Selective hydrogenation of CO2 to formic acid with higher yield in an aqueous medium with a nano-nickel-metal catalyst: reaction parameter optimization by response surface methodology (RSM)†

A highly active mesoporous nano-nickel catalyst was synthesized by the sol–gel method for the selective conversion of CO₂ to formic acid (FA) in an aqueous medium. In this study, CO₂ hydrogenation reactions were performed in a high-pressure autoclave, and the experimental conditions were set by using the response surface methodology (RSM). The RSM analysis was done using a three-factor, one-response, and five-level central composite design (CCD) integrated with the desirability approach. Experiments revealed that under the optimized reaction conditions (200 °C, 60 bar), the obtained formic acid yield was significantly high (2245 μmol g⁻¹ h⁻¹) with 100% catalyst selectivity. The obtained turnover number (TON) was ∼285, significantly higher in an aqueous medium and the presence of a non-noble nickel nano-metal catalyst. Mesoporous nano nickel particles (15–26 nm) facilitated the selective adsorption and splitting of hydrogen molecules to hydrogen radicals, which further reacted with the carbonate ions present in the reaction medium. Na₂CO₃ acted as a promoter, which enhanced the CO₂ adsorption and the formic acid yield. The catalyst recyclability was confirmed by performing the experiments five times and a constant yield of formic acid was found.