Ionic liquids as clean alternative to organic solvents for lipase-catalyzed ester synthesis in non-conventional media
Sustainable process-product development & green chemistry
Modelling, Design & Analysis of Processes with Ionic Liquids (T1-1P)
Keywords: ionic liquids, lipase, ester synthesis, additives, stability
Ionic liquids as clean alternative to organic solvents for lipase-catalyzed ester synthesis in non-conventional media
F.J. Hernández-Fernández, A.P. de los Ríos, , D. Gómez, F. Tomás-Alonso, M. Rubio and G. Víllora*
Department of Chemical Engineering , Faculty of Chemistry, University of Murcia, P.O. Box 4021, Campus de Espinardo, E-30100, Murcia, Spain.
E-mail: gvillora@um.es
This work studies the synthesis of aliphatic esters by transesterification from 1-butanol and vynil esters using Candida antarctica lipase B (CaLB) in ionic liquids. The efficiency of the catalytic action was measured by two parameters, the synthetic rate and the ratio between this parameter and the acyl-donor consumption rate (selectivity). The effect of various parameters such as pH, temperature, acyl-donor and additives on the enzymatic activity and selectivity was studied. The IL medium selected for this study has been 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim+][PF6-]). The reaction was also carried out in a common water-immiscible organic solvent, hexane, in order to use it as reference solvent to study the suitability of this IL for the proposed reaction. It has been observed that at all assayed operating conditions, the enzymatic activity in [bmim+][PF6-] was higher than that obtained in hexane. It was also found a strongly influence of the pH on the synthetic activity and selectivity of the lipase in [bmim+][PF6-], showing a maximum at pH 7, while an increase in the reaction temperature lead to an increase in both parameters in the experimental interval (30 - 70 ºC).
A strong influence of the nature of different vinyl esters (RCOOCHCH2, R= CH3-, CH3CH2-, CH3(CH2)2-, CH3(CH2)8-, CH3(CH2)10-, CH2CH-, CH2C(CH3)-, Ph-) used as acyl-donor on both activity and selectivity was found. When the alkyl chain length of the acyl donor was increased, the synthetic activity of CALB showed a bell curve, with a maximum at an 4-carbon chain length of the vinyl ester (vynil butyrate).
In order to improve the use of this solvent as reaction media, different treatments were carried out on the ionic liquid, such as equilibration with NaHCO3 or Na2CO3 solution until neutral pH, or adding triethylamine to the reaction mixture. The lipase activity was greatly enhanced in the IL treated in these ways.
Finally, the study was extended to seventeen water-immiscible and water-miscible 1,3-dialkylimidazolium based ionic liquids using vinyl butyrate as acyl donor at 30 ºC and pH 7. It has been observed that in all the water-immiscible ionic liquids assayed the enzymatic activity and selectivity were higher than the obtained in hexane. However, in water-miscible ionic liquid the activity result to be smaller than that in hexane. In general, for the ILs based on the same anion, the synthetic activity were gradually enhanced by the increase in hydrophobicity of the ILs, Furthermore, it has been observed that the activity of CaLB is greater in ionic liquids containing anions with lower nucleophilicity.
Moreover, to know the direct influence of the IL medium on the enzyme, the stability of the lipase has been studied by the incubation of the enzyme at 30 ºC. It has been observed that the increase in the enzyme´s half-life seemed to be in agreement with the increase in the hydrophobicity of the ionic liquid. In addition, the enzyme showed potentiality of recycled use in the ionic liquid.
Acknowledgements:
This work was partially supported by the CICYT CTQ2005-09238/PPQ. The authors are thankful to Mr. R. Martínez from Novo España S.A. for the generous gift of enzymes.
Presented Monday 17, 13:30 to 15:00, in session Modelling, Design & Analysis of Processes with Ionic Liquids (T1-1P).
