Straw Alkali Extract Fractionation Of The LCC And Its Structural Characteristics

The alkali extraction liquor of wheat straw contains abundant lignin-carbohydrate complexes (LCCs). A systematical and quantitative method isolating LCCs from wheat straw extraction liquor has been successfully established. Characteristics of lignin, carbohydrate and the linkages between lignin and carbohydrate of LCCs were analyzed by alkaline nitrobenze oxidation (NO), ozonation, periodate oxidation, CP/MAS 13C-NMR and TG/DTA etc.The main conclusions were described as follows:1. A factional isolation method of LCCs from wheat straw alkali extraction liquor by a combination process of solvent-ultrafiltration was successfully established. Three fractions, enriched-lignin LCC, water-soluble LCC and water-insoluble LCC, were obtained with high yield and purity.2. Hollow fiber ultra-filtration showed good effects on removing sodium salts and low molecular weight (<1000) substances and is suitable to the isolation of water-soluble LCCs. The FTIR spectra, CP/MAS 13C-NMR spectra and chemical analysis methods of LCCs entirely proved the existence of linkage between lignin and carbohydrate.3. Significant quantitative differences in the lignin contents of LCCs from ambient temperature extraction liquor are observed. About 55 % of lignin in enriched-lignin LCC from the liquor is condensed lignin, whereas above 90 % of lignins in water-insoluble and water-soluble LCCs are non-condensed lignin. The content ofβ-O-4 structure increases with the content of non-condensed lignin in LCCs, and no significant difference in the stereochemistry ofβ-O-4 structure of LCCs was observed. Enriched-lignin LCC contains etherified G and S units with high content ofα-C=O structure. The contents of non-etherified 5-5',β-βandβ-5 units, and thermal stability of enriched-lignin LCC are higher than those of water-insoluble and water-soluble LCCs.4. The LCCs in ambient temperature liquor mainly contain arabinose, xylose and galactose. Some of arabinose and galactose in side chain of xylan is likely linked to lignin through ester or ether bonds. The degree of substitution of enriched-lignin LCC is high than that of water-insoluble and water-soluble LCCs. The contents of reducing end groups in enriched-lignin LCC are lower than those of water-insoluble and water-soluble LCCs containing substitution groups linked to C2 or C3 position of xylan with low contents.5. High temperature alkali extraction could increase the contents and condensation extent of lignin, and decrease the yields of LCCs. During high temperature alkali extraction, the ratio of erythonic to threonicβ-O-4 structures in water-insoluble LCC decreases with extraction temperature, and lignin-glucose and lignin-uronic acid complexes might be formed during the extraction.6. The influence of high temperature alkali extraction on the linkages between lignin and neutral sugars is almost small, During high temperature extraction, the content of non-reducing end group in enriched-lignin LCC decreases, the L-C bonds could be also formed between reducing end group and lignin, and no significant difference in the characteristics of reducing end group was observed.7. The phenolic substances contents of ambient temperature extraction liquor are high than those of high temperature liquor. High temperature alkali extraction makes the contents of phenolic substances of LCCs decrease. Vanillin and vanillic acid as bridge linkage between lignin and carbohydrate were observed.