Catalytic roles of the acid sites in different pore channels of H-ZSM-5 zeolite for methanol-to-olefins conversion

Abstract

H-ZSM-5 zeolite is a typical catalyst for methanol-to-olefins (MTO) conversion. Although the performance of zeolite catalysts for MTO conversion is related to the actual location of acid sites in the zeolite framework, the catalytic roles of the acid sites in different pore channels of the H-ZSM-5 zeolite are not well understood. In this study, the MTO reaction network, involving the aromatic cycle, alkene cycle, and aromatization process, and also the diffusion behavior of methanol feedstock and olefin and aromatic products at different acid sites in the straight channel, sinusoidal channel, and intersection cavity of H-ZSM-5 zeolite was comparatively investigated using density functional theory calculations and molecular dynamic simulations. The results indicated that the aromatic cycle and aromatization process occurred preferentially at the acid sites in the intersection cavities with a much lower energy barrier than that at the acid sites in the straight and sinusoidal channels. In contrast, the formation of polymethylbenzenes was significantly suppressed at the acid sites in the sinusoidal and straight channels, whereas the alkene cycle can occur at all three types of acid sites with similar energy barriers and probabilities. Consequently, the catalytic performance of H-ZSM-5 zeolite for MTO conversion, including activity and product selectivity, can be regulated properly through the purposive alteration of the acid site distribution, viz., the location of Al in the zeolite framework. This study helps to elucidate the relation between the catalytic performance of different acid sites in the H-ZSM-5 zeolite framework for MTO conversion, which should greatly benefit the design of efficient catalyst for methanol conversion.