Conversion of the CO and CO₂ mixture to alcohols and hydrocarbons by hydrogenation under the influence of the water-gas shift reaction, a thermodynamic consideration

Due to the intervention from the water-gas shift (WGS) reaction (or the reverse one (RWGS)), the hydrogenation of CO (or CO₂) into alcohols and hydrocarbons often displays rather high selectivity to CO₂ (or CO), which makes it rather puzzling to evaluate such conversion processes by using the relatively low selectivity to the target products. Herein, a thermodynamic consideration is made to elaborately evaluate the effect of the WGS/RWGS reaction on the hydrogenation of CO, CO₂, and their mixture to typical alcohols (e.g. methanol) and hydrocarbons (e.g. ethene). The results indicate that for the hydrogenation of CO (or CO₂), although the WGS (or RWGS) reaction, acting as a communicating vessel connecting CO and CO₂, may have a severe influence on the equilibrium conversion of CO (or CO₂), forming a large amount of CO₂ (or CO), it only has a relatively minor impact on the C-based equilibrium yield of the target alcohol/hydrocarbon product. The hydrogenation of CO shows a higher C-based equilibrium yield for the target product than the hydrogenation of CO₂, while the overall C-based equilibrium yield of target product for the hydrogenation of the CO and CO₂ mixture just lies in between. For the hydrogenation of the CO and CO₂ mixture, although the equilibrium conversion of CO and CO₂ may vary greatly with the change in the feed composition, the relation between the overall C-based equilibrium yield of the target product and the feed composition is rather simple; that is, the overall C-based equilibrium yield of alcohol/hydrocarbon product decreases almost lineally with the increase of the CO₂/(CO + CO₂) molar ratio in the feed. These results strongly suggest that the mixture of CO and CO₂ is credible in practice for the production of alcohols and hydrocarbons through hydrogenation, where the overall C-based yield should be used as the major index for the hydrogenation of CO, CO₂, and their mixture.