Catalytic lignin depolymerization to aromatic chemicals. (Image by ZHANG Chaofeng)
Research on lignin depolymerization and its downstream product transformation is aimed at harvesting aromatic compounds from this abundant and renewable biomass resource. The complexity of the lignin structure, various linkages, the high stability of lignin bonds, and the diverse fragments condensation process make lignin depolymerization to monomers a challenging task.
Compared with lignin conversion to liquid fuel with extra H2 consumption, maintaining the aromatic structure and preparing high-value aromatic chemicals from renewable lignin is more profitable.
Prof. WANG and his co-workers presented their ten years' studies on lignin’s catalytic conversion to aromatic chemicals in this article. They introduced the research on protolignin depolymerization via a fragmentation-hydrogenolysis process in alcohol solvents.
Then, focusing on the catalytic cleavage of lignin C–C and C–O bonds, they shed light on a recapitulative adjacent functional group modification (AFGM) strategy for the conversion of lignin models. AFGM strategy begins with the adjacent functional group modification of the target C–C or C–O bond to directly decrease the bond dissociation enthalpy (BDE) of targeted bonds or generate new substrate sites to introduce the cleavage reagent for further conversion.
On the basis of these two concepts from AFGM, they summarized their strategies on lignin depolymerization, which highlighted the effects of lignin structure, catalyst character, and reaction conditions on the efficiency of these strategies.
In short, the key point for lignin depolymerization to aromatics was promoting the lignin conversion and restraining the condensation.
Compared with the complex research on direct lignin conversion, this bottom-up research approach, beginning with lignin model research, could make the conversion mechanism study clear and provide potential methods for the protolignin/technical lignin conversion.
Moreover, one of their perspectives for lignin utilization is that the products from lignin conversion can be used as monomers for artificial polymerization, such as the simple phenol (PhOH) and other potential acid compounds, or that lignin derivative molecules can be used to synthesize high-value synthetic building blocks.
This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China, the China Scholarship Council and DICP.
