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Tuning Solvents for Sustainable Technology Charles A. Eckert,* David Bush, James S. Brown, and Charles L. Liotta Schools of Chemical Engineering and Chemistry and Specialty Separations Center, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
The key to sustainable technology is achieving an economic benefit for environmental improvements. The tunability of benign compounds such as CO2 and water as supercritical and nearcritical fluids offers improved performance and greater flexibility for separation and reaction processes, leading to economic advantages. Examples of such behavior are supercritical CO2 extractions and separations, CO2-expanded polar liquids for separations, and reactions in nearcritical water. In each case the tunability is achieved through density changes or cosolvents and may augment product yield, quality, or rates. Introduction “Sustainable technology” embodies our responsibility for the environment and the most effective way to impact it. What this phrase conveys is both our desire to make the world cleaner and to make better use of natural resources and our recognition that this goal is best realized if it can be rendered economically feasible. In this work, we discuss how supercritical and nearcritical fluids can constitute versatile and useful solvents for sustainable technology. There are three solvents that constitute examples for this purpose: supercritical CO2, near-critical water (NCW), and gas-expanded liquids. The first two are virtually, by definition, environmentally benign, and no further discussion of that aspect is needed. The gasexpanded (usually CO2) liquids also are environmentally favorable because they can replace less favorable solvents or solvent mixtures as well as minimize energy consumption and downstream processing. When these solvents in processes are viewed, we do not specify whether they are solvents for separations or reactions; in fact, we take a more global view of processes. In most chemical processes there is a reaction followed by a separation. Generally, two-thirds or more of both the capital expenditure and the operating costs are for the separation. Years ago, Sherwood made a linear log-log plot of the price of chemicals versus the concentration of the mixture that needed to be separated, and it ran from elemental sulfur to penicillin.1 Since, it has been expanded to interferon and other rare pharmaceuticals, and the linearity still holds.2 However, the separation is almost always interrelated with the reaction, and often modifications in the reaction conditions can result in a more easily (economically) separable mixture. Many process designers are wary of supercritical fluid and related processes for several reasons. (i) First, they have been heavily oversold by their proponents in the past. (ii) Second, they are inherently high-pressure processes, and while these are commonplace in some industries (such as oil or polymers), they are uncomfortable in others (food and pharmaceuticals). * Corresponding author. E-mail:
[email protected]. Fax: 1-404-894-9085.
(iii) Third, the design tools we use and the data we have are often of limited accuracy in the region of a critical point. (iv) Finally, in part because many processes are proprietary, there are only limited applications to point to in the public domain. At least part of these concerns can be addressed by having tools that solve environmental problems, that offer strong technical and economic advantages, and for which we have good phase equilibrium and kinetic data and models. Too often have we seen bad examples, where enthusiasts in their zeal to promote supercritical fluid (SCF) processes neglected obvious engineering steps. An example would be the many reports of kinetic data from closed vessels in which the phase behavior is neither observed nor known. Of course, rate constants have little meaning if the distribution of the reactants between multiple phases is uncertain. What we do here is outline some examples of the use of tunable fluids for both separations and reactions, stressing the technical capability of such fluids as well as our ability to correlate and/or predict their behavior. In so doing we seek to show that these fluids, while not panaceas for every problem, can often offer distinct process and economic advantages. Phase Equilibria Determining the phase equilibria for these processes is not trivial; liquid and supercritical carbon dioxide and NCW have properties substantially different from those of their standard state at ambient conditions. Foremost for process design is an exact knowledge of in what phase the reaction or separation takes place, so that the process can be characterized properly. While a knowledge of phase equilibria does not supply the necessary transport data, it gives the constraints on the necessary size of equipment. The second key role gained from phase equilibria is to give the engineer “knobs” for tuning separations and reactions. In supercritical CO2 processes, we tune with temperature, pressure, or density and, if needed, a cosolvent. The typical method for predicting the phase behavior changes with these variables is an equation of state (EoS). For example, the Peng-Robinson EoS works well at correlating vapor-liquid equilibria between CO2 and typical cosolvents such as methanol and ethanol, useful when one wants to operate in a single-phase region. For
