Research Communications A Carbon Arc Process for Treatment of CF4 Emissions D A N I E L T . C H E N , * ,† M O S E S M . D A V I D , ‡ GEORGE V. D. TIERS,‡ AND JOSEPH N. SCHROEPFER‡ 3M Occupational Health and Environmental Safety Division, 3M Center, Building 260-3B-08, St. Paul, Minnesota 55144-1000, and 3M Corporate Research Laboratories, 3M Center, St. Paul, Minnesota 55144-1000
Light perfluorocarbons, such as carbon tetrafluoride, are produced or emitted from a variety of processes, including manufacture of aluminum and processing of semiconductor devices. At the same time, the long atmospheric lifetime and high global warming potential of such compounds makes them an environmental concern. A new process for the abatement of perfluorocarbon emissions using a carbon arc plasma was investigated. In particular, the conversion of CF4 to C2F4 and higher fluorinated species, including poly(tetrafluoroethylene) (PTFE) was demonstrated. General features of the reaction chemistry are discussed, including primary reactions to form radicals and ions and secondary reactions to form C2F4 and higher compounds. The conversion efficiencies and products obtained in the reported experiments indicate potential applicability of the process for point source emission control of high global warming potential perfluorocarbons.
Introduction Perfluorinated compounds are a class of materials useful for a variety of applications, ranging from semiconductor wafer etching and precision parts cleaning to heat transfer. An often exploited feature of these materials, their chemical stability, also has an environmental consequenceslong atmospheric lifetime. As such, perfluorocarbons represent an environmental burden as potent greenhouse gases. The most potentially damaging of these gases are the light perfluorocarbons, CF4 and C2F6; for example, CF4 has been estimated to have an atmospheric lifetime of between 2300 and 50 000 years (1-3). Although the total contribution of perfluorocarbons to anthropogenic sources of global warming may be small, strategies for their control should be developed. The major emissions of CF4 and C2F6 are from aluminum production, fluorochemical manufacturing, and semiconductor device processing. Several strategies are currently being investigated for mitigation of perfluorocarbon releases, including source reduction (4-7), pressure swing adsorption (8), microwave plasma destruction (9), thermal destruction (10), cryogenic recovery, and membrane permeation (11). Several of these techniques have recently been assessed and reviewed by Mocella et al. (12). * To whom correspondence should be addressed. Phone: (651) 733-3122; fax: (651) 737-3069; e-mail:
[email protected]. † 3M Occupational Health and Environmental Safety Division. ‡ 3M Corporate Research Laboratories. S0013-936X(98)00533-1 CCC: $15.00 Published on Web 08/29/1998
1998 American Chemical Society
FIGURE 1. Experimental apparatus (not to scale). This paper reports on a new alternative process for mitigating fluorocarbon emissions, namely, the conversion of light perfluorocarbons into fluorinated materials with recoverable value. The preliminary work described in this paper demonstrates the feasibility of a plasma arc process for the conversion of CF4 to C2F4, poly(tetrafluoroethylene) (PTFE), and other fluorocarbons. Plasma processes are a class of Advanced Oxidation Technologies (AOTs) gaining recognition for treatment of gaseous and liquid waste. One type of thermal plasma is the carbon arc; in this case, the arc is sustained between graphite electrodes placed in a vacuum and driven by a high current DC welding power supply. Extremely high current densities (106-107 A/cm2) are characteristic of the carbon arc process, leading to evaporation and subsequent ionization of the graphitic carbon; this occurs at the arc spots where the local temperatures can exceed 10 000 K. The resulting gas phase species are highly energetic with local particle velocities exceeding 104 m/s (13); such highly energetic particles are conducive for subsequent gas phase reaction chemistry. While the temperatures within the arc spot are very high, the bulk gas phase temperatures are relatively low; importantly, the plasma gas temperature quenches over a much shorter distance than that corresponding to a bulk gas heating process, such as incineration. Homogeneous gas phase reactions between energetic molecules, electrons, ions, and radicals as well as heterogeneous reactions with carbon electrodes at elevated temperature lead to the efficient conversion of even stable molecules such as CF4. Although there are discrete arc spots on the electrodes, the reaction zone is considerably larger, as these arc spots move over the surface of the electrodes. The process can be operated at atmospheric pressure, though the work described in this report was conducted below 10 Torr. Important practical advantages of the carbon arc process include relatively low capital cost, high conversion efficiencies, low bulk thermal temperature (compared to incineration), and a wide flexibility in reaction chemistries. Interestingly, the conversion of fluorocarbons at high temperature or in plasma was investigated many years ago, primarily directed toward the synthesis of PTFE. In the late 1950s to the mid 1960s, workers at du Pont (14, 15), Dow VOL. 32, NO. 20, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Example GC analysis of gas phase reactor effluent.
