Enhanced catalytic performance of the hollow Mo/HZSM-5 nanocrystal for methane dehydroaromatization

Abstract

Although Mo/HZSM-5 catalyst is widely used in the methane dehydroaromatization (MDA) reaction, the conventional microporous HZSM-5 with long diffusion path often causes the blockage of channels and pores by coke. In this work, a series of hollow HZSM-5 zeolites with different cavity sizes and tunable shell thickness were prepared by varying the alkali treatment time. After being loaded with Mo, the catalytic performances of the hollow catalysts were investigated in MDA reaction. Compared with conventional solid Mo/Z5 catalyst, the hollow Mo/Z5(x) catalysts exhibit higher CH4 conversion and C6H6 selectivity, which show an increasing trend with the increase of inner cavity size and decrease of zeolite shell thickness. Specifically, the hollow Mo/Z5(24) catalyst achieves the maximum CH4 conversion of 13.8% and C6H6 selectivity of 72%. It suggests that the hollow structure is conducive to enhance the accessibility of feed or intermediates to the Brønsted acid sites within zeolites and facilitates the rapid diffusion of the C6H6 from the intraframework channels rather than further polymerization to the coke. Furthermore, this novel structure endows stronger interaction between the Mo species and the acid sites, resisting agglomeration of Mo-species on external zeolite surface at high temperature. This research provides a new insight into exploiting a candidate catalyst for the MDA reaction.