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
H2 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-OCH3 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–1 to three organic intermediates and CO, our model could reproduce
the experimental CO and H2 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.