Research: Science & Education
Chemical Reactions in Supercritical Carbon Dioxide C. M. Wai, Fred Hunt, Min Ji, and Xiaoyuan Chen Department of Chemistry, University of Idaho, Moscow, ID 83844-2343
In the past two decades, there has been considerable interest in utilizing supercritical fluids as solvents for chemical separations (1–4). The development of supercritical fluid extraction (SFE) technologies is mostly due to the environmental regulations and waste disposal costs for conventional solvents. Supercritical fluids have both gas-like and liquid-like properties. The high diffusivity and low viscosity of supercritical fluids enable them to penetrate and transport solutes from solid matrices. Since the solvation power of a supercritical fluid depends on pressure and temperature, one can achieve the optimum conditions for a particular separation process by manipulating the temperature and pressure of the fluid phase. Carbon dioxide is the most widely used gas for SFE because of its moderate critical constants (Tc = 31.3 °C, Pc = 72.9 atm), nontoxic nature, and availability in pure form. In SFE processes, solutes dissolved in supercritical carbon dioxide (sc-CO2) are precipitated by reducing the pressure of the fluid phase. The sc-CO2 is then expanded into a collection vessel to remove the solutes, and the gas is recycled for repeated use. Typical examples of large-scale industrial applications of the SFE technology using sc-CO2 include the preparation of decaffeinated coffee and hop extracts (1). The rapid development of supercritical fluid extraction technology has generated interest in using supercritical fluids as solvents for chemical reactions (5–8). The unique properties of supercritical fluids observed from different SFE experiments may be used to manipulate chemical reactions to make them more efficient or specific. One interesting property of sc-CO2 is its ability to dissolve fluorinated compounds. Laintz et al. reported in 1991 that the solubility of fluorinated metal dithiocarbamates can be 2–3 orders of magnitude higher than their nonfluorinated analogues (9). Many fluorinated polymers are highly soluble in both liquid and sc-CO2; thus, compounds containing functional groups such as fluoroalkyl and fluoroether are considered “CO2-philic”. For this reason, sc-CO2 can be used to replace Freons that are conventionally used as solvents for synthesis of perfluoro polymers. Another unique property of supercritical fluids is their miscibility with gases such as H2 (10). For example, the concentration of H2 in a supercritical mixture of H2 (85 atm) and CO2 (120 atm) at 50 °C is 3.2 M (mol/L), while the concentration of H2 in a conventional solvent such as THF (tetrahydrofuran) under the same pressure is merely 0.4 M. Therefore, heterogeneous reactions involving these gases may become homogeneous reactions in sc-CO2, and the speed and yield of such reactions can be greatly improved. Reactions in supercritical fluids may offer other advantages, including controlling phase behavior and products, increasing speed of reactions, and obtaining specific reaction channels. Utilizing supercritical fluids as environmentally benign solvents for chemical synthesis is one of the new approaches in the “greening” of chemistry. Conceivably, chemical reactions in supercritical fluids may lead to more efficient and specific processes for manufacturing chemical products in the future. This paper describes types of chemical
reactions reported recently in the literature utilizing sc-CO2 as a solvent to illustrate some unique properties of supercritical fluid reaction systems. Hydrogenation and Hydroformylation High concentrations of CO2 in the supercritical fluid phase can be advantageous for chemical reactions that incorporate CO2. For example, in conventional synthesis, hydrogenation of CO2 to formic acid is rendered thermodynamically favorable by the addition of a base in an organic solvent. This reaction is highly efficient in sc-CO2 (Scheme I). Efficient production of formic acid in a supercritical mixture of CO2 and H2, containing a trimethylphosphine complex of Ru II as a catalyst precursor, was reported by Leitner et al. (11). The use of a sc-CO2 phase, in which hydrogen can be dissolved with a much higher concentration, leads to a very high initial rate of reaction—up to 1,400 moles of formic acid per mole of catalyst per hour. The rate of the same reaction under identical conditions in liquid organic solvents is lower by an order of magnitude (12). catalyst, 50 oC, NEt3, sc-CO2
H2 (g) + CO2 (g)
∆Go = 33 kJ mol-1
HCO2H (l) Cl
P(CH3)3 (CH3)3P (CH3)3P
(CH3)3P
H Ru H
P(CH3)3 Ru
or
P(CH3)3
(CH3)3P
P(CH3)3
Cl
Scheme I. Ruthenium(II)-catalyzed hydrogenation of CO2 in the supercritical fluid phase
The formic acid synthesis can be coupled with subsequent reactions of the product. In the presence of alcohol or secondary amines, this system provides a highly efficient “onepot” route to formate ester or formamide. For example, in the presence of methanol, thermal esterification of the formic acid leads to the formation of methyl formate which in sc-CO2 is two orders of magnitude faster than in subcritical systems (eq 1) (13). In the presence of dimethylamine, diethylamine, or n-propylamine, the hydrogenation of sc-CO2 at 100 °C produces N-substituted formamides (eq 2). RuCl2(PMe3)4
CO2 + H2 + CH3OH → HCO2CH3 + H2O (1) NEt3, sc-CO 2
RuCl2(PMe3)4
CO2 + H2 + R2NH → HCONR2 + H2O sc-CO2
(2)
R = CH3, C2H5, and n-C3H7 Synthesis of Organometallic Compounds Poliakoff and coworkers at the University of Nottingham, U.K., have recently investigated a number of organo-
JChemEd.chem.wisc.edu • Vol. 75 No. 12 Decmeber 1998 • Journal of Chemical Education
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Research: Science and Education
metallic systems in sc-CO 2. One interesting system is the UV photolysis of Cp*Ir(CO)2 (Cp* = η5-C5Me5) in sc-CO2, containing H2 or C 2 H 6, leading to the formation of Cp*Ir(CO)(H)R (R = H or C2H5) (eqs 3–5) (14).
