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

B. A. Williams, S. M. Babitz, J. T. Miller, R. Q. Snurr, H. H. Kung*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

161 Scopus citations


Steamed HY or ultrastable Y (H-USY) zeolites are active hydrocarbon cracking catalysts. The high activity of H-USY compared to HY zeolite has been previously explained by the generation of unusually strong and active Brønsted acid sites, or an increase in the number of accessible sites in a micropore diffusion-controlled reaction. However, neither model explains the accumulated literature observations. A model is proposed that incorporates a change in the predominant cracking reaction mechanism as a function of alkane conversion and the very different rates of these mechanisms. Additionally, an oligomeric cracking mechanism is introduced to explicitly account for coking and deactivation of the catalyst. The model is capable of accounting for most literature results. It concludes that the large enhancement in cracking activity by steaming is due to a proportionally smaller increase in external 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 oligomeric cracking and, to a lesser extent, bimolecular cracking on the alkene partial pressure.

Original languageEnglish (US)
Pages (from-to)161-175
Number of pages15
JournalApplied Catalysis A: General
Issue number2
StatePublished - Feb 22 1999


  • Cracking mechanism on zeolites
  • Cracking, effect of acid strength
  • Cracking, effect of diffusion
  • Cracking, effect of steaming
  • Diffusion effect on cracking
  • Hydrocarbon cracking
  • Zeolite Y

ASJC Scopus subject areas

  • Catalysis
  • Process Chemistry and Technology


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