Transition metal substituted polyoxotungstates for the oxygen delignification of kraft pulp Original Research Article

A series of transition metal substituted polyoxotungstates (TMSP) were tested as catalysts for the oxygen delignification of eucalypt kraft pulp. These include Mn substituted polyoxotungstates of Keggin-type, image (n = 1 (1) or n = 2 (2)), sandwich-type image (n = 1 (3) or n = 3 (4)), and the transition metal mono-substituted Keggin-type anions, such as α-[XW11M(H2O)O39]n− (M = CoIII, X = Si (5) and X = B (6); M = RuIV, X = Si (7), and X = P (8)). All TMSP showed a catalytic activity. The experimental results revealed high selectivity of the residual lignin oxidation during oxygen delignification catalysed by 1–4, though the final kappa number of delignified pulps varied significantly. This was related to the removal of hexeneuronic acid residues at low pH diminishing the corresponding pulp kappa number counterpart. A comparison of the kappa number decrease and the delignification selectivity in conventional oxygen-alkaline bleaching process and in the TMSP-catalysed oxygen delignification showed significant advantages of the latter process. The best results, concerning both the delignification degree and the selectivity of delignification, were obtained with TMSP α-[SiW11MnIII(H2O)O39]5− (1), image (4) and α-[SiW11CoIII(H2O)O39]5− (5). The application of catalysts 6–8 was limited by their structural instability under the delignification conditions. The practical aspects of the oxygen delignification catalysed by TMSP are discussed.