Supercritical fluids diffusion controlled reactions

Roberts, C. B. Zhang, J. Brennecke, J. F. Chateauneuf, J. E. Laser Flash Photolysis Investigations of Diffusion-Controlled Reactions in Supercritical Fluids. J. Phys. Chem. 1993a, 97, 5618-5623. 

CB Roberts, J Zhang, JF Brennecke, JE Chateauneuf. Laser flash photolysis investigations of diffusion-controlled reactions in supercritical fluids. J Phys Chem 97 5618, 1993. 

Supercritical fluids show unique physicochemical properties, such as density, diffusivity, solubility, and viscosity all can be easily controlled by changing temperature and pressure. Thus, these fluids are attractive as a useful solvent for chemical reactions and the following purification. Particularly, supercritical C02(scC02) has the advantages of relatively low critical temperature and pressure (critical temperature (71.) = 304.2 K, critical pressure (Pc) = 7.28 MPa), non-flammability, and inexpensiveness. 

Bioreactions. The use of supercritical fluids, and in particular C02, as a reaction media for enzymatic catalysis is growing. High diffusivities, low surface tensions, solubility control, low toxicity, and minimal problems with solvent residues all make SCFs attractive. In addition, other advantages for using enzymes in SCFs instead of water include reactions where water is a product, which can be driven to completion increased solubilities of hydrophobic materials increased biomolecular thermostability and the potential to integrate both the reaction and separation bioprocesses into one step (98). There have been a number of biocatalysis reactions in SCFs reported (99—101). The use of lipases shows perhaps the most commercial promise, but there are a number of issues remaining unresolved, such as solvent—enzyme interactions and the influence of the reaction environment. A potential area for increased research is the synthesis of monodisperse biopolymers in supercritical fluids (102). 

Much of the research discussed in this chapter has demonstrated that sc C02 is a viable alternative solvent for free-radical reactions. Moreover, several of these studies demonstrate that the unique properties of a supercritical fluid, specifically the ability to change important solvent properties such as viscosity by varying pressure (and temperature), can be exploited to manipulate reaction yield and selectivity. Solvent viscosity is particularly important for reactions that are diffusion-controlled or those for which cage-effects are important. 

There is one more unique feature of supercritical fluid solvents that will be a recurring theme in this chapter. Several studies have demonstrated that near the critical point, the density of the solvent about a solute is enhanced relative to the bulk density (solvent/solute clustering). As such, the mobility of the solute may be impeded to an extent greater than expected on the basis of the bulk viscosity. This phenomenon may also affect reactivity for reactions that are diffusion-controlled or for which cage effects are important, particularly near the critical point (vide infra). 

The prospect of using enzymes as heterogeneous catalysts in scC02 media has created significant interest. Their low viscosity and high diffusion rates offer the possibility of increasing the rate of mass-transfer controlled reactions. Also, because enzymes are not soluble in supercritical fluids, dispersion of the free enzymes potentially allows simple separations without the need for immobilization. 

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. 

One of the expected benefits from using enzymes in supercritical fluids (SCFs) is that mass transfer resistance between the reaction mixture and the active sites in the solid enzyme should be greatly reduced if the reactants and products are dissolved in an SCF instead of running the reaction in a liquid phase. It is expected that the high diffusivity and low viscosity of SCFs will accelerate mass-transfer controlled reactions. 

Generally, diffusivity is faster and viscosity lower for supercritical fluids than for liquids. A standard value of the diffusivity of solutes in liquids is roughly 10 cm s [4] the diffusion coefficient of naphthalene in CO2 at 10 MPa and 40 °C is 1.4 10 cm s and the self-diffusion coefficient of CO2 itself is two orders of magnitude higher than that of liquids [9]. The effect of temperature and pressure on the self-diffusivity of CO2 is illustrated in Figure 6. Diffusivity is obviously a major consideration in reactions whether they be homogeneously, heterogeneously, or not catalyzed, and it will determine whether a reaction is controlled kinetically or by diffusion. 

The greater control of physical properties and phase behaviour by both pressure and temperature in these media gives improved possibilities for optimising reaction conditions. For example, the higher concentrations in supercritical fluids vis-a-vis the gas phase, together with superior solute diffusivities vis-a-vis the liquid phase, can increase reaction rates. In some... 

Formation of pyrene excimer (a complex between a photoexcited and a ground-state pyrene molecule Scheme 4) is an extensively characterized and well-understood bimolecular process (35). Because the process is known to be diffusion controlled in normal liquid solutions, it serves as a relatively simple model system for studying solvent effects on bimolecular reactions. In fact, it has been widely employed in the probing of the solute-solute clustering in supercritical fluid solutions (40-42,46,47,160,166-168). (See Scheme 4.)... 

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