Characterization of residual lignin after SO2-catalyzed steam explosion and enzymatic hydrolysis of Eucalyptus viminalis wood chips

The lignin component found in both water insoluble (WI) and water and alkal i insoluble (WIA) fractions derived from SO2-impregnated steam-exploded euc alyptus chips (SEE) was isolated and characterized. Dioxane lignins with a sugar content lower than 2% (w/w) were obtained after each material was tre ated with commercial cellulases. The C9 formulas of both SEE-WI and SEE-WIA dioxane lignins were C9H6.83N0.04O2.24(OCH3)(1.21)(OHaro)(0.56)(OHali)(0.7 7) and C9H8.65N0.29O1.97(OCH3)(0.90)(OHaro)(0.46)(OHali)(1.02), respectivel y. The weight-average molecular weight (M-w) of the SEE-WI lignin correspon ded to 3.85 kDa, whereas the SEE-WIA. lignin had an M-w of 3.66 kDa for the same polydispersity of 2.4. The SEE-WIA lignin was shown to be more therma lly stable than the SEE-WI lignin, requiring temperatures in the range of 5 20 degrees C for complete degradation. FTIR and H-1 NMR analyses of both un treated and peracetylated lignin fractions showed that (a) the alkali insol uble lignin contained a relatively higher degree of substitution in aromati c rings per C9 unit and that (b) alkaline extraction removed lignin fragmen ts containing appreciable amounts of phenolic hydroxyl groups.