PhotocatalyticReduction of Levulinic Acid to γ‑Valerolactoneon Non-noble Metal Bi₂S₃ Catalysts under MildReaction Conditions

Catalytic conversion of levulinic acid (LA) to γ-valerolactone (GVL) is one of the key reactions of biomass utilization, but the relatively high reaction temperature and H₂ pressure always introduce safety and energy consumption issues. In this work, a photocatalytic transfer hydrogenation method was applied to achieve the hydroconversion reaction from LA to GVL under green and mild conditions using non-noble metal Bi₂S₃ materials as the catalyst. Up to 74.8% GVL yield and 76.3% selectivity can be achieved after 36 h of irradiation using the spherical Bi₂S₃ as the catalyst and isopropanol as the hydrogen source, which is significantly higher than any reported results on non-noble metal photocatalysts. The structural characterization and density functional theory calculation results indicated that the appropriate band position and lower adsorption energy toward LA rendered the spherical Bi₂S₃ catalyst highly reactive. Based on the results of electron paramagnetic resonance and density functional theory calculations, a reasonable photocatalytic transfer hydrogenation reaction pathway of LA to GVL was proposed. Bi₂S₃-photocatalyzed dehydrogenation of isopropanol generates H radicals, which react with LA adsorbed on the surface of Bi₂S₃ to form GVL through a hydrogenation-esterification process. The presence of Bi₂S₃ greatly promotes the adsorption of LA and reduces the energy barrier for LA hydrogenation.