Ind. Eng. Chem. Res. 1995,34, 967-970
967
Effect of Water on the Durability of BP04 in the Decomposition of Freon 12 Seiichiro Imamura,* Takao Higashihara, and Kazunori Utani Department
of
Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606,Japan
Water improved the durability of titania silica (TiOdSiOz), phosphate supported zirconia (Pod ZrOa), and boron phosphate (BP04) catalysts in decomposing dichlorodifluoromethane (Freon 12). Although configuration changes of TiOz/SiOz and of POdZrO4 occurred after about 30 h, BP04 retained its activity without any configuration change even after 35 h. Water eliminated inorganic fluorides accumulated on the surface of BP04 presumably i n the form of hydrogen fluoride. Methanol exhibited little reactivating effect on BP04. Two catalyst beds were used in a series, and methanol was completely decomposed on Co304 in the first bed a t 275 "C, a t which Freon 12 did not decompose much. When the remaining Freon 12 was decomposed over BP04 in the second bed, the life of BP04 was prolonged remarkably due to the water produced from methanol in the first bed. Thus only water was effective as a reactivating reagent, and hydrogen atoms of methanol did not work directly.
Introduction Worldwide efforts have been made to decrease the use of chlorofluorocarbons since the United Nations Environmental Protection Protocol for chlorofluorocarbon regulation was adopted in Montreal in 1987. At the same time the development of various processes has been attempted to detoxify chlorofluorocarbons. These processes include reduction with sodium naphthalenide (Oku et al., 1989) or hydrogen (Witt et al., 1981), adsorption and decomposition on activated charcoal (Colussi and Amorebieta, 19871, decomposition by solar energy (Blake, 1988), and so on, although none of them has reached the level of practical application yet. Catalytic decomposition seems to be effective to dispose of a large amount of chlorofluorocarbons (Okazaki and Kurosaki, 1989, 1991; Aida et al., 1990; Jacob, 1990; Miyatani et al., 1992). One of the authors has been engaged in the development of the catalytic decomposition process and found that many solid acids are effective (Imamura et al., 1990) However, inorganic halogen species (especially fluorine species: F2 and HF) produced during the reaction poisoned the catalysts remarkably. Thus, the design of the refractory catalysts against inorganic halogen species is crucial to develop a catalytic decomposition process. The Phosphate group, which is rather inert toward inorganic fluorine species, produces acid sites effective for decomposing chlorofluorocarbons when combined with other elements. Thus we prepared BP04 and POdZrOz (Imamura et al., 1991, 1993a). Although they are the most robust catalysts ever examined,their durability was not enough for practical use. In the course of these investigation it was found that water helped maintain the activity of the catalysts (Imamura et al., 199313). The present paper deals with the more detailed investigation on the effect of water on the catalyst durability in the decomposition of Freon 12.
Experimental Section Catalysts. BPO4 with a BET surface area of 12.6 m21g was prepared by the reaction of boric acid with phosphoric acid and was calcined at 550 "C in air for 3 h (Imamura et al., 1991). Its acid strength as expressed by Hammett acidity coefficient (Ho) was -5.6 < HOI -3.0. TiOdSiOz with a Ti to Si molar ratio of 1 was
prepared by coprecipitating tetraisopropyl titanate(IV) and tetraethyl orthosilicate, followed by calcination at 550 "C in air for 3 h (Imamura et al, 1990). It had a surface area of 291.1 m2/g and acid strength of -8.2 < HOI-5.6. POflrO2 with P/Zr molar ratio of 0.005 was prepared by impregnating phosphoric acid on commercial zirconia and subsequent calcination at 800 "C in air for 3 h (Imamura et al., 1993a). Its surface area was 11.3 m2/g, and the acid strength was -5.6 < HOI -3.0. Co304 with a surface area of 9.6 m2/g was prepared by precipitation from aqueous solution of cobalt(I1) nitrate and was calcined at 550 "C in air for 3 h. Apparatus and Procedure. Reactions were carried out with a tubular quartz flow reactor (inner diameter of 6 mm) under an atmospheric pressure, and the reactor was heated with an electric furnace. The reaction gas mixture (Freon 12 0.60%, 0 2 21.6% He 77.8%)was fed into the reactor at a space velocity (SV) of 