Isolation, Fractionation, Characterization And Modification Of Hemicelluloses From Agricultural And Forestry Biomass

Agricultural and forestry biomass, such as shrubs, sugarcane bagasse, and cereal straws, could be used for the production of high-value materials, chemicals to replace dwindling petrochemical-based feedstocks. However, they are still under-utilized in China today. In order to utilize the agricultural and forestry biomass for high-value materials, the fractional isolation, structural characterization, and chemical modification of hemicelluloses and the isolation of lignin from sugarcane bagasse were investigated, and the results are described as follows:The five hemicellulosic fractions were sequentially extracted with H2O, 1%, 3%, 5% and 8% NaOH aqueous solution from the dewaxed sugarcane bagass, and sub-fractioned into fifteen preparations by a graded ethanol precipitation method at concentration of 15%, 30% and 60% (v/v). Comparison of these hemicelluloses indicated that the smaller sized and more branched hemicelluloses were extracted by the hot water treatment and bound less tightly to cellulose. The larger molecular size and more linear hemicelluloses were dissolved by the alkali treatment, which are rich in xylose. In addition, with an increasing ethanol concentration from 15% to 60%, the Ara/Xyl ratios, molecular weights and thermal stability of hemicellulosic subfraction increased, the content of associated lignin decreased. On the basis of the FT-IR, sugar composition, and 1H and 13C NMR studies, there were no significant differences in the structural features of the precipitated hemicellulosic sub-fractions, which are mainly consisted of L-arabino-(4-O-methyl-D-glucurono)-xylan, whereas the difference may occur in the distribution of branches along the xylan backbone.Water- and alkali-soluble hemicelluloses were isolated with hot water and 10% KOH at 25°C from dewaxed and delignified Salix psammophila, respectively. The alkali-soluble hemicelluloses were then successively sub-fractioned by neutralization and graded precipitation at final ethanol concentrations of 15%, 30%, 45%, 60%, 75% and 90%, respectively. Neutral sugars and molecular weight found that water-soluble hemicelluloses were more branched, and had low molecular weights (6060 g/mol), whereas alkali-soluble hemicelluloses were less branched, and had high molecular weight (17110-85540 g/mol), in which xylose and uronic acid are main sugar component. In addition, the less branched hemicelluloses with large molecules were precipitated in lower ethanol percentages, while with the increasing ethanol concentrations, more branched hemicelluloses with low molecular weights were obtained. These results are in good agreement with the result of the Chapter 2, which indicated that results of graded ethanol precipitation method are applicable to the hemicelluloses of Gramineae and Hardwood. FT-IR, 1H, 13C and 2D HSQC NMR studies the alkali-soluble hemicelluloses had a backbone of D-xylose residues with 4-O-methyl-α-D-glucopyranosyl units.The sequential treatments of dewaxed sugarcane bagasse with hot water and 1% NaOH at 50°C yielded 5.3% and 11.3% water- and alkali-soluble hemicelluloses, respectively. The water- and alkali-soluble hemicelluloses obtained were sub-fractionated on DEAE-cellulose-52 chromatography and obtained six hemicellulosic sub-fractions. Sugar composition and molecular weight analysis revealed that the lower-molecular-weight (14180–43590 g/mol) and more branches of hemicelluloses could be extracted by the hot water, which are rich in glucose, galactose, and xylose, while the higher-molecular-weight (75430-138170 g/mol) and more linear hemicelluloses were able to be dissolved into 1% NaOH aqueous solution, which are rich in xylose. In addition, it was found that with increasing the concentration of NaCl aqueous solution, the hemicellulosic sub-fractions with both higher arabinose to xylose ratio, molecular weight, thermal stability and content of associated lignin were eluted. Based on the FT-IR, sugar composition and 1H and 13C NMR comparative studies, the alkali-soluble hemicellulosic sub-fractions had a classical structure, with a backbone ofβ-(1→4)-linked xylosyl residue substituted with arabinose at C-2 and/or C-3 of main chain, whereas the difference may occur in the distribution of branches along the xylan backbone.The dewaxed cell walls of Populus gansuensis were delignified with NaClO2 and then sequentially extracted with 0.25, 0.5 and 1.0 M KOH aqueous