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144 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 of CO2 to 300% of the their initial volume. Because the metal-ligand construct is tethered to the beads, the catalysts can be readily recovered after the re- action and potentially re-used. Crooks [98] has also tried to address the catalyst recycle issue through design of dendrimer-supported metal catalysts; they have created Pd nanoparticles within dendrimers and employed these to support hydrogenation and other reactions. The outer shell of the dendrimers can be decorated with fluoroalkyl groups and hence, these macrocatalysts can be employed in CO2. Fi- nally, Keurentjes et al. [99] have recently published a method where catalysts are tethered to microporous inorganic supports for use in catalysis in CO2. The strategies employed by these three groups are extremely important, in that each has attempted to preserve the benefits of a homogeneous catalyst while co-opting the primary benefit of a heterogeneous catalyst—the ability to easily recover the valuable metal. For each case, then, some key issues remain to be discussed—does each ‘supported’ catalyst preserve the activity and selectivity of the soluble parent? Are the reactions kinetically controlled or diffusionally limited? How fast does the metal ‘leach’ from the supported catalysts? Eckert [15], Tumas [100] and others have examined the use of phase transitions to allow recycle of catalysts and other valuable components in a CO2 process. Eck- ert has found that addition of CO2 to a mixture of or- ganic and fluorocarbon solvents induces mixing, while removal of the CO2 (by depressurization) rapidly leads to complete phase separation. Consequently, one can employ CO2 as a reversible and benign ‘trigger’ to al- low a catalytic reaction while ultimately allowing seg- regation of the catalyst following reaction. Tumas has examined the use of a ‘pressure trigger’ to attempt to recover the catalyst from a CO2-continuous emulsion. At elevated pressure, a water-in-CO2 emulsion forms where the catalyst is localized in the aqueous micellar cores. Reduction of the pressure breaks the emulsion, leading to a distinct aqueous phase housing the cata- lyst (which could then be re-used). 2.5. Industrial activity: hydrogenation in CO2 Of the relatively small number of patents (1996– 2001) that directly cover hydrogenation in supercrit- ical fluids, two are worthy of special consideration. First, Harrod et al. [101] describe the hydrogenation of fatty acids in supercritical fluids, technology that has formed the basis for a small start-up company in Europe. Likewise, Poliakoff et al. [102] have de- scribed the hydrogenation of a variety of substances in supercritical fluids, technology that has formed the basis/motivation for a pilot scale plant con- structed for Thomas Swan Company (Durham, UK) by Chematur (Karlskoga, Sweden). It should be noted that Chematur, a company known for its supercritical water work (assets in both the US and Europe), has acquired the high pressure-related portion of Rauma (Finland), increasing their capabilities in design of processes capable of handling supercritical fluids. The Thomas Swan facility, which was scheduled to start up in September 2001 (and did in early 2002), will be able to generate 1000 tons per year of prod- ucts, including the results of hydrogenations and Friedel-Crafts acylations and alkylations conducted in supercritical fluids. At this time, it appears that the Swan facility will be used (at least in part) as a pilot-scale or semi-works facility to evaluate the use of supercritical fluids as solvents in various chemical reactions. 2.6. Summary: hydrogenation in CO2 In summary, hydrogenation in supercritical fluids has been extensively investigated over the past decade and it is clear that hydrogenation reactions can be suc- cessfully conducted in CO2 and other fluids. It is not always clear, however, what if any green advantages are obtained via operation in a supercritical solvent, as many authors do not draw comparisons to conven- tional processes. Nevertheless, some generalizations can be made: 1) The primary rationale for use of a supercritical solvent in hydrogenation reactions is the elimina- tion of transport limitations to reaction through enhancement of the solubility of hydrogen at the reaction locus. Hydrogen is poorly soluble in con- ventional hydrocarbon liquids and water and use of CO2 (and propane, to a lesser extent) as the solvent has been shown to enhance H2 solubility and hence improve the efficiency of the reaction. Attaining kinetic control over the reaction can lead to reduced byproduct formation and lower energyPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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