Lignin oxidation processes

A large number of patents describe various procedures for the mainly continuous hydrolysis and oxidation processes, as weU as for the purification steps requited to obtain high grade vanillin. Lignin is degraded either with sodium hydroxide or with calcium hydroxide solution and simultaneously... 

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. 

These findings led to the proposition that the veratryl alcohol is degraded via the quinone intermediates (Figure 5) to CO2 through a series of transformations involving lignin peroxidase, perhydroxy radicals and the NADP-dependent aryl alcohol oxidoreductase. Veratraldehyde, the major product of lignin peroxidase catalyzed veratryl alcohol oxidation, is rapidly reduced back to veratryl alcohol it is the further metabolism of the side products of the oxidative process, viz. the quinones and lactones, that drives the overall transformation towards completion (34). 

Following comparison to sound wood (Figs. 5A and 5B), it was clearly evident that degraded wood had undergone oxidation. This oxidative process could take place on both polysaccharides and lignin (22,23), and could therefore represent a general degradation mechanism of the wood cell wall polymers. 

It has been proposed that quinonoid structures 8) contribute to the color of kraft lignin. If quinonoid groups are present, they must have been formed in an oxidative process whose nature is unknown. 

Elucidation of the Mn coordination sphere allows for further mechanistic insights into the Mn oxidation process specifically the electron transfer pathway from Mn11 to the acceptor (delocalized porphyrin % radical or the Fe center) may take place via the heme propionate ligand [52]. Upon further chelation a (neutral) Mnm complex may diffuse outside the enzyme and into the lignin polymer. 

Blodig W, Smith AT, WA D et al (2001) Crystal structures of pristine and oxidatively processed lignin peroxidase expressed in Escherichia coli and of the W171F variant that eliminates the redox active tryptophan 171. Implications on the reaction mechanism. J Mol Biol 305 851-861... 

More selective catalytic processes leading to desired chemicals directly have to be developed for a midterm period of time. This means mainly oxidation processes in which products such as aromatic aldehydes and acids can be obtained. Examples of the conversion of genuine lignins to syringaldehyde, vanillin, or corresponding acids give yields around 10% [18]. For these reactions, mainly polyoxymetalate catalysts or simple metal salts were used [7]. 

Wu, G. X., Heitz, M., and Chomet, E., Improved alkaline oxidation process for the production of aldehydes (vanillin and syringaldehyde) from steam-explosion Hardwood Lignin. Industrial Eng Chem Res 1994, 33 (3),... 

As a broad generalization the cellulose provides impact resistance and tensile strength, the lignin provides stiffness, and both matrix polymers contribute to hardness and compressive strength. Hydrolytic and oxidative processes that depolymerize and solubilize the wood components will affect the wood properties, often to an extent far greater than might be expected from the limited initial reaction. 

PCA is widely used in complex biotechnological processes as the lignin oxidation. This tool makes possible the differentiation between samples of oxidized lignins. 

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