Oxidation, lignin

Lignox [Lignin oxidation] A pulp-bleaching process using hydrogen peroxide as the oxidant, and a chelating agent. Developed in Sweden in 1990 by Eka Nobel. See also Acetox. 

Stable Free Radicals in Lignin and Lignin Oxidation Products... 

Carbon-0-carbon coupling might, of course, also be considered in the case of lignin oxidation. Because there is no accurate method for determining biphenyl structures in lignins, the extent of such couplings could not be estimated. 

Hedges, J.I. and Mann, D.C. (1979) The characterization of plant tissues by their lignin oxidation products. Geochim. Cosmochim. Acta 43, 1809-1818. 

Fig. 4-1. Examples of classical methods indicating a phenylpropanoid structure of lignin. (A) Permanganate oxidation (methylated spruce lignin) affords veratric acid (3,4-dimethoxybenzoic acid) (1) in a yield of 8% and minor amounts of isohemipinic (4,5-dimethoxyisophtalic acid) (2) and dehydrodiveratric (3) acids. The formation of isohemipinic acid supports the occurrence of condensed structures (e.g., /3-5 or y-5). (B) Nitrobenzene oxidation of softwoods in alkali results in the formation of vanillin (4-hydroxy-3-methoxybenzaldehyde) (4) (about 25% of lignin). Oxidation of hardwoods and grasses results, respectively, in syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde) (5) and p-hydroxybenzaldehyde (6). (C) Hydrogenolysis yields pro-pylcyclohexane derivatives (7). (D) Ethanolysis yields so-called Hibbert ketones (8,9,10, and 11).

Fargues, C., Mathias, A., and Rodrigues, A., Kinetics of vanillin production from om Krafl lignin oxidation. Industrial Eng Chem Res 1996, 35 (1), 28-36. 

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... 

Pepper JM, Casselman BW, Karapally JC (1967) Lignin oxidation Preferential use of cupric oxide Can J Chem 45 3009-3012... 

Larsson S, Miksche GE (1967) Gas chromatographic analysis of lignin oxidation products The diphenyl ether linkage in lignin Acta Chem Scand 21 1970-1971 Larsson S, Miksche GE (1969a) Gaschromatografische Analyse von Ligmnoxydationsprodukten... 

Although Chang and Allan (1) reviewed essentially all the available lignin oxidation work, they observed that most of the research was not mechanistic in nature. Thus, they noted that their proposed mechanisms were based on limited data and were presumed to require possible further modifications. 

Pizzi and colleagues (Chapter 7) have cleverly taken advantage of the special circumstances offered in the kraft lignins from bagasse to develop both cold-setting wood laminating and particleboard adhesives. Lignin oxidations... 

B) atomic carbon nitrogen atom ratio, C/N)a, versus cinnamyfvanillyl phenol ratio (CA ) of the lignin oxidation products from the same samples. 

Alkaline cupric oxidation of coals produced phenolic acids (p-hydroxy and 3,4 hydroxy-benzoic acids, 4 hydroxy l,2 and 4-hydroxy l,3 benzenedicarboxylic acids), which are known as characteristic lignin oxidation products. The results indicate that lignin-like polymers are incorporated into macromolecules of coals and still are identifiable in lower rank coals. 

Many oxidative degradations also have been carried out to break coal down into simpler species (10, 18-21) however, isolation and identification of phenols such as p-hydroxybenzene (I), vanillic (II), and syringic (III) derivatives, which are considered as characteristic lignin oxidation products (1, 4, 17, 22, 23), have not yet been confirmed definitely. 

In general, commonly used oxidants destroy phenolic rings or give complex products (25, 26). Some of the oxidants such as nitrobenzene produce reaction byproducts that may interfere with the analysis of the oxidation products (23, 27, 28). To obtain lignin oxidation products from coals, we resorted to the alkaline cupric oxide oxidation method which has been applied successfully to analysis of lignins in plants (23), fulvic and humic acids (24, 27), and land-derived marine sediments (23). 

As shown in our previous communication (32), most informative was the identification of large amounts of p-hydroxy- and 3,4-dihydroxyben-zoic acids in the oxidation products of low-rank coals (these coals are listed as Samples I and 2 in this chapter). These are regarded as lignin oxidation products. In this study we have found that, while no o- and m-hydroxybenzoic acids were found in the oxidation products of Samples I and 2, all three isomers were identified in small amounts in other rank bituminous coals (Samples 3, 5, 6). From Sample 3, 3,4-dihydroxyben-zoic acid also was isolated in small amounts, however, this compound was not detected in the oxidation product of other bituminous coals (Samples 4, 5, and 6). 

Although one cannot rule out the possibility that plant phenols (3, 22, 35) could be one of the important precursors for coal formation, the present work shows that lignin-like polymers have been incorporated into the macromolecules of coals, and are still identifiable in lower-rank coals. Evidence for this is the identification of p-hydroxy- and 3,4-dihydroxy-benzoic acids, and hydroxybenzenedicarboxylic acids which are known lignin oxidation products. 

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