活性炭负载磷钨酸和硅钨酸催化剂的研究图文.docx

活性炭负载磷钨酸和硅钨酸催化剂的研究图文.docx

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活性炭负载磷钨酸和硅钨酸催化剂的研究图文

AppliedCatalysisA:

General208（2001

7–19

Tungstophosphoricandtungstosilicicacidson

carbonasacidiccatalysts

MarceloE.Chimienti,LuisR.Pizzio,CarmenV.Cáceres,MirtaN.Blanco∗

CentrodeInvestigaciónyDesarrollodeProcesosCatal´ıticos（CINDECA,

UniversidadNacionaldeLa,UNLP-CONICET,47No.257,LaPlata1900,ArgentinaReceived10January2000;receivedinrevisedform9June2000;accepted14June2000

Abstract

Tungstophosphoric（TPAandtungstosilicicacid（TSAcatalystssupportedoncarbonwerestudied.Theywerepreparedbyequilibriumandincipientwetnessimpregnationtechniques.SolutionsofTPAandTSAin50%v/vethanol–waterwereusedtoimpregnatecarbonat20◦C.TheFouriertransforminfraredand31PnuclearmagneticresonancespectroscopiesofTPAandTSAsupportedcatalystsdriedat70◦Cshowedthatthespeciespresentweretheundegraded[PW12O40]3−and[SiW12O40]4−anion,respectively.X-raydiffractionoftheimpregnatedsolidsshowedthesamediffractionpatternsasthesupports,possiblyowingtoahighdispersionofnon-crystallinespecies.ThescanningelectronmicroscopywithenergydispersiveX-rayanalysisofthesupportedacidsshowedauniformdistributionofTPAandTSAinthecarbonparticleswithlowheteropolyacidcontent.InthesampleswithhighcontentorpreparedbyincipientwetnessmethodsmallgrainsofTPAandTSAwerefound.Thesegrainsshowedtobenearlyinactiveforisopropanoldehydration.Thespecificconversionofthecatalystsforthisreactionincreasesregularlywiththeincreaseoftheadsorbedheteropolyacidcontent.©2001ElsevierScienceB.V.Allrightsreserved.

Keywords:

Tungstophosphoricacid;Tungstosilicicacid;Kegginphase;Equilibriumimpregnation;Isopropanoldehydration;Carbon

1.Introduction

Theuseofheteropolyacids（HPAascatalystsforfineorganicsynthesisprocessesisdeveloping.Syn-thesisofantioxidants,medicinalpreparations,vita-minsandbiologicallyactivesubstances,forinstance,havebeenreportedandsomearealreadyappliedinpractice[1].Then,thesecatalystsareveryim-portantforindustriesrelatedwithfinechemicals,asflavors,pharmaceuticalandfoodindustries,amongothers.

∗Correspondingauthor.Tel.:

+54-221-4210711;fax:

+54-221-4254277.

E-mailaddress:

mnblanco@dalton.quimica.unlp.edu.ar（M.N.Blanco.

Heteropolyacidsaremoreactivecatalystsforvariousreactionsinsolutionthanconventionalinor-ganicandorganicacids.Theyareusedasindustrialcatalystforseveralliquid-phasereactions[2–6],asalcoholdehydration[7],alkylation[8]oresterification[9]reactions.Amongheteropolyacids,polytungsticacidsarethemostwidelyusedcatalystsowingtotheirhighacidstrengths,thermalstabilities,andlowreducibilities.

Acidandoxidationreactionscatalyzedbysolidheteropolycompounds（gas–solidandliquid–solidsystemscanproceedviathreemainwaysofreactionnamedsurface,bulktypeI（pseudoliquidandbulktypeIIcatalysis[1–3].

Inpseudoliquidcatalysis,largeamountsofthere-actantswereadsorbedbetweenthepolyanionsinthe

0926-860X/01/$–seefrontmatter©2001ElsevierScienceB.V.Allrightsreserved.PII:

S0926-860X（0000702-X

8M.E.Chimientietal./AppliedCatalysisA:

General208（20017–19

ioniccrystalbyreplacingwaterofcrystallizationorexpandingthelattice,andreactionoccursthere.

InbulktypeIIcatalysis,althoughtheprincipalre-actionmayproceedonthesurface,thewholesolidbulktakespartinthereactionowingtotherapidmi-grationintothebulkofcarrierssuchasprotonsandelectrons.

Insurfacetypecatalysis,thereactionstakeplaceontheinternalporeandexternalsurfacesofsolidcat-alysts.Ingeneral,thereactionrateisproportionaltothecatalystsurfacearea.

Supportingtheheteropolyacidsonsolidswithhighsurfaceareasisausefulmethodforimprovingcatalyticperformanceinliquid–solidandgas–solidsurfaceheterogeneousreactions.Forinstance,Huetal.[10]haveusedcatalystsbasedonheteropoly-acidssupportedonsilicaandalumina,withgoodresults,forthepreparationofoctylphenolandnonylphenol,throughthealkylationreactionofphenolwith1-octeneandnonene,respectively.Thesefinechemicalproductswerewidelyusedassurfactants.Thecatalystsbasedonheteropolyacidshavemanyadvantagesoverliquidacidcatalysts.Theyarenon-corrosiveandenvironmentallybenign,present-ingfewerdisposalproblems.Theirrepeateduseispossibleandtheirseparationfromliquidproductsiseasierthanthehomogeneouscatalysts.

ThenatureoftheinteractionbetweentheHPAandthesupportfundamentallydependsonthecentralandperipheralatomswhichconstitutethepolyanion,thesupport,andtheHPAloadingonthesupport.Itisnecessarytopayattentiontothechangesintheacidstrength,thestructuresoftheaggregates,andthepos-sibilityofdecomposition.

