Oxidation in Supercritical Carbon Dioxide

Supercritical CO2 is a non-polar, aprotic solvent and promotes radical mechanisms in oxidation reactions, similar to liquid-phase oxidation. Thus, wall effects might occur as known, e.g. from olefin epoxidation with 02 or H202 which may decrease epoxide selectivities. The literature covers the synthesis of fine chemicals by oxidation either without catalysts (alkene epoxidation, cycloalkane oxidation, Baeyer-Villiger oxidation of aldehydes and ketones to esters ), or with homogeneous metal complex catalysts (epoxidation with porphyrins, salenes or carbonyls ). Also, the homogeneously catalysed oxidation of typical bulk chemicals like cyclohexane (with acetaldehyde as the sacrificial agent ), toluene (with O2, Co +/NaBr ) or the Wacker oxidation of 1-octene or styrene has been demonstrated. 

Heterogeneously catalysed oxidations, which are mostly carried out between 250 and 400 C, are difficult to reahse under typical SCCO2 conditions because pressures above 500 bar are needed to reach the critical density of CO2 and to take advantage of its liquid-like properties. Operation at lower, more viable pressures is inevitably linked with a feed dilution which lowers the space-time yield and makes the process less attractive. In this case, benefits may occur mostly from its gas-like transfer properties and only reactions that are hmited by mass transfer are a target for possible improvement. Consequently, the full potential of SCCO2 as a reaction medium is utilised only with catalysts that are active at moderate temperatures up to 150 C. 

The benefits of SCF reaction engineering are supposed to be highest for liquid-phase oxidation processes with mass transfer limitations for molecular oxygen. The reported results show some inconsistency with regard to a possible improvement by setting the SCF conditions. 

Partial oxidation of propane with air in CO2 up to 400 C and 113 bar, over supported metal oxide catalysts in a flow reactor, revealed an increase in total 

The continuous flow oxidation of propylene with O2 to propylene glycol in SCCO2 at 138 bar on a Cu/Cu20/Mn02 catalyst showed strong pressure dependence with a maximum selectivity of 95%. Catalyst dissolution and deactivation did not occur over a run time of 50 hours. However, the space-time yields were still one order of magnitude too low for scale-up.  

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Trofimov, T.I. Samsonov, M.D. Kulyako, Y.M. Myasoedov, B.F. Dissolution and extraction of actinide oxides in supercritical carbon dioxide containing the complex of tri-n-butylphosphate with nitric acid, C.R. Chimie 7 (2004) 1209-1213. 

Samsonov, M. D. Trofimov, T. I. Vinokurov, S. E. Lee, S. C. Myasoedov, B. F. Wai, C. M. Dissolution of actinide oxides in supercritical carbon dioxide modified with various organic ligands. In Proceedings of the International Solvent Extraction Conference 2002, ISEC 2002, eds. K. C. Sole, P. M. Cole, J. S. Preston, and D. J. Robinson, SAIMM, Johannesburg, South Africa, 2002, pp. 1187-1192. 

Hou Z, Theyssen N, Leitner W (2007) Palladium nanoparticles stabilised on PEG-modified silica as catalysts for the aerobic alcohol oxidation in supercritical carbon dioxide. Green Chem 9(2) 127-132... 

Caravati M, Grunwaldt J-D, Baiker A (2005) Benzyl alcohol oxidation in supercritical carbon dioxide spectroscopic insight into phase behaviour and reaction mechanism. Phys Chem Chem Phys 7(2) 278-285... 

Oakes RS, Clilford AA, Bartle KD, Petti MT, Rayner CM. Sulfur oxidation in supercritical carbon dioxide dramatic pressure dependant enhancement of diastereoselectivity for sulfoxidation of cysteine derivatives. Chem Commun 1999 247-248. 

Ehiring our investigations of the oxidation in supercritical carbon dioxide, water and carbon dioxide, more than 500 experiments were conducted in the temperature range of 300 -550°C, with reaction times between 10-50 seconds and with the model compounds methanol, ethanol, toluene, triethanolamine, formic acid, acetic acid, and formaldehyde. Usually the differences between reactions in supercritical watCT, supa-critical carbcMi dioxide and wet carbon dioxide are rath small but in some cases they are significant and interesting (to be published elsewhere, see also [5]). Complete conversion as in the case of SCWO can be reached in supercritical CO2 as well [5], so a coupling of extraction and oxidation is of into est for application. 

Oakes, R. S. Clifford, A. A. Bartle, K. D. Pett, M. T. Rayner, C. M. Sulfur Oxidation in Supercritical Carbon Dioxide Dramatic Pressure Dependent Enhancement of Diastereo-selectivity for Sulfoxidation of Cysteine Derivatives. Chem. Comm. 1999,247-248. 

Hou, Z., Theyssen, N. and Leitner, W. (2007). Palladium Nanoparticles Stabilised on PEG-Modified Sihca as Catalysts for the Aerobic Alcohol Oxidation in Supercritical Carbon Dioxide, Green Chem., 9, pp. 127—132. 

Mukhopadhyay, M. and Srinivas, P. (1996). Multicomponent solubilities of reactants and products of cyclohexane oxidation in supercritical carbon dioxide, Ind. Eng. Chem. Res., 35, pp. 4713-4717. 

Sahle-Demessie, E., Gonzalez, M., Enriquez, J., et al. (2000). Selective oxidation in supercritical carbon dioxide using clean oxidants, Ind Eng. Chem. Res., 39, pp. 4858-4864. 

