Studies on the mechanism of alkaline peroxide delignification of agricultural residues

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 H 2 O 2 ⇌ H+ + HOO−. The data are consistent with a mechanism in which H 2 O 2 decomposition products such as ·OH and O·, rather than H 2 O 2 or HOO−, are the primary lignin oxidizing species. During the course of the delignification reaction, O 2 is evolved from the reaction mixture indicating active H 2 O 2 decomposition. At a given concentration of H 2 O 2, the rate of O 2 evolution is proportional to the amount of lignocellulosic substrate present in the reaction mixture. However, the total amount of O 2 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 O 2 per lignin C 9 unit, depending upon the initial concentration of lignocellulosic substrate.