Lignin dissolution in dialkylimidazolium-based ionic liquid-water mixtures.

Lignin dissolution in dialkylimidazolium-based ionic liquid (IL)-water mixtures (40wt%-100wt% IL content) at 60°C was investigated. The IL content and type are found to considerably affect lignin solubility. For the IL-water mixtures except 1-butyl-3-methylimidazolium tetrafluoroborate ([C4C1im]BF4), the maximum lignin solubility can be achieved at 70wt% IL content. Lignin solubility in IL-water mixtures with different cations follows the order 1-butyl-3-methylimidazolium (C4C1im)>1-hexyl-3-methylimidazolium (C6C1im)>1-ethyl-3-methylimidazolium (C2C1im)>1-octyl-3-methylimidazolium (C8C1im)>1-butyl-3-ethylimidazolium (C4C2im)>1-butyl-3-propylimidazolium (C4C3im). For IL mixtures with different anions, lignin solubility decreases in the following order: methanesulfonate (MeSO3(-))>acetate (MeCO2(-))>bromide (Br(-))>dibutylphosphate (DBP(-)). Evaluation using the theory of Hansen solubility parameter (HSP) is consistent with the experimental results, suggesting that HSP can aid in finding the appropriate range of IL content for IL-water mixtures. However, HSP cannot be used to evaluate the effect of IL type on lignin solubility.