Controllable lignin valorization to value-added aromatic chemicals by a UV-assisted ternary heterogeneous photo-fenton catalytic system

Photocatalysis, which leverages sustainable solar energy to facilitate the conversion of lignin under mild conditions, has gained considerable research interest. Therefore, our study introduces a novel heterogeneous photo-Fenton reagent, Fe3O4@GO@TiO2, designed to selectively depolymerize lignin into aromatic chemicals using ultraviolet light as the energy source. This ternary photo-Fenton catalyst could effectively depolymerize approximately 90% of lignin model compounds. The model compounds without or with phenolic OH blocking yield guaiacol exclusively and predominantly guaiacol with aromatic aldehydes, respectively. The lignin degradation mechanism using the photo-Fenton catalyst (Fe3O4@GO@TiO2) involves the formation of an enol ether intermediate, followed by the cleavage of lignin β-O-4 bonds, facilitated by a mild photon-stimulated reduction–oxidation Fenton process. Notably, the reduced GO plays a crucial role in two aspects. Firstly, it transfers and stabilizes the photoexcited electron-hole pairs. Secondly, it promotes the generation of radicals (•OH, •OOH) essential for selective degradation of the lignin side chains. Consequently, the average molecular weight of the lignin depolymerized product is half of that of the original lignin. Furthermore, this photocatalyst demonstrates recyclability, retaining 85% of its initial activity after five-time recycling. This tertiary Fenton-photocatalyst provides a novel approach for the selective depolymerization of lignin interunit bonds into valuable phenolic monomers under ambient conditions.