Oxidation of lignin

Guaiacols. Cresote, obtained from the pyrolysis of beechwood, and its active principles guaiacol [90-05-1] (1) and cresol [93-51-6] (2) have long been used ia expectorant mixtures. The compounds are usually classed as direct-acting or stimulant expectorants, but their mechanisms of action have not been well studied. Cresol is obtained by the Clemmensen reduction of vanillin (3), whereas guaiacol can be prepared by a number of methods including the mercuric oxide oxidation of lignin (qv) (4), the ziac chloride reduction of acetovanillone (5), and the diazotization and hydrolysis of o-anisidine (6). 

Rapid Micromethod for Alkaline Nitrobenzene Oxidation of Lignin... 

The greater rate of oxidation of lignin than of cellulose in the temperature range of 150°C to 250°C with its dominating effect on wood has also been pointed out in various papers (18,19). 

LiP catalyzes the oxidation of a low-molecular-weight redox mediator, veratryl alcohol, which in mrn mediates one-electron oxidation of lignin to generate aryl cation radicals [100]. The radicals facilitate a wide variety of reactions such as carbon-carbon cleavage, hydroxylation, demethylation, and so on. Dezotti et al. [101] reported enzymatic removal of color from extraction stage effluents using lignin and horseradish peroxidases immobilized on an activated silica gel support. 

Table II. Lignin Monomer Composition, Obtained by Nitrobenzene Oxidation of Lignin from Normal and Brittle Fescue Grown at Lusignan (Same as in Table I)...

Products from the direct oxidation of lignin that give information about its structure include acetic, oxalic, and succinic acids, vanillin, vanillic acid, and dehydrodivanillin. Regarding lignin constitution, they are of only minor interest, but the yields in which they are obtained are significant. For vanillin and its derivatives the total yield is about 33% of spruce lignin (24, 36, 37, 38). Two extraordinary products of this direct oxidation deserve special attention—i.e., benzenepentacarboxylic (45) and tricarballylic acids (47), (XXIX) and (XXX). They will be mentioned later in connection with lignenolide (XXVII). 

In addition, EPR spectrometry seems particularly suitable for studying the mechanism of biological and chemical oxidation of lignin, its precursors, and its degradation products. 

Figure 1. Oxidation of lignin model substances in aqueous alkali (0.2N NaOH) 1 mole equivalent of alkali, 70°C.

Oxidation of lignin can occur through chemical or biochemical means (25). Oxidation, especially in the presence of alkali, is a promising approach when the oxidizing agent can be air or oxygen obtained by a molecular sieve process (26,27). 

In studies on the oxidation of lignin that had alternately been methylated at the p-hy-droxyl and benzylic hydroxyl groups, Leopold (1952) concluded that methylation caused the low yield of vanillin obtained in the oxidation. As mentioned in Section 6.6.3, the replacement of the OH group by OMe seriously impedes C-C bond cleavage in the water reaction medium. 

Other compounds that we should consider are those derived from ligno-cellulose. Production of LMW components during oxidation of lignin, a biopolymer found only in terrestrial plants, may be quite high in fresh-waters where the input of ligno-cellulose and other allochthonous DOM is substantial (see Chapter 2). But we know little about LMW byproducts from ligno-cellulose degradation (Benner and Hodson, 1985 Moran and Hodson, 1989). 

Reeves, R.H., Pearl, I.H., "Reaction Products Formed Upon the Alkaline Peroxide Oxidation of Lignin-Related Model Compounds", Tappi, 1965, 48(2), 121. 

Figure 9.45 Eleven dominant phenolic monomers, yielded from the oxidation of lignin using the CuO oxidation method these compounds can be separated into four families p-hydroxyl, vanillyl (V), syringyl (S), and cinnamyl (C) phenols. (Modified from Hedges...

Fig. 3.6 Solvent access surfaces (colors represent electrostatic potentials) showing the exposed tryptophan residue (as yellow van der Waals spheres) involved in oxidation of lignin and other high redox-potential substrates by VP (a) and LiP (b). Lignin can be directly oxidized by VP at the tryptophan radical, while LiP requires the simultaneous presence of VA (synthesized by the fungus) acting as an enzyme-bound mediator [74]. Based on VP and LiP crystal structures (PDB 2BOQ and 1LLP, respectively)...

Fig. 8-7. Chlorine oxidation of lignin to muconic acid structures via o-quinone structures.

Vanillin. Catalytic oxidation of lignin has long been used as a source of vanillin. For the... 

Crestini, C., Pastorini, A., and Tagliatesta, P., Metalloporphyrins immobilized on motmorillonite as biomimetic catalysts in the oxidation of lignin model compounds. Molecular Catalysis A-Chemical 2004, 208 (1-2), 195-202. 

Alves, V, Capanema, E., Chen, C. L., and Gratzl, J., Comparative studies on oxidation of lignin model compounds with hydrogen peroxide using Mn(IV)-Me(3)TACN and Mn(IV)-Me4DTNE as catalyst. J Molecular Catalysis A-Chemical 2003, 206 (1-2), 37-51. 

Sippola, V., Krause, O., and Vuorinen, T., Oxidation of lignin model compounds with cobalt-sulphosalen catalyst in the presence and absence of carbohydrate model compound. Journal Of Wood Chemistry And Technology 2004,24 (4), 323-340. 

Bozell, J. J., Hoberg, J. O., and Dimmel, D. R., Heteropolyacid catalyzed oxidation of lignin and lignin models to benzoquinones. J Wood Chem Technol 2000, 20 (1), 19-41. 

Tarabanko, V E., Fomova, N. A., Kuznetsov, B. N., Ivanchenko, N. M., and Kudryashev, A. V, On the mechanism of vanillin formation in the catalytic-oxidation of lignin with oxygen. Reaction Kinetics Catalysis Lett 1995, 55 (1), 161-170. 

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