Using To
C O 2 Produce
CHEMICAL PRODUCTS S U S TA I N A B LY Great strides have been made in understanding CO 2’s
ERIC J. BECKMAN
solvent properties, yet fundamental aspects
C
TONY FERNANDEZ
remain vague.
arbon dioxide is clearly a greenhouse gas, but if it can be withdrawn from the environment, used in a process, and then returned to the environment “clean”, no environmental detriment accrues. During the past decade, chemists and engineers have been developing industrial or commercial processes to do just that—use liquid CO2 in place of or in combination with organic solvents to create more “green” operations. Great strides have been made in understanding CO2’s solvent properties (see sidebar on page 349A), yet some fundamental aspects are still not well understood. Moreover, materials and catalysts that easily dissolve in CO2 are lacking. Nevertheless, there are many reasons to look at CO2 as a solvent. It is nonflammable, naturally abundant, and, with a threshold limit value (TLV) of 5000 ppm, less toxic than many other organic solvents (1). This TLV for airborne concentration at 298 K is the safe level established for workers with repeated exposure. For example, acetone has a TLV of 750 ppm, and chloroform’s TLV is 10 ppm. CO2’s high TLV and vapor pressure mean that residual CO2 in substrates is not a concern for human exposure; the same is not true for many synthetic and naturally occurring organic compounds. Indeed, CO2 has largely replaced methylene
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chloride as the solvent of choice in coffee decafhuman exposure to toxic compounds and the release feination because it is not burdened by liability conof emissions. However, major obstacles still confront cerns, even though it is not a better or less expensive these companies, such as the design of nonfluorinatsolvent. This safety rationale applies to many CO2ed surfactants that clean effectively in CO2 and combased food-processing schemes. Overall, current petition from other “benign” cleaning technologies in commercial operations that use CO2 as a solvent take which, for example, high-flash-point alkanes and water advantage of either the compound’s low toxicity in are used. Indeed, these start-up ventures must even products designed for inticompete with more efficient mate human contact (e.g., conventional dry-cleaning food) or the fact that it is not equipment that uses perGiven the rapid designated a volatile organchloroethylene (PERC) as the ic compound in products in throughput required by solvent; the volume of PERC which the solvent must be used by dry cleaners in the emitted to the environment United States has dropped the industry, can high(e.g., foaming of thermodramatically over the past plastics). There may be other decade primarily because of pressure systems be advantages also. For examthe introduction of “tighter” ple, DuPont scientists disequipment. developed that will covered that adding CO2 to Microelectronics.The pretetrafluoroethylene (TFE) allow use of CO2 at the paration of an 8-in. silicone enhances the stability of that wafer requires hundreds of notoriously difficult-to-hanindividual process steps, of rates required? dle monomer, although the which approximately half exact mechanism for the involve washing. It has been enhanced stability has not been published (2). estimated that a single fabrication line will use more This article critically examines the current status than 1 million gallons of solvent each year. In photoof work with CO2 and outlines areas in which further lithography, patterned microelectronic components research and development are required. are fabricated by applying a polymer layer to an inorganic substrate using spin coating from solvent, then Using CO2 as a solvent selectively imaging and developing (washing off) the deCleaning. The surface tension of CO2 is much sired pattern. Photolithography currently uses signifilower than that of conventional organic solvents and cant volumes of either solvent or water for developing, actually vanishes as the temperature rises above the and hence, it generates a substantial liquid effluent critical temperature (304 K), at which CO2 becomes stream. Carbon dioxide is a particularly intriguing sola supercritical fluid. CO2 would thus be expected to vent for microelectronics applications because it is enwet and penetrate complex geometries better than vironmentally benign and because its vanishingly low simple liquids. This latter point has prompted the interfacial tension allows it to penetrate very small feasubstantial research effort toward evaluating CO2’s tures. Further, CO2’s small interfacial tension allows ability to clean clothing, mechanical parts, and mithe production of features with higher aspect ratios; the croelectronic components. Although CO2 is a weak high surface tension of water can induce collapse of solvent, it will readily solubilize low-molecular-weight, such features during drying. volatile, nonpolar compounds. Initial work with CO2 for coating and photolithography dates to the mid-1990s, when researchers at However, breakthroughs are needed in the design IBM and Phasex Corp. designed resins specifically of high-pressure cleaning equipment that rapidly for use in CO2-based developing (4). Several fluorineprocesses individual parts. Presently, as