Cs-substituted tungstophosphoric acid salt supported on mesoporous silica Catalysis Today 2000

Cs-substituted tungstophosphoric acid salt supported on mesoporous silica Catalysis Today

CatalysisToday55(2000)

117–124

Cs-substitutedtungstophosphoricacidsaltsupportedon

smesoporoussilica

SaeminChoi?,YongWang,ZiminNie,JunLiu,CharlesH.F.Peden

Paci?cNorthwestNationalLaboratoryMSK8-93,P.O.Box999,Richland,WA99352,USA

Abstract

Inthispaper,wedescribethecharacterizationandcatalyticpropertiesofmesoporoussilicasupportedCs-substitutedtungstophosphoricacidsalt(Cs-TPA/MS)withimproveddispersionoftheactiveclusterscomparedtomaterialsdescribedpreviouslyintheliterature.Inparticular,transmissionelectronmicrographsandtheactivityresultsforamodelalkylationreactionarepresentedasevidencefortheenhanceddispersionandperformance.Inaddition,wedemonstrateimprovementsinthephysicalandthermalstabilityofthesematerialswithCs-substitutionusingvariouscharacterizationtechniques.©2000ElsevierScienceB.V.Allrightsreserved.

Keywords:Tungstophosphoricacid(TPA);Heteropolyacid(HPA);Mesoporoussilica;Cs-TPA

1.Introduction

Demandsforacleanerenvironmenthavecontin-uouslystimulatedthechemicalandpetrochemicalindustriestodevelopalternativecatalystsystemsand/orprocessestomeetmorestringentregulations.Oneparticularareathathasattractedconsiderableattention,recentlyinvolvesthereplacementofHFandH2SO4liquidacidsinthecommercialalkylationunitsbymoreenvironmentallybenignheterogeneoussolidacids[1–3].Althoughcurrenthomogeneouscatalystsareef?cient,theircorrosiveandtoxicna-tureprovidespotentialenvironmentalhazardsandpresentoperationalproblems,includingdif?cultyinseparation,recoveryandreutilization,thatresultsinhighercapitalcosts.Amongmanysolidacidsystems,heteropolyacids(HPA)withKegginanionstructureshavereceivedthemostattentionduetotheirsimple

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Correspondingauthor.

preparationandstrongacidity[4,5].Speci?cally,12-tungstophosphoricacid(H3PW12O40),denotedasTPA,hereafter,isamongthemostextensivelystudied[6–8],sinceitpossessesthehighestBrönstedacidity[9],strongerthanthatof100%sulfuricacid,whichresultsfromminimizedchargeontheanionsurface.However,todate,lowef?ciencyduetolowsurfacearea,rapiddeactivationandrelativelypoorstabilityaresomeofthemajorproblemsassociatedwiththeseTPAsinconventionalbulkacidforms.

Attemptstoimprovetheef?ciencyofthesemate-rialshavebeenmadebysupportingtungstophospho-ricacid(TPA)onvarioushighsurfaceareasupports[10,11]and,morerecently,onmesoporoussilicawithorderedporestructures[12–15].Kapustinetal.[10]reportedthatacidityofthesupportedTPAdecreasedinthefollowingorder:SiO2>?-Al2O3>carbon.TheyconcludedthatthestronginteractionbetweenTPAandcarbonmighthaveresultedinthedecompositionoftheKegginstructure.Likewise,severalreportsintheliteraturehaveidenti?edsilicaasasuitablesupport

0920-5861/00/$–seefrontmatter©2000ElsevierScienceB.V.Allrightsreserved.PII:S0920-5861(99)00231-X

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duetoitsintrinsicinertness[16–18].Recently,meso-poroussilicaknownasMCM-41,?rstdevelopedbyresearchersatMobil[19–20],hasbeenusedtosup-portTPAclusterstotakeadvantageofitsuniformporesizeandhighlyorderedstructures.Morerecently,wehavereportedthatacidneutralizationofthemeso-poroussilicasupportassistedinpreservingtheKegginstructureevenatTPAloadingsaslowas10wt.%[21].Although,wehaveobservedanenhancementinresis-tancetoleachingofTPAbywaterwhenmesoporoussilicawasusedasthesupportinsteadofamorphoussilica,thiswaslikelyduetostericconstraintsratherthanadirectimprovementinthegraftingoftheTPAclustersonthesurface.

