Precisely regulating Brønsted acid sites to promote the synthesis of light aromatics via CO2 hydrogenation

Abstract

Significant progresses have recently been made in the direct conversion of CO2 to aromatics, whereas it remains a big challenge to selectively synthesize light (C6–8) aromatics because of the perplexing role of Brønsted acid sites (BAS) in aromatic synthesis. Herein, a series of composite catalysts comprising Fe-based component and ZSM-5 zeolites with distinct Brønsted acidities were studied to explore the influence of BAS on the light aromatic synthesis and coke formation in CO2 hydrogenation. The results indicated that BAS of ZSM-5 are the main active sites for aromatization, and increasing Brønsted acidity significantly promotes the synthesis of aromatics, especially light aromatics. Further passivation of the external BAS of HZ(25) zeolite by silylation process could inhibit the alkylation of light aromatics and the isomerization of xylene. Consequently, light aromatics account for up to 75 % of aromatics, which is the highest value reported in CO2 hydrogenation, and p-xylene could make up as high as 72 % of xylene. Moreover, a larger density of BAS (>154 μmol g−1), which promote the formation of highly condensed, carbon-rich, and hard-to-oxidize coke, would accelerate the coke formation, degrade their physico-chemical properties, and shorten the catalyst lifetime.