Developing Catalytic Oxygen Delignification for Kraft Pulp: Kinetic Study of Lignin Oxidation with Polyoxometalate Anions

The effect of pH, ionic strength, and temperature on the kinetics of oxidation reactions between residual lignin and polyoxometalate (POM) anions ([AlMnIII(OH2)W11O39]6- and [SiMnIII(OH2)W11O39]5-) in a pulp suspension of unbleached softwood kraft pulp at varying conditions was investigated. According to the results, cation concentration and pH have a remarkable effect on reactivity of the POMs. At cation concentrations <= 0.10 mol L-1, no reaction was observed during the first 5000 s; at 0.30 mol L-1, the reactions were significantly more rapid, and the rate correlated strongly with increasing pH for both POMs. The activation energies (Ea) of the oxidation reactions at 0.30 mo L-1 cation concentration varied between ~27 and 68 kJ mol-1 for [AlMnIII(OH2)W11O39]6-, depending on pH and reaction time; the respective values for [SiMnIII(OH2)W11O39]5- were ~17 to 82 kJ mol-1. The reasons for the observed decrease in Ea along with increasing pH and cation concentration are discussed.