Insights on structure and antioxidant properties of lignin extracted from Eucalyptus by deep eutectic solvent/2-methyltetrahydrofuran biphasic system treatment

A deep eutectic solvent (DES, choline chloride/lactic acid)/2-methyltetrahydrofuran (2-MTHF) biphasic system was proposed to deconstruct the recalcitrance of Eucalyptus under different conditions for efficient extraction of lignin (DES-phase lignin (D-L) and organic-phase lignin (O-L)). The recovered lignin had relatively high total yield (61.65–83.69 %), low molecular weight (2400–1440 g/mol), and high purity. Results showed that the D-L had relatively higher aliphatic OH contents but lower phenolic OH contents than that of corresponding O-L, and the antioxidant activity of the O-L was higher than that of the corresponding D-L. Additionally, the total phenolic OH contents of the recovered lignin (1.92–2.96 mmol/g) were much higher than that of double enzymatic lignin (DEL, 1.29 mmol/g) because of the breakage of β-O-4′ linkages of the lignin in DES phase. Overall, the lignin fractions recovered from the DES and 2-MTHF phase has obvious difference, which is beneficial to the diversified utilization of lignin.