Anothermethodthatcouldpossiblyenhancestabil-ityoftheactiveclustersinsolutionistopreparecata-lystsintheformoftungstophosphoricacid(TPA)salts[8,22,23].Heteropolyacid(HPA)salts,asaresultofpartiallyexchangingprotonswithlargecations,typi-callydemonstratedifferentphysiochemicalpropertiesthanthoseoftheirprecursoracids.Forexample,par-tialsubstitutionofCs+forprotonsrenderbulkTPAswithhighersurfacearea(upto150m2/gcomparedto5m2/g)andimprovedthermalstabilitythantheirparentacids[8].Inaddition,TPAsaltsareknowntobeinsolubleeveninliquidsaspolaraswater.Con-sequently,TPAsaltsshouldbebettersuitedforprac-ticalapplicationsthatmightinvolvepolarreagentsinharshoperatingconditions.However,theirsmallparticlesize(?m)limittheirapplicationforuseascatalystsincommercial?xedbedorslurrytypereac-tors.Anobvioussolutionasoftenappliedinindus-trialpracticeistosupporttheseTPAsaltsonalargerparticlesize(mm)carrier.Unfortunately,preparationofthesecatalystsinanengineeredformischalleng-ingsincedirectaqueousimpregnationisnotfeasible.Forexample,Soled,etal.[23]?rstreportedusingatwo-stepimpregnationinordertodisperseTPAsaltsontoasilicasupport.However,theyobservedthinin-ternalringsofCs-substitutedTPAsalt,knownasanegg-whitedistribution,withinthesilicaextrudatesug-gestingnonuniformdispersionoftheactiveclustersonsilica.

Thepremiseofourwork,someofwhichisde-scribedinthispaperisthat,dispersionofTPAcanbemanipulatedbyadoptingappropriategraftingtechniques.Inessence,weareattemptingtohighlydispersetheactiveCs-TPAsaltspeciesuniformly

onahighsurfaceareamesoporoussilicawithor-deredstructureforimprovedactivityand/oraddedshape-selectivity.OurprimaryobjectivehereistoprepareaseriesofmesoporoussilicasupportedCs-TPAsaltswithhighlydispersedandintactKeg-ginanions,andcomparetheirstructuralandcatalyticpropertiestoaconventionallypreparedsupportedcatalystandtheirbulkcounterparts.Alkylationof1,3,5-trimethylbenzenebycyclohexenewasusedasamodelreactiontoevaluatethereactivitiesofthesecatalysts.

2.Experimental2.1.Catalystpreparation

TPAsolutionswerepreparedusingtheKeggintypeH3PW12O40·nH2O(Aldrich).MCM-41typemesoporoussilicawithmonodimensionalporesof50Åwassynthesizedusingaprotocolreportedelse-where[19,20].Mesoporoussilicawas?rsttreatedwitha0.1MHNO3solution(10mlofsolutionpergramofsilica)andstirredfor15minat80?Cbeforedryinginvacuumat110?Covernight.Then,itwascalcinedat540?Cfor1hbeforeusingasthesupportmaterial.PreparationofhighlydispersedCs-TPAonmesoporoussilica,denotedasCsx-TPA/MS(x=Csstoichiometry,MS=mesoporoussilica),hereafter,isexplainedindetailelsewhere[24].Forcomparison,supportedCs-TPAsamplesusingthetwo-stepim-pregnationmethodreportedbySoledetal.[23]werealsoprepared.Intheir?rststep,Cs2CO3(Aldrich,99.9%)wasimpregnatedbyaqueousincipientwet-nessontomesoporoussilica,driedat110?Covernightandcalcinedat300?Cfor2h.Followingthis,TPAwasimpregnatedusingasimilaraqueousincipientwetnesstechnique,driedat110?Covernightandcalcinedat300?Cfor2h.SupportedTPA(withoutCs-substitution)catalystswerepreparedbyaqueousincipientwetnessimpregnationontomesoporoussil-ica,driedat110?Covernightandcalcinedat300?Cfor2h.BulkCs-TPAmaterialswerepreparedbyaddingtheCs2CO3solutiondropwisetotheTPAsolutionwhilestirring[22].Theresultingprecipitatewasdriedat110?Covernightinvacuumandcalcinedat300?Cfor2h.

