Comparing physicochemical characteristics and depolymerization behaviors of lignins derived from different pretreatment processes

Lignin valorization is an important part of increasing the economic viability of sustainable biorefineries. Various methods have been proposed for lignin depolymerization. However, the relationship between lignin structure and its depolymerization behavior hasn't been extensively investigated. This research aims to clarify how the structure and composition of lignins derived from different pretreatment processes affect the downstream catalytic conversion. Herein, optimal organosolv lignin yield above 50% in three different solvent systems were obtained. Meanwhile, three types of hydrolyzed lignin were used as reference lignins. Various lignins were comprehensively characterized by elemental analysis, FT-IR, TGA-DSC and Py-GC/MS techniques. Compared with hydrolyzed lignin, organosolv lignin has higher purity, wider functional group distribution, more uniform structure, as well as a lower G/S ratio. Moreover, lignin depolymerization was carried out with a non-noble metal catalyst Ni/Al₂O₃ and without the addition of H₂. The results showed that transetherification and alkylation reaction were enhanced significantly for organosolv lignin. Total depolymerized aromatics yield from organosolv lignin was 4 to 11 times higher than that of hydrolyzed lignin-rich residues. Notably, Methanol-extracted lignin obtained an optimal yield of 29.7 wt% of monomeric aromatics. Methanol can effectively protect benzylic carbocations formed during organosolv pretreatment, thus minimizing the formation of CC bonds.