Polycrystalline ZSM-5@Beta core–shell composites fabricated by a seed-directed approach: A catalyst to boosting light olefin production by ordered hierarchical cracking

Abstract

Featuring engineerable reaction pathway, ordered hierarchical cracking over core–shell structured zeolites (core@shell) plays a pivotal role in achieving the high-efficient conversion of low-value oil fractions into important petrochemical building blocks such as ethylene and propylene. However, the synthesis of core@shell zeolite composites with specific desirable textures is technically challenging. Herein, a novel embryo seed-breeding (ESB) approach was developed to synthesize polycrystalline ZSM-5@Beta core–shell composites (pZ@B) for boosted light olefin production. For the first time, pZ@B composites composed of a polycrystalline ZSM-5 core and a submicron-sized Beta shell was reported, which shows enhanced inner-diffusion properties and improved hydrothermal stabilities than that of conventional ZSM-5@Beta composites. In pZ@B-catalyzed reations, bulky molecules undergone a preliminary cracking into intermediate products within the Beta shell, followed by the direct diffusion into the micropores of the ZSM-5 core where they could be cracked selectively into ethylene and propylene. Compared with physically mixed zeolites and hierarchical ZSM-5, pZ@B composites showed the highest conversion and yield of ethylene and propylene in the cracking reaction of both decalin and practical industrial heavy oil. The superior catalytic activity of pZ@B can be attributed to the high accessibility of acid sites and the ordered hierarchical cracking space derived from the core–shell structure.