10.1021/ie000396n CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/2000
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Figure 2. Dielectric constant of water.8 Figure 1. Phase diagram of water.8
extraction of solids from matrixes, the Peng-Robinson EoS is also adequate, provided one has the critical parameters and vapor pressure of the substance of interest. Because most solids of interest decompose at temperatures below Tc, the critical parameters must be estimated. We have developed an alternate technique for estimating solubilities using melting points, enthalpies of fusion, and the solvatochromic parameters R, β, and π*.3 Melting points and enthalpies of fusion are abundant in the literature and are also easy to measure. Many of the solvatochromic parameters are also available4,5 or can be measured provided that the compound can be separated by gas chromatography.6 We take advantage of the fact that, at high pressures (above 30 MPa), supercritical carbon dioxide behaves more like a liquid. We then predict the infinite-dilution activity coefficients of the solute in this hypothetical liquid. The model is then easily scaled to lower pressures because the solubility is directly proportional to the pure CO2 compressibility. In gas-expanded liquid, particularly those swelled with CO2, the important variable is pressure, which is quite a sensitive control of the concentration of CO2 in the liquid process. Therefore, we need a model to calculate accurately the equilibria of the CO2 between the liquid and gas phase as well that of any solute between the solid state and the liquid phase. For simple systems, equations of state can give good semiqualitative results, such as the CO2-toluene-phenanthrene system.7 For a more complex system, like CO2methanol-syringealdehyde-vanillin, we have found that an EoS can give nonsensical roots or multiple roots. Typically, a γ-φ approach works better for this type of system. Temperature is the key variable to tune NCW processes (processes using hot liquid water in the temperature region from ∼200 °C up to the critical point at 374 °C). Figure 1 shows that the density of NCW is about 25% less than ambient. However, the main effect is on the dielectric constant, which is shown in Figure 2. Liquid-liquid equilibria between water and organics is quite difficult to predict. There is a limited relationship between the solubility of organic in water at 25 °C and the upper critical solution temperature (UCST) of the mixture, but it is accurate to only (70 °C. EoS are able to predict the organic-rich end of the T-x diagram (see Figure 3) but fail by orders of magnitude on the
water-rich end. They also tend to overpredict the UCST. Modified UNIFAC9,10 can predict both ends provided the molecule is not too complex, i.e., toluene, but it performs worse than the Peng Robinson-Stryjek Vera (PRSV) EoS for the acetophenone-water system. To understand the equilibrium phenomena and develop better models, we and others have studied these systems, particularly pure and modified supercritical fluids, using solvatochromic indicators.12-16 We have now measured π* and R for water up to 275 °C, as shown in Figure 4. Selective Extractions with Cosolvents The low viscosity and tunable density of supercritical CO2 make it an attractive solvent for extractions, but it is often too poor a solvent to use in commercial applications except when health, environmental, or marketing issues preclude the use of traditional organic solvents. Pressurized carbon dioxide has been used successfully on an industrial scale as an environmentally benign alternative solvent to hazardous organic compounds for the decaffeination of coffee and the extraction of flavor compounds from hops. In these cases, CO2 was chosen primarily because its presence could be tolerated in the final product, unlike the solvents that it replaced. To broaden the applicability of supercritical CO2 without sacrificing the tunable density and low viscosity, cosolvents are added to improve the solvent power of supercritical CO2. Cosolvents have their greatest effects when there is a strong, specific