Tunable solvent systems

Blasucd, V.M., Husain, Z.A., Fadhel, A.Z., Donaldson, M.E., Vyhmeister, E., Pollet, P., Hotta, C.L. and Eckert, C.A. (2010) Combining homogeneous catalysis with heterogeneous separation using tunable solvent systems. Journal of Physical Chemistry A, 114 (11), 3932-3938. 

A recent proposal concerns mixed organic-aqueous tunable solvents (OATS) such as dimethyl ether-water, the solubility of which for substrates can be influenced by a third component such as carbon dioxide. CO2 acts as a antisolvent and as a switch to cause a phase separation and to decant the phases from each other (preferably under pressure). This behavior makes the operation of bi- or multiphase homogeneous catalytic processes easier and more economic the preferential dissolution at modest pressure of carbon dioxide causes phase separation which results in large distribution coefEcients of target molecules in biphasic organic-aqueous systems. This extraordinary behavior lead to a sophisticated flow scheme (Figure 6) [7]. 

Higher rates Different selectivities Fewer reaction steps Additional safety Enhanced separation Continuous-flow operation Miscibility with gases, rapid mass transfer Weak coordination, pressure tuning In-situ protection of amines No toxicity, inertness, good heat transfer Tunable solvent properties, multiphase systems Multiphase systems, mass transfer... 

SCFs are an environmentally friendly alternative to organic solvents as media for biocatalysis. A key feature of biocatalysis in SCFs is the tunability of the medium [75]. Enzymatic activity in SCFs has been proven and well documented [76]. Limiting factors, which may affect enzymatic activity in supercritical solvent systems, have been identified and are well characterized. A major limitation to the broader use of SCFs is their inability to dissolve a wide range of hydrophilic and ionic compounds, which greatly impedes their ability to carry out biolransformation with polar substrates. The interest in water-in-SCF microemulsion as reaction media stems from the fact that in such systems high concentrations of both polar and apolar molecules can be dissolved within the dispersed aqueous and continuous SCF phases, respectively. 

The difficulty in direct synthesis of mesoporous transition metal oxides by soft templating (surfactant micelles) arises from their air- and moisture-sensitive sol-gel chemistry [4,10,11]. On the other hand, mesoporous silica materials can be synthesized in nimierous different solvent systems (i.e., water or water-alcohol mixtures), various synthetic conditions (Le., acidic or basic, various concentration and temperature ranges), and in the presence of organic (Le., TMB) and inorganic additives (e.g., CT, SO, and NOs ) [12-15]. The flexibility in synthesis conditions allows one to synthesize mesoporous silica materials with tunable pore sizes (2-50 nm), mesostructures (Le., 2D Hexagonal, FCC, and BCC), bimodal porosity, and morphologies (Le., spheres, rods, ropes, and cubes) [12,14,16-19]. Such a control on the physicochemical parameters of mesoporous TM oxides is desired for enhanced catalytic, electronic, magnetic, and optical properties. Therefore, use... 

Experimental limitations initially limited the types of molecular systems that could be studied by TRIR spectroscopy. The main obstacles were the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long pathlengths and/or multipass cells could be used without interference from solvent IR bands. Pimentel and co-workers first developed a rapid scan dispersive IR spectrometer (using a carbon arc broadband IR source) with time and spectral resolution on the order of 10 ps and 1 cm , respectively, and reported the gas phase IR spectra of a number of fundamental organic intermediates (e.g., CH3, CD3, and Cp2). Subsequent gas phase approaches with improved time and spectral resolution took advantage of pulsed IR sources. 

An ionic liquid can be used as a pure solvent or as a co-solvent. An enzyme-ionic liquid system can be operated in a single phase or in multiple phases. Although most research has focused on enzymatic catalysis in ionic liquids, application to whole cell systems has also been reported (272). Besides searches for an alternative non-volatile and polar media with reduced water and orgamc solvents for biocatalysis, significant attention has been paid to the dispersion of enzymes and microorganisms in ionic liquids so that repeated use of the expensive biocatalysts can be realized. Another incentive for biocatalysis in ionic liquid media is to take advantage of the tunability of the solvent properties of the ionic liquids to achieve improved catalytic performance. Because biocatalysts are applied predominantly at lower temperatures (occasionally exceeding 100°C), thermal stability limitations of ionic liquids are typically not a concern. Instead, the solvent properties are most critical to the performance of biocatalysts. 

These structures are extensively described in the current literature (Fanum, 2008 Friberg, 1976 Birdi, 2002 Holmberg, 2004 Somasundaran, 2006). Even within the same phases, their self-assembled structures are tunable by the concentration for example, in lamellar phases, the layer distances increase with the solvent volume. Lamellar structures are found in systems such as the common hand soap, which consists of ca. 0% soap + 20% water. The layers of soap molecules are separated by a region of water (including, salts etc.) as a kind of sandwich. The x-ray diffraction analysis shows this structure very clearly. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals. Similar phases and characteristics can be observed in immiscible diblock copolymers. 

In this chapter, we provide an overview of our recent research on solvent extraction of fission products based on ILs. Since the inception of fhe IL-based extraction system for metal ions in 1999, this approach has proven to be highly efficient for the extraction of metal ions. The success of the IL-based extraction systems lies in its ionicity, tunability, ion exchangeability, and nonvolatility. Although this review is focused on the recent research activities of our research groups, we hope to convince readers that the separation of metal ions based on IL is a fascinating research arena. Existing achievements and many anticipated future advances in this area will have fundamental and practical impacts on separation sciences. 

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