CATALYTIC CYCLES AND SELECTIVITY OF HYDROCARBON CRACKING ON Y-ZEOLITE-BASED CATALYSTS

Environmental concerns have created the need for selective catalysts t hat increase the yield of desirable products (e.g., isobutylene from h ydrocarbon cracking units) or reduce the production of polluting bypro ducts. The development of selective catalysts may be facilitated by un derstanding the chemical factors controlling the rates of the various catalytic cycles available to the reactants and:products. We have deve loped a kinetic model based on carbenium and carbonium ion surface che mistry for isobutane cracking and extended it to 2-methylhexane cracki ng over USY-based catalysts. Catalytic cycles for isobutane cracking t hat include initiation reactions lead to olefin production, while cycl es that include hydride ion transfer reactions lead to paraffin produc tion. The overall chemistry of the major catalytic cycles is the same for isobutane and 2-methylhexane cracking, although additional reactio n pathways are available for the larger 2-methylhexane molecule. Paraf fins 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 us to build catalytic cycles, such models help ide ntify similarities and differences in reactivity patterns for various reactants.