英文

Effectsofsurfacemodificationoncoking,deactivationandpara-selectivityofH-ZSM-5zeolitesduringethylbenzenedisproportionationWen-HuaChena,Tseng-ChangTsaib,1,Sung-JengJonga,2,QiZhaoa,Chung-TaTsaia,c,IkaiWangd,Huang-KueiLeec,Shang-BinLiua,∗aInstituteofAtomicandMolecularSciences,AcademiaSinica,P.O.Box23-166,Taipei106,Taiwan,ROCbDepartmentofChemicalEngineering,I-ShouUniversity,Kachsiung840,Taiwan,ROCcInstituteofMaterialsScienceandManufacturing,ChineseCultureUniversity,Taipei111,Taiwan,ROCdDepartmentofChemicalEngineering,NationalTsinghuaUniversity,Hsinchu300,Taiwan,ROCReceived24January2001;receivedinrevisedform9April2001;accepted22June2001AbstractTheeffectsofcokingandsurfacemodificationofH-ZSM-5zeolitesonenhancementofpara-diethylbenzeneselectivityduringdisproportionationofethylbenzenehavebeeninvestigated.Surfacemodificationsofthezeolitesbysilicachemicalvapordeposition(Si-CVD)andbystepwiselepidineadsorptionand/orSi-CVDtreatmentswereexamined.Theacidity,sorptioncapacity,stability,andcokecontentofthesampleswerecharacterizedbyvarioustechniques,suchasadsorption,TGA,IR,129XeNMR,andsolid-state27Aland31PMASNMR.Theperformanceofthepara-selectiveprocesswasstudiedbythecorrelationbetweentheobservedpara-selectivityandconversion.Theworkingprincipleresponsibleforthepara-selectivefeaturewasfoundduetothecombinedeffectsofdiffusionlimitationsandinactivationofexternalactivesites.Attheextremeoflowconversion,thefeaturedependsmostlyontheformereffect,whereasthelatterbecomesprogressivelyimportantwithincreasingconversion.Athighconversion,whilebotheffectsareimportant,diffusioncontrolledlimitationsassociatedwithsterichindrancesneartheporemouthsplaysthepredominantrole,especiallytowardssaturatingsurfaceSiO2loading.©2002ElsevierScienceB.V.Allrightsreserved.Keywords:Shapeselectivity;Si-CVD;Coking;H-ZSM-5zeolite;NMR1.IntroductionPara-diethylbenzene(p-DEB)isahigh-valueddesorbentcommonlyusedinp-xyleneadsorptiveseparationprocesses[1]andthekeystartingmaterialofvinylstyrene,whereasitsisomers,o-andm-DEB,havelowermarketvalues.Althoughp-DEBcanbeproducedbytheethylbenzene(EB)disproportionation,amodelreactionwidelyusedforthecharacterizationofcatalyticactivityofzeolites[2–5],theyieldisnormallylimitedbythethermodynamicequilibriumcompositions.Thedevelopmentandimprovementofpara-selectiveprocessnotonlysimplifytheproductionschemesbutalsosignificantlyreducetheproductioncosts.Thus,researchonpara-selectivityenhancementisanimportantandchallengingtaskwithgreatindustrialdemands.Manypara-selectiveprocesseshavebeendevelopedforimportantindustrialapplications,forexamples,alkylation,disproportionationandisomerizationofmonoalkylbenzene,usingavarietyofdifferentcatalystmodificationtechniques,suchassurfacedepositionofSiO2[6–20]oroxidesofP,BorMg[8,20–29],orbypre-coking[30–33].Inspiteofextensiveresearchinthisfield,thenatureoftherelatedshapeselectiveprocessisstillnotwellunderstoodandthesubjectofongoingdebates[8,14,19,33–38].Itwasproposedthatthepara-selectivefeatureisprovokedbydiffusionlimitationsduetoporenarrowing/blockingand/orbyinactivationofexternalaciditywhichinhibitsthesecondary,non-shapeselectiveisomerizationreactionsonthesurfaceofthecatalyst.Theobjectivesofthisstudyistoinvestigatehowcokinginterplaywiththetypesandextentsofcatalystsurfacemodificationtreatmentsandtheirrespectiverolesduringthedecisiveworkingprinciplesofthepara-selectiveprocess.Inparticular,theeffectsofcokedeposition,surfacemodificationbysilylationand/ormolecularadsorption,andreactionconditionsontheactivityandp-DEBselectivityofH-ZSM-5zeolitesduringEBdisproportionationwereexamined.ThenatureandacidityofthecatalystswerecharacterizedbydiffusereflectanceFourier-transforminfraredspectroscopy(DRIFTS),solid-state27Almagic-angle-spinning(MAS)NMRand31PMASNMRspectroscopyoftheadsorbedphosphineoxideprobemolecules.Whereasthevariationsofsorptioncapacityandthelocation/concentrationofcokedepositsinthecatalystsweredeterminedbyxenonadsorption,129XeNMR,andthermogravimetricanalysis(TGA),respectively.2.Experimental2.1.MaterialsThepowdered,binderlessparentH-ZSM-5zeolite(Si/Al=15;denotedasHZSM5)wasobtainedcommercially(StreamChemical),itsstructureandframeworkcompositionswereconfirmedbypowderX-raydiffraction(XRD)and29SiMASNMR.TheamountofNH3retainedinthesampleat473Kwas0.79mmolg−1catalyst.Anaveragecrystallinesizeof0.5_mforthezeolitesamplewasdeterminedbyelectronmicroscopy.Researchgradereagentswereobtainedcommerciallyandwereusedwithoutfurtherpurification.2.2.Silicachemicalvapordeposition(Si-CVD)Surfacemodified,silylatedsampleswerepreparedbychemicalvapordepositionofsilica(Si-CVD),usingTEOSasthesilylationagent,followingtheproceduresreportedearlierbyWangetal.[8].PelletizedparentHZSM5zeolite(10–20mesh;ca.4g)wasfirstpackedintoahome-builtreactor,followedbycalcinationat723Kinair(flowrate100ml/min)for8h,thenbroughtto413KunderstreamofN2gas(100ml/min).Silylationtreatmentofsampleswereconductedbypassingmixtureof4wt.%TEOSintoluenethroughthecatalystbedat413KunderN2carriergas(12mlh−1)forvariousdepositiontime(td),subsequentlyfollowedbycalcinationinflowingair(10