interaction with a solute, such as a hydrogen bond.18,19 The solubility of the protic solute, naphthol, in a SCF is enhanced severalfold by basic cosolvents capable of accepting protons, such as ethanol or an amine. Similarly, cosolvents can be used for selective extraction or separation of solutes: For example, acridine and anthracene differ in structure only by the replacement of a carbon atom with a (basic) nitrogen. The selectivity of SCF CO2 for dissolving acridine over anthracene is increased by a factor of 3 with the addition of 1% methanol cosolvent, because of hydrogen bonding of the methanol proton to the amine.20 Crystallization Just as polar organic cosolvents are used to improve the solvent power of supercritical CO2, CO2 can be added as an antisolvent to polar organic solvents. CO2 is miscible with many organic solvents at pressures from
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Figure 3. Liquid-liquid equilibria for acetophenone (1)/water (2) at 6.8 MPa.11
compounds, we chose to separate a racemic mixture of mandelic acid as our model system. First, the two enantiomers were converted to the two diastereomeric salts, (R)-(+)-R-phenethylammonium (R)-(-)-mandalate and (R)-(+)-R-phenethylammonium (S)-(+)-mandalate with (R)-R-phenethylamine. The two salts have a melting point difference of almost 80 °C, probably because of the differences in hydrogen bonding in the crystalline structures of the salts.23 These diastereomers were successfully separated by GAS crystallization with a diastereomeric excess of over 90%.24 Reactive Separation Figure 4. π* (1) and R (b) measurements for water from 25 to 275 °C. Data from Lu et al.17
10 to 100 bar.21,22 As CO2 pressure is increased, the solvent power of the resulting organic/CO2 liquid mixture is diminished and solutes precipitate. These liquid mixtures, known as gas-expanded liquids or GELs, have been of increasing interest for crystallization applications. Gas antisolvent (GAS) crystallization has been used to crystallize polymers, produce nanoparticles, and form microspheres used in controlled drug delivery. Because crystallization can be induced and controlled with CO2 pressure, this provides an alternative to temperature-controlled crystallizations that gives precise control, avoids fouling on heat-transfer surfaces, and eliminates refrigeration systems. A limitation of traditional antisolvents is the downstream separation of the mixed solvent system after crystallization is completed. When using CO2 as an antisolvent, this is simplified to a pressure decrease and the bleed of gaseous CO2. Chiral Separations CO2 antisolvent processes are especially attractive for pharmaceutical applications, where residual organic solvents are unacceptable and where the tuning gives fine control of crystal nucleation and growth. To view GAS as a potential process for the separation of chiral
Because the dissolving power of SCFs is so easily tuned with density, the large capital expense of pressurized equipment can be offset by the savings from coupling a reaction and a separation into a single process unit. For example: SCFs can be used to selectively remove soluble intermediate products from reactants before subsequent reaction to unwanted byproducts can take place. An example of this type of reactive separation is a new route to poly(ethylene terephthalate) (PET). Normally, ethylene oxide (EO) is hydrated to ethylene glycol and the water then reremoved in the condensation polymerization. In the new process, EO is reacted directly with terephthalic acid to give mono(2-hydroxyethyl terephthalate) (MHET), polymerizable to PET. The SCF process for the precursor reduces heat- and mass-flux loads in the polymerizer, increases the rate of the polymerization reaction, and reduces the required water removal by half. MHET was synthesized by the esterification of terephthalic acid (TA) and EO in the presence of a quaternary ammonium salt catalyst. The desired MHET was removed from the involatile bed of terephthalic acid by continuous extraction with SCF before subsequent reaction to the diester could take place (Figure 5).25 The SCF dimethyl ether (Tc ) 126.9 °C and Pc ) 52.4 bar) was tuned with temperature and pressure to remove the monoester from the bed without also dissolving appreciable catalyst or terephthalic acid.
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Figure 5. Ethoxylation of terephthalic acid (TA) and selective removal of mono 2-hydroxyethyl terephthalate (MHET).
Figure 6. Variation of the rate constant for HI pyrolysis at 321.4 °C with pressure.
Figure 8. Diels-Alder reaction of maleic anhydride-isoprene (30 °C): [, data of Dewar;29 9, data of Grieger;30 2, data of Snyder.31
Figure 7. Diels-Alder reaction of substituted butadiene with maleic anhydride.
