Kinetic Investigation on Selective CO₂ Hydrogenation to Formic Acid over Rhodium Hydrotalcite (Rh-HT) Catalyst

Abstract—

A heterogeneous catalyst, rhodium hydrotalcite (Rh-HT), was synthesized, characterized, and explored for kinetic studies for the hydrogenation of CO₂ to formic acid using molecular hydrogen in an autoclave. The catalyst was efficient for the selective formation of formic acid at a moderate temperature with up to 5 catalytic cycles without any significant loss in catalytic activity. The detailed kinetic investigations were performed by determining the rate of formic acid formation as a function of time, catalyst amount, total pressure, partial pressure of CO₂, partial pressure of H₂, reaction volume, v/v ratio of the mixed solvent of methanol and water, agitation speed, and temperature in a wide range of variation. The rates were found to be dependent on all these parameters. The formic acid formation rate followed the first-order kinetics with respect to the partial pressures of CO₂ and H₂, and catalyst amount. The best reaction conditions obtained from the kinetic parametric optimisation were, 50 bar total pressure (1/1 p/p, CO₂ and H₂); 60°C temperature; a mixture of methanol:water solvent (5/1 v/v, 60 mL); 24 h time; and 500 rpm agitation speed to get a TON of 15840 for formic acid with no additional base. The thermodynamic performance of the heterogeneous catalyst Rh-HT was appreciably associated with highly negative entropy. The performance of the catalyst was effectively enhanced by the mixture of water and methanol as a solvent. The mechanistic routes for CO₂ hydrogenation to formic acid are proposed and discussed based on the kinetic and experimental observations involving the role of the molecular effect of water used in the solvent.