CF3 S O
O
Htta hν
[Cp*Ir(CO)2] + H2 → [Cp*Ir(CO)(H)2] + CO
(3)
hν
[Cp*Ir(CO)2] + C2H6 → [Cp*Ir(CO)H(C2H5)] + CO (4) hν
[Cp*Ir(CO)(H)2] + C2H6 → [Cp*Ir(CO)H(C2H5)] + H2 (5) The authors also demonstrated that Cp*Ir(CO)2 can be impregnated into a polyethylene film, on the basis of the fact that ethylene can be plasticized in sc-CO2. UV irradiation of this impregnated polyethylene film caused changes in IR spectra consistent with C–H activation and formation of [Cp*Ir(CO)(H)(polymer)]. These findings indicate that this process may provide a means of manufacturing supported catalysts (15). Metal Chelation and Extraction in Supercritical CO2 Metal ions are not soluble in sc-CO2 because of the charge neutralization requirement and the weak solute– solvent interactions. However, when metal ions are bound to organic ligands and form neutral metal chelates, they may become quite soluble in sc-CO2. Wai and coworkers were first to report that copper ions in solid materials, such as Celite or cellulose-based filter paper, can be effectively extracted by sc-CO2 containing a fluorinated dithiocarbamate chelating agent, bis(trifluoroethyl)dithiocarbamate (16 ). The fluorinated chelating agent was shown to form a much more soluble Cu chelate than that from the nonfluorinated analogue diethyldithiocarbamate in sc-CO2. A number of chelating systems, including dithiocarbamates, β-diketones, organophosphorus reagents, and macrocyclic compounds, have been reported recently for chelation and, occasionally, selective extraction of metal species in sc-CO2 (17). This in situ chelation–supercritical fluid extraction technique may have a wide range of applications in toxic metal remediation, metal processing, and materials production. Of particular interest is the ability of sc-CO2 to extract uranyl ions from nitric acid solutions in the presence of tributyl phosphate (TBP) (18). The efficiency of extracting uranium from nitric acid solutions with TBP in sc-CO 2 is comparable to that in kerosene. The uranyl ions extracted in sc-CO2 are probably in the form of UO2(NO3)2?2TBP, similar to that in kerosene. Reprocessing of spent nuclear fuels is conventionally done by dissolving the fuel elements in 3–6 M HNO3, followed by solvent extraction with TBP dissolved in an organic solvent such as kerosene, for the separation and extraction of uranium and plutonium. However, the acid and the organic waste containing fission products and transuranic elements produced by this process are major environmental problems for the nuclear industry. It is possible that CO2 may be used to replace kerosene for the extraction of uranium and plutonium, thus eliminating the production of organic solvent wastes. Current research has also demonstrated that UO3 crystals can be dissolved directly in sc-CO 2 containing a fluorinated β-diketone, 4,4,4-trifluoro-1-(2-thienyl)-1,3butanedione (Htta), 1642
according to the following reaction (19): UO3 + 2 Htta = UO2(tta)2 + H2O
(6)
In the presence of TBP, this dissolution process is rapid, probably owing to the formation of highly soluble UO 2(tta)2?TBP adduct in the fluid phase. The possibility of dissolving UO3 directly in sc-CO2 suggests that acid dissolution may not be needed if a supercritical-fluid-based technology can be developed for reprocessing spent nuclear fuels. This technology may result in a significant reduction in the volume of waste production by the nuclear industry. Preparation of Inorganic Nanoparticles Synthesis of nano-sized spherical titanium dioxide particles in sc-CO2 (20) was demonstrated by reactions between titanium alkoxides and water, using an anionic surfactant Zony FSJ (F(CF2 CF2) zCH2CH2O) x P(O)(ONH4) y , where x = 1 or 2, y = 2 or 1, and z = 1–7. Zonyl FSJ was used to stabilize water dispersion in sc-CO2.. The solubility of titanium alkoxides in CO2 appears to be parallel with their vapor pressure, which is dependent on the oligomerization of the unhydrolyzed alkoxides. A metal alkoxide dissolved in sc-CO2 can be decomposed at the surface of a substrate to form a thin homogeneous oxide layer (21). Very thin (
