Freons, supercritical

Solvent-assisted decaffeination of coffee can result in residues of solvent reaching the consumer.208 The use of chlorinated hydrocarbon solvents such as chloroform,209 methylene chloride, trichloroethylene,208 and difluoromonochloromethane (Freon),210 will probably be replaced by compounds already found in roasted coffee. The use of an ethyl acetate and 2-butanone mixture leaves a 26-ppm residue in green coffee, but zero residue in roasted coffee.211 Other solvent compounds used or suggested for coffee improvement or decaffeination include propane, butane,212 carbon dioxide,213 214 acetone215 dimethyl succinate,2161,1-dimethoxymethane, and 1,1-dimethoxyethane.217 Of all these, supercritical carbon dioxide, ethyl acetate, and methylene chloride are the solvents most used currently in decaffeination processes. 

Instead of performing the polymerization reactions in supercritical C02 (we do not yet have access to this technology), we chose the dispersion polymerization in 1,1,2-trichlorotrifluoroethane (Freon 113), to serve as a model for supercritical C02. We only used the short diblock copolymers for the polymerization of styrene in Freon 113, since it is known that such diblock copolymers are the most efficient steric stabilizers. 

These block copolymers can act as effective steric stabilizers for the dispersion polymerization in solvents with ultralow cohesion energy density. This was shown with some polymerization experiments in Freon 113 as a model solvent. The dispersion particles are effectively stabilized by our amphi-philes. However, these experiments can only model the technically relevant case of polymerization or precipitation processes in supercritical C02 and further experiments related to stabilization behavior in this sytem are certainly required. 

Enzymes can express activity in supercritical and near-supercritical fluids, such as carbon dioxide, freons (CHF3), hydrocarbons (ethane, ethylene, propane) or inorganic compounds (SFe, N2O). The choice of supercritical fluids is often... 

The supercritical fluid carbon dioxide, C02, is of particular interest This compound has a mild (31°C) critical temperature (Table 1) it is nonflammable, nontoxic, and, especially when used to replace freons and certain organic solvents, environmentally friendly. Moreover, it can be obtained from existing industrial processes without further contribution to the greenhouse effect (see Air pollution). Carbon dioxide is fairly miscible with a variety of organic solvents, and is readily recovered after processing owing to its high volatility. It is a small linear molecule and thus diffuses more quickly than... 

Extraction using a supercritical fluid (CO2, N20 or CHC1F2 Freon-22) is a well-known process in the food industry (cf. 6.1). Extractors used for analytical purposes operate on the same principle. They incorporate a highly resistant tubular container into which the sample is placed (in solid or semi-solid form) with the supercritical fluid. Two modes of operation are employed ... 

Absorption spectra of 2-nitroanisole in supercritical C02, N20, Freon-13, ammonia and C02-methanol mixtures were obtained on a Cary model 1605 spectrophotometer operated in the dual beam mode. The gases used as supercritical solvents were of the highest purity available from the supplier (Matheson) and were further filtered prior to use. The mixed solvent system of C02-methanol was obtained from Scott Speciality Gases (15.4 wt% methanol), and other mixtures were made in the laboratory. Spectra of 2-nitroanisole in n-pentane, methanol, tetrahydrofuran and acetonitrile (Burdick A Jackson) were obtained using quartz cells with a 1-cm light path and with a pure solvent blank in the reference beam. Vapor phase and supercritical fluid spectra were obtained using an air reference. 

At more "liquid-like" densities, the solvatochromic shifts in supercritical fluids approach those observed in the corresponding liquids. Figures 1 and 2 depict the pressure dependence of the wavelength of the absorption maximum of 2-nitroanisole in supercritical CO2, N2O, CC1F3 (Freon-13), and NH3. These measurements reflect the effect of pressure (fluid density) on the cybotatic region of these solvents. It is clear that the fluid density affects the cybotatic region, as evidenced by the shift in the absorption maximum with pressure and it is also evident that the magnitude of the shift is fluid dependent. 

The data indicate that as the reduced density of a fluid is increased the cybotatic region becomes more polar/polarizable, with an especially large effect for supercritical NH3. The low it values for Freon-13 at these densities are similar to those of the perfluoro-alkanes ( 5). The values for supercritical C02 and N20 range between those of the perfluoro-alkanes to that of the n-alkanes (n-heptane, etc.) 5). Ammonia gives it values which range from approximately that of n-heptane to that of ethyl acetate or tetrahydrofuran ( 5) over the density range studied. 