Maayan, G., Ganchegui, B., Leitner, W, et al. (2006). Selective aerobic oxidation in supercritical carbon dioxide catalyzed by the H5PV2M010O40 polyoxometalate, Chem. Commuru, 21, pp. 2230-2232. 

ER Birnbaum, RM Le Lacheur, AC Horton, W Tumas. MetaUoporphyrin-catalyzed homogeneous oxidation in supercritical carbon dioxide. J Mol Cat A 139 11-24,... 

Karmee SK, Roosen C, Kohlmann C, Lutz S, Greiner L, Leitner W. Chemo-enzymatic cascade oxidation in supercritical carbon dioxide/water biphasic media. Green Chem. 2009 11 1052-1055. 

Cyclohexane oxidation has been studied in supercritical carbon dioxide medium for homogenizing the initial reaction mixture to produce cyclohexanone and cyclohex-anol as the chief reaction products. Conversions obtained are low compared to the liquid phase oxidation because of dilute concentrations of the reactants. Cyclohexanone is more selectively formed and favored by both pressure and temperature. Results suggest that the reaction in supercritical CO2 medium can be gready manipulated (Srinivas and Mukhopadhyay, 1994). 

Uncatalysed oxidation of cycloalkanes and alkylarenes by molecular oxygen with acetaldehyde as sacrificial co-reductant occurs efficiently in supercritical carbon dioxide under mild multiphase conditions.252... 

An efficient and convenient methodology for the aerobic oxidation of alcohols catalysed by sol-gel trapped perruthenate and promoted by an encapsulated ionic liquid in supercritical carbon dioxide solution has been reported. The reaction is highly selective and useful for substrates otherwise difficult to oxidize.263 A four-component system consisting of acetamido-TEMPO-Cu(C104)2-TMDP-DABCO has been developed for aerobic alcohol oxidation at room temperature. The catalytic system shows excellent selectivity towards the oxidation of benzylic and allylic alcohols and is not deactivated by heteroatom-containing (S, N) compounds. The use of DMSO as the reaction medium allows the catalysts to be recycled and reused for three runs with no significant loss of catalytic activity.264... 

Randolph, T. W. Blanch, H. W. Prasnitz, J. M. Enzyme-Catalyzed Oxidation of Cholesterol in Supercritical Carbon Dioxide. AIChE J. 1988, 34, 1354-1360. 

Hou Z, Theyssen N, Brinkmann A et al (2005) Biphasic aerobic oxidation of alcohols catalyzed by polyethylene glycol)-stabilized palladium nanoparticles in supercritical carbon dioxide. Angew Chem Int Ed 44(9) 1346—1349... 

Campestrini S, Carraro M, Ciriminna R et al (2005) A mechanistic study on alcohol oxidations with oxygen catalysed by TPAP-doped ormosils in supercritical carbon dioxide. Adv Synth Catal 347(6) 825-832... 

Wang X, Yang H, Feng B et al (2009) Functionalized polyethylene glycol)-stabilized palladium nanoparticles as an efficient catalyst for aerobic oxidation of alcohols in supercritical carbon dioxide/poly(ethylene glycol) biphasic solvent system. Catal Lett 132 34-40... 

The oxidation of hydrogen to hydrogen peroxide in supercritical carbon dioxide is a case in point Hancu, D., and Beckman, E. J. Green Chemistry, 2001, 3, 80. 

Of the 18 systems, some of which are unstable and must be generated in the reaction has been accomplished for at least 15, but not in all cases with a carbon-carbon double bond (the reaction also can be carried out with other double bonds ). Not all aUcenes undergo 1,3-dipolar addition equally well. The reaction is most successful for those that are good dienophUes in the Diels-Alder reaction (15-60). The addition is stereospecific and syn, and the mechanism is probably a one-step concerted process, as illustrated above, " largely controlled by Frontier Molecular Orbital considerations. " In-plane aromaticity has been invoked for these dipolar cycloadditions. " As expected for this type of mechanism, the rates do not vary much with changes in solvent, " although rate acceleration has been observed in ionic liquids. " Nitrile oxide cycloadditions have also been done in supercritical carbon dioxide. There are no simple rules... 

Oxidation. Oxidation of cyclohexene and norbomene with Mo(CO) -r-BuOOH in supercritical carbon dioxide gives vic-diols. Some alkenes (e.g., cyclooctene) afford epoxides but vinylnaphthalenes and cii-stilbene undergo oxidative cleavage to the aromatic aldehydes. 

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. 

Subra P, Castellani S, Ksibi H, Garrabos Y. Contribution to the determination of the solubility of p-carotene in supercritical carbon dioxide and nitrous oxide experimental data and modeling. Fluid Phase Equilibria 1997 131 269. 

A combination of sc carbon dioxide and light has been used in flow reactors to make some metal complexes not possible by other methods, for example, cyclopentadi-enylMn(CO)2(H2) and Cr(CO)s(ethylene).179 A man ganese hydride has been added to olefins in carbon dioxide.180 A zeolite in sc carbon dioxide selectively adsorbs 2,7-dimethylnaphthalene so that it can be separated from the desired 2,6-dimethylnaphthalene.181 The latter is oxidized to the dicarboxylic acid for conversion to a highermelting analogue of poly(ethylene terephthalate). A pillared clay made in supercritical carbon dioxide had a higher... 

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