Enzymatic esterifications and transesterifications in AOT-based gels with different composition

The enzyme-catalysed esterification between racemic 2-octanol and hexanoic acid using lipase immobilized in two different microemulsion-based gels has been investigated. The gels differ in the composition of the microemulsion used for gelation. Furthermore, transesterifications between vinyl butyrate and racemic 2-octanol or between vinyl butyrate and racemic menthol have been studied. The phase boundary of the L(2) phase, consisting of water droplets in oil, of the system water-AOT-decane has been outlined. Two microemulsions, one in the low-conducting region and one in the high-conducting region, were chosen for gelation and enzyme immobilization. The reactions were performed at 298.2 K by immersing the sectioned gel in hexane, containing the substrates. The conversion and selectivity of the esterification between 2-octanol and hexanoic acid were independent of the gel composition, but the initial reaction rate differed for the gels used. The transesterification between vinyl butyrate and 2-octanol was very fast, resulting in a maximum conversion after only 10 min. A subsequent hydrolysis, of the formed (R)-(-)-2-octyl butyrate, took place due to nonequilibrium conditions. A final conversion of (R)-(-)-2-octyl butyrate, of 0.45, was reached after 2 h. The enantiomeric purity of all the products was very high in all the studies. (C) 1998 Elsevier Science B.V.