Water-Mediated Electronic-State Modulation Enables Selective C–OCH3 Cleavage in Catalytic Hydrodeoxygenation

Abstract

Solvents can dictate reaction pathways in heterogeneous catalysis, but the molecular-scale mechanisms that enable their selective promotion of bond cleavage remain unclear. Here, we show that water acts as an electronic-state modulator on a Ni/γ-Al2O3 catalyst, switching guaiacol conversion from aromatic ring hydrogenation to demethoxylation. Under the aqueous phase (130 °C, 2 MPa H2), guaiacol conversion reaches 96.0% with 89.3% cyclohexanol selectivity, whereas in heptane, conversion drops to 45.3% and cyclohexanol selectivity is only 19.4%. Water significantly enhances cyclohexanol selectivity and accelerates the reaction rate more than tenfold. Combined kinetics, isotopic labeling, and DFT calculations reveal that water transforms chemisorbed hydrogen into electrophilic H3O+ (charge +0.98 |e|). This hydronium ion serves as a hydrogen shuttle that selectively attacks the electron-rich –OCH3 group, shifting the relative energy difference between C–O cleavage and phenyl hydrogenation from +0.42 eV (without water) to –0.49 eV, thereby inverting the energetic preference. Kinetic orders of H2 (water: 1.0; heptane: 0.5) and large kinetic isotope effects (1.6–4.9) confirm H3O+ as the active hydrogen donor in the rate-determining step. This water-mediated electronic modulation generalizes to other oxygenated aromatics, establishing H3O+ as a tunable electrophilic shuttle for selective hydrodeoxygenation.