Water Splitting Integrated with Self-Transfer Hydrogenolysis for Efficient Demethoxylation of Guaiacols to Phenols over the Ni/MgO Catalyst

This work demonstrates the upgrading of lignin-derived monomers through a cascade demethoxylation, aqueous-phase reforming reaction, eliminating the need for external hydrogen supply. The core of this research lies in the use of neat water as both reaction medium and the hydrogen donor over a multifunctional Ni/MgO catalyst, which is responsible for water splitting, aqueous-phase reforming of in situ generated methanol, and selective cleavage of the C–O bond, finally establishing an efficient one-pot approach achieving a high yield of phenols. Reaction mechanism studies proved that the initial H* source came from water by its splitting on the surface of the Ni/MgO catalyst, which triggered the fracture of the aromatic ether bond to afford phenols and CH3O*. The subsequent aqueous-phase reforming of CH3O* and OH* generated more hydrogen and further accelerated the hydrodeoxygenation (HDO) process. A high conversion of 87.8% with a selectivity of 88.9% for phenol could be achieved at 190 °C from guaiacol. Thanks to the interesting water-splitting mechanisms and strong metal–support interaction (SMSI), Ni/MgO exhibited significantly enhanced stability compared to the previously reported nanoporous Ni catalysts. Further, with real lignin as the substrate, 16.3 wt % combined yield of phenol and 4-methylphenol could be acquired under optimized conditions. Overall, this “H2-free” approach offers a promising alternative to conventional biorefinery processes, addressing the challenges of hydrogen sourcing and economic feasibility.