Real time measurement and control of thermodynamic water activities for enzymatic catalysis in hexane

The esterification reaction of geraniol with acetic acid catalyzed by immobilized Candida antarctica lipase B was studied in hexane using a pervaporation-assisted batch reactor. The effect of thermodynamic water activity (aw) on the initial reaction rate was investigated at aw ranging from 0.02 to 1.0. The aw was monitored on-line in real time. aw was actively controlled throughout the reaction by using highly water-selective membrane pervaporation. This novel combination of aw sensing and control eliminates changes in aw during the reaction even in the initial phase of relatively rapid water release during an esterification. No chemicals are introduced for aw control, and no purge gases or liquids are needed. A maximum in the initial reaction rate was found approximately at aw = 0.1. The initial reaction rate declined quickly at higher aw, and dropped precipitously at lower aw.