Outer-Sphere CO Release Mechanism in the Methanol-to-Syngas Reaction Catalyzed by a Ru-PNP Pincer Complex.

Methanol can be used as a surrogate molecule for CO and H in the synthesis of a large variety of chemicals. In this work, the mechanism for the methanol-to-syngas reaction catalyzed by a Ru-PNP complex was studied using density functional theory. In the proposed mechanism, the CO is directly released from the methyl formate intermediate, forming a Ru-OCH species. The preference for this pathway compared to others proposed in literature was supported by a microkinetic model constructed from the computed Gibbs free energies and coupled to a liquid-vapor batch reactor describing the gas phase composition. After including energy corrections of ≤6 kcal mol to three organic intermediates and CO, our model could reproduce the experimental CO and H turnover numbers over the time previously reported. Further, this model was used to evaluate the influence of solvent polarity and methanol concentration on the formation of products and catalyst resting states. These results suggest that in methanol, CO formation is limited by the organic reaction thermodynamics, whereas in toluene, it is limited by Ru-CO formation. Overall, this work shows the potential of microkinetic models to benchmark reaction mechanisms and computational methods and provide the relevant information required for catalyst design.