A thermodynamic study of the lipase-catalyzed transesterification of benzyl alcohol and butyl acetate in supercritical carbon dioxide media

The equilibrium constant for the lipase-catalyzed transesterification reaction (benzyl alcohol + butyl acetate = benzyl acetate + 1-butanol), using supercritical carbon dioxide (SCCO2) as the solvent, has been measured as a function of temperature (T = 303.15 to 318.15 K) at the pressure p = 10.0 MPa. At T = 298.15 K, the equilibrium constant K = 0.238 ± 0.020 and the standard molar Gibbs energy change ΔrG°m=3.56±0.22 kJ mol−1; the values for the standard molar enthalpy and standard molar entropy changes ΔrH°m and ΔrS°m, respectively, are zero within experimental error. The value of the equilibrium constant for this reaction in SCCO2 was compared with values determined by carrying out the reaction in organic solvents. A time course study of this reaction has also been carried out in supercritical carbon dioxide, n-hexane, toluene, and neat media (no solvent added) at the temperature T = 303.15 K. The time course data show that the reaction proceeds more rapidly in SCCO2 media than in the other three solvent systems and also that the reaction is at or near equilibrium within 3 h in the SCCO2 solvent system. However, the value of the equilibrium constant for the reaction carried out in SCCO2 is ≈30% smaller than the values of the equilibrium constants for the reactions carried out in hexane and toluene. Some advantages of the use of supercritical carbon dioxide as a solvent for enzyme-catalyzed reactions are discussed.