Lignin reactions in oxygen delignification catalysed by Mn(II)-substituted molybdovanadophosphate polyanion

The lignin oxidation in the oxygen delignification/bleaching process catalysed by Mn(II) substituted heptamolybdopentavanadophosphate polyanion (HPA-5-Mn-II) was studied using monomeric lignin model compounds and Eucalyptus globulus dioxane lignin adsorbed on the bleached sulphite pulp. The analysis of reaction products after lignin models oxidation in the presence HPA-5-Mn-II and the parent heptamolybdopentavanadophosphate polyanion (HPA-5) showed similar qualitative and quantitative patterns, indicating the identical lignin oxidative delignification mechanisms with both catalysts. The higher delignification selectivity of the pulp in the reaction system HPA-5-Mn-II/O-2 than with HPA-5/O-2 was explained by the protective effect of Mn-II in the heteropolyanion towards oxidative degradation of polysaccharides. The study of lignin adsorbed on bleached sulphite pulp, in the HPA-5-Mn-II/O-2 delignification system, showed the cleavage of beta-O-4 linkages as the major contribution to the lignin depolymerisation. The formation of Calpha-O-polysaccharide linkages in beta-O-4 structures of lignin during the delignification was observed.