The effects of oxygen on the degradation of lignin model compounds and residual lignin

Abstract: The aim of this work was to achieve a further understandingof the reactivity of lignin structures and residual ligninduring oxygen bleaching. The kinetics of lignin model compoundssuch as diguaiacyl stilbene, propylguaiacol,dipropylbiguaiacol, enol ether and ?-aryl ether structuresunder oxygen bleaching conditions have been studied. Anincrease in oxygen partial pressure results in a higher rate ofdegradation and reaches a maximum inthe vicinity of pH 11.The reactivity of the different lignin model compounds withoxygen in alkaline media has been compared at industrial oxygenprebleaching conditions. Generally, phenolic structures with aconjugated side chain, such as stilbene and enol ethers, reactrapidly during oxygen bleaching. Structures likepropylguaiacol, ?-aryl ethers, and degradation products,such as vanillin and gualacol, are more resistant. The reactionmechanisms involved are probably of the radical type becausenew crosslinked structures can be formed with even lessreactivity.The addition of transition metal ions or Mg+ lowered therate of degradation of propylguiacol measured at pH 11. Theaddition of H202 increased the rate of degradation.The results of a modified enzyme hydrolysis of pulp indicatethat the isolated residual lignin may have a spatialheterogeneity in the fibre. Sequential isolation of residualkraft lignin indicates that each fraction becomes progressivelyless modified. lncnased methoxyl content in combination with adecrese of the phenolic content indicates a less fragmentedlignin structure.Characterisation of the isolated residual lignin from kraft,kraft-oxygen and kraft-oxygen-oxygen pulps imply that theoxygen treated lignin structures are dissolved from the fibreand the remaining residual lignin in the pulp fibre is onlyslightly chemically altered.Keywords:Oxygen bleaching, Residual lignin, Ligninmodel compounds, Degradation, Kinetics, Transition metal ions,Hydrogen peroxide, Molecular composition.