Sastri, M. V. C.

Varadarajan, T. K.

Viswanathan, B.

English

NOPR

Indianlournai of ChemistryVol. 15 , March1977,pp.202-204CatalyticDecompositionof Formic Acid on Zinc MolybdateT. K. VARADARAJAN, B. VISWANATHAN & M. V. C. SASTRI*Departmentof Chemistry.Indian Instituteof Technology,Madras600036Received28 June 1976; accepted9 August 1976formateout iso-thermo-1.505 1.0CO NT ACT TIME (SECONOS)oo60zo~ 40>z8*~~ 20The thermogravimetricanalysesof zincand of the samewith MoOawerecarriedthermally with an automaticrecordingbalance (Stanton model HT-SM).Differentialthermal analyseswere made usinga recordingDTA apparatus,at heatingrates of5°, 10° and 20°Imin. Ignited aluminawas usedas the referencematerial.The IR spectraof pure zinc formateand thecatalystafter exposureto fonnic acidvapourwererecordedin KBr on a BeckmanIR-12 spectrometer.ResultsCatalyticdecompositionfformicacid- Thecata-lytic decompositionof formicacidhasbeencarriedout on zinc molybdateand zinc oxidein the tem-peraturerange240-300°as a function of contacttime. Both thesecatalystshave been found topromotethe dehydrogenationreactionexclusively.The concentration-timeplots at varioustempera~turesfor the reactionon zinc molybdateareshownin Fig. 1. The values of the activationenergyand frequencyfactor for the decompositionoffonnic acid on zinc oxide and zinc molybdatecatalysts calculated from Arrh'nius plots are:E and frequencyfactor (zinc oxide catalysts)21·9 kcal/moleand 6'7X 109 min-1 respectively;.•.(1)Studiesonthecatalyticdecompositionofformicacidon ZnMo04 showedthatthedehydro-genationreactionproceedsthroughaformateintermediate,identifiedtobezincformate.Thermo-gravimetric,DTA and IR spectroscopicdatasupportthis conclusion.X-ray analysisof thecatalystafterthereactionwithformicacidshowthatthecatalystbreaksdownintoits componentoxides,ZnOandMo03, underthereactionconditions.IICAfAL YTIC decompositionof formic acid,a test reaction for dehydrogenationandd hydration activities,has been shown1top.roceethroughth f rmationof surfaceformatespe i sacc rdingto Eq. (1).H H20+COI I B~I A C /HCOO, ~ /, ,Ci 0 OH '\..I -- Hz+COzDepeding on the prevailingexperimentalcondi-tionsei herstepA or B (orC) canbe rate-control-ling. n acidic oxideslike Al20a,Si02, Ti02 theadsorptoncanbeeitherdissociativeor non-dissocia-ti.ve2-4• On all otheroxides,it hasbeenshownbyIR spetroscopicmethodthat the reactionproceedsthroug the dissociativeadsorptionof formicacid1•The resentinvestigationon the decompositionof for ic acid on zinc molybdate(ZnMo04) hasbeenm tivatedby thefollowingreasons:(i)ZnMoO4has be n found to be a good dehydrogenationcatalys for the decompositionof isopropylalcoholSandis, herefore,an appropriatesystemfor investi-gating the parallelismbetweenthis reactionandthe de mpositionof formicacid claimedby MarsetaU. (ii) It is of interestto studythe natureofthe int rmediatesurfaceformateformed on thistypeof ernaryoxides6•Materi Is and MethodsZinc oxide (preparedfrom zinc oxalate) wasmixed horoughlywith molybdenumoxide (certi-fiedAC quality,FischerScientificCo.,NewJersey,USA) i 1:1 molar ratio and the mixture heatedat 500°for 24 hr. The productthus obtainedwasconfirmd as ZnMo04 by chemicalanalysisand IRspectra.8.TheiecompOSitionstudieswere carried out ina flow type steadystate reactor at atmosphericplressur in the temperature range 240°-300°.Thepr uctscollectedundersteadystateconditionswerea alysedby Orsat analysisand by gaschro-matogrphy.