Investigation of Hydrogenation of Formic Acid to Methanol using H2 or Formic Acid as a Hydrogen Source

Production of methanol (MeOH) from CO2 is strongly desired as a key chemical feedstock and a fuel. However, the conventional process requires elevated temperature and pressure, and high temperature restricts the productivity of MeOH due to equilibrium limitations between CO2 and MeOH. This paper describes the efficient hydro- genation/disproportionation of formic acid (FA) to MeOH by using iridium catalysts with electronically tuned ligands and by optimizing reaction conditions. An iridium complex bearing 5,5′-dimethyl-2,2′- bipyridine in FA hydrogenation achieved MeOH selectivity with H2 of up to 47.1% for FA hydrogenation under 4.5 MPa of H2 in the presence of H2SO4. The final concentration of MeOH of 3.9 M and a TON of 1314 were obtained in 12 M FA aqueous solution including 10 mol % of H2SO4 at 60 °C under 5.2 MPa of H2. Even under atmospheric pressure without introduction of external hydrogen gas, the FA disproportionation under deuterated conditions produced MeOH with 15.4% selectivity. Furthermore, the isotope effect and NMR studies revealed mechanistic insight into the catalytic hydrogenation of FA to MeOH.