the size of a and silicon-containing polymers were examined, and conventional cleaning bath increases (for atmosa photoacid generator was used to develop the pheric operation), the unit cost of treatments drops; patterns. the opposite is true for a high-pressure operation because the cost of the pressure vessel rapidly increasOber and colleagues have also designed a photoes as the internal volume increases. lithography system that uses CO2 (5). In this case, the Because CO2 is a weak solvent, any cleaning that polymer was spun-cast onto a substrate from a conrequires the solubilization of polar, inorganic, or highventional solvent, and a photoacid generator added. molecular-weight material will require the use of The system was masked, patterned using 193-nm raCO2-soluble auxiliaries such as surfactants and chelatdiation, and developed with CO2, producing features ing agents (3). The discoveries in the early 1990s that as small as 0.2 µm. certain fluorinated compounds are CO2-philic quickDeSimone has also proposed using fluorinated ly advanced the design of such auxiliaries. In the fucopolymers in photolithography (6); both negative and ture, CO2-philic auxiliaries most likely will include positive resist systems have been described. Internonfluorinated building blocks, because fluorinated estingly, some fluorinated materials are both highly materials are both expensive and environmentally CO2-soluble and known to be relatively transparent to suspect. radiation in the 130–190-nm range, which are the waveBoth Micell, Inc., and Global Technologies have relengths to be used in next-generation systems. cently commercialized CO2 fabric cleaning in the DeSimone and colleagues have also described a freeUnited States. Using CO2 for cleaning fabrics limits meniscus coating methodology using CO2 to apply 348 A
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CO2’s solvent properties In the late 1960s, Giddings suggested that, on the basis of a simple correlation between solubility parameter (a way to relate the probability of mixing based on energy changes and molar volume) and critical pressure, CO2’s solvent power should be similar to that of pyridine (1). Subsequent calculations during the early 1980s using CO2’s equation of state indicated instead that CO2’s solubility parameter should approach that of normal alkanes (2). Unfortunately, early calculations invariably inflated the solubility parameter (3). Later work by Johnston’s group demonstrated that polarizability/volume is a better measure of solvent power, and by this standard, CO2 is judged to be a feeble solvent, in line with a large body of experimental evidence (4–7 ). Consequently, although CO2 will readily solubilize low-molecular-weight, volatile compounds, it cannot dissolve meaningful amounts of polar or high-molecular-weight materials—including, unfortunately, commercial surfactants, chelating agents, and organometallic catalysts—at economically reasonable pressures. During this same period, researchers found that silicones and fluorinated materials were miscible with CO2 at pressures well below those of alkanes of comparable chain length (8–12 ). In 1992, DeSimone and colleagues published the first reports that describe a truly “CO2-philic” material, a fluorinated polyacrylate (13 ). Researchers have subsequently applied fluorinated compounds widely in the design of CO2-soluble catalyst ligands and chelating agents. In the late 1990s, Beckman proposed a design of CO2-philic materials that incorporated earlier conclusions reached by other groups (14–19). Here, CO2-philic materials should include functional groups with bonds that allow relatively free rotation such as carbon−carbon single bonds, low solubility parameter, and Lewis base groups, which provide loci for specific interactions with CO2 such as ketones or esters. Beckman and colleagues demonstrated the effectiveness of the hypothesis by designing highly CO2-soluble ether−carbonate copolymers and modifying silicones to enhance their CO2 solubility (14, 15). Thus, great strides have been made in understanding the solvent properties of CO2, yet a fundamental understanding of CO2–solute thermodynamic behavior is still lacking. For example, the miscibility pressures of
polymers to inorganic substrates, which potentially eliminates the significant volumes of solvent currently used for that purpose (6). Both Texas Instruments and Micron Technology have patented processes in which inorganic chemistry is performed in CO2 to support cleaning and processing of silicone wafers (7, 8). Interest in using CO2 in microelectronics processing is clearly growing. Efforts to date have focused on using mixtures and cosolvents to overcome
poly(vinyl acetate) (PVAc) in CO2 are hundreds to thousands of bars lower than those for poly(methyl acrylate), an isomer of PVAc (18, 19)! The effect is enormous, yet the underlying mechanism for this behavior is unknown. Poly(fluoroacrylates) are the most CO2-philic materials known, but their high cost renders their application problematic. In addition, most fluorinated polymers investigated have been found to be poorly soluble in CO2 at moderate pressures. If one could, from first principles, design a nonfluorinated, truly CO2-philic material, this would greatly enhance the potential for industrial applications.
