Supercritical water, aqueous organic solvents

The application of SCF as reaction media for enzymatic synthesis has several advantages, such as the higher initial reaction rates, higher conversion, possible separation of products from unreacted substrates, over solvent-free, or solvent systems (where either water or organic solvents are used). Owing to the lower mass-transfer limitations and mild (temperature) reaction conditions, at first the reactions which were performed in non-aqueous systems will be transposed to supercritical media. An additional benefit of using SCFs as... 

Extraction into another phase, using water-immiscible organic solvents, a second aqueous phase, or supercritical fluids. The last of these techniques, with supercritical CO2 as extractant, did not prove feasible on the extractive fermentation of 2-phenylethyl alcohol cell harvesting was required prior to extraction [15]. No further data on the use of supercritical extractive bioconversions using whole cells was found. 

A product may be removed from its producing cell by five main possible techniques. Evaporation occurs via stripping, (vacuum) distillation or by membrane-supported techniques such as pervaporation and transmembrane distillation. Extraction into another phase includes the use of water-immiscible organic solvents, supercritical fluids, or an aqueous two-phase system. In addition, the... 

During the printing of this volume, DuPont Company announced the introduction of the first generation of perfluoroplastics produced by a new process called Process G. This process utilizes supercritical carbon dioxide as the polymerization medium in contrast to water or organic solvents. The reader can find a review of this technology in Sec. 5.10 of this book. There are key advantages of Process G relative to aqueous medium polymerization. 

Extraction from Aqueous Solutions Critical Fluid Technologies, Inc. has developed a continuous countercurrent extraction process based on a 0.5-oy 10-m column to extract residual organic solvents such as trichloroethylene, methylene chloride, benzene, and chloroform from industrial wastewater streams. Typical solvents include supercritical CO9 and near-critical propane. The economics of these processes are largely driven by the hydrophihcity of the product, which has a large influence on the distribution coefficient. For example, at 16°C, the partition coefficient between liquid CO9 and water is 0.4 for methanol, 1.8 for /i-butanol, and 31 for /i-heptanol. 

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

The techniques discussed up to now use C02 as the mobile phase for substrates and products. Naturally, this restricts the applications to relatively non-polar and/or volatile components with sufficient solubility in the supercritical medium. An intriguing alternative for processing highly polar substrates are inverted aqueous systems. In this approach, a C02-philic catalyst resides in the non-polar C02 phase, while water-soluble substrates and products are contained in the aqueous layer [58, 59]. A very attractive and unique feature of the scC02/H20 system is that the stationary supercritical phase is never depressurized and hence the large energy input required for recompression is avoided. Furthermore, the aqueous solution is not contaminated with any organic solvent or catalyst residues, which is particularly important if the product is a fine chemical intended for direct further use in aqueous solution. 

Supercritical water (SCW) presents a unique combination of aqueous and non-aqueous character, thus being able to replace an organic solvent in certain kinds of chemical synthesis. In order to allow for a better understanding of the particular properties of SCW and of its influence on the rate of chemical reactions, molecular dynamics computer simulations were used to determine the free energy of the SN2 substitution reaction of Cl- and CH3C1 in SCW as a function of the reaction coordinate [216]. The free energy surface of this reaction was compared with that for the gas-phase and ambient water (AW) [248], In the gas phase, an ion-dipole complex and a symmetric transition... 

Organic solvents at STP and under supercritical conditions are the most common extractants for soil organic matter. Supercritical C02 and, to a lesser extent, N20 have also been used to extract both native and organic contamination from soil. Humus is extracted using aqueous solutions, but otherwise, water is rarely used to extract organic compounds from soil. A list of common soil organic matter extractants is given in Table 12.2. 

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. ... 

Purification. Where an aqueous system or aqueous purification is employed, water can be left with traces of organic solvents, such as toluene, which may prohibit river disposal. Supercritical-fluid techniques can be used for final purification. 7 8... 

Water. Water long used mainly for aqueous, generally ionic chemistry, more recently became attractive to replace organic solvents particularly under supercritical conditions (see discussion below). It is abundant, easy to purify, not flammable or toxic, and easily available in large quantities, and, therefore, its use is most economical. Product isolation in many cases can be a simple phase separation, and because of its large heat capacity, water offers an easy temperature control. The... 

Enzymatic reactions in non-aqueous solvents are subjected to a wide interest. A particular class of these solvents is the supercritical fluid (1) such as carbon dioxide that has many advantages over classical organic solvents or water no toxicity, no flammability, critical pressure 7.38 Mpa and temperature 31°C, and allowing high mass transfer and diffusion rates. 

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