A comparative study of biocatalysis in non-conventional solvents: Ionic liquids, supercritical fluids and organic media

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Abstract
The catalytic activities of cutinase immobilized on zeolite NaY and Candida antarctica lipase B immobilized on an acrylic resin (Novozym 453) were measured in a model transesterification reaction in three imidazolium cation-based ionic liquids (RTILs), supercritical ethane (sc-ethane), sc-CO2 and n-hexane, at a water activity (aW) of 0.2 and 0.7. The transesterification activity of cutinase was highest and similar in 1-n-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), sc-ethane and n-hexane, more than one order of magnitude lower in sc-CO2, and increased with an increase in aW. Hydrolysis was not detected in sc-fluids and n-hexane, and was observed in RTILs at aW 0.7 only. Both initial rates of transesterification and of hydrolysis of Novozym decreased with an increase in aW. sc-CO2 did not have a deleterious effect on Novozym activity, which was as high as in sc-ethane and n-hexane. The low reaction rates obtained in this case in RTILs suggested the existence of internal diffusion limitations absent in the cutinase preparation where the enzyme is only adsorbed at the surface of the support. sc-CO2 did not adversely affect the catalytic activity of cutinase suspended in [C4mim][PF6], suggesting a protective effect of the RTIL. In the case of Novozym, a marked increase in the rate of transesterification was obtained in the [C4mim][PF6]/sc-CO2 system, compared to the RTIL alone. This may reflect improved mass transfer of solutes to the pores of the immobilization matrix due to a high concentration of dissolved CO2 and a reduction in viscosity of the RTIL.