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CO2 eliminates transport limitations while two-phase operation permits use of moderate pressure. To date, the ionic liquids being explored as solvents are primarily based on imidazolium or pyridinium cations (some work has also been conducted on phos- phonium ILs). Whereas these ionic liquids (ILs) are proposed as benign solvents (owing to their near-zero vapor pressures), it must be remembered that the tox- icity and fate (in the environment) of such materials is currently not known. Brennecke et al. [92] have recently observed that the toxicity of the butyl im- idazolium hexafluorophosphate salts towards Daph- nia magna is similar to that shown by benzene or dichloromethane, where toxicity of the IL did not de- pend strongly on the nature of the anion. We expect more such studies in the future in this area. In ad- dition, because large-scale manufacturing processes for these solvents have yet to be established, the im- pact of such processes on the environment is also not known. In summary, the current crop of ILs may ul- timately be judged to be benign solvents or they may not. 2.4.4. Homogeneous hydrogenation and material synthesis Watkins has explored a novel means by which to apply homogeneous hydrogenation in CO2 to creation of metal nanoparticles and thin metal films. Watkins has found that certain metal complexes exhibit mil- limolar solubility in CO2 at pressures below 100 bar. Exposure of these complexes to hydrogen under mild conditions reduces the metal to the zero valent state, inducing nucleation of pure metal. Watkins first em- ployed this reaction to create small metal particles within polymer monoliths [93]. The complex is added to CO2, and this solution brought into contact with the polymer, which swells accordingly. Hydrogen is then introduced, which reduces the complex within the polymer, forming the nanoparticles. Recently, Nazem et al. [94] and Howdle et al. [95] have examined the impregnation of polymers with silver particle pre- cursors, performing the reduction in-situ to form the nanoparticle-impregnated material. In Howdle’s work, the polymers involved (polylactic acid and analogues) were found to resist attachment by bacteria owing to the antibacterial properties of silver. Use of nanoparti- cles allowed for useful antibacterial properties despite low loadings of silver. Watkins has further extended [96] this concept into the realm of green chemistry by adopting the pro- cess for use in creating thin metal films. In the micro- electronics industry, thin metal films can be generated on an inorganic substrate via vapor deposition, or via dip coating and reduction from an aqueous solution. The former can only be applied to volatile precursors, while the latter route produces very large volumes of metal-contaminated aqueous waste. Watkins has found that homogeneous hydrogenation of metal complexes in CO2 allows generation of conformal metal films on substrates with sub-micron features and that the only waste produced is a low molecular weight alkane byproduct. Small trenches and pits can be easily coated because CO2’s low interfacial tension permits wetting of even complex features. Watkins has demonstrated this concept with platinum, palladium and nickel—a recent paper [96a] shows that the concept can be ex- tended to copper as well. This technology is undeniably green, and could be readily applied to a variety of metal film applications, particularly if it can be demonstrated that metal depo- sition can be targeted (patterned). 2.4.5. How does one economically recover a catalyst and/or a product from CO2 ? Catalyst recycle is a more pressing need for su- percritical fluid processes (owing to the custom de- sign of CO2-philic ligands) than conventional analogs, while also presenting a more difficult problem. Homo- geneous catalysts are designed to provide enhanced selectivity and kinetic control of reactions, yet with- out effective recycle their added cost can prevent eco- nomical scale-up. Consequently, any green advantages gained through use of CO2 as a solvent are more than counteracted by the green and economic disadvan- tages incurred by use of a homogeneous catalyst. As such, investigations into means by which to recover homogeneous catalysts from CO2 play a vital role in enhancing the viability of green chemistry in CO2. For example, a collaboration between Tumas and the DeSimone group has investigated the design of metal catalysts that are tethered to crosslinked, polyfluoroacrylate polymer beads [97]. As noted ear- lier, fluoroacrylate polymers are the most CO2-philic materials yet identified; while the crosslinked ver- sions employed by Tumas cannot dissolve (they are, after all, crosslinked), they will swell in the presence E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 143PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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