Solvent supercritical

A supercritical fluid (SCF) is any compound above its critical point, which is the maximum in both temperature and pressure at which a gas and liquid can coexist. Above the critical point, isothermic compression yields a continuous increase in density without condensation to a liquid state. All substances theoretically have a critical point, but many experience thermal degradation well before reaching it. 

Carbon dioxide is the most employed substance by far in supercritical fluid processes (SCCO2). Like water, carbon dioxide is an environmentally attractive solvent. The ability to control its solvating power by simple swings in temperature and pressure makes it an ideal medium for homogeneous catalysis. The main problem compared with conventional systems is that such swings, especially in pressure, require costly recompression and care must be taken to control the temperature whilst the pressure swing is occurring so that mixed liquid and gas phases do not form. 

SCCO2 is largely used to process food (extraction or fractionation), but other applications, such as the fluoropolymer synthesis by DuPont, hydrogenation or alkylation by Thomas Swan, coatings by Union Carbide, and polyurethane processing by Crain Industries, are still in development [111]. The application of supercritical fluids (SCFs) as reaction media with homogeneous catalysts has been mainly investigated on a laboratory scale. 

Most examples so far have concentrated on SCCO2 as the phase containing substrates and/or products, corresponding to the mobile phase in continuous flow operation. More recently, the reverse situation, where the catalyst is retained in the SCCO2 phase, has also found increasing interest. These systems have been referred to as inverted biphasic catalysis. 

Using the CESS approach, the rates and selectivities for the hydroformylation of 1-octene and other long-chain alkenes can be commercially attractive [64]. An 

A major area of concern in chemical processes is the use of volatile organic compounds as solvents. Generally, the solvent in which a reaction is run is not consumed in the reaction, and there are unavoidable releases of solvent into the atmosphere even in the most carefully controlled processes. Further, the solvent may be toxic or may decompose to some extent during the reaction, thus creating waste products. 

The use of supercritical fluids represents a way to replace conventional solvents. Recall that a supercritical fluid is an unusual state of matter that has properties of both a gas and a liquid, (Section 11.4) Water and carbon dioxide are the two most popular choices as su- 

As a further example, para-xylene is oxidized to form terephthalic add, which is used to make polyethylene terephthalate (PET) plastic and polyester fiber (Section 12.8, Table 12.5)  

This commercial process requires pressurization and a relatively high temperature. Oxygen is the oxidizing agent, and acetic acid (CH3COOH) is the solvent. An alternative route employs supercritical water as the solvent and hydrogen peroxide as the oxidant. This alternative process has several potential advantages, most particularly the elimination of acetic acid as solvent. 

The use of supercritical fluids represents a way to replace conventional solvents. Recall that a supercritical fluid is an unusual state of matter that has properties of both a gas and a liquid, qqo (Section 11.4) Water and carbon dioxide are the two most popular choices as supercritical fluid solvents. One recently developed industrial process, for example, replaces chlorofluorocarbon solvents with liquid or supercritical CO2 in the production of polytetrafluoroethylene ([CF2CF2] , sold as Teflon ). Though CO2 is a greenhouse gas, no new CO2 need be manufactured for use as a supercritical fluid solvent. 

A significant observation was made in 1994 to show a marked increase in the rate of hydrogenation of C02 to formic acid derivatives in supercritical C02,72 which could be further enhanced by adding appropriate solvents.73-75 Photoreduction76 and hydrogenation in supercritical C02 combined with ionic liquids54 were also performed. 

Promising results were observed in Friedel-Crafts alkylation77 and epoxidation.78 Higher rates or better selectivities were found for hydroformylations in supercritical C02.79-84 Simple trialkyl phosphines, for examples, were shown to provide highly active Rh catalysts.81 Hydroboration showed enhanced regioselec-tivity.85 The Wacker reaction performed in alcohol-supercritical C02 exhibits high reaction rates and markedly increased selectivity toward methyl ketone.86 

Results with respect to polymerization are summarized in a review.87 Norbomene was shown to give high-m polymer in supercritical carbon dioxide88 in contrast to the high-frans polymer formed in protic solvents. Supercritical conditions enhanced selectivities in oligomerization89 and polymerization processes.90 

Aydin K. Sunol and Sermin G. Sunol Department of Chemical Engineering University of South Florida, Tampa, FL, USA 

Significant and steady inroads towards wider and more effective utilization of supercritical fluids have been made over the last two decades, especially for high value added differenti- 

Critical conditions for various inorganic supercritical solvents  

Critical conditions for various organic supercritical solvents  

F igure 21.1.1. Pressure-temperature and pressure-density behavior of matter. 

Critical conditions for various inoiganic supercritical solvents  

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Watei has an unusually high (374°C) ctitical tempeiatuie owing to its polarity. At supercritical conditions water can dissolve gases such as O2 and nonpolar organic compounds as well as salts. This phenomenon is of interest for oxidation of toxic wastewater (see Waste treatments, hazardous waste). Many of the other more commonly used supercritical fluids are Hsted in Table 1, which is useful as an initial screening for a potential supercritical solvent. The ultimate choice for a specific appHcation, however, is likely to depend on additional factors such as safety, flammabiUty, phase behavior, solubiUty, and expense. 