5900 or 6000 h-l. Space velocity was defined by the ratio of the volumetric feed rate of the reaction mixture (at room temperature and under an atmospheric pressure) to the catalyst bed volume. The effective catalyst bed volume was 1 mL in all experiments, and the catalyst bed densities (g of catalyst/mL of bed volume) were as follows: BP04 (0.651, TiOdSiO2 (0.451, POflrO2 (0.901, and Co304 (0.71). When water or methanol was introduced into the reaction gas mixture, the amount of oxygen was decreased by the amount corresponding to the water or methanol added; after oxygen was passed into a saturator of water or methanol, it was mixed with helium containing Freon 12. The reacted gas was analyzed by a gas chromatograph at the exit of the reactor. Analyses. Freon 12, methanol, CO, and C02 were determined with a Shimadzu GC-12A gas chromatograph equipped with a flame ionization detector at a column temperature of 120 "C. After CO and CO2 were separated with an activated charcoal column (1m), they were converted to methane with a Shimadzu MNT-1 methanizer and were determined. The column packing for the analysis of Freon 12 and methanol was Chromosorb 101 (1m). Acid strength was determined in dry benzene by the use of the following Hammett indicators: methyl red (pKa = 4.8), methyl yellow (pKa = 3.3), benzeneazo-
0888-5885/95/2634-0967$09.00/00 1995 American Chemical Society
968 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995
Time (h) Figure 1. Conversion of Freon 12 in the presence (0,A, 0 ) and absence ( 0 ,A, m) of 1%of water. [Freon 121 = 0.6%, LO21 = 21.6% (20.6% in the presence of 1%of water), [He] = 77.8%. Reaction temperature and SV are shown in the parentheses below. (A) TiOd Si02 (550 "C, 5900 hell; (A)TiOdSiOz (550 "C, 6000 h-l), (0)Pod ZrOz (550 "C, 6000 h-l); (a)POdZrOz (500 "C, 6000 h-l); (0) BPO4 (550 "C,6000 h-l); ( 0 )BP04 (550 "C, 5900 h-l). ( 0 )Yield of COZ; (v)yield of CO in the decomposition of Freon 12 on BP04 in the presence of 1%of water.
diphenylamine (PKa = 1.5) dicinnamalacetone (PKa = -3.01, benzalacetophenone (PKa = -5.61, and anthraquinone (PKa = -8.2). A Rigaku Denki Geigerflex 2012 X-ray analyzer and a Shimadzu ESCA 750 spectrophotometer were used for X-ray diffraction analysis and ESCA analysis, respectively.
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2 8 (degree)CuK a Figure 2. X-ray diffraction patterns of (A) TiOdSiOz before reaction and (B) after reaction for 33 h in the presence of 1%of water.
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Results and Discussion Decomposition of Freon 12 in the Presence and Absence of Water. Figure 1shows the effect of water on the durability of TiOdSiO2, POdZrOz, and BP04 in the decomposition of Freon 12. They are the typical acid catalysts effective for decomposing Freon 12; BP04 was the most durable and POdZrOs was the second among the various catalysts examined in the past works. All these catalysts, however, deactivated during the reaction in the absence of water, and even the conversion of Freon 12 on BP04 dropped to 20% after 30 h. The causes for the deactivation are elimination of B in the form of volatile BF3 for BPO4 (Imamura et al., 19911, elimination of Si02 for TiOdSiOz (Imamura et al., 19901, and formation of zirconium fluorides for POdZrO2 (Imamura et al., 1993a). When 1%of water was introduced, lives of all catalysts were prolonged. Although Freon 12 conversion on POdZrO2 decreased slightly after 10 h, it recovered again to 100% after 20 h. The conversion on TiOdSiO2 began to decrease after 30 h. A 100% of conversion was attained on BP04 during the reaction for more than 30 h. The configuration change of the catalysts after the reaction in the presence of water was investigated by an X-ray diffraction technique, and the results are shown in Figures 2-4. TiOdSiO2 did not show any resolved diffraction peak before the reaction (Figure 2A), while a diffraction pattern due to anatase-type T i 0 2 was clearly observed after 33 h (Figure 2B). This configuration change of TiOdSiOa was exactly the same with that observed in the reaction without water (Imamura et al., 19901,indicating that fine Ti02 particles aggregated into large crystals due to the elimination of the surrounding Si02 by inorganic fluorine species. The acid strength of TiOz/SiOz decreased from -8.2 < HO5 -5.6 (before the reaction) to -3.0 < Ho 5 1.5 after the reaction.
