Bamboo Hemicelluloses Extraction And Their Structural Characterization

With the increasing scarcity of fossil fuels, the research gradually focus on the biomass conversion both at home and abroad, especially the research of transforming agricultural and forestry wastes into the renewable products such as bio-ethanol, xylitol, and a large number of other chemicals. Bamboo is a common type of lignocellulosic renewable resources, which are widely distributed in China and many other Asian countries. Bamboo is mainly used in construction industry, papermaking, textile industry, cardboard manufacturing, food industry and bio-energy. The content of hemicellulose in bamboo accounts for20-30%, therefore, bamboo hemicellulose is also considered as a source of materials for papermaking, food and non-food industry.Hemicellulose is the second largest component of lignocellulose in bamboo, it is not homogeneous in structure and is a heteropolysaccharide composed of different pentose and hexose units. There are some researches on bamboo hemicellulose, however, it’s still very hard to completely describe its structure due to its complex chemical structure. Thus, to some extent, limit its application in industry.In this paper, the bamboo of3-month-old and18-month-old were selected to be the materials for hemicelluloses extraction. Fractional isolation and ethanol fractional precipitation were used to extract the hemicelluloses from the bamboo stems and bamboo leaves, respectively. Chemical and instrumental analysis methods were used to comparatively study the structural characteristics of the bamboo stem hemicelluloses and leave hemicelluloses.The extraction rate of bamboo stem hemicelluloses by alkali fractionation was61.50%-67.21%. The water soluble hemicelluloses in eighteen-month-old were lower than that in three-month-old. The alkali soluble hemicelluloses dissolved more in the low concentrations alkali solution for the three-month-old while the content of alkali soluble hemicelluloses was high in the high alkali concentration solution for the eighteen-month-old. The contents of the uronic acids of the hemicelluloses from bamboo stems aged three months and eighteen months were0.22%-4.45%and0.23%-5.12%, respectively. The neutral sugar compositions analysis showed that hemicelluloses from bamboo stems comprised of mainly D-xylose, followed by arabinose, D-galactose, mannose, glucose, and a small amount of ribose. Periodate oxidation results showed that there had a large number of hemicellulose types which only consumed periodate without formic acid produced, indicating the presence of1→2、1←2,6、1→、1→4,6linkages in the stem.The extraction rate of bamboo leave hemicelluloses by alkali extraction and ethanol fractional precipitation was67.83%-78.70%. With increasing ethanol concentration, the yield of hemicelluloses gradually increased and then decreased. When the ethanol concentration was40%and60%, the yield of hemicelluloses were relatively high. There had a certain purification effect of the hemicelluloses using ethanol fractional precipitatoin. The results of uronic acid analysis showed that the contents of the uronic acids of the hemicelluloses from bamboo leaves aged three months and eighteen months were1.28%-3.24%and1.48%-3.40%, respectively. The neutral sugar compositions analysis showed that hemicellulose fractions from bamboo leaves comprised of mainly xylose, followed by L-arabinose, galactose, mannose, glucose, and a certain amount of rhamnose. But the content of rhamnose in eighteen-month-old leaves was very small.The results of FT-IR,1H,13C NMR combined analysis showed that the acetyl-can be retained with hot water extraction and destroyed by saponification with lye. From the spectra of FT-IR,1H、13C NMR, we can inferred that β-(1→4)-D-linked xylose was the main backbone linkage of bamboo stem hemicelluloses, and there were braches of α-L-arabinofuranose connected at C-2and/or C-3of xylan, and the glucuronic acid connected to xylan at C-2through4-O-methyl-. There may also be glucomannan structure type in the water soluble hemicelluloses which was constituted of glucan as the main chain and D-mannose as the branched chain. The hemicellulose in the bamboo leaves was mainly arabinoxylan whose structure and composition were the same as in the stem hemicelluloses. With increasing of the ethanol concentration, the main chain of hemicellulose obtained was shorter, and contained more branched-chain, and the acetyl substituent was less. The hemicelluloses of the18-month-old bamboo leave were used to carry out electron microscopy(SEM) analysis to investigate the appearance and the morphology of the hemicelluloses. The results showed that the floc structure was more for the hemicelluloses with more branched-chain content. The low concentration of ethanol precipitated hemicelluloses had more rectangular structure. With the increasing of ethanol concentration, the hemicelluloses became more a irregular pieces, and finally turned into a spherical structure.The thermal gravimetric analysis(TGA) was used to study the pyrolysis characteristics of bamboo stem and leave hemicelluloses. The results indicated that the hemicelluloses pyrolysis could be divided into three stages with the main pyrolysis occurred from180to400℃. TG-DTG curve showed that the maximum weight loss occurred between250and300℃. The more the branched-chain content in the hemicelluloses, the poor thermal stability was. The residue amount increased with the increasing content of the linked lignin which also increased the thermal stability of the hemicelluloses.