Catalytic Cycles and Selectivity of Hydrocarbon Cracking on Y-Zeolite-Based Catalysts

Abstract

Environmental concerns have created the need for selective catalysts that increase the yield of desirable products (e.g., isobutylene from hydrocarbon cracking units) or reduce the production of polluting byproducts. The development of selective catalysts may be facilitated by understanding the chemical factors controlling the rates of the various catalytic cycles available to the reactants and products. The authors have developed a kinetic model based on carbenium and carbonium ion surface chemistry for isobutane cracking and extended it to 2-methylhexane cracking over USY-based catalysts. Catalytic cycles for isobutane cracking that include initiation reactions lead to olefin production, while cycles that include hydride ion transfer reactions lead to paraffin production. The overall chemistry of the major catalytic cycles is the same for isobutane and 2-methylhexane cracking, although additional reaction pathways are available for the larger 2-methylhexane molecule. Paraffins and olefins with three or more carbon atoms can be produced from 2-methylhexane by cycles that include both initiation and hydride ion transfer reactions and the paraffin to olefin ratio cannot be greater than 1. By allowing one to build catalytic cycles, such models help identify similarities and differences in reactivity patterns for various reactants.