References (1) Giddings, J. C.; Meyers, M. N.; McLaren, L.; Keller, R. A. Science 1969, 162, 67. (2) Allada, S. R. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 344. (3) Orr, F. M.; Heller, J. P.; Taber, J. T.; Card, R. J. CHEMTECH 1983, 482. (4) McFann, G. J.; Howdle, S. M.; Johnston, K. P. AIChE J. 1994, 40, 543. (5) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction, 2nd ed.; Butterworth-Heinemann: Boston, 1994. (6) Brunner, G. Gas Extraction; Steinkopff Verlag: Darmstadt, Germany, 1994. (7) Harris, T. V.; Irani, C. A.; Pretzer, W. R. U.S. Patent 5,045,220, Sept. 3, 1991. (8) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51. (9) Iezzi, A.; Bendale, P.; Enick, R. M.; Turberg, M.; Brady, J. Fluid Phase Equilib. 1989, 52, 307. (10) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127. (11) Hoefling, T. A.; Newman, D. A.; Enick, R. M.; Beckman, E. J. J. Supercrit. Fluids 1993, 6, 165. (12) Hoefling, T. A.; Newman, D. A.; Enick, R. M.; Beckman, E. J. J. Supercrit. Fluids 1993, 6, 205. (13) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945. (14) Sarbu, T.; Styranec, T.; Beckman, E. J. Nature 2000, 405, 165. (15) Fink, R.; Hancu, D.; Valentine, R.; Beckman, E. J. J. Phys. Chem B 1999, 103, 6441. (16) Meredith, J. C.; Johnston, K. P.; Seminario, J. M; Kazarian, S. G.; Eckert, C. A. J. Phys. Chem 1996, 100, 10,837. (17) Kazarian, S. G.; Vincent, M. F.; Bright, F. V.; Liotta, C.; Eckert, C. A. J. Am. Chem. Soc. 1996, 118, 1729. (18) Rindfleisch, F.; DiNoia, T. P.; McHugh, M. A. J. Phys. Chem. 1996, 100, 15,581. (19) Kirby, C. F.; McHugh, M. A. Chem. Rev. 1999, 99, 565.
CO2’s feeble solvent power without having to resort to CO2-philic materials. Clearly, future technical challenges have to include the design of acceptable— from both a technical and environmental perspective—CO2-philic materials for use in microelectronics processing. Indeed, we do not possess a firm understanding of the underlying molecular foundation that describes high CO2 solubility and transparency to a particular wavelength of radiation. Will these unSEPTEMBER 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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derlying mechanisms ultimately conflict with one another? Further, given the rapid throughput required by the industry, can high-pressure systems be developed that will allow use of CO2 at the rates required? These questions have yet to be answered.
Advantages of using CO2 as a solvent
Carbon dioxide also provides chemical advantages that can ultimately translate to the prevention of waste when applied strategically. FIGURE 1
The anthraquinone route to hydrogen peroxide Hydrogen peroxide is currently produced via hydrogenation, then oxidation of a 2-alkyl anthraquinone in an organic solvent. Using an organic solvent coupled with a liquid–liquid extraction step against water to recover the product creates a significant contamination issue, which is currently remediated with energy-intensive distillation. Filtration
H2
Hydrogenation
Drying
Working solution regeneration
H2O
Oxidation
Air
Degassing
Extraction H2O2 solution
Source: Adapted with permission from Reference (9 ).
