Catalytic cleavage of lignin C O and C C bonds

Lignin is a unique and abundant renewable organic carbon resource in nature with an abundant aromatic structure, thus, is being regarded as the potential renewable bank to prepare aromatic chemicals. Therefore, in recent decades, researches on lignin depolymerization and the transformation of its down-stream products have drawn enormous attention from academia to industry worldwide, aiming at harvesting aromatic compounds. Due to the complex lignin linkages among the random polymerized sinapyl alcohol (S-unit), coniferyl alcohol (G-unit) and p-coumaric alcohol (H-unit) with various robust CC and CO bonds, lignin depolymerization to easy-to-handle aromatic monomers with acceptable conversion and selectivity is still a highly challenging task. In this chapter, focusing on lignin catalytic depolymerization to aromatic chemicals, we herein first provide our understanding about the scientific question and strategic foundation process in lignin depolymerization, and then clarify them by mainly presenting our recent studies. Firstly, we introduce our research on direct protolignin depolymerization via a fragmentation–hydrogenolysis process in alcohol solvents. Then, focusing on the catalytic cleavage of lignin CC and CO bonds, we shed light on a recapitulative adjacent functional group modification (AFGM) strategy for the conversion of lignin models and apply the established methods in the real lignin conversion via a bottom-up research approach. AFGM strategy begins with the adjacent functional group modification of target CC or CO bond to directly decrease the bond dissociation enthalpy (BDE) of targeted bonds or to generate new substrate sites to introduce the cleavage reagent for further conversion. Furthermore, the critical point for lignin depolymerization to aromatics is not only promoting the lignin conversion, but also restraining the fragments recondensation, which requires the foundation of efficient strategies taking into account the effects of lignin structure change during pre-abstraction and reaction, catalyst character, and reaction conditions. In addition, our perspectives for lignin utilization are that the products from lignin conversion can be used as monomers for artificial polymerization, or using lignin derivative molecules to synthesize high-value synthetic building blocks for medicine and material, which needs a deeper understanding about lignin chemistry itself, new engineering technologies to intensify the conversion process, and fantastic ideas from different subjects to extend the relative research fields.Download : Download high-res image (298KB)Download : Download full-size image