Ionic supercritical carbon dioxide

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. 

Carbon dioxide, 4 803-822. See also CO2 entries Ionic liquid/CC>2 mixtures Supercritical carbon dioxide in acid gas stream, 23 605 adsorption of mixtures with ethane on zeolite 5A, 7 603 from anaerobic digestion, 3 701 from ascorbic acid metabolism, 25 771 in beer, 3 582t... 

Supercritical fluids (e.g. supercritical carbon dioxide, scCCb) are regarded as benign alternatives to organic solvents and there are many examples of their use in chemical synthesis, but usually under homogeneous conditions without the need for other solvents. However, SCCO2 has been combined with ionic liquids for the hydroformylation of 1-octene [16]. Since ionic liquids have no vapour pressure and are essentially insoluble in SCCO2, the product can be extracted from the reaction using CO2 virtually uncontaminated by the rhodium catalyst. This process is not a true biphasic process, as the reaction is carried out in the ionic liquid and the supercritical phase is only added once reaction is complete. 

Another environmental issue is the use of organic solvents. The use of chlorinated hydrocarbons, for example, has been severely curtailed. In fact, so many of the solvents favored by organic chemists are now on the black list that the whole question of solvents requires rethinking. The best solvent is no solvent, and if a solvent (diluent) is needed, then water has a lot to recommend it. This provides a golden opportunity for biocatalysis, since the replacement of classic chemical methods in organic solvents by enzymatic procedures in water at ambient temperature and pressure can provide substantial environmental and economic benefits. Similarly, there is a marked trend toward the application of organometal-lic catalysis in aqueous biphasic systems and other nonconventional media, such as fluorous biphasic, supercritical carbon dioxide and ionic liquids. ... 

Thus, the use of catalysts in new green reaction media such as ionic liquids, fluorous solvents, and supercritical carbon dioxide has become a viable alternative to those discussed within the chapters. 

This chapter covers the recent advances in amidocarbonylations, cyclohydrocarbonylations, aminocarbonylations, cascade carbonylative cyclizations, carbonylative ring-expansion reactions, thiocarbonylations, and related reactions from 1993 to early 2005. In addition, technical development in carbonylation processes with the use of microwave irradiation as well as new reaction media such as supercritical carbon dioxide and ionic liquids are also discussed. These carbonylation reactions provide efficient and powerful methods for the syntheses of a variety of carbonyl compounds, amino acids, heterocycles, and carbocycles. 

Brown, R.A., Pollet, R, McKoon, E., Eckert, C.A., Liotta, C.L., and Jessop, P.G., Asymmetric hydrogenation and catalyst recycling using ionic liquid and supercritical carbon dioxide, /. Am. Chem. Soc., 123,1254-1255,2001. 

Liu, F., Abrams, M.B., Baker, R.T., and Tumas, W., Phase-separable catalysis using room temperature ionic liquids and supercritical carbon dioxide, Chem. Commun., 433-434,2001. 

The miscibility behavior of ionic liquids and organic solvents is also rather unpredictable dichloromethane and THF mix with, e.g., [BMIm][ Tf2N], whereas alkanes and ethers do not, and ethyl acetate seems to be a borderline case [35]. Supercritical carbon dioxide (scC02) does not mix with ionic liquids such as [BMIm][PF6] and [OMIm][BF4], but is absorbed in the ionic liquid phase in huge amounts (up to a molar fraction of 0.7) [36]. No ionic liquid dissolves in the C02 phase. 

After performing the bioconversion in an ionic liquid, the product needs to be recovered and the biocatalyst and the ionic liquid recycled. Relatively volatile products can be removed by evaporation. Alternatively, immiscible organic solvents can be used to extract the product, and the biocatalyst can be recycled as a suspension in the ionic liquid phase [58]. A more elegant, green method, which avoids the use of volatile organic solvents altogether, involves the use of supercritical carbon dioxide as the extractive phase [96, 147, 148]. 

J. A. Laszlo, D. L. Compton, Chymotrypsin-catalyzed transestrification in ionic liquids and ionic liquid/ supercritical carbon dioxide. In Ionic Liquids, R. D. Rogers, K. R. Seddon, Eds., ACS Symposium Series Vol. 

Various nonconventional reaction media have been intensively studied in recent years, including water [34], supercritical COj [35], Jluorous hiphasic [36], and ionic liquids [37] alone or in hquid-liquid biphasic combinations. The use of water and supercritical carbon dioxide as reaction media fits with the current trend toward the use of renewable, biomass-based raw materials, which are ultimately derived from carbon dioxide and water. 

