Ｈ?S２?TＯ?S２?Tによるリグニンの淡色化

Abstract

Neutral H2O2 bleaching of various unbleached pulps under the mild conditions was carried out. A portion of lignin in the pulps was oxidatively degraded to the water-soluble products. The increased color removal observed when high-yield pulps were bleached, demonstrates the lebelling-off effect encountered in H2O2 bleaching. But, the brightness of chemical pulps increased up to about 70% during the bleaching. Milled wood lignin and dioxane lignin, isolated from ground pulp or chemical ground pulp, were degraded with H2O2 in a neutral homogeneous system at 20°C. The change of absorbance at 350 nm or 457 nm of the lignins shaves the levelling-off in brightness gain, being the same as the bleaching of high-yield pulps. But, the absorbance of thiolignin and lignosulfonate at the same wave length decreased during H2O2 treatment. The chromophores in all lignin preparations employed were more rapidly degraded than the aromatic nuclei. Attempts are reported in this report to characterize the reaction of model compounds with H2O2 at pH 7.0. The formation of colored products from β-aryl ketones in model compounds employed was identified. 2-Methoxy-p-quinone and 2-methoxy-p-hydroquinone were important intermediates in the formation of colored materials from vanillin or acetovanillone. Based on the above-mentioned results, it can be concluded that the levelling-off in the brightness gain of high-yield pulps by neutral H2O2 bleaching is at least due to the formation and/or the stabilization of new H2O2-resistant chromophores via p-quinone or p-quinoid structures. It is also suggested that the quinoid structures are formed from color-contributing carbonyl groups in the reaction of lignin with H2O2.