Analysis of products from the oxidation of technical lignins by oxygen and H3PMo12O40 in water and aqueous methanol by size-exclusion chromatography

One kraft lignin and two lignosulfonates were oxidized in aqueous acidic solutions containing a polyoxometalate (POM). The degradations were carried out in H2O or MeOH/H2O mixtures in the presence of oxygen. The treatment with aqueous H3PMo12O40 led to the dissolution of the studied lignins in the acidic medium (pH 1–2) and to the formation of up to 6.5 wt% vanillin and 6.2 wt% methyl vanillate based on the weight of dry lignin. The lignin oxidation products were analyzed by size-exclusion chromatography (SEC). For this purpose, a SEC method was developed, which allows the analysis of kraft lignin, lignosulfonates, and reaction products thereof without the need to remove the homogeneous catalyst. This method allows the direct observation of depolymerization and repolymerization reactions and hence the provision of a tool for studying the underlying chemistry. It has been demonstrated that the depolymerization of kraft lignin in water is accompanied by counterproductive condensation reactions. These repolymerization reactions were effectively prevented by addition of methanol, which couples competitively with lignin intermediates.