Catalytic consequences of hydrogen addition events and solvent-adsorbate interactions during guaiacol-H2 reactions at the H2O-Ru(0 0 0 1) interface

Abstract

Catalytic reactions of biomass-derived phenolics and H2 occur on transition metal surfaces via competitive C–O cleavage and ring saturation pathways, with both requiring multiple hydrogen addition events before forming their respective rate limiting transition states. These events are markedly affected by solvent chemical identity, with polar protic solvents ionizing hydrogen adatoms (H*) to interfacial protons (H+) and opening up new catalytic routes. Here, we establish the reaction coordinate space for guaiacol-H2 reactions on Ru(0 0 0 1) using density functional theory and describe the atomic-scale effect of a polar protic solvent, H2O. Coupled H+ and H* attack leads to quasi-equilibrated enol and keto intermediates as the precursors for C–O cleavage and ring saturation, respectively. For C–O cleavage, H2O solvent enables a lower energy pathway via concomitant transfer of the hydroxyl H+ to the methoxy oxygen during C–OCH3 cleavage, forming a charge separated [Ru(s)–(C6H5O−)⋯(H+)⋯OCH3]† transition state and reducing the barrier by up to 0.8 eV as compared to unassisted C–OCH3 cleavage. For ring saturation, H* attack onto an unsaturated meta carbon is rate limiting with no direct solvent participation, suggesting that protic polar solvents selectively promote the C–O cleavage pathway. Taken together, we show that activating guaiacol for either C–O bond cleavage or ring saturation product formation depends on the reactive hydrogen identity (H* or H+), enol/keto isomerization equilibrium, and accessibility of the proton assisted Car–OCH3 cleavage transition state. All such factors are tunable via changes to the solvent or metal identity.