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Decomposition of chlorofluorocarbons in the presence of water over zeolite catalyst
8.ENVIRONMENTAL Applied Catalysis
ELSEVIER
B: Environmental
9 (1996) 167-177
Decomposition of chlorofluorocarbons in the presence of water over zeolite catalyst ’ Masahiro Tajima a, Miki Niwa a’*, Yasushi Fujii b, Yutaka Koinuma ‘, Reiji Aizawa ‘, Satoshi Kushiyama ‘, Satoru Kobayashi ‘, Koichi Mizuno ‘, Hideo Ohachi ’ a Department
ofMaterials Science, Facul@ of Engineering, Tottori Unimv-sir)?,Koyama, Tottori, 680, Japan
’ Tosoh Corp., Shin-nanyou, Yamaguchi 746, Japan ’ National Institute for Resources and Environment, Agency of Industrial Science and Technology, Onogawa, Tsukuba, Tharaki 305, Japan Received
18 July 1995; revised 28 November
1995; accepted
11 December
1995
Abstract The decomposition of chlorofluorocarbons (CFCs) in the presence of water was examined over a variety of solid acid catalysts. More than 40% of the conversion of CFC was observed on HY zeolite, H-mordenite, H-ZSM-5, y-Al,O,, and SiO,-TiO, catalysts, and the selectivity to CO and CO, was nearly 100% except on y-Al,O,. Although the H-mordenite had the highest activity among the tested catalysts, it was gradually deactivated during the reaction due to the elimination of Al atoms from the zeolite framework. A good relationship was found between the reactivity on H-mordenite and the bond energy of C-Cl in compounds of Ccl,, CCl,F, CCl,F,, and CClF,, suggesting that the rate controlling step was the cleavage of the C-Cl bond.
1. Introduction The decomposition of chlorofluorocarbons (CFCs) is an important reaction in order to protect the ozone layer surrounding the earth. However, it is difficult due to the extremely high stability of CFC molecules [l]. Although CFCs as detergents and solvents have been replaced by hydrochrolofluorocarbon (HCFC) or hydrofluorocarbon (HFC), remaining stocks of CFCs must be converted into harmless compounds.
* Corresponding author. Tel.: (+ 81-857) 3 15256; fax: (+ 8 l-857) 3 10881; e-mail: [email protected] ’Previously submitted on 8 March 1995. 0926.3373/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO926-3373(96)0001 1-2
168
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Catalysis B: Enoironmental9 (1996) 167-I 77
The thermal decomposition of CFCs is impractical, thermodynamic equilibrium, as shown in Eq. (1).
because of unfavorable
CC1,F + C + Cl, + CIF AG = 650kJ/mol
(1)
However, it is thermodynamically possible to decompose CFCs in the presence of such compounds as H,O, 02 and H,, because the change of free energy (AG) of the scheme (21, (31, and (4) at 1000 K show negative values. Values of the free energy of CCl,Fs were obtained from ref. [2]. CC1,F + 2H,O CC1,F + 0,
+ CO, + 3HCl+
HF AG = -441.37kJ/mol
+ CO, + Cl, + CIF AG = - 306.67 kJ/mol
CC1,F + 4H, =+ CH, + 3HCl+
HF AG = - 411.78 kJ/mol
(2) (3) (4)
Okazaki et al. studied the decomposition of CFC13 (CClF,) with H,O over various metal oxides unsupported and supported on activated carbon, and found that Fe,O,/carbon was an active catalyst at temperatures above 723 K; however, the gradual deactivation was also reported [3]. Karmakar and Greene studied the oxidation of CFC12 (CCl,F,) over Yzeolites (H-Y and Cr-Y) [4]. The initial activities of these catalysts were high, but the long term deactivation tests exhibited a serious drop in the catalytic activity because of the partial destruction of the zeolite crystalline structure. The decomposition of CFCs in the presence of H, also has been reported [5-71. Okazaki et al. studied the hydrodechlorination of CFCl13 (C,Cl,F,) over the NiO-Cr,O, catalysts which contained more than 70% Cr, and observed nearly 100% of the conversion even after a running time of 5.25 h [5]. In this study, H,O was used for the decomposition reaction, because H,O is easily available, and HCI and HF stoichiometrically produced by the decomposition are easily removed by neutralization. We examined several solid catalysts for the reaction, and found the H-type zeolite as an active catalyst. Moreover, we studied its possibility as an industrial catalyst. The purpose of this investigation is therefore to reveal the active catalyst in the decomposition of CFCs in moisture conditions.
