Toward the High-Yield Generation of Vanillic Acid from Lignin by Selective C–C Cleavage over CuO Nanoparticles

The selective cleavage of C–C bonds in lignin for scalable production of phenolic biochemicals constitutes a major obstacle, owing to lignin’s inherent structural complexity, the poor selectivity of existing catalysts, and the complicated separation process. Herein, we report a strategy to overcome this by improving the lignin side-chain C–C selective cleavage using a CuO nanoparticles (CuO NPs) catalyst through a one-pot oxidation process under mild conditions. The CuO NPs (<10 nm) were prepared by super-slow in situ precipitation of Cu2+ on GO. Morphology analysis revealed the presence of both CuO NPs and single Cu atoms, which exhibited an average size of 8 nm. Under mild conditions (140 °C, 1 atm), the CuO NPs/GO catalyst facilitated the selective cleavage of lignin Cα–Cβ side chains, resulting in over 95% lignin conversion and 10.8 wt % pure vanillic acid (VA), a record efficiency for isolated VA production. The high selectivity could be attributed to the presence of CuO NPs on the GO support. DFT simulations suggest that the CuO NPs decrease the formation energy barrier of active [O] species significantly, resulting in selective oxidation of the double bond in the lignin-derived intermediate, enol ether. By elucidating the key factors governing bond selectivity, our findings provide a foundation for the rational design of C–C bond oxidation catalysts in biomass valorization. The economic analysis based on a daily lignin feedstock of 500 tons demonstrates an estimated gross margin of ∼20%.