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 (MoVI2O5) 2 (MO2) O3P–C (O) (CH2-4–C5–NH4) –PO32−7 M = V (III), Cr (III) and Mn (III), in catalytic transfer hydrodeoxygenation (CTHDO) reactions. These complexes were immobilized on anatase TiO2 to form bifunctional catalysts that combine tunable redox and acidic properties, along with high structural stability. The POMo@TiO2 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–TiO2 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 (TiO2) 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.
Pardhi et al. (Fri,) studied this question.