| The kinetics and mechanistic pathways of the reaction between 1,1'-methylenebis(2-hydroxy-3-methoxy-5(2-carboxyethyl)benzene) (MBB) and alkaline hydrogen peroxide were investigated in the absence and presence of catalytic levels of added transition metal ions (Fe, Mn, Cu, ferricyanide). This compound was chosen to be representative of the condensed phenolic structural units which may be present in the residual lignin of unbleached softwood kraft pulps.; All reactions were conducted in a teflon-lined reactor under the following standard conditions: 5:1 molar ratio of H(,2)O(,2) to MBB, pH 11.0-11.4, 45(DEGREES)C. The consumption of MBB and H(,2)O(,2), and the forma- tion of MBB degradation products were followed over the course of each reaction by various analytical methods. In addition, peroxide decomposition was monitored independently of its consumption by the substrate by continuously measuring the volume of oxygen evolved during each reaction.; The addition of the catalysts was found to change the reaction orders (with respect to hydrogen peroxide concentration) for MBB oxidation and H(,2)O(,2) decomposition to different extents. The effect was most pronounced in the Cu catalyzed reaction. This effect is explained in terms of the overall mechanisms of phenol oxidation and H(,2)O(,2) decomposition reactions.; The results indicate that transition metal ions perform two central roles in the alkaline hydrogen peroxide oxidation of phenolic lignin units. First, they catalyze the decomposition of H(,2)O(,2) into reactive intermediates. And second, these metals participate in the oxidation of phenols via phenoxy radical formation. Both reactions can be catalyzed to different extents by different metal ion species. In the presence of the Fe and Mn additives, H(,2)O(,2) decomposition was catalyzed to a greater extent than MBB oxidation. In contrast, in the ferricyanide reaction the ratio of the rates of MBB oxidation to H(,2)O(,2) decomposition was slightly greater than in the reaction without added catalysts. The practical significance of these findings is discussed with reference to peroxide delignification of pulp.; The added catalysts did not induce major changes in the reaction stoichiometries or the types of reaction products formed during MBB degradation. The nature of the observed product mixture is consis- tent with the intermediacy of cyclohexadienone hydroperoxides which result from reaction of superoxide (O(,2)('-)) or oxygen with the phenoxy radical of MBB. A unique primary route of MBB degrada- tion, accounting for about one-fourth of the reacted MBB, was found and discussed in this study. (Abstract shortened with permission of author.)... |
