Comparative study for the separation and depolymerization behavior of lignin from different separation methods

Due to its natural phenolic structure, lignin is the promising feed stock for the production of value-added phenolic monomers. However, the poor understanding of the relationship among separation methods, lignin structure and depolymerization behavior hinders the regulation of phenolic monomers production. In this work, lignin samples were separated by four methods (organic solvent, ionic liquids, deep eutectic solvents, acid-alcohol) using different strategies and their structure difference were characterized and verified by various methods (FTIR, NMR, nitrobenzene oxidation). It was found p-toluenesulfonic acid (PTS) and deep eutectic solvents (DES) were effectively solvents for lignin separation and for structure protection. Highest lignin separation of 99.42% (PTS) and recovery/purity of 92.93%/91.76% (DES) were obtained. The less condensed lignin also showed better performance on mono-phenols production (26.52%). Whereas the condensation C-C bonds formed during separation would hinder the depolymerization and lead to a lower phenolic monomers yield. The research results can provide guidance for the rational selection of lignin separation technology and subsequent depolymerization to produce phenolic monomers.