Combined Computational and Experimental Investigation on the Mechanism of CO2 Hydrogenation to Methanol with Mn-PNP-Pincer Catalysts

Abstract

A theoretical and experimental mechanistic study is presented for the homogeneously catalyzed CO2 hydrogenation to methanol, using an Mn-PNP-Pincer catalyst in the presence of a Lewis acid cocatalyst and alcohol as a solvent. Quantum chemical computations at the density functional theory and DLPNO-CCSD(T) level of theory suggest the presence of a formate resting state as the most stable intermediate. The concerted activation of dihydrogen via a proton shuttle mechanism and decomposition of a hemiacetal intermediate is computed to define the turnover-determining transition state. The resulting energy span is calculated as 34.5 kcal mol–1 at the DLPNO-CCSD(T) level of theory. An Eyring plot reveals the experimental barrier of the reaction at 31.4 kcal mol–1 under catalytic turnover conditions, showing a good agreement with a slight overestimation of the computational model. Concentration–time profiles of the involved species also locate experimentally the rate-determining states (RDSs) of the reaction in the hydrogenation of a formate ester intermediate to methanol. The measured kinetic isotope effects for the use of H2/D2 and EtOH/D are in agreement with hydrogen splitting as the RDS, giving further support to the computed mechanism. These insights provide guidance and reference for future improvement of hydrogenation catalysts based on abundant 3d metals for the CO2-based production of methanol.