Direct CO₂ Hydrogenation to Aromatics using ZnO/t‐ZrO₂ and Zeolite Bi‐Functional Catalysts

The potential of producing aromatics by CO₂ hydrogenation over a combination of a ZnO/t-ZrO₂ high temperature methanol synthesis catalyst and zeolites was explored. The simultaneous methanol synthesis and dehydration allowed the methanol synthesis equilibrium to be exceeded. High hydrogen partial pressures favoured the methanol synthesis equilibrium and thus CO₂ conversion but lead to a high hydrogenation activity that lowered the selectivity towards aromatics. To favour the aromatics selectivity, operating at relatively low temperatures and pressures was required, where the conversion was low. By using H-ZSM-5 and operating at low temperature and pressure (320 ºC and 10 bar) the hydrogenation activity was moderated resulting in an aromatics selectivity of 33 carbon mol%, at a CO₂ conversion to hydrocarbons of 4.5 %. An analysis of the hydrocarbon pool mechanism suggested that the achievable upper limit for aromatics selectivity was 45-52 carbon mol% for H-ZSM-5. To reach a higher carbon-based yield of aromatics, more methylation of the aromatic species or dehydrogenation of the olefins is necessary. The substitution of H-ZSM-5 with the larger pore zeolite H-ZSM-12 resulted in the formation of more highly methylated products, such as penta- and hexamethylbenzene, due to the larger pores of H-ZSM-12 enhancing the aromatic cycle.