Theoretical study of the mechanism for CO₂ hydrogenation to methanol catalyzed by trans-RuH₂(CO)(dpa)

In this work, the reaction mechanism for the conversion of CO₂ and H₂ to methanol has been researched by density functional theory (DFT). The production of methanol from CO₂ and H₂ is catalyzed by a univocal bifunctional pincer-type complex trans-RuH₂(CO)(dpa) (dpa = bis-(2-diphenylphosphinoethyl)amine). The reaction mechanism includes three continuous catalytic processes: (1) CO₂ is converted to formic acid; (2) formic acid is converted to formaldehyde and water; (3) formaldehyde is converted to methanol. By computing the catalytic processes, we have shown that the rate-limiting step in the whole process is the direct cleavage of H₂. The calculated largest free energy barrier is 21.6 kcal/mol. However, with the help of water, the free energy barrier can be lowered to 12.7 kcal/mol, which suggests viability of trans-RuH₂(CO)(dpa) as a catalyst for the direct conversion of CO₂ and H₂ to methanol.