abstract
The catalytic response of Mn(III)-substituted polyoxometalates (MSP) of series [XW11MnIII(H2O)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 SiW11MnIII, possessing relatively high redox potential (Edegrees = +0.65 V) and being partially re-oxidised by dioxygen during the pulp bleaching. Under the conditions of SiW11MnIII catalysed bleaching (100 degreesC, 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 SiW11MnIII 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 SiW11MnIII 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 SiW11MnIII 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 SiW11MnIII/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. (C) 2002 Elsevier Science B.V. All rights reserved.
keywords
MODEL COMPOUNDS; OXIDATION; HETEROPOLYANIONS;
subject category
Chemistry; Environmental Sciences & Ecology
authors
Gaspar, A; Evtuguin, DV; Neto, CP
our authors
Carlos Pascoal Neto
Raiz Director