Mechanistic Insights into the Effects of Structural Characteristics on Lignin Depolymerization in Acidic Deep Eutectic Solvents

Converting lignin into high-value chemicals is an effective valorization strategy. However, the relationship between lignin structures and their depolymerization behaviors in acidic deep eutectic solvents (DESs) remains unclear. This study characterized hardwood, softwood, and herbaceous lignins and investigated their depolymerization in an acidic DES (choline chloride (ChCl):p-toluenesulfonic acid (p-TsOH)). Results demonstrated that β–O–4 linkage content, which is positively correlated with oil yield, is one of the key determinants of depolymerization extent. Model compound experiments revealed the condensation mechanism of syringyl (S) units in a strongly acidic DES, where S-type aldehyde-containing fragments are highly prone to condensation. Additionally, demethylation of S-type phenolic fragments generates polyphenolic species, further promoting condensation. The depolymerization products exhibited notable antioxidant activity. This work clarifies structure–depolymerization relationships and condensation mechanisms in acidic DESs, offering a theoretical foundation for targeted lignin conversion into aromatic chemicals and polymer precursors.