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2.2.Characterization

KegginstructuresofbulkandsupportedsampleswereexaminedusingaVector22(Bruker)FT-IRspec-trometerequippedwithaDTGSdetector.ThespectrawerecollectedatroomtemperatureundercontinuousN2purgewithaspectralresolutionof1cm?1.Forsamplepreparation,ca5%samplematerialwasmixedinKBrandthenpressedintoa5mmODdiscwaf?eusingahandpress.AplainKBrspectrumwasusedasbackground.

TheTGA/DTAexperimentswereperformedusingaNetzschSTA409C.Transmissionelectronmicroscopy(TEM)wasperformedonaJEOL1200microscopeat120kV.TheTEMsampleswerepreparedbyground-ingthepowderanddispersingitonaTEMcoppergridwithsuspendedcarbon?lms.Thecompositionwasalsoanalyzedusingtheenergydispersivespec-troscopytechnique(EDS).

BETsurfaceareaandporesizedistributionsweredeterminedfromBJHandmulti-pointBETmethods,respectively,usingaQuantachromeAutosorb6-Bgassorptionsystem.

SelectedsampleswereexaminedforleachingofTPAbywaterafterstirringvigorouslyfor2hat50?Cinawaterbath.AHewlett-Packard8453UVspec-trometerequippedwithanautosamplerwasusedtocollecttheabsorbancespectraat267nm.2.3.Catalyticproperties

Thecatalyticpropertieswereevaluatedusingaprobereaction,theliquidphasealkylationof1,3,5-trimethylbenzene(mesitylene,Aldrich,98%)withcyclohexene(Aldrich,99%),asreportedintheliterature

[23,25]:

Approximately50mgofbulkorsupportedcatalystwasloadedintoa7mmODvialandcalcinedat300?CunderN2for2h.Underaninertenviron-ment(Argon),2gofpremixed5wt.%cyclohexenein1,3,5-trimethylbenzenesolutionwasaddedandthenreactedinan80?Cwaterbathwhilebeingvigorously

stirred.Afteracertainperiodoftime,thereactionvialwaswithdrawntoanicebathforcoolingandcentrifugedforfurthersampling.TheproductswereseparatedusingaJ&WDB-5capillarycolumnandanalyzedusingaHP5890AGCequippedwitha?ameionizationdetector.

3.Resultsanddiscussion

3.1.CharacterizationofsupportedCs-TPAcatalystsPrimarystructuresofthesupportedcatalystswereidenti?edbycomparingtheirFT-IRabsorbancebandstothoseofbulkTPA,tungstophosphoricacidsalt(Cs-TPA),andmesoporoussilica(Fig.

1).

Fig.1.Infraredspectraof(a)tungstophosphoricacid(TPA);(b)bulkCs2.5H0.5PW12O40(Cs2.5-TPA);(c)50Åmesoporoussil-ica;(d)‘conventional’[23]50wt.%TPA/MS;(e)‘conventional’50wt.%Cs2.5-TPA/MS,and(f)improved50wt.%Cs2.5-TPA/MS.

