Glycerol Degradation

Studies show that the measured composition of the product mixture at constant temperature depended on the water density (Fig. 7.7). This was taken as an indication that these products could be formed by competing ionic and free-radical reaction pathways. Usually in gas-phase kinetics the product composition changes with temperature because of the different activation energies and, to a minor extent with pressure, mainly because of the concentration effect on bimolecular elementary reaction steps. In water, the drastic dependence on pressure is likely a consequence of the competition between reactions with different polarity. Free radical reaction rates (involving large free radicals beyond the RRKM high-pressure limit, see, for example, [25]) should decrease with pressure as a result 

For the modeling of the decomposition of glycerol in near- and supercritical water on the basis of elementary reactions, we assumed a combination of a free-radical thermal decomposition model and an acid-catalyzed ionic decomposition model [128]. This is supported by some studies in the hterature, which also include the 

3) This non-Arrhenius behavior of acid- or base-catalyzed reactions is mainly a consequence of the nonconsideration of the H concentration in the rate law, see also [57]. 

The radical part of the reaction mechanism was developed similar to other radical mechanisms in supercritical water [61, 62], which are only modifications of free-radical mechanisms at low pressures. The reaction classes considered are initiation reactions, P-scissions, hydrogen-transfer reactions, radical isomeriza-tions, radical additions, radical dehydratizations, radical substitutions, and radical-termination reactions. 

The ionic part of the reaction mechanism is totally based on assumptions. There is no literature known to us, in which similar reaction systems are 

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The temperature dependence of ionic reactions at constant pressure in near-and supercritical water shows a typical non-Arrhenius behavior. If free-radical reactions are important for the global rate, the Arrhenius plot may even become more comphcated by the superposition and interaction of the two mechanisms. Figure 7.8 shows the Arrhenius plot of the glycerol degradation in near- and supercritical water at 45 MPa. Here, the overlay of ionic and free-radical reactions is responsible for such an unusual shape. (For details see [128].)... 

Figure 7.8 Arrhenius plot of the global rate constant of glycerol degradation (first-order kinetics, 45MPa, water/ glycerol ratio 199) from experimental results and from model calculations. In addition, the ionic product for water at 45 MPa is given [31].

Today the lack of knowledge about solvent effects and of data about activation volumes of single reaction steps prevents the development of good models for reactions in supercritical fluids, especially supercritical water. Here, a lot of fundamental studies are necessary. In the future, when these data might become available, kinetic modeling will be a powerfully tool to understand and describe complex chemical reactions. This is demonstrated in the studies of glycerol degradation presented here. These models will be a helpful tool to project processes in supercritical fluids into industrial applications. 

Evidently, Cs-compounds are utilized via the dikinase reaction (at least with lactate and pyruvate as substrates), gluconeogenetic reactions, hexose monophosphate and diphosphate pathways. It is suggested that glycerol degradation proceeds through the formation of glycerol-3-phosphate and dihydroxyacetone (Stjemholm and Wood, 1963). In cells growing on... 

Fig. 5.8. Glycerol degradation pathways by lactic acid bacteria (Ribdreau-Gayon et al., 1975)...

The identity of the moiety (other than glycerol) esterified to the phosphoric group determines the specific phosphoHpid compound. The three most common phosphoHpids in commercial oils are phosphatidylcholine or lecithin [8002-45-5] (3a), phosphatidylethanolamine or cephalin [4537-76-2] (3b), and phosphatidjlinositol [28154-49-7] (3c). These materials are important constituents of plant and animal membranes. The phosphoHpid content of oils varies widely. Laurie oils, such as coconut and palm kernel, contain a few hundredths of a percent. Most oils contain 0.1 to 0.5%. Com and cottonseed oils contain almost 1% whereas soybean oil can vary from 1 to 3% phosphoHpid. Some phosphoHpids, such as dipaLmitoylphosphatidylcholine (R = R = palmitic R" = choline), form bilayer stmetures known as vesicles or Hposomes. The bdayer stmeture can microencapsulate solutes and transport them through systems where they would normally be degraded. This property allows their use in dmg deHvery systems (qv) (8). 

The sweet water from continuous and batch autoclave processes for splitting fats contains tittle or no mineral acids and salts and requires very tittle in the way of purification, as compared to spent lye from kettle soapmaking (9). The sweet water should be processed promptly after splitting to avoid degradation and loss of glycerol by fermentation. Any fatty acids that rise to the top of the sweet water are skimmed. A small amount of alkali is added to precipitate the dissolved fatty acids and neutralize the liquor. The alkaline liquor is then filtered and evaporated to an 88% cmde glycerol. Sweet water from modem noncatalytic, continuous hydrolysis may be evaporated to ca 88% without chemical treatment. 

The fatty acids released on triacylglycerol hydrolysis are transported to mitochondria and degraded to acetyl CoA, while the glycerol is carried to the liver for further metabolism. In the liver, glycerol is first phosphorylated by reaction with ATP. Oxidation by NAD+ then yields dihydroxyacetone phosphate (DHAP), which enters the carbohydrate metabolic pathway. We ll discuss this carbohydrate pathway in more detail in Section 29.5. 

LPA, i.e. monoacyl-glycerol-3-phosphate, can be formed and degraded by multiple metabolic pathways (Fig. 1). Depending on the precursor molecule and respective pathway, the fatty acid chain in LPA differs in length, degree of saturation and position (sn-1 or sn-2), which has an influence on biological activity. LPA... 

Wang Y, Kim YM, and Danger R. In vivo degradation characteristics of poly(glycerol sebacate). 

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