Lignin formation required

Woody plants can synthesize anywhere from 15-30% of all biomass as lignin. Hence, equivalent amounts of phenylalanine are required at some point, i.e., during lignin formation, large amounts of ammonia are recycled as a consequence of PAL activity. While plants have no serious problems in obtaining glucose as a carbon source, supplied abundantly through photosynthesis,... 

The second class comprises conventional solids, defined by a chemical formula, but whose property requirements are very minimal. In this class we included lignin, cellulose, mannan, galactan, xylan, arabinan and the biomass. The properties specified in the database include molecular weight, heat of formation, solid molar volume, and solid heat capacity. 

The acetylated lignin, dissolved in 1.0 ml of dioxane containing 5 mg of an internal standard, is treated with 1.0ml of a dioxane-pyrrolidine (1 1, v/v) solution which initiates the aminolysis reaction. The rate of the 1-acetylpyrrolidine formation is followed by periodically injecting 1-2/d of the reaction mixture into the gas chromatograph. The initial rapid phase is generally complete within 20 min, but a total of 60min is required to complete the deacetylation process. 

The rate of 1-acetylpyrrolidine formation is followed as in the case of acetylated lignin, but the overall process is much slower and a 6-7-h reaction period may be required as illustrated in Fig. 7.2.1A for Norway spruce wood meal. 

In contrast, the periodate oxidation method is a relatively simple process and is unaffected by the presence of carbohydrates. Construction of a kinetic curve is not necessary in the periodate oxidation procedure, which requires only determination of the maximum amount of methanol formation for the calculation of phenolic hydroxyl content. Thus, for routine analysis, the periodate oxidation method may require only measurement of the amount of methanol liberated after 2 and 3 days reaction for isolated lignin and wood samples, respectively (Gierer et al. 1964, Yang and Goring 1980). 

The formation of uniform network polymers from lignin requires that phase separation prior to gel formation remain limited. The process of component demixing is controlled by both an enthalpic and an entropic contribution to Gibbs free energy. Whereas, the entropic factor requires the use of low molecular weight fractions for uniform gel formation, the enthalpic parameter necessitates chemical modification. The type of modification needed depends on the medium in which gel formation is performed. Examples of improving the solubility and compatibility with various reaction media are cited. 

Control of Precipitate Formation. The problem with the character of the precipitated HML proved to be easily resolved with the particular lignin under study. When the addition of the glacial acetic acid was carried out at temperatures only as high as 50 °C, the reaction product was granular and did not retain large quantities of water in a semisolid mass. However, this condition required a redispersion of the solid after the oxalic acid catalyst had been added. This could be accomplished with mixers designed to homogenize and disperse materials in liquids. 

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