The Study Of Lignin Dissolution In The Imidazolium-based Ionic Liquid-water Mixtures

We have studied lignin dissolution in ionic liquid (IL)-water mixtures (IL content of40-100wt%) at60oC. Dialkylimidazolium-based ILs with six types of cations (varying alkylchain from C1to C8) and five types of anions (methanesulfonate, acetate, bromide,dibutylphosphate and tetrafluoroborate) were investigated. Process of lignin dissolution wasfirstly screened by optical microscopy technologies, which verifies that lignin can be fullydissolved in the used IL-water mixtures with IL content≥60wt%except for1-butyl-3-methylimidazolium tetrafluoroborate ([C4C1im]BF4). Solubility of lignin in theIL-water mixtures was experimentally determined. The results indicated there are greatinfluences of IL content and type on lignin solubility. For the used IL-water mixtures except[C4C1im]BF4, the maximum solubility of lignin can be achieved at IL content of70wt%. Inthe case of the mixtures of water and ILs with acetate anion (MeCO2-), the order of ligninsolubility is [C4C1im]+>[C6C1im]+>[C2C1im]+>[C8C1im]+>[C4C2im]+>[C4C3im]+. For theILs with the cation of [C4C1im]+, the order of lignin solubility is MeSO3->MeCO2->Br->DBP-.Hansen solubility parameters (HSPs) of ILs and lignin were measured using the inverse gaschromatography (IGC) method, and theory of HSP was used to predict and explain thebehavior of lignin dissolution in the IL-water mixtures. The assessment using the HSP theoryis consistent to the experimental results, suggesting that it can help to find a good mixedsolvent system of IL and water. This work will favor understanding and optimizing thedelignification and pretreatment of lignocellulosic biomass using IL-water mixtures.