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Energy & Fuels 1997, 11, 260-266
Improved Method for Producing Catalytic Carbon and the Potential for Increasing Its Use in Commercial Applications† C. W. Kruse,‡ A. A. Lizzio,* J. A. DeBarr, and C. A. Feizoulof§ Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820 Received November 8, 1996. Revised Manuscript Received January 14, 1997X
This paper describes an improved method for producing a catalytic carbon, which was first produced in the late 1960s. The new activated carbon (AC) removes and destroys organic pollutants in aqueous solutions. To determine the effects of altering the pore structure and surface chemistry of activated carbons, carbons differing in the amount of functional groups on their surfaces were prepared in three steps: (1) oxidizing AC with boiling nitric acid, (2) washing oxidized AC with water to remove the acid, and (3) heating oxidized AC to temperatures beteween 100 and 925 °C. The surfaces of the products were characterized by determining the amount of CO2 and CO evolved during temperature-programmed desorption. Depending on the desorption temperature, these modified carbons showed enhanced adsorptive and/or catalytic properties that included (1) carbon molecular sieves for separating oxygen from nitrogen, (2) increased capacity for adsorbing sulfur dioxide, (3) stronger adsorption of p-nitrophenol from water, and (4) catalysis of dehydrochlorination reactions. A dehydrohalogenation catalyst produced by the oxidation/ desorption steps was found to be similar to one prepared in the 1960s by oxidizing AC with air at 500-700 °C. The dehydrohalogenation catalyst produced by either the old method or the new method involves an oxidized surface that has been exposed to a 500-700 °C temperature range. This carbon catalyst retains modified adsorptive properties of the AC from which it is produced. It can be used both to adsorb pollutants from liquid or gaseous streams and to convert them to recyclable products.
Introduction The research described herein is timely because of the confluence of three circumstances: (1) the discovery of an alternate way to make and perhaps regenerate oxidized activated carbon (OAC) catalysts, (2) developments that may portend the availability of less expensive activated carbon (AC) in the United States (U.S.) from which OAC catalysts could be manufactured at lower prices than previously projected, and (3) a high level of interest in developing better ways to deal with displaced organic compounds that threaten the environment, especially organic halides. ACs and OAC catalysts have the potential to solve a number of environmental problems associated with misplaced organic chlorides. Chlorinated compounds in drinking waters and aquatic environments have become a significant topic for study by scientists concerned about effects of direct toxicity and/or carcinogenicity on human and aquatic life.1 A correlation between surface water chlorination and cancer mortality rates in humans has been shown to be statistically significant.2 The chlorination of surface waters has been shown to * Author to whom correspondence should be addressed. † A preliminary account of this work has appeared: Kruse, C. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (1), 451-455. ‡ Visiting Professor at China University of Mining and Technology, Beijing, People’s Republic of China. § Now affiliated with Camp Dresser & McKee Inc., Chicago, IL 60606. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Hanson, D. EPA study points to health risks of dioxins and similar compounds. Chem. Eng. News 1994, May 30, 13-14. (2) Kalmaz, E. V.; Kalmaz, G. D. Chemosphere 1981, 10, 1163-1175.