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Fig.2.Transmissionelectronmicrographsof(a)Cs2.5-TPA/MSpreparedfromapublishedmethod[23]and(b)ourimproved,newlysynthesizedCs2.5-TPA/MSmaterial.

Bulktungstophosphoricacid(H3PW12O40)andtungstophosphoricsalt(Cs2.5H0.5PW12O40)showthecharacteristicIRbandsatca1080cm?1(P–Ointhecentraltetrahedra),984cm?1(terminalW=O),897cm?1and812cm?1(W–O–W)associatedwiththeasymmetricvibrationsintheKegginpolyanion(Fig.1aandb);however,theCs-TPAcatalystsaredistinctivelycharacterizedbyasplitintheW=Oband.ThisdoubletbecomesmoreprominentastheCsstoichiometryinthecatalystincreases(notshownhere),suggestingadirectinteractionbetweenthepolyanionandCs+exists.SimilarobservationshavebeenreportedforthecaseofW=OvibrationsinCu1.5PW12O40anions[26].Thesamedistinguish-ablefeatureswereobservedforthe50wt.%TPA/MS(Fig.1d)andCs-TPA/MScatalysts(Fig.1eandf),indicatingthattheprimaryKegginstructureispre-servedaftersupportingitontomesoporoussilica.Thus,thenewmaterialspreparedinthisstudyhadintactpolyanionstructuresand/orspeciesonthesil-icasurface.Similarresults(notshown)wereobtainedevenforthesupportedmaterialswithlowerHPAloadings(10wt.%)anddifferentCsstoichiometry(CsxH3?xPW12O40,x=1).Furthermore,31PNMRresultshavecon?rmedthat[PW12O40]3?weretheonlyspeciespresentonthesupportasevidencedbyasingle31PNMRpeakatachemicalshiftofca?15ppmreferencedtoa0ppmresponsefrom85%H3PO4.Thiswasthecaseforboththenovel50wt.%Cs-TPAmaterialsnewlypreparedinthisstudyaswellasthosewepreparedusingpublished[23]methods.WehavepreviouslyreportedthattheKegginstruc-tureremainsintactonmesoporoussilicaevenatTPAloadingsaslowas10wt.%[21].Inparticular,noad-ditional31PMRpeaks,forexample,atca?13ppm,generallyattributedtoadefect(P2W21O716?)Kegginstructure[27,28],wasevident.

ThedispersionofCs-TPAonmesoporoussilicacanbeinferredfromtheTEMresultsillustratedinFig.2,andfromEDSanalysis(notshown).Themate-rialwepreparedusingapreviouslypublishedmethod[23]resultedinasegregatedphase,whereCs-TPAisnotuniformlydispersed(Fig.2a),whereasournovelmaterialconsistsofuniformlydispersedCs-TPAsaltonmesoporoussilica(Fig.2b).Asaforementioned,directimpregnationusingaCs-TPAsolutionwasnotpossible,sinceCs-TPAisnotsolubleinanysolvent.TheprevioussyntheticapproachusedbySoledandcoworkers[23]consistsofatwo-stepimpregnation,wherethesupportwas?rstimpregnatedwithaqueousCs2CO3solutionpriortoaqueousTPAimpregna-tion.However,asindicatedinFig.2a,thepreviouslypublishedtwo-stepimpregnationresultedinpoordispersionoftheactivespecies,duelikelytothehighmobilityofCsduringTPAimpregnation,andhencepooractivity.Incontrast,oursyntheticapproach[24]leadstoenhanceddispersionasevidencedinFig.2b.

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Fig.3.Thermalgravimetricanddifferentialthermalanaly-sisoftungstophosphoricacid(TPA,solidlines)andbulkCs2.5H0.5PW12O40(Cs2.5-TPA,dashedlines).