Reactions Transition-state theory provides the link between chemical kinetics and thermodynamics that permits us to treat rate processes by equilibrium methods. Basically we assert that the reactants pass through a transition state that for most reactions may be considered to be in thermodynamic equilibrium with the reactants. For example, The pyrolysis of HI was studied at elevated pressures by Kistiakowsky,26 who found that the rate increased with solution density. These gasphase results were shown to be well predicted by a thermodynamic model, the virial equation of state; see Figure 6.27 Similarly, thermodynamic models are useful in the liquid phase as well, especially for kinetic solvent effects. The Diels-Alder reaction provides an excellent example because it is a well-characterized second-order reaction, as shown in Figure 7. The reaction is termed “nonpolar” by organic chemists because the polarity of the transition state is not markedly different from that of the substrates. If data for this reaction are treated with regular solution theory, which characterizes well-nonpolar solutions, good predictions of kinetic solvent effects are found over a fairly wide variety of solvents, ranging from low to medium polarity. In the example shown below in Figure 8, it fails only for nitromethane and acetonitrile.28 Because the transition state is pseudoaromatic, one can apply yet another type of thermodyamic treatment to the series of reactions shown in Figure 9, that of linear free energy relationships. In this case, McCabe and Eckert showed that the Hammett equation gives agreement with data in many solvents ranging over close to 5 decades of rate to better than (20%,32 as shown below in Figure 9.
Figure 9. Solvent and substitutent effects on the rate of the Diels-Alder reaction of substituted butadienes with maleic anhydride at 30 °C.
Thus, the link between kinetics and thermodynamics is well established and is extraordinarily useful because both the experimental techniques and the modeling methods for thermodynamics are more advanced and more predictive than those for chemical kinetics. These same approaches bear up well when we apply them to reactions in near-critical fluids and SCFs. Tuning with Cosolvents The strong specific interactions between solutes in SCFs and polar organic cosolvents are used not only to enhance solute solubilities18,19 but also to tune chemical reactions. Cosolvent interactions (usually hydrogen bonds) with one product can be used to tailor product distributions, or those with a transition state can be used to alter rates. One example is the tautomeric equilibrium of the Schiff base 4-methoxy-1-(N-phenylforminidoyl)-2-naphthol shown in Figure 10. In this reaction, the carbonyl oxygen in the keto form can be stabilized by a strong proton donor. This reaction was run in SCF ethane with various cosolvents, and an example of the results is shown in Figure 11. Ethanol is a relatively weak proton donor (in large part because of its self-association), trifluoroethanol (TFE) is stronger, and hexafluoro-2-propanol (HFIP), which has virtually no self-association, is stronger yet.33 The same approach is also effective for rates of reaction. The thermal cis-trans isomerization of 4-(diethylamino)-4′-nitroazobenzene (DENAB) has been mea-
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Figure 13. Phase-transfer-catalyzed C-C bond formation in SCF.
Figure 10. Keto-enol tautomerization of the Schiff base 4-methoxy-1-(N-phenylforminidoyl)-2-naphthol.
Figure 14. log KW (ionization constant of water).43
NCW Figure 11. Use of cosolvents to tailor product distribution for Schiff base equilibrium in SCF ethane at 35 °C.33
Figure 12. Phase-transfer-catalyzed halide exchange in SCF.
sured in supercritical CO2, ethane, and fluoroform.34 Even tiny amounts of polar and protic cosolvents permit significant tuning of the rate. Heterogeneous Reactions (PTC) Reactions between immiscible species, such as a nonpolar organic and a salt, are carried out either in an environmentally undesirable solvent, which is often high boiling and thus hard to remove, or with a phasetransfer catalyst (PTC). Generally, these PTCs are counterions that are soluble in nonpolar phases, such as quaternary ammonium salts; they act both to shuttle the reactive species between the immiscible phases and to lower the activation energy of the reaction. In the early 1990s, sales of products made with phase-transfer catalysis in the U.S. exceeded $10 billion/yr.35 The tunability and ease of removal make supercritical CO2 an attractive candidate as a replacement solvent in these PTC systems. The first example of a phase-transfer catalytic process between a solid salt phase and a SCF solvent was the nucleophilic displacement of benzyl chloride by a bromide ion in the presence of tetraheptylammonium bromide, as shown in Figure 12.36,37 PTC in supercritical fluids can also be used for carbon-carbon bond formation as in the reaction of ethyl bromide, benzyl cyanide, and potassium carbonate (Figure 13).38 Thus, SCF PTC provides a variety of opportunities for heterogeneous reaction processes with the distinct advantages of a cheap, nonflammable, environmentally benign solvent with tunable solvent power, enhanced mass transfer, and facile solvent removal.
