Synergistic Redox-Acid Functionality in Mixed-Metal Polyoxometalate–TiO ₂ Catalysts for Fuel Additive Production

Hydrodeoxygenation (HDO) has emerged as a vital strategy for the synthesis of value-added chemicals, particularly in the valorization of lignin-derived compounds. Such processes require catalysts containing both acidic and redox-active sites. Polyoxomolybdates (POMos), possessing sizable Brønsted acidic sites, have shown great promise as catalysts toward such processes. Incorporation of additional redox-active sites in such compounds can further enhance their catalytic activity by generating a greater number of sites for substrate activation. In this study, we explore the catalytic activity of a series of three isostructural 3d-4d mixed-metal polyoxomolybdates [(Mo VI 2 O 5 ) 2 (MO 2 ){O 3 P–C(O)(CH 2 -4–C 5 –NH 4 )–PO 3 } 2 ] −7 [M = V(III), Cr(III) and Mn(III)], in catalytic transfer hydrodeoxygenation (CTHDO) reactions. These complexes were immobilized on anatase TiO 2 to form bifunctional catalysts that combine tunable redox and acidic properties, along with high structural stability. The POMo@TiO 2 composites were designed and synthesized to investigate their efficiency in the CTHDO of lignin-derived vanillin (4-hydroxy-3-methoxybenzaldehyde) to 2-methoxy-4-methylphenol (MMP or creosol), under hydrogen-free conditions and using isopropanol (IPA) as both the solvent and the in situ hydrogen donor. Comprehensive physicochemical characterizations, including powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy–energy dispersive X-ray spectroscopy, high resolution transmission electron microscopy, and Brunauer–Emmett–Teller and Barrett–Joyner–Halenda (BET-BJH) studies, confirmed uniform dispersion, enhanced surface area, and strong POMo–TiO 2 interfacial interactions. Among the series, the vanado-molybdate composite exhibited superior catalytic performance, achieving the highest vanillin conversion of 94.9% and MMP selectivity of 99.6%, at an optimal temperature of 180 °C after 10 h of reaction time. Kinetic studies followed pseudo -first-order behavior, highlighting the balance between redox-driven hydrogen transfer and acid-catalyzed C–O bond cleavage. All of the catalysts showed excellent recyclability and structural integrity over multiple runs, as confirmed by Raman spectroscopic studies, demonstrating their robustness under liquid-phase reaction conditions. The remarkable stability and catalytic activity emphasize their viability as sustainable, non-noble-metal catalysts for hydrodeoxygenating lignin-derived bio-oil model compounds, fostering progress in green catalytic technologies. This study elucidates the structure–activity relationships governing polyoxometalate-metal oxide (TiO 2 ) synergy and establishes an effective, sustainable route for upgrading lignin-derived monomers to value-added fuel precursors, highlighting the importance of both acidic as well as versatile redox sites for improved catalytic selectivity performance.