Enhancing Lignin Model Compound Depolymerization Using Mediator-Enzyme Catalysis: A Sustainable Approach to C–C Bond Cleavage

The effective cleavage of C–C/C–O bonds in linkages of lignin with mild conditions and a green process was still a huge challenge for lignin valorization. Herein, the mediator–enzyme systems utilized in the depolymerization of lignin were developed. The C–C oxidation of β-O-4 lignin models was achieved through a mediator–enzyme systems. The carboxylic acid/ester-lipase was employed for the oxidation of ketone compounds. In the C–C bond oxidation process, Novozym 435 was used as the catalyst, and H2O2 was used as the oxidant in the solvent of ethyl acetate for the reaction. The reaction conditions were optimized, and excellent conversion was achieved. The mediator-enzyme was active for the oxidative cleavages of various β-O-4 lignin model compounds substituted by different functional groups. Furthermore, Novozym 435 was employed as a recyclable biocatalyst, demonstrating its stability confirmed by SEM, FTIR, and XPS. This study introduces a mediator-enzyme strategy for lignin model compound depolymerization.