CO2 cannot be oxidized. A reaction in almost any organic solvent using air or O2 as the oxidant—the least expensive and most atom-efficient route—will form waste byproducts arising from oxidation of the solvent. Consequently, oxidation reactions in CO2 have been investigated extensively over the past decade. For example, because CO2 is inert toward oxidation and is nonflammable, it is perhaps the only organic solvent that could be considered for the direct reaction of hydrogen and oxygen to form H2O2. Hydrogen peroxide is currently produced via hydrogenation, then oxidation of a 2-alkyl anthraquinone in an organic solvent (see Figure 1) (9). Using an organic solvent coupled with a liquid–liquid extraction step against water to recover the product creates a significant contamination issue, which is currently remedied with energy-intensive distillation. Gelbein has estimated that one-third of the cost of H2O2 production can be tied directly to anthraquinone and solvent regeneration (10); approximately 150,000 tons of anthraquinone are produced each year simply to support this consumption. Hancu and Beckman examined generating H2O2 in CO2 in a single step from H2 and O2 using a CO2-soluble palladium catalyst (9). The green aspects 350 A
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of this process include the elimination of the organic solvent, anthraquinone and its byproducts, the distillation train and its associated energy input, and several unit operations and their associated energy input. The process could be run continuously and the product recovered from CO2 without a large pressure drop, rendering the process economics more favorable. Previous work on the direct route to H2O2 focused on balancing safety and productivity, in which most of the patented processes used water as the reaction medium to maintain safety. However, because the solubility of H2 and O2 in water is so low, the productivity of these processes has not been sufficient to merit scale-up. Hancu showed that by using a CO2-soluble catalyst, the reaction could be run in CO2 without transport limitations and in a nonexplosive concentration regime in which reaction rates are fast. Although future work is needed to optimize catalyst performance and lifetime, this is a good example of how homogeneous hydrogenation in CO2 can accomplish process and environmental goals. Baiker and colleagues have also used mixtures of O2 and H2 in CO2 to generate propylene oxide from propylene (11), another process in which significant waste streams could be eliminated using CO2. CO2 reduces cross-contamination during liquid– liquid extractions. A number of large-scale chemical processes use biphasic (water–organic) mixtures; hydrogen peroxide production and hydroformylation of low-molecular-weight alkenes are but two examples (9). In any contact between aqueous and organic phases, some cross-contamination is inevitable. The aqueous phase will require subsequent remediation to eliminate the organic contamination, whereas the organic phase may require drying to allow further use in the process. Although CO2 will “contaminate” an aqueous phase upon contact in a process, a mixture of CO2 and water clearly does not require remediation. CO2 is generally immune to free-radical chemistry. Because CO2 does not support chain transfer to solvent during free-radical-initiated polymerization, it is an ideal solvent for such polymerizations, despite typically being a poor solvent for high-molecularweight polymers. In chain transfer, a growing chain (with a terminal radical) abstracts a hydrogen from a solvent molecule, terminating the first chain. This solvent-based radical may or may not support further initiation, and hence, chain transfer to solvent can lead to a diminished molecular-weight polymer and polymerization rate. Research conducted during the 1990s, primarily by DeSimone and co-workers, showed that CO2 is inert toward polymer-based free radicals (12). Indeed, DuPont has constructed a semiworks facility in North Carolina to produce fluoropolymers in CO2, and the company has received several patents on the polymerization of fluoromonomers in CO2 (13). (Interestingly, most of the fluoromonomer polymerizations under investigation by DuPont are precipitation polymerizations because many fluoropolymers are insoluble in CO2 [14].) Using CO2 also stabilizes the monomer TFE and eliminates the
hydrogenation of metal complexes in CO2 generates conformal metal films on substrates with submicrometer features, and that the only waste produced is a low-molecular-weight alkane byproduct. Small trenches and pits can be easily coated because of CO2’s easy wetting of even complex features.
