催化剂多孔材料的制备过程.docx
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催化剂多孔材料的制备过程.docx
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催化剂多孔材料的制备过程
ProcessingofCatalysts,PorousMaterialsandAerogels
ProcessingofHighPerformanceCatalysts
INTRODUCTION
Becauseofthepurityofmetalalkoxidesandwellunderstandingofthegelationmechanisms,sol–gelprocessingusingmetalalkoxideshasbeenwidelyusedforthepreparationofsupportedmetalormetaloxidecatalysts.Althoughmostoftheworkconcerningsol–gel-preparedcatalystshavebeenfocusedonthecatalystssupportedonsilicaand/oralumina,but,recently,bimetallicandmixedoxidecatalystspreparedthroughthesol–gelprocessinghavealsobeenextensivelystudied.Accordingly,catalystsreportedinthischapterwillbeclassifiedintofourgroupsasfollows;silicasupportedcatalysts,aluminasupportedcatalysts,bimetalliccatalystsandmixedoxidecatalysts.
SILICASUPPORTEDMETALORMETALOXIDECATALYSTSBYSOL–GELTECHNIQUES
Tetraethoxysilane(TEOS)hasbeengenerallyusedasaprecursorofSiO2powderinsol–geltechniques,sincegelationmechanismsofTEOSwerewellrevealedbyDislich(1971),Teichneretal.(1976),Mazdiyasnietal.(1969)andSakka(1982).ThegelationmechanismsofTEOSmixedwithethanol,waterandasmallamountofacid,asacatalystforhydrolysisofTEOS,consistoftwosteps;thefirststepisthehydrolysisofTEOSandthesecondstepisacondensationprocess,asfollows:
(22-1)
(22-2)
Thestructuralframeof–Si–O–Si–generatedinreaction(22-2)expandsthree-dimensionallytobetransparentwetgels.Thewetgelsthusobtainedaredriedandcalcinedattemperatureshigherthan500°Ctobesol–gelSiO2powder.BETsurfacearea,porevolumeandporesizeofSiO2powderareimportantfactorstoevaluatecatalystcarriers,andcanbemodifiedtosomeextentbyanadditionofeitheracidsorbasesintheTEOSsolutionduringgelation(Sakka,1988).Onecanusethesol–gelSiO2powderforacatalystpreparationbyconventionalimpregnationmethodusinganaqueoussolutionofmetalsaltssuchasmetalnitratesandchlorides.Thecatalystspreparedwiththeseprocessing,ofcourse,belongtoakindofsol–gelcatalyst.
Moreattractivetechniqueswasdeveloped,however,inthemiddleof1980s,whereasmallamountofanothermetalalkoxide,M(OR)x,wasaddedintoTEOSsolutioninordertogenerateanewframeconsistingof–Si–O–M–O–Si–inSiO2gelsastheprecursorofsilicasupportedmetalormetaloxidecatalysts.Mostofthemetalsintheperiodictableyieldtheiralkoxidesatleastinthelaboratoryscale(Bradleyetal.,1978),butforcatalyticallyactiveelementssuchasNiCo,Cuand/orMotheiralkoxidesareexpensiveandoccasionallycontaminatedwithNaandClions,whichsignificantlyaffectuponthecatalyticperformances.Oneoftheconvenientmethodstoobtainmetalalkoxidesistodissolvemetalnitratesorhydroxidesintoethyleneglycol(EG)at~80°C,reportedbyYoda(1973).AcomplexcomposedofNiionsandethyleneglycol,denotedasNi-EG,wasfoundtobeformedduringstirringthesolutionofNi(NO3)2dissolvedinEGasfollows(Uenoetal.,1983;Tohjietal.,1984):
(22-3)
ThepresenceoftheNi-EGcomplex,adivalentmetalalkoxide,inthesolutionwasevidencedby13C-NMRandEXAFSmeasurements,butitwasdifficulttoisolatetheNi-EGcrystallitesfromthesolution.TEOSwasaddedintothesolutioncontainingtheNi-EGdivalentalkoxide,thenthefollowingreactionoccurredduringstirringthesolutionat~80°C:
