Recent Advances in Depolymerization and High-Value Utilization of Lignin: A Review

Given the global greenhouse effect caused by excessive CO2 emissions and fossil energy exploitation, cleaner energy sources are crucial for modern developments. Lignin present in biomass is being considered to replace petroleum due to its abundance and reproducibility. Although the depolymerization of lignin to produce high-value chemicals has gained attention, it encounters significant challenges. Lignin possesses a high molecular weight, and the structural changes during separation often diminish its potential for producing value-added chemicals. This study reviews the recent research on lignin depolymerization using three traditional methods, including hydrolysis, pyrolysis, and hydrogenolysis. First, the lignin hydrolysis is presented, where its properties and mechanisms under various conditions are detailed. Subsequently, the pyrolysis mechanism, products, and pyrolysis methods (catalytic pyrolysis, microwave pyrolysis, molten salt pyrolysis) are discussed in detail. In addition, this study provides a comprehensive summary of hydrogenolysis in terms of both solvent selection and catalytic system. Finally, it addresses challenges and proposes solutions for future lignin value-added research.