Kinetic study on delignification of kraft-AQ pine pulp with hydrogen peroxide catalyzed by Mn(IV)-Me4DTNE

The kinetics of delignification of a kraft-AQ southern pine pulp with hydrogen peroxide catalyzed by [LMn(IV)(mu-O)(3)Mn(IV)](ClO4)(2) (1), where L = 1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane was studied. The degree of delignification was significantly improved by using the catalyst. The pulp was bleached for 2 hours at 80 degrees C, in 10 % consistency with 2 % NaOH, 4 % H2O2 and 60 ppm catalyst charges on pulp (O.D.). Kappa number of the pulp was reduced from 31.6 to 16.8 corresponding to a degree of delignification of approximately 4%, while GE brightness was increased from 24.2 to 44.7. At the same time, viscosity of the resulting pulp was reduced from 31.1 mPa.s to 20.1 mPa.s compared to the reduction from 31.1 mPa.s to 20.1 mPa.s in the uncatalyzed bleaching under the same reaction condition. This indicates that the degradation of the carbohydrates was moderate in the catalyzed bleaching compared to the uncatalyzed bleaching. The delignification was found to follow pseudo first order kinetics with respect to kappa number, i.e., residual lignin, in the initial phase and quickly slowed down after 30 minutes (residual phase) under all the reaction temperatures investigated. The delignification rate constants in the initial phase were 0.17, 0.18, and 0.21 min(-1) at 50, 60, and 80 degrees C, respectively. Degree of delignification at the delignification time of 30 minutes is approximately 40 % at 80 degrees C. The possible delignification mechanism was discussed on the basis of the kinetic studies and lignin model compound experiments.