Ind. Eng. Chem. Res. 1993,32, 3146-3149
3146
Decomposition of Dichlorodifluoromethane on P04-ZrO2 Catalyst Seiichiro Imamura,' Hideaki Shimizu, Toshiki Haga, Seizaburo Tsuji, and Kazunori Utani Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Makoto Watanabe Department of Industrial Chemistry, Chubu University, Kasugai 487, Japan
The activity and durability of phosphate-zirconia composite catalysts were examined in the decomposition of dichlorodifluoromethane (Freon 12). Combination of phosphate group with zirconia produced acid sites which were effective for decomposing Freon 12: 100% conversion of Freon 12 was attained at a temperature as low as 300 "C on the phosphate-zirconia catalyst which had a superacid nature. The phosphate group also protected the surface of zirconia from poisoning by inorganic fluorides produced during the reaction. An addition of water helped maintain the activity of the catalyst by eliminating inorganic fluorides accumulated on its surface.
Introduction As prevention of the release of chlorofluorocarbons (Freons) is an important assignment to protect the global atmospheric environment of the earth, it is urgent to develop an effective method to detoxify used or excess Freons. Catalytic decomposition is one of the promising processes for disposal of Freons (Aida et al., 1990; Jacob, 1990; Miyatani et al., 1992; Okazaki and Kurosaki, 1989, 1991). One of the authors has been engaged in the development of active catalysts to detoxify Freons and found that many acidic solid catalysts are effective (Imamura et al., 1990). However, as inorganic fluorides produced during the reaction poison and deactivate these catalysts rapidly, practical catalytic processes for decomposing Freons have not yet been developed. The most resistant catalyst examined in our past works was BP04 (Imamura et al., 1991a,b). As fluorine is the most electronegative of all the elements in the periodic table, catalysts composed of metallic elements react with fluorine readily. On the other hand, the constituent elements of BP04 are all nonmetallic and fluorine cannot attack the BP04 so easily;thus the BP04had considerable durability. However, boron also reacted with inorganic fluorines, although slowly,during prolonged use, and the framework of this catalyst was destroyed due to the elimination of B in the form of volatile BF, (presumably as BFd. This result led us to abandon boron and utilize the Po4 group to combine with other elements to induce an acid property. Zirconia is reported to be very durable in decomposing Freons (Mizuno et al., 1990), and the combination of zirconia with PO4 produces acid sites (Kagiya et al., 1963). Thus the present report deals with an investigation on the performance of POr-ZrOz composite catalysts to decompose dichlorodifluoromethane (Freon 12).
Experimental Section Materials. Two kinds of phosphate-zirconia catalysts were prepared. Commercial ZrOz (BET surface area 12.6 m2/g) was dispersed into deionized water containing known amounts of diammonium hydrogenphosphate. The mixtures were heated to dryness with an evaporator, followed by calcination in air at 800 O C for 3 h. The series of these catalysts were denoted as P04IZr02-A. Sodium hydroxide (3 N) was added to an aqueous solution of zirconium(1V) oxynitrate until the pH of the
solution was 11. The resultant precipitate was filtered, washed with deionized water five times, and dried at 200 "C overnight. It was dispersed into 2 N aqueous phosphoric acid with stirring for 30 min, followed by filtration and calcination in air for 3 h at prescribed temperatures. This catalyst was denoted as POJZr02-B. These catalysts were molded into a disk under a pressure of 20 MPa and were cut into about 8-14 mesh size before use. Freon 12 in He (0.67 vol % ) was obtained from Nippon Fine Gas co. Benzene and cyclohexane were dried with calcium chloride, followed by refluxing with sodium metal overnight and subsequent distillation. Other reagents were used as obtained commercially. Apparatus and Procedure. Reactions were carried out under atmospheric pressure by the use of a flowreactor made of an alumina tube whose outer diameter was 10 mm and inner diameter was 6 mm. Freon 12 in He was mixedwithoxygen(Freonl20.60%,0~21.2%,He78.2%), and this reaction gas was introduced into the reactor at a space velocity (the volume of the gas introduced into the catalyst at a reaction temperature divided by the volume of the catalyst) of 6000 h-l. The alumina tube reactor without catalyst decomposed 1.4 and 3.0% of Freon 12 at 500 and 550 "C, respectively. Analyses. Freon 12, CO, and COz 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 C02 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 was Chromosorb 101 (1 m). Acid strength of POdZrOz was determined in dry benzene by the use of the Hammett indicators methyl red (pKa = 4.8), methyl yellow (PKa = 3.31, benzeneazodiphenylamine (PKa = 1.51, dicinnamalacetone (PKa = -3.01, benzalacetophenone (pK. = -5.61, and anthraquinone (PK. = -8.2), or in dry cyclohexane by using p-nitrotoluene (pK. = -11.35), nitrobenzene (pK, = -12.14), m-nitrochlorobenzene (pK. = -13.16), and 2,4-dinitrochlorobenzene (PKa -14.52). 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.
