Synthesis of hierarchical SAPO-34 to improve the catalytic performance of bifunctional catalysts for syngas-to-olefins reactions

Bifunctional process integrating methanol synthesis and methanol-to-olefins conversion provides a new synthetic strategy for lower olefins (C2C4=), whereas designing matchable zeolite components is still challenging due to the harsh reaction conditions. Here, a hierarchical SAPO-34 zeolite with a sandglass-like hollow structure is synthesized from an aluminum-rich precursor gel, which provides an abundance of crystal defects during zeolite crystallization. The bifunctional catalyst obtained by integrating the hierarchical SAPO-34 and ZnOZrO2 oxide offers a C2C4= selectivity of 80% at 25% CO conversion with excellent stability in syngas conversion. We demonstrate that in the presence of high-pressure hydrogen, a moderate density of acid sites is the prerequisite for obtaining high C2C4= selectivity in syngas/methanol conversion, because the olefins are easily hydrogenated into alkanes by excessive Brønsted acid sites. The hierarchical architecture significantly prolongs the lifetime of bifunctional catalysts, by facilitating C2C4= desorption and slowing down the coking rate.