Highly efficient Ru-decorated CeO2 for photocatalytic hydrogenation and cyclization of levulinic acid to γ-valerolactone†

Abstract

The photocatalytic biomass transformation into valuable chemicals and fuels is interesting but challenging. The levulinic acid (LA) to γ-valerolactone (GVL) transformation has been explored under conventional thermal conditions. Reports on the photocatalytic hydrogenation of LA to GVL are rare, obtaining comprehensive information on the complete reduction process is challenging. Herein, CeO₂ was synthesized via the hydrothermal method and decorated with varying wt% of Ru to form Ru/CeO₂. The physical characteristics of the catalysts were confirmed through PXRD, TEM, and XPS analyses. The light absorption capacity of CeO₂ and Ru-decorated CeO₂ (specifically 0.5Ru/CeO₂ and 1Ru/CeO₂) was characterized using UV-visible spectroscopy. Additionally, the band structure of CeO₂ and 1Ru/CeO₂ was examined using VB-XPS and UPS analysis. Decorating CeO₂ with Ru improved charge separation and enhanced visible light absorption capacity. The visible light active 1Ru/CeO₂ catalyst achieved ∼99% conversion of LA to GVL under 15 W blue LED illumination at 0.2 MPa hydrogen. A mechanistic investigation through control experiments revealed that electrons facilitated the reduction of the ketonic group, while C–O cleavage in the acidic group (–COOH) is by the holes, which forms a carbonyl cation or radical, followed by immediate cyclization to GVL. The findings elucidate the active sites and demonstrate the recyclability of 1Ru/CeO₂ for selective LA hydrogenation under visible light. The work is significant from a sustainable chemistry perspective and crucial for the sustainable production of valuable chemicals.