b-Axis-Oriented ZSM-5 Nanosheets for Efficient Alkylation of Benzene with Methanol: Synergy of Acid Sites and Diffusion

ZSM-5 nanosheets are promising catalysts for the catalytic reactions controlled by diffusion limitations. This study reveals its significant application in the alkylation of benzene with methanol. The b-axis-oriented ZSM-5 nanosheets with similar acid property but varied thicknesses of about 30, 90, and 300 nm were prepared to investigate the effect of thickness on their catalytic properties for alkylation reactions. Comparative results demonstrate that the sample with a thickness of 30 nm exhibits higher benzene conversion, xylene selectivity, and methyl selectivity (up to 97%), accompanied by an ultralong lifetime (up to 1000 h, 10 times longer than that of the sample with a thickness of 300 nm) and lower byproduct ethylbenzene selectivity. This is ascribed to the shortened straight channel length, increased specific surface area, and enlarged mesopore volume that significantly facilitate the diffusion of reactants and products, increase the accessibility of acid sites, and decrease the coke formation. Moreover, compared with conventional ZSM-5 nanocrystals, ZSM-5 nanosheets deliver a substantially extended lifetime due to fewer framework defects. Most significantly, this study unravels the diffusion effect on ethylbenzene selectivity over ZSM-5 nanosheets with different thicknesses and illustrates the role of strong Brønsted acid sites in the dynamic changes of ethylbenzene selectivity. In light of the above analysis, we developed a precoking strategy and an introducing-heteroatom strategy to precisely tailor the catalyst acidity, further suppressing the ethylbenzene formation (<0.3%) while maintaining long-term stable operation (>300 h).