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Ind. Eng. Chem. Res. 2001, 40, 4484
Response to Comment on “Effect of Local Composition Enhancements on the Esterification of Phthalic Anhydride with Methanol in Supercritical Carbon Dioxide” Joan F. Brennecke Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana 46556
Sir: Iwai and co-workers have provided an extended analysis of our data on the rates of esterification of phthalic anhydride with methanol in supercritical CO2. Their conclusions are consistent with the conclusions that we presented in the original paper, i.e., that the increased local composition of methanol around phthalic anhydride enhances the reaction rate. Using standard transition-state theory (note that transition-state theory was invoked in our original paper to analyze the pressure dependence of the rate constants1), Iwai and co-workers regress two empirical parameters from our experimental data. These parameters are R, used to describe the dependence of the local composition on pressure, and ∆Hq, the enthalpy difference between the transition state and the reactants. With these regressed parameters, they are, not surprisingly, able to describe our experimental data with the transition-state-theory model. However, please note that the expression they suggest for the pressure dependence of the local composition (eq 4) is valid only for the limited pressure range of the experimental data and only for small concentrations. It gives unrealistically high values (i.e., infinity) of the local-to-bulk composition ratio at low pressures. Obviously, the local composition must go through a maximum and decrease toward the bulk value in the limit of low pressure, as we showed in our recent publication.2 In addition, Iwai and coworkers interpret the local density that we described in eq 4 of our original publication as meaning the bulk density of a solution composed of a mixture with the correct local composition. However, numerous researchers (e.g., see references in ref 3) have shown that the local density of even a pure supercritical fluid around a dilute solute can be significantly greater than the bulk value. It was our original intention that these values (for instance, as found in ref 4) be used for the local
densities. We anticipate that Iwai and co-workers would have been able to fit our experimental data equally well if they had followed this route as their analysis requires the two fit parameters. Finally, we would like to call attention to our more recent paper on the topic of local composition effects on reactions in supercritical and subcritical fluids.2 In this paper, we explore local composition effects on an energy transfer reaction over a very wide pressure range, showing a maximum in the reaction rate at a pressure corresponding to approximately 0.4Fc. Moreover, using integral equation calculations (as an alternative to the molecular simulations suggested by Iwai and co-workers), we show that the local composition of one reactant around the other goes through a maximum at essentially the same density, more clearly demonstrating the effect of local compositions on reactions. Literature Cited (1) Ellington, J. B.; Park, K. M.; Brennecke, J. F. Effect of Local Composition Enhancements on the Esterification of Phthalic Anhydride with Methanol in Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1994, 33, 965. (2) Roek, D. P.; Chateauneuf, J. E.; Brennecke, J. F. A Fluorescence Lifetime and Integral Equation Study of the Quenching of Naphthalene Fluorescence by Bromoethane in Super- and Subcritical Ethane. Ind. Eng. Chem. Res. 2000, 39, 3090. (3) Brennecke, J. F.; Chateauneuf, J. E. Homogeneous Organic Reactions as Mechanistic Probes in Supercritical Fluids. Chem. Rev. 1999, 99, 433. (4) Knutson, B. L.; Tomasko, D. L.; Eckert, C. A.; Debenedetti, P. G.; Chialvo, A. A. Local Density Augmentation in Supercritical Solutions: A Comparison Between Fluorescence Spectroscopy and Molecular Dynamics Results. In Supercritical Fluid Technology: Theoretical and Applied Approaches in Analytical Chemistry; Bright, F. V., McNally, M. E. P., Eds; ACS Symposium Series 488; American Chemical Society: Washington, DC, 1992; p 60.
IE010397V
10.1021/ie010397v CCC: $20.00 © 2001 American Chemical Society Published on Web 09/08/2001
