Oxygen bleaching of kraft pulp catalysed by Mn(III)-substituted polyoxometalates

Abstract

The catalytic response of Mn(III)-substituted polyoxometalates (MSP) of series [XW 11Mn III(H 2O)O 39] n− (X=B, Si, P) on the oxygen bleaching of eucalypt kraft pulp was investigated. All MSP tested showed catalytic activity in the delignification, which depended on MSP redox potentials and rates of their re-oxidation with dioxygen. The best delignification results were obtained with SiW 11Mn III, possessing relatively high redox potential ( E°=+0.65 V) and being partially re-oxidised by dioxygen during the pulp bleaching. Under the conditions of SiW 11Mn III catalysed bleaching (100 °C, 2 h, pH 4) the selectivity of delignification was higher than that in the conventional oxygen–alkaline bleaching. The delignification mechanism was elucidated by kinetic studies and by oxidation experiments with SiW 11Mn III using monomeric lignin model compounds (homovanillyl and homoveratryl alcohols). The delignification rate showed a pseudo first order with respect to lignin and a non-integer, lower than one, order for SiW 11Mn III and O 2. The effective activation energy of the delignification was about 76 kJ/mol. It was suggested that two consecutive one-electron oxidations of lignin substructures by SiW 11Mn III is the delignification rate limiting reaction step. The oxidation of phenolic lignin structures was much faster (more than 10 times) than non-phenolic units, i.e. the oxidative delignification is determined by reactivity of phenolic units. The delignification time profile in the reaction system SiW 11Mn III/O 2 showed a fast initial delignification period corresponding to 35–40% lignin removal followed by a slow delignification period. These features were explained by different reactivity of phenolic and non-phenolic lignin units as well as by the competitive coupling reactions of one-electron oxidised lignin substructures.