(569bu) CO2 Hydrogenation to Formic Acid (FA) in an Aqueous Medium: Catalyst Development, Reaction Parameter Optimization, and Reaction Mechanism

Carbon capture, sequestration, and conversion into valuables are global challenges to achieving net zero. The conversion of CO₂ to valuable chemicals is one innovative way to control CO₂ concentration. CO₂ hydrogenation to FA recently emerged as a promising route for CO₂ utilization. FA is an important chemical used in the leather, pharmaceutical, rubber, painting, and dye industries. The compound annual growth rate (CAGR) of FA is ~4%. CO₂ and H₂ can also be stored in the form of FA. Previously, few studies have discussed CO₂ hydrogenation to FA in the presence of noble metal complex catalysts in an ionic liquid. However, a big issue is the catalyst cost, ionic liquid availability, and product separation. Therefore, developing a heterogeneous catalytic process for in an aqueous medium is more attractive. In addition, quantifying product formation, CO₂ conversion, and the fundamental understanding of reaction mechanisms is crucial.

In this study, 10 wt.% Ni/Al₂O₃ and 10wt.%Ni/AC (activated carbon) catalysts were synthesized by impregnation method, and their physicochemical properties were characterized by various techniques such as BET, XRD, TGA, TPR, SEM, and TEM, etc. The CO₂ hydrogenation reaction was performed in aqueous medium in an autoclave reactor. The effect of important reaction parameters such as temperature (50-300^oC), CO₂ to H₂ pressure ratio (0.2-2.3), and catalyst concentration (0.2-1 g/10 ml solution) were optimized by using the response surface methodology (RSM). Results demonstrated that both the catalysts exhibited very promising performance in the hydrogenation of CO₂ and obtained a very high yield (2011.92-2143.7 μmol g^-1 h^-1) of FA at the optimum reaction temperature (200^oC), and a total pressure of 60 bar. Comparatively, the yield was higher for 10 wt.% Ni/Al₂O₃ catalyst. The surface area, catalyst morphology, active sites, and interaction of CO₂ and hydrogen with the catalyst played a crucial role in the yield of FA.