CO2 Hydrogenation to Methanol Using Dinuclear Iridium Catalysts under Gas-Solid Phase Conditions

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.



[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.


Yuichiro Himeda

Prime Senior Researcher at the National Institute of Advanced Industrial Science and Technology(AIST)