S0887-0624(96)00198-3 CCC: $14.00
produce high levels of trihalomethanes (THMs).3 Approximately 98% of drinking water systems in the United States use chlorination. More than 300 chlorinated organic compounds have been identified in chlorinated potable waters, cooling waters, and sanitary effluents by the U.S. Environmental Protection Agency (EPA).4 Less expensive ACs and AC catalysts are needed to provide cost-effective capture, concentration, and decomposition of pesticides and THMs.5 Halogenated organic compounds account for a major portion of toxic and persistent hazardous wastes. A goal of the U.S. EPA is to develop improved methods for destroying organic compounds. Representatives of low molecular weight compounds encountered in hazardous wastes include CCl4, CHCl3, CH2Cl2, C2H4Br2, and CCl3NO2.6 Potential uses of OAC catalysts go beyond the problem of chlorinated organics. AC catalysts have been shown recently to catalyze other elimination reactions including the dehydration of alcohols, the deamination of amines, and the dehydrosulfurization of mercaptans.7 (3) Bellar, T. A.; Lichtenberg, J. J.; Droner, R. C. J. Am. Water Works Assoc. 1974, 71, 40-42. (4) U.S. EPA. Preliminary assessment of suspected carcinogens in drinking water. Report to Congress, 1975; p 3. (5) Graham, J. R.; Ramaratnam, M. Recovering VOCs using activated carbon. Chem. Eng. 1993, 100 (Suppl.), 6-12. (6) Harden, J. M.; Ramsey, G. G. Catalytic dehydrohalogenation: A chemical destruction method for halogenated organics. Report (EPA/ 600/2-86/113; Order No. PB87-133104/GAR), 1986. (7) Kruse, C. W.; Lytle, J. M.; Rostam-Abadi, M.; Chou, M.; Feizoulof, C.; Fatemi, S. M.; Beaulieu, P.; Schreine, J.; Roginski, R.; Zajac, G. Final Technical Report to the Illinois Clean Coal Institute. Sept 1, 1989, through Aug 31, 1991.
© 1997 American Chemical Society
Production of Catalytic Carbon
Opportunities for developing specialty chemicals and advanced materials from coal were recently reviewed.8 One- to four-ring aromatics from coal may be necessary to feed the rapidly growing engineering applications of aromatic polymer materials. Development of highperformance carbon materials, such as carbon fibers, graphites, and advanced adsorbents for environmental applications, is another approach for moving coal’s carbon into nonfuel uses.8-10 Carbons suitable for methane storage are being produced.11,12 Carbon molecular sieves (CMS) manufactured for commercial pressure swing adsorption (PSA) systems are now competing with zeolites for air separation applications. Illinois coals have been shown to be good feedstocks for making molecular sieves.13,14 European and Japanese researchers are reportedly testing carbons for adsorption of radon.15,16 There are biotechnology applications involving carbon as a support.17 Commercial use of AC to reduce SO2, NOx, and mercury in the flue gas from waste incineration plants is well-advanced in Germany.18,19 Carbon-oxygen complexes control many of the properties of ACs.20,21 An OAC catalyst of a type that is the focus of this paper was first described in the 1960s patent literature.22 It was initially produced by air oxidation of carbon blacks and ACs in the 500-700 °C temperature range. A catalytic carbon of this type, regardless of the method used to prepare it, is hereafter designated OAC500-700. It was effective for the dehydrochlorination of alkyl halides, and it also promoted polymerization of chloromethyl aromatics.23 Incorporating potassium salts into the catalyst inhibited skeletal rearrangements of olefins produced from monochloron-alkanes.24 Continuous vapor phase dehydrochlorination of mixed linear C13-C14 monochlorides gave >95% conversion for 690 h (73.5 g of alkyl chlorides/g of catalyst) with OAC500-700 produced from Darco activated (8) Song, C. S.; Schobert, H. H. Fuel 1996, 75, 724-736. (9) Economy, J.; Foster, K.; Andreopoulos, A.; Jung, H. CHEMTECH 1992, Oct, 597-603. (10) Thwaites, M. W.; Stewart, M. L.; McNeese, B. E.; Summer, M. B. Fuel Process. Technol. 1993, 34, 137-145. (11) Quinn, D. F.; MacDonald, J. A. Carbon 1992, 30, 1097-1103. (12) Sun, J.; Rostam-Abadi, M.; Lizzio, A. A.; Rood, M. J. Gas Sep. Purif. 1996, 10 (2), 91-96. (13) Lizzio, A. A.; Rostam-Abadi, M. Fuel Process. Technol. 1993, 34, 97-122. (14) Lizzio, A. A.; Feizoulof, C. A.; Rostam-Abadi, M.; Vyas, S. N. Carbon 1996, in press. (15) Kinner, N. E.; Malley, J. P.; Cement, J. A.; Fox, K. R. J. Am. Water Works Assoc. 