Selective and Direct Hydrogenation of CO2 for the Synthesis of Formic Acid over a Rhodium Hydrotalcite (Rh‐HT) Catalyst

Abstract

AbstractRhodium hydrotalcite (Rh‐HT), a heterogeneous catalyst was synthesized, characterized and investigated for CO2 hydrogenation. The catalyst was effectual for the CO2 hydrogenation and selective formation of formic acid at a moderate temperature and was efficiently recycled up to 5 times without any significant loss in activity. A TON of 15840 for formic acid was selectively obtained at 50 bar pressure (1:1, p/p, CO2 and H2) and 60 °C temperature in a mixture of methanol:water (5:1 v/v, 60 mL) as a solvent in 24 h with no additional base. The effect of various reaction parameters was investigated for the reaction rate and CO2 conversion. The formic acid formation rate follows the 1st order kinetic trend regarding the catalyst amount and partial pressure. The determined activation parameters from the temperature dependence rate of formic acid were Ea=33.5 ± 2.5 kJmol−1, ΔH#=30.9 ± 2.5 kJmol−1, ΔS#=‐275 ± 5 Jdeg−1mol−1. The presence of water as a solvent together with methanol have effectively enhanced the performance of the catalyst. The mechanistic routes for CO2 hydrogenation to formic acid are proposed and discussed by the kinetic and experimental observations.