(22-4)
Detectionofdiethoxyethane,(C2H5O)–(CH2CH2)–(OC2H5),byNMRwasaproofoftheprogressofreaction(22-4),andthe“–Si–O–Ni–O–Si–”frameexpandedthree-dimensionallytoyieldgreenandtransparentgelswhereNiatomswerehomogeneouslydispersedina–Si–O–Si–framework,provedbyEXAFSmeasurementsofthelocalstructurearoundNiatoms(Tohjietal.,1984).ThegelswereusedasprecursorofsilicasupportedNicatalyst;dryingandcalciningthegels,followedbyreductioninflowingH2atvarioustemperatures.TheparticlesizeofNimetalinthereducedcatalystswassharplydistributedmeasuredbyTEMandSAXS(Uenoetal.,1983),andwaswellcontrolledbytheamountofNiloadedinsilicasupports.Theselectivityintopropylalcoholduringthehydrogenationofpropionaldehydewasfoundtoincreaseonsol–gelpreparedNi/SiO2catalystswithNiparticlessmallerthan5nmindiameter.ItshouldbenotedthatsomepartsofNiatomsloadedwerestillinsidethemicroporestructuresofSiO2supportsevenafterthereductionbyH2,andwerenotexposedtoreactantgaseswhenemployedfortheheterogeneouscatalyticreaction(Tamagawaetal.,1987).
Thesol–geltechniquesforacatalystpreparationwereextensivelydevelopedbyNiwaetal.(1986a).Theyusedvariouskindsofdiolcompounds,includingethyleneglycol(EG),forpreparationofsilicasupportedRuandRu–Cucatalysts;diolcompoundsemployedwerebutanediol,cyclohexanediol,hexylenediol,pinacolandpentanediol.BETspecificsurfaceareasofthesilicasupportedRu–Cucatalystspreparedbysol–geltechniqueswerefoundtodependuponthediolcompoundsused,asgiveninTable22-1(Niwaetal.,1986b).TheyconcludedthatdialswithprimaryC–OHstructureprovidedSiO2powderpossessingthehighersurfacearea,anddialshardtobehydrolyzedyieldedSiO2powderwiththelargerporesize.SilicasupportedRu–Cucatalystspreparedbysol–gelmethodexhibitedahighactivityandselectivityforthehydrogenationofbenzenetocyclohexene;morethan30%yieldsofcyclohexenewereobtained(Niwaetal.,1986b).Sodesawaetal.havealsopreparedsilicasupportedCucatalystsbysol–geltechniques,usingcoppernitratedissolvedinethyleneglycol,inordertouseforthedehydrationofmethanolintomethylformate(Sodesawaetal.,1986)andforthesteamreformationofmethanolintoCOandH2(Sodesawaetal.,1991).
Schubertetal.areoftheopinionthatamoregeneralapproachtocontroldispersionofthemetalcompoundduringsol–gelprocessingistheuseofchemicallyadjustablelinksbetweenthemetalionsandtheSiO2support(Breitscheideletal.,1991;Schubertetal.,1989a,1989b).ThefirststepistopreparemetalcomplexesdescribedLnM–X–(CH2)3Si(OC2H5)3,whereLnrepresentsanorganicligandcoordinatingtotheinterestedmetal,andmetalacetylacetonatesM(acac)xoracetatesM(OAc)xareoftenemployedasLnM.Themetalsaltsweredissolvedinethanol,andthenspecifiedamountsofX–(CH2)3–Si(OC2H5)3wereaddedintotheethanolsolutiontoproduceLnM–X–(CH2)3–Si(OC2H5)3.ThesecondstepisthereactionofLnM–X–(CH2)3–Si(OC2H5)3withTEOSinEq.(22.5),accompaniedwithpolycondensationreactionofTEOSinthepresenceofasmallamountofNH3ascatalyst.
