PHYSICOCHEMICAL AND THERMAL CHARACTERIZATION OF ALKALI-SOLUBLE LIGNINS FROM WHEAT-STRAW

Four alkali-soluble lignin fractions were extracted from dewaxed wheat straw with 3% sodium hydroxide at 45 degrees C for 2, 5, 12, and 15 h , respectively. The pure alkali-soiuble lignin preparations LA were ob tained by using a two step precipitation method instead of the traditi onal ether precipitation procedure. The physico-chemical properties an d structural features of the isolated pure lignin fractions were chara cterized by UV, FT-IR, C-13 NMR spectroscopy, and thermal analysis. Th e lignin in fraction LA-1 is mainly composed of beta-O-4 ether bonds. The less common beta-5 and beta-beta carbon-carbon linkages are also p resent in the lignin structural units. The results obtained showed tha t the lignin fractions appeared to be strongly associated to hydroxyci nnamic acids such as p-coumaric and ferulic acids and glucuronic acid or 4-O-methylglucuronic acid.