Submitted to: Intl Symposium on Supercritical Fluid Chromatography and Extraction
Publication Type: Proceedings
Publication Acceptance Date: 4/8/2000
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Thermodynamic analysis of the effect of solvent media on the enzyme catalyzed conversion of acids and alcohols to a favored ester enantiomer requires precise measurement of the associated differential thermodynamic functions. The magnitude of these thermodynamic functions and the associated enantioselectivity (E) have been evaluated in the condensed liquid phase; however, a comparison of these results for a model esterification reaction in supercritical fluid media has not been evaluated. 3-methyl-2-butanol was transesterified with vinyl octanoate in supercritical carbon dioxide (SC-CO2) using Novozym 435 as the active lipase, yielding an excess of the R-enantiomer of 3-methyl-2-butyl octanoate. A novel stirred reaction vessel was constructed from a high pressure gauge snubber and placed in a commercial SFE instrument to facilitate measurement of E as a function of temperature in SC-CO2. Using pressures between 14-22 MPa over a temperature range of 45-90 deg C, the thermodynamic functions and E values could be determined for the above reaction. Thermodynamic functions were evaluated via the Arrhenius dependence of the ln E. E was lower and the magnitude of the differential activation Gibbs free energy, enthalpy, and entropic term were found to all be decidedly less negative than the values displayed for these functions in liquid solvents over the same range of temperatures. This was found to be true when comparing results obtained using SC-CO2 as a solvent with those obtained for running the same reaction in n-hexane, despite the similarity in solubility parameters for the two solvent media. The lower E value obtained for the model reaction in SC-CO2 suggests that moderation should be applied in promoting the use of enzymatic conversions.