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 th e esterification of phthalic anhydride with methano in supercritical c arbon 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 tran sition-state theory suggest a less than 2-fold decrease for that incre ase in pressure. Therefore, we attribute the apparently large values o f the rate constants at lower pressures to an actual increase in the l ocal concentration of the methanol around the phthalic anhydride at th ose conditions. To further support this interpretation we present solv atochromic measurements of the local composition of methanol around a solute in supercritical fluid solutions at the temperatures and pressu res used in the kinetic experiments. We present plausible evidence tha t the observed kinetic measurements can be explained by the local conc entration of the methanol around the phthalic anhydride being signific antly higher than the bulk concentration when operating in the supercr itical fluid mixture.