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input, although in the case of typically exothermic hydrogenations, energy removal is more important than energy addition. 2) A key point that arises if one examines the recent literature is that one does not need to create a single phase (of SCF, substrate and hydrogen) to create a situation where transport limitations can be elim- inated [65,67,68]. For example, one can attain ki- netic control over the reaction simply by ensuring that a significant amount of CO2 is present in the liquid phase (maintaining a gas phase of CO2 /H2 ). Here the CO2 functions as a diluent (and viscosity reducer) that enhances the solubility of hydrogen in the lower phase. The enhanced hydrogen solubility more than makes up for the dilution effect from the CO2. While elimination of the resistance owing to transport of H2 into the liquid phase does not by definition create kinetic control over the reaction (resistances owing to diffusion to and within the catalyst also exist), the previous work has shown that the solubility of H2 in the liquid is typically the limiting factor. The use of CO2 as the ‘H2 solubility enhancing diluent’ could have broad ramifications on the practicality for conducting hydrogenations in supercritical fluids, in that it could make the use of benign (and non-flammable) CO2 more viable. For example, Harrod [61], as well as others, has employed propane as supercritical solvent solely to enable formation of a single phase with sub- strates whose solubility in CO2 is poor. It may be possible to both employ CO2 as the ‘diluent’ and eliminate transport limitations to reaction, render- ing the reaction more efficient while avoiding the flammability problems inherent to propane. The use of CO2 as ‘diluent’ could also render the an- thraquinone process described by Hancu [72] much more economically efficient as well as greener. This situation obviously best applies to liquids (or low melting solids) that are relatively non-volatile. The use of a two-phase (liquid–vapor) mixture can also help with heat transfer, as the boiling of the liquid can be employed to absorb excess heat. 3) Regarding asymmetric hydrogenations, the key green advantages to this work seem to be the elimination of organic solvent and improved se- lectivity. However, the results in the literature have not established that significantly greater se- lectivities are likely to be obtained solely through replacement of a conventional solvent with a su- percritical fluid (primarily CO2 ). Solvent polarity does impact selectivity, so it is possible that reac- tions will be identified where use of CO2 provides selectivity benefits. Most of the work on asym- metric hydrogenation has employed homogeneous catalysts; catalyst lifetime and recovery are unre- solved issues in this area. 4) The poisoning of noble metal catalysts via the for- mation of CO from CO2 and H2 could seriously impact the economic viability of hydrogenation processes conducted in carbon dioxide. Subra- maniam [28] has begun to elucidate the effect of various process parameters on this process; more research in this area is clearly merited. 2.7. Hydroformylation in CO2 Hydroformylation, the reaction of hydrogen and CO with an alkene to form aldehydes (Scheme 1), is prac- ticed industrially (the ‘oxo’ process) on an enormous scale using alkenes of various chain lengths [13]. In one form of the process, cobalt is fed to a reactor containing the oxo gas (H2 and CO) and the alkene, where a reaction takes place to form the cobalt hydro- carbonyl, the active catalyst species. Alkene is then converted to aldehyde in the liquid phase (the liquid is either a mixture of alkene substrate and alkane sol- vent or simply the alkene alone). The reaction takes place under rather severe conditions, 200–300 bar and temperatures between 410 and 450 K. The reaction produces the needed aldehyde(s), as well as residual alcohols and alkane. The useful products are recov- ered and the remainder combusted. The selectivity of the process is ≈85% to the aldehyde products. The catalyst is recovered as a cobalt ‘sludge’ and regener- ated/recycled. In a variation on the basic oxo process, a water soluble cobalt catalyst is employed which can be recovered via retention in the aqueous phase at the end of the process. Hence, the reaction is biphasic in nature—poor solubility of higher alkenes limits this E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 145 process to C2 –C4 alkenes. Scheme 1.PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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