2-METHYLHEXANE CRACKING ON Y-ZEOLITES - CATALYTIC CYCLES AND REACTIONSELECTIVITY

We have formulated a quantitative kinetic model for analyzing the cata lytic cracking of 2-methylhexane over USY-zeolite-based catalysts. The model is based on carbocation chemistry which includes carbenium ion initiation, isomerization, olefin desorption, beta-scission, oligomeri zation, and hydride ion transfer reactions. It describes the complex p roduct distribution at different reaction conditions and for catalysts with different Bronsted acid strengths. Three catalytic cycles domina te this reaction and determine activity and selectivity: the initiatio n/desorption, initiation/beta-scission, and hydride ion transfer/beta- scission cycles. The rates of these cycles decrease with increasing st eaming severity, which reduces catalyst acid strength. The overall sit e time yield for 2-methylhexane cracking decreases as severity of stea ming increases. Because the cycles do not produce excess paraffins, th e paraffin to olefin ratio is always lower than 1. beta-Scission react ions follow initiation and hydride ion transfer reactions and are impo rtant reactions of 2-methylhexane cracking. Olefin adsorption-desorpti on reactions determine surface coverage of carbenium ions, and althoug h these reactions are in quasi-equilibrium, they play a crucial role i n influencing the rates of other surface processes. (C) 1997 Academic Press