Additive-FreeReductive Cleavage of Lignin Model Compoundsby Ethanol-Sensitized Titania under Visible Light: A Sustainable Approachto β‑O‑4 Bond Cleavage

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The selective cleavage of the β-O-4 bond in ligninmodelcompounds is a crucial step in realizing the full potential of transforminglignin to value-added aromatic compounds. Herein, we report an additive-free,visible-light-driven ligand-to-metal charge transfer (LMCT)-mediatedstrategy for the reductive cleavage of β-O-4 lignin model ketonesin ethanol, utilizing TiO2 as the photocatalyst. We foundthat ethanol can act as a sensitizer and forms an LMCT complex onthe surface of titania, which enables the reductive cleavage of theβ-O-4 ketone under blue light (440 nm). The LMCT-sensitizedtitania showed moderate conversion (58%) for the reductive cleavageof β-O-4 ketone. Additionally, we observed that nitrogen dopingof titania further enhances the visible light absorption and improvesthe conversion of the β-O-4 ketone up to 91%. EPR studies indicatethe 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 cleavageof β-O-4 lignin model ketone to form value-added aromatic compoundsunder mild conditions without the use of an additional reductant byusing ethanol (reaction solvent).