Three new symmetrical 1,3-dialkylimidazolium ionic liquids including 1,3-di(n-butyl)imida- zolium hexafluorophosphate ([DnBIM][PF6]), 1,3-di(iso-butyl)imidazolium hexafluorophosphate ([DiBIM][PF6]) and 1,3-di(s-butyl)imidazolium hexafluorophosphate ([DsBIM][PF6]) were designed and synthesized. The esterification reaction of l-menthol with acetic anhydride catalyzed by Pseudomonas cepacia lipase was used as a model reaction to investigate the activity, reactivity and stability of the lipase in different media. The results indicated that the new ionic liquids as reaction medium were obviously better than classical ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate and organic solvent hexane. Among these, the [DiBIM][PF6] is of best biocompatibility and was then selected as medium of the model reaction. Moreover, various factors on the conversion of l-menthol including reaction temperature, molar ratio of the two substrates, the amounts of the ionic liquid, water content and reuse of the lipase were investigated in detail. Under optimal reaction conditions, the conversion of l-menthol was about 97.4%. Its initial reaction rate, the equilibrium conversion of l-menthol and the half-lifetime of the lipase in the [DiBIM][PF6] medium were about 12.7, 4.6 and 15.1-fold those obtained in hexane. Because the three ionic liquids used in the study are isomeric compounds, the fact also confirmed that besides hydrophobicity and nucleophilicity the spatial configuration of ionic liquids should be considered a key factor effecting enzyme performance in ionic liquid.

Key words: ionic liquid, lipase, menthol, menthyl acetate