Expanded applications are making SCFs the green solvent of the 21st century.

The United States produces millions of tons of pollution each year and spends billions of dollars controlling it. This data indicates that sustainable economic growth will require more than end-product environmental monitoring of existing industrial processes. Rather, the worldwide focus on technology development must include new industrial processing methods that support pollution prevention at the source. Such a change in production methodology will bring numerous immediate and long-term benefits—including financial—as fewer capital investments will be necessary for future environmental remediation.

The U.S. Environmental Protection Agency (EPA) recently created a Green Chemistry Program to support “benign by design” principles in the design, manufacture, and use of chemicals and chemical processes. This Design for the Environment (DfE) program features R&D efforts related to innovative technologies to assist industries with the development of environmentally benign products and processes. This initiative harmonizes with the Pollution Prevention Act of 1990, which was created to focus on source reduction of pollutants—a concept that is often overlooked because of the industrial focus on waste management and pollution control.

In 1992, EPA’s Office of Pollution Prevention and Toxics (OPPT) teamed up with the National Science Foundation (NSF) to jointly fund worldwide green chemistry research. Since its inception in 1977, the OPPT has been responsible for assuring that chemicals for use or sale do not pose any adverse effects to human health or the environment. To date, the OPPT–NSF partnership has awarded tens of millions of dollars in grants for fundamental research in green chemistry to groups throughout the world. Much of the recent funding has been directed at research that exploits the unique properties of supercritical fluids (SCFs) as an alternative to traditional solvents.

Under the Federal Resource Conservation and Recovery Act (RCRA), industries that use organic solvents must comply with strict regulations concerning on-site storage, recycling and disposal, and off-site waste transport. Along with the Federal Clean Air Act (CAA), these regulations are intended to suppress soil, air, and water pollution that could result from excessive solvent evaporation or improper disposal. It would be an extremely attractive proposition to have media that would serve as a versatile solvent, without carcinogenic properties or the potential for environmental degradation. Indeed, this goal has been realized with the advent of SCF technology.

FIGURE 1: Pressure–temperature phase diagram for a pure substance

SCFs possess properties that are intermediate between liquids and gases. This unique phase is obtained through the exertion of pressures and temperatures greater than the critical point (Figure 1). Near the critical point of a fluid, minute changes in pressure or temperature significantly alter the physicochemical properties of the SCF (e.g., density, diffusivity, or solubility characteristics). This is especially important for synthetic applications, in which reaction conditions (e.g., selectivity, rates, pathways) may be sensitively manipulated. Such reaction control is impossible using traditional organic solvents. Furthermore, because of the deleterious effects that many organic solvents have on the environment and/or health, media such as halogenated hydrocarbons (e.g., chloroform, dichloromethane) are being phased out of use and benign replacements are being developed. Supercritical carbon dioxide (sc-CO₂) is an attractive alternative because it is inexpensive and poses no threat to the environment or human health. However, depending on the application, a variety of other SCFs may be more attractive; Table 1 (below) lists common fluids that have been used for applications as diverse as extraction/chromatography, inorganic and organic synthesis, catalysis, materials processing, and even dry cleaning. (See also "Unifying Chromatography" April 2001 TCAW.)

Extraction and Chromatography
Although SCFs were discovered more than 100 years ago, it wasn’t until the 1970s that they were used commercially—to decaffeinate coffee. Since then, SCF media have been used successfully to extract analytes from a variety of complex compounds through manipulation of system pressure and temperature. By comparison, conventional methods (e.g., Soxhlet extraction and vacuum isolation) are more complicated and time and energy intensive. In general, conventional methods have a tendency to generate crude extracts consisting of deteriorated constituents or to prematurely remove volatile components.

TABLE 1: Comparison of the critical constants for commonly used fluids Fluid Critical Temperature (°C) Critical Pressure (atm) Carbon dioxide (CO2) 31.1 72.8 Methane (CH4) –82.1 45.8 Ethane (C2H6) 32.3 48.2 Propane (C3H8) 96.7 41.9 Argon (Ar) –122.3 48.0 Nitrous oxide (N2O) 36.5 72.5 Water (H2O) 374.1 218.3

The limiting property of sc-CO₂ is that it is only capable of dissolving nonpolar organic-based solutes. However, the addition of small amounts of a cosolvent such as acetone has been shown to significantly improve the solubility of relatively polar solutes. More recently, solubility of ionic compounds such as aqueous metal salts has been enhanced through inverse micelle formation using fluorinated surfactants. DeSimone and colleagues have performed much of the research related to surfactant design, which has been exploited for “solvent-free” dry-cleaning applications (1).

SCF extraction has also been applied to environmental remediation such as removing PCBs and other organics from water and soil (2). To extract metal contaminants, a chelating agent is commonly added to the fluid, with the soluble metal complex being removed from the SCF following system depressurization.