1993, 85 (6), 75-86. (16) Nakayama, Y.; Nagao, H.; Mimura, M.; Yamasita, H.; Tanaka, E.; Hosokawa, K. X Abstractss21st Biennial Conference on Carbon, State University of New York at Buffalo, June 13-18, 1993; pp 402403. (17) Rittmann, B. E. Water Resourc. Res. 1993, 29, 2195-2202. (18) Brueggendick, H.; Pohl, F. G. Operating experience with Steag’s activated carbon processes-a/c/tTM in European waste incineration plants; Steag AG: Essen, Germany, 1993. (19) Tsuji, K.; Shiraishi, I. Proceedings of the EPRI SO2 Control Symposium, Washington D.C.; 1991; pp 307-324. (20) Boehm, H. P.; Bewer, G. The role of surface oxides in the gasification of carbon and the thermal stability of functional groups. In 4th London International Conference on Carbon and Graphite; 1976; pp 344-360. (21) Leon y Leon D.; Carlos, A.; Radovic, L. R. Interfacial chemistry and electrochemistry of carbon surfaces. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Dekker: New York, 1994; Vol. 24, Chapter 4, pp 213-310. (22) Kruse, C. W.; Ray, G. C. Dehydrohalogenation of hydrocarbon halides. U.S. Pat. 3,240,834, March 15, 1966. (23) Mahan, J. E.; Reusser, R. E.; Kruse, C. W. Dehydrohalogenation process. U.S. Pat. 3,352,935, Nov 14, 1967. (24) Kruse, C. W. Polymerization. U.S. Pat. 3,437,695, April 8, 1969.
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charcoal.25 Engineers advising the inventors of this catalyst believed the costs would be too high for onetime use in a commercial plant. Regeneration was achieved with a steam-nitrogen mixture at 750 °C, but predicting the time needed to attain precisely the desired amount of burnoff was difficult. Too much gasification shortened catalyst life. The regeneration method and the projected catalyst cost were among the factors that frustrated commercial development of OAC catalysts in the late 1960s. Experimental Procedures Acquisition, Preparation, and Characterization of Carbon Adsorbents for Adsorption Tests. A commercial granular activated carbon (GAC), Filtrasorb 400 (F-400), manufactured from a bituminous coal, was obtained from Calgon Carbon Corp. A 16 × 20 mesh size fraction (U.S. Standard Size) was used to prepare samples with various surface chemistries. The sized GAC was acid leached with 10 bed volumes of 1.0 M hydrochloric acid and 10 bed volumes of 1.0 M nitric acid (HNO3) to remove mineral impurities. The carbon was then rinsed with 20 bed volumes of deionized water followed by 5 bed volumes of a carbonate solution buffered at pH 6. The carbon was then rinsed with 5 bed volumes of distilled-deionized water. It was placed in a fluidized bed reactor (FBR), heated to 950 °C at 20 °C/min in ultrahigh purity (UHP) nitrogen gas, and fluidized at 950 °C for 30 min. The sample was then allowed to cool in UHP nitrogen. A 100-g portion of this degassed carbon was oxidized using 500 mL of boiling 1.0 M HNO3 for 30 min in a round-bottom boiling flask. Four samples were prepared from this HNO3-oxidized carbon to study whether a relationship between the presence of surface oxygen and adsorption of p-nitrophenol exists. Samples of this carbon relatively rich in surface oxygen groups were placed in a FBR and heated to 405, 525, 725, and 950 °C at 20 °C/min in a flowing stream of UHP nitrogen to produce four samples with differing amounts of oxygen remaining on the surface. The HNO3 oxidation, desorption steps were repeated on an AC made from IBC-109, a low-sulfur, high-chlorine Herrin (Illinois No. 6) coal from the Illinois Basin Coal Sample Program (IBCSP).26 A 0.25 in. × 10 mesh sample of IBC-109 was charred in a 5-cm i.d. FBR, a larger version of the 2.5-cm i.d. FBR described in more detail in the next paragraph. In a typical run, a 200-g quantity of coal was heated to 600 °C at 20 °C/min and held there for 15 min in a nitrogen atmosphere. The nitrogen flow rate was 10 L/min. A 10 × 30 mesh sample of this char was partially gasified. Gasification was conducted in a 2.5-cm FBR previously used for mild gasification of char and char desulfurization.27 The reactor was constructed from a 61-cm length of type 446 stainless steel, schedule 40 pipe. A distributor plate, constructed from porous Hastelloy-X stainless steel having an average pore size of 10 µm, was located 23 cm from the bottom of the pipe. The space below the plate was filled with 0.64cm ceramic Raschig rings, which served as a gas preheateer. Type 446 stainless steel caps were used to seal both ends of the reactor and to allow the reacting gas to enter and exit through 0.64-cm tubing. For each experiment, an amount of char, usually about 10 g, was charged to the reactor. The char was heated at 20 °C/min in nitrogen up to 870 °C, at which temperature it was gasified in 50% CO2/50% H2O at a flow rate of 2 L/min for 2.5 h. The gasified char was allowed to (25) Kruse, C. W. Unpublished Phillips Petroleum Company report. 1969, used by permission. (26) Harvey, R. D.; Kruse, C. W. J. Coal Qual. 1988, 7, 109. (27) Stephenson, M. D.; Williams, A. D.; Ellis, S. L.; Sprague, J. A.; Clauson, G. L. In Processing and Utilization of High Sulfur Coals II; Chug, Y. P., Caudle, R. D., Eds.; Elsevier: Amsterdam, 1987; pp 192201.
