Polyoxometalate Delignification of Birch Kraft Pulp and Effect on Residual Lignin

To advance the understanding of delignification with polyoxometalates (POMs) that have been explored for use in bleaching of chemical pulps, the transformation of lignin during anaerobic treatment of birch kraft pulp with an equilibrated POM mixture composed of Na5(+2)[SiV1(-0.1)MoW10(+0.1)O40] was investigated. The conversion factor between the Klason lignin and the kappa number corrected for the hexenuronic acid (HexA) contribution gradually increased, indicating loss of lignin oxidizability. Comparative analysis of residual lignins isolated from pulps of decreasing kappa number showed that lignin undergoes changes that include a sharp reduction in the content of PhOH groups, a gradual demethylation, and a high increase in carbonyl groups. The results indicated that the POM treatment of kraft pulps leads to the loss of aromaticity, most likely caused by the conversion of aromatic rings to quinone moieties. The 2D NMR studies revealed the disappearance of the correlations assigned to stilbene structures formed during kraft pulping, and the weakening of those assigned to the native lignin bonds. The GPC studies showed a gradual lignin depolymerization.