Lignin oxidation reactions

For the kinetic study, an approach of the lignin oxidation reaction was made for a reaction of pseudo-first order, as shown in Eq. 3. 

The aromatic nature of lignin contrasts with the aliphatic stmcture of the carbohydrates and permits the selective use of electrophilic substitution reactions, eg, chlorination, sulfonation, or nitration. A portion of the phenoUc hydroxyl units, which are estimated to comprise 30 wt % of softwood lignin, are unsubstituted. In alkaline systems the ionized hydroxyl group is highly susceptible to oxidative reactions. 

Sodium chlorite has also been used for treatment and removal of toxic and odorous gases such as hydrogen sulfide and mercaptans. Chlorine dioxide from chlorite is also useful for microbial and slime control in paper mills and alkaline paper machine systems (164,165). The use of sodium chlorite in textile bleaching and stripping is well known. Cotton is not degraded by sodium chlorite because the oxidation reactions are specific for the hemiceUulose and lignin components of the fibers. 

Notes on the reagents. The permanganate reagent oxidizes and dissolves the lignin and any tannins. The 2-methylpropan-2-ol enhances the wetting of the ADF fibres by the permanganate. The acetic acid in the buffer solution is to neutralize the alkali formed in the oxidation reaction ... 

During the rotting of plant materials, different phenolic compounds can be identified (14, 16, 18, 28) as shown in Figure 1. These are aromatic compounds with side chains of one or three carbon atoms. They may be derived from the polymeric lignin by cleavage or oxidation reactions whereby the side chain is shortened. 

In all reactions, wood of white fir Abies concolor) has been used. For the alkaline nitrobenzene reactions, extractive-free —20+40 mesh heart-wood sawdust containing 28.0% Klason lignin was used. Sequential extraction of the original sawdust with alcohol-benzene, 95% ethanol, and hot water gave extractives amounting to 4.9, 0.5, and 1.2%, respectively. In the other oxidation reactions, nominal 5 -in. wood chips, commonly used in pulping procedures, were employed. The mixed sap wood-heart-wood chips contained 26.5% Klason lignin and sequential extractives of 3.3, 0.5, and 3.2%, respectively. 

The occurrence of polycarboxybenzenes in all oxidation reactions is of interest in studying lignin structure. These results do not indicate whether the basic structures were originally present in lignin or result from condensation. However, further improvement of the GLC technique to resolve all 11 polycarboxylic acids is desirable to determine which of the various isomers normally appear in the oxidations. 

Catechol melanin, a black pigment of plants, is a polymeric product formed by the oxidative polymerization of catechol. The formation route of catechol melanin (Eq. 5) is described as follows [33-37] At first, 3-(3, 4 -dihydroxyphe-nyl)-L-alanine (DOPA) is derived from tyrosine. It is oxidized to dopaquinone and forms dopachrome. 5,6-Dihydroxyindole is formed, accompanied by the elimination of C02. The oxidative coupling polymerization produces a melanin polymer whose primary structure contains 4,7-conjugated indole units, which exist as a three-dimensional irregular polymer similar to lignin. Multistep oxidation reactions and coupling reactions in the formation of catechol melanin are catalyzed by a copper enzyme such as tyrosinase. Tyrosinase is an oxidase con-... 

Steelink C (1966) Stable free radicals in lignin and lignin oxidation products In Marton J (ed) Lignin structure and reactions Adv Chem Ser 59 51-64... 

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