TABLE22-1.SURFACEAREASOFSILICASUPPORTEDRu–CuCATALYSTSPREPAREDPYSOL–GELTECHNIQUES(NIWAETAL.,1987)
Diolcompounds
Kindsofdiol
KindsofC-OH
Surfacearea(m2/g)
Pinacol
α
Tertiary,tertiary
177
2,3-Butanediol
α
Secondary,secondary
504
1,2-Cyclohezanediol
α
Secondary,secondary
500
1,2-Butanediol
α
Secondary,primary
621
Ethyleneglycol
α
Primary,primary
794
Hexylenediol
β
Tertiary,secondary
441
3-Methyl-1,3-butanediol
β
Tertiary,primary
455
2,4-Pentanediol
β
Secondary,secondary
509
1,3-Butanediol
β
Secondary,primary
712
1,3-Propanediol
β
Primary,primary
825
1,4-Cyclohexanediol
γ
Secondary,secondary
850
1,4-Butanediol
γ
Primary,primary
592
(22-5)
ThemostimportantthingisinthechoiceofX,whichwillcoordinatetothemetalatomand,atthesametime,anchorthemetalatomto–Si–O–Si–netframethrough–(CH2)3–spacer.AccordingtotheirconclusionthebestchoiceofXstronglydependsuponthemetalsaltsemployed,andforCo(OAc)2,Cu(OAc)2,Ni(OAc)2,Pd(acac)2andPt(acac)2thebestchoiceistheethylenediaminederivativeH2NCH2CH2NH–(CH2)3–Si(OC2H5)3.WhileforAgNO3,H2N–(CH2CH2)3–Si(OC2H5)3orNC(CH2CH2)3–Si(OC2H5)3isveryeffectiveforanchoringthemetalionsto–Si–O–Si–frame.Finalstepisthecalcinationoftheresultantgelsat500°Cinair,followedbythereductionwithH2atanappropriatetemperaturedependinguponthemetals.MetalparticlesinthecatalystsarewellcontrolledinsizeduetohighdispersionofmetalcompoundsinthegelsthroughM–O–Silinksformedduringpolycondensationreaction.
Lopezetal.comparedthedeactivationresistanceofsilicasupportedRucatalysts,preparedbysol–geltechniquesandaconventionalimpregnationmethod,forhydrogenationofbenzenetocyclohexaneando-xyleneinto1,2-dimethylchlorohexaneat100°C(Lopezetal.,19911992a,1992b).Thesol–gelcatalystswerepreparedbyadditionofRuCl3,dissolvedinaqueoussolutionofNH3,intoTEOSmixedwithethanol,followedbygelationat80°C.Theyreportedthatthedeactivationofsol–gelRucatalystswasimprovedbyalmost50timeshigherthanthatoftheimpregnatedcatalysts.Inordertoelucidatetheimproveddeactivationofsol–gelcatalysts,thelocalstructuresaroundRuionswerecharacterizedbyvariousspectroscopictechniquessuchasX-raydiffraction,SAXS,UV-VISabsorptionandradialdistributionanalysis.Conclusionstheyobtainedwereasfollows;mostoftheRuionsinthedriedsol–gelcatalystswereoccludedin–Si–O–Si–network,andsomepartofRuionsmigratedtothesilicasurfaceuponcalcinationandsegregatedintosmall-sizeRuO2particles.WhereastheRuionsintheimpregnatedcatalystsdepositedonthesilicasurfaceduringdrying,andcoagulatedtoyieldlargesizeRuO2particlesuponthecalcinationsinair.AmodelproposedforlocalstructurearoundRuionsinthesol–gelcatalystsisdrawninFigure22-1(a),whereRuionsaresurroundedwithfourSiO4tetrahedra;twotetrahedracoordinatedbyvertexandtwobyedges(Lopezetal.,1993).Abinitiocalculationswerecarriedoutinordertoestimatethe
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