Investigation of Kraft Lignin Solubility in Protic Ionic Liquids and Their Aqueous Solutions

In this study, the potential of alkanolammonium-based protic ionic liquid (PIL) aqueous solutions as solvents of Kraft lignin was evaluated. The results showed that the PILs' anion plays a key role in lignin dissolution, while the cation plays a secondary role. The presence of extra hydroxyl groups in the anion structure negatively affects lignin solubility, while the increase of the cation alkyl chain favors this process. Using mechanical agitation, all tested PILs required 8 h to reach saturation, except for lactate-based PILs (at least 24 h). The efficiency of lignin dissolution was improved with ultrasounds achieving saturation in less than 4 h. Finally, the recyclability of two PILs was herein demonstrated for at least three cycles. Tris(2-hydroxyethyl)ammonium lactate presented the best ability to dissolve Kraft lignin (>47 wt %) among the tested PILs. This result shows the capacity of PILs as excellent media to dissolve lignin.