4442
Ind. Eng. Chem. Res. 2000, 39, 4442-4444
Commentary on Supercritical Fluids: Research and Applications About 20 years ago, a meeting of the High Pressure Committee of the AIChE almost went out of business because it appeared unlikely that high-pressure research would lead to new commercial processes. Lowpressure catalysts had been developed for making polyethylene, the air and ammonia industries were mature, and all that appeared to be left was hot isostatic pressingsand that was not exactly big business. Supercritical fluids were known only from some patent literature from Germany, and they did not appear to be of significant commercial interest to justify an extensive research effort. Nevertheless, one of the younger memberssDick Grieger-Block of Wisconsins convinced the Committee that sessions on supercritical fluids would be of some interest to AIChE members, and a few sessions were therefore planned for a future AIChE Meeting. Those first sessions were held in New Orleans in 1980sunfortunately without Dick Greiger-Block, who had passed away. The AIChE Meeting Program Committee was apparently unimpressed by the potential of supercritical fluids and assigned a small room for the sessions. To everyone’s surprise, more than 200 people tried to jam into that small room; supercritical fluids had obviously “arrived”! The discussions in those first sessions were devoted mainly to phase equilibria and extractions.1 However, there were also papers on making silica aerogels and carrying out organic reactions. After New Orleans came the halcyon days of supercritical fluids, when many university and industrial laboratories started research in this area. These laboratories had to fabricate their own high-pressure equipment because very little high-pressure equipment was available commercially. Nevertheless, many papers on supercritical fluid extraction were published, with the majority reporting solubility data and measurements of physical properties. New applications were proposed and, perhaps unfortunately, supercritical fluids were offered as a panacea for many industrial problems. Here was a classic case of a solution in search of problems! Many of the proposed applications were proprietary, some were not even patented, and some were quite speculative. It soon became quite clear that there would be no particular advantage to using supercritical fluids in many of the proposed processes. Distillation, solvent extraction, adsorption, fractional crystallization, membrane separation, preparatory-scale chromatography, or even zone refining were often cheaper than supercritical fluid extraction for several reasons: (i) the obvious one that high-pressure equipment is more expensive, and (ii) the other methods were more established and required little research and development effort in order to apply them. Nevertheless, the flood of research continued unabated. Despite this effort, however, supercritical fluid extraction remains a relatively “new” technology even today. Each supercritical process essentially requires a new design, with concomitant frontend costs and contingency factors.2 There are cases, of course, when the supercritical process is more economical and some when processing cost is not a major consideration (high-value-added products, for example).
There are also cases where environmental or health considerations dictate the use of benign supercritical solvents and some where the marketability of the product (coffee that is “naturally decaffeinated with sparkling effervescence”) or its quality are enhanced by supercritical processing. Finally, there are some cases where the traditional processes are simply inadequate and the use of supercritical fluids offers a viable solution. This may be because the product is thermally labile or morphologically unique or when solvent contamination must be avoided.3 Until the early 1980s, large-scale applications of supercritical fluids were confined to fossil fuel processing (the ROSE process) and food processing (coffee and tea decaffeination and hops extraction), the latter mainly in Germany. Toward the end of this period, the German companies had expanded into smaller volume applications in the food industry (aromas and additives) and the drug/cosmetics industries. Also by the late 1980s, large plants dedicated to coffee decaffeination and hops extraction were built in the U.S., and there were several small-scale plants related to drugs/cosmetics/natural products in Japan, Korea, India, and elsewhere. The emphasis in all cases was on the ability of supercritical fluids to extract components from complex substrates, aided by government regulations and consumer demands that encouraged the replacement of organic solvents. More recently, plants have been constructed in the Far East for the extraction of phytochemicals and neutraceuticals from natural products such as ginseng. In these applications, supercritical fluids provide better quality products by also selectively separating contaminants such as pesticides. New applications such as dry cleaning and degreasing of precision parts and electronics are also on the verge of being commercialized. In these applications, chlorinated solvents are being replaced by supercritical carbon dioxide because of increasing environmental and health concerns. Also, foam expansion and particle generation have been developed commercially and do not employ the extractive capabilities of supercritical fluids. CO2 is an excellent substitute for