<Original>Characterization of Steam-Exploded Wood II : Chemical Changes of Wood Components by Steam Explosion

Abstract

Chemical changes of main components in wood (cellulose, hemicellulose and lignin) by steam-explosion process have been elucidated by ^1H- and ^<13>C-NMR, Gas chromatography, GPC and thermal softening property. By steam-explosion hemicelluloses were rapidly hydrolyzed to lower molecular weight products. Almost all hemicelluloses (27.9%) in Shirakanba wood were hydrolyzed to oligosaccharides to be extracted with water by only one min. steaming at 20 kg/cm^2, and 53.7% of hemicelluloses were converted to monosaccharides by 8 min. steaming at 28 kg/cm^2. Monosaccharides obtained by 2 min. steaming at 28 kg/cm^2 of Shirakanba wood were composed of 61.1% of xylose and only 4.9% of glucose, and the yields were in accord with original composition of hardwood hemicelluloses. Cellulose in non-crystalline area was partially hydrolyzed, and micelle length was decreased to about 2000 Å by 8 min. steaming at 28 kg/cm^2. However, cellulose was not hydrolyzed to glucose, and non-crystalline cellulose would be annealed and transformed to crystalline celluslose. Thus the crystallinity and micelle width of cellulose were increased by steam explosion treatment. However, when reaction time was increased, the yields of furfural and 5-hydroxymethylfurfural derived from polysaccharides were increased. Lignin was degraded slower than hemicelluloses. The yield of lignin was 29.2% in maximum by 8 min. steaming at 28 kg/cm^2, and the molecular weight of lignins obtained were decreased to Mw=2100 and 1100 by 2 min. and 16 min. steaming, respectively. A degradation mechanism of lignin by steam explosion was presumed to be homolytic cleavage of aryl ether linkage.