Fundamentals of o×ygen delignification. Part II. Functional group formation/elimination in residual kraft lignin

Abstract

The complex interaction of softwood residual kraft lignin with oxygen at elevated temperatures, pressures, and pH's was investigated by isolating and systematically oxidizing the lignin, as a function of time and temperature. The isolation of the oxidized lignin was carried out using a multistep 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. 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 (80-100°C), 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 on lignin despite the idealized twophase homogeneous conditions. The technological ramifications of these data imply that a three-phase oxygen delignification system, operating at temperatures below 100°C, causes only minor oxidative changes to the structure of the lignin in 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.

Structural modification of eucalypt pulp lignin in a totally chlorine-free bleaching sequence including a laccase-mediator stage

“…Oxidized units have been detected in lignin from pine pulp based on G-unit C 2 -H 2 and C 6 -H 6 correlations (Balakshin et al 2003 (Asgari and Argyropoulos 1998;Gierer 2000). These functional groups have also been detected after laccase mediator treatment using 13 C and 31 P NMR and FTIR spectroscopy (Sealey and Ragauskas 1998;Poppius-Levlin et al 1999b;Chakar and Ragauskas 2004).…”

Application of Solid-State 13C NMR Spectroscopy and Dipolar Dephasing Technique to Determine the Extent of Condensation in Technical Lignins

“…The relative amount of condensed structures in lignin has most often been determined by various oxidative degradation methods [6,8,9] or by 31 P NMR spectroscopy [1,4,7,[10][11][12]. However, only free phenolic units of lignin can be investigated by these methods.…”

Multistage oxygen delignification of high-kappa pine kraft pulp with peroxymonosulfuric acid (Px)

“…For the two or three stage oxygen delignification trials, the first stage was performed at 85°C and the following at 95°C in accordance with the finding of Iribarne and Schroeder (1997), in the sense that the degree of delignification at the same pulp viscosity loss may be increased at higher temperature in the second oxygen stage. This has been explained by lower activation energy of high-reactive lignin compared to carbohydrates and the need to oxidize the residual low-reactive lignin at higher temperatures (Asgari and Argyropoulos 1998).…”