Understanding structural changes in the lignin of Eucalyptus urophylla during pretreatment with an ionic liquid-water mixture

Lignin is the second most abundant natural biopolymer found on earth. For sustainable use of lignin, the development of effective techniques for its depolymerization is necessary. In this study, the depolymerizations of enzymatic mild acidolysis lignin (EMAL) and Alcell lignin (AL), isolated from the eucalyptus urophylla using an ionic liquid (IL)-water mixture, were analyzed and compared. The results showed that the addition of a certain amount of water to IL could enhance the efficiency of lignin hydrolysis. The maximum solubilities of EMAL (41.5 wt.%) and AL (37.1 wt.%) were obtained in the [C(2)C(1)im][OAc](85%)-water and [C(2)C(1)im][OAc](15%).water solutions, respectively. The structures of regenerated lignin fractions are changed according to the proportion of water added to the IL, and were thoroughly characterized by GPC, FT-IR, 2D-HSQC NMR, and TGA. The results indicated that the molecular weight of regenerated lignin decreased, especially in the AL fractions, which were dissolved due to a high proportions of water (35-85 wt.%) in the mixture. The increased syringyl (S) to guaiacyl (G) ratio (S/G) of EMAL indicated the preferential breakdown of G-unit lignin. For AL, the S-unit lignin was easy to be removed by the mixture, indicating different mechanism for the lignin depolymerization. The study provides a better understanding of IL-water mixture as a solvent for dissolving lignin, thus encouraging further use of lignin.