The thermodynamics analysis and experimental validation for complicated systems in CO₂ 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.