solution. The successive treatments together resulted in dissolution of 83.7% of original hemicelluloses. The solubilized hemicelluloses were further fractionated into six hemicellulosic sub-fractions by an iodine-complex precipitation technique. Neutral sugar composition and molecular weight analysis showed that, for each extract, the hemicellulosic sub-fractions that precipitated with aqueous potassium iodide-iodine had lower overall uronic acid/xylose ratios and higher molecular weights than those remaining in the solution. However, Chapter 2 indicated the highly branched xylan fractions to be of higher molecular weight than their less branched counterparts. These inconsistencies might reflect structural differences among hemicelluloses from different botanical origin, but they might also be due to differences in the techniques and solvent conditions employed during measurement of the molecular weight such as solvent quality, chain aggregation events. FT-IR, 1H, 13C and 2D HSQC NMR spectroscopy analysis indicated that the alkali-soluble hemicelluloses of Populus gansuensis had a structure composed of the (1→4)-linkedβ-D-xylopyranosyl backbone with 4-O-methyl-α-D-glucuronic acid attached to C-2 of the xylose residues.Rapid phthaloylation and succinylation of wheat straw hemicelluloses with phthaloyl dichloride and succinyl chloride in N,N-dimethylformamide/lithium chloride system using N-bromosuccinimide as a catalyst was achieved for only 5 minutes at 50°C by microwave irradiation. The degree of substitution ranged between 0.43 and 1.47. The effect of the molar ratios of xylose units in hemicelluloses to phthaloyl dichloride or succinyl chloride on the degree of substitution was investigated. It was found that the reactivity of succinyl chloride was higher as compared to phthaloyl dichloride under similar experimental conditions. 13C NMR studies revealed that the esterification occurred at the C-3 and C-2 positions in theβ-D-xylose units of hemicelluloses, but preferentially at C-3 position. The thermal stability of the modified hemicelluloses was lower than that of the unmodified hemicellulosic polymers. Hemicellulose-based hydrophobic biomaterials with degrees of substitution ranging from 0.46 to 1.54 were synthesized under mild conditions in homogeneous media (N,N-dimethylformamide-lithium chloride) by reacting the native wheat straw hemicellulosic polymers with lauroyl chloride using 4-dimethylaminopyridine as a catalyst. Other catalysts such as N-bromosuccinimide, N-methyl pyrrolidine, Nmethylpyrrolidinone, and pyridine were also investigated. Under optimum reaction conditions (2 equiv of lauroyl chloride and triethylamine per hydroxyl group, 5% 4-dimethylaminopyridine, 40°C, 35 min), a high DS value of 1.54 was obtained. The biomaterials were characterized by FT-IR spectroscopy and 13C NMR spectroscopy as well as by thermal analysis. The results showed that the lauroylation occurred preferably at the C-3 hydroxyl group ofβ-D-Xylp units in the hemicelluloses, and the thermal stability of the hydrophobic polymers increased by esterification. The modified polymers could be found promising for making environmentally friendly thermoplastics, such as making the degraded films for food packages.Sequential treatments of dewaxed bagasse with H2O, 1%, 3%, 5% and 8% NaOH aqueous solution at 55°C for 3 h solubilized 78.5% of the original lignin. The five isolated lignin fractions were subjected to a comprehensive structural characterization by UV, FT-IR, and 1H, 13C and 2D HSQC NMR spectroscopies analysis. The result showed that the bagass lignin fractions are typical grass lignins composed of syringyl, guaiacyl, and a small amount of p-hydroxyphenyl units. p-Coumaric is linked to lignin by ester bonds, whereas ferulic acid is linked to lignin by both ether and ester bonds. No significant differences were found in the weight-average molecular weights (3040-3790 g/mol) of the four alkali-soluble lignins. In addition, the 1%, 3% and 5% NaOH-soluble lignin fractions were rich in syringyl units and contained large amounts of noncondensed ether structures, whereas the 8% NaOH-soluble lignin fractions were rich in guaiacyl lignins. It was found thatβ-O-4 ether bonds are the major linkages between the lignin structural units together with common carbon-carbon linkages such asβ-β′,β-1, andβ-5. The complete characterization of the lignins is important for the understanding of the lignin structural features as well as for possible utilization of such natural materials.