Operating a CO2-based process economically
Although CO2 is considered to be a “green” solvent, operating any process at high pressure typically involves higher costs than those operated at 1 atmosphere. Some simple rules of thumb can keep the cost of a CO2-based process as low as possible. FIGURE 2
Solid–fluid phase behavior Solid–fluid phase behavior for (a) CO2–naphthalene and (b) liquid–liquid phase behavior for CO2–hexane demonstrate that liquid–liquid phase systems may be more beneficial than liquid–solid. (a) 250
Pressure (bar)
200
150 35 °C 40 °C 45 °C
100 50 0.00
0.05
0.10
0.15
0.20
Naphthalene mole fraction (b) 140
40 °C 80 °C 120 °C
120 Pressure (bar)
need for fluorinated solvents. Further, because the conventional process for creating many fluoropolymers uses an aqueous emulsion process requiring an environmentally suspect surfactant, the CO2-based process has another advantage by eliminating the surfactant. Hence, in fluoropolymer polymerization, CO2 provides environmental, safety, and product advantages. CO2 is miscible with gases in all proportions above 31 °C. The rate of most processes in which a gas reacts with a liquid is limited by the rate at which the gas diffuses to the active site—either within a catalyst particle or simply to the liquid reactant. Gases such as H2, O2, and CO are poorly soluble in organic liquids and water. Therefore, in many two- and threephase reactors, the rate-limiting step is gas diffusion across the gas–liquid interface. Supercritical CO2 eliminates these transport problems by enhancing the gas concentration at the reaction locus. Attaining kinetic control over the reaction can lead to reduced byproduct formation and lower energy input. A key point is that there is no need to generate a single phase of CO2, substrate, and gas in order to create a situation in which transport limitations are eliminated. Several researchers have attained kinetic control over the reaction simply by ensuring that a significant amount of CO2 is present in the liquid phase (15). Here, the CO2 functions as a diluent and viscosity reducer that enhances the solubility of gases in the lower phase. Using CO2 as the “gas solubility-enhancing diluent” could have broad ramifications for the practicality of conducting such reactions in CO2. Hydrogenation is widely used in industry at scales ranging from grams per year to tons per hour (9). Several hydrogenations (e.g., synthesis of unsaturated fatty acids, reduction of fatty esters to alcohols) are conducted commercially in organic solvents. Replacing these solvents with benign CO2 will reduce liability arising from flammability and toxicity issues and potential fugitive emissions of volatile organic compounds. Poliakoff and colleagues examined the hydrogenation of various substances in supercritical fluids (16), which led to technology that formed the basis of a pilot-scale plant started early this year by the Thomas Swan Co. in the United Kingdom. The plant will be able to generate 1000 tons per year of products generated by hydrogenations and Friedel− Crafts acylations and alkylations conducted in supercritical fluids. Watkins has found that certain metal complexes exhibit millimolar solubility in CO2 at pressures less than 100 bar. Exposing these complexes to H2 under mild conditions reduces the metal to the zero valent state, inducing nucleation of pure metal. Watkins has extended this concept into the realm of green chemistry by using it to create thin-metal films (17 ). In the microelectronics industry, thin-metal films can be generated on an inorganic substrate via vapor deposition or via dip coating and reduction from an aqueous solution. The former method can only be applied to volatile precursors, whereas the latter route produces very large volumes of metal-contaminated aqueous waste. Watkins has found that homogeneous
100 80 60 40 20 0 0.0
0.2
0.4
0.6
0.8
1.0
Hexane mole fraction Source: Adapted with permission from Reference (5 ) in sidebar.
