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

Abstract

The equilibrium constant for the lipase-catalyzed transesterification reaction (benzyl alcohol + butyl acetate = benzyl acetate + 1-butanol), using supercritical carbon dioxide (SCCO 2) 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 Δ r G° m=3.56±0.22 kJ mol −1; the values for the standard molar enthalpy and standard molar entropy changes Δ r H° m and Δ r S° m, respectively, are zero within experimental error. The value of the equilibrium constant for this reaction in SCCO 2 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 SCCO 2 media than in the other three solvent systems and also that the reaction is at or near equilibrium within 3 h in the SCCO 2 solvent system. However, the value of the equilibrium constant for the reaction carried out in SCCO 2 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.