2355
Ind. Eng. Chem. Res. 1991,30, 2355-2358 for prediction of vapor-liquid equilibria at 250-425 K (16 pages). Ordering information is given on any current masthead page.
Literature Cited Bondi, A. Physical Properties of Molecular Crystals, Liquids and Classes; Wiley: New York, 1968 Chapter 14. Fredenslund, Aa.; Jones, R. L.; Prausfiitz, J. M. AIChE J. 1975,21, 1086. Fredenslund, Aa.; Gmehling, J.; Rasmussen, P. Vapor-Liquid Equilibria Using UNIFAC; Elsevier: Amsterdam, 1977; pp 1-380. Gmehling, J.; Onken, U. Vapor-Liquid Equilibrium Data Collection; DECHEMA Chemistry Data Series 1; DECHEMA: Frankfurt, 1977; pp 1-698.
Tiegs, D.; Gmehling, J.; Raamussen, P.; Fredenslund, Aa. Ind. Eng. Chem. Res. 1987,26, 159.
Henrik K.Hansen, Peter Rasmussen Aage Fredenslund* Znstitut for Kemiteknik The Technical University of Denmark DK-2800Lyngby, Denmark
Martin Schiller, Jiirgen Gmehling Technische Chemie Universitat Oldenburg 0-2900Oldenburg, Germany Received for review January 28, 1991 Accepted July 25, 1991
Decomposition of Dichlorodifluoromethane on BPOI Catalyst Catalytic decomposition of dichlorodifluoromethane (Freon 12) was carried out on BPO, catalyst, and the durability of this catalyst against inorganic fluorines, the reaction products, was investigated. The decomposition of Freon 12 proceeded on the acid sites of the BPO,, although their acid strength was not very large. Although the BPO, catalyst had the highest durability among the catalysts ever examined, it was also deactivated during prolonged use. The analysis by ESCA (electron spectroscopy for chemical analysis) indicated that inorganic fluorines attacked the boron to produce volatile boron fluorides, which were eliminated from this catalyst during the reaction. CaO absorbed inorganic chlorines and fluorines. Thus the efficiency of the BP04-catalyzed decomposition of Freon 12 was improved by the combined use of CaO which protected the BPO, catalyst from poisoning by inorganic fluorines. There is a great concern about the depletion of the ozone layer caused by chlorofluorocarbons (CFC's). Although their production will be diminished in the future owing to the United Nations Environmental Protection Protocol for CFC regulation adopted in Montreal, Canada, in 1987, some of them are still being used now, and the preserved CFC's will inevitably continue to increase. Thus an efficient method to destruct CFC's must be urgently developed. CFC's can be destructed by various methods: reduction by use of sodium naphthalenide (Oku et al., 1988, 1989), laser-induced decomposition (Hudgens, 1977; Zitter et al., 1990), catalytic hydrogenolysis (Witt et al., 1981), and catalytic decomposition (Colussi and Amorebieta, 1987). Of these methods, catalytic decomposition seems most practical. Chlorotrifluoromethane and 1,1,2-trichloro1,2,2-trifluoroethane were readily decomposed over Fe203 supported on activated charcoal above 450 "C in the presence of water vapor (Okazaki and Kurosaki, 1989). Gold supported on A1203showed high activity in decomposing dichlorodifluoromethane (Aida et al., 1990). Hydrogen fluoride and molecular fluorine were produced during the decomposition of these CFC's. Although these inorganic fluorines are highly reactive and cause the deactivation of the catalysts, there have been scarce data on the durability of the catalysts used for the decomposition of CFC's. Previously we reported that multiple-step catalytic combustion is effective to dispose of chlorinated organic compounds (Imamura et al., 1989a,b). This process is composed of the following three steps. The first step is a catalytic decomposition of chlorinated organics into inorganic chlorines and a carbonaceous moiety (mainly to CO and COJ. The second step i3 the absorption of inorganic chlorines by basic oxides such as CaO to protect the combustion catalysts to be employed in the last step where complete combustion of the carbonaceous moiety
is accomplished. This process could be applied to the combustion of dichlorodifluoromethane (Freon 12), and the active catalysts in the first step were acidic oxides such as zeolite, mordenite, silica-alumina, and titania-silica. Although these acid catalysts, especially titania-silica, were durable against inorganic chlorines, they were deactivated rapidly during the decomposition of Freon 1 2 due to an attack by inorganic fluorines (Imamura et al., 1990). We, therefore, investigated the activity and durability of various catalysts including acid catalysts other than those mentioned above, precious-metal catalysts, transition-metal oxides, and so forth (Imamura et al., 1991). The examination of these catalysts led us to conclude that it was impossible to design robust catalysts against fluorines by using metals or silicon as starting elements. As a nonmetal acid catalyst, supported sulfuric acid was found to be active. However, sulfur escaped as sulfur oxides during the reaction. Finally, it was found that a phosphate and boron-based compound, BPO,, was the only promising catalyst. The present paper deals with the catalytic performance of the BPO, catalyst in the decomposition of Freon 12. The recovery of fluorines and chlorines by CaO and its effect to protect the BPO, catalyst from attack by inorganic fluorines are also described.
