Supercritical biphasic systems

These alternative processes can be divided into two main categories, those that involve insoluble (Chapter 3) or soluble (Chapter 4) supports coupled with continuous flow operation or filtration on the macro - nano scale, and those in which the catalyst is immobilised in a separate phase from the product. These chapters are introduced by a discussion of aqueous biphasic systems (Chapter 5), which have already been commercialised. Other chapters then discuss newer approaches involving fluorous solvents (Chapter 6), ionic liquids (Chapter 7) and supercritical fluids (Chapter 8). 

Biphasic systems that contain the catalyst in the supercritical phase and the substrates/products in a second liquid phase can also be implemented. With water as the polar phase, these inverted systems are particularly attractive for the conversion of highly polar and/or low-volatile hydrophilic substrates with limited solubility in typical SCFs such as scC02. 

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. ... 

Webb, P.B.and Sellin, M.F. and Kimene, T.E. and Williamson, S. and Slawin, A.M.Z. and Cole-Hamilton, D.J. (2003). Continuous Flow Hydroformylation of Alkenes in Supercritical Fluid-Ionic Liquid Biphasic System. J. Am. Chem. Soc., 125, 15577-15588. 

Biphasic systems containing an ionic liquid and supercritical CO2 have been used effectively for catalytic hydrogenation of alkenes. The ionic liquid phase containing the catalyst could be reused (2/6). 

Biphasic systems consisting of ionic liquids and supercritical CO2 showed dramatic enhancement in the operational stability of both free and immobilized Candida antarctica lipase B (CALB) in the catalyzed kinetic resolution of rac- -phenylethanol with vinyl propionate at 10 MPa and temperatures between 120 and 150°C (Scheme 30) 275). Hydrophobic ionic liquids, [EMIM]Tf2N or [BMIM]Tf2N, were shown to be essential for the stability of the enzyme in the biotransformation. Notwithstanding the extreme conditions, both the free and isolated enzymes were able specifically to catalyze the synthesis of (J )-l-phenylethyl propionate. The maximum enantiomeric excess needed for satisfactory product purity (ee >99.9%) was maintained. The (S)-l-phenylethanol reactant was not esterified. The authors suggested that the ionic liquids provide protection against enzyme denaturation by CO2 and heat. When the free enzyme was used, [EMIM]Tf2N appeared to be the best ionic liquid to protect the enzyme, which... 

A mixed reaction medium, composed of scC02 and ILs, has been defined as a new biphasic system by Advanced Industrial Science and Technology (AIST), and used for selective and efficient CC synthesis. For example, 1-alkyl-3-methylimidazolium salts represent a suitable system when used under supercritical conditions for the synthesis of CCs [156] from epoxides and C02. Kanawami et al. [159] have reported that the use of 1-octyl-3-methylimidazolium tetrafluoroborate under supercritical conditions resulted in a 100% conversion into PC, with 100% selectivity, within only a few minutes (Equation 7.15). 

The use of sc C02 instead of toluene as a solvent leads to some rate enhancement in these two systems, although it is clear that this activity is still not practical for most nonpolar, nonvolatile substrates. Significant improvements to the biphasic water/supercritical C02 system were accomplished by forming H20/C02 emulsions using newly developed surfactants (Jacobson et al., 1999). Three different surfactants were used that form water in C02 (w/c) or C02 in water (c/w) emulsions (1) anionic surfactant perfluoropolyether ammonium carboxylate, (2) cationic Lodyne 106A, and (3) nonionic poly(butylene oxide)-h-poly(ethylene oxide). The low interfacial tension, y, between water and C02 (17 mNm-1 at pressures above 70 bar), which is significantly lower than water/alkane systems (30-60 mNm-1),... 

Fluoromethylbenzoic acids, metallation, 9, 26-27 Fluoro(phenyl) complexes, with platinum(II), 8, 482 Fluorosilanes, elimination in fluorinated alkene activation, 1, 732 in fluorinated aromatic activation, 1, 731 and hydrodefluorination, 1, 748 Fluorosilicate anions, hypercoordinated anions, 3, 484 Fluorotoluenes, metallation, 9, 21 Fluorous alkylstannanes, preparation, 3, 820 Fluorous biphasic system, as green solvent, 12, 844 Fluorous ligands, with supercritical carbon dioxide, 1, 82 Fluorous media... 

Biphasic Systems with Supercritical Carbon Dioxide... 

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. 

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. 

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