FIGURE 3. Single ion traces for specific fluorocarbon fragments from mass spectrometry. (a) Ion m/z ) 169.00 (168.70-169.70) corresponding to C3F7+. (b) Ion m/z ) 131.00 (130.0-131.70) corresponding to C3F5+. (c) Ion m/z ) 69.00 (68.70-69.70) corresponding to CF3+. (16), and MIT (17) demonstrated the feasibility of forming PTFE in plasma processes; more recently, Swanepoel and Lombaard have filed a patent on a similar process (18). The principal motivation for this earlier work was to develop an economical method of producing tetrafluoroethylene, the monomer for PTFE, rather than any environmental considerations. This work forms the basis for more detailed studies of the feasibility of plasma processes for the mitigation of fluorocarbon emissions.
Experimental Section Equipment. The equipment used in this series of experiments is shown in Figure 1. The power supply was a Miller Maxstar 151. A graphite anode and cathode were milled to form a matching cone and cup configuration. In the configuration used, gap width was small and controlled; consumption of the electrodes did not strongly influence the gap space as there was a large effective area at the chosen gap width. The cone measured 10 cm in height with a basal diameter of 5 cm. The anode-cathode assembly was fixed 3238
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into a wafer processing chamber (Plasma-Therm Systems Inc.); the chamber dimensions were 66 cm in diameter and 18 cm in height. The reactant gases were fed through the center of the graphite cone via 1/4-in. copper tubing. Procedure. The anode and cathode were configured, and the chamber cover was lowered. The chamber was first evacuated to ∼5 mTorr; the flow of CF4 was then initiated and allowed to stabilize. A variety of flow rates were investigated ranging from 200 to 800 standard cm3/min. To initiate the arc, the chamber pressure was initially 1 Torr or below. After striking the arc, a steady-state current flow was rapidly achieved, with a typical current flow of 60 amp at a potential of 20-30 V as measured at the power supply. Arc stability was highly dependent on the gas-phase pressure and the reactor configuration, as discussed below. After achieving a steady-state plasma, the gas-phase pressure was raised to ca. 5 Torr by reducing the vacuum pumping speed. Steady-state operation for 10-15 min was readily achievable. The product analyses of this report were obtained at an arc current of 62 amp at 23 V, with a CF4 flow of 800 standard cm3/min at a pressure of 5.6 Torr.
Although an exhaustive discussion of the reaction chemistry is outside the scope of this paper, some general features are worthy of discussion. If the carbon arc process is to be used for light perfluorocarbon conversion, a potential target reaction is the conversion of tetrafluoromethane to tetrafluoroethylene followed by polymerization to PTFE:
CF4(g) + C(g) f C2F4(g) nC2F4(g) f (C2F4)n(s)
FIGURE 4. XRD pattern of solid residue showing presence of PTFE. Analysis. Gas phase samples were analyzed by gas chromatography/mass spectrometry (GC/MS) using a HP5890/5972 system. Separation was achieved using a 30-m DB-5MS column (0.25 mm i.d., 1.0 µm film) column with a temperature ramp between -40 (2 min hold) and 230 °C (10 °C/min). The separated components were subjected to electron bombardment at 70 eV. Full-scan unit resolution mass spectra from 29 to 500 m/z were recorded. Additional gas chromatography experiments were used to confirm CF4 conversion using an HP5880A chromatograph equipped with a thermal conductivity detector. Solid-phase product was sampled and analyzed by X-ray diffraction (Philips, copper KR radiation) and FTIR (Bomem MB102).
Results and Discussion Gas phase pressure and gap spacing influenced arc stability, with a larger dielectric constant (related to pressure) and larger gap width being detrimental. During operation, the generation of arc spots that moved over the surface of the electrodes could be clearly seen. The preliminary nature of this work precluded optimization; it is evident that more efficient reactor configurations could be envisioned. Gas phase samples were analyzed by GC/MS. A typical result from gas chromatography is shown in Figure 2, which shows the presence of several compounds in addition to the starting material. Quantitative analysis by gas chromatography showed an average conversion of 23% of CF4, with C2F4 being the primary product (21.6%). The space velocity under standard conditions is 40 min-1; at the reactor conditions (pressure of 5.6 Torr), the effective space velocity is approximately 5430 min-1. The presence of high molecular weight materials is consistent with CF4 being broken down in the presence of carbon to produce CF2 fragments. Figure 3 shows the single ion traces for specific fragments corresponding to C3F7+ (m/z ) 169, Figure 3a), C3F5+ (m/z ) 131, Figure 3b), and CF3+ (m/z ) 69, Figure 3c); various saturated and unsaturated species in the gas phase products are identified. Interestingly, species up to perfluorodecane can be clearly identified. Higher unsaturated compounds are also present, although less readily identifiable. Although quantitative results are difficult to extract, it is clear that species concentration diminishes with higher molecular weight, consistent with the idea that CF2 units are building chain length. Analysis of the solid white material deposited on the walls of the chamber shows the presence of PTFE, as confirmed by FTIR and XRD (Figure 4). The PTFE produced from these experiments was in powder form and loosely attached to the reactor walls, indicating that some polymerization may be taking place in the bulk gas phase in addition to polymerization at the reactor walls.