Experimental Section Materials. The BPO, was prepared by the reaction of boric acid with phosphoric acid and was calcined at 550 "C in air for 3 h (Ozaki, 1980). Its structure was determined as BPOl with an X-ray diffraction analyzer, and ita BET surface area (S,) was 12.6 m2/g. Ita particle size as determined by a SEM technique was about 0.6 pm for the primary particles and about 100 pm for the secondary particles, respectively. The primary particles aggregated to form large secondary particles. CeOz (S, 80.7 m2/g) was prepared by precipitation from aqueous cerium(II1) nitrate, followed by calcination at 550 "C in air for 3 h (Imamura
0888-588519112630-2355$02.50/0 0 1991 American Chemical Society
2356 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 Table I. Decomposition of Freon 12' conversion, %, at reaction catalvst temo. "C 0.1 2 5 6 48.5 CeOz 500 94.3 (100)b (94.8) Nb20, 500 98.0 93.3 10.0 (93.6) (89.2) (90.0) 91.9 71.0 33.5 a-Al2Oa 500 98.6 (100) (87.9) (73.0) (80.3) 81.9 82.2 BPO, 500 80.0 (76.8) (77.5) (74.1) BPO, 550 100 96.8 93.3 92.1 (88.7) (94.3) (98.5) (96.3) ~
time, h, of 8 10
~~
0
I
22.7 (81.0) 89.3 90.8 (93.0) (94.1)
"[Freon 121 = 0.06%; [O,] = 21.2%; [He] = 78.2%; BPO, = 1 mL; space velocity = 5900 h-l. The values in parentheses are selectivity to CO + COP.
and Ando, 1989). Benzene was purified by refluxing overnight over sodium metal, followed by distillation. Freon 12 (0.74% in He), CaO, Nb205 ( S , 7.8, m2/g), activated a-A1203( S , 138.5 m2/g), Hammett indicators, and other reagents were obtained commercially. Apparatus and Procedure. Reactions were carried out under atmospheric pressure with a tubular flow reactor made of alumina (outer diameter 10 mm; inner diameter 6 mm). The catalysts were molded into a disk under a pressure of 40 MPa and were cut into about 8-14 mesh size. One milliliter of the catalysts was charged in the reactor, and a thermocouple was placed in the center of the catalyst bed. Quartz wool was stuffed into the reactor tube from both sides to keep the catalysts packed densely. The reactor was heated with an electric furnace. Freon 12 in He was mixed with oxygen, and this reaction gas mixture (Freon 12 0.60%; O2 21.2%; He 78.2%) was introduced into the reactor at a space velocity of 5900 h-' at prescribed temperatures. The space velocity was measured at ambient temperature. The reacted gas was analyzed for CO, COz, and the remaining Freon 12. The reaction was not affected by any mass-transfer limitation (Imamura et al., 1990). In the absence of the catalysts with the aluminum tube reactor, only 1.4% and 3.0% of Freon 12 were decomposed at 500 and 550 "C, respectively. Analyses. Freon 12, CO, and C02were determined with a Shimadzu GC-12A gas chromatograph equipped with a flame ionization detector at a column temperature of 120 "C. The column packing for the analysis of Freon 12 was Chromosorb 101 (1m). After CO and C02were separated with an activated-charcoal column (1m), they were converted to methane with a Shimadzu MNT-1 methanizer and were determined. The acid amount on the BPO, was determined by titration with n-butylamine in dry benzene in the presence of the following Hammett indicators; methyl red (pK, = 4 8 , methyl yellow (pK, = 3.3), benzeneazodiphenylamine (pK, = 1,5), dicinnamalacetone (pK, = -3.0), benzalacetophenone (pK, = -5.6), and anthraquinone (pK, = -8.2). The X-ray and ESCA analyses were carried out with a Rigaku Denki Geigerflex 2012 X-ray analyzer and a Shimadzu ESCA 750 spectrophotometer, respectively. The amount of fluorine on the surface of the used BPO, catalyst was determined with the ESCA spectrophotometer after the used catalyst was evacuated at 300 "C for 30 min. The boron that escaped from the BPO, during the reaction was trapped with 100 mL of water. The boron in this aqueous solution was converted to BF4- by reaction with hydrofluoric acid, followed by complex formation with methylene blue. This complex was determined spectrophotometrically (Utsumi et al., 1965).
