Solvents carbon dioxide, supercritical

The high-pressure phase behavior of polymer-solvent-supercritical carbon dioxide systems was investigated experimentally The polymers used were poly(methyl methacrylate), polystyrene, polybutadiene, and poly(vinyl ethyl ether) at concentrations ranging from 5 to 10% in mixtures with toluene or tetrahydrofuran. The experiments were conducted for temperatures from 25 to 70°C and pressures up to 2200 psi in a high-pressure cell (Kiamos and Donohue, 1994). 

Finally, it should be noted that Lewis acids and bases can also be used in other non-conventional media, as described in Chapter 7, e.g. fluorous solvents, supercritical carbon dioxide and ionic liquids by designing the catalyst, e.g. for solubility in a fluorous solvent or an ionic liquid, to facilitate its recovery and reuse. For example, the use of the ionic liquid butylmethylimidazolium hydroxide, [bmim][OH], as both a catalyst and reaction medium for Michael additions (Fig. 2.45) has been recently reported [151]. 

Potentially, supercritical carbon dioxide (SCCO2) is the ideal green solvent. It is non-toxic for both humans and the environment. It is chemically inert under most conditions, whether they be non-flammable or non-protic, and it is inert to radical and oxidizing conditions. This gas can be obtained in large quantities as a by-product of fermentation, combustion, and ammonia synthesis and it is relatively cheap, particularly compared with conventional solvents. Supercritical carbon dioxide presents other practical advantages as well, such as the possibility of achieving product isolation to total dryness by simple evaporation. 

The increased interest in recent years in supercritical fluids as media for chemical reactions produced the need to characterize them in terms of their solvation abilities towards various kinds of solutes. The most widely used supercritical fluid is carbon dioxide, with a critical temperature T0 = 304.2 K (31 °C) and pressure P0 = 7.39 MPa (74 atm). Being a non-polar solvent, supercritical carbon dioxide (SCCD) can be modified in order to solubilize polar solutes by inclusion of a polar co-solvent, such as methanol. Aniline derivatives have been used as probes for the determination of the polarity of neat and modified supercritical fluids. 

Unlike liquid solvents, supercritical carbon dioxide does much more than dilute the coating to a low spray viscosity - it changes the nature of the atomization mechanism. Thus, low spray viscosity is less important than with conventional spray methods, and many new factors contribute to the spray-ability of the coating. 

Often a safer solvent can be substituted for a more hazardous solvent. Safer can be either, or both, of the goals of reducing flammability or toxicity. Two publications have described green solvents. Supercritical carbon dioxide has proved to be an excellent substitute for some organic solvents, particularly if newly designed polymers are added, which can increase solute solubility. ... 

Environmentally green solvents. Supercritical carbon dioxide presents an environmentally benign medium for polymerizations (and other chemical operations), minimizing pollution from organic solvents and facilitating the isolation of the polymeric product. 

A major disadvantage of Kraus s procedure in terms of an environmentally benign method involved the use of benzene (or alternatively acetonitrile) as solvent. Supercritical carbon dioxide (SC-COj) was recently reported as an alternative solvent and was applied to the benzophenone-mediated photoacylation of 1,4-benzoquinone (5) with benzaldehyde or butyraldehyde, respectively. " High yields of up to 81% were achieved at higher CO pressure, or with the addition of 5% t-butyl alcohol as co-solvent (Table 88.12) the latter is necessary due to the limited solubility of the quinone in SC-COj. Despite these achievements, this technique suffers from technical disadvantages (e.g., high pressure or small reactor volume), especially for a large-scale application. 

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