0888-5885/93/2632-3146$04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 3147 A
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Figure 1. Decomposition of Freon 12 on POJZr02-A. [Freon 121 = 0.06%;[Ozl = 21.2%; [He] = 78.2%; space velocity = 6000 h-l; ( O , O , A) temperatures at which the conversion of Freon 12 was 10, 50, and 90%, respectively; ( 0 )BET surface area; (A)surface P/Zr (molar ratio) as determined by ESCA analysis.
Results a n d Discussion Decomposition of Freon 12 on BPOdZr02-A. Figure 1 shows the result of the decomposition of Freon 12 on POdZr02-A with varying amount of PO4 loading. The activity was expressed by the temperatures of 10,50, and 90% conversion of Freon 12. The changes in the BET surface area and the surface P/Zr molar ratio are also shown. The activity increased with an increase in P/Zr molar ratio to a maximum at a P/Zr ratio of 0.005. The acid strength as expressed by Hammett acidity coefficient (Ho) was 3.3 < HO5 4.8 for P/Zr ratio less than 0.005 and -5.6 < HO I-3.0 in the higher P/Zr region. As the decomposition of Freon 1 2 is catalyzed by acid sites (Imamura et al., 1990), combination of PO4 with Zr is favorable to induce the activity due to acid sites. Thus the POdZr02-A with low P/Zr (low acidity) will have only low activity. The rather low activity of the catalysts with P/Zr ratio higher than 0.005 may be due to their decreased surface area as shown in the figure even though their acid strengths were the same. Figure 2A shows the X-ray diffraction pattern of Pod/ ZrO2-A with a P/Zr molar ratio of 0.005. The diffraction pattern coincided well with that of ZrO2, and no clear peak corresponding to zirconium phosphates such as ZrPz07, Zr3(P04)4, ZrP207, Zr(PO3)4, or Zr3 (Pod4 was observed (McClune, 1982). Durability of the P04/Zr02-A is shown in Figure 3, together with that of the most robust catalyst (BPO4) examined (Imamura et al., 1991a,1991b). The conversion of Freon 12 on P04/Zr02-A (P/Zr = 0.005; surface P/Zr = 0.09) dropped after 5 h and reached an almost constant value of 23%. X-ray analysis of this catalyst indicated the presence of ZrF4 after the reaction, which was assumed to be the cause of the activity loss (Figure 2B). The durability of the catalyst with P/Zr ratio of 0.2 (BET surface area = 6.8 m2/g; surface P/Zr = 2.5) was also investigated. Although the initial conversion on this catalyst was low, it was not deactivated so rapidly and more than 35% conversion was maintained after 30 h. Little change in its bulk configuration occurred (Figure 2C), probably because its surface was protected by the large amount of resistant phosphate group. BP04 was active and considerably durable. However, it deactivated remarkably after 20 h due to the complete destruction of its framework caused by the loss of B as BF, (Imamura et al., 1991b). Thus POJZrO2 had higher performance than BP04 for prolonged use. Decomposition of Freon 12 on BP04/Zr02-B and the Effect of Water. As the decomposition of Freon 12 proceeds on the acid sites of the catalysts, it is assumed that the stronger the acidity of the catalyst, the higher the
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28 CuKa Figure 2. X-ray diffraction patterns of (A) POJZrOrA (P/Zr = 0.005) before reaction, (B) POJZr02-A (P/Zr = 0.005) after reaction for 29 h, (C) POdZrOz-A (P/Zr = 0.2) after reaction for 33 h. (0) ZrFd. Reaction conditions are given under Figures 1 and 3.
