Direct Formic Acid Production by CO2 Hydrogenation with Ir Complexes in HFIP under Supercritical Conditions

Development of hydrogen energy carriers is crucial for society. Reversible (de)hydrogenation using carbon-based materials, particularly formic acid (FA), has been widely studied. Typically produced under basic conditions through CO2 hydrogenation, formate salt is an energetically favorable form, but its dehydrogenation is challenging. This study found an equilibrium between formic acid dehydrogenation (FADH) and CO2 hydrogenation under high-pressure conditions, facilitated organic solvent suppression of dehydrogenation, and accelerated hydride formation on an Ir catalyst. These conditions allow for the direct production of FA from CO2 and H-2 in nonbasic 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) using a pentamethylcyclopentadienyl iridium (Cp*Ir) catalyst featuring a 4,4 '-diamino-2,2 '-bipyridine ligand (4DABP). Under mild conditions (50 degrees C, 1 MPa; CO2:H-2 ratio = 1:1), the catalyst achieved a turnover number (TON) of 2084 in 2 h. The use of supercritical CO2 further increased the TON to 6100, producing a 0.12 M FA solution after 96 h. This study presents a novel method for the direct production of formic acid from CO2 and H-2, indicating new possibilities in the development of hydrogen energy carriers.