2 8 (degree)CUK a Figure 3. X-ray diffraction pattern of (A) PO&rOz before reaction and (B) after reaction for 30 h in the presence of 1% of water. ( 0 ) ZrF4-HF.1.5HzO; (A)ZrF4eH20; (H)ZrF4.HF.3H20. The assignment of these fluorides may not be necessarily correct (see the text).
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Figure 4. X-ray diffraction patterns of (A) BPO4 before reaction and (B) after reaction for 35 h in the presence of 1%of water.
Water seemed not to have worked effectively although it prolonged the life of TiOz/SiOz. ks shown in Figure 3B, fluorides of zirconium was produced after 30 h; although an exact assignment was difficult, these peaks were assumed to be due to ZrF4.HF.1.5H20, ZrFcH20, and ZrF4.HF .3H20 (McClune, 1967). Water also could not protect this catalyst completely, although an apparent catalyst activity was maintained. Acid strength of this catalyst did not decrease after the reaction (-5.6 < HOI -3.0). The X-ray diffraction pattern of BP04 was exactly the same before and after the reaction for
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 969
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Time(h) Figure 6. Effect of the concentration of water on the decomposition of Freon 12 over BPO4. [Freon 121= 0.6%, LO21 = 21.6% minus water concentration, [He] = 77.8%, SV = 6000 h-l. water = (+) o%, (0)1%, (0)2%, (A) 4%.
35 h (Figure 41, and its acid strength also did not change (-5.6 < HOI -3.0). These results together with the fact that 100% conversion of Freon 12 was maintained during the reaction indicated that BPO4 was completely protected from poisoning by inorganic fluorine species in the presence of water. The yield of CO plus C02 on BP04 in the presence of 1%of water did not reach loo%, and a small amount of an unknown compound was found to be produced. It was not trichlorofluoromethane (Freon 11)nor chlorotrifluoromethane (Freon 13)which were supposed to be formed by disproportionation of Freon 12. However, as the focus of this work was on the durability of the catalysts, no further analysis was carried out on this unknown product. Effect of water on the durability of BP04 will be discussed further. Effect of Water and Methanol in the Decomposition of Freon 12 on BP04. Deactivation of the catalysts is caused by an attack of inorganic fluorine species. Water may eliminate inorganic fluorides accumulated on the surface of BP04, thus prolonging its life. It may also hydrolyze Freon 12 on BPO4, leading to the acceleration of the reaction. Thus the above two possibilities were examined. Figure 5 shows the effect of the concentration of water. Reactions proceeded to almost the same extent in the presence of 1%of water and in the absence of water. However, a further increase in water (2% and 4%) tended to suppress the reaction. Although the cause for the retarding action of water was not known now, the reaction was not accelerated by hydrolysis of Freon 12. After BP04 was used for the reaction in the absence of water at 550 "C for 10 h (SV of 6000 h-l), the surface composition was analyzed with an ESCA spectrophotometer. The molar ratio of B:P:F was 0.49:1:0.2, indicating that fluorine accumulated on the surface. This catalyst was again used in the presence of 1%of water for 2 h, and the surface composition was analyzed. The ratio of B:P:F was found to be 0.48:1:0.08. Therefore fluorides on the surface of BP04 were eliminated by water. As metal fluorides are rapidly hydrolyzed to form HF at high temperatures (Warf et al., 19541, the fluorides on the surface of BP04 were probably transformed into HF and were eliminated. To further see the effect of water, Freon 12 was decomposed on BP04 with an intermittent introduction of 1%of water (Figure 6). The arrows in the figure show the point of the introduction of water for 1h. The activity of BP04 was maintained at a considerably high level by this method.