FoodApphca.tlons, Carbon dioxide, a nontoxic material, can be used to extract thermally labde food components at near-ambient temperatures. The food product is thus not contaminated with residual solvent, as is potentially the case when usiag coaveatioaal Hquid solveats such as methylene chloride or hexane. In the food iadustry, CO2 is not recorded as a foreign substance or additive. Supercritical solvents not only can remove oils, caffeiae, or cholesterol from food substrates, but can also be used to fractionate mixtures such as glycerides and vegetable oils iato aumerous compoaeats. 

In some cases, the solids themselves are subjected to extraction by a solvent. For example, in one process used to decaffeinate coffee, the coffee beans are mixed with activated charcoal and a high-pressure stream of supercritical carbon dioxide (carbon dioxide at high pressure and above its critical temperature) is passed over them at approximately 90°C. A supercritical solvent is a highly mobile fluid with a very low viscosity. The carbon dioxide removes the soluble caffeine preferentially without extracting the flavoring agents and evaporates without leaving a harmful residue. 

In Fig. 10 the experimental data [35, 37] showing the effect of temperature and supercritical solvent density on the solubility of adamantane in dense (supercritical) carbon dioxide at various temperatures are reported. 

The data of Smith [35] is reported graphically in Fig. 11 and shows the effect of pressure on the solubility of adamantane in various supercritical solvents (carbon dioxide, methane, and ethane) at 333 K. 

A graphical representation of diamantane solubility data [36] in various supercritical solvents (carbon dioxide and ethane at 333 K and methane at 353 K) is shown in Fig. 12. 

Trends of solubility enhancement for each diamondoid follow regular behavior like other heavy hydrocarbon solutes in supercritical solvents with respect to variations in pressure and density [38, 39]. Supercritical solubilities of... 

Huang, X. W., Elbashir N. O., and Roberts, C. B. 2004. Supercritical solvent effects on hydrocarbon product distributions from Fischer-Tropsch synthesis over an alumina-supported cobalt catalyst. Industrial Engineering Chemistry Research 43 6369-81. 

Demex [Demetallization by extraction] A process for removing metal compounds from heavy petroleum fractions, after vacuum distillation, by solvent extraction and supercritical solvent recovery. The solvent is typically a mixture of octanes and pentanes. Developed jointly by UOP and the Institute Mexicano del Petroleo seven units were operating in 1988. Hydrocarbon Process., 1988, 67(9), 66. 

Another interesting application of high-pressure tubes is the in-situ investigation of reactions in supercritical solvents such as carbon dioxide. For example, the iridium-catalyzed enantioselective hydrogenation of imines was investigated in a sapphire tube at 313 K [32]. 

C M. Gordon, W. Leitner, Homogeneous Catalysis in Supercritical Solvents as a Special Unit Operation, in , B. Comils, W.A. Herrmann, D. Vogt, I. Horvath, H. Oli-vier-Bourbigon, W. Leitner, S. Mecking (Eds.), Multiphase Homogeneous Catalysis. Wiley-VCH, 2005. 

The catalyst can be treated with a solvent to extract hydrocarbon deposits. The most straightforward solvent to use is isobutane, which has been shown to restore catalytic activity only partially. Supercritical solvents have been tested, but they also lead to only partial restoration of the activity. Supercritical alkylation to remove the deposits in situ has been shown in Section III.D.l to be less effective. It is unlikely that this method of operation will lead to a competitive process. 

X. Huang, N. O. Elbashir and C. B. Roberts, Supercritical Solvent Effects on Hydrocarbon Product Distributions from Fischer-Tropsch synthesis over an Alumina-Supported Cobalt Catalyst, Ind. Eng. Chem. Res., 2004, 43, 6369-6381. 

Solvation properties, of supercritical solvents, 14 80-81 Solvatochromic materials, 22 708t Solvatochromic probes, 26 853—855 Solvatochromic spectral shifts, 23 96 Solvatochromy, 20 517 Solvay, 7 641 Solvay process, 15 63... 

Supercritical regime, 11 756 Supercritical solvents, solvation properties of, 14 80-81... 

Supercritical solutions are characterized by very low solvent densities. As a result, they possess the interesting feature that solubility is determined more by solute-solute than solute-solvent interactions. Thus we were able to express the solubilities of naphthalene and a series of indole derivatives in four different supercritical solvents (C2H4, C2H6, C02 and the highly polar CHF3) in the same functional format, only the numerical coefficients varying from one to another.57 Solute-solvent interactions do occur,58 but solubility can be represented quite... 

Supercritical solvents have a number of advantages which make them excellent reaction media, such as the low cost, non-toxicity, and low viscosity. These advantages have meant that they are increasingly utilized in reactions. Supercritical solvents can be described as fluids with attributes of both liquids and gases. Solubility of the solute in the fluid depends on the vapour pressure of the solute however, addition of different polar/nonpolar compounds can change the solubility. What makes the supercritical solvent to be so imique is that properties, such as solubility, can be tuned by varying the pressure, so the solvent becomes more gas-like or liquid-like [46]. 

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