Studies on the mechanism of alkaline peroxide delignification of agricultural residues

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Abstract

Alkaline solutions of hydrogen peroxide partially delignify wheat straw and other lignocellulosic materials, leaving a cellulosic residue that is highly susceptible to enzymatic digestion by cellulase. The delignification reaction is strongly dependent upon the pH of the reaction mixture, with an optimum at pH 11.5–11.6, pKa for the dissociation H2O2 ⇌ H+ + HOO. The data are consistent with a mechanism in which H2O2 decomposition products such as ·OH and Omath image·, rather than H2O2 or HOO, are the primary lignin oxidizing species. During the course of the delignification reaction, O2 is evolved from the reaction mixture indicating active H2O2 decomposition. At a given concentration of H2O2, the rate of O2 evolution is proportional to the amount of lignocellulosic substrate present in the reaction mixture. However, the total amount of O2 evolved is inversely proportional to the amount of substrate present, indicating that some of the peroxide oxygen becomes incorporated into lignin degradation products. The amount of peroxide oxygen incorporated can range as high as 2 O2 per lignin C9 unit, depending upon the initial concentration of lignocellulosic substrate.

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