A ruthenium-inserted hydrotalcite (Ru-HT) heterogeneous catalyst: kinetic studies on the selective hydrogenation of carbon dioxide to formic acid

Abstract

Kinetic studies have been carried out for the base-free hydrogenation of CO₂ to formic acid using a heterogeneous ruthenium-inserted hydrotalcite (Ru-HT) catalyst. An impressive turnover number (TON = 11 389) was achieved for formic acid under the optimized reaction conditions using a methanol : water mixture as the solvent (5 : 1, v/v) with 60 bar total pressure at 60 °C for 24 hours. The rates were determined directly in terms of the formation of formic acid with time. Kinetics were performed by carrying out experiments concerning the amount of catalyst, the individual partial pressures of CO₂ and H₂, the total pressure, temperature, agitation speed, reaction volume and v/v ratio of the mixed solvent. The rate of formic acid formation was first order on the amount of catalyst and partial pressures of CO₂ and H₂. The best reaction conditions achieved from the kinetic parametric optimization were: 100 mg catalyst, 30 bar pCO₂, 30 bar pH₂, 60 °C temperature, 800 rpm agitation speed and methanol–water (5 : 1, v/v) solvent. The computed activation parameters obtained from the temperature-dependent rate of formic acid formation were E_a = 34.5 ± 2.5 kJ mol⁻¹, ΔH^# = 32 ± 2.5 kJ mol⁻¹ and ΔS^# = −384 ± 5 J deg⁻¹ mol⁻¹. The presence of water in the mixed solvent effectively enhanced the reaction rate, which is characteristically observed due to its molecular effect. Two mechanistic routes for CO₂ hydrogenation to formic acid are proposed and discussed based on the kinetic and experimental observations. The studied parameters were found to be significantly effective to increase the rate of reaction appreciably.