chlorofluorohydrocarbons in swelling polymers and has been utilized to expand foams. Particle generation using supercritical fluids has been employed in pharmaceutical process development and in the making of paints and coatings. In addition, the dyeing of textiles using supercritical fluids has also been commercialized. These examples of supercritical processing are likely to become increasingly common in the future, particularly in the case of particle production, solvent replacement, and reactions to obtain “tailor-made” materials with desired properties, morphology, and purity. Some of these are discussed below. Extraction and Purification Extraction and purification of nutraceuticals, food supplements, and active ingredients for pharmaceuticals will continue to be areas of importance in research as well as the application of supercritical fluids.3 As a result, the knowledge base in solubility, extractability,
10.1021/ie000915m CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/2000
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4443
purity, and quality of these materials will continue to expand as more products are commercialized. The phase behavior of the solute species in the supercritical fluid must be known precisely for reliable process design, and the considerable effort in this area is likely to continue. However, there will be an increasing need for masstransfer, process design, and simulation studies for these supercritical processes. Moreover, the need for acceptance of these sometimes “new” products by the consumer will determine their commercial success. Solvent Replacement and Green Chemistry A significant amount of research on supercritical fluids has been devoted to their use as alternative solvents that are environmentally benign. In particular, the use of water and CO2 as solvents for traditional organic syntheses is receiving and will continue to receive much attention. The design of new chemical pathways using these and other supercritical solvents will also be of much interest. The potential for exploiting the “tunable” nature of the properties of supercritical solvents (with and without cosolvents) will probably be realized commercially in applications other than the oxidation of organic wastes in supercritical water.4 Supercritical fluids are promising media for homogeneous as well heterogeneous reactions. In particular, reactions in near-critical and supercritical fluids will be able to exploit the enhanced rates, improved mass transfer, increased selectivity and yield, and ease of separation of reaction products.5,6 The technical feasibility of phase-transfer catalysis, selective oxidations, cycloadditions, enzymatic reactions, and enantioselective syntheses has already been demonstrated and will likely be exploited in commercial applications in the future.
reactions has been demonstrated in many studies.9 What is more interesting is that polymer molecular weight and molecular weight distribution, as well as microstructure and morphology, can be controlled in these reactions. Supercritical fluids such as CO2 also lower the glass transition temperature of the polymer, which can facilitate the impregnation of the polymer by additives, drugs, dyes, and other polymers. The viscosity of the polymer in the molten or dissolved states can be lowered in the presence of supercritical fluids, which can lead to new methodologies for the production of polymer films, coatings, fibers, foams, membranes, composites, and porous structures.10 Finally, supercritical fluids also offer the potential for the processing of inorganic materials, via synthesis reactions or from decomposition of soluble precursors. Thin metal films, magnetic oxides, silicon nanowires, and other materials have been produced by deposition, microemulsion, and sol-gel techniques using supercritical fluids.11 Analytical Applications The use of supercritical fluids as mobile phases in a chromatograph was demonstrated almost 40 years ago. Since then, commercial chromatographs have been marketed and supercritical fluid chromatography has found its place as an analytical technique in many applications.12 Furthermore, supercritical fluid extraction is extensively used in sample preparation. There is an extensive literature on the development of packings, columns, modifiers, detectors, and methods, discussion of which is beyond the scope of this brief commentary. It is sufficient to note here that research and development in this area will continue to flourish in the future. Other Applications
Particle Production The manufacture of uniform-sized organic and inorganic particles using the RESS process has already been commercialized on a pilot-plant scale. The supercritical antisolvent process (and its many variations including GAS, SAS, PCA, SEDS, etc.) has also been the subject of many studies to make uniform particles.7 These processes are of considerable interest in the pharmaceutical, paint, and polymer industries. Supercritical fluids offer a wide range of possibilities in the making of particles. Active fragrance and flavor agents, as well as drugs, can be microencapsulated within a coating of protective material using RESS. The supercritical process eliminates the potential for solvent contamination and is able to microencapsulate many materials that are difficult to treat with existing techniques. It has also been demonstrated that morphology can be controlled by adjusting nucleation and growth during the synthesis of nanoparticles of inorganic materials in supercritical fluids,8 and this is an area that is likely to receive much attention in the future. Materials Processing Processing in supercritical fluids offers routes to “novel” materials, especially in the case of polymeric materials, where properties such as morphology can be tailored to specific applications. The use of supercritical fluids as polymerization media to replace conventional organic solvents and as reactants in polymerization
