Effectofsteamingonthedefectstructureandacidcatalysis ofprotonatedzeolites R.A.Beyerlein a ,C.Choi-Feng c ,J.B.Hall b ,B.J.Huggins b andG.J.Ray b a USDepartmentofEnergy,ChemicalSciencesDivision,ER-142,19901GermantownRoad,Germantown,MD20874,USA b AmocoResearchCenter,POBox3011,Naperville,IL60566,USA c AmocoPolymers,Inc.,4500McGinnisFerryRd.,Alpharetta,GA30202,USA ThecatalyticpropertiesofultrastableY(USY)aredirectlyinfluencedbythezeolitedestructionwhichoccursduringformation ofUSYandduringsubsequenthydrothermaltreatment.Anewpictureoftheformationandevolutionofmesoporesduringhydro- thermal treatment has emerged from recent electron microscopy studies on hydrothermally dealuminated USY materials. Laboratory steam treatments give rise to an inhomogeneous distribution of mesopores, which occurs concomitantly with further zeolite dealumination. Such inhomogeneities are observed among different USY grains as well as within single grains. In regions withhighdefectconcentration,mesopores``coalesce''toformchannelsandcrackswhich,uponextendedhydrothermaltreatment, ultimately define the boundaries of fractured crystallite fragments. The predominant fate of aluminum ejected from lattice sites appearstobecloselyassociatedwithdarkbandswhichoftendecoratethesenewlyformedfractureboundaries.High-silicaYmate- rials,havinglittleornononframeworkAl,exhibitpoorcatalyticactivity.Theresultsofrecentstudiesprovidecompellingevidence thatthecriticalnonframeworkAlspeciesare(1)highlydispersed,and(2)quitepossiblyexistascationicspeciesinthesmallcagesof dealuminatedH-Y.InvestigationsofLewisacidityinmildlydealuminatedzeolitesindicatethattheoriginofthehighcatalyticactiv- ityisasynergisticinteractionbetweenBrnsted (framework) and highly dispersedLewis (nonframework) acid sites.The enhanced cracking, isomerization activity associated with the presence of highly dispersed nonframework Al species is not reflected in direct measuresofsolidacidity,as,forexample,bycalorimetry,orbyNMRspectroscopy.Theenhancedactivityofmildlysteamedproto- natedzeolites isnot duetoanincreaseinacidityofthebridginghydroxylorBrnstedsites. Keywords: zeoliticsolidacids;mechanismsofdealumination;zeolitedefectstructure;acidcatalysis 1.Introduction The catalytic properties of ultrastable Y (USY) are directly influenced by the zeolite destruction which occurs during formation of USY and during subsequent hydrothermal treatment. For ultrastable, high-silica, FAU framework materials prepared by steam dealumi- nation,interpretationofcatalyticdataiscomplicatedby the presence of entrained, nonframework aluminum (NFA) species. While the individual and collective roles of framework and nonframework aluminum species are not well understood, it is now clear that the presence of some nonframework Al is essential for the strong solid acidity exhibited by high-silica H-Y [1^3]. These critical nonframeworkspeciesareprobablyisolated.Whilethey arenoteasilysubjecttodirectobservation,theexistence ofisolatedNFAspeciesindealuminatedmaterialsisnot indoubt. The importance of certain nonframework Al species in the development of strong acidity in protonated (proton-exchanged) zeolites is not limited to H-Y. A review of recent literature shows a consensus that devel- opment of ``enhanced activity'' in mildly steamed HZSM-5 is also critically dependent on the presence of nonframeworkAl[4^6]. Knowledge of framework geometry is essential for understanding overall reactivity patterns for hydrocar- bon conversions over these open framework solid acids. The well known features of ``molecular traffic manage- ment'' exhibited by these materials are not always lim- itedtomolecularsieving,thatisreactantorproductsize exclusion effects. For example, at reaction temperatures of 400^500  C, the large-pore zeolites, dealuminated H- Y (H-ultrastable Y) and dealuminated mordenite, each catalyzetheisomerizationofisobutaneto n-butane[7,8]. Under similar conditions, the medium-pore system HZSM-5 produces relatively little n-butane, but instead yields much methane and propylene [8,9]. The dramati- cally different product selectivities in the latter case are attributed to the more severe spatial restrictions of the medium-pore ZSM-5, which tend to inhibit bimolecular processesinvolvingbulkyreactionintermediates. Ever since the rapid commercialization in the early 1960's of a zeolite-catalyzed process for gas oil cracking [10,11], zeolites have comprised the predominant solid acid catalyst. The characterization and application of high-silica, protonated zeolites in fluid catalytic crack- ing has been reviewed by Scherzer [12]. A broader over- view of the use of zeolites in hydrocarbon processing is givenbyMaxwellandStork[13].Recently,alargenum- ber of potential zeolite applications in the synthesis of intermediates and fine chemicals have been demon- strated[14,15]. Despite high industrial and academic interest, the nature of the active site in solid acids remains largely unresolved. In systems of interest such as a protonated zeolite,achloridedorfluoridedalumina,orsulfatedzir- conia,weareunabletoquantifythedistributionorrela- TopicsinCatalysis4(1997)27^42 27 Ä J.C.BaltzerAG,SciencePublishers