EFFECT OF LOCAL COMPOSITION ENHANCEMENTS ON THE ESTERIFICATION OF PHTHALIC-ANHYDRIDE WITH METHANOL IN SUPERCRITICAL CARBON-DIOXIDE

We report the pressure effect on the bimolecular rate constants for the esterification of phthalic anhydride with methano in supercritical carbon dioxide at 40 and 50-degrees-C. We observe as much as a 25-fold decrease in the bimolecular rate constant based on bulk concentrations when increasing the pressure from 97.5 to 166.5 bar. Calculations of the anticipated pressure effect on the rate constant according to transition-state theory suggest a less than 2-fold decrease for that increase in pressure. Therefore, we attribute the apparently large values of the rate constants at lower pressures to an actual increase in the local concentration of the methanol around the phthalic anhydride at those conditions. To further support this interpretation we present solvatochromic measurements of the local composition of methanol around a solute in supercritical fluid solutions at the temperatures and pressures used in the kinetic experiments. We present plausible evidence that the observed kinetic measurements can be explained by the local concentration of the methanol around the phthalic anhydride being significantly higher than the bulk concentration when operating in the supercritical fluid mixture.