Supercritical Fluids as Media for Chemical Reactions

Jessop, P. G. Leitner, W. Supercritical fluids as media for chemical reactions. In Chemical Synthesis Using Supercritical Fluids, eds. P. G. Jessop and W. Leitner, Wiley -VCH, New York, 1999, pp. 9-13. 

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. 

Supercritical Fluids as Media for Chemical Reactions Table 1.1-2 Selected organic supercritical fluids. 

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant may be adjusted over the continuum of gas-like to Hquid-like densities by varying pressure and temperature. Subsequently, these changes can be used to affect reaction conditions. A review encompassing the majority of studies and apphcations of reactions in supercritical fluids is available (96). 

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. 

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. 

Microemulsions tremendously expand the potential applications of supercritical fluids as reaction media for chemical reactions. By themselves, near-critical and supercritical fluids are much weaker solvents than the typical organic liquid. However, microemulsions create a highly polar region that is capable of solvating polar catalysts, reactants, or products. The unique aspects of the near-critical or supercritical continuous phase offer many advantages over their liquid-phase analogs. 

Y Ikushima. Supercritical fluids as novel media for chemical reactions. Koatsuryoku no Kagaku to Gijutsu 6(2) 86-93, 1997. 

In general, the properties of supercritical fluids make them interesting media in which to conduct chemical reactions. A supercritical fluid can be defined as a substance or mixture at conditions which exceed the critical temperature (Tc) and critical pressure (Pc). One of the primary advantages of employing a supercritical fluid as the continuous phase lies in the ability to manipulate the solvent strength (dielectric constant) simply by varying the temperature and pressure of the system. Additionally, supercritical fluids have properties which are intermediate between those of a liquid and those of a gas. As an illustration, a supercritical fluid can have liquid-like density and simultaneously possess gas-like viscosity. For more information, the reader is referred to several books which have been published on supercritical fluid science and technology [1-4],... 

Supercritical fluids continue to attract interest as media for catalysis and chemical reactions because of their unique properties. They offer possible strategies for more economical chemical processes by increasing reaction rates, prolonging catalyst lifetime, and simplifying downstream processing. In addition to these considerations, the current trend towards pollution prevention in the chemical processing industry and the attendant search for environmentally benign solvents is... 

We now turn attention to a completely different kind of supercritical fluid supercritical water (SCW). Supercritical states of water provide environments with special properties where many reactive processes with important technological applications take place. Two key aspects combine to make chemical reactivity under these conditions so peculiar the solvent high compressibility, which allows for large density variations with relatively minor changes in the applied pressure and the drastic reduction of bulk polarity, clearly manifested in the drop of the macroscopic dielectric constant from e 80 at room temperature to approximately 6 at near-critical conditions. From a microscopic perspective, the unique features of supercritical fluids as reaction media are associated with density inhomogeneities present in these systems [1,4],... 

In addition to fluorous solvents and ionie liquids, supercritical fluids sc-fluids, scf s), sueh as supercritical carbon dioxide (se-C02), constitute a third class of neoteric solvents that can be used as reaction media. Although sc-fluids have been known for a long time and have been advantageously used as eluants in extraction and chromatography processes (see Sections A.6 and A.7 in the Appendix), their application as reaction media for chemical processes has become more popular only during the last decade. Some of their physical properties and the supercritical conditions necessary for their existence have already been described in Section 3.2 (see Figure 3-2 and Table 3-4) see also references [209, 211-220, 224-230] to Chapter 3 for reviews on sc-fluids and their applications (particularly for SC-CO2 and SC-H2O). 

Supercritical fluids, especially SCCO2 (cf. Section 3.1.13), find increasing interest as environmentally friendly reaction media with unique properties for chemical reactions [285]. The problem of insufficient solubility of the ligand complexes has been solved by an approach similar to fluorous biphasic catalysis (cf. Section 3.1.1.2.1) [286-289]. 

Different uses of supercritical fluid (SCF) solvents in chemical separation processes have been of considerable research interest since the 1970s. The basic principles of SCF extraction engineering and a number of applications for this technology are described in several review papers [1,2]. As a new field related to SCF technology, the application of supercritical solvents as reaction media attracts increasing attention, especially for catalytic reactions. In such processes, the SCF may either actively participate in the reaction or function solely as the solvent for the reactants, catalysts, and products. 

Supercritical (sc) fluids have received considerable attention recently as new reaction media for chemical syntheses [30]. In particular, carbon dioxide in its supercritical state (SCCO2 T = 31 °C, = 7.37 MPa) appears to be a very attractive... 

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