The thermodynamics analysis and experimental validation for complicated systems in CO 2 hydrogenation process

Catalytic conversion of CO2 into chemicals and fuels is an alternative to alleviate climate change and ocean acidification. The catalytic reduction of CO2 by H2 can lead to the formation of various products: carbon monoxide, carboxylic acids, aldehydes, alcohols and hydrocarbons. In this paper, a comprehensive thermodynamics analysis of CO2 hydrogenation is conducted using the Gibbs free energy minimization method. The results show that CO2 reduction to CO needs a high temperature and H2/CO2 ratio to achieve a high CO2conversion. However, synthesis of methanol from CO2 needs a relatively high pressure and low temperature to minimize the reverse water–gas shift reaction. Direct CO2 hydrogenation to formic acid or formaldehyde is thermodynamically limited. On the contrary, production of CH4 from CO2 hydrogenation is the thermodynamically easiest reaction with nearly 100% CH4 yield at moderate conditions. In addition, complex reactions with more than one product are also calculated in this work. Among the considered carboxylic acids (HCOOH, CH3COOH and C2H5COOH), propionic acid dominates in the product stream (selectivity above 90%). The same trend can also be found in the hydrogenation of CO2 to aldehydes and alcohols with the major product of propionaldehyde and butanol, respectively. In the process of CO2 hydrogenation to alkenes, low temperature, high pressure, and high H2 partial pressure favor the CO2 conversion. C4H6 is the most thermodynamically favorable among all considered alkynes under different temperatures and pressures. The thermodynamic calculations are validated with experimental results, suggesting that the Gibbs free energy minimization method is effective for thermodynamically understanding the reaction network involved in the CO2 hydrogenation process, which is helpful for the development of high-performance catalysts.