Study on the residual lignin in Eucalyptus globulus sulphite pulp

The residual lignin (L-R sample) was isolated from unbleached acid sulphite pulp from Eucalyptus globulus with kappa number 18.2 by acidolysis and structurally characterized by wet chemistry and NMR techniques. The main structural features of L-R were compared with lignin isolated from sulphite spent liquor (L-SSL sample) and dioxane lignin (DL sample) from eucalypt wood. L-R contains less sulphonic groups (4.4%) compared to L-SSL (11.3%), and its molecular weight (2200 Da) is very close to that of DL (2600 Da). A part of sulphonic groups is located at the benzylic carbon in beta-O-4' and beta-5' structures. L-R revealed ca. 20% lower abundance of beta-O-4' structures than DL, but ca. 40% higher abundance of these structures than L-SSL. The degree of condensation of L-R was higher than that of DL but lower than that of L-SSL. The condensed structures in L-R mainly originate from C6-linked syringyl units. The structural peculiarities of L-R consisted of the relatively high proportion of syringyl units compared to DL and the strong structural association with he via benzyl ether linkages. The differences in the structure of residual lignins from eucalypt sulphite and kraft pulps have been discussed.