Another environmentally benign solvent with tunable properties is NCW. The special properties of water in the near-critical region make it a favorable solvent for carrying out certain chemical reactions. Sisken and coworkers39 have suggested that NCW has approximately the properties of ambient-temperature acetone. As the temperature of water increases from 25 to 300 °C, its density decreases from 1 g/cm3 at ambient conditions to about 0.7 g/cm3 40 and its dielectric constant decreases by a factor of ∼4.41 The solubility properties of water are most valuable. The dielectric constant is still high enough for it to dissolve and even ionize salts but low enough to dissolve organics. Polar organics of any sort are completely miscible, and even hydrocarbons dissolve to a large degree.42 For example, n-heptane is 5 orders of magnitude more soluble in water at 350 °C than at 25 °C. Water also dissociates to a greater extent at higher temperatures: Kw increases by more than 3 orders of magnitude from 25 to 250 °C (Figure 14),43 providing hydronium and hydroxide ions that may catalyze chemical reactions. This increased concentration of hydronium and hydroxide ions eliminates or reduces the need to neutralize conventional acid or base catalysts and dispose of the resulting salts, which can amount to 5-10 kg/kg of product. The ionization constant can be further increased with pressure. If kilobar pressures are applied, the ionization constant of water can be increased several more orders of magnitude.43 Besides the alkylation of phenol and cresol with alcohols,44 we have been able to conduct Friedel-Crafts acylations in near-critical water without added catalyst. In this type of reaction, acid chlorides, acid anhydrides, or carboxylic acids react with aromatics to form arylalkyl ketones, conventionally in the presence of Lewis acids such as aluminum chloride, ferric chloride, and zinc chloride. Phenol can be converted with acetic acid to produce 2’-hydroxyacetophenone, 4′-hydroxyacetophenone, and phenyl acetate with no added catalyst (Figure 15). The reaction could be extended to di- and trihydroxybenzenes as substrates and other organic acids as
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Figure 15. Acetylation of phenol in NCW without added catalyst.
than the reactants, NCW can be tuned to an intermediate solubility between reactants and products so that, as products are formed, they fall out of solution, driving the reaction to completion. Summary
Figure 16. Simplified reaction scheme of major condensation products in the conversion of phenylacetaldehyde. PAA: phenylacetaldehyde. TPhBz: 1,3,5-triphenylbenzene.46
Figure 17. Homogeneous aqueous/organic reaction in NCW with separation by cooling and decanting.
