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In theory, one could also conduct reactions across a CO2 –solid interface (other than heterogeneous catal- ysis) and a CO2–organic liquid interface, although lit- tle work has been reported to date. The one notable example here is the work by Eckert’s group [15], where a phase transfer catalyst (PTC) is used to pro- mote the displacement reaction of benzyl chloride with solid potassium bromide (no reaction occurs in the absence of the PTC). Brennecke [296] found that a simple esterification reaction conducted in a biphasic CO2/organic mixture, proceeded to a greater degree of conversion, possibly because the product partitioned preferentially to the upper, CO2-rich phase. In order to render any of these interfacial reactions practical, the thermodynamics of the system must be well under- stood. Clearly, the extent to which reactants, products, byproducts and solvents partition between the phases will determine the rate of reaction and the ability to re- cover both products and catalysts. In the case of ionic liquids, data and or models on the pVT and mixture behavior is entirely lacking and hence, partitioning be- havior must still be determined experimentally. 5. Formation of fine particles using carbon dioxide The controlled formation of particles (or powders) is important to several disparate industries, including those that manufacture pigments, pharmaceuticals and catalysts. Needless to say, these diverse applications mandate a diverse set of specifications for the produc- tion of such particles. Not surprisingly, supercritical fluids (and carbon dioxide in particular) have made in- roads into particle production to varying degrees, with penetration more significant in some industries versus others. In particular, the benign properties of carbon dioxide (vis-à-vis intimate contact with humans) have created substantial interest within the pharmaceutical production community for use of CO2 in the genera- tion of therapeutic particulate products. In some cases, the use of CO2 is proposed to supplant the use of or- ganic solvents, and hence such a process could rightly be termed green processing. In other cases, the use of CO2 (plus auxiliaries, as will be described below) might actually be less ‘green’ than a current process, but the characteristics of the product are superior, pro- viding a performance rather than an environmental ad- vantage. Further, because regulatory approval on new products or processes (in the pharmaceutical indus- try) can require years to obtain, the industrial impact of CO2 processing of pharmaceutical powders make not occur for some time (if at all, naturally). However, recent industrial investment (by entities in the phar- maceutical industry) in supercritical fluid technology suggests that the level of interest remains high. 5.1. Production of particles using carbon dioxide: RESS The earliest particle formation process using CO2 as the solvent is probably the oft-cited paper by Han- nay and Hogarth in the 19th century, where depressur- ization of a CO2-based solution created a precipitate ‘like snow’ (see Ref. [1] for description). During the 1980s, researchers at Battelle’s Pacific Northwest Laboratories created the rapid expansion of supercrit- ical solution (RESS) process, where a solution (here, of solid in supercritical alkane) was sprayed through a nozzle (where the outlet was at atmospheric pres- sure), creating fine particles [297]. Other researchers have explored the use of RESS to form particles since then, both from an experimental and theoretical standpoint [298]. As mentioned previously, CO2 is not a particularly powerful solvent and hence, many of the solutes one might like to process using RESS require very high pressures (500 bar and above) to dissolve even small quantities of material—high CO2 throughput will be needed to produce relatively small amounts of particles. The high CO2 throughput (with its associated costs, capital and operating) has effec- tively inhibited the use of RESS on a commercial ba- sis. This has rendered RESS generally less interesting than some competing CO2 -based particle formation technologies; these will be described below. The most successful (from a developmental, if not yet truly commercial point of view) particle forming processes are those that have taken what is known about CO2 ’s thermophysical properties and applied these characteristics strategically. For example, as has been mentioned previously, it is well known that CO2 is a rather feeble solvent—while problematic when attempting to use CO2 in a RESS process, this characteristic is quite useful when CO2 is employed as a non-solvent to induce precipitation of a solute from organic solvent. Further, whereas high pres- sure is required to create dilute solutions of large E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 177PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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