Short communication Synthesis of esters in ionic liquids The effect of vinyl esters and alcohols Antonia P. de los Rı ´os a , Francisco J. Herna ´ ndez-Ferna ´ ndez b , F. Toma ´ s-Alonso a , Demetrio Go ´ mez a , Gloria Vı ´llora a, * a Department of Chemical Engineering, Faculty of Chemistry, University of Murcia, P.O. Box 4021, Campus of Espinardo, E-30100 Murcia, Spain b Departament of Chemical and Environmental Engineering, Polytechnic University of Cartagena, Paseo Alfonso XIII, 42, Cartagena, E-30203, Spain 1. Introduction The use of biocatalysts in non-aqueous environments has received growing attention during the last two decades due to the advantages they offer, such as the possibility of carrying out processes involving hydrolytic enzymes which are thermodynami- cally unfavourable in water (e.g. transesterification), and their ability to increase the solubility of organic substrates [1]. Among these non-conventional media, ionic liquids (ILs) have recently appeared as clean alternatives to classical organic solvents for a wide variety of biochemical processes [2–6]. Ionic liquids, organic salts which are liquid close to room temperature, normally consist of an organic cation (e.g. dialkylimidazolium, tetraalkylammomium) and a polyatomic inorganic anion (e.g. tetrafluoroborate, hexafluoropho- sphate). From an environmental point of view, the most important properties of ionic liquids are their negligible vapour pressure and their good chemical and thermal stabilities [7]. Additionally, the physical–chemical properties of ionic liquids, such as their hydrophobicity, density, viscosity, melting point, polarity and solvent properties, may be tunable by appropriate modification of the cation and/or anion [2]. These features are important because it might be possible to design an optimal IL for each specific enzymatic reaction system [2]. Lipase-catalyzed reactions in ILs have been seen to have several potential advantages over conventional organic reaction media, including increased synthetic activity, (enantio)s- electivity and stability [8–13]. The aim of the present work was to study the ability of free Candida antarctica lipase B (CALB) to catalyze the synthesis of organic esters, which are commonly used in the perfumery, flavour and pharmaceutical industries, by transesterification from vinyl esters and alcohols in two water-immiscible ILs, 1-butyl-3-methylimida- zolium hexafluorophosphate ([bmim + ][PF 6 À ]) and 1-octyl-3- methylimidazolium hexafluorophosphate ([omim + ][PF 6 À ]) at low water content. Vinyl esters were used as the acylating agent for transesterification because they offer an effective solution to overcome equilibrium since the enol co-product is immediately transformed into acetaldehyde [14] (see Scheme 1). Variables affecting the performance of CALB in transesterifica- tion reactions in ionic liquids, such as temperature and pH, were studied. Furthermore, the role of vinyl esters (RCOOCHCH 2 , R CH 3 –, CH 3 CH 2 –, CH 3 (CH 2 ) 2 –, CH 3 (CH 2 ) 8 –, CH 3 (CH 2 ) 10 –, CH 2 CH–, CH 2 C(CH 3 )–, Ph–) and alcohols (ROH, R CH 3 –, CH 3 (CH 2 ) 3 –, CH 3 (CH 2 ) 5 –, CH 3 (CH 2 ) 7 –) in lipase-catalyzed ester synthesis in ionic liquids was investigated. Process Biochemistry 43 (2008) 892–895 ARTICLE INFO Article history: Received 12 February 2008 Received in revised form 13 April 2008 Accepted 14 April 2008 Keywords: Ionic liquids Lipase Esters Activity Selectivity Green chemistry 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-methylimida- zolium cations ([bmim + ] and [omim + ]) and the hexafluorophosphate anion ([PF 6 À ]). 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 À ] as reaction model. An increase in the rate of butyl butyrate synthesis in [bmim + ][PF 6 À ] was observed as the temperature was increased from 30 to 70 8C, 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 À ]. ß 2008 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +34 968 367 363; fax: +34 968 364 148. E-mail address: aprios@um.es (G. Vı ´llora). Contents lists available at ScienceDirect Process Biochemistry journal homepage: www.elsevier.com/locate/procbio 1359-5113/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2008.04.012