Kinetic and Mechanistic Investigation on Catalytic CO ₂ Hydrogenation to Formic Acid Over Copper Hydrotalcite (Cu‐HT) Catalyst

ABSTRACT Kinetic modeling for the plausible mechanism of hydrogenation of CO 2 to formic acid over copper hydrotalcite (Cu‐HT) catalyst was done, and derived rate equations were compared with the experimental rates. Mechanistic pathways were suggested and examined based on experimental facts and kinetics regarding the role of water. The best reaction condition for the kinetic parametric optimization was 62 bar pressure (1:1, pCO 2 /pH 2 ), 60°C temperature, water–methanol mixture as a solvent (1:5, 60 mL, v/v), and stirring speed of 500 rpm in 24 h, giving a TON of 2.97 × 10 3 with no additional base. The formic acid formation rate was determined by varying different values of catalyst amount, time, temperature, partial pressure of CO 2 and H 2 , total pressure, mechanical pressure, mixed water–methanol solvent ratio (v/v), reaction volume, and stirring speed. The rate was found to be independent of stirring speed above 500 rpm and hence, the kinetic experiments were performed at 800 rpm to minimize mass‐transfer effects. Catalytic performance improved in mixed water–methanol solvent systems. The rate of formic acid production followed the first order kinetics with regard to catalyst amount and pCO 2 /pH 2 partial pressure. The thermodynamic performance of the catalyst was found to be associated with E a = 35.7 ± 2.5 kJ mol −1 , ΔH ‡ = 33.2 ± 2.5 kJ mol −1 , and ΔS ‡ = − 278 ± 5 J mol −1 K −1 (highly negative entropy). Four catalytic cycles were observed with no loss of catalytic activity.