Progress in directed biomass depolymerisation under the “lignin-first depolymerisation” strategy

Climate change, arising from the excessive use of fossil fuel resources, has evolved into a pressing global concern, with an emphasis on the development of renewable energy solutions. Biomass has been distinguished by the renewable, abundant, widely distributed, and environmentally friendly representative materials of clean energy sources. Among them, the organic wealth of nature primarily consists of cellulosic biomass, including cellulose (30%-50%), hemicellulose (20%-35%), and lignin (15%-30%). Lignin can also be positioned as the second-largest biomass resource. The solitary polymer can be distinguished with an aromatic backbone, indicating the potential to yield essential commodities, such as fuels, bio-based materials, and aromatic compounds. A sustainable solution to the prevailing oil crisis can then be provided for solid waste disposal in industry. Consequently, the strategic and value-added utilization of lignin can be indispensable for the long-term sustainability of biomass as a primary energy source. The phenylpropane monomers are closely linked through C-O and C-C bonds in the molecular structure of lignin. A three-dimensional reticulated macromolecular structure can then be formed to impose the depolymerization. The catalytic disruption of C-O and C-C bonds can be used within the phenylpropane structure. The concept of "Lignin‐first depolymerization" has gained notable attention in recent years. Lignin can also be effectively separated and further depolymerized into monomers, dimers, or other oligomers. In contrast to traditional biomass treatment, "Lignin-first depolymerization" can offer protection for the lignin backbone structure and circumvent condensation reactions in the depolymerized lignin. The high-value utilization of lignin can be realized to facilitate the efficient separation of cellulose, hemicellulose, and lignin. Importantly, the anti-degradation barrier was disrupted in biomass, leading to a holistic hierarchical utilization of the diverse components. The influencing factors (the source of lignin, catalysts, and solvents) have been determined in the conversion of lignin and product distribution. In this study, a systematic review was undertaken on the recent global progress in the preferential depolymerization of lignin. A comparison was also conducted on the acid, alkali, noble metal, and non-precious metal catalysts. The reaction mechanisms of these catalysts then underscored the efficacy of metal-loaded catalysts. Particularly, the conversion of lignin phenylpropane units was promoted using the breakage of inter-unit C-C or C-O bonds, thereby leading to the formation of valuable aromatic compounds. A comprehensive investigation was extended to explore the effects of various solvents (including aqueous, alcohol, and low eutectic solvents) on the lignin release rate, carbohydrate removal rate, slurry macrostructure, and monomer yield. The reaction mechanisms of each solvent type were also summarized in the lignin depolymerization. The alcohol solvents can be expected to break the ester bond between lignin and hemicellulose, and then to shield cellulose and hemicellulose from potential damage. This dual functionality of alcohol solvents can greatly contribute to the high-value utilization of biomass. In summary, the challenges associated with biomass depolymerization were outlined in the research directions. Effective depolymerization of lignin monomers can also be required for the synergistic combination of catalysts and solvents, which are identified as the two hot topics in this field. The findings can provide a roadmap to advance the sustainable utilization of biomass.