Mechanisms by which molecules are solubilized in supercritical CO2 have been studied extensively and are being increasingly understood. It has been demonstrated that fluoroalkanes, fluoroethers, poly(dimethylsiloxane), and even carbonates offer special opportunities for CO2-philicity.13 This can be exploited in the synthesis of fluorochemicals and fluoropolymers and in the design of amphiphilic molecules specifically for supercritical CO2. The latter include surfactants to promote water/CO2 macro- and microemulsions that offer many opportunities to do aqueous chemistry in the supercritical phase. As a result, bioreactions involving enzymes, peptides, and proteins can now be carried out in supercritical CO2. These techniques can also be used to make chelating agents to recover metal atoms, to carry out heterogeneous reactions such as phasetransfer catalysis and biphasic chemistry in fluorous systems, and to develop new synthesis routes for nanomaterials in supercritical CO2. Finally, investigations are underway to use supercritical CO2 to improve the uniformity, purity, and feature size of microelectronic devices. These investigations include the use of chemical fluid deposition from CO2-soluble precursors to make thin metal films and core-shell materials for magnetic recording. It has been suggested that the future of supercritical fluid processing depends on the technology going beyond these niche markets and into mainstream processing.2 The argument is that this technology would never gain acceptance in chemical manufacturing until it was
4444
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
applied to high-volume products with low capital costs. Although we would welcome such applications, it seems more likely that the “niche” markets described in this commentary will make up a significant fraction of the manufacturing sector in the future. Supercritical fluids have already made inroads into these markets, and their contributions are likely to continue to increase. The variety of these “niche” applications and the increasing amount of research devoted to understanding the mechanisms of supercritical processes attest to the “maturing” of this technology and to the increasing likelihood that it will be a significant contributor to manufacturing in the future. Literature Cited (1) Paulaitis, M. E., Penninger, J. M. L., Gray, R. D., Davidson, P., Eds. Chemical Engineering at Supercritical Conditions; Ann Arbor Science: Ann Arbor, MI, 1983. (2) Chordia, L. 8th International Symposium on Supercritical Chromatography and Extraction, St. Louis, MO, July 1998. (3) Perrut, M. New Challenges for Supercritical Fluids. Proceedings of the 5th Meeting on Supercritical Fluids, Nice, France, 1998; Instituto National Polytechnique De Lorraine: Vandoeuvre Cedex, France, 1998; Vol. 1, pp 1-4. (4) Tester, J. W.; Marrone, P. A.; DiPippo, M. M.; Sako, K.; Reagan, M. T.; Arias T.; Peters, W. A. Chemical Reactions and Phase Equilibria of Model Halocarbons in Sub- and Supercritical Water. J. Supercrit. Fluids 1998, 13, 225-240. (5) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at Supercritical Conditions: Applications and Fundamentals. AIChE J. 1995, 41, 1723-1778.
(6) Brennecke, J. F.; Chateauneuf, J. E. Homogeneous Organic Reactions as Mechanistic Probes in Supercritical Fluids. Chem. Rev. 1998, 99, 433-452. (7) Debenedetti, P. G.; Tom, J. W.; Yeo, S.-D.; Lim, G. B. Application of Supercritical Fluids for Production of Sustained Delivery Devices. J. Controlled Release 1993, 24, 27-34. (8) Hakuta, Y.; Adschiri, T.; Hirakoso, H.; Arai, K. Chemical Equilibria and Particle Morphology of Boehmite in Sub- and Supercritical Water. Fluid Phase Equilib. 1999, 158-160, 733739. (9) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Dispersion Polymerizations in Supercritical Carbon Dioxide. Science 1994, 265, 356-357. (10) Watkins, J. J.; Rao, V. S.; Pollard, M. A.; Russell, T. P. In Phase Transitions in Polymer Blends and Block Copolymers Induced by Selective Dilation with Supercritical Carbon Dioxide; Kiran, E., Debenedetti, P. G., Peters, C. J., Eds.; NATO Advanced Study Institute Series; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1999. (11) Johnston, K. P.; Jacobsen, G. B.; Lee, C. T.; Meredith, C.; Da Rocha, S. R. P.; Yates, M. Z.; DeGrazia, J.; Randolph, T. W. Microemulsions, Emulsions, and Latexes in Supercritical Fluids. In Chemical Synthesis in Supercritical Fluids; Jessup, P., Leitner, E., Eds.; Wiley-VCH: Weinheim, Germany, 1999. (12) Taylor, L. The Future Impact of Supercritical Fluid Chromatography on Packed Columns, Modified Fluids, and Detectors; ACS Symposium Series 670; American Chemical Society: Washington, DC, 1997; pp 134-153. (13) Beckman, E. J.; Sarbu, T.; Styranec, T. J. CO2-Philic Polymers: Beyond Fluoropolymers and Silicones. The 5th International Symposium on Supercritical Fluids, Atlanta, GA, April 2000.
Amyn S. Teja and Charles A. Eckert* School of Chemical Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0100 IE000915M
