Preferential Synthesis of Toluene and Xylene from CO2 Hydrogenation in the Presence of Benzene through an Enhanced Coupling Reaction

Abstract

The conversion of CO2 into high-value-added chemicals, especially aromatics, is appealing for the alleviation of carbon emissions. However, preferential production of valuable aromatics including benzene, toluene, and xylene (BTX) from CO2 is still a great challenge through direct hydrogenation routes due to the complicated side reactions. Herein, we apply a composite catalyst containing metal oxides (ZnZrOx) and Zn-modified zeolites (Zn/ZSM-5) to selectively synthesize toluene and xylene through CO2 hydrogenation in the presence of benzene. A total selectivity of toluene plus xylene as high as 92.8% is achieved with C1–5 alkane selectivity of less than 4%, as the CO2 conversion reaches 21% and the benzene conversion reaches 22.2%. We demonstrate that the appropriate acid properties of zeolites play a pivotal role in the coupling reaction, which can be readily adjusted by the introduction of Zn. The increase of moderate Zn-induced Lewis acid sites with consumption of strong Brønsted acid sites not only promotes the formation of intermediate alkylation species but also facilitates the recovery of occupied active sites by regulating the successive transformation of formate intermediates and desorption of aromatic products. The enhanced coupling effect results in the elevation of both reactant conversion and specific aromatics products’ (toluene and xylene) selectivity. This work may provide an efficient strategy for the synthesis of high-value aromatics through coupling reactions from CO2 hydrogenation.