acylating agents. Owing to the lower stability of the intermediate electrophile (acyl versus alkyl cation), the yields obtained in the acylation reactions have been lower than those in the alkylation reactions.45 Near-critical water can also serve as a reaction medium for C-C bond formation reactions conventionally catalyzed by added base, such as the aldol condensation of phenylacetaldehyde (PAA; Figure 16). Without the addition of any catalyst, a high selectivity for the primary condensation product at reasonable conversion can be achieved in appreciable reaction times (less than 1 h).46 At higher reaction times, subsequent condensation reactions give rise to ring closure and higher molecular weight products. The potential advantages of reactions run in NCW include replacing environmentally undesirable catalysts, eliminating unwanted byproducts, recycling, improving selectivity, and eliminating mass-transfer limitations by changing from heterogeneous to homogeneous systems. The tunable solubility of organics in NCW allows for practical and facile combinations of reactions with separations. Often this could be as simple as cooling and decanting. A simplified schematic flow sheet of a typical process is shown below in Figure 17. Homogeneous aqueous/organic reactions can be run at near-critical conditions. For reactions that are equilibrium-limited and where the products are less soluble
Tunable fluids can be useful tools for the design of sustainable development; by their benign nature and process flexibility, they offer both a cleaner environment and a more efficient, less costly medium for separations and reactions. The tunability can be used to optimize yield, selectivity, or conversion of reactions as well as to offer opportunities for novel catalysis. In separations, one may design for better loading or selectivity, with improved mass transfer, ease of solvent removal, or facilitated downstream processing. Creative applications may include coupling of reactions and/or catalysis with separations for process advantages. As with any new technique, implementation will require reliable mathematical models for process design and optimization to constrain contingency. For these tunable fluids, often classical design methods, such as cubic EoSs, may be limited in applicability, either because the highly asymmetric systems encountered in SCF extraction do not lend themselves to corresponding states theory or because they do not account for the strong chemical forces such as hydrogen bonding. Because our research community understands better the complex behavior of these fluids, we shall see the development of more versatile and reliable mathematical models for rational design and implementation. This is our aspiration for sustainable technology. Literature Cited (1) Sherwood, T. K.; Pigford, R. L.; Wilke, C. R. Mass Transfer; McGraw-Hill: New York, 1975. (2) Keller, G. E., II. Separations, New Directions for an Old Field; AIChE Monograph Series No. 17; AIChE: New York, 1987. (3) Bush, D.; Eckert, C. A. Prediction of Solid-Fluid Equilibria in Supercritical Carbon Dioxide Using Linear Solvation Energy Relationships. Fluid Phase Equilib. 1998, 150-151, 479-492. (4) Abraham, M. H. Scales of Solute Hydrogen-bonding: Their Construction and Application to Physicochemical and Biochemical Processes. Chem. Soc. Rev. 1993, 22, 73-83. (5) Abraham, M. H.; Andonian-Haftvan, J.; Whiting, G. S.; Leo, A.; Taft, R. S. Hydrogen Bonding. Part 34. The Factors that Influence the Solubility of Gases and Vapours in Water at 298 K, and a New Method for its Determination. J. Chem. Soc., Perkin Trans. 2 1994, 1777-1790. (6) Li, J.; Zhang, Y.; Dallas, A. J.; Carr, P. W. Measurement of Solute Dipolarity/Polarizability and Hydrogen Bond Acidity by Inverse Gas Chromatography. J. Chromatogr. 1991, 550, 101134. (7) Dixon, D. J.; Johnston, K. P. Molecular Thermodynamics of Solubilities in Gas Antisolvent Crystallization. AIChE J. 1991, 37, 1441-1449. (8) Harvey, A. H.; Klein, S. A. NIST/ASME Steam Properties, 2.01 ed.; NIST: Washington, DC, 1996.
