Structural variation of eucalyptus lignin in a combination of hydrothermal and alkali treatments

[Display omitted] •Hydrothermal and alkali treatments were combined to fractionate lignin.•An increase in pretreatment severity facilitated the release of lignin.•The β-O-4 linkage was cleaved gradually with the increased temperature.•Mw of lignin decreased from 1630 to 510g/mol as temperature increased. In this work, the structural features of the lignin isolated with 2% NaOH at 90°C for 2.5h from the hydrothermally pretreated eucalyptus fibers at different temperatures (100–200°C) for different times (15–60min) were thoroughly investigated. Results showed that the hydrothermal pretreatment facilitated the separation of alkali lignin from the pretreated fibers. It was found that the linkages of β-O-4, β-β, and β-5 decreased gradually with the increase of hydrothermal severity. Furthermore, decreased molecular weights (1630–510g/mol), associated carbohydrates contents (1.99–0.05%) and aliphatic OH contents (3.37–0.65mmol/g), and increased phenolic OH contents (0.71–2.98mmol/g) and thermal stability of the alkali lignins were observed with the increase of the hydrothermal severity.