Supercritical oxygen solubility

The process employs the supercritical fluid carbon dioxide as a solvent. When a compound (in this case carbon dioxide) is subjected to temperatures and pressures above its critical point (31°C, 7.4 MPa, respectively), it exhibits properties that differ from both the liquid and vapor phases. Polar bonding between molecules essentially stops. Some organic compounds that are normally insoluble become completely soluble (miscible in all proportions) in supercritical fluids. Supercritical carbon dioxide sustains combustion and oxidation reactions because it mixes well with oxygen and with nonpolar organic compounds. 

In the wet oxidation process, materials partially or completely dissolve into a homogeneous, condensed-phase mixture of oxygen and water, and chemical reactions between the material and oxygen take place in the bulk water phase. This condensed-phase makes wet oxidation an ideal process to transform materials which would otherwise be non-soluble in water to a harmless mixture of carbon dioxide and water. Since oxidation reactions are also exothermic, the high thermal mass of supercritical water makes this reaction medium better suited for thermal control, reactor stability, and heat dissipation. The purpose of this research was to establish a new method for selectively oxidizing waste hydrocarbons into new and reusable products. 

In their test system, the researchers used the ionic liquid l-butyl-3-methylimidazol-ium hexafluorophosphate (bmim)(PF6), which is stable in the presence of oxygen and water, with naphthalene as a low-volatility model solute. Spectroscopic analysis revealed quantitative recovery of the solute in the supercritical CO2 extract with no contamination from the ionic liquid. They found that CO2 is highly soluble in (bmim)(PF6) reaching a mole fraction of 0.6 at 8 MPa, yet the two phases are not completely miscible. The phase behavior of the ionic liquid-C02 system resembles that of a cross-linked polymer-solvent system (Moerkerke et al., 1998), even though... 

SCFs offer a nonaqueous environment which can be desirable for enzymatic catalysis of lipophilic substrates. The lipophilic substance cholesterol is 2 to 3 orders of magnitude more soluble in CX>2-cosolvent blends than in waterQ). In CO2 based blends, it may be oxidized to cholest-4en-3one, a precursor for pharmaceutical production using an immobilized enzyim(22). The enzyme polyphenol oxidase has been found to be catalytically active in supercritical CO2 and fluoroform (22). The purpose of using a SCF is that it is miscible with one of the reactants-oxygen. Lipase may be used to catalyze the hydrolysis and interesterification of triglycerides in supercritical OO2 without severe loss of activity(24). These reactions could be integrated with SCF separations for product recovery. 

In addition to the design of the solubility properties, the reactivity of organome-tallic species toward CO2 [13] (and many other potential supercritical reaction media) must be considered as important criteria for the choice of the catalyst. For example, the bisallyl ruthenium complex shown in Table 1 cannot be utilized as a precursor for ring-opening metathesis polymerization (ROMP) in SCCO2, because the insertion of CO2 into the Ru-allyl bond prevents the initiation mechanism [14]. Metal-mediated oxygen transfer to form CO and phosphine oxide was found to lead to deactivation of the [Ni(cod)2]/PMe3 (cod = 1,5-m-cycloocta-diene) catalyst system [15]. On the other hand, the reactivity of CO2 with metal... 

Additional benefits of the supercritical carbon dioxide extraction are 1) oxygen-free system prevents oxidation of the extract 2) low temperature minimizes the thermal degradation 3) microbes or their spores are not soluble, hence aseptic extracts are obtained and 4) solvent-free extract is obtained because CO2 is gas at ambient and is not retained by the extract. 

The environmentally benign nature of carbon dioxide comes from its very stable molecular bonds, which in turn do not provide high polarity. In fact, a carbon dioxide molecule has only a weak quadrupole moment, due to minor charge separation on oxygen and carbon atoms. Hence, the molecular interaction with most polar and heavy substances of interest is minor, providing only a weak solvent power. If needed, a small amount of cosolvent (also termed as entrainer or modifier) is added to enhance polarity and affinity with solutes. In many applications, however, the design limitation is the solubility of the substance in supercritical carbon dioxide. Therefore, the solubility data are essential both for the initial feasibihty study and final process design. 

The very good solubility of organic compounds in supercritical water opens up the possibility of extracting hazardous compounds e.g. from soil. Such an extraction was combined with a SCWO process, in which the oxygen is produced elec-trochemically [122, 123]. 

Several classes of chemical reactions are possible in microemulsions formed in supercritical fluids. Catalytic hydrogenation or oxidation reactions using molecular hydrogen and oxygen as reactants are particularly well suited for these studies as both are very soluble in supercritical fluid solvents. A potentially useful role for these oxidation reactions is the destruction of hazardous chemical wastes or contaminated materials. 

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