitrate solutionsby the slowaddition of analar ammonia(BDH) keepingthesystemstirred. A zincoxidepastemadewithdistil-led waterwasaddedto the mixtureof freshlypre-cipitatedthoroughlywashedFe(OH)3and Cr(OH)aandheatedfor 10hr on a water-bathwith vigorousstirring. After the reactionthe solid was filtered,dried overnightat 110°and calcinedat 550°for5 hr.Binary oxides, Zn/Cr, Zn{Fe and Cr/Fe werepreparedby the slurry methodas adoptedforZn-Cr-Fesystemand driedand calcinedas.before.For the preparationof mechanicalmixturesofZn{Cr+Zn{Fe,. Zn{Fe+Fe/Cr and Zn{Cr+Fe{Cr,the binary systemsZn/Cr, Fe{Crand Zn{Fewerepowderedseparatelyin a mortar, mixed to getthe respectivecombinations,groundin an agatemortar and heatedat 550°for 5 hr. Reactionswerecarriedout in a flowtypereactorfunctioningat atmosphericpressure.The packing of thereactorand experimentalprocedurehas beende~scribedin an earlierpaper6• The IR spectrainKBr wererecordedon a Perkin-Elmer257spectro-meter.ResultsandDiscussionPropan-2-01forms acetonewhen reactedovertheZn-Cr-Fecatalyst. The IR spectrumof a freshcatalystindicatesa broad band around900-1000cm-lwhich actuallyvanishesafter reactionwithpropan-2-01at 400°. The regeneratedcatalystexhibitsthe originalbandat the sameposition.Theselectivityofthe oxidizingagentis attributedto thepresenceof surfacemetaloxygenbondswithdouble bond characterI,2.These groups act aschemicaloxidizersandarereplenishedby molecularoxygen. This doublebondedoxygensituateda.tthe surfaceprojectsout of the catalystplane6andthereforeit is very likely that this oxygenspeciesis reducedfirst. 'Reactionof thesubstratewiththe oxygenof thecatalystis typical of an oxidationmechanismofthe oxyreductivetype7 which has been proposedfor catalystscontaininga metal-oxygenbondhavingdouble-bondcharacter.Reaction of propan-2-01with the oxygenof the catalystmust be takingplaceasa firststepin thecaseof Zn-Cr-Fe.The influenceof the double-bondedoxygeninthe dehydrogenationof alcoholin the presenceofoxygenin the feed has also been investigated.After 15 min. of reactionof alcoholmixed withoxygenover the catalystthe IR spectrumof thecatalyst recorded at room temperatureshowsthat the absorptionis shifted to a higherwavenumber(1050-1000em-I)as comparedto the freshsample(900-1000em-I).Therearereportsthatbydecreasingthenumberofterminaloxygens,thedoublebondcharacterof themetal-oxygenbond increasesand the correspond-ingbandis shiftedtowardshigherwavenumberss-IO.In the presentcase(i.e.with the sampleafter 15min.reactionof a mixtureof alcoholandoxygen)it is quite possiblethat the decreasein numberof terminaloxygenbondsmay be the causefortheshiftin theband;towardshigherwavenumbers.If we assumethat therearetwo terminaloxygen589\\,INDIAN J. CHEM., VOL. 15A,JULY 1977bon4sto startwith, thenthe followingmechanism(Eg.ll) may be considered: TABLE 1- d-VALUES OF VARIOUS SAMPLES0'45640·40840'39810·25540·15740,45640,40840·39810·25540·12320,64550,45560·40710'30810·26420,40840,30700'27240·23120·21010,40560,40840,38700·24360'21620'47440·43830,40710'38200'33830·53100'42290'39810·3626(a) Zn-Cr-Fe(b) Zn/er +Fe/Cr(c) Zn/Fe + Fe/Cr(d) Zn/Cr +Zn/Fe(f) Zn/Fe(g) Fe/Cr(e) Zn/CrFrom the aboveobservations,it can