Operate at high concentrations. One way to minimize the cost of a CO2-based process is to minimize equipment size. Given that CO2 is typically a solvent rather than a reactant, the most obvious approach is to minimize the amount of solvent flowing through the process. Consequently, one should try to choose or design substrates that exhibit high solubility in CO2. In addition, those processes that use CO2 as a minor component are likely to be favored economically. Another way to minimize equipment size arises from the typical phase behavior of compounds in CO2. As shown in Figure 2a, the typical phase diagram SEPTEMBER 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of a crystalline solid in CO2 has an essentially pure solid phase in equilibrium with a solution. Given that the solid phase cannot be processed, one obviously makes use of the solution in which CO2 is the major component. For liquid–liquid phase behavior as seen in Figure 2b, a CO2-rich phase exists in equilibrium with a substrate-rich phase. Because CO2 has been FIGURE 3
Process schematic for coffee decaffeination using CO2 One example of product recovery without a high pressure drop is liquid–liquid extraction against water. The CO2-based coffee decaffeination process uses a water–CO2 extraction to recover the caffeine. Decaffeination is environmentally problematic with typical organic solvents, yet benign with CO2. Makeup CO2
Coffee extractor
Caffeinerich CO2
Caffeine-lean CO2
Fresh water Water column
Green moist coffee
Reverse osmosis Concentrated caffeine and water
Decaffeinated green coffee Caffeine-rich water Source: Adapted with permission from Reference (7) in sidebar.
shown to substantially lower the viscosity of solutions, the substrate-rich phase can actually be pumped and processed. Thus, one can operate at lower pressure as well as at higher concentration. Consequently, it may be beneficial to use liquid–liquid rather than liquid–solid phase systems. A good example is the swelling and plasticization of polymers by CO2. In certain polymer–CO2 mixtures, very high degrees of swelling (>25% in polyacrylate–CO2 mixtures, for example) can be observed at pressures of 100 bar or less (18). The relatively low pressures required for high degrees of swelling may be one reason that applications in which CO2 is the minor component have been successfully commercialized, whereas those using dilute polymer solutions have proven less economically tractable. For example, polyurethane flexible slabstock foam has been produced via the “one-shot” process since the late 1950s (19). For decades, the preferred blowing agent was either a chlorofluorocarbon or methylene chloride, which were simply emitted to the atmosphere during foam formation. The 1986 Montreal Protocol forced foam producers to introduce alternative compounds, such as pentane and hydrofluoropropane. Yet, these hydrocarbons do not fully ameliorate the emissions problem and are flammable. In the late 1980s and early 1990s, Crain Industries created the CarDio process, in which liquid CO2 (3–5% by weight) is injected into the polyol stream at pres352 A
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sures above the vapor pressure of CO2 (20). The pressure is reduced in several steps, with the final step using a “gate-bar” assembly that expands the mixture to 1 atmosphere and spreads it evenly onto the moving belt. Plants operating with the CarDio process in Europe and the United States replace a large volume of organic solvent that would have been emitted to the atmosphere with CO2 and have little additional energy input. The first patent proposing the use of CO2 as the blowing agent for polyurethane foam was filed in 1959 (21); it has been known for decades that CO2 will swell polyols sufficiently to create foam. Yet, it was only after perfection of the gate-bar assembly in 1991 that Crain was able to successfully scale up a CO2-based polyurethane foam line. Thus, the success of a green, CO2-based chemical process can depend as much on mechanical design as on chemical design. Swelling a polymer with CO2 will lower viscosity significantly. Kazarian and colleagues exploited this effect in a novel way to greatly enhance the mixing kinetics of a CO2-incompatible dye with a polymer (22). In this work, the dye and polymer want to mix, but the rate of the dye’s infusion into the polymer is glacially slow. Carbon dioxide plasticizes the polymer (while not actually dissolving very much, if any, of the dye), lowering the viscosity and allowing fast blending. The dyeing of fabric and fibers using CO2 has been extensively examined in Europe and the United States (23); here again, the dye and polymer are thermodynamically compatible, while the dye is sparingly soluble in CO2. The green aspect is that the energy required for mixing and the aqueous waste stream commonly associated with dyeing operations are reduced. The remaining challenges in this process are mostly mechanical: How does one design a treatment chamber that allows fast charging, fast sample changeover, and rapid dyeing? This situation is analogous to continuous polyurethane production using CO2—the chemical challenges were overcome long before the mechanical issues were settled. Howdle and colleagues recently expanded