3.2.Catalyststability

Animportantpotentialbene?tofsupportingTPAonoxidesupports(includingMS)isenhancedther-malstabilityfortheTPAsalts.Assuch,thethermalstabilityofbulkTPAandCs-TPAwasinvestigatedusingTGA/DTAandFT-IRtechniquesandthere-sultsareshowninFigs.3and4.Atabout600?C(Fig.3),bulkTPAseemedtoundergoanexothermicdecomposition[4],

H3PW12O40→1P2O5+12WO3+3H2Oresultinginaweightlossof<2%above300?Cwhile

thebulkCs2.5-TPAdidnotshowanysigni?cantweightchange.Initialweightlossof?11%(below300?C)forthebulkTPAwasprobablyduetodehydrationofthesample.Unfortunately,TGA/DTAtechniqueswerenotsuitabletoaccuratelyanalyzethesupportedmaterials.Therefore,FT-IRwasusedtoexaminethestabilityofbulkandsupportedmaterialsafterather-maltreatmentat600?C(Fig.4).InagreementwiththeTGA/DTAresultsjustdescribed,theinfraredspec-trumofbulkCs2.5-TPA(Fig.4b)showsthecharacter-isticbandsoftheKegginstructure,whilethatofbulkTPA(Fig.4a)clearlyindicatesthatdecompositionoc-curredabove600?CasevidencedbythedisappearanceandshiftofcharacteristicIRbands.Afterannealingto600?Cfor10min,eventhesupportedCs-TPA(Fig.4c

andd)showthecharacteristicIRbandsoftheKegginstructure.Previously,wehavereportedthatthether-malstabilityofHPAisenhancedby75?C(to585?C),whenTPAissupportedonmesoporoussilica[21].Itisworthnotingthatthethermalstabilityofthesup-portedTPAisenhancedfurtherevenwithasingleCs-substitution.However,theenhancementinthermalstabilityforthesecatalystsshouldbecorrelatedwiththelossofacidicgroupsand/orprotonsforproperevaluationofchangesintheircatalyticproperties(seebelow).

Thestabilityoftheactivespeciesinsolutionhasalsobeenofconcernforsolidacids,speci?callyforthesupportedmaterials[8,21].Silicahasbeenidenti-?edasoneofthemostsuitablecarriermaterialsforsupportingTPA[16].However,silicawasalsopointedouttohavearelativelyweakinteractionwithTPAspeciesduetoitsinertsurface.Thiscanresultinsig-ni?cantleachingofTPAinpresenceofapolarsolvent.Wehavepreviouslyreportedthattheorderedstruc-tureofmesoporoussilicawiththeproperporesizemitigatedleachingofTPAfromsilicainwaterlikelyduetostericconstraints[21].Inotherwords,poresizerelativetothesizeofaTPAclusterwascrucialinmitigatingleaching.InordertoexaminetheeffectofCs,wehavetestedtheleachresistanceofbothbulkandsupportedCs-TPAmaterialswithvaryingCscon-tent(Fig.5).Supportedmaterialsshowthesametrendasthebulkmaterialwhereresistancetoleachingim-provedsigni?cantlywithincreasingCsstoichiome-try.ConsideringthefactthatoneofourpurposesinusingCswastoanchortheTPAspeciestothesil-icasupportmaterial,itisworthnotingthatasmallamountofCsreadilyenhancesitsstabilityinaqueoussolution.

3.3.Catalyticproperties

Catalystactivitieswereevaluatedusingthealkyla-tionoftrimethylbenzene(mesitylene)bycyclohexeneasamodelreaction.CatalyticpropertiesofselectedcatalystsandtheirbulkcounterpartaresummarizedinTable1.Thenovel50wt.%Cs2.5-TPA/MSmaterialsynthesizedinthisstudywasabout?vetimesasactiveasthatmadefromapreviouslypublishedmethod[23],andwasalsomoreactivethanthebulkCs2.5-TPA.TheprimarypurposeforadoptingtheCs=2.5for