Kinetics of Functional Group Formation/Elimination in Residual Kraft Lignin During Oxygen Delignification, 1997 Pulping Conference Proceedings

The complex interaction of softwood residual kraft lignin with oxygen at elevated temperatures, pressures and pH, was investigated by isolating it and systematically oxidizing it, as a function of time and temperature. The isolation of the oxidized lignin was carried out using a multi-step procedure developed specifically to recover all species. Detailed quantitative 31 P NMR measurements, provided for the first time three-dimensional plots describing the formation and/or elimination of the various functional groups. Oxidative demethoxylation reactions were found to induce the formation of catechols which are, most likely, reactive intermediates of the oxidation. The rate of carboxylic acid formation and guaiacyl phenol elimination was found to follow two distinct phases; a rapid initial phase followed by a slower phase. The elimination of guaiacyl phenolic units was described by a second order rate law while the kinetics of formation of carboxylic acids were of zero order. More specifically, the rate of carboxylic acid group formation was found to dramatically increase as the reaction temperature increased; analogous to the profiles obtained for the elimination of guaiacyl phenolic units. At reaction temperatures typical of conventional commercial oxygen delignification installations (80-100EC), only minor oxidation was found to occur within the lignin. At these temperatures, increasing the reaction time did not significantly aid the formation of carboxylic acid groups onto lignin despite the idealized two phase homogeneous conditions. The technological ramifications of these data imply that a three-phase oxygen delignification system, operating at temperatures below 100EC cause only minor oxidative changes on the structure of the lignin on the fibre. It was also possible to isolate the effect of alkali and temperature on the structure of residual kraft lignin through experiments in which pressurized oxygen was replaced by nitrogen. These data revealed a concealed set of alkali induced condensation and fragmentation reactions that operate under oxygen delignification conditions.