Roles of Alkenes and Coke Formation in the Deactivation of ZSM-5 Zeolites During n-Pentane Catalytic Cracking

Abstract

In order to reveal the roles of alkenes and coke formation in catalyst deactivation, n-pentane catalytic cracking was carried out over ZSM-5 zeolites at 650 °C under atmosphere for 100 min on stream, and the operating conditions including the SiO2/Al2O3 ratio of ZSM-5 zeolites, weight hourly space velocity, reactant partial pressure and carrier gas flow rate were tailored. Three indexes i.e. catalyst half-life, average selectivity to ethylene plus propene, and coke burning temperature were defined to qualitatively and quantitatively evaluate the influences of operating conditions on catalytic stability, alkenes formation and coke location. It was found that catalyst stability, alkenes formation and coke location were closely related to each other: Higher alkenes selectivity, more coke on the external surface of ZSM-5 zeolites, and faster catalyst deactivation. This can be attributed to the specific texture of ZSM-5 zeolites. Since the external surface was free from steric hindrance, the coke on the external surface tended to grow bigger and was enhanced by increasing alkenes selectivity. Thus, an increase of alkenes selectivity enhanced the formation of coke on the external surface of ZSM-5 zeolites, which blocked pore openings, accelerated catalyst deactivation and resulted in a low yield of ethylene plus propene of 1.7 wt% after 100 min on stream in n-pentane catalytic cracking; while, a decrease of alkenes selectivity promoted the in-migration of coke to the micro pores of ZSM-5 zeolites, which kept certain pore openings alive, maintained the catalytic activity against the same or even higher coke extent, and led to a high yield of ethylene plus propene of 43.0 wt% after 100 min on stream in n-pentane catalytic cracking.