Studies on the Oxidative Decomposition of Lignin

Abstract

Peroxide materials (hydrogen and sodium peroxides) are now used very successfully in the bleaching of certain types of pulp such as ground wood or semichemical pulp. Peracetic acid is of obvious interest as an oxidative bleaching agent. A significant characteristic of peracetic acid is that the addition of this acid in the ordinary bleaching system results in increasing brightness, up to a high limit.A study of the rate of consumption by hard-wood chemical ground pulp of oxidants such as hydrogen and sodium-peroxides or peracetic acid indicated that peracetic acid is an effective bleaching agent for high yield pulp, and that the bleaching is highly effective at ordinary temperatures in short times.The delignification of the chemical ground pulp by sodium peroxide or peracetic acid was studied. Residual lignin present in the pulp was more rapidly degraded by peracetic acid than by sodium peroxide under the same conditions. It was also found that the open aromatic aldehyde (vanillin or syringaldehyde) -yielding moieties of residual lignin are more effectively attacked during bleaching of chemical ground pulp by the former than by latter.Peracetic acid lignin prepared from dioxane lignin, which was isolated from un-bleaching chemical ground pulp, possessed interesting qualities. It was partially water- or ether-soluble, and a high total phenolic hydroxyl and carboxyl and a low methoxyl content. Combined evidences from UV and IR absorption spectra of dioxane lignin and its degradation product by peracetic acid, and from potentiometric titration and elemental analysis data suggest the demethoxylation and the ring opening of aromatic nuclei in the lignin to muconic acid structures via catechol structural elements by peracetic acid. Syringyl nuclei of dioxane lignin was more preferentially degraded than guaiacyl nuclei of it during peracetic acid oxidation.To gain a better understanding the mechanism of pulp bleaching by peracetic acid, the chemical degradation by this agent of aromatic compounds structurally related to lignin was carried out. Products thus obtained were vanillin, vanillic acid, Protocatechualdehyde, 2-methoxy-p-hydroquinone, 2-methoxy-p-benzoquinone and muconic acid from vanillin, vanillyl alcohol, acetovanillone or guaiacylmethyl carbinol under the conditions of peracetic acid bleaching.