High-pressure fluid phase equilibria

Organometallic chemistry, chemistry of compounds containing metal–carbon bonds or compounds in which an organic molecule (sometimes with a net negative charge) is bonded to a metal atom through an oxygen or nitrogen atom, is one of the most rapidly growing areas of chemical research. Organometallic compounds are being extensively utilized as reagents in preparation and processing of advanced inorganic materials, as catalysts in production of a wide variety of chemicals and as chemotherapy drugs. Supercritical fluid (SCF) science and technology is another rapidly growing field due to the interesting and desirable properties of SCFs as solvents. The combination of organometallic chemistry and SCFs is a relatively new research area with significant potential. Some applications include (1) use of organotransition metal complexes as homogeneous catalysts for reactions in SCFs [D.A. Morgenstern, R.M. LeLacheur, D.K. Morita, S.L. Borkowsky, S. Feng, G.H. Brown, L. Luan, M.F. Gross, M.J. Burk, W. Tumas, Supercritical carbon dioxide as a substitute solvent for chemical synthesis and catalysis, in: P.T. Anastas, T.C. Williamson (Eds.), Green Chemistry: Designing Chemistry for the Environment, American Chemical Society, Washington, DC, 1996, p. 132 and G.P. Jessop, T. Ikariya, R. Noyori, Homogeneous catalysis in supercritical fluids, Science 269 (1995) 1065], (2) impregnation of polymers with various organometallic complexes from SCF solutions for property enhancement or for subsequent in-situ chemical transformations within such matrices [J.J. Watkins, T.J. McCarthy, Polymer/metal nanocomposite synthesis in supercritical CO2, Chem. Mater. 7 (1995) 1991, and A.I. Cooper, S.G. Kazarian, M. Poliakoff, Supercritical fluid impregnation of polyethylene films, a new approach to studying equilibria in matrices; the hydrogen bonding of fluoroalcohols to (η5-C5Me5)Ir(CO) and the effect on CH activation, Chem. Phys. Lett. 206 (1993) 175], (3) decomposition of organometallic complexes in SCFs for formation of inorganic powders with controlled size distribution [M. Barj, J.F. Bocquet, K. Chhor, C. Pommier, Submicronic MgAl2O4 powder synthesis in supercritical ethanol, J. Mater. Sci. 27 (1992) 2187], (4) SCF extraction of heavy metals from various matrices by formation of organometallic complexes [K.E. Laintz, C.M. Wai, C.R. Yonker, R.D. Smith, Extraction of metal ions from liquid and solid materials by supercritical carbon dioxide, Anal. Chem. 64 (1992) 2875]. At the University of Connecticut, our research efforts are concentrated on evaluation of technical and economical feasibility of some of these applications. The three primary research thrusts in our group have been the utilization of supercritical carbon dioxide (scCO2) as a solvent in rhodium catalyzed homogeneous hydroformylation reactions [D.R. Palo, C. Erkey, Homogeneous catalytic hydroformylation of 1-octene in supercritical carbon dioxide using a novel rhodium catalyst with fluorinated aryl phosphine ligands, Ind. Eng. Chem. Res. 37 (1998) 4203], impregnation of polyurethane foams with organometallic oxidants from scCO2 solutions and subsequent vapor phase polymerization in these foams for production of electrically conductive composite foams [Y. Fu, D.R. Palo, C. Erkey, R.A. Weiss, Synthesis of conductive polypyrrole/polyurethane foams via a supercritical fluid process, Macromolecules 30 (1997) 7611], and investigation of extraction of heavy metals from aqueous solutions by compound formation using scCO2 [J. Murphy, C. Erkey, Copper(II) removal from aqueous solutions by chelation in supercritical carbon dioxide using fluorinated β-diketones, Ind. Eng. Chem. Res. 36 (1997) 5371]. Advances in these areas greatly depend on our understanding the interactions of SCFs and organometallic complexes and how these interactions affect a particular application. The subject matter of this review is extraction of heavy metals from aqueous solutions in the presence of SCFs. Since solvent extraction of heavy metals is utilized on a commercial scale, the replacement of organic solvents by SCFs has been the major driving force behind our research efforts. Therefore, this review was prepared to highlight the areas important for commercial scale application of the technology. In Section 1, an introduction to solvent extraction of metals is given. A brief introduction to the possible advantages of using SCFs is also presented in the same section. The fundamentals of extraction with different types of extractants (cation exchangers, solvating extractants and ion-pair extractants) are given in Sections 2, 3 and 4, together with the studies in the literature on metal extraction using SCFs for each type of extractant. Thermodynamics of extraction is particularly emphasized due to its governing role in the economical feasibility of a large scale process. The experimental methods that are utilized in evaluation of thermodynamic behavior of such systems are provided in Section 5. The current methods to recycle the extractants are presented in Section 6. The kinetics of extraction is described in Section 7 where no studies using SCFs have been reported to date and a brief conclusion is provided in Section 8.

