Carbon dioxide , supercritical

Carbon dioxide has a conveniently low critical point (31 °C, 7.39 MPa), and supercritical CO2 has become the most widely used fluid where supercritical solvent properties are required, as it is also inexpensive and nontoxic. The solvent powers of supercritical fluids generally increase with increasing density, which can be regulated at will by varying the pressure. The absence of a gas-liquid interface and associated surface tension in a supercritical fluid enables the fluid to penetrate porous solids freely, and also to 

Supercritical CO2 at temperatures of 31-50 °C and pressures of about 10 MPa is now widely used as a nontoxic extractant of excess fats from foodstuffs and in decaffeinating coffee, but its largest scale future use is likely to be the enhancement of recovery of oil that cannot be extracted from wells by conventional techniques. Supercritical CO2 is finding increasing favor as a solvent for chemical syntheses, for example, in the radical-promoted polymerization of fluoroacrylic monomers in homogeneous solution, for which ozone layer-destroying chlorofluorocarbons had been the only effective solvents previously.  

Although supercritical CO2 is an effective solvent for oils, fats, and similar substances, it is a poor one for nonvolatile hydrophilic (water-loving) substances such as proteins or metallic salts. Adding water as such to the supercritical CO2 is of little help, as the solubility of water in it is limited. Johnson and co-workers overcame the latter limitation by forming water-in-C02 emulsions with the aid of an added nontoxic perfluoropolyether surfactant that forms reverse micelles around the water microdroplets, in effect combining the special properties of supercritical CO2 with the solvent power of water. These emulsions can dissolve a variety of biomolecules at near-ambient temperatures, without loss of their biological activity. 

Polymerization, including radical polymerization, in supercritical CO2 has been reviewed. It. should be noted supercritical CO2 while a good solvent for many monomers is a very poor solvent for polymers such as the (meth)acrylates and S. As a consequence, with the exception of certain fluoropolymcrs and polymerizations taken to very low conversion, most polymerizations in supercritical CO2 are of necessity precipitation, dispersion or emulsion polymerizations. 

Several studies have been directed towards determining the kinetics of radical polymerization in supercritical CO2 using PLP (Section 4.5.2). While some early rcsults suggested that /rpfCOi) for MMA was not significantly different to / p(bulk), more recent work has shown that A-pfCOo) for MMA and various acrylate esters (MA, DA ) are significantly reduced from values for 

For safe and long-term storage, carbon dioxide must be injected more than 800 m below the Earth s surface. At that depth, the gas becomes a supercritical fluid. Such fluids have the gas-like characteristic of low viscosity and the liquidlike characteristic of high density. Supercritical behaviour exists only when temperature and pressure both reach, or exceed, their respective values at the so-called critical point . Every substance has its own critical temperature, above which the gas cannot be liquefied no matter how high the pressure. For carbon dioxide, the critical point lies at 31 °C and 7.4 MPa. Supercritical fluids have properties similar to those of liquid solvents and are employed commercially to extract soluble substances. For example, supercritical carbon dioxide is used to remove caffeine from coffee. 

In the next section, we specialize these formulas to the case of supercritical carbon dioxide. 

In Fig. 6, we show the thickness of the boundary layer as a function of the stream velocity of supercritical CO2 for temperatures of 325 K (solid line) and 375 K (dashed line) at a fixed pressure of 300 bars. In this regime, the density of CO2 is 0.94 g/cc and its viscosity is 7.4 x lO g/cm-s. The thickness of the laminar sublayer is seen to depend inversely on the stream velocity. This figure indicates that for a stream velocity of 200 cm/s, the boundary layer thickness is about 2 microns. 

To compare the cleaning characteristics of supercritical carbon dioxide with that of air, we have plotted in Fig. 8 the radius of the smallest particle (in the range of a few microns) that can be removed with air at 1 atmosphere and a temperature of 300 K. It shows that even for a relatively high velocity of 1000 cm/s, the minimum radius is 0.2 microns. This may be compared to 0.005 microns for supercritical CO2. Requirements of such fast flow for air will probably force the use of jets. This raises an additional complication of having to install a mechanism to sweep the wafer clean. The sweeping motion will most certainly lead to additional contamination from lubricated joints, etc. Such complications do not arise for the case of supercritical CO2, given the relatively low velocities needed. 

Flgiire 7. The theoiy of rolling develqied for tuiinilent flow conditions can used to calculate (at 300 bars) the radius of the smallest particle that can be moved as a function of the stream velocity for 325 K (solid line) and 375 K (dashed line). Notice the sensitivity to temperature. Velocities of 100 cm/s to 200 cm/s are needed to move particles with a radius of about a tenth of a micron. 

The most common use of scC02 is in the extraction of caffeine from coffee or tea, nicotine from tobacco, and essential oils from plants. The isolation of products is simple, with the evaporation of the solvent with no residue. Another important application is in supercritical fluid chromatography (SFC). 

