Abstract

Hydrogenation of CO2 to energy-rich products over heterogeneous metal catalysts has gained much attention due to their commercial applications. Specifically, the first-row transition metal catalysts are very rarely reported and discussed for the production of formic acid from the hydrogenation of CO2. Herein, hydrotalcite supported copper metal has shown activity and efficiency to produce formic acid from the hydrogenation of CO2, without adding any additional base or promoter and was effectively recycled 4 times after separating by simple filtration without compromising the formic acid yield. Hydrotalcite supported copper-based catalyst (Cu-HT) was synthesized through the coprecipitation method and used as a heterogeneous catalyst for the hydrogenation of CO2. The precise copper metal content determined by ICP in Cu-HT is 0.00944 mmol. The catalyst afforded maximum TOF, 124 h-1 under the employed reaction conditions: 100 mg catalyst, 60 °C, 60 bar total pressure of CO2/H2 (1:1, p/p) with 60 mL of mixed methanol:water (5:1, v/v) solvent. Cu-HT catalyst was synthesised and thoroughly characterized by FT-IR, PXRD, SEM, TEM, XPS and BET surface area. The first-order kinetic dependence with respect to the catalyst amount, partial pressures of CO2, and of H2 was observed and a plausible reaction mechanism is suggested.

Background: CO2 hydrogenation to energy-rich products over heterogeneous metal catalysts has gained much attention due to their commercial applications. Specifically, the first-row transition metal catalysts are very rarely reported and discussed for the production of formic acid from the hydrogenation of CO2.

Objective: The aim is to investigate the heterogeneous catalyst systems, using solid soft base hydrotalcite supported Cu metal-based catalyst for effective and selective hydrogenation of CO2 to formic acid.

Methods: The Cu –HT catalyst was synthesized and characterized by FT-IR, PXRD, SEM, TEM, XPS and BET surface area in which the precise copper content was 0.00944 mmol. The Cu-HT catalysed hydrogenation of CO2 was carried out in the autoclave.

Results: The Cu-HT catalyst afforded maximum TOF of 124 h-1 under the employed reaction conditions: 100 mg catalyst, 60 °C, 60 bar total pressure of CO2/H2 (1:1, p/p) with 60 mL of mixed methanol: water (5:1, v/v) solvent, without adding any additional base or promoter and was recycled 4 times by simple filtration without compromising the formic acid yield. Formation of formic acid was observed to depend on the amount of the catalyst, partial pressures of CO2 and H2, total pressure, temperature and time.

Conclusion: Cu-HT based heterogeneous catalyst was found to be efficient for selective hydrogenation of CO2 to formic acid and was effectively recycled four times after elegantly separating by simple filtration.

Keywords: Formic acid, carbon dioxide, hydrogenation, copper hydrotalcite, heterogeneous catalyst.

Graphical Abstract