Acid strength
Figure 1. Acidity of BPO,. Acid strength is expressed by Hammett acidity coefficient Ho.
Results and Discussion Decomposition of Freon 12 on the Active Catalysts. Table I shows the results on the decomposition of Freon 12 on the BPO,, Ce02,NbzO5, and a-A1203at 500 "C. The latter three catalysts were chosen as the most durable catalyst group ever examined. The result of the reaction on the BPO, at 550 "C is also shown in the table. C02was the main reaction product in all cases. Ce02 was deactivated rapidly, and Nb205also lost its activity markedly after 5 h. Although the activity and selectivity to CO + C02 on the BPO, was not very high compared to those of the other three catalysts, its activity did not decrease at all even after 5 h. When the reaction temperature was raised to 550 "C, the initial conversion of Freon 12 was 100%. The BPO, retained high activity even after 10 h although a gradual deactivation occurred, whereas the conversion on a-A1203,which was the most durable metal oxide catalyst ever examined, dropped to 22.7% after 8 h at 500 "C. Figure 1 shows the acid amount on BPOe It contained mainly the acid sites with weak acid strength. The observed strongest acidity expressed by the Hammett acidity coefficient (Ho) was -3.0, and the amount of the acid site with this acid strength was not very large. However, these weak acid sites were enough to decompose Freon 12 because the conversion of Freon 12 and BPOl at 550 "C was almost 100% in the absence of oxygen, indicating that acid-catalyzed decomposition of Freon 12 occurred (Imamura et al., 1990). Recovery of Inorganic Halogens by CaO and Protection of the BPOl from Attack by Inorganic Fluorines. In order to recover inorganic halogens produced during the reaction, 1mL of CaO was charged between the two BPO, catalyst beds (1-mL volume each) in the same reactor. Figure 2 shows the decomposition of Freon 12 on this catalyst system at 550 "C. A 100% conversion of Freon 12 was attained even after 10 h. The initial selectivity to CO + COi was very low, which suggested that CO or COPwas trapped by basic oxide, CaO, in the form of CaC03. Therefore, CaO is effective to trap CO or C02 produced during the combustion of organic compounds. However, the selectivity to CO + C02 gradually increased, and exceeded 100% after 2 h. After the selectivity reached a maximum, it approached 100%. The amount of CO plus COPtrapped initially (dashed area in the figure) coincided well with the CO plus C02which were released over 100% selectivity after 2 h. This means that although CO and C02 were trapped by CaO to form CaC03 in the initial stage of the reaction, fluorines or chlorines attacked CaC03 and replaced Cog to release it as C02 and/or CO.
Ind. Eng. Chem. Res., Vol. 30,No. 10, 1991 2357
1
5
0
1
-
OO
5
10
Time (h)
Figure 2. Decomposition of Freon 12 on BPO, at 550 "C. [Freon 121 = 0.06%;[O,]= 21.2%;[He] = 78.2%;apace velocity = 5900 h-l; 1 mL of CaO was charged between two BP04 beds of 1-mL volume each in the same reactor. %electivity to CO + COz. A
1
II
P I B A
I
I
I
28
CuKa
Figure 3. X-ray diffractionpatterns of CaO before reaction (A) and after 10 h of reaction (B): CaF, ( O ) , CaClF (A), and CaCl, (0).
