Acid-Assisted Hydrogenation of CO2 to Methanol in a Homogeneous Catalytic Cascade System

A catalytic cascade system for CO 2 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(CH 2 CH 2 PPh 2 ) 3 ]/Sc(OTf) 3 /Ir( tBu PCP)(CO) was found to be the most active for CO 2 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(CH 3 CH 2 PPh 2 ) 3 ](H)(CO)](OTf), Ir( tBu PCP)(H)(CO)(OTf), and [Ir( tBu PCP)(H)(CO) 2 ](OTf) were formed as the major metal-containing species. Notably, [Ru[P(CH 3 CH 2 PPh 2 ) 3 ](H)(CO)](OTf) was found to be inactive for CO 2 hydrogenation, limiting the productivity of the reaction.