Visible-light-driven efficient photocatalytic oxidation of 5-hydroxymethyl-furfural over a metal-free recyclable anthraquinone derivative

Visible-light-driven upgrading of biomass and its derivatives into value-added chemicals under mild reaction conditions is an economical and sustainable strategy for biomass valorization. In this contribution, industriallyaccessible anthraquinone and its derivatives (AQs) have been developed as efficient metal-free photocatalysts for the photo-oxidation of 5-hydroxymethylfurfural (HMF). Especially, a quantitative furans selectivity containing 86.0 % of 2,5-diformylfuran (DFF) and 13.6 % of 5-formyl-2-furancarboxylic acid (FFCA) was implemented at HMF conversion of 88.8 % over 2,6-dibromoanthraquinone (2,6-DB-AQ) catalyst under visible light illumination. In addition, the homogeneous 2,6-DB-AQ catalyst can be readily separated and recycled from the reaction mixture by a "water-assisted" precipitate method. Photophysical characterizations indicated that the introduction of bromine substituents on AQ enhances its visible light absorption ability, decrease its band gap energy and enables the catalyst with a more stable photoexcitation state. It has also been demonstrated that the lower photoexcitation energy of O2 over 2,6-DB-AQ endowed the catalyst with better capacity for the activation of O2 than that of AQ. These contributions should be in charge of the excellent catalytic activity of 2,6-DB-AQ for the photo-oxidation of HMF. This work demonstrates an efficient and recyclable organo-photocatalyst for the visiblelight-mediated valorization of biomass-derived alcohols and aldehydes to value-added chemicals.