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Table 1. Effect of Nitric Acid Treatment on CMS Properties (IBC-102, 700 °C) adsorbed sample
outgas temp (°C)
O2 (cm3/g)
N2 (cm3/g)
O2/N2
control HNO3, 80 °C HNO3, 80 °C HNO3, 80 °C HNO3, 80 °C
130 25 130 160 180
5.12 0.30 2.22 2.91 4.72
3.71 0.03 0.31 0.48 1.83
1.38 10.00 7.16 6.06 2.58
cool in nitrogen. This activated carbon was then treated with boiling 1.0 M HNO3 as described above for the Calgon carbon. The HNO3-oxidized carbon was desorbed to 475, 525, 575, and 625 °C. Adsorption Capacity of Carbon Adsorbents in Distilled-Deionized Water. The procedures were those described in a previous paper.28 Adsorption isotherms of p-nitrophenol (PNP) on the partially desorbed the samples produced at 405, 525, 725, and 950 °C from F-400 and at 475, 525, 575, and 625 °C from IBC-109 were obtained at pH value of 6. 1. A 0.01 M stock solution of PNP was prepared. 2. A small sample (about 2 g) of GAC was dried and pulverized in a SPEX 8000 Mixer/Mill for 5 min. 3. The pulverized carbon was weighed into 10 125 mL serum bottles in the following quantities: 0, 0, 2, 4, 5, 6, 7, 8, 10, and 12 mg. 4. Ten millililters of 0.01 M PNP was pipetted into a 1.0 L volumetric flask and diluted to 1.0 L, giving a 0.0001 M PNP solution. Three drops of hydrochloric acid was added to this solution. 5. One hundred milliliters of 0.0001 M PNP solution was pipetted into each of the 10 serum bottles. 6. The bottles were sealed with Teflon-lined rubber septa, placed on an end-over-end tumbler, and allowed to equilibrate. 7. The bottles were removed from the tumbler, and the carbon was removed from the solution by vacuum filtration. 8. The pH of each solution was adjusted to g11 or more using concentrated sodium hydroxide. 9. The concentrations of PNP remaining in the solutions were determined using a spectrophotometer. Preparation of Oxidized/Partially Desorbed Carbons for CMS Tests. A 65 × 100 mesh fraction of IBC-102, a Colchester (Illinois No. 2) coal from the IBCSP26 that had been heated in N2 at 700 °C for 0.5 h (the control in Table 1), was treated for 2.5 h with 10 M HNO3 to the temperatures shown. For the desorption step the temperature was ramped quickly to the target temperature and held there for 2 h while purging with an inert gas. Where indicated, the char was activated by steam gasification before the HNO3 oxidation. SO2 Adsorption Capacity. The capacity of carbons after 6 h using 2500 ppmv of SO2, 7% H2O, and 5% O2 was measured using a Cahn TG-131 thermogravimetric analyzer (TGA) system. Temperature-Programmed Desorption (TPD). In a typical TPD experiment, 500 mg of sample was placed in a quartz tube reactor under a stream of UHP argon flowing at 100 cm3/min. The sample was heated at 10 °C/min to 950 °C and held isothermally at 950 °C for 5 min. A portion of the gas exiting the reactor was diverted to a quadrupole gas analyzer to determine the relative amounts of CO and CO2 desorbing from each sample. Dehydrochlorination Catalyst Preparation and Testing. (A) Catalyst Preparation. A catalyst effective for catalysis of elimination reactions was produced from an activated char made from IBC-109 [a Herrin (Illinois No. 6) coal from the IBCSP] by subjecting 100 g of the AC in a roundbottom flask to (1) refluxing with 500 mL of 1.0 M HNO3 for 30 min, (2) washing the treated carbon with distilled water (28) Feizoulof, C.; Snoeyink, V.; Kruse, C. W. Abstracts of Paperss21st Biennial Conference on Carbon, State University of New York at Buffalo, June 13-18, 1993; pp 367-368.
