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142 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 to take advantage of the favorable properties of CO2 (with respect to hydrogenation) while avoiding some of the negative process issues by employing a gas–liquid rather than one-phase system. For exam- ple, it is known that H2 is poorly soluble in most organic liquids and hence it is expected that a hy- drogenation in organic solvent would be transport limited. If one knows the fundamental kinetic param- eters of the reaction, one should be able to predict at what [H2] to [substrate] ratio the reaction could be controlled by the underlying kinetics, and hence calculate the target [H2] for the reaction in the pres- ence of CO2. If the substrate is a liquid, one should be able to find conditions where a two-phase sys- tem (H2 –CO2 -substrate) exists, yet where substantial amounts of hydrogen are dissolved in the lower phase. As described previously, liquid–liquid phase diagrams of CO2 and larger molecules are typically asym- metric and hence operation at high concentrations of substrate is possible at relatively lower pressures. Further, the catalyst would be required to dissolve in a mixture of (primarily) substrate and CO2, suggest- ing that one might not have to fluorinate the catalyst to achieve solubility in the proper phase. Thus, by operating in the two-phase region, one could oper- ate at lower pressure with the original catalyst while also eliminating the need for the organic solvent and the transport resistance to reaction. Ideal substrates would be those that are relatively high in molecular weight, or are polar, yet are also liquids (or low melt- ing solids, where CO2 can depress the melting point [83]). Another interesting possibility would, in fact, in- volve functionalization of the catalyst (fluorination) to allow better solubility in CO2 while also operating in the two-phase regime. Here, the presence of the CO2 in the lower phase would serve to not only allow higher hydrogen concentrations but would also solubilize the catalyst. Upon removal of the CO2, the catalyst would precipitate, allowing recycle. This would present the CO2-based analogy to recent work by Gladysz et al. [84], where a fluorinated catalyst was developed that was insoluble in the reaction solvent, but dissolved upon heating. Hence, temperature was used as the re- versible trigger to allow catalyst use and recovery. Re- cently, it has indeed been shown that CO2 itself could also be employed as a reversible solvation trigger [85]. 2.4.3. Chemical rationale for homogeneous catalysis The final reason for conducting a homogeneous hydrogenation in CO2 is the premise that use of CO2 would alter the selectivity of the reaction in a positive way. Xiao, for example [86], examined the asymmet- ric hydrogenation of tiglic acid (2-methyl-2-butenoic acid) in CO2 using a ruthenium catalyst; ee’s (enan- tiomeric excess’s) in CO2 were essentially no better than those found for the same reaction in methanol. Tumas [87] examined the hydrogenation of dehy- droamino acids in CO2 using a cationic rhodium catalyst—here the fluorinated counteranion (3,5 bis(trifluoromethyl phenyl) borate (BARF) or triflate) enhanced solubility of the catalyst in CO2. Tumas found somewhat better ee’s for some substrates in CO2 versus hexane or methanol, but overall the per- formance of CO2 was comparable to that of the other organic solvents. Leitner [88] has used chiral iridium catalysts to perform the hydrogenation of imines in CO2. The catalysts were modified (using fluoroalkyl ponytails) to permit solubility in CO2. Enantiomeric excesses in CO2 were comparable to those found for the same reaction in dichloromethane, while rates were found to be much higher for some substrates in CO2 versus CH2 Cl2 . Recently Tumas [89] and Jessop [90] explored the use of biphasic mixtures of ionic liquids and car- bon dioxide to perform hydrogenations. Ionic liquids are salts (typically ammonium or phosphonium) that exhibit melting temperatures near or below room temperature. Ionic liquids behave as polar solvents, yet exhibit vanishingly small vapor pressures. In both the Tumas and Jessop studies, a CO2-insoluble cat- alyst was dissolved in the ionic liquid, which is then brought into contact with a mixture of CO2, substrate and hydrogen. As has been shown by Brennecke [91], ionic liquids absorb large amounts of CO2 (mole fractions >0.5) at pressures below 100 bar. Further, the ionic liquid does not measurably dissolve in CO2. Consequently, both Tumas and Jessop were able to conduct reactions in the ionic liquid at very high rates (the high CO2 swelling allowed for high H2 solubil- ity), where the product could be stripped from the ionic liquid into CO2 and the catalyst retained in the ionic liquid for recycle. Note that this is an analogy of the two-phase CO2 /H2 /substrate mixture mentioned above, where the high swelling of the lower phase byPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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