The reactions occurring on the graphite surface and those in the bulk phase will likely be very different. In the high temperature arc region, difficult (e.g., breakdown of stable bonds) reactions can occur; the resulting radicals and ions recombine under cold bulk phase conditions, promoting condensation and polymerization. It should also be noted that the principal arc reactions are primarily taking place at the cathode spots. The reactions taking place with CF4 as a feed and solid graphite may include reactions such as those described below:
primary reactions: formation of radicals and ions and CF2 units C(s) f C(g)
(vaporization of carbon)
C(g) + CF4 f 2CF2
(gas phase carbon reacting with CF4)
C(s) + CF4 f 2CF2
(solid carbon reacting with CF4)
CF4 + e- f CF3• + F-
(electron impact dissociation and ionization)
CF3• + e- f CF2 + FCF2 + e- f CF• + Fsecondary reactions: formation of C2F4 and higher compounds (examples) 2CF2 f C2F4 CF• + CF3• f C2F4 CF2 + C2F4 f C3F6 C3F6 + CF2 f C4F8 In practice, a variety of reactions in addition to those above take place in the carbon arc process, most of them dominated by free radical chemistry. Such reactions include the stepwise dissociation of CF4 into CF2. The literature suggests that CF2 fragments are the fundamental building blocks for the formation of oligomeric fluorocarbons under high temperature conditions (19, 20). The carbon arc process offers wide flexibility in the conversion of fluorochemicals to various products depending on the co-reactants fed into the system. Many chemistries that will be active have not yet been examined, such as the addition of hydrocarbon co-reactants. The high reactivity of gas phase hydrocarbons relative to graphite may allow lower plasma energies to be used and higher conversions to be achieved. One can certainly envision a slate of partially fluorinated hydrocarbons (HFCs) resulting from a carbon arc or other plasma process. The principal challenge in employing a carbon arc or other thermal plasma process for control of perfluorocarbons is to VOL. 32, NO. 20, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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increase the selectivity to the desired reactions. It is likely that continued investigation into the quenching of activated species will yield advances toward this goal.
Acknowledgments The authors thank 3M Corporate Research and 3M Specialty Materials Division for supporting this work. Important analytical support was provided by Brian Lynch, Gerry Lillquist, and Bill Peterson. In addition, the insights of Allen Siedle, Fred Behr, Gerald Bauer, David Kracht, and Lou Errede are gratefully acknowledged.
Literature Cited (1) Ravishankara, A. R.; Solomon, S.; Turnipseed, A. A.; Warren, R. F. Science 1993, 259, 194-199. (2) Cicerone, R. J. Science 1979, 206, 59-61. (3) Zander, R.; Solomon, S.; Mahieu, E.; Goldman, A.; Rinsland, C. P.; Gunson, M. R.; Abrams, M. C.; Chang, A. Y.; Salawitch, R. J. Belg. Geophys. Res. Lett, 1996, 23, 2353-2356. (4) Tabereaux, A. T.; Richards, N. E.; Satchel, C. E. Light Met. (Warrendale, PA) 1995, 325-333. (5) Williams, J. D. Mater. Res. Soc. Symp. Proc. 1997, 447, 43-48. (6) Streif, T.; DePinto, G.; Dunnigan, S.; Atherton, A. Semicond. Int. 1997, 20, 129-134. (7) Langan, J.; Maroulis, P.; Ridgeway, R. Solid State Technol. 1996, 39, 115-122. (8) Tom, G. M.; McManus, J.; Knolle, W.; Stoll, I. Mater. Res. Soc. Symp. Proc. 1994, 344, 267-272.
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(9) Hartz, C. L.; Bevan, J. W.; Jackson, M. W.; Wofford, B. A. Environ. Sci. Technol. 1998, 32, 682-687. (10) Mawle, P. Eur. Semicond. 1995, 5, 17-19. (11) Chernyakov, I.; Hsiung, T.; Schwarz, A.; Yang, J. Air Products, U.S. Patent 5,730,779, March 24, 1998. (12) Mocella, M. T. Mater. Res. Soc. Symp. Proc. 1997, 447, 29-34. (13) Johnson, P. C. The Cathodic Arc Plasma Deposited Thin Films. In Thin Film Processes II; Vossen, J. L., Kern, W., Eds.; Academic Press: Boston, MA, 1991; pp 209-280. (14) Denison, J. T.; Edlin, F. E.; Whipple, G. H., E. I. du Pont deNemours and Company, U.S. Patent 2,852,574, September 16, 1958. (15) Farlow, M. W., E. I. duPont de Nemours and Company, U.S. Patent 3,081,245 March, 12, 1963. (16) Von Tress, W. R. The Dow Chemical Company, U.S. Patent 3,133,871, May 19, 1964. (17) Baddour, R.; Bronfin, B. Ind. Chem. Process Des. Dev. 1965, 4, 162. (18) Swanepoel, J.; Lombaard, R. Atomic Energy Corporation of South Africa Limited, European Patent EP648530A1, October 14, 1994. (19) Errede, L. J. Org. Chem. 1962, 27, 3425-3430. (20) Weigert, F. J. J. Fluorine Chem. 1993, 65, 67-71.
Received for review May 22, 1998. Revised manuscript received July 30, 1998. Accepted August 3, 1998. ES980533W
