Fractionation of Lignin with Aqueous Organic Solvents: A Step Closer to Sustainable Wood Biorefinery

Recent rapid developments in forest biomass valorisation have highlighted lignin as a key value driver for the economic sustainability of modern biorefining. Sugar-based biorefinery hydrolysis lignin (HL) from wood biomass has an immense potential as an alternative for fossil-based aromatic feedstock. However, impurities, heterogeneous structures and limited solubility in common solvents are the main challenges associated with HL functionalisation. Aqueous organic solvent fractionation represents a promising approach to improve on HL properties and subsequently increase its valorisation options. In this study, for the first time, biorefinery HL derived from birch was purified and fractionated with water-EtOH and -THF mixtures across the whole co-solvent concentration range at ambient temperature. The highest lignin solubility was achieved with 70 wt% THF mixture, which dissolved a remarkable 81 wt% of lignin available in HL. Moreover, fractionated lignin purity was further increased to 95 wt% when employing an extra water-washing step. In general, fractions soluble in THF mixtures had high dispersity, syringyl-to-guaiacyl (S/G) subunit ratio and aliphatic OH-groups, whereas EtOH fractions had narrower molecular weight distributions, lower S/G subunit ratio and higher concentration of phenolic OH-groups. A brief description of the solvent effects at the molecular level was given using molecular dynamics simulations which revealed specific solvation between the dissolved lignin and solvent mixture. Protocol presented in this paper enables to purify HL and control the physicochemical properties of the soluble lignin fractions by simply altering the composition of the solvent mixture, and therefore, expanding the possibilities in valorisation of HL.