Poliakoff and co-workers developed a catalytic hydrogenation process which has been commercialized by Thomas Swan and Co. for the manufacture of trimethylcy-clohexanone by Pd-catalyzed hydrogenation of isophorone (Equation 4.29) [52], 

Oxazolidinones are useful heterocyclic compounds in organic synthesis. They have a wide range of applications in asymmetric syntheses as chiral reagents and, since they have good antibacterial properties, in medicinal chemistry [53]. Oxazolidinones can be synthesized in traditional solvents such as acetonitrile [54] or DMF [5 5], but it is more environmentally friendly to use scC02 [56]. In the reaction an internal propargyl alcohol, carbon dioxide, and a primary amine participate in a cycloaddition reaction under supercritical conditions to give 4-alkylene-l,3-oxazoli-din-2-ones (Equation 4.30). 

Iijima and Yamaguchi published an efficient and regioselective carboxylation of phenol in supercritical carbon dioxide in the presence of aluminum bromide to form salicylic acid (Equation 4.31) [59]. They also reported the K2C03-catalyzed direct synthesis of salicylic acid from phenol and scC02 [60]. 

Enzymatic esterification of P-citronellol with lauric acid to obtain citronellol laurate was studied in traditional organic solvents (n-heptane, 2-methyl-2-butanol, ethyl methyl ketone, acetone) and compared with scC02 (Equation 4.32) [61]. 

Several workers have reported on the effects of pressure on the rate of reaction and product distribution in Diels-AIder reactions in SCCO2. Paulaitis and Alexander carried out the cycloaddition of maleic anhydride and isoprene in CO2 at pressures of 80-430 bar and at three temperatures of 35, 45, and 60 °C [25]. They observed that the rate constant increased greatly ilear the critical pressure, and at 200 bar or above it was similar to that obtained in a solution of ethyl acetate. 

For the parallel Diels-Alder addition of methyl acrylate and cyclopentadiene, Kim and Johnston estimated the difference in the partial molar volumes of the endo and exo transition states in SCCO2, and found that the selectivity was related to the t(30) polarity parameter of the solvent [26]. Large negative partial molar volumes were measured near the critical point for several systems by Eckert et al. [41]. Further work is needed to understand the unusual reaction rates and selectivities that can appear near the critical point. 

There is significant interest in enzymes as they have proven to be powerful and environment-friendly natural catalysts for the polymerization of water-soluble monomers that can function under milder reaction conditions than those used in traditional free radical polymerization techniques. Hence, the combination of SCCO2 and water as reaction medium is a significant advancement made by Villarroya et al. 

In this context it is interesting to note the recent reports of fluorous catalysis without fluorous solvents [68]. The thermomorphic fluorous phosphines, P[(CH2)m(CF2)7CF3]3 (m=2 or 3) exhibit ca. 600-fold increase in n-octane solubility between -20 and 80 °C. They catalyze the addition of alcohols to methyl propiolate in a monophasic system at 65 °C and can be recovered by precipitation on cooling (Fig. 7.20) [68]. Similarly, perfluoroheptadecan-9-one catalyzed the epoxidation of olefins with hydrogen peroxide in e.g. ethyl acetate as solvent [69]. The catalyst could be recovered by cooling the reaction mixture, which resulted in its precipitation. 

Presumably this technique can be applied to other examples of (organometal-lic) catalysis. We also note that catalysis can also be performed in supercritical carbon dioxide (scC02) as solvent (see next section). 

High gas solubility Weak solvation High diffusion rates Ease of control over properties Good mass transfer Readily available Possible heat-transfer problems 

One of the most widely established processes using SCCO2 is the decaffeination of coffee. Prior to widespread use of this process in the 1980s the preferred extraction solvent was dichloromethane. The potential adverse health effects of chlorinated materials were realized at this time and, although there was no direct evidence of any adverse health effects being caused by any chlorinated residues in decaffeinated coffee there was always the risk, highlighted in some press scare stories. Hence the current processes offer health, environmental and economic advantages. 

This can be efficiently achieved provided the cell walls are broken down prior to the extraction process. 

The use of SCCO2 as a reaction solvent is an area of current significant research activity. The previous lack of attention is at least in part due to the difficulties of carrying out such high-pressure experiments in university 

Hydrogenation is one of the most well-studied synthetic reactions in 

Christopher M. Rayner and R. Scott Oakes (Sections 4.1 and 4.2) Toshiyasu Sakakura and Hiroyuki Yasuda (Sections 4.3 and 4.4) 

High gas solubility Weak solvation Possible heat-transfer problems 

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Fig. 3. Effect of using either liquid or supercritical carbon dioxide on the textural properties of titania aerogels calcined at the temperatures shown. (—), dried with Hquid carbon dioxide at 6 MPa and 283 K (-------), dried with supercritical carbon dioxide at 30 MPa and 323 K. Reproduced from Ref. 36.

