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

A ZnO/Al2O3 catalyst synthesized by impregnating gamma-alumina from a zinc nitrate solution and calcination at 400 degrees C is studied in the hydrogenation of carbon oxides. During heating in a stream of a 2.4% H-2/N-2 mixture to 400 degrees C, ZnO undergoes partial reduction. The activity of the reduced catalyst is studied in a range of 300-400 degrees C at 5 MPa and a space velocity of 6000 NL kg(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 degrees C, the selectivity for oxygenates (methanol and dimethyl ether, in terms of methanol) decreases from similar to 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 degrees C, the catalyst exhibits an insignificant activity in the case of using H-2/CO and almost no activity in the case of H-2/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 degrees 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.