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 model for H2 and CO 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 pincer complex has been studied using DFT and CCSD(T) calculations with methanol and toluene as the solvent. In the proposed mechanism, the CO is directly released from the methyl formate byproduct, forming a Ru-OCH3 intermediate. This reaction is preferred in toluene compared to methanol due to the lower polarity of the organic products and the lower stability of the Ru-alkoxy intermediates. This mechanism differs from previous proposals going through a Ru-CO2CH3 intermediate. The computed Gibbs free energy barriers for the different mechanisms were compared to experimental data using a microkinetic model coupled to a liquid-vapor batch reactor model designed from reported experimental setups. After refining the organic reaction thermodynamics consistent with CCSD(T)/cc-pVTZ method corrections, only our proposed mechanism shows a good agreement with the experimental H2 and CO formation. Our findings herein demonstrate the usefulness of microkinetic modeling to support reaction mechanisms by direct comparison of computational and experimental data. In addition, the proposed mechanism rationalises the decarbonylation reaction in a way that can be easily extended to other carbonyl substrates and acid-base catalysts.