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.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
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