Photocatalytic Reduction of Levulinic Acid to γ-Valerolactone on Non-noble Metal Bi₂S₃ Catalysts under Mild Reaction 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.