Recently, the hydrogenation of CO₂ to methanol has been receiving increased attention.^[1] In this presentation, we describe a novel approach for hydrogenation of CO₂ to methanol performed in the gas-solid phase catalyzed by dinuclear iridium complexes at low temperature.^[2]

Recently, we reported homogenous hydrogenation of CO₂ to methanol using iridium complexes in water in the presence of sulfonic acid.^[3] However, the catalysis at 8 MPa and 70 °C provided mainly formic acid and methanol with only TON of 8.

The homogeneous CO₂ hydrogenation by piconlinamide based mononuclear catalyst (1) in water provided only formic acid at 60 °C and 4 MPa of H₂/CO₂ (3:1) (Fig. 1A). The dinuclear catalyst (2) in water gave small amount of MeOH along with production of formic acid. Although the dinuclear catalyst may produce methanol from CO₂, the productivity was unsatisfactory probably due to the equilibrium limitation.

Therefore, we tested an unconventional gas-solid phase reaction conducted in the absence of a solvent, because formic acid in water is hardly hydrogenated but easily forms H₂ and CO₂ by dehydrogenation. Unfortunately, the mononuclear catalyst (1) under gas-solid phase conditions did not provide any product. To our delight, the reaction by dinuclear catalyst (2) under gas-solid phase conditions led to the effective production of methanol without contamination with CO and CH₄ (Fig. 1B). The catalytic system produced methanol under milder reaction conditions (30 °C, 5 MPa (TON 2.0) and 0.5 MPa, 70 °C (TON 3.0)). In addition, a final turnover number of 113 was obtained upon reusing the catalyst at 60 °C and 4 MPa. The catalytic activities of the dinuclear catalyst were dependent on the relative configuration of each active species.

As a result, only the combination of a multinuclear catalyst and gas-solid phase reaction conditions exhibits remarkable performance for the CO₂ hydrogenation to methanol. It has been attributed to hydride complex formation upon exposure to H₂ gas, suppression of the liberation of formic acid under gas-solid phase reaction conditions, and intramolecular multiple hydride transfer to CO₂ by the multinuclear catalyst. These studies provide novel insights into the hydrogenation under gas-solid phase conditions and the effect of multinuclear active sites toward the hydrogenation of CO₂ catalyzed by complex catalyst.

References:

[1] Onishi N. et al. Coord. Chem. Rev. 2018, 373, 317. Chem Catal. 2022, 2, 242.

[2] Kanega R. et al. J. Am. Chem. Soc., 2021, 143, 1570.

[3] Sordakis K. et al. Chem. Eur. J. 2016, 22, 15605.