Therefore, the efficiency of CaO to absorb inorganic halogens was not impaired even if CO or CO, was present. In fact, no CaC03was detected by an X-ray analysis of the used CaO after 10 h (Figure 3). To examine the ability of fluorines to replace chlorines trapped by CaO, CaCl, was charged instead of CaO in the same catalyst system described above. After the decomposition of Freon 12 at 550 "C for 2 h, however, no replacement of the chlorine by fluorines occurred. Therefore, chlorines and fluorines produced during the decomposition of Freon 12 randomly attacked CaO, producing various calcium halides as are shown in Figure 3. Although the recovery of more valuable fluorine in the form of CaF, is desirable, formation of mixed calcium halides cannot be avoided. In order to improved the efficiency of the decomposition of Freon 12, three reactors were used in a series. One milliliter each of BPO, was charged in the first and the third reactors, and 10 mL of CaO was charged in the second reactor. The temperature of the CaO reactor was maintained at 500 "C,and the two BPO, reactors were at 550 "C. Figure 4 shows the change of the conversion of Freon 12 at the exit of the first and the third reactors together with the selectivity to CO plus COBat the exit of the third reactor. With this reaction system, the conversion of Freon 12 at the exit of the third reactor was maintained at 100% after 29 h. However, the conversion at the exit of the first reactor dropped markedly after 16
Time (h) Figure 4. Decompositionof Freon 12 on BPO, at 550 "C. A reactor with 10 mL of CaO (500 "C) was placed between two reactors with 1 mL of BP04 each. Other reaction conditions are the same aa shown in Figure 2. Conversion of Freon 12 at the exit of the first BP04 bed (0) and the second BPO, bed (0);selectivity to CO + COz at the exit of the second BP04 bed (A).
h, and was 11% after 40 h. The X-ray diffraction analysis of the BPO, in the first reactor indicated no change in its configuration after 10 h. Boron scarcely eluted from the catalyst; analysis of the water through which the reacted gas was passed for 10 h in a separate experiment indicated the presence of only 0.026% of B (based upon the BP04 initially charged). Therefore, the loss of B was not the cause of the deactivation of the BPO, after 10 h. The surface composition of the BPO, was determined by an ESCA analysis. The peaks used for the calculation of the surface composition were the isll2 peak (binding energy = 188 eV) for B, and the 2s112 peak (189 eV), 2p1,, peak (135 eV), and 2p3/, peak (134 eV) for P. The surface composition of the fresh BPOl was B / P = 0.5711. The BPO, used for 10 h of reaction contained fluorine on its surface: the surface composition was B / P / F = 0.54111 0.13. As the only available peak for B (Isllz) was superimposed by the 2sIl2peak of P, the exact ratio of B to P could not be obtained. Although the composition determined above was not accurate, the presence of fluorine on the surface of the used BPO, was clearly shown. Therefore, the coverage of the BPO, surface by fluorine might be the cause for the slight deactivation during 10 h of the reaction, although details were not known. The ESCA analysis of the surface of the BPO, in the first reactor after 40 h showed the absence of fluorine. However, the molar ratio of B to P was found to be 0.063; almost all B was eluted from the catalyst. Thus the marked deactivation after 16 h was due to the loss of B, leading to the destruction of the catalyst frame. The effect of the reaction conditions on the efficiency of the reaction and on the durability of the BP04 was investigated. Decreasing the space velocity from 5900 to 3000 h-' resulted in 100% conversion of Freon 12 at 500 "C whereas the conversion at a space velocity of 5900 h-' was 80% at the same reaction temperature. An injection of 0.13% of water (hydrogen or proton source) had no effect on Freon 12 conversion and durability of the BPO, at 550 "C. Although a 100% conversion of Freon 12 was attained until 6 h at 650 "C,successive reaction led to a marked deactivation of the BPO, presumably due to the destruction of the catalyst frame (Figure 5). When the reaction temperature was decreased to 400 "C, the initial conversion was only 70%. However, drastic deactivation of BPO, did not occur even after 40 h, although the conversion of Freon 12 gradually decreased. Thus it may be advantageous to carry out the reaction at lower temperatures at the expense of the conversion to avoid drastic destruction of the catalyst frame. The ultimate conversion of Freon 12 would be improved by employing several BPO,
