Enhanced hydrocarbon cracking activity of Y zeolites

It has been known for a long time that faujasite zeolites that have been st eam-dealuminated to form USY zeolite are much more active than the unsteame d parent material for hydrocarbon cracking. A popular model to explain this phenomenon is the generation of active sites of unusually high activity. I n this paper, a brief review of recent advances towards understanding this phenomenon is presented. Evidence suggests that steaming does not generate sites of new chemical or catalytic properties for cracking. An alternate mo del is summarized that incorporates a change in the dominant cracking react ion mechanism as a function of conversion, and raises the possibility that the rates of bimolecular and oligomeric cracking are influenced by pore dif fusion.