The roles of acid strength and pore diffusion in the enhanced cracking activity of steamed Y zeolites

Steamed HY or ultrastable Y (H-USY) zeolites are active hydrocarbon crackin g catalysts. The high activity of H-USY compared to HY zeolite has been pre viously explained by the generation of unusually strong and active Bronsted acid sites, or an increase in the number of accessible sites in a micropor e diffusion-controlled reaction. However, neither model explains the accumu lated literature observations. A model is proposed that incorporates a chan ge in the predominant cracking reaction mechanism as a function of alkane c onversion and the very different rates of these mechanisms. Additionally, a n oligomeric cracking mechanism is introduced to explicitly account for cok ing and deactivation of the catalyst. The model is capable of accounting fo r most literature results. It concludes that the large enhancement in crack ing activity by steaming is due to a proportionally smaller increase in ext ernal surface area of the zeolite crystals and possibly a small increase in the specific initiation activity of each site. These small changes lead to a much larger overall effect because of the sensitive dependence of oligom eric cracking acid, to a lesser extent, bimolecular cracking on the alkene partial pressure. (C) 1999 Elsevier Science B.V. All rights reserved.