Ind. Eng. Chem. Res. 1991, 30, 2358-2359
2358 100,
A\
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6
I
I
-I-
I
I
I
I
1
Colussi, A. J.; Amorebieta, V. T. Heterogeneous Decomposition of Trichlorofluoromethane on Carbonaceous Surfaces. J. Chem. SOC.,Faraday Trans. 1 1987,83, 3055-3059. Hudgens, J. W. In Situ Studies of Infrared Multiple Photon Laserinduced Decomposition of CF& and CFC13. J. Chern. Phys. 1978, 68, 777-778. Imamura, S.; Ando, M. Oxidation of Tristearin on Manganese/Cerium Composite Oxide. Znd. Eng. Chem. Res. 1989,28,1452-1456. Imamura, S.; Ikeda, T.; Ishida, S. Multiple-step Catalytic Combustion of 1,2-Dichloroethane. Nippon Kagaku Kaishi 1989a, 139-144. Imamura, S.; Tarumoto, H.; Ishida, S. Decomposition of 1,2-Dichloroethane on TiOz/SiOz. Ind. Eng. Chem. Res. 198913, 28, 1449-1452. Imamura, S.; Shiomi, T.; Ishida, S.; Utani, K.; Jindai, H. Decomposition of Dichlorodifluoromethane on Ti02/SiOz. Znd. Erg. Chem. Res. 1990,29, 1758-1761. Imamura, S.; Imakubo, K.; Fujimura, Y.Catalytic Decomposition of Dichlorodifluoromethane-A Study on the Catalysts Durable Against Fluorine. Nippon Kagaku Kaishi 1991,645-647. Okazaki, S.; Kurosaki, A. Decomposition of Chlorofluorocarbons by the Reaction with Water Vapor Catalyzed by Iron Oxide Supported on Activated Carbon. Chem. Lett. 1989,1901-1904. Oku, A.; Kimura, K.; Sato, M. Chemical Decomposition of Chlorofluorocarbons by Reductive Dehalogenation Using Sodium Naphthalenide. Chem. Lett. 1988, 1789-1792. Oku, A.; Kimura, K.; Sato, M. Complete Destruction of Chlorofluorocarbons by Reductive Dehalogenation Using Sodium Nephthalenide. Znd. Eng. Chem. Res. 1989,28, 1055-1059. Ozaki, A. Ed. Shokubai Chosei Kagaku; Kodansha: Tokyo, 1980, pp 254-258. Utsumi, S.; Ito, S.; Isozaki, A. Determination of a Trace Amount of Boron by Extraction-Spectrophotometric Method. Nippon Kagaku Kaishi 1965,86,921-925. Witt, S. D.; Wu, E. C.; Loh, K. L.; Tang, Y. N. Heterogeneous Hydrogenolysis of Some Fluorocarbons. J. Catal. 1981,71,270-277. Zitter, R. N.; Koster, D. F.; Choudhury, T. K.; Cantoni, A. Kinetics and Mechanisms of the CO, Laser Induced Decompositions of CFC1, and CFzCl2. J. Phys. Chem. 1990,94, 2374-2377.
Seiichiro Imamura,* Ken-ichiro Imakubo Setsuo Furuyoshi Department of Chemistry Kyoto Institute of Technology Matsugasaki, Sakyo-ku, Kyoto 606, J a p a n
Hitoshi Jindai Nippon Fine Gas Co. Ltd. 1 - 4 Takasago, Takaishi, Osaka 592, J a p a n
Literature Cited Aida, T.; Higuchi, R.; Niiyama, H. Decomposition of Freon-12 and Methyl Chloride over Supported Gold Catalysts. Chem. Lett. 1990, 2247-2250.
Received for review April 15, 1991 Revised manuscript received July 2, 1991 Accepted July 26, 1991
CORRESPONDENCE A Note on Differing Characterization of the Mechanisms for Glucose to Pyruvate Conversion Sir: In our recent paper in this Journal (Happel et al., 1990), our final example was the conversion of glucose to
pyruvate, as presented by Seressiotis and Bailey (1988). In this correspondence we wish to state that what we described as a “discrepancy” between the two papers involves no error or inconsistency. The matter has been resolved after further discussion with the authors and we believe that the conclusions may be of interest to readers con0888-5885/91/2630-2358$02.50/0
cerned with this type of problem. Both papers had as their object the complete listing of all minimal or (as we say) direct mechanisms for the conversion of glucose to pyruvate. In Happel et al, (1990) this conversion was assumed to be a combination of two linearly independent reactions advancing at independent rates. In contrast, Seressiotis and Bailey (1988) view the conversion as having alternative forms that could be de@ 1991 American Chemical Society