Figure 1. TPD profiles of HNO3-oxidized chars desorbed at 25-925 °C and oxygen. until the pH of the wash water remained constant, (3) drying the treated carbon at 100 °C, (4) heating the HNO3-treated carbon to 525 °C in a flow of nitrogen gas, and (5) cooling the desorbed carbon to room temperature under a nitrogen gas atmosphere. An air-oxidized carbon was prepared from steam-activated IBC-102 char in a porcelain crucible over a Bunsen burner until red hot. The maximum temperature monitored was 640 °C for the material made for the TGA test shown in Figure 1f. A sample weight loss of 10% occurred. (B) Dehydrochlorination Procedures. Fifty milliliters of 1,1,2,2-tetrachloroethane was transferred into a microevaporator. The sample was then heated to assist vaporization, and the vapor was passed over a bed of the OAC to be tested. The bed was held at 450 °C. A flow of nitrogen pushed the vapor through the hot bed of char. The collected product was analyzed using a Varian ion trap GC/MS. In a few experiments, the HCl from the condenser was trapped in a 0.1 M NaOH solution via a trap adapter with a bubbler. The NaOH was then back-titrated with 0.1 HCl, and the HCl evolved was calculated. The conversions were 73% for the HNO3-oxidized IBC-109 activated char that was desorbed to 575 °C, and 80% for the air-oxidized sample of the same IBC109 activated char.
Results and Discussion The discovery of an improved method for producing the OAC500-700 dehydrohalogenation catalyst was a byproduct of adsorptive studies begun in 1991 on AC.28 The objective of that work was initially to study the effects of an AC’s surface chemistry on aqueous adsorptive properties. The discovery described in this paper was triggered by an unexpected enhanced strength of adsorption of p-nitrophenol (PNP) on an activated carbon that had been oxidized with HNO3 and then heated to partially remove oxygen complexes. For a series of HNO3-oxidized carbons desorbed to 405, 525, 725, and 950 °C, respectively, the material desorbed to 525 °C displayed a high 1/n value (Figure 2a) for the Freundlich equation which is described as
qe ) KCe(1/n) where K and 1/n are the Freundlich constants. In this equation, the term 1/n is representative of the AC’s strength of adsorption for the adsorbate. The 525 °C sample’s slope was 0.27 at a pH value of 6, while those
Production of Catalytic Carbon
Figure 2. Freundlich equation plots for PNP on HNO3oxidized AC desorbed to the temperature shown.
