Structural Characterization and Thermal Stability Analysis of Lignin from Flaxseed Hulls

To understand the structural and thermal properties of lignin in flaxseed hulls, milled wood lignin (MWL), lignin-carbohydrate complex (LCC), and acid-soluble LCC (LCC-AcOH) from flaxseed hulls, obtained by ball milling pretreatment in combination with solvent extraction, were characterized for monosaccharide composition, molecular mass, chemical structure and thermodynamic stability. The results showed that flaxseed hulls contained four types of lignin monomers: G, C, S and H, with G-type being predominant. The extraction rate of lignin from linseed hulls was 0.12%, with a purity of 95.54%, and the lignin contained 4.46% carbohydrates, with a main chain consisting of glucose units. In the three fractions, lignin and carbohydrate were linked together through benzodioxane rings. The predominant linkages for lignin in MWL and LCC-AcOH were β-O-4, β-β and β-5 bonds. MWL exhibited the highest thermal stability, while LCC had the lowest thermal stability. The results of this study provide a theoretical basis and data support for understanding the thermal degradation mechanism of lignin in flaxseed hulls and for its development and utilization.