Fractional isolation and characterization of lignin and hemicelluloses from Triploid of Populus tomentosa Carr.

The sequential three-step extraction procedure performed on fast-growing poplar, Triploid of Populus tomentosa Carr., with 96:4 dioxane:water, 50:50 dioxane:water and 80:20 dioxane:water containing 1% NaOH at 85 °C yielded 11.41% lignin and 14.78% hemicelluloses based on poplar dry mass. The structural features of the three isolated lignin fractions were comparatively examined by nitrobenzene oxidation, FT-IR, 31P NMR and 1H–13C 2D NMR spectroscopies. The results showed that the three lignin fractions, isolated sequentially with neutral and alkaline dioxane, were all GSH-type lignin. The ratio of non-condensed syringyl to guaiacyl was 2.7, 3.2, and 1.3 in the 96:4 dioxane:water, 50:50 dioxane:water, and alkaline dioxane lignin extracts, respectively. The contents of phenolic hydroxyl groups in the three lignin fractions obtained from 31P NMR spectra were 1.2, 0.7, and 0.87 mmol/g, respectively. The results also indicated that the 50:50 dioxane:water lignin extracts contained a relatively higher amount of aliphatic hydroxyls and a lower amount of non-condensed syringyl hydroxyl groups than the other two fractions.