of the remaining carbons were 0.22 ( 0.02. When the PNP adsorption was repeated on a second series of partially desorbed carbons, the slopes were 0.15 for 475 °C, 0.19 for 525 °C, 0.23 for 575 °C and 0.21 for 625 °C. Material desorbed to 575 °C was found to have the strongest adsorption in this series (Figure 2b). The catalytic properties of OAC500-700 produced by the 1960s air oxidation method were under study at the time the adsorptive studies were begun in 1991.29,30 This stronger adsorption in the (575 °C range took place in the temperature range for which oxidation of carbonaceous materials with air was known since the 1960s to produce carbon catalysts that are effective for elimination reactions.22-24 The similarity of temperature range for both phenomena raised the question whether this increase in the strength of adsorption might be related to chelation of the host molecule and correlate with catalytic activity. Vapor phase dehydrohalogenation of 1,1,2,2-tetrachloroethane using HNO3-oxidized, desorbed activated carbon as catalyst confirmed the catalytic activity of this material. The conversions were 72% using a HNO3-oxidized carbon that had been desorbed to 575 °C and 80% for the air-oxidized carbon. Catalyst activity has been known since the 1960s to be destroyed by heating to the 800 °C range. Are the functional groups remaining on internal surfaces (micropore surfaces) at this temperature not only the right groups but also positioned at the precise distances from one another to provide not only a chelation of host molecules but also enhanced catalytic properties? Although TPD data in the literature31 and (29) Fatemi, M.; Kruse, C. W.; Lagman, R. T. Abstracts of Paperss21st Biennial Conference on Carbon, Buffalo, NY, 1993; pp 505-506. (30) Kruse, C. W.; Lizzio, A. A.; Feizoulof, C. A.; DeBarr, J. A.; Fatemi, S. M. In Proceedings of the 22nd Biennial Conference on Carbon, San Diego, CA, July 1995; pp 538-539. (31) Otake, Y.; Jenkins, R. G. Carbon 1993, 31, 109.
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TPD profiles generated on samples from our study do not answer the mechanism question, they do confirm the similarity of oxygen functional groups on carbons made according to the two methods (Figure 1b,e). The notion that catalyst sites are in the pores and may involve functional groups located on opposing pore walls has been advanced on the basis of the following observations.32 First, dehydrohalogenation of monochloro-n-alkanes over properly prepared OAC500-700 occurs without significant carbon skeleton rearrangement. This property was of special interest to the catalyst developers in the 1960s because the biodegradability of detergents to be produced from the olefins depended on preserving the linearity of the carbon chains. The speculation at that time was that dehydrohalogenation might be taking place in narrow confines of a pore achieving the desired result by keeping the chain in a linear configuration in the segment of the chain undergoing the elimination reaction. Second, desorption of oxygen functional groups appears to change the pore dimensions of HNO3-oxidized carbons. Lizzio et al.14 examined CMS properties of oxidized/thermally desorbed carbons. The adsorption of both oxygen and nitrogen, kinetic diameters 3.46 and 3.68 Å, respectively, is markedly diminished by HNO3 oxidation (Table 1). The adsorption capacity is restored in proportion to the amount of the oxygen functionalities removed at successive desorption temperatures, however, not at the same rate for the two gases. The restoration takes place more rapidly for oxygen, the smaller of the two, resulting in a high selectivity (higher ratio) needed for pressure swing gas separation processes. The conventional thinking is that a partial filling of pores by the functional groups that are introduced by HNO3 oxidation accounts for a narrowing of the pores. However, the possibility that the porenarrowing action of these functional groups is due in part to a shrinkage of the structure by pulling the pore walls closer together, especially if they are slit shaped, has not been ruled out. Carboxyl groups, the groups that are the primary ones being removed in the stage of oxygen desorption that brings a HNO3-oxidized carbon to a catalytic state, are groups with a strong capacity to hydrogen bond and form anhydrides that could shrink the structure if the carbon network is flexible enough for such deformation. An increase in the proportion of pores having the correct dimensions and opposing functional groups could increase the strength of adsorption of molecules occupying positions between them. A sketch of the type of arrangement that might facilitate catalysis without rearrangement is shown in Figure 3. The ability to “fine tune” pore dimensions could be as important in the catalytic processes taking place in pores as it is known to be in making molecular sieves for specific applications. It might also be important for other adsorption phenomena, including SO2 adsorption, which is increased by desorption of oxygen functional groups at temperatures higher than those used to make the catalyst (Table 2).33,34 The adsorption properties of the starting AC are modified but not lost by making it a catalyst. Most of (32) Kruse, C. W.; Fatemi, M.; Feizoulof, C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39, 601-605. (33) Lizzio, A. A.; DeBarr, J. A. Fuel 1996, 75 (13), 1515-1522. (34) DeBarr, J. A. Ph.D. Thesis, University of Illinois, in progress.
