Application of the amount of oxygen consumption to the investigation of the oxidation mechanism of lignin during oxygen-alkali treatment

The dioxygen consumption by kraft lignin and several lignin model compounds during oxygen-alkali treatments were directly analyzed using a dioxygen fl owmeter. The average dioxygen consumption by 200 g of kraft lignin was about 3 moles. Because this value was as much as those obtained for monomeric phenolic lignin model compounds, guaiacol and vanillyl alcohol, it was postulated that not only phenolic but also nonphenolic moieties in kraft lignin are extensively oxidized. The dioxygen consumption by 0.5 moles (one equivalent of aromatic units) of a dimeric lignin model compound, guaiacylglycerol-β-guaiacyl ether (GG), was also similar to that for 1 mole of guaiacol and vanillyl alcohol, regardless of the type of the aromatic moiety, which supports the above postulation. The most plausible mechanism for the oxidation of nonphenolic moieties is the oxidation of side chains of residual β-O-4 substructures by active oxygen species. By this mechanism, nonphenolic moieties in kraft lignin and GG are converted into corresponding phenolic moieties, and the oxidation by dioxygen progresses. Part of this article was presented at the 13th International Symposium on Wood, Fiber, and Pulping Chemistry (13th ISWFPC), Auckland, New Zealand, May 2005