Catalysis
The use of SCFs for catalytic processes has been shown to overcome many of the chemical, engineering, and environmental difficulties associated with conventional processes. Homogeneous catalysis is generally preferred to heterogeneous catalysis because it offers greater rates and selectivities. However, the drawback of this methodology is the difficulty in separating products. By comparison, reactions involving SCFs offer the best opportunity for separation of reaction products and removal of solvent from the system—accomplished through simple system depressurization.

Because hydrogen and organic substrates are soluble in SCFs, a single phase is created that eliminates mass-transfer considerations. The complete miscibility of supercritical fluids with permanent gases, enhanced mass-transfer properties, and additional safety provided by a nonflammable solvent are aspects that make SCFs, especially sc-CO₂, very attractive as a benign solvent for hydrogenations and other catalytic processes. Even biphasic catalysis has made use of SCF technology, with the co-addition of organic salts known as ionic liquids to assure catalyst miscibility.

Materials Synthesis
Nanometer metal powders are expected to have applications as burn rate modifiers in propellants and components in fuel air explosives, energetic structural materials, and high-density explosives (3). Powders of some transition metals and their alloys are used in thick-film technology for the production of conductive pastes for hybrid integrated circuitry and the metallization of multilayer ceramic (MLC) capacitors. Metal powders are prepared by a variety of methods such as powder mixing/calcination, metal-organic decomposition from nonaqueous solutions, and precipitation from aqueous solutions of metal salts (4). However, these methods generally give a nonuniform size distribution that requires milling of the agglomerated powders. Spray pyrolysis has also been used to generate metal alloy particles with diameters in the 100–1000 nm range (5). However, only in the past two years have researchers begun to use SCFs as a medium for nanoparticle growth.

Once a component is dissolved in a supercritical fluid, the particles may easily be isolated from the fluid by decreasing the system pressure. If the medium is sc-CO₂, gaseous CO₂ is released from the system (often being recycled), and the dissolved components are deposited as extremely fine particles because of the rapid expansion of the supercritical solution (RESS). Another method for nanoparticle formation uses microemulsions, whereby an aqueous metal salt solution, reducing agent, and surfactants are added to the SCF. The resultant metal nanoparticles are deposited by RESS after the SCF is vented from the system. Particles formed through this simple procedure are shown to be free of atomic incorporation and are extremely homogeneous in size (6).

Recently, chemical vapor deposition (CVD) has also used SCF technology for the growth of thin films. Supercritical fluid transport CVD (SFT-CVD) allows relatively nonvolatile precursors to be introduced into the deposition chamber, as long as they are soluble in the SCF (7). By comparison, traditional thermal CVD methods may only use volatile precursors. Complex films such as BaTiO₃ and YBCO have been successfully deposited using SFT-CVD by dissolving stoichiometric amounts of metal -diketonate precursors within the SCF. The deposition of these films using traditional CVD processes is much more complex because more than one solid or liquid precursor must often be used, each possessing unique volatility.

Safety and the Future
The obvious unappealing aspect of dealing with SCFs is the relatively high-pressure conditions that must be used. However, this problem has been circumvented by the use of flow reactors analogous to those reported by the Poliakoff research group at the University of Nottingham (8).

Flow reactors also offset the problem created by altering the critical temperature of the fluid by dissolution of solutes; that is, with batch reactors, the critical temperature of the reaction mixture may change significantly as the reaction proceeds.

Furthermore, if one uses an autoclave, it is typically small to reduce the danger associated with large volumes at high pressure. To scale up a reaction carried out in a flow reactor, the reactor is simply run for a longer period of time with in situ, real-time spectroscopic monitors, if desired.

Hence, it can no longer be claimed that reactions in SCFs are either too dangerous and/or expensive to carry out. Adjustments in both areas have allowed SCF methodology to improve many important processes and will continue to open this approach to an array of unexplored areas of chemistry.

References

1. Carson, T.; Wells, S. L.; DeSimone, J. M. Surfactant Sci. Ser. 2001, 100, 129.
2. Wagner, J. New and Innovative Technologies for Mixed Waste Treatment; EPA Office of Solid Waste, U-915074-01-0, Aug 1997; www.epa.gov/radiation/mixed-waste/mw_pg11.htm.
3. Gurganus, T. B. Adv. Mater. Process. 1995, 148, 57.
4. Hayashi, A.; Ushijima, A.; Nakamura, Y. Process for the production of silver-palladium alloy fine powder. U.S. Patent 4,776,883, 1988.
5. Pluym, T. C.; Kodas, T. T.; Wang, L-M; Glicksman, H. D. J. Mater. Res. 1995, 10, 1661.
6. Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631–2632.
7. Fernandes, N. E.; Fisher, S. M.; Poshusta, J. C.; Vlachos, D. G.; Tsapatsis, M.; Watkins, J. J. Chem. Mater. 2001, 13, 2023.
8. Banister, J. A.; Lee, P. D.; Poliakoff, M. Organometallics 1995, 14, 3876.