Chemical, structural, and thermal characterizations of alkali-soluble lignins and hemicelluloses, and cellulose from maize stems, rye straw, and ricestraw

Treatment of dewaxed maize stems, rye straw, and rice straw with 1 M NaOH a t 30 degreesC for 18 h resulted in a dissolution of 78.0, 68.8, and 82.1% o f the original lignin, and 72.1, 72.6, and 84.6% of the original hemicellul oses, respectively. The three alkali lignin fractions and three hemicellulo sic preparations and the corresponding residues (mainly cellulose) were cha racterized by both degraded methods, such as alkaline nitrobenzene oxidatio n and acid hydrolysis, and non-destructive techniques, e.g. ultraviolet (UV ), Fourier transform infrared (FT-IR), carbon-13 nuclear magnetic resonance spectroscopies (C-13-NMR), and gas permeation chromatography (GPC). It was found that the three lignin preparations contained substantial amounts of non-condensed guaiacyl and syringyl units with fewer p-hydroxyphenyl units, and had weight-average molecular weights between 3280 and 3890 g mol(-1). The two hemicellulosic preparations, obtained from maize stems and rye stra w, were dominant in glucuronoarabinoxylans. While the hemicelluloses presen t in rice straw were mainly composed of alpha -glucan and kappa -arabino-(4 -O-methyl-D-glucurono)-D-xylan. The thermal analysis of the polymers showed that hemicelluloses degraded in first place, while lignin showed less degr adation, and therefore, its structure was more stable. Cellulose, on the ot her hand, showed an important degradation process, mainly between 250 and 3 30 degreesC, and its thermal stability is lower than that of lignin, but hi gher than that of hemicelluloses. (C) 2001 Elsevier Science Ltd. All rights reserved.