Comparative study on the catalytic behaviors of zeolites with different diffusion limitation in ethane aromatization

Abstract

The design of zeolite-based catalysts with reduced diffusion limitation has attracted much attention for the enhanced activity and stability in light alkane aromatization. Here we employed three types of zeolite-based catalysts including micron zeolite, solid nano zeolite, and hollow zeolite with nano size to investigate the effects of diffusion length and pore structure of zeolites on their mass transport efficiency and catalytic performance during ethane aromatization. It was indicated that the shortened diffusion length and increased mesoporosity of Zn/hollow-HZSM-5 endow it with substantial improvement in mass transport, leading to enhanced accessibility to active acid sites and rapid escape of formed lighter aromatics rather than further polymerization to form coke. Contrarily, the solid micron zeolites with long diffusion path experience strong diffusion limitation, which extends the contact time of reactants and products with the active acid sites within zeolites thus resulting in severe coke deactivation. After being subjected to ethane aromatization for 400 min, the aromatics selectivity toward Zn/hollow-HZSM-5 catalyst retained 50%, which is almost threefold with respect to that of the solid Zn/micro-HZSM-5 and Zn/nano-HZSM-5. Based on the Thiele modulus, it can be suggested that shortening diffusion length and introducing mesopores into zeolites can effectively alleviate diffusion resistance, then enhance their catalytic activity and stability toward ethane aromatization.