ISOBUTANE CRACKING OVER Y-ZEOLITES .2. CATALYTIC CYCLES AND REACTION SELECTIVITY

We describe the activity and selectivity of isobutane cracking on Y-zeolite-based catalysts in terms of catalytic cycles composed of initiation, beta-scission, oligomerization, olefin desorption, isomerization, and hydride ion transfer reactions involving carbenium ions. Olefin desorption and isomerization reactions are at quasi-equilibrium, with the former reactions determining the surface carbenium ion coverages. Microcalorimetric measurements of ammonia adsorption show that catalyst steaming leads to a decrease in the number and strength of acid sites. Decreasing the strength of Bronsted acid sites results in decreasing surface carbenium ion coverages and decreasing rates of all reactions. The hydride ion transfer cycles are affected most by catalyst steaming, leading to lower paraffin selectivities. Lower temperatures and higher conversions increase carbenium ion coverages and favor hydride ion transfer cycles, leading to higher paraffin selectivities. (C) 1995 Academic Press, Inc.