Accurate Measurement of Oxygen Consumption during Oxygen-alkali Treatment of Kraft Lignin, Pulp and Lignin Model Compounds

In order to directly analyze the progress of lignin oxidation during oxygen-alkali bleaching, we designed a device by which oxygen consumption of lignin samples can be precisely measured by following the accurate amount of oxygen supply to the reactor. Oxygen consumption of isolated softwood kraft lignin and several lignin model compounds was measured by the device when these substances were subjected to oxygen-alkali treatments. The result showed that a unit of aromatic ring present in the kraft lignin consumes approximate 3 oxygen molecules. It was suggested that not only phenolic but also non-phenolic units in the kraft lignin consume oxygen. Model study by the use of guaiacylglycerol-β-guaiacyl ether, which carries both phenolic and non-phenolic aromatic ring, confirmed the occurrence of extensive oxidation on non-phenolic aromatic ring. Because the simple monomeric lignin model compound of non-phenolic type does not consume any oxygen when it is subjected to oxygen-alkali treatment alone, co-oxidation was suggested for the oxidation mechanism of non-phenolic aromatic ring.