An efficient and convenient methodology for the aerobic oxidation of alcohols catalysed by sol-gel trapped perruthenate and promoted by an encapsulated ionic liquid in supercritical carbon dioxide solution has been reported. The reaction is highly selective and useful for substrates otherwise difficult to oxidize.263 A four-component system consisting of acetamido-TEMPO-Cu(C104)2-TMDP-DABCO has been developed for aerobic alcohol oxidation at room temperature. The catalytic system shows excellent selectivity towards the oxidation of benzylic and allylic alcohols and is not deactivated by heteroatom-containing (S, N) compounds. The use of DMSO as the reaction medium allows the catalysts to be recycled and reused for three runs with no significant loss of catalytic activity.264... 

As shown in Figure 1.2, the solvent strength of supercritical carbon dioxide approaches that of hydrocarbons or halocarbons. As a solvent, C02 is often compared to fluorinated solvents. In general, most nonpolar molecules are soluble in C02, while most polar compounds and polymers are insoluble (Hyatt, 1984). High vapor pressure fluids (e.g., acetone, methanol, ethers), many vinyl monomers (e.g., acrylates, styrenics, and olefins), free-radical initiators (e.g., azo- and peroxy-based initiators), and fluorocarbons are soluble in liquid and supercritical C02. Water and highly ionic compounds, however, are fairly insoluble in C02 (King et al., 1992 Lowry and Erickson, 1927). Only two classes of polymers, siloxane-based polymers and amorphous fluoropolymers, are soluble in C02 at relatively mild conditions (T < 100 °C and P < 350 bar) (DeSimone et al., 1992, 1994 McHugh and Krukonis, 1994). 

Blanchard LA, Brennecke JF (2000) Recovery of organic products from ionic liquids using supercritical carbon dioxide. Ind Eng Chem Res 40(l) 287-292... 

Lozano, P., T. de Diego, D. Carrie, M. Vaultier and J.L. Iborra, Continuous Green Biocatalytic Processes Using Ionic Liquids and Supercritical Carbon Dioxide, Chemical Communications, 692-693 (2002). 

Reetz, M.T., W. Wiesenhofer, G. Francio and W. Leitner, Continuous Flow Enzymatic Kinetic Resolution and Enantiomer Separation Using Ionic Liquid/Supercritical Carbon Dioxide Media, Advanced Synthesis Catalysis, 345, 1221-1228 (2003). 

In biphasic reactors or two-phase partitioning bioreactors (TPPB), the substrate is located mostly in the immiscible phase and diffuses to the aqueous phase. The enzyme catalyzes conversion of the substrate at the interface and/or in the aqueous phase. The product/s of the reaction then may partition to the organic phase. The system is self-regulated, as the substrate delivery to the aqueous phase is only directed by the partitioning ratio between the two phases and the enzymatic reaction rate [53]. The use of ionic liquid/supercritical carbon dioxide for enzyme-catalyzed transformation is gaining attention [69]. 

Zhao GY, Jiang T, Han BX, et al. Electrochemical reduction of supercritical carbon dioxide in ionic liquid l-n-butyl-3-methylimidazolium hexafluorophosphate. J Supercrit Fluids. 

Reetz, M.T., Wiesenhoefer, W., Francio, G., and Leitner, W., Biocatalysis in ionic liquids batchwise and continuous flow processes using supercritical carbon dioxide as the mobile phase, Chem. Commun. 992-993 (2002). 

Other non-classical reaction media [96] have, in recent years, attracted increasing attention from the viewpoint of avoiding environmentally unattractive solvents and/or facilitating catalyst recovery and recycling. Two examples, which readily come to mind, are supercritical carbon dioxide and room temperature ionic liquids. Catalytic hydrogenation in supercritical C02, for example, has... 

Finally, it should be noted that Lewis acids and bases can also be used in other non-conventional media, as described in Chapter 7, e.g. fluorous solvents, supercritical carbon dioxide and ionic liquids by designing the catalyst, e.g. for solubility in a fluorous solvent or an ionic liquid, to facilitate its recovery and reuse. For example, the use of the ionic liquid butylmethylimidazolium hydroxide, [bmim][OH], as both a catalyst and reaction medium for Michael additions (Fig. 2.45) has been recently reported [151]. 

Chapter 7 addresses another key topic in the context of green chemistry the replacement of traditional, environmentally unattractive organic solvents by greener alternative reaction media such as water, supercritical carbon dioxide, ionic liquids and perfluorous solvents. The use of liquid/liquid biphasic systems provides the additional benefit of facile catalyst recovery and recycling. 

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