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140 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 anthraquinone with an inexpensive oligomer (such as a short chain polypropylene oxide or silicone) that would (a) not raise cost significantly; (b) transform the crystalline, high melting alkyl AQ to a low melting (or amorphous) derivatized AQ that would; (c) swell significantly with CO2 at moderate pressures (<100 bar); allowing (d) a low viscosity liquid phase with significant hydrogen solubility. This would render the oxidation process more tractable as well, as one could employ air (instead of O2), where the nitrogen would by and large remain in the upper gas phase. Hence, a CO2-based version of the AQ process could be rendered greener (through elimination of the solvent waste and energy load reduction) while not detracting from the economics. As noted in Section 1.5, a key future research issue that will impact heterogeneous hydrogenations in CO2 is the lifetime of the catalysts, particularly the widely used palladium catalysts. The literature contains ex- amples of successful hydrogenations over Pd in CO2 and also examples where the rapid formation of CO led quickly to catalyst poisoning and de-activation. Sub- ramaniam et al. has recently presented a rationale [28] for the seemingly contradictory results in the recent literature. They showed (using high pressure FT-IR) that CO forms very quickly (within minutes) on Pd in a mixture of CO2 and H2 and then over much longer times alters its mode of binding to reduce catalyst ac- tivity. Temperature is a key parameter in this process, where temperatures >343 K seem to greatly acceler- ate the process. Longer residence times (as would be experienced in batch reactors or CSTRs) also enhance the rate of poisoning. 2.4. Homogeneous hydrogenation in CO2 2.4.1. CO2-soluble catalyst design Clearly, the most pressing issue one must deal with to conduct a homogeneous hydrogenation in a supercritical fluid is that of catalyst and substrate sol- ubility. Carbon dioxide is without question the most popular solvent of those with a readily accessible (<370 K) critical temperature. However, CO2 is also a feeble solvent [74,75], whose inability to effectively solvate compounds of interest has greatly inhibited commercial development in the past. While many metal-containing catalysts exhibit low solubility in carbon dioxide at moderate pressures, simple metal carbonyls are known to be miscible with CO2 under relatively mild conditions [30,76] and as such have been used successfully to catalyze reactions in car- bon dioxide. In general, if the catalyst in question is relatively volatile liquid, chances are good that it will exhibit accessible (<500 bar) miscibility pressures in carbon dioxide. For the case of those metal catalysts whose ligand design renders them poorly soluble in CO2, work per- formed since 1990 [77–79] has identified a number of functional groups that are decidedly ‘CO2-philic’, such that derivatization of catalyst ligands with such groups enhances the solubility of catalysts in CO2 to the point where homogeneous hydrogenation reactions are feasible. The most widely used of the CO2-philic groups for catalyst ligand preparation are (CF2)’s, used in –(CH2)x(CF2)y–CF3 ‘ponytails’, where x ranges generally from 0 to 2 and y ranges from 0 to 6. The use of such groups creates a complex optimization prob- lem for those wishing to scale up such processes: • The solubility of the catalyst is sensitive to the length (and number) of the fluorinated ponytails— longer (or more) tails tends to lower the pressure re- quired to solubilize a given concentration of catalyst [14,80,81]; lower operating pressure means lower capital investment. At the same time, increasing the percentage of fluorine in the catalyst raises the cost owing both to synthetic cost and increased catalyst molecular weight. The presence of the fluorines in the ligands can affect the electronic environment of the metal, either enhancing or detracting from the efficiency of catalysis. • It has recently been shown that low molecular weight fluorinated sulfonate surfactants (PFOS and analogues, see p. 32) persist in the environment [48,82]. If restrictions associated with PFOS type materials are extended to cover other low molecu- lar weight fluorinated compounds, this would fur- ther raise the cost involved with use of fluorinated catalysts. Whereas conducting homogeneous hydrogenation in an alkane lessens problems owing to the weak sol- vent power of CO2, the added liability due to the flammability of the mixture has dampened enthusiasm for such reactions. As mentioned previously, one must be aware that running a hydrogenation reaction in CO2 can create byproducts owing to reaction of hydrogenPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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