Time (h)
Figure 3. Decomposition of Freon 12 on (0) POJZrOrA (P/Zr = 0.005) at 500 OC, (0) POJZr02-A (P/Zr = 0.2) at 550 OC, and (0) BPOr at 550 OC. Reaction conditions are the same aa those shown in Figure 1except for the reaction temperatures. The method of the preparationof BPO, is described elsewhere (Imamuraet al., 1991a). Table I. Acid Strength and Surface Area of PO4/ZrOrB acid strength surface area (me/g) calcination temp ("C) 500 600 700 800 900
-13.16 < Ho 5-12.14 -13.16 < Ho 5-12.14 -12.14 < Ho 5-11.35 -8.2 < Ho I - 5 . 6 -8.2 < Ho 5-5.6
192.3 114.7 91.2 68.2 23.1
activity. Thus it is expected that the lifetime of the catalyst can be prolonged when the reaction temperature is lowered by employing the catalyst with strong acidity. Mukaida reported that phosphate-supported zirconia starting from zirconium(1V)oxynitrate has acid sites whose acid strength is in the range of those of superacids (stronger than 100% sulfuric acid, HO-11.9) (Mukaida et al., 1991). Thus we prepared superacid P04/Zr02-B catalyst according to his method. Table I shows the acid strength and BET surface area of POdZr02-Bcalcined at various temperatures. The surface P/Zr molar ratio of these catalysts as determined
3148 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 400
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Figure 6. Effect of calcination temperature on the activity of POJ ZrO2-B (-) in the absence of water; (- - -) in the presence of 1vol % water. Tlo, Tw,and Tw are the temperatures at which 10,50, and 90 % of Freon 12 waa decomposed,respectively. Reaction conditions are given under Figure 1.
c I 20 CuKa Figure 4. X-ray diffraction patterns of (A) POJZr02-B before reaction, (B) POJZr02-B after reaction for 27 h, and (C) P04/Zr02-B ZrO2; after reaction for 29 h in the presence of 1 vol % water. (0) (A)unknown phase; ( 0 ) ZrFI. Calcination temperature of POJ ZrO2-B was 600 O C . Reaction conditions are given under Figures 1 and 7.
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Figure 5. Decomposition of Freon 12 on (0) P04/Zr02-B, (0) Pod/ ZrO2-B in the presence of 1vol % water, (0) ZrO2, and (A)BPO4. Reaction conditions are given under Figure 1. POd/Zr02-Band ZrO2 were calcined at 600 "C. ZrO2 was prepared by precipitation from aqueous zirconium(1V) oxynitrate.
by an ESCA technique was 0.48. The HOof P04/Zr02-B calcined at 500 and 600 "C was lower than -12.14; thus, they were superacids. The one calcined at 700 "C also exhibited strong acidity. The HOof the zirconia prepared in the same way (calcination temperature 600 "C) without PO4 was higher than 4.8. X-ray diffraction analysis of POdZr02-B calcined at 600 "C showed the ambiguous presence of ZrO2 in addition to an unknown phase which had a strong peak at a 28 of 30.3" (Figure 4A). Freon 12 was decomposed completely at a temperature as low as 300 "C on P04/Zr02-B calcined at 600 "C (Figure 5), the lowest temperature attained with various acid catalysts; the result of the decomposition of Freon 12 on BP04 is shown as a comparison. Deactivation of the POr/ZrOz seemed to be caused by the formation of zirconium fluorides (Figure 2B). If the
surface fluorines are eliminated as HF by some hydrogen sources during the reaction, active sites will be regenerated and the lifetime of this catalyst will increase. On the basis of this assumption, the effect of water (1vol %) was also examined. It is shown that water slightly decreased the activity of P04/Zr02-B. Zirconia without PO4 which was calcined at 600 "C also showed a comparatively high activity. This is surprising because Freon 12 could be decomposed effectively only on acid catalysts and usual combustion catalysts such as C0304 and Mn203 were ineffective (Imamura et al., 1990). Commercial zirconia used to prepare P04/Zr02-A did not have high activity and decomposed only 26.5% of Freon 12 a t 500 "C. The difference in their activities seems to come partly from the difference in their surface area; the BET surface area of commercial ZrO2 was 12.6 m2/g and that of the ZrO2 prepared in this work (at 600 "C) was 114.7 m2/g. However, a ZrO2 catalyst which was prepared by the present method and calcined at 900 "C decomposed 90% of Freon 12 at 375 "C although its surface area was small (14.3 m2/g). Thus another possibility is that a slight difference in the structure of ZrOis caused by the different preparation techniques leads to the remarkable difference in their activity; however, details are not known now. Figure 6 shows the effect of calcination temperature on the activity of P04/Zr02-B. The activity was again evaluated by the temperatures at which 10,50, and 90% conversion of Freon 12 were attained. The calcination temperature did not have such a profound effect on the activity of the catalysts in the absence of water, although high conversion of Freon 12 (shown by Tw)is difficult on the POdZr02-B calcined at 900 "C. This result is confusing because acid strength seems to play no or a minor role if any: P04/Zr02-B calcined at 500 "C or 600 "C had stronger acidity than the one calcined at 800 "C and, yet, their activities were almost the same or rather low. In addition, BET surface area decreased as the calcination temperature was increased, which also should have a retarding effect on the reaction. However, the lack of kinetic investigation left the detail of these phenomena unknown. Water had a detrimental effect especially on the activity of PO4/ ZrOz-B calcined at high temperatures. It may compete with Freon 1 2 in interacting with the active sites of the catalysts. Figure 7 shows the durability of P04/Zr02-B calcined at 600 "C in the presence (1vol % ) and absence of water at 400 "C. Deactivation of the catalyst occurred after 5 h in the absence of water, and the conversion after 20 h
Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 3149 and the effect of water on the durability of POr/ZrOz, BP04, and other catalysts are now under investigation.