Natural Products. Various methods have been and continue to be employed to obtain useful materials from various parts of plants. Essences from plants are obtained by distillation (often with steam), direct expression (pressing), collection of exudates, enfleurage (extraction with fats or oils), and solvent extraction. Solvents used include typical chemical solvents such as alcohols and hydrocarbons. Liquid (supercritical) carbon dioxide has come into commercial use in the 1990s as an extractant to produce perfume materials. The principal forms of natural perfume ingredients are defined as follows the methods used to prepare them are described in somewhat general terms because they vary for each product and suppHer. This is a part of the industry that is governed as much by art as by science. 

Conventional nitrocellulose lacquer finishing leads to the emission of large quantities of solvents into the atmosphere. An ingeneous approach to reducing VOC emissions is the use of supercritical carbon dioxide as a component of the solvent mixture (172). The critical temperature and pressure of CO2 are 31.3°C and 7.4 MPa (72.9 atm), respectively. Below that temperature and above that pressure, CO2 is a supercritical fluid. It has been found that under these conditions, the solvency properties of CO2 ate similar to aromatic hydrocarbons (see Supercritical fluids). The coating is shipped in a concentrated form, then metered with supercritical CO2 into a proportioning airless spray gun system in such a ratio as to reduce the viscosity to the level needed for proper atomization. VOC emission reductions of 50% or more are projected. 

FIG. 22-22 Schematic diagram of the Kraft process for producing decaffeinated coffee using supercritical carbon dioxide (McHugh and Ktukonis, op. cit.). 

THE EFFECT OF PROTON-DONATING MODEFIER ON THE SOLUBILITY ENHANCEMENT OF TRIS(p-DIKETONATO) CHROMIUM(HI) IN SUPERCRITICAL CARBON DIOXIDE... 

Attention should be drawn to a very interesting possibility, viz subjecting drugs before their use as teas to pressure, followed by rapid release of the pressure (so-called PEX procedure), in order to achieve a kind of opening up and thereby improving the liberation of many constituents during the preparation of the tea. The use of supercritical carbon dioxide in this connection is particularly suitable [8]. 

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. 

Above the critical temperature and pressure, a substance is referred to as a supercritical fluid. Such fluids have unusual solvent properties that have led to many practical applications. Supercritical carbon dioxide is used most commonly because it is cheap, nontoxic, and relatively easy to liquefy (critical T = 31°C, P = 73 atm). It was first used more than 20 years ago to extract caffeine from coffee dichloromethane, CH2C12, long used for this purpose, is both a narcotic and a potential carcinogen. Today more than 10s metric tons of decaf coffee are made annually using supercritical C02. It is also used on a large scale to extract nicotine from tobacco and various objectionable impurities from the hops used to make beer. 

The dense fluid that exists above the critical temperature and pressure of a substance is called a supercritical fluid. It may be so dense that, although it is formally a gas, it is as dense as a liquid phase and can act as a solvent for liquids and solids. Supercritical carbon dioxide, for instance, can dissolve organic compounds. It is used to remove caffeine from coffee beans, to separate drugs from biological fluids for later analysis, and to extract perfumes from flowers and phytochemicals from herbs. The use of supercritical carbon dioxide avoids contamination with potentially harmful solvents and allows rapid extraction on account of the high mobility of the molecules through the fluid. Supercritical hydrocarbons are used to dissolve coal and separate it from ash, and they have been proposed for extracting oil from oil-rich tar sands. 

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. 

Diels-Alder Reaction in Supercritical Carbon Dioxide (sc-COi)... 

POLYCAPROLACTONEINTO REPOLYMERIZABLE OLIGOMERS IN SUPERCRITICAL CARBON DIOXIDE... 

Details are given of the enzymatic transformation of polycaprolactone into repolymerisable oligomers in supercritical carbon dioxide. The object was to establish a sustainable chemical recycling system for polycaprolactone. 14 refs. 

Leitner W (1999) Reactions in Supercritical Carbon Dioxide (SCCO2). 206 107-132... 

Among the SCFs, supercritical carbon dioxide (SCCO2) provides additional benefits [73], since it is environmentally benign, inexpensive, available in large quantities, nonflammable, and exhibits low toxicity. Its critical pressure is relatively low (73.4 bar) and it has an ambient critical temperature (31.3 °C). CO2 can be easily removed from reaction mixtures by depressurization [74]. 

Studies of reversed micelles dispersed in supercritical fluids have shown their ability to solubihze hydrophihc substances, including biomolecules and dyes, opening the door to many new applications [60,61]. In particular, solutions of reversed micelles in liquid and supercritical carbon dioxide have been suggested as novel media for processes generating a minimum amount of waste and with a low energy requirement [62]. 

Discuss the advantages and disadvantages of using supercritical carbon dioxide and water as solvents in place of organic solvents. 

Polymerization of methyl methacrylate in supercritical carbon dioxide with PDMS based stabilizers A study on the effect of stabilizer anchor groups... 

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