Supercritical carbon dioxide extraction of metals from aqueous solutions: a review

Advances with supercritical fluids [review]

This paper summarizes representative developments in the field of polluted soil/sediment decontamination by supercritical extraction (SCE). The broad accomplishments of SCE through sustained research and its applications as a remedial technology are outlined. This technology basically employs green supercritical fluids (SCFs) such as carbon dioxide (SCCD) and water (SCW) for the extraction of volatile organic compounds (VOCs) and persistent organic pollutants (POPs), which include polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and various pesticides from environmental matrices. Comprehensive data on solute solubilities in SCFs, partition equilibrium of solutes between a solid matrix and SCFs, solute desorption from the matrix, and solute separation from the SCF provide a solid basis to develop models regarding a SCE process. Significant experimental results and various proposed models to interpret the extraction phenomena from soils/sediments are presented. SCE has been developed to be applied as an environmental remedial technology after its successful use in analytical chemistry. This development has occurred because of a need for a rapid, safe and cheap cleanup method. Conventional techniques for the extraction of environmental matrices (e.g. liquid solvent extraction, thermal desorption) are time and solvent/energy consuming. The review attempts to summarize the representative studies about SCE of organic contaminants, advantages and the drawbacks of SCE compared to other remedial technologies, and economic aspects. A short section on the future direction for research in this field and general trends toward commercial applications are also included. This review is also intended as a guide for conducting a remediation study in a systematic and stepwise fashion for determination of the effectiveness of SCE technology in conjunction with other cleanup technologies. Systematically conducted and well-documented remedial studies are important in the investigation/feasibility and design processes. The applicability and inherent limitations of SCE technology to determine its specific suitability are also discussed.

Supercritical extraction of contaminants from soils and sediments

Electro-oxidation of benzyl alcohol was conducted by electrolysis in an ionic liquid/supercritical CO2 (IL/SC CO2) two-phase system in an undivided cell at 318.2 K and up to 10.3 MPa, and two ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), were used as the solvents and electrolytes. It was demonstrated that benzyl alcohol could be efficiently electro-oxidized to benzaldehyde. [Bmim][BF4] was more effective medium for the electro-oxidation of benzyl alcohol. The effects of various factors, such as pressure and the concentration of benzyl alcohol, on the reaction were studied as [Bmim][BF4] was used. The Faradic efficiency (FE) and selectivity of benzaldehyde increased with the pressure of CO2 when the pressure was lower than about 9.3 MPa, while the FE decreased as the pressure was increased further. This interesting phenomenon could be explained reasonably on the basis of solubility difference of the reactants and products...

Electro-oxidation of Benzyl Alcohol in a Biphasic System Consisting of Supercritical CO2 and Ionic Liquids

A new apparatus is described for easy and quick determination of partition coefficients in a system containing an aqueous and a supercritical phase, and an organic substance. The partition coefficient in this case is defined as the ratio of molar fractions of a substance in two different phases in equilibrium. The apparatus consists of a high-pressure cell of 200-mL volume. Equilibrium is attained by recirculation of the fluid phases. Samples can be taken from either phase by using six-way sampling valves. Quantitative analysis is carried out either by UV-spectroscopy or by gas chromatography. In the measurements, carbon dioxide was used as the supercritical fluid in studies of partitioning behavior of the organic compounds, phenol, benzoic acid, benzyl alcohol, 2-hexanone, vanillin, and caffeine. The experiments were carried out at temperatures ranging from 313 to 333 K and pressure of 8 to 30 MPa. Partition coefficients between 0.2 and 1.5 were found for phenol which roughly match the data previously reported by other authors. Partition coefficients of benzyl alcohol and benzoic acid were found to be in a similar range, whereas those of 2-hexanone turned out to be between 10 and 140. The partition coefficients obtained ranged from 0.02 to 0.25 for caffeine and 0.2 to 3 for vanillin.

Partition coefficients of organic substances in two-phase mixtures of water and carbon dioxide at pressures of 8 to 30 MPa and temperatures of 313 to 333 K

In the two phase system of water and supercritical carbon dioxide, the partitioning behavior of two single solutes, aniline and benzaldehyde, was measured at temperatures of 313, 323 and 333 K and pressures up to 30 MPa using a recirculation view cell apparatus. Samples were taken from both phases and analyzed either by UV-spectroscopy or gas chromatography. The partition coefficient in this case is defined as the ratio of molar fractions of a substance in the carbon dioxide-rich phase and the aqueous phase in equilibrium. Partition coefficients between 0.21 and 3.03 were found for aniline and 2.5 up to 62.9 for benzaldehyde. In addition to the single solute measurements, the distribution behavior of an aqueous solution of phenol, benzyl alcohol, cyclohexanol and 2-hexanone was examined at 323 K. While the alcohols were entrained in the fluid phase, the other compounds were retained in the aqueous phase. Analogous to high pressure binary solid fluid phase equilibria, an enhancement factor is introduced to describe partitioning equilibria. Differences in the distribution behavior of single solutes are discussed in terms of the enthalpy of hydration.

Partition coefficients of aromatic organic substances in two-phase mixtures of water and carbon dioxide at pressures from 8 to 30 MPa and at temperatures of 313 to 333 K. Part II

Karlheinz Brudi

Papers

Partition coefficients of organic substances in two-phase mixtures of water and carbon dioxide at pressures of 8 to 30 MPa and temperatures of 313 to 333 K

Partition coefficients of aromatic organic substances in two-phase mixtures of water and carbon dioxide at pressures from 8 to 30 MPa and at temperatures of 313 to 333 K. Part II