110th Anniversary: Ionic Liquid Promoted CO2 Hydrogenation to Free Formic Acid over Pd/C

Direct hydrogenation of CO2 into formic acid is an atom-economic reaction, but it is thermodynamically limited, and the production of free formic acid is challenging. Herein, we report a protocol of CO2 hydrogenation to free formic acid over Pd/C in ionic liquid (e.g., 1-butyl-3-methylimidazolium acetate, [Bmim][OAc]) without base under mild conditions. Detailed study indicates that [Bmim][OAc] plays multiple roles in the reaction process with activating CO2, modifying the Pd nanoparticles to improve their activities, and stabilizing generated formic acid. This Pd/C-[Bmim][OAc] catalytic system is very effective for the production of free formic acid at 40 °C, affording a production rate of 233.5 mmol·g–1.h–1 and turnover number (TON) of 594, and the concentration of free formic acid in the reaction solution could reach up to 5.20 mmol/L.