Supercritical solvents, examples

In some cases, the solids themselves are subjected to extraction by a solvent. For example, in one process used to decaffeinate coffee, the coffee beans are mixed with activated charcoal and a high-pressure stream of supercritical carbon dioxide (carbon dioxide at high pressure and above its critical temperature) is passed over them at approximately 90°C. A supercritical solvent is a highly mobile fluid with a very low viscosity. The carbon dioxide removes the soluble caffeine preferentially without extracting the flavoring agents and evaporates without leaving a harmful residue. 

Another interesting application of high-pressure tubes is the in-situ investigation of reactions in supercritical solvents such as carbon dioxide. For example, the iridium-catalyzed enantioselective hydrogenation of imines was investigated in a sapphire tube at 313 K [32]. 

A third example can be taken from analytical chemistry. Absorption and resonance Raman spectra of phenol blue were measured in liquid and supercritical solvents to determine the solvent dependence of absorption bandwidth and spectral shifts. Good correlation between absorption peak shift and resonance Raman bands and between Raman bands and bandwidth of C-N stretching mode were observed while anomalous solvent effect on the absorption bandwidth occnrred in liquid solvents. Large band-widths of absorption and resonance Raman spectra were seen in supercritical solvents as compared to liquid solvents. This was dne to the small refractive indices of the supercritical solvents. The large refractive index of the liqnid solvents only make the absorption peak shifts withont broadening the absorption spectra (Yamaguchi et al., 1997). 

Supercritical solvents can be used to adjust reaction rate constants (k) by as much as two orders of magnitude by small changes in the system pressure. Activation volumes (slopes of In k vs P) as low as —6000 cm3/mol were observed for a homogeneous reaction (97). Pressure effects can also be pronounced on reversible reactions (17). In one example the equilibrium constant was increased from two- to sixfold by increasing the solvent pressure. The choice of supercritical solvent can also dramatically affect an equilibrium constant. An obvious advantage of using supercritical fluid solvents as a media for chemical reactions is the adjustability of the reaction kinetics and equilibria owing to solvent effects. 

Free convection is fluid flow, induced by density gradients owing, for example, to temperature gradients. In gas extraction the supercritical solvent is subject to density variation with only slight changes in pressure and temperature. Furthermore, flow velocities within the processing equipment are low, so that flow owing to free convection may be important. Therefore, conditions for free convective flow must be considered in such types of systems. For isothermal vertical plates ... 

In another process, patented by Berens [67], the additive must again be solubilized in the supercritical solvent, as in Sand s process but, in this case, the patent is mainly concerned with reactive monomers that successively polymerize, modifying the host polymer. Nevertheless, in some examples (with a film of polyurethane), the problem of solid-drug impregnation is also considered. 

SCF). This fluid does not any more have a free surface, that characterizes a liquid as opposed to a vapour, but may serve as a useful solvent just the same. Some substances that are gases at ambient conditions can be compressed by high pressures to become supercritical fluids and solvents, a well-known example being carbon dioxide, used extensively as an extractant for foodstuffs and pharmaceuticals. Some physical properties—the critical temperature Tc, pressure Pc, and density dc—of supercritical solvents are shown in Table 3.3. 

Reid and others(11. 121 have shown that supercritical solvents exhibit varying degrees of specificity towards a particular specie. Furthermore, the small number of SC solvents available limits the potential use SC extraction. The use of entrainers or mixtures of solvents, may remove the limitation imposed by the narrow choice of likely solvents. Moreover, it is possible that through the proper choice of entrainer and solvent the desired chemical activity can be adjusted to improve the selectivity of the solvent. For example, mixtures of solvent gases with entrainers can permit a modification of critical properties as well as chemical properties, so that P and T adjustment can be used to maximize some physical property of the system(2). 

Supercritical fluids are found in numerous applications thanks to their properties which vary with temperature and pressure. Supercritical fluids are put in contact with various compounds which also have physico-chemical properties dependant on temperature and pressure. Consequently, mixtures of these compounds with the supercritical solvent must be expected to behave in a complex way. For a binary mixture, for example, several types of phase equilibrium exist solid-fluid for low temperatures, solid-fluid-liquid when temperature rises, and liquid-fluid. 

Extraction may also be performed with supercritical solvents. Ionic liquids have been described as designer solvents that is, their properties can be adjusted to suit the requirements of a particular process. Properties such as melting point, viscosity, density, and hydrophobicity can be modified by simple changes in the structure of the ions (Table 12.4). For example, the melting points of... 

In recent years there has been much interest in the use of supercritical fluids (SCFs) as replacements for conventional liquid solvents, particularly in separation science, but also as reaction media. In addition to their environmental benefits, SCFs have further advantages over conventional liquid solvents, and these are briefly outlined in Section 2. The remainder of the chapter describes the use of SCFs as a medium for NMR spectroscopic studies. First we look briefly at motives for such NMR studies and the techniques employed. We then examine in more detail chemical shifts and nuclear spin relaxation in SCFs. The lower relaxation rates associated with SCFs and consequent sharper lines obtained for quadrupolar nuclei make SCFs excellent solvents. Section 8 describes some NMR studies of organometallic reactions in SCFs. Here the miscibility of supercritical solvents with gaseous reagents proves to be a tremendously useful feature in, for example, homogeneous catalysis. Finally we comment on future possibilities for NMR studies in SCFs. 

In principle many other solvent combinations could be used in biphasic chemistry although the main driving force in this area is to provide environmental benefits. For example, ionic liquids have been combined with supercritical solvents for the hydroformylation of 1-octene.30 Since ionic liquids are have no vapor pressure and are essentially insoluble in supercritical C02, the product can be extracted from the reaction using C02 virtually uncontaminated by the rhodium catalyst. 

Among the practical applications of supercritical extraction is the use of supercritical carbon dioxide as the solvent in a number of processes. Carbon dioxide has several favorable properties as a supercritical solvent. It is nontoxic, low-cost, and noncorrosive. Its critical temperature is 304.2 K, which is near ambient. These properties are especially desirable in food processing, for the extraction of food components that must not be exposed to high temperatures. Examples are the removal of caffeine from coffee and the extraction of oil from beans and corn. 

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