IL-SC-CO2 Biphasic Systems

Typically, the SC-CO2 and IL system form a biphasic mixture that contains the enzyme in a denser phase, which is the IL phase whereas the lighter phase acts as a carrier for the substrates and products. In enzyme-catalyzed reactions in a biphasic mixture, the immobilized enzymes are suspended in the IL phase, and reaction substrates are dissolved in SC-CO2. The substrates diffuse from the bulk of the SC-CO2 phase into the two-phase interface, followed by partitioning between the two phases and diffusion into the IL phase toward the active site of the enzyme. The products are then released in the IL phase and extracted by SC-CO2 (de los Rios et al. 2007a). 

The extraction of the product from the ILs with SC-CO2 is the most important advantage of the biphasic systems. Typically, the effectiveness of SC-CO2 for extraction depends on the phase behavior of the binary IL-SC-CO2 system. The solubility of CO2 in the IL is important to allow for contact between the CO2 and the products. The dissolved CO2 also decreases the viscosity of the IL and therefore improves mass transfer. It has been reported that CO2 is soluble in every IL, whereas ILs are not soluble in the gaseous CO2 phase, even at high pressures (Blanchard et al., 2001). Such systems have been successfully used in the synthesis of esters. 

Although the IL-SC-CO2 system enhances the stability of the enzyme, a reduction in reaction rates was observed in the biphasic system due to portioning and mass transfer restrictions between the two phases. By adjusting the appropriate composition of cations and anions in the IL, the activity and selectivity of the enzyme, and the mass transfer between phases can be improved. However, as the reaction proceeds and more products are formed, the interaction between these products and the IL may cause a significant increase in the solubility of the IL in the SC-CO2 (Wu et al., 2003, 2004). 

The development of a continuous system, in which immobilized enzymes can be used and both solvents can be recycled, would decrease overall costs. Hernandez et al. (2006) and de los Rios et al. (2007b) developed a process combining an IL-SC-CO2 system with membrane technology to synthesize vinyl propionate from vinyl propionate and 1-butanol at 50°C and 80 bar in a recirculating bioreactor with the presence of coated immobilized C. antarctica lipase B within the IL. It was found that the selectivity of the lipase increased with the use of ILs, compared to that achieved with supercritical carbon dioxide tested alone. 

Adachi, Y., and B. C. Y. Lu. 1983. Supercritical Fluid Extraction with Carbon Dioxide and Ethylene. Fluid Phase Equilibria 14 147-156. 

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Enzyme transformations in ILS/SC-CO2 biphasic systems, whereby enzyme molecules are immobihzed in the IL phase and substrates/products are transported by the SC-CO2 phase, are described as a way for carrying out clean synthetic chemical processes to produce pure products [52]. 

Kinetic resolution of 1-phenylethanol catalyzed by CALB was carried out in the IL/SC-CO2 biphasic system. To prevent undesirable reactions and ensure better conversion of (R)-1-phenylethanol, [bmim][PF,5] was chosen for this kind of experiments. Because of the possible direct and indirect effects of the pressure on the activity of biocatalyst its influence was studied between 6 and 36.5 MPa. At aU conditions examined a biphasic reaction medium was attained, which is illustrated in Figure 8.5. The enzyme was suspended in the IL phase, where the reaction took place. The substrates and products resided largely in the supercritical phase, which was also the extractive phase. 

The outlook of syntheses, based on using SC-CO2 or appropriate IL/SC-CO2 biphasic system for developing integral green chemical processes due to the physical and chemical characterishcs of these neoteric solvents and the enhanced enzyme catalytic properties seems promising. 

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