Figure 6. Effect of the intermittent introduction of 1%of water. [Freon 121 = 0.6%, [021= 21.6% (20.6% in the presence of water), [He] = 77.8%. (0)Intermittent introduction of water for 1 h a t the point indicated by arrows, reaction temperature = 550 "C, SV = 6000 h-1. (A) In the absence of water, reaction temperature = 550 "C, SV = 5900 h-l.
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Time(h) Figure 7. Decomposition of Freon 12 on BPO4 in the presence of 0.8%of methanol at 620 "C. [Freon 121 = 0.6%, [ 0 2 1 = 20.8%, [He] = 77.8%, SV = 6000 h-l. ( 0 )Conversion of Freon 12; (A) yield of COz; (0)yield of CO.
However, as a slight conversion decrease occurred in the later stage of the reaction, continuous supply of water seems favorable. BP04 has the best durability among many catalysts examined so far because it is composed of nonmetallic elements which are rather unreactive toward fluorine, and the drastic deactivation due t o the destruction of its skeleton occurs rather slowly. Therefore, if the surface inorganic fluorides are eliminated by water continuously and immediately after they are produced, BP04 can retain fresh active surface without deactivation. Wasted chlorofluorocarbons may be contaminated with other hydrocarbon solvents. In such case both chlorofluorocarbons and hydrocarbon solvents must be destructed simultaneously. It is deduced that hydrocarbons can be hydrogen sources to reactivate the catalysts. Therefore, Freon 12 was decomposed on BP04 in the presence of methanol (0.8%) as a model hydrocarbon, and the result is shown in Figure 7. As the reaction was a little suppressed in the presence of methanol, the reaction temperature was increased t o 620 "C.It was confirmed beforehand that methanol was completely decomposed at this temperature. High Freon 12 conversion was maintained for a longer time in the initial stage (until 20 h) in the presence of methanol than in its absence. However, a drastic deactivation of the catalyst occurred after 20 h. The volume of this catalyst decreased remarkably after the
970 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995
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Figure 8. Decomposition of Freon 12 in the presence of 0.8%of methanol using tandem catalyst beds. The result a t the first bed charged with 2 mL (effective catalyst bed volume) of Co304 a t 275 "C. [Freon 121 = 0.6%, LO21 = 20.8%, [He] = 77.8%, SV = 3000 h-l. An additional 1mL O f C0&4 was charged at the point shown conversion of Freon by an arrow. (A) Conversion of methanol; (0) 12.
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Time(h) Figure 9. Decomposition of Freon 12 in the presence of 0.8% of methanol using tandem catalyst beds. The result at the second bed charged with 1 mL (effective catalyst bed volume) of BPO4 a t 610 "C. SV = 6000 h-*. The reaction gas composition is the same Conversion of Freon 12; (A)selectivity as shown in Figure 8. (0) to COz; (0)selectivity to CO. The selectivities of COz and CO are based upon Freon 12 and methanol decomposed.