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4621 (9) Gmehling, J.; Li, J.; Schiller, M. A Modified UNIFAC Model. 2. Present Parameter Matrix and Results for Different Thermodynamic Properties. Ind. Eng. Chem. Res. 1993, 32, 178-193. (10) Gmehling, J.; Li, J.; Schiller, M. A Modified UNIFAC (Dortmund) Model. 3. Revision and Extension. Ind. Eng. Chem. Res. 1998, 37, 4876-4882. (11) Brown, J. S.; Hallet, J. P.; Bush, D.; Eckert, C. A. LiquidLiquid Equilibria for Binary Mixtures of Water + Acetophenone, 1-Octanol, Anisole, and Toluene from 370 to 550 K. J. Chem. Eng. Data 2000, 45, 846-850. (12) Yonker, C. R.; Frye, S. L.; Kalkwarf, D. R.; Smith, R. D. Characterization of Supercritical Fluid Solvents Using Solvatochromic Shifts. J. Phys. Chem. 1986, 90, 3022-3026. (13) Kim, S.; Johnston, K. P. Clustering in Supercritical Fluid Mixtures. AIChE J. 1987, 33, 1603-11. (14) Kelley, S. P.; Lemert, R. M. Solvatochromic Characterization of the Liquid Phase in Liquid-Supercritical CO2 Mixtures. AIChE J. 1996, 42, 2047-2056. (15) Hafner, K. P.; Pouillot, F. L. L.; Liotta, C. L.; Eckert, C. A. Solvatochromic Study of Basic Cosolvents in Supercritical Ethane. AIChE J. 1997, 43, 847-850. (16) Maiwald, M.; Schneider, G. M. Solvatochromism in Supercritical Fluids. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 960964. (17) Lu, J.; Brown, J. S.; Eckert, C. A.; Liotta, C. L. Solvatochromic Study of Saturated Water from 25 °C to 250 °C. AIChE Annual Meeting, Dallas, TX, 1999; Paper 84j. (18) Ekart, M.; Bennett, K. L.; Ekart, S. M.; Gurdial, G. S.; Liotta, C. L.; Eckert, C. A. Cosolvent Interactions in Supercritical Fluid Solutions. AIChE J. 1993, 39, 235-248. (19) Eckert, C. A.; Bergmann, D. L.; Tomasko, D. L.; Ekart, M. P. Modeling Solutions Containing Specific Interactions. Acc. Chem. Res. 1993, 26, 621-627. (20) Van Alsten, J. G.; Eckert, C. A. Effect of Entrainers and of Solute Size and Polarity in Supercritical Fluid Solutions. J. Chem. Eng. Data 1993, 38, 605-10. (21) Jennings, D. W.; Lee, R.-J.; Teja, A. S. Vapor-liquid equilibria in the carbon dioxide plus ethanol and carbon dioxide plus 1-butanol systems. J. Chem. Eng. Data 1991, 36, 303-307. (22) Chang, C. J.; Chiu, K.-L.; Day, C.-Y. A New Apparatus for the Determination of P-x-y Diagrams and Henry’s Constants in High-Pressure Alcohols with Critical Carbon Dioxide. J. Supercrit. Fluids 1998, 12, 223-237. (23) Zingg, S. P.; Arnett, E. M.; McPhail, A. T.; Bothner-By, A. A.; Gilkerson, W. R. Chiral Discrimination in the Structures and Energetics of Association of Stereoisomeric Salts of Mandelic Acid with R-Phenethylamine, Ephedrine, and Psuedophedrine. J. Am. Chem. Soc. 1988, 110, 1565-1580. (24) West, K. N.; McCarney, J. P.; Griffith, K. N.; Liotta, C. L.; Eckert, C. A. CO2 as an Anti-Solvent for Chiral Separations. AIChE Annual Meeting, Dallas, TX, 1999; Paper 114g. (25) Brown, J. S.; Lesutis, H. P.; Lamb, D. R.; Bush, D. M.; Chandler, K.; West, B. L.; Liotta, C. L.; Eckert, C. A.; Schiraldi, D.; Hurley, J. S. Supercritical Fluid Separation for Selective Quaternary Ammonium Salt Promoted Esterification of Terephthalic Acid. Ind. Eng. Chem. Res. 1999, 38, 3622-3627. (26) Kistiakowsky, G. B. Homogeneous Gas Reactions at High Concentrations. 1. Decomposition of Hydrogen Iodine. J. Am. Chem. Soc. 1928, 50, 2315-2332. (27) Eckert, C. A.; Boudart, M. On the Use of Fugacities in Gas Kinetics. Chem. Eng. Sci. 1963, 18, 144-147. (28) Eckert, C. A. Molecular Thermodynamics of Chemical Reactions. Ind. Eng. Chem. 1967, 59, 20-32.
