Bleaching of kraft pulp by oxygen in the presence of polyoxometalates

A new approach for pulp bleaching using oxygen in the presence of a polyoxometalate (POM), such as the heptamolybdopentavanadophosphate anion (HPA-5), in aqueous and nonaqueous media is presented. The selective oxidation of residual lignin in the pulp/POMO2 system is catalyzed by HPA-5 oxidized species. The reduced forms of HPA-5 after the reaction with lignin are continuously regenerated by re-oxidation with oxygen, enabling the use of the catalyst in multiple cycles of pulp bleaching. The organic matter dissolved in the bleaching effluents is readily oxidized to carbon dioxide. The method described here achieves, for the same degree of delignification, higher yield and intrinsic viscosity of pulp than those obtained by alkaline-oxygen bleaching. The POM bleaching was tested in POM EP, POM ED and POM EpD sequences. The pulps obtained have brightness degrees and mechanical properties slightly lower than those obtained by the standard DEpDEpD ECF sequence.