Successive organic solvent fractionation and structural characterization of lignin extracted from hybrid poplar by deep eutectic solvent for improving the homogeneity and isolating narrow fractions

Abstract

Deep eutectic solvent (DES) treatment emerged as an efficient way to separate lignin at a moderate condition via acidolysis and hydrophobic interaction. In the attempt to realize lignin’s upgrading, this work employs successive organic solvent fractionation in DES-extracted lignin to obtain fragments with a narrow molecular weight distribution and relatively homogeneous structure. The polydispersity was decreased from 2.26 to 1.35. Purity and thermal stability were enhanced. Ethyl acetate-extracted lignin possessed the highest content of phenolic hydroxyls (3.23 mmol/g) but the lowest content of aliphatic hydroxyls of 1.10 mmol/g β-O-4 linkages were partly cleaved during DES separation and fragments containing more C–C bonds tended to be dissolved in ethyl acetate through hydrophobic interaction. The successive fractionation performed effectively in obtaining lignin fragments with diversity in main linkages and functional groups. This approach provided lignin fractions with versatility in preparation and application in thermos-plastics, lignin-derived carbon materials, antioxidants and aromatic fuels and chemicals.