Optimizing Ethylene Production through Enhanced Monomolecular β-Scission in Confined Catalytic Cracking of Olefin

Abstract

Confined catalytic cracking of olefins on shape-selective zeolites involves a complex reaction network with multiple β-scission types. Herein, grand canonical Monte Carlo and molecular dynamics simulations were adopted to confirm the inferior adsorption and superior diffusion of 1-pentene in the H-ZSM-5 zeolites at a reaction temperature between conventional catalytic cracking and steam cracking operating temperatures, which was the favorable condition for the monomolecular cracking pathway to improve the ethylene selectivity. Subsequently, the feasibility of improving ethylene production via enhancing the monomolecular reaction pathway was confirmed through repetitive experiments in which the catalytic cracking of 1-pentene was carried out over H-ZSM-5 zeolites. More notably, the ethylene selectivity reached a maximum of 36.2% and the ethylene/propylene ratio exceeded 1, which meant that optimizing ethylene production could be achieved by increasing the temperature of the catalytic cracking, at a milder condition than that of steam cracking. On the basis of density functional theory calculations at high temperature and kinetics analysis, it was rationalized that the dominant β-scission type evolved as the reaction temperature increased. Under the confined effect of zeolites, bimolecular pathways were suppressed while monomolecular pathways were enhanced, and even the primary (ethyl) carbenium ion-involving monomolecular pathway by rare assembly of 1-pentene was activated. Such an observation provides a feasible approach to the ethylene production via olefin-confined cracking and enriches the connotation of carbocation chemistry in zeolites.