Research Status of Lignin Oxidative and Reductive Depolymerization

Lignin is a natural phenolic polymer and the second most abundant component next to cellulose in almost all plant biomass. However, only 2% of lignin is applied to industrial production due to the complexity of lignin structure. Therefore, the importance of comprehensive utilization of lignin should be addressed. It is a very important and promising approach for oxidative and reductive depolymerization of lignin polymer into aromatic compounds in lignin valorization. Oxidative depolymerization of lignin can significantly reduce the bond energy of main chemical bonds in lignin, which promotes the conversion of lignin into highly functionalized lignin monomer, such as vanillin, syringaldehyde, homovanillin. The reductive depolymerization can remove the oxygen-containing functional groups of the lignin and facilitate the transformation of lignin into low-oxygen and oxygen-free bio-oil, which can be applied to high caloric value bio-fuel. Besides, condensation reaction is conspicuously suppressed during reductive depolymerization. A brief introduction of the lignin structure unit, connecting and the recent progress in oxidative and reductive depolymerization of lignin are reviewed intensively. In. addition, the catalytic mechanism for the depolymerization of lignin is also discussed. Furthermore, forthcoming research emphasis and directions of lignin depolymerization are proposed at the end of the review based on existing problems in this area.