Influence of Cation Chain Length and Anion Identity in Imidazolium-Based Ionic Liquids on Lignin Depolymerization via Acid Hydrolysis

In this work, the ability of a series of imidazolium-based ionic liquids (ILs) to depolymerize Kraft lignin through acid hydrolysis was evaluated. ILs featuring two-, four-, and six-carbon alkyl chains combined with [Cl−], [BF4−] and [CH3COOH−] anions were studied to determine the influence of cation and anion structure. The twelve ILs were synthesized and characterized by FT-IR and 1H/13C NMR spectroscopy. Results indicate that both the anion and cation significantly affect depolymerization efficiency; specifically, longer alkyl chain lengths correlated with higher conversion percentages. Anion efficacy followed the order: [Cl−] > [CH3COOH−] > [BF4−]. Furthermore, reaction temperature did not show a significant impact on conversion within the studied range. Spectroscopic data suggest that bond dissociation follows a Brønsted acid-catalyzed mechanism, evidenced by the reduction of phenolic components and guaiacyl/syringyl units in the recovered lignin samples.