Supercritical systems catalytic properties

Table 1.6 Comparison of catalytic properties for biphasic and supercritical reaction systems...

The combination of ionic liquids with supercritical carbon dioxide is an attractive approach, as these solvents present complementary properties (volatility, polarity scale.). Compressed CO2 dissolves quite well in ionic liquid, but ionic liquids do not dissolve in CO2. It decreases the viscosity of ionic liquids, thus facilitating mass transfer during catalysis. The separation of the products in solvent-free form can be effective and the CO2 can be recycled by recompressing it back into the reactor. Continuous flow catalytic systems based on the combination of these two solvents have been reported [19]. This concept is developed in more detail in Section 5.4. 

In this report, the nse of ionic Uquids/supercritical carbon dioxide in biocatalytic reactions has been extensively reviewed. Properties of supercritical carbon dioxide, ionic liquids and ionic liqnids/supercritical carbon dioxide biphasic systems have been analysed. Representative examples of the enzyme catalytic reactions in IL/ scCOj biphasic systems have been included. Finally, the effect of the biphasic systems on activity, selectivity and stability of enzymes has been carefully analysed. 

Heterogeneous catalysis takes place in multiphase systems. If a new phase, the supercritical phase, is selectively introduced, remarkable change can be expected. The effects caused by the introduction of a supercritical phase depend on many parameters, such as fluid properties, reaction conditions, and affinity. Successful supercritical phase heterogeneous catalytic reactions can be realized, as long as these parameters are carefully controlled. With greater activities, catalyst lifetimes, or selectivities than for gas phase or liquid phase reactions, some supercritical phase reactions have industrial potentid. 

This chemical and physico-chemical behavior of the binary H2O-CO2 mixture [38] suggests that water is an attractive liquid to be combined with supercritical carbon dioxide in multiphase catalysis. CO2/H2O systems have adequate mass-transfer properties, especially if emulsions or micro-emulsions can be formed ([39] and refs, therein). The low pH of aqueous phases in the presence of compressed CO2 (pH ca. 3-3.5 [40]) must be considered and the use of buffered solutions can be beneficial in the design of suitable catalytic systems, as demonstrated for colloid-catalyzed arene hydrogenation in water-scC02 [41]. 

Potentially, these advantages allow for high reaction rates, high rates of transport of substrate over the catalyst, and low leaching of both the catalyst and the IL. Besides, the favorable properties of CO2 in terms of very low toxicology, few environmental hazards, process safety, and low material costs make it especially attractive for green and sustainable synthetic processes. There would appear to be excellent potential for the application of SILP systems with supercritical flow in a variety of different catalytic reactions. 

Beneficial reaction conditions for biphasic catalytic systems can be created using mixtures of water with supercritical CO2 that have rather unique and controllable phase properties. 

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