SOLVENT EFFECTS ON REACTIONS IN SUPERCRITICAL FLUIDS

Supercritical fluid (SCF) solvents represent a unique and powerful reaction medium because of the strong pressure dependence of their densities and solvent properties (nu, eta, delta, epsilon, and D). This offers the capability of controlling the solvent properties and reaction rates and selectivities through pressure manipulations. Transition-state theory was used herein to interpret the effect of pressure on the overall reaction rate as an apparent activation volume that can be further decomposed into various contributions: DELTA-V(app) = -RT(d ln r/dP) = DELTA-V(int)k + DELTA-V(comp)k + DELTA-V(diff)k + DELTA-V(elec)k + DELTA-V(p)pi, accounting for mechanical pressure effects, solvent compressibility, diffusional limitations, electrostatic interactions, and phase behavior effects, respectively. The present investigation emphasized the latter three and evaluated their contributions as DELTA-V(diff)k = 1090 cm3/mol for benzyl phenyl ether pyrolysis; DELTA-V(elec)k = -3700, -2600, and -220 cm3/mol for hydrolysis of dibenzyl ether, phenethyl phenyl ether, and guaiacol, respectively; and DELTA-V(p)pi = 1600 cm3/mol for dibenzyl ether pyrolysis in toluene.