Mechanisms of oxidative degradation of carbohydrates during oxygen delignification

Environmental concerns have heightened interest in chlorine-free bleaching sequences. Oxygen-alkali systems are of particular interest since the by-products are environmentally benign. Unfortunately, the use of oxygen as a bleaching chemical degrades carbohydrates as well as the lignin resulting in a lower pulp strength and yield. Obtaining a better knowledge of the reaction mechanisms involved in oxygen delignification will help achieve the long term goal of this project, promoting lignin degradation while preserving carbohydrates.;Five different oxygen species are present under oxygen delignification conditions: Oxygen, superoxide anions, hydrogen peroxide, hydroxide anions, and hydroxyl radicals. Presently it is uncertain which oxygen species is most responsible for degrading carbohydrates during oxygen delignification. Carbohydrate model compounds were reacted with each of the various oxygen species to determine which species are responsible for the degradation of carbohydrates and to propose reaction mechanisms.;The experiments conducted in this research suggest that hydroxyl radicals are responsible for the degradation of carbohydrates during oxygen delignification. Hydroxyl radical experiments yielded significant degradation to all six carbohydrate model compounds used in this study. However, molecular oxygen, superoxide anions, hydrogen peroxide, and hydroperoxyl anions do not appear to degrade carbohydrates directly.;The major degradative pathway for the oxidative cleavage of carbohydrates by hydroxyl radicals is proposed to be a substitution reaction between hydroxyl radicals and carbohydrates at the anomeric position. Experimental and theoretical evidence supports this mechanism for all four carbohydrate model compounds in which products were identified. In each case, appearance of the most abundant reaction products support the proposed substitution reaction.;The carbohydrate degradation by hydroxyl radicals is not limited to soluble model compounds. Hydroxyl radicals produced in UV/HOOH experiments degraded both cellulose compounds investigated in this research.;This research suggests that even a small concentration of hydroxyl radicals in oxygen delignification can be very detrimental to the end product. Hydroxyl radicals can simply cleave any anomeric linkage in a carbohydrate chain. The cleavage is not dependent on temperature or oxygen pressure. Once the hydroxyl radical attacks the anomeric position the carbohydrate chain is broken. Because this cleavage can occur at any position in the carbohydrate chain, pulp properties including viscosity and strength will be greatly reduced.