Novel Mechanoenzymatic Strategy for Lignin Depolymerization

Constitutive lignin phenolic monomers have gained significant attention in recent years due to their potential applications in the production of high-value chemicals. However, generating phenolic monomers remains challenging, as lignin, the precursor, is hard to depolymerize. In this work, we introduce a novel depolymerization strategy combining mechanochemistry and enzymes to depolymerize lignin into phenolic monomers. Both model lignin and natural lignin compounds were successfully converted to active monomers by using this strategy. The lignin model compounds were first subjected to a ball mill supplemented with laccase (0.1 GL/g), NaBr (2.5 equiv), and CuBr2 (0.2 equiv). Afterward, the reaction mixture was milled at 400 rpm for 180 min and refluxed in acetone (10 equiv) for 90 min to extract degradation products. The yields of aromatic acid and phenol were up to 83–94 and 85–90%, respectively. During the process, laccase oxidized Cα–OH into Cα═O and activated Cβ-H on lignin. Bromine attacked Cβ-H to form α-carbonyl alkyl bromine. Finally, the Cα–Cβ bond was activated through the hydroxyl ketone intermediate, followed by its fracture and formation of aromatic monomers. Natural lignin from birch or straw was also efficiently degraded to valuable aromatic compounds using this strategy. The development of this effective strategy to depolymerize lignin into functional aromatic monomers under mild, green, and efficient reaction conditions is of great significance to the utilization of lignin as a renewable resource. It has the potential to provide a sustainable source of high-value chemicals for pharmaceutical, material, and energetic industries.