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150 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 Scheme 5. medium. Regarding epoxidations, the direct genera- tion of propylene oxide from propylene would be the most significant ‘green’ advance to be made in this area, yet use of anything but oxygen (or air) as the oxidant is currently too expensive. Wacker chemistry (the oxidation of an alkene to a ketone using a PdCl2/CuCl2 catalyst) has also been examined using CO2 as the sole solvent. Li et al. [120] examined the oxidation of 1-octene in CO2 and found that operation in a mixture of CO2 and methanol led to higher selectivity to the methyl ketone than operation in either CO2 or methanol alone. Because the phase behavior of the system was not measured, the effects reported by Li cannot be completely explained. For example, while it is known that the PdCl2 and CuCl2 catalysts are soluble in methanol and poorly soluble in CO2, it is not clear as to their solubility in the mixture of MeOH and CO2. Li et al. also examined the oxidation of acrylic acid to the analogous 3,3 dialkoxy propionate using a similar catalyst system. In early 2002, Subramaniam et al. [121] published the results of an interesting study on homogeneous ox- idation performed in mixtures of carbon dioxide and conventional organic solvents (primarily acetonitrile). This study showed vividly that one can use judicious mixtures of solvent and CO2 to truly optimize the per- formance of a reaction. Here, use of CO2 alone ne- cessitated high pressures (hundreds of bar to dissolve both substrate and catalyst) and the low polarity of pure CO2 provided a non-ideal medium for the cata- lyst. On the other hand, while use of pure acetonitrile allowed operation at one atmosphere and provided the catalyst with a suitably polar environment, the solubil- ity of oxygen in the liquid phase was poor. When the right mixture of acetonitrile/CO2 was employed, the catalyst activity was high, and all components (oxy- gen, substrate and catalyst) dissolved at pressures of only tens of bar. Study of more examples of this type of system may yield processes that are both greener than current methods and economically practical, particu- larly if one can ultimately eliminate the need for the organic solvent and work with neat liquid substrates. 2.8.2. Industrial activity: oxidations in supercritical fluids In a 1997 patent [122], Pitchai et al. (ARCO Chem- ical Co., now Lyondell Chemical Co., a leading pro- ducer of propylene oxide via the co-product process) a hydroperoxide (such as t-BuOOH). Liquid CO2 will dissolve in molar quantities in water, forming carbonic acid. Beckman and Eckert each showed that a biphasic CO2 /H2 O2 /water mixture will also form percarbonate (upon the addition of appropriate amounts of base) and hence will epoxidize olefins, here cyclohexene oxide (Scheme 5). Addition of base is critical for achieving high ac- tivity. In general, sodium hydroxide is more effective than bicarbonate (likely as it raises the pH more effec- tively). Given Beckman’s results, it would appear that percarbonate is formed both via reaction of H2O2 and bicarbonate and via direct reaction between CO2 and H2 O2 . Further, because the reaction is biphasic, addi- tion of a CO2-philic surfactant enhanced the rate dra- matically, as would be expected. Likewise, addition of a phase transfer catalyst (a tetraalkyl ammonium halide) also enhanced the rate. These epoxidations are intriguing as they employ only water, CO2 and H2O2 as reactants and a catalytic amount of base. The primary drawback to this route is that hydrogen peroxide, although usually considered a commodity chemical, is currently too expensive to use as an oxidant to produce PO. A number of other researchers have examined the oxidation of alkenes to epoxides using a variety of chemical strategies in carbon dioxide. Birnbaum [117], for example, employed a fluorinated (and hence CO2 -soluble) porphyrin catalyst to oxidize cyclohex- ene to cyclohexene oxide. Not surprisingly, Birnbaum found that the selectivity was significantly higher in CO2 than in organic solvent, as operation in CO2 does not produce solvent oxidation products. Loeker [118] examined the oxidation of olefins in CO2 us- ing oxygen and aldehydes as sacrificial co-oxidants. Here the reaction was heterogeneous, although it was the steel walls of the high-pressure reaction vessel that were employed as the catalyst. Finally, Haas and Kolis [119] found that one could readily oxi- dize olefins in CO2 using t-butyl hydroperoxide and a soluble Mo(CO)6 catalyst as an oxygen transferPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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