Studies on the Oxidation Decomposition of Lignin

Abstract

The color improvement in a commercial bleaching of high-yielding pulps is due to the fact that the chromatophoric groups responsible for discoloration are chemically combined with the constituents and are removed from the pulps when these are treated with the bleaching agents, such as sodium peroxide, hydrogen peroxide or organic per-acids. It is also discussed that some of the groups is converted to the lighter colored groups during sodium peroxide bleaching.To gain a better understanding the mechanism of pulp bleaching, the chemical degradation by above agents of aromatic compounds structually related to softwood lignin was carried out in a dilute sodium peroxide solution (pH's 12, 7 and 2) at 30°C. for a belonged period.Products thus obtained in this way were vanillin, vanillic acid, 2-methoxy-p-hydroquinone, 2-methoxy-p-benzoquinone, muconic acid, maleic acid and oxalic acid from vanillyl alcohol, vanillylethyl ether, acetovanillone and guaiacylmethyl carbinol at pH 12. It also appears that the formation of 2-methoxy-p-hydroquinone and 2-methoxy-p-benzoquinone from vanillin was accelerated in an alkaline sodium peroxide solution. On the other hand, the oxidation products, such as vanillin, vanillic acid and protocatechualdehyde were formed from vanillyl alcohol, vanillylethyl ether, acetovanillone and guaiacylmethyl carbinol by treating the substructes in a sodium peroxide solution adjusted to pH 2. It was further identified the formation of all oxidation products, which were derived from under above conditions, from aromatic compounds employed in a neutral sodium peroxide solution.These results indicate that vanillyl alcohol and its ether groups of the lignin residue present in pulp would be selectively converted to 2-methoxy-p-benzoquinone and the polymer via vanillin and 2-methoxy-p-hydroquinone during the course of alkaline sodium peroxide bleaching.