Harnessing Phase-Dependent Acidity and Redox Activity of Ru-Decorated CePO₄ Catalysts for Biofuel Production from Lignin Model Compounds and Lignin Bio-oil

Lignin valorization offers a sustainable route to biofuel production, addressing global energy demands and environmental concerns. In this study, Ru-decorated cerium phosphate (CePO4) catalysts with distinct crystalline phases (hexagonal, monoclinic, and mixed) were synthesized to investigate the influence of phase-dependent properties on catalytic transfer hydrogenation (CTH) of phenethyl phenyl ether (PPE), a lignin model compound, and catalytic hydrogenation of lignin bio-oil derived from softwood biomass into cycloalkanes. The hexagonal phase (CePO4–H) emerged as the most effective support, offering superior Lewis/Bronsted acidity, redox activity (Ce3+/Ce4+), and Ru nanoparticles dispersion. The Ru(2%)/CePO4–H offered 99% conversion under CTH conditions, selectively cleaving β-O-4 ether linkages in PPE to produce ethylbenzene and cyclohexanol as the sole products under mild conditions (150 °C, 6 h, 1 MPa N2). The catalyst afforded a near-quantitative yield of saturated cyclic hydrocarbons from PPE under catalytic hydrogenation conditions (150 °C, 6 h, 1 MPa H2). Furthermore, Ru(2%)/CePO4–H offered nearly complete conversion (99%) of lignin bio-oil into fully saturated cyclic C6–C9 products (suitable for aviation fuel applications) with up to 99% selectivity under catalytic hydrogenation conditions (220 °C, 3 MPa H2). Mechanistic insights revealed that the enhanced catalytic performance arises from phase-dependent surface acidity, electron-enriched Ru0 sites, and strong metal–support interactions, facilitating efficient C–O bond cleavage and hydrodeoxygenation/hydrogenation of PPE. The green chemistry metrics further substantiate the reduced environmental impact of this catalytic process. This work highlights the transformative potential of phase-engineered Ru(2%)/CePO4–H catalysts for lignin bio-oil upgrading. It provides a blueprint for designing robust catalysts with tunable properties for scalable, sustainable biofuel production.