Oxygen bleaching of kraft pulp with polyoxometalates and laccase applying a novel multi-stage process

A series of manganese substituted polyoxotungstates, [SiW11MnIII(H2O)O39]5− and [PW11MnIII(H2O)O39]4−, and [SiW11VVO40]5− were applied as catalysts for the oxygen delignification of unbleached eucalypt kraft pulp with laccase of Trametes versicolor. Unlike to modest results obtained in the Laccase-Mediator System (LMS) at 45–60 °C (lignin oxidation and catalyst re-oxidation occurred at the same stage), a sustainable delignification with removal of about 50% of residual lignin was achieved with [SiW11MnIII(H2O)O39]5− and [SiW11VVO40]5− when the kraft pulp treatment was carried out with polyoxometalate at 110 °C (lignin oxidation stage) and with laccase at 45 °C (catalyst re-oxidation stage) in separate stages. The use of [PW11MnIII(H2O)O39]4− in this multi-stage process was limited by the low re-oxidation rate with laccase. The best selectivity on the pulp delignification was found with polyoxoanion [SiW11MnIII(H2O)O39]5−, whereas [SiW11VVO40]5− was the most effective in the oxidative delignification. The influence of process factors on the POMs re-oxidation, such as the amount of laccase, oxygen pressure and temperature is discussed. UV–vis and 51V NMR studies indicated that POMs maintained stable after redox turnovers during the pulp delignification.