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124 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 byproducts [13]. Oxidation reactions in CO2 have con- sequently been investigated extensively over the past decade (see Section 2.8). Because CO2 is inert towards oxidation and is also non-flammable, CO2 is one of the very few organic solvents that could be considered for the direct reac- tion of hydrogen and oxygen to form hydrogen per- oxide [14]. This process has been under investigation for over two decades, yet traditional organic solvents are not sufficiently inert/safe, while water exhibits pro- ductivity disadvantages. 1.4.2. CO2 is benign and hence cross-contamination of the other phase during liquid–liquid extraction is not really contamination There are a number of large-scale chemical processes that employ biphasic (water–organic) mixtures—H2 O2 production and hydroformylation of low molecular weight alkenes are but two examples [13]. In any contact between aqueous and organic phases, some cross-contamination is inevitable. The aqueous phase will require subsequent remediation to eliminate the organic contamination, while the or- ganic phase may require drying to allow further use in the process. While CO2 will ‘contaminate’ an aqueous phase upon contact in a process, a mixture of CO2 and water clearly does not require remediation (the CO2 phase may, of course, require drying for further use). Consequently, CO2 exhibits a particular advantage in processes where a biphasic reaction or liquid–liquid extraction against water is required. Eckert et al. [15] have, for example, investigated the use of phase transfer catalysts in CO2/water mixtures. Further, the coffee decaffeination process employs a liquid–liquid extraction between CO2 and water to recover the extracted caffeine [16]. 1.4.3. CO2 is an aprotic solvent Clearly, CO2 can be employed without penalty in cases where labile protons could interfere with the reaction. 1.4.4. CO2 is generally immune to free radical chemistry Because carbon dioxide does not support chain transfer to solvent during free-radically initiated poly- merization, it is an ideal solvent for use in such polymerizations, despite the fact that it is typically a poor solvent for high molecular weight polymers. In chain transfer, a growing chain (with a terminal rad- ical) abstracts a hydrogen from a solvent molecule, terminating the first chain. The solvent-based radi- cal may or may not support further initiation, and hence chain transfer to solvent can lead to diminished molecular weight and diminished polymerization rate. Research conducted during the 1990s (primarily by DeSimone et al.) showed that CO2 does not support chain transfer, as it is inert towards polymer-based free radicals [17]. Other researchers have examined small-molecule free radical chemistry in CO2 to be viable as well [18]. Indeed, it is likely that most of the polymerizations currently conducted by DuPont in its semi-works facility are precipitation polymerizations, where the improved control over molecular weight and the enhanced safety inherent to use of TFE/CO2 mixtures (see Section 1.2) more than makes up for any difficulties caused by polymer precipitation during the reactions. 1.4.5. CO2 is miscible with gases in all proportions above 304 ◦K The rate of most processes where a gas reacts with a liquid is limited by the rate at which the gas diffuses to the active site (either within a catalyst particle or simply to the liquid reactant). Gases, such as hydro- gen and oxygen, are poorly soluble in organic liquids and water and hence in many two- and three-phase reactors, the rate is limited specifically by the rate at which the gas diffuses across the gas–liquid interface. Although phase separation envelopes exist with gases at lower temperatures, liquid CO2 can absorb much higher quantities of H2 or O2 than typical organic solvents or water [19]. Hence, one can elim- inate the dependence of the rate on gas transport into the liquid phase by employing CO2. Although conventional wisdom might claim that this effect is achieved only through creation of a single phase (of CO2, gaseous reactant and liquid substrate), recent work in the literature shows that one can achieve high gas solubility and hence high rate while remaining two-phase (see Section 2). It should be remembered that CO2 will exhibit total miscibility with gases >304 K only if those gases also exhibit critical temperatures 304 K. This includes commonly used reactant gases such as H2, O2 and CO,PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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