(29) Dewar, M. S. J.; Pyron, R. S. Nature of the Transition State in Some Diels-Alder Reactions. J. Am. Chem. Soc. 1970, 92, 3098. (30) Grieger, R. A.; Eckert, C. A. Mechanistic Evidence for the Diels-Alder Reaction from High-Pressure Kinetics. J. Am. Chem. Soc. 1970, 92, 7149-7153. (31) Snyder, R. B.; Eckert, C. A. Chemical Kinetics at a Critical Point. AIChE J. 1973, 19, 1126-1134. (32) McCabe, J. R.; Eckert, C. A. High-Pressure Kinetic Studies of Solvent and Substituent Effects on Diels-Alder Reactions. Ind. Eng. Chem. Fundam. 1974, 13, 168-178. (33) Dillow, A. K.; Hafner, K. P.; Yun, S. L. J.; Deng, F.; Kazarian, S. G.; Liotta, C. L.; Eckert, C. A. Cosolvent Tuning of Tautomeric Equilibrium in Supercritical Fluids. AIChE J. 1997, 43, 515-524. (34) Dillow, A. K.; Brown, J. S.; Liotta, C. L.; Eckert, C. A. Supercritical Fluid Tuning of Reactions Rates: the Cis-Trans Isomerization of 4,4′-Disubstituted Azobenzene. J. Phys. Chem. A 1998, 102, 7609-7617. (35) Starks, C. M.; Liotta, C. L.; Halpern, M. Phase-Transfer Catalysis: Fundamentals, Applications, and Industrial Perspectives; Chapman & Hall: New York, 1994. (36) Boatright, D. L.; Suleiman, D.; Eckert, C. A.; Liotta, C. L. Solid-Supercritical Fluid Phase-Transfer Catalysis. ACS 207th National Meeting, San Diego, CA, 1994. (37) Dillow, A. K.; Yun, S. L. J.; Suleiman, D.; Boatright, D. L.; Liotta, C. L.; Eckert, C. A. Kinetics of a Phase-Transfer Catalysis Reaction in Supercritical Fluid Carbon Dioxide. Ind. Eng. Chem. Res. 1996, 35, 1801-1806. (38) Culp, C. W.; Chandler, K.; Lamb, D.; Eckert, C. A.; Liotta, C. L. Phase Transfer Catalysis in Supercritical Fluids AIChE Annual Meeting, Miami Beach, FL, 1998. (39) Kuhlmann, B.; Arnett, E.; Sisken, M. Classical Organic Reactions in Pure Superheated Water. J. Org. Chem. 1994, 59, 3098-3101. (40) Smith, J. M.; Van Ness, H. C. Introduction to Chemical Engineering Thermodynamics, 4th ed.; McGraw-Hill: New York, 1987. (41) Akerlof, G. C.; Oshry, H. I. The Dielectric Constant of Water at High Temperatures and in Equilibrium with Vapor. J. Am. Chem. Soc. 1950, 72, 2844-2847. (42) Connolly, J. F. Solubility of Hydrocarbons in Water Near the Critical Solution Temperature. J. Chem. Eng. Data 1966, 11, 13-16. (43) Marshall, W. L.; Franck, E. U. Ion Product of Water Substance. New International Formulation and Its Background. J. Phys. Chem. Ref. Data 1981, 10, 295-304. (44) Chandler, K.; Eason, B.; Liotta, C. L.; Eckert, C. A. Phase Equilibria for Binary Aqueous Systems from a Near-Critical Water Reaction Apparatus. Ind. Eng. Chem. Res. 1998, 37, 3515-3518. (45) Brown, J. S.; Gla¨ser, R.; Liotta, C. L.; Eckert, C. A. Acylation of Activated Aromatics without Added Acid Catalyst. Chem. Commun. 2000, 1295-96. (46) Gla¨ser, R.; Brown, J. S.; Eckert, C. A.; Liotta, C. L. Basecatalyzed reactions in near-critical water for environmentally benign chemical processing. Prepr. Symp.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44, 385-388.
Received for review April 7, 2000 Revised manuscript received July 14, 2000 Accepted July 15, 2000 IE000396N
