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Fractional separation and structural characterization of lignins and hemicelluloses by a two-stage treatment from rice straw

Rice straw was separated into two lignin fractions and two hemicellulosic preparations through a two-stage treatment. Pre-treatment with ethanol–H2O (60/40, v/v) under acid catalyst (0.2 N HCl) at 70°C for 4 hr degraded 48.8, 24.9, and 23.3% of the original lignin, hemicelluloses, and cellulose, respectively. Following treatment with 2% H2O2 at pH 11.5 for 16 hr at 45°C resulted in a dissolution or degradation of 44.7, 55.2, and 12.1% of the original lignin, hemicelluloses, and cellulose, respectively. By this two-stage process, 93.5% of the original lignin and 88.2% of the original hemicelluloses were separated from rice straw. In comparison, the isolated two lignin fractions and two hemicellulosic preparations were characterized for their chemical composition, physico-chemical properties, structural features, and thermal stability. Both of the lignin fractions contained higher amounts of noncondensed guaiacyl units than those of noncondensed syringyl units, together with small amounts of p-hydroxyphenyl units. Pre-treatment with acidic organosolv, under the condition used, favored solubilization of the small molecular size of hemicelluloses, which were mainly composed of α-glucan, while the post-treatment with alkaline peroxide, under the condition given, enhanced dissolution of large molecular size of hemicelluloses, which comprised l-arabino-(d-glucurono)-d-xylan as the major constituent. The thermal analysis showed that the two hemicellulosic fractions were substantially decomposed between 200 and 330°C, while the lignin decomposition process covered a large temperature range, between 200 and 600°C, indicating more thermal stability of the lignin polymers.