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

Abstract

Treatment of dewaxed maize stems, rye straw, and rice straw with 1 M NaOH at 30 °C for 18 h resulted in a dissolution of 78.0, 68.8, and 82.1% of the original lignin, and 72.1, 72.6, and 84.6% of the original hemicelluloses, respectively. The three alkali lignin fractions and three hemicellulosic preparations and the corresponding residues (mainly cellulose) were characterized by both degraded methods, such as alkaline nitrobenzene oxidation and acid hydrolysis, and non-destructive techniques, e.g. ultraviolet (UV), Fourier transform infrared (FT-IR), carbon-13 nuclear magnetic resonance spectroscopies ( 13C-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 straw, were dominant in glucuronoarabinoxylans. While the hemicelluloses present in rice straw were mainly composed of α-glucan and k-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 degradation, and therefore, its structure was more stable. Cellulose, on the other hand, showed an important degradation process, mainly between 250 and 330 °C, and its thermal stability is lower than that of lignin, but higher than that of hemicelluloses.