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Figure 3. Conceptualized dehydrohalogenation site in a poresan E2 elimination. Table 2. Variation of SO2 Capacity with Oxygen Content (IBC-102, 900 °C; H2O; 860 °C) sample
outgas temp (°C)
SO2 capacity (mg/g)
oxygen (wt %)
control 45% HNO3 2.5 h 45% HNO3 2.5 h 45% HNO3 2.5 h 45% HNO3 2.5 h
120 525 725 925
176 26 91 241 287
1.1 16.4 5.9 1.6 0.5
plates. Each compartment has it own discharge equipment so that it can be moved and extracted independently from the others. The first compartment removes particulates, gaseous heavy metals, and dioxins and furans. The second compartment removes sulfur oxides (SO2/SO3), and the third removes halogenated hydrogen (acid gases). The adsorbent in each compartment can be replaced according to its capacity, providing the most efficient use of the carbon. The adsorption unit is designed to hold a supply of fresh carbon in a hopper at the top of the bed, which continually replenishes the supply. The unit’s unique, patented features ensure the safe and efficient use of activated carbon while operating at temperatures between 100 and 150 °C. The ISGS demonstrated in 1994 that a low surface area AC (300 m2/g.18 The spent sorbent from STEAG’s a/c/t process is not regenerated in one configuration, and regeneration may not be necessary with other processes using lower cost, lower surface area AC (Figure 4). STEAG’s proprietary fixed bed design is shown in Figure 5.37 The flue gas flows horizontally through a fixed bed. The bed consists of three vertical compartments of activated carbon separated by perforated (35) Kruse, C. W.; Chou, M. I. M.; Feizoulof, C.; Fatemi, M.; Beaulieu, P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37, 556-563. (36) Beaulieu, P. L.; Fatemi, S. M.; Tumidalsky, J. B.; Kruse, C. W.; Chou, M.; Feizoulof, C. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37, 1965-1970. (37) Brueggendick, H.; Gilgen, R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (1), 399-403.
(38) Kruse, C. W.; Lizzio, A. A.; DeBarr, J. A.; Bhagwat, S.; RostamAbadi, M.; Lytle, J. M. Producing activated char for cleaning flue gas from incinerators. Final Technical Report to the Illinois Clean Coal Institute, Dec 1995; Subgrant 22CRGTSK592C. (39) Smisek, M. Manufacture of active carbon; Applications of active carbon. In Active Carbon; Smisek, M., Cerny, S., Eds.; Elsevier: Amsterdam, 1970; Chapters 2 and 5, pp 42, 256-257. (40) U.S. EPA. Standards of performance for new stationary sources and emission guidelines for existing sources. U.S. Environmental Protection Agency, 40 CFR Part 60, Oct 31, 1995.
Production of Catalytic Carbon
Energy & Fuels, Vol. 11, No. 2, 1997 265
Figure 5. STEAG’s a/c/t process.
granular carbon for fixed beds could occur. The availability of low-cost AC could then become the base from which to launch the production of cost-effective catalytic carbons to satisfy the needs of large scale adsorbent/ catalyst systems designed both to remove and to decompose toxic pollutants found in a variety of liquid and gaseous streams, chlorinated organic compounds in particular. The tradeoffs both technically and economically to achieve a balance between catalytic properties and adsorptive properties must be evaluated. Fresh, dry OAC was shown in previous work to remove 37 volatile organic compounds (VOCs) from aqueous solutions.7,41 These are VOCs for which federal and state discharge permits usually require screening. Some of the analytes were more strongly adsorbed than others. Important factors for future commercial development of OAC500-700 include (1) how it ranks in competitive tests with other catalysts, (2) comparative disposal costs, and (3) availability and pricing. Porous glass (unglazed porcelain),42 silica gel,43-46 4 B-18 crown ether-potassium chloride on silica gel,47 alcoholic potassium hydroxide, and the new Calgon Carbon catalyst, Centaur, are among its rivals. Extensive comparative tests of OAC500-700 and other catalysts will be necessary to determine the strengths and weaknesses of each material for specific applications. The disposal costs must be weighed against regeneration costs. Incineration (41) Beaulieu, P. L.; Fatemi, S. M.; Tumidalsky, J. B.; Kruse, C. W.; Chou, M.; Feizoulof, C. Prepr. Pap.sAm. Chem Soc., Div. Fuel Chem. 1992, 37, 1965-1991. (42) Lycourghiotis, A. React. Kinet. Catal. Lett. 1976, 5 (4), 453457. (43) Misono, M. J. Catal. 1973, 30 (2), 226-234. (44) Lycourghiotis, Vattis, A. D.; Katsanos, N. A. Z. Phys. Chem. (Wiesbaden) 1981, 126 (2), 259-267. (45) Mochida, I.; Watanabe, H.; Uchino, A.; Fujitsu, H.; Takeshita, K.; Furuno, M.; Sakura, T.; Nakajima, N. J. Mol. Catal. 1981, 12 (3), 359-364. (46) Suarez, A. R.; Mazzieri, M. R. J. Org. Chem. 1987, 52 (6), 11451147. (47) Fujitsu, H.; Takagi, T.; Mochida, I. Bull. Chem. Soc. Jpn. 1985, 58, 1589-1590.
