Features of Carbon Dioxide and Monoxide Hydrogenation in the Presence of ZnO/Al₂O₃ and ZnO

Abstract

A ZnO/Al₂O₃ catalyst synthesized by impregnating gamma-alumina from a zinc nitrate solution and calcination at 400°C is studied in the hydrogenation of carbon oxides. During heating in a stream of a 2.4% H₂/N₂ mixture to 400°С, ZnO undergoes partial reduction. The activity of the reduced catalyst is studied in a range of 300–400°С at 5 MPa and a space velocity of 6000 NL ${\text{kg}}_{{{\text{cat}}}}^{{ - 1}}$ h⁻¹. The main product of CO hydrogenation is methanol. In addition, the methanol dehydration and CO methanation reactions occur. Water formed during methanol dehydration provides the formation of CO₂ via the CO steam reforming reaction. With an increase in temperature from 300 to 400°C, the selectivity for oxygenates (methanol and dimethyl ether, in terms of methanol) decreases from ~74 to 56%, while the selectivity for hydrocarbons (methane, ethane, ethylene, propane) increases from 1 to 14%. The main products of CO₂ hydrogenation are CO and H₂O. The formation of oxygenates and a small amount of methane, in addition to CO, is observed. Water formed in a significant amount during CO₂ hydrogenation adversely affects the dehydration of methanol. In methanol synthesis at 240°C, the catalyst exhibits an insignificant activity in the case of using H₂/CO and almost no activity in the case of H₂/CO₂. Data on CO and CO₂ hydrogenation in the presence of ZnO/Al₂O₃ are consistent with the results for precipitated ZnO. In addition, at a pressure of 3 or 5 MPa and a temperature of 344 or 364°C, the content of oxygenates in the case of CO hydrogenation is 4–5 times higher than that in the case of CO₂ hydrogenation. Analysis of the dependence of the relative selectivity for oxygenates on the contact time leads to the conclusion that, in the presence of zinc oxide, methanol is formed from both CO and CO₂.