Defective sites in ZSM-5 zeolite synthesized by n-butylamine template facilitating uniform meso-microporosity by alkali-treatment

Abstract

Two ZSM-5 zeolites (denoted as BZ and TZ series), respectively synthesized by using n-butylamine (BA) and tetrapropylammonium (TPA+) as template, were transformed to pertinent hierarchical zeolites (BZ-AT and TZ-AT) by post-treatment via alkali-treatment. The both ZSM-5 zeolites have distinct defective sites, which are crucial importance for their properties in terms of forming meso-microporosity and performance in the methanol to olefin reaction. By alkali treating ZSM-5 zeolites (BZ-80 and TZ-80) with comparable SiO2/Al2O3 of 80, the hierarchical zeolite BZ-80-AT proves higher Desilication Efficiency than TZ-80-AT. Notably, BZ-80-AT features a uniform distributed meso-microporosity with mesopore size centered around 10 nm, larger external surface area and mesopore volume, while TZ-80-AT displays a centrally hollow structure with cavity size of more than 200 nm in diameter. Characterization results demonstrate more silanol nests in BZ-80, facilitating uniform desilication to create uniform mesopore in BZ-80-AT crystals. In contrast, the more internal silanols in TZ-80, along with the connatural distorted framework aluminium generate the hollowed structure in TZ-80-AT. Much should be owed to its specific micro/mesoporosity rendering better diffusivity, BZ-80-AT exhibits higher propylene and butylene selectivity as well as longer lifetime in the MTP reaction.