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

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.