Direct synthesis of liquid hydrocarbons from CO2 and H2 over bifunctional ZnCrOx-НZSM-5 combined and composite catalysts in a single reactor

Abstract

A one-stage synthesis of gasoline fraction from CO₂ and H₂ was carried out on Zn–Cr–oxide-zeolite combined and composite catalysts. It was found that after oxidative and reduction treatment the catalysts contain ZnO and ZnCr₂O₄ phases. The one-step coprecipitation of zinc and chromium salts in the presence of zeolite allows obtaining an active bifunctional composite catalyst, each particle of which contains Zn–Cr oxide and zeolite components. A synergistic catalysis due to a closer distance of metal and acid sites increased the efficiency of such a catalyst in CO₂ hydrogenation. It was found that the optimal temperature of oxidative activation of the composite catalyst is 400 °C. With an increase in the temperature of oxidative treatment of the composite catalyst from 400 to 500 °C, part of the zinc leaves the structure of the non-stoichiometric spinel and leads to a decrease in the basicity of the catalyst, which is responsible for the activation of CO₂ on surface oxygen vacancies. Agglomerated ZnO particles blocked the pores and reduced their volume, as well as the specific surface of the catalyst. At the same time, the total acidity of the catalyst decreased. ZnO-species, which have hydrogenating activity, under the reaction conditions removed alkenes from the reaction cycle and thereby reduced the C₅₊–selectivity of the composite catalyst. Among the studied catalysts, the ZnCrO_x-НZSM-5/Al₂O₃(400) composite catalyst in the tail-gas recirculation mode at 380 °C and 10 MPa demonstrated high CO₂ conversion of 25% and high selectivity for C₅₊–HCs of 53 wt% with a high content of isoalkanes 71 wt% for 60 h. The methane content in the tail-gas did not exceed 3 vol%. The results obtained will be used to develop approaches to creating effective catalysts for one-stage CO₂ hydrogenation into valuable petrochemical products.