Submitted to: Intl Symposium on Supercritical Fluid Chromatography and Extraction
Publication Type: Abstract Only
Publication Acceptance Date: 4/8/2000
Publication Date: N/A
Citation: N/A 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 (Gibbs free energy, enthalpy, and entropy). 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. In this study, 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 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 sa function of temperature in SC-CO2. Using pressures between 14-22 MPa over a temperature range of 6 - 90 deg C, the thermodynamic functions and E values could be determined for the above reaction. E was lower and the magnitude of the differential activation Gibbs free energy, enthalpy, and entropic term, as well as E, were found to all be decidedly less negative than the values displayed for these functions in liquid solvents over the same range of temperatures. 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 in supercritical fluid media, despite their many cited advantages.