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Acknowledgment
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This research was supported by a Grant in Aid for Developmental Scientific Research in 1991(No. 03555171) and a Grant in Aid for Scientific Research Project on Formulation and Management on Man-Environment System in 1992 (No. 04202126) from the Ministry of Education, Culture, and Science, Japan. The authors thank Dr. Mukaida of Muroran Institute of Technology for helpful discussion.
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Time (h)
Figure 7. Decomposition of Freon 12 on (0) POJZrOz and (0) POJZr02-B in the presence of 1 vol % water at 400 "C. (A)Yield of C02 and ( 0 )yield of CO in the presence of 1vol % water. PO$ Zr02-Bwas calcined at 600 OC. Reaction conditions are given under Figure 1 except for the reaction temperature.
dropped to below 205%. This deactivation was assumed to be caused by the extensive formation of ZrF4 as is shown by the result of the X-ray analysis (Figure 4B). Decrease in the conversion of Freon 1 2 also occurred in the presence of water within 10 h. However, constant conversion of Freon 1 2 (60% or higher) was maintained after the initial deactivation. The decomposed Freon 12 was mainly converted to CO and COZ;however,the material balance could not necessarily be attained and it was found that an unknown compound was produced. The X-ray analysis indicated almost no change in the bulk configuration of the catalyst after 29 h in spite of the presence of a small amount of ZrFl (Figure 4C). Thus, as expected, hydrogen source helped maintain the activity of the catalyst by eliminating fluorine accumulated on the catalyst surface. The acid strength decreased from -13.16 < HoI-12.14 to -5.5 < HO5 -3.0 after the reaction for 29 h in the presence of water. It was also found that the acid strengths of all catalysts calcined from 600 to 900 "C decreased to -5.6 < HO I -3.0 within several hours irrespective of the presence or absence of water. The reason for the partial decrease in the activity of POdZrOz-B (calcined at 600 "C) in the presence of water is not known. A slight fluorination of the catalyst as shown by Figure 4C may be one cause, and the decrease in its acid strength, which occurred rather rapidly, may be an another factor. Unfortunately, the present result did not show any advantage of employing superacid catalyst. It seems that the acid strength of PO$ZrOz does not have to be so high to decomposeFreons owing to the peculiar activity of ZrOz, although it is clear from our previous works that acidic catalysts play an important role. Moreover, even if superacid catalyst is employed, it readily loses its superacidity during the reaction. However, it is evident that the phosphate group is robust enough against inorganic fluorine (Imamura et al., 1991a,b). Thus, by employing the phosphate group to protect active zirconia and produce an additional active site (acid sites) in an optimum procedure, an active and durable POr/ZrOz will be produced. As the combined use of hydrogen sources is effective to regenerate the active site of the PO4/ZrO2, it seemspossible to developa practical process for detoxifying Freons. However, much is left to be solved, e.g. to ensure complete detoxification of Freons without forming possible harmful by-products just shown before. The effect of the method of the preparation of POdZrOz on its durability
Nomenclature ESCA: electron spectroscopy for chemical analysis Freon: chlorofluorocarbon Freon 12: dichlorodifluoromethane Ho:Hammett acidity coefficient POdZrOz: zirconia impregnated with phosphate group POdZr02-A: commercial zirconia impregnated with phosphate group POdZr02-B: zirconia prepared in this work and impregnated with phosphate group 8,: BET surface area
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