| The oxidizing efficacy of hydrogen peroxide catalyzed by sodium molybdate (PM) and chlorine dioxide (D) were investigated at pH 3--5. Hardwood kraft pulps were delignifed by ODEP and OD/PME P where 25% of the oxidizing equivalents (OXE) from ClO2 were replaced by H2O2. It was demonstrated that for pulps containing low concentrations of transition metals, the OD/PME P partial sequence afforded superior overall bleaching efficacy. However, slightly inferior performance was obtained for pulps rich in transition metals, particularly iron.; Three lignin model compounds, two biphenyl dimers and a monomer were oxidized under acidic conditions by hydrogen peroxide catalyzed by sodium molybdate (PM) and by chlorine dioxide (D). The dimers were treated in dioxane/water (30:70, v/v) and ethanol/water (10:90, v/v) and somewhat surprisingly almost all of the H2O2 oxidation products were also obtained from ClO2 treatment. Oxidation of the monomer was compared in a completely aqueous system and again the chromatograms of reaction products were very similar. It was concluded that similar to ClO 2, the oxodiperoxomolybdenum (VI) complexes generated in PM abstract an electron from the aromatic nuclei to form radical cations that rearranged to phenoxy and benzyl radicals. Further reaction of these radicals lead to ring opening and cleavage of the alpha--beta bond in the sidechain. |
