MCM-56 stabilization synthesis using auxiliary tetraethylammounium ions: Its role to inhibit surface Si-O-Al bridged linkage and retain highly delaminated structure

摘要

MCM-56 is one important member of MWW type zeolites, which possesses an unique 2D framework structure consisting of one or two MWW monolayers. Due to its high external surface area and large external acid concentration, MCM-56 zeolite exhibits excellent potential catalytic performance in alkylation of aromatics. However, it's a transient product and momentarily convert into MCM-49 zeolite. Herein, we present a facile strategy for stable synthesis of highly delaminated MCM-56 zeolite, denoted MCM-56(D), by using the dual-template system composed of hexamethyleneimine (HMI) and tetraethylammonium hydroxide (TEAOH). The characteristics of extended stable period and higher delamination degree distinguish MCM-56(D) from traditional MCM-56. The phase stabilization period is prolonged by two days or more, which is dependent on the increase of TEAOH amount. The results indicate that HMI acts as a directing agent to promote crystallization of MWW topology, while TEAOH has been proven to be a crystallization regulator by hindering Si-O-Al bridges formation in interlayers, thus forming a highly exfoliated MCM-56(D). Compared with the traditional MCM-56, the external surface area and mesoporous volume (213.6 m(2) /g, 1.20 cm(3) /g) of MCM-56(D) increased by more than 60% and 183%, respectively, naturally possessing more accessible Bronsted acid sites favourable for bulky molecule reaction, so it shows a high conversion in cracking of 1, 3, 5-triisopropylbenzene. In the liquid-phase alkylation of benzene with ethylene, MCM-56(D) performs higher reactivity and slower deactivation than MCM-22 and MCM49 under a WSHV (Ethylene) of 2 h(-1). This dual-template method ensures MCM-56(D) a practicable catalyst for processing bulky molecules in petrochemical industry.