this work into biomedicine (24). Here, CO2 was used to swell an aliphatic polyester, depressing its glass-transition temperature to well below room temperature. A temperature- and shear-sensitive enzyme was then mixed with the swollen polymer, and on depressurization, the enzyme was found to be dispersed throughout the now foamed polymer and to have retained its activity. Operate at the lowest pressure possible. Operating a process at high pressure is more expensive than at 1 atmosphere, owing to equipment design and construction and the need for additional safety features. Further, the capital cost of a high-pressure process is not linear with pressure, because the pressure ratings of certain vital equipment, such as flanges, change in discrete steps (e.g., 60 bar, 100 bar). Clearly, these caveats strongly argue for operating at the lowest pressure possible. The chemical design of reactants and/or substrates is critical—using CO2-philic functional groups in the design of substrates or catalysts can greatly lower operating pres-
sure. As mentioned previously, another way is to use concentrated liquid mixtures with CO2 as the minor component; the asymmetry of the liquid–liquid phase envelope provides for lower pressures. Recover products without high-pressure drops. An often cited advantage of CO2 as a solvent is that reducing the pressure to 1 atmosphere completely precipitates any dissolved material, rendering product recovery easy. This may be true, but the strategy raises costs, because either the CO2 must be recompressed before reuse or the makeup CO2 compressed. Gas compression is energy-intensive and expensive! One example of product recovery without a high pressure drop is liquid–liquid extraction against water, a process that is environmentally problematic for typical organic solvents yet benign with CO2. Indeed, the CO2-based coffee decaffeination process uses a water–CO2 extraction to recover the caffeine, allowing the CO2 to move in a loop at relatively constant pressure (Figure 3). Operate the process continuously. The rationale for operating in a continuous mode is that the equipment can be smaller while maintaining high productivity. Although this is usually straightforward for liquid substrates, it can be much more difficult for processing solids at high pressure. Indeed, a viable means currently does not exist for introducing and removing solids continuously from a high-pressure (>100 bar) process. Those commercial CO2-based processes that use solids run in either batch or semibatch mode. In the late 1980s, Chiang and colleagues at the University of Pittsburgh developed a CO2-based process for the cleaning of coal that introduced it into the process continuously as a water slurry (25 ). If a water slurry of solid substrate is tolerable, this is a useful means for continuously introducing solids into a high-pressure process. Recover and reuse homogeneous catalysts and CO2-philes. The discovery of CO2-philes in the early 1990s allowed the exploration of several processes that had been heretofore untenable owing to CO2’s feeble solvent power. Highly CO2-soluble surfactants and catalyst ligands became available, leading to important discoveries regarding chemistry in CO2. However, the new CO2-philes are significantly more expensive than their CO2-phobic counterparts and should be recycled. Thus, recycling of CO2-philes not only makes good economic sense, but also is more sustainable than simply disposing of them.
A final caveat on phase behavior It cannot be overemphasized that knowing phase behavior in CO2-based systems and its results are crucial to understanding both the effects of pressure and temperature on the outcome and the optimization of operating the process. Tester and Danheiser, for example, showed that a lack of knowledge of the phase behavior in a CO2-based reaction can lead to completely spurious conclusions about CO2’s efficacy as a solvent (26). Phase behavior is not only important at the beginning of a reaction/process, but it is likely to change as the reaction proceeds—the best reaction path may be contingent on using the opti-
mal phase behavior path. Finally, although conventional wisdom predicts that single-phase behavior is always desired, two-phase systems (in oxidations and hydrogenations) may actually improve performance, if one accounts for both reaction and economic efficiency. Thus, attention must always be paid to the economic viability of processes that use CO2 as a reactant and/or solvent. Although CO2-based processes are generally thought to be green, their benefits will never be realized if the cost of such processes surpasses conventional analogues. New applications are constantly emerging in which both process and molecular design reduces the cost of CO2-based processing, allowing green and profitable chemistry. Eric Beckman is the Bayer Professor of Chemical Engineering at the University of Pittsburgh. His research interests include designing highly CO2-soluble materials and polymers for use in tissue engineering and using CO2 as a raw material. Address correspondence to Beckman at the Chemical Engineering Department, University of Pittsburgh, Pittsburgh, PA 15261 or
[email protected].
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