Towards understanding the enhanced cracking activity of steamed Y zeolites

Ultrastable Y (H-USY) zeolite, prepared by steam treatment of Y zeolite, is a very active hydrocarbon cracking catalyst. However, the extent of enhanc ement in activity compared to a non-steamed sample depends on the reaction condition. A model that has been proposed to explain this behavior is summa rized. The model incorporates the three different mechanisms for hydrocarbo n cracking, and the dependence of their rates on the partial pressures of r eactants and products and temperature. Depending on the reaction condition, such as hydrocarbon pressure, temperature, and conversion, the predominant cracking reaction mechanism may differ. The change in the predominant mech anism may also be a result of the proportionally small increase in external surface area caused by the steaming-induced structural destruction of the zeolite particles. However, these relatively small changes can lead to a mu ch larger overall effect on the cracking rate because of the sensitive depe ndence of oligomeric cracking, and to a lesser extent, bimolecular cracking on the alkene partial pressure. (C) 1999 Elsevier Science B.V. All rights reserved.