One way of improving SFE efficiency is by using a more suitable SF to extract the target analyte. Unfortunately, the choice of fluids other than CO, is restricted by the desire to have reasonable critical parameter values and costs, chemical inertness, low toxicity and little environmental impact. The use of supercritical N2O has proved to increase the extraction efficiency for high-molecular weight PAHs and chlorinated dibenzo-p-dioxins from fly ash and sediment [52]. This extractant, however, does not always improve the extraction efficiency [53] also, it can be explosive in the presence of reactive organics. Other polar fluids such as CHCIF, (Freon-22) have exhibited increased efficiency in the extraction of nitrated and non-nitrated PAHs from particulate matter in diesel exhaust [54]. Freon-22 has also been found to allow significantly fast and effective extraction of... 

Use of acidic COz is clearly undesirable for the extraction of basic compounds, and N20 has been used effectively for a number of amines (Mathias-son et al. 1989 Ashraf-Khorassani and Taylor 1990). Similarly, C02 was ineffective for extraction of 1-nitropyrene from diesel exhaust particulate matter although this could be accomplished effectively using the freon CHC1F2 (Paschke et al. 1992). In conclusion, it seems safe to state the obvious no single supercritical fluid is likely to be optimal for the extraction of structurally diverse analytes. 

Freon 13 becomes supercritical more easily than methanol and it is polar (Table 13-1, p. 139). Why is methanol usually used ... 

Sanchez, F.G., Bursey, M.M., 2002. Transient nature of rhizosphere carbon elucidated by supercritical freon-22 extraction and 13C NMR analysis. Forest Ecol. Manag. 169, 177-185. 

Even though the extraction efficiency for some analytes can be increased by change of extraction fluid, carbon dioxide is by far the most common compound used (98% of all applications). It has low critical parameters, it is nonexplosive, nontoxic, and environmentally benign. Alternatives have been proposed such as alkanes and freons but they have never been widely accepted due to health and safety risks for the former and ozone depletion by the latter. One of the few competitors to carbon dioxide is nitrous oxide, however, it might cause explosion in contact with high amounts of organic material. Supercritical carbon dioxide has a polarity similar to that of n-hexane, and consequently for the extraction of more polar analytes an organic modifier such as methanol or acetonitrile (1-5%) has to be added to increase the polarity (see the section Modifiers ). To maintain supercriticality for two-component fluids, somewhat different conditions have to be applied, but normally there is no problem at the conditions under which SFE is normally carried out. 

There is, however, one major problem with CO2 as a mobile phase and that is its low polarity. Thus only relatively non-polar analytes can be dissolved in CO2. Moreover, in columns packed with silica-based material there are always residual adsorptive sites. In reversed-phase HPLC the mobile phase deactivates these sites, but the CO2 is not polar enough to do that. As a consequence, the more polar analytes are adsorbed and these are then eluted as severely tailing peaks or are not eluted at all. It should be mentioned here that reports on more inert packings have been published (Li, Malik and Lee, 1994). There are some supercritical mobile phases other than CO2 that can be used, but those that are realistic to use are all non-polar. The only alternatives are the freons, of which chlorine-free freons are considered to be less harmful to the environment (Blackwell and Schallinger, 1994). 

Recent work by several research groups has shown that supercritical fluids can be superior to other solvents for several chemical processes. For example, DeSimone has demonstrated the ability of supercritical CO2 to replace Freons in the free radical polymerization of fluorinatkl acrylate monomers. 34) Noyori has shown that significant rate enhancements can be achieved in supercritical carbon dioxide relative to other solvents for the homogeneous catalytic hydrogenation of carbon dioxide to either formic acid or its derivatives in the presence of triethylamine or triethylamine/methanol respectively, (equation 1). (55-57) As discussed below, we have recently demonstrated that improved enantioselectivities can be achieved in supercritical carbon dioxide for the catalytic asymmetric hydrogenation of several enamides. 5 8)... 

SEC has also been used for the separation of phenols [105-107]. The supercritical fluid normally used is carbon dioxide with some modifier, for example, methanol or chlorodi-fluoromethane (Freon 22) [106]. Berger and Deye tested binary and tertiary supercritical mixtures, among them methanol/carbon dioxide mixtures containing very polar additives [107]. Ong et al. used chlorodifluoromethane as the supercritical fluid [105]. In most studies, UV detection was used and did not measure up to the required sensitivity. To bypass this problem, an online system with SPE cormected to the SFC instrmnentation was designed. Some of these systems are presented in the following section. 

Many other probes have been used to study supercritical fluid-cosolvent mixtures, including the charge transfer complexes Fe°(l,10-phenanthroline)3 " and Fe (2,4-pentadionate)3 (for C02-methanol mixtures) (154), Nile red dye (for Freon-13, Freon-23, and CO2 with the cosolvents methanol, THF, acetonitrile, and dichloromethane) (155), benzophenone (for ethane with the cosolvents... 

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