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

In this work, the structural features of the lignin isolated with 2% NaOH at 90 °C for 2.5 h from the hydrothermally pretreated eucalyptus fibers at different temperatures (100–200 °C) for different times (15–60 min) 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 (1630–510 g/mol), associated carbohydrates contents (1.99–0.05%) and aliphatic OH contents (3.37–0.65 mmol/g), and increased phenolic OH contents (0.71–2.98 mmol/g) and thermal stability of the alkali lignins were observed with the increase of the hydrothermal severity.