Transition metal substituted polyoxotungstates for the oxygen delignification of kraft pulp

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, α-[SiW(12−n)MnnIII(H2O)nO(40−n)](4+n)− (n = 1 (1) or n = 2 (2)), sandwich-type α-B-[(PW9O34)2Mn(4−n)IIMnnIII(H2O)2](10−n)− (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), α-B-[(PW9O34)2MnIIMn3III(H2O)2]7− (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.