Aqueous - supercritical fluids

Jason Bonilla R, James BR, Jessop PG (2000) Colloid-catalysed arene hydrogenation in aqueous/supercritical fluid biphasic media. Chem Commun 11 941-942... 

Biocatalytic reaction in aqueous-supercritical fluids biphasic systems have also been investigated and some examples are shown further. 

Asymmetric reduction of ketones by crude alcohol dehydrogenase from G. candidum was carried out in aqueous-supercritical fluids biphasic systems (Figure 3.20) [30]. It was found that the lowered pH caused by scCO irreversibly inactivated the enzyme. In order to overcome this undesired side effect of scCO, basic salts were added to the reaction mixture. Sodium bicarbonate was found to be the best among several tested. For the reduction of acetophenone, adding sodium bicarbonate improved the )deld from 4% to 25%. The asymmetric reduction of o-fluoroacetophenone with the immobilized... 

Carboxiation of pyrole by decarboxylase from B. megaterium PYR 2910 in aqueous supercritical fluids biphasic systems [29]. 

Supercritical Extraction. The use of a supercritical fluid such as carbon dioxide as extractant is growing in industrial importance, particularly in the food-related industries. The advantages of supercritical fluids (qv) as extractants include favorable solubiHty and transport properties, and the abiHty to complete an extraction rapidly at moderate temperature. Whereas most of the supercritical extraction processes are soHd—Hquid extractions, some Hquid—Hquid extractions are of commercial interest also. For example, the removal of ethanol from dilute aqueous solutions using Hquid carbon dioxide... 

I. J. Koski, B. A. Jansson, K. E. Markides and M. L. Lee, Analysis of prostaglandins in aqueous solutions by supercritical fluid exti action and clrromatogi aphy , 7. Pharm. Biomed. Anal. 9 281-290 (1991). 

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. 

Supercritical fluid extraction (SFE) is generally used for the extraction of selected analytes from solid sample matrices, but applications have been reported for aqueous samples. In one study, recoveries of 87-100% were obtained for simazine, propazine, and trietazine at the 0.05 ug mL concentration level using methanol-modified CO2 (10%, v/v) to extract the analytes, previously preconcentrated on a C-18 Empore extraction disk. The analysis was performed using LC/UV detection. Freeze-dried water samples were subjected to SFE for atrazine and simazine, and the optimum recoveries were obtained using the mildest conditions studied (50 °C, 20 MPa, and 30 mL of CO2). In some cases when using LEE and LC analysis, co-extracted humic substances created interference for the more polar metabolites when compared with SFE for the preparation of the same water sample. ... 

To date most of the work which has been done with supercritical fluid extraction has concentrated on the extraction of analytes from solid matrices or liquids supported on an inert solid carrier matrix. The extraction of aqueous matrices presents particular problems [276-278]. The co-extraction of water causes problems with restrictor plugging, column deterioration, and phase separation if a nonpolar solvent is used for sample collection. Also, carbon dioxide isay have limited extraction efficiency for many water soluble compounds. 

For the analysis of organic additives in polymeric materials, in most cases, prior extraction will be necessary. Depending on the nature of the additive, many different approaches are employed. These include soxhlet extraction with organic solvent or aqueous media, total sample dissolution followed by selective precipitation of the polymer leaving the additive in solution, assisted extraction using pressurised systems, ultrasonic agitation and the use of supercritical fluids. In trace analysis, solid phase extraction (SPME) from solution or solvent partition may be required to increase the analyte concentration. 

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). 

The second approach was taken by practicing liquid chromatographers. They routinely dealt with thermally labile, highly polar molecules and frequently sacrificed resolution, and speed in their separations because of the aqueous mobile phases that were required. With the enhanced diffusion and decreased viscosity of supercritical fluids over liquids, chromatographic run-time and resolution could be improved when supercritical fluids were used. But solubility in pure carbon dioxide mobile phases, which has the solvating powers from hexane to methylene chloride under normal density ranges, was a problem for these polar molecules. To compensate for this, experimentalists started working with mixed mobile phases. These mixed phases were based on... 

The use of supercritical fluids and aqueous-based extractants is discussed in Section 13.8... 

With the widening use of the liquid-liquid extraction for the separation of complex mixtures into their components, it has been necessary to develop fluids with highly selective characteristics. The metallurgical, nuclear, biotechnolgy and food industries are now major users of the technique, and many of the recent developments have originated in those fields. Some of the characteristics and properties of two classes of fluids of increasing importance-supercritical fluids and aqueous two-phase systems are described in this section. 

Although the general principles of separation processes are applicable widely across the process industries, more specialised techniques are now being developed. Reference is made in Chapter 13 to the use of supercritical fluids, such as carbon dioxide, for the extraction of components from naturally produced materials in the food industry, and to the applications of aqueous two-phase systems of low interfacial tensions for the separation of the products from bioreactors, many of which will be degraded by the action of harsh organic solvents. In many cases, biochemical separations may involve separation processes of up to ten stages, possibly with each utilising a different technique. Very often, differences in both physical and chemical properties are utilised. Frequently... 

Young and Weber [397] presented an equilibrium and rate study of analyte-matrix interactions in SFE in aqueous matrices, while correlation of SFE with supercritical fluid chromatography (SFC) in aqueous media has been reported by Yu et al. [398]. Tena et al. [399] screened PAHs in soil by on-line fiber-optic-interfaced SFE spectrometry. 

Leitner W (1999) In Knochel P (ed) Reactions in Supercritical Carbon Dioxide (scC02) in Modern Solvent Systems. Top Curr Chem 206 107 Morita DK, David SK (1998) Chem Commun, p 1397 Wegner A, Leitner W (1999) Chem Commun, p 1583 Sellin M, Cole-Hamilton DJ (2000) J Chem Soc, Dalton Trans, p 1681 Solinas M, Pfaltz A, Leitner W (2004) J Am Chem Soc 126 16124 Leitner W, Scurto AM (1998) Imobilization of Organometallic Catalysts using Supercritical Fluids. In Cornils B, Herrmann WA (eds) Aqueous Organometallic Catalysis. WUey, Weinheim, p 664... 

Recent experimental results on thermodynamic properties of high pressure supercritical fluids have opened up the possibility to study combustion and flames at very high pressures and in unusual environments. Stationary diffusion flames have been produced up to 2000 bar in dense aqueous mixed fluid phases. 

P. O. Chye, P. C. Kiat, K. L. Hian and F. Y. L. Sam, Analysis of nitroaromatics in aqueous samples by capillary supercritical fluid chromatography. International Journal of Environmental Studies, 1992, 41, 17-25. 

The single largest use of ammonia is its direct apphcation as fertdizer, and in the manufacture of ammonium fertilizers that have increased world food production dramatically. Such ammonia-based fertilizers are now the primary source of nitrogen in farm soils. Ammonia also is used in the manufacture of nitric acid, synthetic fibers, plastics, explosives and miscellaneous ammonium salts. Liquid ammonia is used as a solvent for many inorganic reactions in non-aqueous phase. Other apphcations include synthesis of amines and imines as a fluid for supercritical fluid extraction and chromatography and as a reference standard in i N-NMR. 

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