Comparison of lipase-catalyzed enantioselective esterification of (±)-menthol in ionic liquids and organic solvents

Enantioselective esterification of (±)-menthol was studied using Candida rugosa lipase (CRL) in ionic liquids (1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIM][PF6]) and 1-butyl-3-methyl-imidazolium tetraflouroborate) and organic solvents of different hydrophobicities. Propionic anhydride was employed as an acylating agent. Because the enzyme showed comparable conversion yield and enantioselectivity in [BMIM][PF6] and hexane in a 24-h reaction, more work focused on these two reaction media. Comparison of the activity, stability and enantioselectivity of CRL was carried out by examining the effects of the mole ratio of substrates, temperature, incubation time and enzyme recycling. It was found that temperature control was more crucial in the ionic liquid than in hexane to reach high conversion and enantioselectivity. The ionic liquid system showed an advantage of using less acid anhydride to achieve higher (±)-menthol conversion yield and better enantioselectivity. Moreover, during an incubation of 4–60 days in the ionic liquid, CRL activity was 2.5 times higher than its initial value, while that in hexane decreased to less than 60% in 2 days. In addition, the enzyme showed potentiality of recycled use in the ionic liquid. These advantages of the ionic liquid suggest that it would be used as a green alternative to organic solvents for the enantioselective esterification of (±)-menthol.