Investigation of [Emim][OAc] as a mild pretreatment solvent for enhancing the sulfonation efficiency of alkali lignin

The chemical recalcitrance and complex structure of lignin macromolecules make them difficult to meet the needs of real production processes and limit their high-value utilization. Hence, an efficient and economic method to break the recalcitrance under mild conditions is meaningful and purposeful. In this study, 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) was employed to pretreat alkali lignin (AL). Then the dissolving properties and behaviors of AL in [Emim][OAc] were investigated. In addition, the apparent standard Gibbs energy change (triangle G degrees), the standard apparent entropy change (triangle S degrees), and the apparent standard enthalpy energy change (triangle H degrees) of lignin in [Emim][OAc] were estimated by using the experimental solubility data. The regenerated lignin (RL) from [Emim][OAc] was also analyzed by Fourier transform infrared spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), GPC, TG analysis and P-31-NMR spectroscopy. Importantly, the water solubility and sulfonated degree of RL were increased by 14.6% and 33.6%, respectively. Such results imply that [Emim][OAc] pretreatment could improve the sulfonation efficiency and make the development of lignin-based products a real possibility.