*PreseI)taddress:Head,MaterialsScienceResearchCentre, Fig. 1- Concentration-timeplotsfor the decompositIonofIndian Instituteof Technology,Madras600036. formicacidon ZnMoO.202IIIII I!I I II------------------------VARADARAJAN etat.: FORMIC ACID DECOMPOSITION ON ZINC MOLYBDATEu . ~_..(0.7Fig. 3 - Zinc formate decompositionkinetics accordingtocontracting cube equation1001.880Fig.4-Kineticsof zinc formatedecompositioninpresenceofMoO.according toAvrami-Erofeyevequation2.40-Cl4•A_~240't2.0260'cLO1.$40 60TIME (MINUTES)1.00.1-1.6-2.00.2°-2.00.$+0.4-1.20.50.6,--,0.4-~iiI90.3I.-::Jvaluesof the kineticparametersderivedareinclud-edin Table1.DiscussionCatalyticactivityof zinc molybdate- Zinc oxide,zinc molybdateand the latter dopedwith lithiumo0.27-0A-p.9'"-0.8~§ -1.2LoetFig. 2 - Zinc formate decompositionkinetics according toAvrami-Erofeyevequationr:jf-O_4"~-o.8~E and frequencyfactor (zincmolybdatecatalyst)26·2kcal/moleand 2·42x1011min-1respectively.Thermaldecwnpositionofzincformate:(a) Thermo-gravimetricmeasurements- The kinetics of iso~thermaldecompositionof zincformatehave beenstudiedin the temperaturerange220-300°. Theresultswereanalysedthroughvariouskineticequa-tionsbasedon differentmodelsand thedatahavebeenfoundto obey(i)theAvrami-ErofeyevEq. (2)-log (l-at)=(kt)" ...(2)whereat is the fractiondecomposedat timet and(ii) contractingcube Eq. (3)1-(1-at)1/3=kt ...(3)The correspondingplotsareshowninFigs.2and3.The abovevaluesof k have beenutilizedin theconstructionof Arrhenius type of plotsfor theevaluationof activationenergies,givenin Table1.(b)Differentialthermalanaylsis-The differentialthermalanalysisof zincformateshowedtwoendo-thermicpeaksaround150°andat 300°. The firstpeak correspondsto thedehydrationprocessandthe secondto the decompositionof zinc formateto zinc oxide. The energyof activationaswellasthe frequencyfactor have beencalculatedbythe methodproposedby KissingerSusingEq. (4)dT E (E)2 log Tm-Iog/lt= 2.303RTm+log RA ...(4)whereTm is the temperatureat which the peakmaximumoccurs. This type of analysishasbeencarriedout by Trillo etal.10 for the decompositionof theformatesof manganeseandiron. Thevaluesof E and A obtainedfrom the linear plot of[2 log Tm- logdTfdt]versus1fTmfor the decom-positionof zincformatearegivenin Table1.Thermaldecompositionf zincformatein presenceof MoOa- Molybdenumis knownto formdimericcarboxylatesonly in the oxidationstate+2. Theformationof an analogousformatespeciesundertheexperimentalconditionsemployedin thepresentstudyfor the decompositionof HCOOH is highlyunlikely. Attempts have been made, therefore,to preparecomplexformatesof the typeZn[MoOa-(HCOO)a].xHaOor Zn[Moa05(HCOO),].xHaOcon-tainingboth Z~+and M06+ions. The correspond-ing oxalatecomplexesare knownll. Our attemptsto preparesuch formatecomplexesresultedonlyin the formationof a mixtureof zinc formateandmolybdicoxide. The kineticsof decompositionofthis mixturewasstudiedfor comparisonwith thekineticsof decompositionof purezinc formate.As with the latter, only the Avrami-Erofeyevand the contractingcube equationswere foundapplicableto thekineticdata (Figs.4 and5). TheTABLE 1-KINETIC PARAMETERSFOR THE DECOMPOSITIONOF ZINC FORMATEWITH AND WITHOUTMoOsPure zinc formate Zinc formate with MoOsMethod-Contractingcube equationAvrami-ErofeyevEquationDifferential thermal analysisActivationenergy(kcal/mole)20-721·523-8Frequencyfactor(min-1)6'2x10·3·8X 1073,8X 108Activationenergy(kcal/mole)28·327'930,5Frequencyfactor(min-1)8,5X 10'2·3X 101117'4xl011203INDIAN J. CHEM., VOL. 15A, MARCH 1977TABLE 2- X-RAY DIFFRACTIONPATTERNZnO Mo03-- I I d (A) Id (A)m1·240w1·443w'359756 7'982 -4 72 271ill626ill-6 5vs20S023 0 81vs60S060081ill6 3 ZnMoO~ ZnMoO( after reactionwith formic acidI d (A) Id (A)w1·435 w1·2 (ZnO)w53369S514 Mo .)ill- 762ill72 ill7 7 03)99 a)2 4 S2- 86413 3vs3, 063 1s0627 024 8 vs5· 36s7 0zaoOcFig. ~- Kinetics of zinc formatedecompositionin presence,f MoO. accordingto contracting cube equationAcknowledgementThis work forms part of a researchprojectsupportedby theDepartmentof ScienceandTechno-logy, Governmentof India, New Delhi.Referenceszinc molybdatecatalysttaken after exposuretoformic acid vapourat 2600• The spectrum[vmax2910w (vC-H),1670sh (vCO), 1580br (vasO-C-O),1400,1370,1360w (vsO-C-O),760s(1' O-C-O)cm-I]agreescloselywith that recordedfor pure zincformate. The vMo-O bandsat 910,950 and 980cm-1 arenot affectedin the least,thus confirmingthenon-involvementof thesebondsin thecatalyticdecomposition.The X-ray diffractionpattern (Table2) of thespentzinc molybdatecatalystafter beingusedinseveralreactionruns showedthat the left-overmaterialwas a mixture of ZnO and Mo03• Thecharacteristiclines found in the originalZnMoO<lwereabsentin the usedcatalyst. This againcon-firmsthat zinc molybdateis unstableundercondi-tions of catalysisof formic acid decompositionand breaksdown into its componentoxides. Inthis respect,it differsfrom manganesemolybdate(MnMoO<l)whichremains tablewithits spinelstruc-ture intact underthe sameconditions6•1. MARS, P., SCHOLTEN,J. J. F. & ZWIETERING,P., Adv.Catalysis,14 (1963),35.2. NOTa, Y., FUKUDA, K., ONISHI, T. & TAMARU, K.,Trans. Faraday Soc., 63 (1967),2300.3. DALMAI, G., FRACHONDE PRADEL, A. & IMELIK, B.,Proc. 2nd Intern. Congr.Catalysis,Paris, 1960,p. 865.4. MUNUERA,G., J. Catalysis,18 (1970),19.5. VARADARAJAN,T. K., VISWANATHAN,B. & SASTRI,M.V. C.,Indian J. Ch~m.,14A (1976),in press.6. RAJARAM, P., VISWANATHAN,B., SASTRI, M. V. C. &SRINIVASAN,v., Indian J. Chem.,12 (1974),1267.7. BROWN,R. G., DENNING,J., HALLETT, A. & Ross, S. D.,Spectrochim.Acta, 26A (1970),963.8. CLARK, G. M. & DOYLE, W. P., Sp~ctrochim.Acta, 2:::(1966),1441.9. KISSINGER,H. E., Analyt. Chern.,29 (1957),1702.10. CRIADO,J. M., GONZALEZ,F. & TRILLO, J. M., An. Real.Soc.Espan. Pis. Quim., Ser.Bll (1969),905.11. MITCHELL, P. C. R., Quart.Rev., 20 (1966),103.1008040 GOTIME (MINUTES)200" 't.oO.0.6oxidehavebeenfoundto catalysethedehydrogena-tion f formic acid exclusively,yielding carbondioxi e and hydrogen.The fact that dopingthezinc olybdatecatalyst,with lithiumoxide,whichlower its Fermi level,producesno changein theselectvity of the decomposition,showsthat thelatter doesnot involveany electron-exchangeo -twee the catalystand the substrate.Further,if themolybdateionswereactivein thedecomposi-tion f formicacid,asis thecasein thedecomposi-tion f isopropylalcoholon the samecatalyst5the hydrationreactionshouldalsohaveoccurredto s me extent. The completeabsenceof thisreacton is a clearindicationthat the activecons-titne t in zinc molybdatethat is responsiblefordeco positionof formicacidis justzincoxidewhichoper es through the intermediateformationofzinc ormate. The fact that the activationenergyfor t e decompositionof zinc formatein presenceof m lybdicoxide(,....,28kcaljmole)is alm0stequalto t at observedfor the decompositionof formicacid On zinc molybdate(,.......26kcaljmole)stronglysuggststhatthedehydrogenationreactionproceedsthro h theformationof zincformateasthe inter-medite on this catalyst. This conclusionreceivesinde ndentsupportfrom the IR spectrumof the204

Catalytic Decomposition of Formic Acid on Zinc Molybdate

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Catalytic Decomposition of Formic Acid on Zinc Molybdate

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