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

Selective conversion of CO₂ into a specific hydrocarbon is highly desirable but rather challenging. Herein, a bifunctional ZnCrO_x/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 ZnCrO_x 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 ZnCrO_x/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.