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Synergetic effect of dilute acid and alkali treatments on fractional application of rice straw

Sun, Shaolong, Chen, Weijing, Tang, Jianing, Wang, Bing, Cao, Xuefei, Sun, Shaoni, Sun, Run-Cang
Biotechnology for biofuels 2016 v.9 no.1 pp. 217
alkali treatment, biofuels, biomass, biorefining, byproducts, cellulose, condensation reactions, enzymatic hydrolysis, ethanol production, fractionation, glucose, hemicellulose, lignin, lignocellulose, rice straw, sodium hydroxide, sulfuric acid, synergism, temperature, xylooligosaccharides
BACKGROUND: The biorefinery based on an effective and economical process is to fractionate the three primary constituents (cellulose, hemicelluloses, and lignin) from lignocellulosic biomass, in which the constituents can be respectively converted into high-value-added products. In this study, a successive treatment with dilute acid (0.25–1.0 % aqueous H₂SO₄, 100–150 °C, 0.5–3.0 h) and alkali (1.5 % aqueous NaOH, 80 °C, 3 h) was performed to produce xylooligosaccharides (XOS), high-purity lignin, and cellulose-rich substrates to produce glucose for ethanol production from rice straw (RS). RESULTS: During the dilute acid pretreatment, the maximum production of XOS (12.8 g XOS/100 g RS) with a relatively low level of byproducts was achieved at a relatively low temperature (130 °C) and a low H₂SO₄ concentration (0.5 %) for a reaction time of 2.0 h. During the alkali post-treatment, 14.2 g lignin with a higher purity of 99.2 % and 30.3 g glucose with a higher conversion rate by enzymatic hydrolysis were obtained from the successively treated substrates with 100 g RS as starting material. As the pretreatment temperature, H₂SO₄ concentration, or time increased, more β-O-4 linkages in lignins were cleaved, which resulted in an increase of phenolic OH groups in lignin macromolecules. The signal intensities of G₂ and G₆ in HSQC spectra gradually reduced and vanished, indicating that a condensation reaction probably occurred at C-2 and C-6 of guaiacyl with the side chains of other lignin. CONCLUSIONS: The present study demonstrated that the successive treatments with dilute acid and alkali had a synergetic effect on the fractionation of the three main constituents in RS. It is believed that the results obtained will enhance the availability of the combined techniques in the lignocellulosic biorefinery for the application of the main components, cellulose, hemicelluloses, and lignin as biochemical and biofuels.