Pressure Control of Reactions in Supercritical Fluids: Thermodynamics and Kinetics

Abstract

In a supercritical fluid (SCF) solvent, large variations in solvent properties (diffusivity, density, viscosity, dielectric, etc.) may be caused by small changes in pressure or temperature, especially in the region near the critical point. This variability makes SCF's attractive as solvents for reactions (enzymatic or otherwise), because we can hope to “tune” the solvent properties by appropriately adjusting pressure and temperature in order to optimize our process. Several enzymatic reactions using SCF solvents have been reported in the literature; however, the effects of pressure, temperature, and cosolvent concentrations on enzyme-catalyzed reactions and enzyme stability in SCF's are complicated and are still poorly understood. Some insight may be gained, however, by examining the physical chemistry and thermodynamics of SCF's in greater detail. Changes in temperature or pressure affect maximum achievable reaction rates by affecting both the bulk concentration of reactants (through changing solubilities) and by changing the reaction rate constants themselves. Large changes in rate constants can result from small pressure or temperature changes. These changes, expressed as activation volumes and activation energies, can be very large compared to those typically seen in liquids. Both positive and negative activation volumes have been reported in the literature; we will report positive activation volumes as large as 7 liters/mole for reactions in supercritical ethane. A pressure and temperature dependent effect of particular interest in reactive systems is local density augmentation, or clustering, of solvents and solutes. Effective local concentrations in supercritical fluids may be several times higher than those in the bulk, and the resulting effect on reaction rates may be substantial.