Solvent Properties of SCFs

There are several potential advantages which may be realized with the use of supercritical fluids as solvents for chemical reactions from the standpoint of reactivity and selectivity. As many of the examples discussed in this chapter illustrate, the unique features of SCFs can be exploited to control the behavior (i.e. kinetics and selectivity) of many chemical processes in a way not possible with conventional liquid solvents. 

Changes in reaction rates arising from direct effect of temperature and pressure on the kinetics of a reaction are governed by transition state theory, and 

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The ability to fine tune solvent properties of SCFs through changes in operating conditions can be exploited in a wide range of applications, such as extraction, fractionation, and reaction processes, where flexibility in process implementation offers the researcher/processor a gamut of possibilities. 

It was also found that equilibrium conversion decreases sharply with increasing temperature. In order to exploit the unique solvent properties of SCF s, operating temperatures are usually restricted to the 1.0-1.2 range. A simple criterion for... 

This chapter reviews the field of heterogeneous catalytic reactions in SCFs [5-8]. By exploiting the unique solvent properties of SCFs, it may be possible to enhance reaction rates while maintaining or improving selectivity. The following benefits can be expected. 

Baron Cagniard de la Tour [244] made the first reported observation of the occurrence of a supercritical phase in 1822. Tables 3.12 and 3.13 summarise some of the main useful features of SCFs. Several properties of SCFs make them ideal candidates as solvents for industrial extraction processes [245,246],... 

With traditional solvents, the solvent power of a fluid phase is often related to its polarity. Compressed C02 has a fairly low dielectric constant under all conditions (e = 1.2-1.6), but this measure has increasingly been shown to be insufficiently accurate to define solvent effects in many cases [13], Based on this value however, there is a widespread (yet incorrect ) belief that scC02 behaves just like hexane . The Hildebrand solubility parameter (5) of C02 has been determined as a function of pressure, as demonstrated in Figure 8.3. It has been found that the solvent properties of a supercritical fluid depend most importantly on its bulk density, which depends in turn on the pressure and temperature. In general higher density of the SCF corresponds to stronger solvation power, whereas lower density results in a weaker solvent. 

The fundamental properties of SCFs and their relation to organometallic catalysis have been reviewed extensively in recent years, and will not be re-iterated here [1, 7]. The term supercritical indicates that the substance used as reaction medium or solvent is heated and compressed beyond its critical temperature and pressure. For C02, which is the most widely used SCF in hydrogenation reactions, these values are Tc=31.04°C and pc=73.83 bar. Owing to the complex... 

Note that, compared to conventional liquid solvents, SCFs are not always a panacea. They have both merits and disadvantages. Many chemical reactions are better performed in ordinary fluid solutions. However, chemistry of the reaction in SCFs still is a young and fully unexplored scientific field. We need deeper understanding of the microscopic and macroscopic properties of SCFs. The industrial... 

Supercritical fluids (SCFs) are best known through their use for the decaffeination of coffee, which employs supercritical carbon dioxide (scCC ). In this chapter, we will demonstrate that SCFs also have many properties that make them interesting and useful reaction media. Firstly, the physical properties of SCFs will be explained, then the specialist equipment needed for carrying out reactions under high temperatures and pressures will be described. Finally, we will discuss issues relevant to the use of SCFs as solvents for reactions. 

The unusual solvent properties of supercritical fluids (SCFs) have been known for over a century (1). Just above the critical temperature, Tc, forces of molecular attraction are balanced by kinetic energy and fluid properties, including solvent power, exhibit a substantial pressure dependence. Many complex organic materials are soluble at moderate pressures (80 to 100 atmospheres) and SCF solvent power increases dramatically when the pressure is increased to 300 atmospheres. The pressure responsive range of solvent properties thus attainable provides a tool for investigating the fundamental nature of molecular interactions and is also being exploited in important areas of applied research (2,3). 

Supercritical fluids (scf) are highly compressed liquids or gases. The latter already have an established role in "clean extraction (substitution of chlorinated/organic solvents) on an industrial scale (e. g. decaffeination of coffee and tea, extraction of hops, spices, etc.). The specific physical and chemical properties of scf make them particularly suitable for a variety of other applications, e. g. reactions, powder technology and impregnation. 

Table I. Properties of SCF solvents versus n-hexane at a constant value of solvent strength as defined by Ex for phenol blue(f)...

The RESS process relies on the solvent properties of carbon dioxide. Because CO2 is a nonpolar molecule, this process will be mainly efficient and interesting for micronizing nonpolar molecules. For this reason, a preliminary study on the solubility of the compounds with pressure and temperature is necessary. As usual, the solvent polarity can be modified and enhanced by adding to the supercritical CO2, small quantities of an organic cosolvent. This is primarily because the solvent power of an SCF is strongly dependent on its density, which can be adjusted by small variations of pressure and temperature (11). 

Since naphthalene-SCF mixtures have been so widely studied, it is instructive to consider the solubility behavior of just one of these systems, the naphthalene-C02 system, to highlight the solvent properties of a supercritical fluid solvent. In chapter 1 the effects of pressure and temperature on the solubility of naphthalene in ethylene were described. The solubility behavior is quite similar in carbon dioxide, in trifluoromethane, or even in xenon, although each system achieves its own absolute solubility level of naphthalene. 

The interest in SFRs has seen a tremendous increase over the last few years, because the special properties of SCFs make them particularly attractive solvents for modem synthetic chemistry. We should be aware of the fact, however, that the idea of using SCFs as reaction media has been emerging ever since the discovery of this peculiar state of matter early in the nineteenth century by Baron Charles Cagniard de LaTour, an experimental physicist in France [67]. 

The ability of supercritical fluids (SCFs) to act as solvents has been known for well over a century [1] yet the most significant developments in the application of this technology have taken place in the last few decades. Among their most recent applications, SCFs have been employed as media for chemical reactions. As a reaction medium, the SCF may either participate directly in the reaction or simply act as a solvent for the various chemical species. The physical properties of SCFs are highly dependent on pressure and temperature which makes it possible to fine-tune the reaction environment. These characteristics are unique to SCFs and provide the potential to tune the reaction environment in order to optinuze reaction rate and selectivity. 

SCFs may be used in the same way as other ordinary solvents taking into account their different properties and behaviors. Supercritical fluids can replace liquids solvents in many processes, such as extractions from solids (leaching), countercurrent multistage separations, chromatographic separations, and others, provided the solvent properties of the SCFs are adequate. 

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