Supercritical fluid tunability

Supercritical fluid (SCF) with the beneficial effects of both liquid- and gas-phase chemistry is an emerging reaction medium for many scientific and technical reasons. The reaction rate and selcectivity are readily tunable by a subtle change in pressure and temperature. 

Supercritical fluids have many features that render their use attractive in synthetic chemistry and separations. Their tunable physical properties allow reactions to be carried out under a variety of conditions and, in some cases, the selectivities and rates of reactions may be altered. The list of reactions that have been carried out in SCFs and compared with those in conventional solvents is continually growing. 

Supercritical fluid possesses low viscosity and high diffusivity and its solvent power is easily tunable. ... 

Due to its compressibility in the liquid (near the critical point) and in the supercritical fluid state, the dielectric constant and density, and thus the solvent quality of C02, are tunable with pressure and temperature (Keyes and Kirkwood, 1930). As illustrated in Figure 1.2, this compressibility provides for control of the density and therefore solvent-dependent properties such as dielectric constant and overall solvent strength (Giddings et al., 1968). While supercritical C02 can have high liquidlike densities, it shares many of the... 

High-pressure processes have been widely applied in the polymer industry. Near-critical and supercritical fluids (SCFs) are in particular used owing to their easily tunable density, which enhances the control of polymer solubility and their good separability from polymer material [1], SCF solvents (e.g. scC02) offer a potential advantage for separation process. The solubility of different polymeric material in SCFs can be systematically varied by changing operating conditions. Several... 

We have recently reviewed the use of vibrational spectroscopy in supercritical fluids [2] and the theme common to most of our projects is the use of spectroscopy for real-time optimisation of processes in supercritical solution. Such optimisation is considerably more important in supercritical fluids than in conventional solvents because the tunability of the fluids results in a greater number of parameters which can affect the outcome of a reaction. Thus, the chances of hitting the optimal conditions purely by trial and error are much less in supercritical solution than in conventional reactions. Below, we give three examples of our approach, synthesis of polymers, transition metal hydrogen compounds, and the use of flow-reactors. 

Analytical Supercritical Fluid Extraction and Chromatography Supercritical fluids, especially CO2, are used widely to extract a wide variety of solid and liquid matrices to obtain samples for analysis. Benefits compared with conventional Soxhlet extraction include minimization of solvent waste, faster extraction, tunability of solvent strength, and simple solvent removal with minimal solvent contamination in the sample. Compared with high-performance liquid chromatography, the number of theoretical stages is higher in... 

SCFs have a tunable density that may offer further advantages in reaction and processing applications. This tunability is illustrated in Fig. 1 for carbon dioxide. Near the critical point, even small changes in the temperature or pressure of carbon dioxide dramatically affect its density. Similarly, the viscosity, dielectric constant, and diffusivity are also tunable parameters, which allows specific control of systems involving supercritical fluids. 

The tunability of solvency with temperature and pressure as illustrated in Figs. 1 and 2 is a key advantage of cleaning with supercritical fluids. This allows optimization of conditions to extract a particular material from a part and then selection of other conditions in the recycle reactor to separate it from the SCF. As an example, hexane has a solubility much like CO2 near the critical conditions. At higher pressures, carbon dioxide acts like acetone, a more polar solvent. A good rule of thumb is that if low molecular weight materials are soluble in hexane, they are soluble in CO2 at pressures just above the critical point. As pointed out by DeSimone,t °l however, polymers exhibit a different behavior. 

Thus, taken as a whole, the tunable densities and tunable solubilities, the availability of cosolvents as property modifiers, the ability to utilize fluids that are gases under ambient conditions, and the possibility of employing fluids that are environmentally friendly (e.g., CO2 or water) (the unique properties of supercritical fluids) make SFE a valuable alternative to normal liquid extraction processes. 

The supercritical fluids exhibit gas-like viscosities, diffusivities, and liquid-like densities. These favorable transport properties lead to enhanced mass transfer, permeation, and wetting characteristics. The mass transfer limited multiphase reactions will benefit from reduction of a number of phases, as in the case of most oxidation, hydrogenation, or replacement of the more viscous liquid phase with a supercritical or a less viscous expanded liquid phase. The mobility combined with tunability results in effective maintenance of catalyst activity in heterogeneous catalysis. 

Because of their tunable properties, supercritical solvents provide a useful medium for enzyme-catalyzed reactions.f The mechanism of enzyme-catalyzed reactions is similar to the mechanism described for solid-catalyzed reactions. External as well as internal transport effects may limit the reaction rate. Utilizing supercritical fluids enhances external transport rate due to increase in the diffusivity and therefore mass transfer coefficient. Internal transport rate depends on the fluid medium as well as the morphology of the enzyme. Supercritical fluids can alter both. 

Enzyme reactions have been successfully operated in a variety of organic solvents (Table 8.4) as well as in supercritical fluids (e.g., carbon dioxide and fluoroform) and gases. - The latter two categories offer some intriguing possibilities and potential advantages relative to solvents, including enhanced substrate diffusivity, tunable solvent phase properties (via temperature and pressure), reduced solvent... 

The use of supercritical fluids as reaction media has received increased attention in recent years. The pressure-tunable physical and fransport properties of a supercritical fluid may be exploited to find an optimal reaction medium characterized by liquid-like density and heat capacity, yet significantly better (more gaslike) transport properties. The interest in SCF-based processes is evidenced by several recent reviews involving homogenous (1,2), immobilized (3), and hetero-... 

The examples discussed demonstrate that the unique nature of SCFs provides a means of dialing up the selectivity of a chemical process in a manner that is impossible using conventional solvents (i.e. by manipulation of temperature and pressure). SCF solvents such as CO2 and H2O, the latter of which has not yet been exploited as a solvent for organic photochemistry, are especially attractive as they are environmentally benign alternatives to a number of classical solvents which pose hazards to either health or the environment. Coupled with the tunable properties of a supercritical fluid, these solvents emerge not only as viable alternatives to conventional organic solvents, but in some cases at least, superior alternatives. Finally, a point which has not been fully emphasized in this chapter is that SCF solvents are superb tools for probing... 

Carbon dioxide(C02) has emerged these days as the most promising alternative for a green solvent. It provides several benefits It is non-flammable, non-toxic, environmentally benign, and inexpensive. Also, CO2 is a tunable fluid, i.e., its density and solubility can change by controlling pressure and temperature. Supercritical fluid technology has been applied in many and diverse fields... 

Ionic liquid Supercritical fluids (e.g. CO2) Organic co-solvent not needed Tunability of the solubility characteristics of the ionic liquids Presence of CO2 reduces ionic liquid s viscosity Ionic liquid costs High pressure apparatus needed... 

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