The methanol production by hydrogenation of CO2 has been receiving increased attention.[1] However, significant obstacles, such as low CO2 conversion and selectivity, still remain for heterogeneous catalysts due to the equilibrium limitations under the harsh operating temperatures. Therefore, a great deal of research effort has been made to develop alternative routes that circumvent the inherent barriers of the methanol production from CO2. Herein, we describe catalytic hydrogenation of CO2 to methanol using multinuclear iridium complexes in a gas–solid phase reaction under mild reaction conditions.[2] In our previous study, we have reported that CO2 hydrogenation in acidic aqueous solution provided mainly to formic acid (FA) along with a small amount of methanol even under high pressure conditions.[3] We concluded that the equilibrium limitation between FA and CO2/H2 in water suppresses further hydrogenation of FA to methanol. In order to circumvent the equilibrium limitation, we envisioned novel approach for methanol production catalyzed by dinuclear iridium complex under gas-solid phase conditions. To our delight, when the reaction was performed using dinuclear iridium complex, methanol was produced with high selectivity under mild reaction conditions (Table). On the other hand, using mononuclear analog produced a negligible amount of methanol. It is worth noting that methanol was generated catalytically, even at low pressure and low temperature(30 °C, 5 MPa (TON 2.0) and 0.5 MPa, 70 °C (TON 3.0)). A final turnover number of 113 was obtained on recycling of the catalyst at 60 °C and 4 MPa of H2/CO2 (3:1). The reactivity under mild conditions is due to (i) facile formation of a hydride complex upon exposure to H2, (ii) circumvention of the liberation of FA from the formato complex using unconventional gas-solid phase reaction conditions, and (iii) multiple intramolecular hydride transfer to CO2 by the multinuclear catalysts. This catalytic system opens the door to new possibilities for hydrogenation of CO2 to methanol under mild reaction conditions and heterogenous catalysis of molecular catalyst.

Methanol production by CO2 hydrogenation using dinuclear iridium catalysts under gas-solid phase conditions

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