Aqueous two-phase system formed by alkanolammonium-based Protic Ionic Liquids and acetone: Experimental data, thermodynamic modeling, and Kraft lignin partition

Abstract

Aqueous two-phase systems (ATPS) formed by Protic Ionic Liquids (PILs) are a potential alternative to recovery PILs after their use in pretreatment step and valorization of lignocellulosic biomass. Besides, ATPS based on PILs may be applied to Kraft lignin fractionation toward a more sustainable environment. In this study, nine alkanolammonium-based PILs were synthesized using organic acids (formic, acetic, propionic, lactic, and glycolic) as precursors. These PILs were combined with acetone to form ATPS at low temperatures, and the partition of Kraft lignin was tested using these same systems. The presence of an extra hydroxyl group in the PILs’ anionic part increased their ability to form ATPS, due to the increase in their acidity. Also, increasing the alkyl chain length of the PILs’ anionic part impairs the formation of ATPS. In contrast, the behavior was observed for the cationic part, due to the increase in the PILs’ hydrophobicity nature. Regarding the partition of Kraft lignin applying 2-hydroxyethylammonium-based PILs, the increase in the alkyl chain length of the PILs’ anionic part leads to stronger PILs-lignin interactions, which favored the lignin’s migration to the bottom phase (PILs-rich phase), and lower partition coefficient values, as follows: formate < acetate < propionate anions. The presence of an extra hydroxyl group in the PILs’ anionic part (lactate and glycolate-based PILs) impaired PILs-lignin interactions, and consequently, increased the partition coefficient, compared to propionate and acetate-based PILs. Finally, the increase in the PILs’ cationic part leads to the improvement in PILs-lignin interactions, and lower partition coefficient values were observed, following the sequence for lactate-based PILs: tris(2-hydroxyethyl)ammonium < bis(2-hydroxyethyl)ammonium < 2-hydroxyethylammonium; and the following sequence for propionate-based PILs: tris(2-hydroxyethyl)ammonium ≈ bis(2-hydroxyethyl)ammonium < 2-hydroxyethylammonium. The glycolate-based PIL showed the highest partition coefficient value, which is related to its small alkyl chain length and the presence of an extra hydroxyl group, making PIL-lignin interactions weaker. The results showed that acetone emerges as a phase-forming agent to remove, at least partially, the lignin dissolved in PILs aqueous solutions, allowing their recycle and reuse after the pretreatment step, for example. Finally, the NRTL activity coefficient model was used to correlate tie-line compositions, and the interaction parameters were estimated for each system determined.