(97c) Tuning the Reactivity of Hydrogen Species at Solvent-Metal Interfaces for the Activation of Strong Polar Bonds: Hydrodeoxygenation of Phenolics

Production of hydrocarbon fuels and value-added chemicals from lignin derived phenolic monomers requires the selective removal of oxygen rather than the undesired ring saturation during hydrodeoxygenation catalysis. For catalysis at solvent-metal interfaces, solvent molecules play critical catalytic roles, including stabilizing/destabilizing intermediates and transition states as well as directly participating in catalytic turnovers. Despite their obvious manifestation, there is a lack of atomistic descriptions on these interfacial properties and the resulting mechanistic consequences. Here, we rationalize kinetic isotopic experiments with density functional theory calculations to probe the effects of solvent-metal interfaces on the catalytic transformations for phenol and guaiacol over a Ru catalyst. At the water-Ru interface, hydrogen adatoms (H*) can be ionized to interfacial protons (H⁺). To elucidate the periodic trends and co-adsorbate effects on Brønsted acidity of H* at water-metal interfaces, we constructed a cubic Born-Haber thermochemical construct, which showed that the H* acidity is determined by a combination of the metal work function and H* binding energy. Over a Ru catalyst, we show that the presence of both negatively and positively charged hydrogen species at the water-Ru interface consequently accelerates C‒O bond activation in phenolic monomers relative to non-polar interfaces (i.e. cyclohexane-Ru and vapor-Ru) by (1) promoting the partial hydrogenation of the phenolic monomer’s aromatic ring and (2) altering the most abundant reactive intermediate. Overall, these fundamental, atomistic-scale solvent effects can be leveraged to improve catalytic hydrodeoxygenation performance by tuning the relative stability of H⁺ vs. H* species at the solvent-transition metal interface through active site and solvent environment selection. As such, by combining a theoretical explicit solvent model and kinetic isotopic measurements, we demonstrate that the atomistic-scale movement of protons and electrons at the solvent-metal interface improves the catalytic hydrodeoxygenation performance as well as the activation of polar bonds (e.g. carbonyl conversion to alcohols).