appears an option for OAC500-700 and Centaur that is not available for other catalysts listed. It remains to be seen how much the price of Centaur (about $2.50/lb today) will decrease with large scale production. The estimated production capacity in the United States of AC, which has N2 BET surface areas from 500 to 2500 m2/g, was estimated in 1990 to be 46 000 metric tons48 and was projected in 1994 to reach 149 000 tons in the 1998-2003 time frame.49 Retrofitting only a fraction of the existing incinerators with carbon-based systems would create a demand greater than exists today for all types of AC. Because of the variety of ACs marketed today in relatively small volumes, the prices reflect fine chemicals prices, often in dollars per pound. With the arrival of dedicated facilities for producing 80 000 ton/year or more of one type of AC, the prices can be expected to decline, whatever the grade of AC marketed. The future cost of OAC500-700 should parallel the decreasing prices of AC marketed in the United States. The high cost of many technically feasible systems for protecting the environment prevents serious consideration of their use. Providing the database on lower cost adsorbents and catalysts will promote their commercial availability in the future. Conclusions A new way to make, and perhaps “fine tune”, the catalytic properties of oxidized AC has been demonstrated. Because the low-temperature oxidation/desorption method is not only more readily managed than air oxidation for producing OAC500-700 catalyst, it might be used to regenerate the catalytic activity of fixed beds in-place if economics dictate regeneration. The availability of lower cost OAC500-700 in the United States is (48) Baker, F. S.; Miller, C. E.; Repik, A. J.; Tolles, E. D. KirkOthmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 1992; Vol. 4, pp 1015-1037 (pp 1022-1023 for production figures). (49) Fredonia Group Inc. Activated Carbon Markets, 3570 Warrensville Center Road, Suite 201, Cleveland, OH 44122-5226, 1994.
266 Energy & Fuels, Vol. 11, No. 2, 1997
projected on the basis of the probability that lower cost ACs from which to produce the catalysts will become available in the United States. This prediction assumes that, given time for public reaction, Americans will not be satisfied to accept a higher level of emissions than Europeans and that the best available technology will become the fixed bed technology. Acknowledgment. This work was supported, in part, by grants made possible by the U.S. Department of Energy (DOE) Cooperative Agreement No. DE-FC2292PC92521 and the Illinois Coal Development Board (ICDB) and the Illinois Clean Coal Institute (ICCI) and managed by F. I. Honea, D. D. Banerjee, and H. Feldmann. Neither the authors and the ISGS nor any
Kruse et al.
of its subcontractors nor the U.S. DOE, Illinois Department of Energy and Natural Resources, ICDB,or ICCI, nor any person acting on behalf of these organizations assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information, apparatus, method or process disclosed in this paper. We acknowledge contributions by M. Fatemi, M. M. M. Chou, I. Demir, S. Carlson, G. Zajac, and R. T. Lagman. The views and opinions expressed herein do not necessarily state or reflect those of the U.S. Department of Energy. We also acknowledge the in-kind contribution of funds by the Alternative Feedstock Development program of Amoco Corp. EF9601982
