Aqueous-supercritical carbon dioxide oxidation

Antibody catalysts, role in environmentally benign synthesis of chemicals, 125-126 Aqueous-supercritical carbon dioxide medium, phase-transfer catalytic oxidation, 144-145 Arene cw-dihydrodiols, biocatalytic conversion of aromatics to optically pure synthons for pharmaceutical industry, 180-195... 

Like supercritical carbon dioxide, supercritical water is a very interesting substance that has strikingly different properties from those of liquid water. For example, recent experiments have shown that supercritical (superfluid) water can behave simultaneously as both a polar and a nonpolar solvent. While the reasons for this unusual behavior remain unclear, the practical value of this behavior is very clear It makes superfluid water a very useful reaction medium for a wide variety of substances. One extremely important application of this idea involves the environmentally sound destruction of industrial wastes. Most hazardous organic (nonpolar) substances can be dissolved in supercritical water and oxidized by dissolved 02 in a matter of minutes. The products of these reactions are water, carbon dioxide, and possibly simple acids (which result when halogen-containing compounds are reacted). Therefore, the aqueous mixture that results from the reaction often can be disposed of with little further treatment. In contrast to the incinerators used to destroy organic waste products, a supercritical water reactor is a closed system (has no emissions). 

Well ordered mesoporous silicate films were prepared in supercritical carbon dioxide.[218] In the synthesis in aqueous or alcoholic solution, film morphology of preorganized surfactants on substrate cannot be fully prescribed before silica-framework formation, because structure evolution is coincident with precursor condensation. The rapid and efficient preparation of mesostructured metal oxides by the in situ condensation of metal oxides within preformed nonionic surfactants can be done in supercritical CCU- The synthesis procedure is as follows. A copolymer template is prepared by spin-coating from a solution containing a suitable acid catalyst. Upon drying and annealing to induce microphase separation and enhance order, the acid partitions into the hydrophilic domain of the template. The template is then exposed to a solution of metal alkoxide in humidified supercritical C02. The precursor diffuses into the template and condenses selectively within the acidic hydrophilic domain of the copolymer to form the incipient metal oxide network. The templates did not go into the C02 phase because their solubility is very low. The alcohol by-product of alkoxide condensation is extracted rapidly from the film into the C02 phase, which promotes rapid and extensive network condensation. Because the template and the metal oxide network form in discrete steps, it is possible to pattern the template via lithography or to orient the copolymer domains before the formation of the metal oxide network. 

Shimizu et al. (2005) studied the dissolution of rare earths using supercritical carbon dioxide containing TBP complexes with nitric acid and water. By diluting the TBP HN03 H20 complex with anhydrate TBP, they succeeded in preventing the generation of aqueous droplets accompanied by the dissolution of metal oxides. More than 99% of yttrium and europium were dissolved after static operation for 120 min at 15 MPa and 60 °C. 

Supercritical carbon dioxide represents an inexpensive, environmentally benign alternative to conventional solvents for chemical synthesis. In this chapter, we delineate the range of reactions for which supercritical CO2 represents a potentially viable replacement solvent based on solubility considerations and describe the reactors and associated equipment used to explore catalytic and other synthetic reactions in this medium. Three examples of homogeneous catalytic reactions in supercritical CC are presented the copolymerization of CO2 with epoxides, ruthenium>mediated phase transfer oxidation of olefins in a supercritical COa/aqueous system, and the catalyic asymmetric hydrogenation of enamides. The first two classes of reactions proceed in supercritical CO2, but no improvement in reactivity over conventional solvents was observed. Hythogenation reactions, however, exhibit enantioselectivities superior to conventional solvents for several substrates. 

Supercritical carbon dioxide is widely used as an extracting agent, especially in the food industry. Contaminats are also extracted from aqueous solutions or solids by CO2 [3]. Aftac extraction, the hazardous compound is in C02 not in the water phase. Therefore, in an optimum incineration process, the compounds would be oxidized in the extracting agent CO2 as a second step of an integrated process [4,S]. 

Enzymatic oxidation in a supercritical fluid medium has also been demonstrated for the formation of cholest-4-ene-3-one from cholesterol in supercritical carbon dioxide by Randolph et aL [32]. Many cholesterol oxidases showed catalytic activity. That isolated from Streptomyces spp. gradually degraded, but gave reaction rates comparable with those sustained in aqueous systems whilst it was yet catalytic. That from Gloecysticum chrysocrea is chemically stable at 35" C and 100 bar, and yielded reaction rates 75 times faster than in water (5 x 10 phosphate, pH 7, and 5% v/v propanol [34]) which were further increased by the addition of aggregating cosolvents. 

Bhise s patent [53] describes a process for preparing and separating ethylene glycol from an aqueous solution of ethylene oxide using supercritical carbon dioxide. Supercritical fluid extraction (T < 100°C p < 295 bar) removed most of the ethylene oxide and a little water into carbon dioxide, which formed directly the feedstock for carbonation (catalysed by methyl triphenyl phosphon-ium iodide at 20-90°C). Sufficient water was then supplied for hydrolysis. Carbon dioxide, both that produced in the hydrolysis step and the original solvent, was partially vented for recycle, liberating the ethylene glycol product and the reaction catalyst. 

Tachiwaki, T., Takase, Y., Sugimoto, J., Oda, M., Kawanaka, S. An experimental study of supercritical fluid drying of Y-Ba-Cu oxides powder from aqueous alcohol suspension using carbon dioxide. Part. Sci. Technol. 16,109-124,1998... 

At temperatures above 500 °C more than 90 % of the organic bonded bromine was converted into bromide. By adding oxygen complete breakdown to carbon dioxide, bromide and water is possible. Under supercritical water oxidation conditions more than 99 % of the organic bonded bromine was found in the aqueous phase. A formation of bromine, hydrogen bromide and dioxines as in thermal decomposition was not observed. 

---