A Hydrogen-Free Ni/MgO Catalyst System for Aqueous-Phase Hydrodeoxygenation of 4-Propylguaiacol

This study focuses on the catalytic transformation of a lignin model compound, 4-propylguaiacol, without external hydrogen. The reaction using a Ni/MgO catalyst and water as the solvent, conducted at 220°C under 5 bar N 2 for 2 h, achieved 97.8% conversion and 94.8% selectivity toward phenolic products. During the reaction, the methoxy group in 4-propylguaiacol is cleaved from the aromatic ring at Ni active sites and subsequently reacts with hydrogen radicals generated from water dissociation to form methanol. The methanol then undergoes aqueous-phase reforming, providing additional in-situ hydrogen to drive the hydrodeoxygenation process. The Ni/MgO catalyst showed good recyclability over three consecutive cycles, with only a slight decline in activity. Furthermore, this catalyst was applied to the depolymerization of real lignin, and the effects of reaction parameters on product yield were investigated. Among the tested conditions, a maximum monomer yield of 76.7 wt% was obtained at 280°C under 5 bar N 2 for 6 h. Graphical This work presents a sustainable strategy for producing phenolic monomers from lignin-derived compounds using non-noble metal catalysts without external hydrogen.