The physical properties that are important for reactions in supercritical fluids (SCFs) are (a) phase behavior, (b) diffusion rates and (c) interactions between reagents, products and transition states with the fluid. First, phase behavior is important in bringing reagents into the same phase to allow them to react more effectively. For example light gases, such as hydrogen, are often miscible with SCF substances. Second, phase behavior can be important in separating the product of a reaction, possibly also stopping the reaction at an intermediate stage or pushing a reversible reaction to completion. The phase behavior of the reacting system must therefore be understood. At the minimum it will be necessary to know whether the reaction is in a single phase, as is usually required. This will often be a matter of being at a sufficiently high pressure. It will also be important to estimate the critical temperature, T,, of the multicomponent reacting mixture in order to know whether the system is a liquid (below T,) or an SCF (above T,). The difference in behavior in passing through T, is not dramatic, but as the temperature moves below T, the medium becomes less compressible and loses the control by pressure characteristic of a supercritical fluid [l]. Because it has been covered in chapter 1.2, phase behavior will not be further discussed in this section.
Diffusion coefficients are typically higher in SCFs than in liquids. This is partly because the substances used as the solvent, such as carbon dioxide, have typically lighter and smaller molecules than organic liquid solvents and partly because the density of an SCF is typically less than a liquid. Consequently, reactions controlled by diffusion may be faster than in a liquid, giving the advantage of smaller process plant size. However, in the region of the critical point, diffusion coefficients can show an anomalous lowering, which can effect reaction rates. The behavior of diffusion coefficients is therefore discussed in Section 1.3.1 and its effect on reactions in Section 1.3.2.
For reactions controlled by activation, solvation effects on the reagents and the transition state can affect the equilibrium coefficient for the formation of the transition state and therefore the reaction rate. Equilibria can also be affected by solvation effects on reagents and products. In a supercritical fluid, solvation effects can be controlled by density, and therefore at constant