Tailored Lignin Fractions via Ionic Liquid Pretreatment for Sustainable Polymer Systems

The valorization of advanced biorefinery lignins remains a significant challenge, owing to the presence of residual carbohydrates. These lignin-associated carbohydrates hinder lignin purification, reduce its homogeneity, and complicate chemical modifications, ultimately limiting the efficient conversion of lignin into high-value products such as chemicals and materials. This study presents a protic ionic liquid-based lignin fractionation process developed using softwood biomass. Triethylammonium methane sulfonate ([N222H][OMS]) was used to fractionate Pinus sylvestris, yielding two distinct fractions: a low-molecular-weight lignin fraction (LF) and a high-molecular-weight lignin fraction (HF). The extracted fractions were comprehensively characterized using nuclear magnetic resonance (NMR) to quantify changes in interunit linkages (β-O-4, β-5, and β-β) and hydroxyl group distribution, whereas methanolysis gas chromatography/mass spectrometry (GC/MS) was used to quantify residual carbohydrates. The fractionation process achieved LF and HF yields of approximately 70.32% and 17.58%, respectively. Further analysis revealed that the HF contained 59.92 ± 2.12 mg/g carbohydrates, whereas the LF contained only 27.37 ± 1.13 mg/g. These findings underscore the effectiveness of the protic ionic liquid fractionation process in reducing carbohydrate impurities and enhancing lignin purity, paving the way for the more efficient utilization of lignin in value-added applications.