Varieties of Lignin

However, for practical approaches of effective utilization of lignins, it is important to pay attention to industrial lignins such as by-products of pulping processes 

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Chemistry ofDelig niiic tion. The chemistry of delignification is complex and, despite the extensive Hterature, not completely understood. A variety of lignin model compounds have been studied and the results compared with the observed behavior of lignin during pulping (1,10—12,16). 

The products of low temperature pyrolysis are char and low molecular components, see pathway 1 in Figure 53. At moderate temperature levels, the formation of a variety of lignin monomers (see Figure 49) occurs via pathway 2. And at high temperatures (> 500°C), fragmentation reactions take place, forming CO, H2, and reactive vapours. [67]... 

The phenolic hydroxyl contents of a variety of lignins are shown in Table 7.2.2. For untreated wood, the value is generally less than 15%. It is noteworthy that the phenolic hydroxyl contents reported for MWLs show considerable variability. A large part of the variability can be attributed to lack of uniformity in the efficiency of the milling process used in the MWL preparation (Adler 1977). 

To better understand the nature of the condensed structures in lignins, researchers examined the application of the Mannich reaction to probe these structures [40]. The Mannich reaction was applied to a large variety of lignin-model compounds to selectively and quantitatively block the available aromatic C3 and C5 positions [40]. Quantitative P NMR provided a determination of the units that bear no substituents at the aromatic C3 and/or C5 positions. 

H Kutsuki, A Enoki, and MH Gold. Riboflavin-Photosensitized Oxidative Degradation of a Variety of Lignin Model Compounds. Photochem and Photobiol 32 1-7, 1983. 

Cleavage of Carbon—Carbon Bonds. Under appropriate conditions, the propanoid side chain in lignin maybe mptured to form three-, two-, or one-carbon fragments. This carbon—carbon fragmentation occurs in a variety of laboratory treatments and technical processes such as in bleaching of chemical pulps with CI2, CIO2, and O2, in microbial degradation (15), and in photooxidation (16). 

Wood (qv) is arguably the oldest building material used by humans to constmct their dweUings. It is a natural product obtained from trees, used in both stmctural and decorative appHcations. The chemical composition of wood is largely cellulose (qv) and lignin (qv). Today there are a variety of composite or reconstituted wood products, such as plywood, particle board, wood fiber boards, and laminated stmctural beams, where small pieces of wood or wood fiber are combined with adhesives to make larger sheets or boards (see Laminates). 

In addition to carbonates and/or phosphates, a variety of substances have also been used to act as sludge conditioners —e.g. starches, lignins, polyacrylates etc. Other substances, such as nitrites and sulphates are claimed to have some value in the suppression of caustic cracking (see Section 5.1). 

Through these works, Wan has conclusively demonstrated that the photodehydration of hydroxybenzyl alcohols is a general reaction, and a wide variety of quinone methides can be photogenerated and detected using this method. Quinone methide photogeneration via this method has been shown to have importance in the photochemistry of Vitamin B641,42 and in model lignins 43... 

Quinone methides play an important role in lignification. They are produced directly, as intermediates, when lignin monomers, be they hydroxycinnamyl alcohols, hydroxy-cinnamaldehydes, or hydroxycinnamates, couple or cross-couple at their 8- positions. A variety of postcoupling quinone methide rearomatization reactions leads to an array of structures in the complex lignin polymer (Fig. 12.2). 

Plant metabolism can be separated into primary pathways that are found in all cells and deal with manipulating a uniform group of basic compounds, and secondary pathways that occur in specialized cells and produce a wide variety of unique compounds. The primary pathways deal with the metabolism of carbohydrates, lipids, proteins, and nucleic acids and act through the many-step reactions of glycolysis, the tricarboxylic acid cycle, the pentose phosphate shunt, and lipid, protein, and nucleic acid biosynthesis. In contrast, the secondary metabolites (e.g., terpenes, alkaloids, phenylpropanoids, lignin, flavonoids, coumarins, and related compounds) are produced by the shikimic, malonic, and mevalonic acid pathways, and the methylerythritol phosphate pathway (Fig. 3.1). This chapter concentrates on the synthesis and metabolism of phenolic compounds and on how the activities of these pathways and the compounds produced affect product quality. 

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