be statedthat thepresenceof the IR bandat 900-1000cm-1,indicativeof the presenceof M= 0, doesnot seemto be very importantfor the catalyticbehaviourof thesystems. It maybeexpectedthatthechangeof the characterand the numberof bondson themetal atomsmight lead to a changeof catalyticactivity. However, the present results do notappearto SUPPOItsucha generalization,as thereare caseslike Zn/Cr+FejCr mixtuffs where theIR spectrumindicatesa differencein the type ofmetaloxygenbondsfromthosepresentin Zn-Cr-Feand yet exhibitthe samepatternof activity.References1. BARRACLOUGH, C. C, LEWIS, J. & NYHOLM, R. 5., J.chem.Soc., (1969),3552.2. CLARK, G. M. & DOYLE, W. P., Spectrochem.Acta, 22(1966),1441.3. TRIFIRO, F., CENTOLA, P., PASgUON, I. & ]IRU, p"Proceedingsofthefourthinternationalcongressoncatalysis,Paper 18, Moscow, 1968.4. BATIST, PH. A. & DER KINDEREN, A. H. W. M., J.Catalysz:s,12 (1968),45.5. KURIACOSE, J. C, DANIEL, C. & SWAMINATHAN, R.,Indian]. Chem.,7 (1969),367.6. MASAMICHI, A. & ETSURO, E., J. Catalysis,35 (1975),278.7. KEULKS, G. W., ]. Catalysis,19 (1970),232.8. COTTON, F. A. & WING, R. M., Inorg. Chem.,4 (1965),867.9. MITCHELL, P. C. H., Q. Rev.,20 (1966),103.10. MITCHELL, 1'. C. H. & TRIFIRO, F., J. chem.Soc. (A)(1970),3183.80... (1)7060+5040o 02/ALCOHOL 0·2 MOLE RATIO• N2/ ALCOHOL 0·2 MOLE RATIOWT OF CAT.: 3GMC.T. : 1SECTEMP.: 400'c-- FIRST SET- --- SECONO SET302010TIME (MIN)1-,- Eflect of time on conversiollof plUjxUl-2-o1HFigw<.9<l~ 3wu'"wa.~ 7wzo•...~ 6<l8is 5;;;'"w>z:3 4/+M",.e samplesof Zn/Cr, Fe/Cr and Zn/Fe of 1:1osition were preparedand the IR spectraof t e threesamplesrecorded. The IR spectraofZn/ rand Fe/Cr exhibitabandat thE,samepositionas i Zn-Cr-Fe catalyst. ZnjFe fails to exhibita b nd in the regionof interest. This suggeststhat Zn/Cr or Cr/Fe or both the systemsmay berespnsiblefor theobservedactivityof the.Zn-Cr-Fecata st.T e mechanicalmixturesof all the threephasesof n/Cr, Zn/Fe and Cr/Fe were preparedandcalci ed at 500°. None of the three mixtures,i.e. n/Cr+Fe/Cr, Fe/Cr+Zn/Fe andZn/Fe+ZnjCrgaveanybandin theIR regionof interest.Al the three systemsZn/Cr, Zn/Fe and Fe/Crand their three mechanicalmixtureswere testedfor t e reactionof propan-2-01.Only the systemZn/C +Fe/Cr showscatalyticactivity anda trendin th variationof activitywith thedurationof therun hat is similar to that exhibitedby the Zn-Cr-F catalyst(Fig.1). Thecompletesystemwhichis ac ivein Zn-Cr-Fecatalystmightnothaveformedin t e ZnjCr+FejCr mechanicalmixture to givean I band and it is clear that the mechanicalmixt re containsonly the activephases,i.e. Zn/Crand e/Cr whichby themselvesgaveIR bands.Th. data for the X-ray analysisof the varioussampesusingiron filteredCo-KIXradiationaregivenin T ble1. X-ray datafor themechanicalmixture(b) a d ternaryoxidesystem(a) revealthat theydo n t differ in structuralfeatures.590III I I I IHiI :11111 I 'I 11111'1'1II' ii', 1I111.~MlI"lllIlUl'

Nature & Role of Metal-Oxygen Bond in the Activity of a Zn-Cr-Fe Oxide Catalyst

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Nature & Role of Metal-Oxygen Bond in the Activity of a Zn-Cr-Fe Oxide Catalyst

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