Additive-Free Reductive Cleavage of Lignin Model Compounds by Ethanol-Sensitized Titania under Visible Light: A Sustainable Approach to β-O-4 Bond Cleavage

The selective cleavage of the β-O-4 bond in lignin model compounds is a crucial step in realizing the full potential of transforming lignin to value-added aromatic compounds. Herein, we report an additive-free, visible-light-driven ligand-to-metal charge transfer (LMCT)-mediated strategy for the reductive cleavage of β-O-4 lignin model ketones in ethanol, utilizing TiO₂ as the photocatalyst. We found that ethanol can act as a sensitizer and forms an LMCT complex on the surface of titania, which enables the reductive cleavage of the β-O-4 ketone under blue light (440 nm). The LMCT-sensitized titania showed moderate conversion (58%) for the reductive cleavage of β-O-4 ketone. Additionally, we observed that nitrogen doping of titania further enhances the visible light absorption and improves the conversion of the β-O-4 ketone up to 91%. EPR studies indicate the in situ generation of Ti³⁺ under blue light irradiation, which likely enables the reductive cleavage of β-O-4 ketone. Our work demonstrates a promising potential for the reductive cleavage of β-O-4 lignin model ketone to form value-added aromatic compounds under mild conditions without the use of an additional reductant by using ethanol (reaction solvent).