reaction for 33 h, and ESCA analysis revealed that almost all boron was eliminated (B:P:F = 0:1:0.05). Thus methanol contributed little as a hydrogen source to regenerate the catalyst surface, although it will produce water on decomposition. To effectively utilize the water produced from methanol, two catalyst beds were used in a series. The first bed was charged with 2 mL of Cos04 and was operated a t 275 "C, and the second bed with BP04 at 610 "C. It was expected that Freon 12 was not decomposed so much to produce corrosive inorganic fluorine species at 275 "C. The time course of the conversions of methanol and of Freon 12 at the exit of the first bed is demonstrated in Figure 8. Methanol was decomposed almost completely, while the conversion of Freon 12 was about 10%. As the methanol conversion decreased during the reaction probably due to the deactivation of Co304, an additional 1mL of Cos04 was charged a t the point shown by an arrow in the figure. Figure 9 shows the result at the second catalyst bed. Almost 100% of Freon 12 conversion was maintained even after 30 h, and, in this case, a good material balance for carbonaceous part (CO plus C o d was maintained. The X-ray diffraction pattern of BP04 aRer the reaction was exactly the same as that before the reaction, although it is not shown in the figure. The
acid strength (-5.6 < HO5 -3.0, before reaction) also did not decrease. This shows that the water produced at the first bed acted as a reactivating agent for BP04 in the second bed and that the hydrogen atoms contained in methanol did not work directly. The reason why water produced from methanol on BP04 did not work effectively is not known. The water-production rate may be low to keep its high concentration for eliminating the surface inorganic fluorides. Another explanation is that even if water is produced, it escapes from the catalyst before it diffuses to the sites of the catalyst occupied by fluorides. In conclusion, water remarkably prolongs the life of the catalysts, especially that of BP04, by eliminating inorganic fluorides from their surface in the form of HF. The development of the catalytic decomposition process for chlorofluorocarbons has thus attained a step forward.
Acknowledgment This work was supported by a research grant from Nippon Life Insurance Foundation.
Literature Cited Aida, T.; Higuchi, R.; Niiyama, H. Decomposition of Freon 12 and Methyl Chloride over Supported Gold Catalysts. Chem. Lett. 1990,2247-2250. Blake, D. M. Solar Thermal Technology for the Destruction of CFC Waste. Znt. J . Refrig. 1988, 11, 239-242. Colussi, A. J.; Amorebieta, V. T. Heterogeneous Decomposition of Trichlorofluoromethane on Carbonaceous Surface. J . Chem. SOC.,Faraday Trans. 1 1987,83, 3055-3059. Imamura, S.; Shiomi, T.; Ishida, S.; Utani, K.; Jindai, H. Decomposition of Dichlorodifluoromethane on TiOdSiOz. Znd. Eng. Chem. Res. 1990,29,1758-1761. Imamura, S.; Imakubo, K; Furuyoshi, S.; Jindai, H. Decomposition of Dichlorodifluoromethane on BP04. Ind. Eng. Chem. Res. 1991,30,2355-2358. Imamura, S.; Shimizu, H.; Haga, T.; Tsuji, S.; Utani, K.; Watanabe, M. Decomposition of Dichlorodifluoromethane on PO4-ZrO2 Catalyst. Ind. Eng. Chem. Res. 1993a, 32, 3146-3149. Imamura, S.; Higashihara, T.; Jindai, H. Reactivating Effect of Water on Freon 12 Decomposing Catalysts. Chem. Lett. 1993b, 1667-1670. Jacob, E. Method for Converting Gaseous Organic Halocompounds to Carbon Dioxide and Hydrogen Halides. Ger. DE 3841847, 1990; Chem. Abstr. 1990,113, 196942~. Miyatani, D.; Shinoda, K.; Nakamura, T.; Ohta, M.; Yasuda, K. Catalytic Decomposition of CFC-112 and CFC-113 in the Presence of Ethanol. Chem. Lett. 1992, 795-798. 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. Okazaki, S.; Kurosaki, A. A Process for the Catalytic Decomposition of Halofluorocarbons. Eur. Pat. Appl EP 412456, 1991; Chem. Abstr. 1991, 115, 135502d. Oku, A,; Kimura, K.; Sato, M. Complete Destruction of Chlorofluorocarbons by Reductive Dehalogenation Using Sodium Naphthalenide. Ind. Eng. Chem. Res. 1989,28, 1055-1059. Warf, J. C.; Cline, W. D.; Tevebaugh, R. D. F'yrohydrolysis in the determination of fluoride and other halides. Anal. Chem. 1954, 26, 342-346. Witt, S. D.; Wu, E. C.; Loh, K. L.; Tang, Y. N. Heterogeneous Hydrogenolysis of Some Fluorocarbons. J . Catal. 1981, 71,270277.
Received for review July 11, 1994 Accepted November 10, 1994 @
139404285 Abstract published i n Advance A C S Abstracts, February 1, 1995. @
