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1、TopicsinCatalysisVol.15,No.24,2001161SiteisolationandcooperationeffectsintheammoxidationofpropanewithVSbOandGa/H-ZSM-5catalystsSharifahBeeDerouane-AbdHamida,∗,GabrieleCentib,PremPalaandEricG.Derouanea,∗aLeverhulmeCentreforInnovativeCatalysis,TheUniversityofLiverpool,Dep
2、artmentofChemistry,POBox147,LiverpoolL693BX,UKE-mail:derouane@liverpool.ac.ukbDipartamentodeChimicaIndustrialeedIngegneriadeiMateriali,UniversityofMessina,SalitaSperone31,98166Messina,ItalyTheperformancesofaconventionalmixedoxidecatalyst,VSbyOx,andofanovelzeolite-basedc
3、atalyst,Ga-modifiedH-ZSM-5(Ga2Z27),arecomparedfortheammoxidationofpropane.Inbothcases,theeffectsofreactiontemperatureandammoniatopropaneratiowereinvestigated.Increasingproductivitytoacrylonitrileisobservedwhenthetemperatureisraisedandtheammoniatopropaneratioincreased.The
4、maximumyieldsinacrylonitrileare8.5and11.4%fortheunpromotedVSbyOxandGa2Z27catalysts,respectively,atcomparablepropaneconversions(ca.30%).TheactivityandproductivitytoacrylonitrileofGa2Z27areabout50%ofthoseofVSbyOx,at500◦C,intheabsenceofanyadditionalpromoters.Asreportedprev
5、iously,propeneandcarbonoxides,COandCO2,wereobservedtobemajorby-productswhenusingVSbyOxcatalysts,10–20and20–40%,respectively.Bycontrast,theGa2Z27catalystshowsaverylowselectivitytocarbonoxides(1%orless)andaratherhighandinterestingselectivitytoC4hydrocarbons,consistingmain
6、lyofisobutaneandisobutene.Theseobservationscanbeexplainedbydifferencesinthereactionmechanismsdescribingtheammoxidationofpropaneonthesetwosystems.ForVSbyOx,theclassicalmechanismapplies.Propeneistheprimaryreactionproductwhichiseasilyoxidisedtocarbonoxides.ForGa2Z27,propan
7、eisinitiallyactivatedtoaprotonatedpseudo-cyclopropane(PPCP)transitionstateonbifunctionalsitesinvolvingdispersedgalliumspeciesandBrønstedacidsitesofthezeolite.ThePPCPtransitionstatecanevolveinseveralwaysleadingtopropene,C2–C4hydrocarbons,andN-containingcompounds.Isolatio
8、nofgalliumspeciesisnecessaryforGa/H-ZSM-5inordertogeneratethebifunctional,Ga3+–H+,activesites,whereasvanadiums
