Aqueous organic solvent fractionation as means to improve lignin homogeneity and purity

Lignin's heterogeneous polymeric structure limits its utilisation in high-value applications. This paper describes a protocol for kraft lignin precipitation fractionation from aqueous ethanol, acetone and propyleneglycol monomethyl ether solution by adding water as nonsolvent. The solvent-insoluble fractions possessed high molar mass (Mw 4300-18900 g/mol), high polydispersity (2.2-5.6), high nitrogen (0.15-0.43%) and low sulphur (0.7-1.5%) content. Even lignin aggregation was observed in acetone-based process. The molar mass of the precipitated fractions depended on the amount of water added in the precipitation step. The highly soluble lignin fractions possessed very low molar mass (Mw 1100-1200 g/mol), more free phenolic hydroxyl groups (3.95-4.05 mmol/g), carboxylic acid groups (0.56-0.71 mmol/g), higher amount of sulphur (1.6k2.9%) and more carbohydrates (5.3-6.1%) than other lignin fractions. The presented protocol provides a selection of green solvents for a simple and easily scalable and tunable lignin fractionation process to produce lignin with constant quality and high purity.