Acid-Assisted Hydrogenation of CO<sub>2</sub> to Methanol in a Homogeneous Catalytic Cascade System
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A catalytic cascade system for CO2 hydrogenation to MeOH under acidic conditions is described. The reaction uses three catalysts which promote stepwise formation and conversion of formic acid and formate ester intermediates. The activities and decomposition pathways of different catalyst candidates for each step were investigated. The combination Ru(H)2[P(CH2CH2PPh2)3]/Sc(OTf)3/Ir(tBuPCP)(CO) was found to be the most active for CO2 hydrogenation to MeOH. An overall TON of 428 was achieved after 40 h at 155 °C in EtOH, resulting in a high concentration of MeOH (1.07 M). Catalyst speciation studies upon completion of the reaction indicated that the carbonyl complexes [Ru[P(CH3CH2PPh2)3](H)(CO)](OTf), Ir(tBuPCP)(H)(CO)(OTf), and [Ir(tBuPCP)(H)(CO)2](OTf) were formed as the major metal-containing species. Notably, [Ru[P(CH3CH2PPh2)3](H)(CO)](OTf) was found to be inactive for CO2 hydrogenation, limiting the productivity of the reaction.
