Enhanced lignin degradation by a two-step acidic protic bio-based ionic liquid pretreatment method

In this paper, a series of acidic protic bio-based ionic liquids including N,N-dimethyl hydrogensulfate ([DMEA][HSO₄]), N,N-dimethylacetate ([DMEA][OAc]), and N,N-dimethyl hydrochloride ([DMEA][Cl]) were prepared by ion exchange method. The kraft lignin was pretreated to cleave its ether bond under the mild condition (80–160 °C, 30–150 min), and then converted into bio-oil by hydrothermal liquefaction (280 °C, 4 h). The results showed that ionic liquid pretreatment could effectively destroy the macromolecular structure of lignin and realize its preliminary depolymerization. Among which, pretreatment temperature and acidity of ionic liquid greatly influenced lignin degradation and deoxidation rate. The strong acid protic bio-based ionic liquid [DMEA][HSO₄] was selected for pretreatment process based on the characterization of ionic liquids acidity. The degradation rate and deoxidation rate of lignin were up to 25.58% and 37%, respectively, at 160 °C, 60 min. The analysis of lignin degradation products showed that acidic protic ionic liquids could selectively break the β-O-4 bond in the lignin structure to obtain high yield of vanillin (59.16%). The acidity of ionic liquids had not only a certain selectivity for degradation products but also a great influence on modifying the structure of regenerated lignin to upgrade the reaction conditions of subsequent hydrothermal liquefaction. It could provide a potential method for efficient degradation of lignin and obtaining high value-added chemicals. Graphical abstract: [Figure not available: see fulltext.].