Characterization of lignin-carbohydrate linkages in the residual lignins isolated from chemical pulps of spruce (Picea abies) and beech wood (Fagus sylvatica)

To investigate the linkage types between carbohydrates and lignin, residual lignins were isolated from three different unbleached pulps [kraft, alkaline sulfite anthraquinone methanol (ASAM), and soda with anthraquinone (AQ) and methanol] of spruce and beech wood and then characterized by oxidation with 2,3-Dichloro-5,6-dicyanobenzoquinone and followed by Prehm's methylation. In residual lignins, sugar moieties were bound to lignins via benzyl ether bonds. In particular, galactose and mannose are predominantly linked to lignin fragments in residual lignins of spruce wood, while xylose and galactose are favored in the formation of LC bonds in the residual lignins of beech wood. In the case of hexoses, primary hydroxyl groups (C6 position) preferentially take part in benzyl ether linkages. Hydroxyl groups in the C2 and C3 positions of xylose participate in LC bonds and a small portion of arabinose was notably connected to lignin via the C5 position. Approximately seven or eight sugars were connected in soda/AQ/methanol residual lignin per 100 C9 lignin units, while the frequencies of LC bonds in kraft and ASAM residual lignins were distinctively less at one to three sugars per 100 C9 lignin units.