Unveiling the ability of protic and aprotic ionic liquids to dissolve and modify Kraft lignin.

• Kraft lignin solubility measurements were conducted in four AILs and five PILs. • Experimental solubility curves were obtained for the IL aqueous solutions. • Pyrrolidinium octanoate is the best option to solubilize Kfrat lignin (77.7 wt%) • An increase in β-β signals was observed, possibly due to condensation reactions. • β-O-4 linkages within the lignin structure were cleaved by PyrrOc and [Bmim]MeSO 4. Lignin, a renewable macromolecule abundant in lignocellulosic biomass, holds significant promise as a renewable source for producing biofuels, biomaterials, and fine chemicals, offering a potential avenue to reduce dependence on petroleum-derived products. However, the lignin complex structure poses challenges for its effective utilization, often requiring some special treatments. In recent decades, ionic liquids (ILs) have been pointed out as promising candidates for lignin processing due to their appealing properties, which include low vapor pressure/flammability, high chemical and thermal stability, and efficacy as lignin solvents. ILs may be categorized as protic (PILs) or aprotic ionic liquids (AILs), and these two classes offer distinct advantages. PILs are readily synthesized by mixing an acid and a base in stoichiometric proportions, forming a mixture containing neutral species that remain in equilibrium with cations and anions. In contrast, AILs present cations with enduring positive charges and are not in equilibrium with their precursors as the PILs, though AILs are frequently more expensive and challenging to produce. Within the biorefinery context, the ability to dissolve Kraft lignin is pivotal for developing new strategies and technologies. Moreover, chemical transformations may occur during lignin dissolution in IL solutions. In this study, the solubility of Kraft lignin was evaluated in four neat imidazolium-based AILs, five neat pyrrolidinium-based PILs, and in aqueous solutions of these ILs, at 313.2 K. The highest solubility value ((77.71 ± 1.03) wt%) was observed for pyrrolidinium octanoate (PIL), while 1-butyl-3-methylimidazolium methyl sulfate exhibited the highest solubility ((73.93 ± 1.27) wt%) among the studied AILs. Whenever possible, the solubilities measured in the present work were critically compared with data available in the literature. Finally, lignin recovered from the IL solutions was analyzed by Gel Permeation Chromatography (GPC), Fourier Transform Infrared (FTIR), and Nuclear Magnetic Resonance (NMR) spectroscopy to assess the solvent ability to induce structural modifications in this macromolecule. The analysis revealed that ILs effectively cleaved β-O-4 linkages in lignin molecules, and concurrent condensation reactions might increase the precipitated lignin molecular weight. [ABSTRACT FROM AUTHOR]