| Over the years, hydrogen peroxide has been used to boost the brightness of pulp at the end of a bleaching sequence. However, the degree of delignification achieved by a hydrogen peroxide stage is usually modest. That is because in alkaline condition dissociated hydrogen peroxide reacts with chromophores in pulp as a nulceophile that does not oxidize residual lignin to a significant extent. In order to enhance the reactivity of hydrogen peroxide as an oxidant, a binuclear manganese complex Mn(IV)2-Me4DTNE was added. As a result, the degree of delignification was significantly improved, at the mean time, the strength properties of the pulp were better preserved than traditional hydrogen peroxide delignification.; In order to better understand the mechanism of this delignification process, lignin model compound oxidation and kinetics of pulp delignification were studied. In addition, residual lignin was isolated from the pulp before and after delignification and then characterized by GPC, FTIR, and 1H- 13C 2D NMR. Shown by the model compound study, hydrogen peroxide is able to oxidized 1-(3,4-dimethoxyphenyl)ethanol, E-diphenylethene, and 1-(3,4-dimethoxyphenyl)-1-propene to a considerable extent when catalyzed by Mn(IV)2-Me4DTNE, indicating that the reactivity of hydrogen peroxide as an oxidant is significantly improved. Indeed, shown by the kinetic study of pulp delignification, the degree of delignification as well as the rate of delignification was greatly improved when catalyst was applied. In addition, the catalyzed delignification process benefits from the concerted reaction mechanism that regulates the formation of hydroxyl radicals responsible for the sever damages to the fibers. As a result, the strength properties were well preserved. Shown by the characterization of residual lignin before and after the delignification, residual lignin was oxidized to a good extent which accounts for the significant degree of delignification. The process simulation of this catalyzed delignification process by WinGEMS4.0 showed that this process was profitable if the cost of the catalyst was kept below {dollar}330/kg. |
