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 H₂ 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 °C, 1 MPa; CO₂:H₂ 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 H₂, indicating new possibilities in the development of hydrogen energy carriers.