The Role of Ionic Liquids in Direct Synthesis of Formic Acid from CO2 Hydrogenation on Ru Complexes: A Theoretical Study

Due to high thermodynamic stability, the direct generation of formic acid by CO2 hydrogenation is not easy to achieve experimentally. However, when Nakahara and coworkers studied the equilibrium of formic acid reversibly decomposing into CO2 and H2, they found that using imidazolium formate ionic liquid as an additive could shift the reaction equilibrium to the formic acid side. Subsequently, imidazolium acetate ionic liquid and imidazolium bicarbonate ionic liquid have also been experimentally proven to be able to be used for CO2 hydrogenation to directly produce formic acid. In order to investigate the mechanism of action of ionic liquids in the process of CO2 catalyzed hydrogenation to formic acid, we performed DFT calculations. The results showed that, after the hydrogenation of CO2 to formic acid, the ionic liquids and formic acid molecules form adducts through hydrogen bonding, and then stabilize the product formic acid. The further use of methyl to replace H at the position of the cation R3 of the ionic liquids can improve the ability of the ionic liquids to stabilize formic acid, which also supports the experimental work of Nakahara and coworkers. In addition, among the three ionic liquids, the imidazolium acetate ionic liquid had the best stabilizing effect on formic acid, and the second best is the imidazolium formate ionic liquid, while the imidazolium bicarbonate ionic liquid has a relatively weak stabilizing ability.