The Kinetics of Lignin Reactions During Oxygen Bleaching. IV. The Reactivities of Different Lignin Model Compounds and the Influence of Metal Ions on the Rate of Degradation

The reactivities of different lignin model compounds with oxygen in alkaline media have been compared. Generally, phenolic structures with a conjugated side chain like stilbene and enol ethers react very rapidly during oxygen bleaching, whereas structures like propylguaiacol and β-aryl ethers are more resistent. Some of the reaction mechanisms involved are probably of a radical type because new crosslinked structures can be formed with even lower reactivity. The influence of metal ions on the reactivity has also been studied. We have found that Mg2+ and transition metal ions like Mn2+, Cu2+ and Fe2+ influence the rate of degradation as well as the rate of dimerization. Most of the metal ions gave a lower rate of degradation as measured at pH 11. The addition of a small amount of hydrogen peroxide to the oxygen alkali system increases the rate of degradation.