Selective Conversion of CO2 to Trimethylbenzene and Ethene by Hydrogenation over a Bifunctional ZnCrOx/H-ZSM-5 Composite Catalyst

Selective conversion of CO2 into a specific hydrocarbon is highly desirable but rather challenging. Herein, a bifunctional ZnCrOx/H-ZSM-5 composite catalyst was designed, which can selectively convert CO2 into trimethylbenzene (TriMB) and ethene by hydrogenation. The selectivities to aromatics and light olefins reach 64.6 and 26.1%, respectively, at a CO2 conversion of 17.5%. In particular, TriMB accounts for 57.4% of the aromatic products, while ethene takes up 83.9% of all light olefins. Various characterization and DFT calculation results reveal that the CO2 conversion keeps to the tandem methanol-mediated reaction route, viz., CO2 is first hydrogenated to methanol-related intermediates over the ZnCrOx moiety and the H-ZSM-5 moiety then carries on the further conversion of emerged intermediates to hydrocarbons via the hydrocarbon pool mechanism. The 10-ring channels of the ZSM-5 framework combined with an appropriate quantity and distribution of acid sites conduce to the formation of TriMB and meanwhile inhibit the C9-aromatics from further methylation; in addition, compared with propene and butene, ethene survives well from further evolution due to the higher energy barriers for ethene methylation and dimerization. All of these endue the ZnCrOx/H-ZSM-5 composite catalyst with a high selectivity to TriMB and ethene in the CO2 hydrogenation. The insight shown in this work may give valuable clues for the design of efficient catalysts in the conversion of CO2 to a specific hydrocarbon product.