Schwegleretal.[11]havedescribedthecontrolledadsorptionanddesorptionoftungstophosphoric（TPAandtungstosilicicacids（TSAaqueoussolutions,usingcharacterizedactivatedcarbonsupports.TheyfoundthatinthecatalystspreparedtheamountofHPAloadedontoactivatedcarbonismuchhigherthantheamountremainingafterextractionwithor-ganicsolventsorwater.Theobservedleachingshouldbetakenintoaccountwhenusingthesesystemsascatalystsintheliquidphase.

Activatedcarbonhasbeenfoundtobeabletoen-trapacertainamountofheteropolyacids.TheHPAthusentrappedwerehardlyremovedevenbyextrac-tionwithhotwaterorhotmethanol[12].Entrapped

catalystshaveprovedtobeconvenientforliquid-phaseandvapor-phasereactions.

Inapreviouspaper[13],itwasstudiedtheequi-libriumadsorptiononactivatedcarbonofTPAandTSA,bothfromsolutionsinethanol–water50%v/v.Itwasfoundthattheadsorptionstrengthwashigherfortheformeracidthanitwasforthelatter.Inthecatalystsobtainedthespeciespresentwerethean-ions[P（SiW12O40]3（4−withundegradedKegginstructure.

ThispaperaimstostudytheinteractionbetweenTPAorTSAandcarbon,incatalystspreparedbyequi-libriumimpregnationandleachedwithethanol–watersolvent,andalsointhesamplesobtainedbyHPAad-ditiontotheoriginalcatalystsbymeansofincipientwetnessmethod.Thespeciespresentinallsolidshavebeencharacterizedbydifferentphysicochemicaltech-niques.Also,thecatalystactivityintheisopropanoldehydrationreactionhasbeendetermined.2.Experimental2.1.Materials

TheimpregnatingsolutionswerepreparedfromH3PW12O40·nH2OTPA（BDHandH4SiW12O40·nH2OTSA（Mallinckrodt,respectively.Amixtureofdemineralizedwaterandethanol96%（Soria,involumetricratio1:

1,wasusedassolvent.

Thesupportusedwascommercialwood-basedacti-vatedcarbon（Cgroundtoameanparticlesizeof1mm,withasurfaceareaof960m2/g,aporevolumeof0.33cm3/gandaverageporediameterbelow2nm.Beforebeingutilized,thismaterialwaswashedfirstinsolutionofNaOH0.1mol/dm3,theninHCl0.1mol/dm3toeliminatethesolubleacidicandalkalineim-purities.Finally,itwaschemicallytreatedinsolutionofHNO330%w/vheatedbyrefluxfor2h.Thetreat-mentwithnitricacidmakesthecarbonmoreacidic[11]andshiftedtheisoelectricpointfrom6.9to2.7.TheKlevelwasanalyzedbeforeandafterthewash-ing,thevalueswere0.026and0.006%Kw/w.So,theproceduretocleanupthewoodbasedactivatedcarbonhadbringtheKleveldowntonearzerolevel.

Thematerial,afterthewashingandchemicaltreat-ment,presentedasurfaceareaof806m2/g,aporevolumeof0.26cm3/gandtheaverageporediameterbelow2nm.

M.E.Chimientietal./AppliedCatalysisA:

General208（20017–199

2.2.Catalystspreparation

2.2.1.Equilibriumimpregnation

ThecatalystsbasedonTPAorTSAsupportedoncarbonwereobtainedusingtheequilibriumimpreg-nationtechnique.Theimpregnationexperimentswereperformedat20◦Ccontacting1gofsupportwith4mlofsolutionunderconstantstirringfor72h,whichwasatimelongerthanthatrequiredtoreachequili-briumintheadsorption–desorptionprocess.ThiswasensuredinadsorptionkinetictestspreviouslymadewithsolutionsofTPAandTSAinconcentrationsof131and160mgW/cm3,respectively.Adsorbedtung-stenconcentrationwascalculatedonthebasisofthedecreaseoftungstenconcentrationinthesolution,bymeansofamassbalanceandassumingnovariationinthesolutionvolume.

Theinitialconcentrationofthesolutionsemployedherecoveredarangeof6–146mgW/cm3forbothhet-eropolyacids.TheadsorptiontestswerecarriedoutatthepHreachedbythesolutionwhendissolvingeachacidinethanol–water,whichapproximatelyvar-iedfrom2.5to1.1astheconcentrationofeitherHPAincreased.Aftertheselectedcontacttime,solidandsolutionwereseparatedbycentrifugationfortheirana-lysis.ThesamplesobtainedwerenamedM1PtoM6PandM1StoM6S.

Thetungstenconcentrationinthesolutions,bothbeforeandaftercontactingthecarbon（CiandCe,respectively,wasdeterminedbyatomicabsorptionspectrometry.Thecalibrationcurvemethodwasused,withstandardspreparedinthelaboratory.Theequip-mentusedwasanILModel457spectrophotometer,withsinglechannelanddoublebeamandmonochro-matorof330mmfocaldistance.Thelightsourcewasahollowmonocathodelamp.Theanalyseswerecarriedoutatawavelengthof254.9nm,bandwidth0.3nm,lampcurrent15mA,phototubeamplifica-tion800V,burnerheight4mmandacetylene–nitrousoxideflame（11:

14.

Ontheotherhand,itwasobservedthatthesurfaceareaandporevolumevaluesofthecatalystsweresimilartothoseofthesupport.

2.2.2.Soluteremoval

SamplesofcarbonwereimpregnatedwithTPAorTSAsolutions,followingthepreviouslydescribedtechnique,thusobtainingsamplesnamedM7Pand

M7