How Solvent Mediates the Catalytic Fate of 2-Methoxyphenol in Hydrodeoxygenation Catalysis?

Kinetic and isotopic assessments combined with theoretical calculations shed light on the mechanistic differences in terms of the number of hydrogen addition steps and the electronic charges of reactive hydrogen in the catalytic sequence leading to the Ar-OCH₃ scission of 2-methoxyphenol occurring at various interfaces of vapor-Ru, cyclohexane-Ru, and water-Ru. At vapor-Ru and cyclohexane-Ru interfaces, the initial hydrogen adatom (H*) additions on the phenyl ring disrupt its aromaticity, as a step required to occur before kinetically relevant Ar-OCH₃ scission on an ensemble of Ru sites. The number of H* addition events prerequisite to the formation of the kinetically relevant Ar-OCH₃ scission transition state, however, is larger at the cyclohexane-Ru interface than those at the vapor-Ru and water-Ru interfaces. The preferential solvation of 2-methoxyphenol derived reaction precursors and H* weakens their adsorption on Ru sites, which leads to a decrease in the activation barrier for the third H* addition step and in turn a larger number of H* additions. At the water-Ru interface, water as a polar protic solvent promotes the Ar-OCH₃ scission by enabling a mechanistically distinct catalytic route─an adjacent water molecule from the solvent layer assists with the intramolecular proton shuttling from the hydroxyl group (−OH) to the vicinal methoxy group (−OCH₃) of 2-methoxyphenol, evolving a highly charged transition state that is further stabilized by the water solvent layer, thereby leading to Ar-OCH₃ scission rate constants that are much higher than those at the vapor-Ru interface and, when comparing at the same H₂ chemical potential, higher Ar-OCH₃ scission turnovers. These distinct mechanistic details of the Ar-OCH₃ scission at various reaction interfaces, inferred from experiment and theory, illustrate how a solvent either solvates reaction intermediates and transition states or directly participates in the formation of kinetically relevant transition states, leading to changes in the free energy landscape, thus rewriting the catalytic fates in hydrodeoxygenation reactions.