Water Activity Effects on Geranyl Acetate Synthesis Catalyzed by Novozym in Supercritical Ethane and in Supercritical Carbon Dioxide

The esterification reaction of geraniol with acetic acid catalyzed by Novozym was studied in supercritical ethane (sc-ethane) and in supercritical carbon dioxide (sc-CO2). Water activity (a W) had a very strong effect on enzyme activity, with reaction rates increasing up to a W = 0.25 and then decreasing for higher a W. Salt hydrate pairs could not prevent changes in a W during the course of reaction but were able to control a W to some extent and had a beneficial effect on both initial rates of esterification and conversion in sc-ethane. The enzyme was more active in sc-ethane than in sc-CO2, confirming the deleterious effect of the latter already observed with some enzymes. Temperatures between 40 and 60 °C did not have a strong effect on initial rates of esterification, although reaction progress declined considerably in that temperature range. For the mixture of 50 mM acetic acid plus 200 mM geraniol, 100% conversion was achieved at a reaction time of 10 h at 40 °C, 100 bar, an a W of incubation of 0.25, and a Novozym concentration of 0.55 mg cm-3 in sc-ethane. Conversion was below 50% in sc-CO2 at otherwise identical conditions. With an equimolar mixture of the two substrates (100 mM), 98% conversion was reached at 10 h of reaction in sc-ethane (73% conversion in sc-CO2). Keywords: Novozym 435; Candida antartica lipase B; enzymatic esterification; supercritical fluids; geranyl acetate; geraniol; water activity