Synthesis of esters in ionic liquids: The effect of vinyl esters and alcohols

Abstract

Free Candida antarctica lipase B (CALB) was successfully applied to catalyzing the synthesis of alkyl esters by transesterification from vinyl esters and alcohols in ionic liquids based on 1-alkyl-3-methylimidazolium cations ([bmim +] and [omim +]) and the hexafluorophosphate anion ([PF 6 −]). First, the variables affecting the performance of CALB in transesterification reactions in ionic liquids, temperature and pH, were studied, choosing the synthesis of butyl butyrate in [bmim +][PF 6 −] as reaction model. An increase in the rate of butyl butyrate synthesis in [bmim +][PF 6 −] was observed as the temperature was increased from 30 to 70 °C, while the optimal pH was 7. Secondly, the role of vinyl esters and alcohols in lipase-catalyzed ester synthesis in ionic liquids was investigated. When the number of carbon atoms of the vinyl ester was increased, the optimum activity was found for six carbon atoms. The influence of the alkyl chain length of the alcohol was tested, the optimal length being four carbon atoms. The rate of catalyzed ester synthesis of esters was found to be closely dependent on the solvent medium, being higher in [omim +][PF 6 −].