Stable silica-modified zirconia with tunable acidity for enhanced catalytic transfer hydrogenation of levulinic acid and esters to γ-valerolactone

Abstract

γ-Valerolactone (GVL) is a valuable bio-based chemical, solvent and fuel additive derived from levulinic acid, a key platform chemical from lignocellulosic biomass. Catalytic transfer hydrogenation (CTH) of levulinic acid using secondary alcohols as hydrogen donors presents a sustainable alternative to conventional hydrogenation with molecular hydrogen and can be efficiently carried out with inexpensive oxides. Here, we demonstrate how controlled silica incorporation onto zirconia provides a route to tailor acidity and thus direct reactivity in the CTH of levulinic acid and its esters to GVL. Silica-doped zirconia catalysts with varying Si loadings were synthesised via colloidal deposition and comprehensively characterised using ICP-OES, TEM/EDX, XRD, BET, NH3-TPD, pyridine-adsorbed DRIFTS, XPS and NEXAFS. Moderate silica incorporation enhanced surface area, stabilised the tetragonal ZrO2 phase, and increased total acidity, and most importantly, altered the Brønsted-to-Lewis acid balance that dictated the reactivity. Ethyl levulinate conversion was favoured over Lewis acid-rich catalysts, whereas LA conversion required higher Brønsted acidity. The optimal catalyst (6 wt% Si) delivered 80% GVL yield from levulinic acid at 190 °C in 4 hours. Isopropyl levulinate was identified as a side-product that can also convert to GVL via CTH, though less efficiently. The 6 wt% Si/ZrO2 catalyst exhibited excellent stability across three consecutive cycles without calcination, demonstrating resistance to leaching, a major drawback of heterogeneous catalysts in liquid-phase reactions, as well as to carbon deposition. This study demonstrates that silica doping provides an effective means of tuning zirconia acidity, resulting in catalysts that combine good stability with practical applicability in sustainable chemistry.