2. Experimental 2.1. Materials Na-Y zeolite (atomic ratio of Si/Al = 2.81, H-Y (10) and H-ZSM5 (11) were obtained from Tosoh obtained from Nihon Ketzen Co. SiO, and TiO, Kagaku Co. SiO,-TiO, (Si/Ti = 1) was purchased Fe,O, was prepared by calcining iron(II1) hydroxide, Kanto Kagaku Co., at 723 K for 2 h in air. ZrO,
zeolite (2.81, H-mordenite Corp. y-Al,O, pellet was was obtained from Kanto from Nakarai Tesque Co. which was obtained from was prepared by calcining
M. Tajima et al. /Applied Catalysis B: Environmental 9 (1996) 167-177
169
zirconium hydroxide, which was obtained by hydrolysis of ZrOCl,, at 723 K for 2 h in air. These particles of catalysts were grained and sieved in lo-20 mesh. 1,2-Trichloro- 1,2,2-trifluoroetane (CFC113) and carbontetrachloride were obtained from Nakarai Tesque Co. Chlorotrifluoromethane (CFC 131, dichlorodifluoromethane (CFC 12) and trichlorofluoromethane (CFC 11) were supplied from Asahi Glass Co. 2.2. Apparatus
and procedure
The decomposition reactions were carried out at atmospheric pressure using a tubular flow reactor made of quartz (12 mm O.D., 9 mm I.D.>. 1 g of catalyst was charged in the reactor which was heated with an electric furnace. A gas mixture (CFCs, 1000 ppm; H,O, 3000-16000 ppm; N, or dry air, balance) was flown into the reactor at 30 1 hh ’g-cat- ’ of WHSV; unless otherwise described, air and 4000 ppm of the H,O were flown into the reactor. The exhaust gas was then dissolved in a KOH solution to remove HCl and HF produced in the reaction. 1 ml of the exhaust gas was collected before the KOH solution trap, and unconverted CFCs, and produced CO and CO, were analyzed respectively by gas chromatographs equipped with a flame ionization detector (FID) as follows. The CFCs were separated with a chromosorb 101 (2 m) column at 373 K, and then detected by an FID detector. CO and CO, were separated with an activated charcoal column (1 m) at 373 K, converted into methane with H, on a ruthenium catalyst, and detected by an FID detector. The X-ray diffraction was measured with a Rigaku Denki Geigerflex RAD-II A X-ray analyzer. The X-ray photoelectron spectroscopy (XPS) was measured with a Shimazu ESCA-750 spectrophotometer. Samples were mounted with Scotch double-stick tape. Elemental atomic ratios were calculated from integrated peak intensities of A12p, Si2p, and Fls which were corrected based on the reported cross-sectional area [S]. Transmission infrared spectra of pyridine adsorbed on H-mordenite were measured with a JEOL JIR-40 spectrophotometer. A thin wafer of H-mordenite was mounted in a vacuum-tight infrared cell. After the sample was evacuated at 773 K for 1 h, pyridine was adsorbed at 573 K for 1 h at the equilibrium pressure of 10 ton-, followed by evacuation at 423 K; the spectra for adsorbed pyridine were then measured at room temperature.
3. Results 3.1. Decomposition Table CFCl13
of CFCs over several catalysts
1 shows the activity of several catalysts for the decomposition of in the presence of water vapor at 773 K. Because the conversion
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Table 1 Catalytic activities of CFCl13 Catalyst
HY NaY H-mordenite H-ZSM-5 -w&O, SiO, -TiO, FeA SiO, ZQ TiO,
decomposition
Conversion of CFCI 13 (%I 85 15 98 88 85 40 16 8 0 0
Catalysis B: Environmental 9 (1996) 167-I 77
at 773 K a Conversion C,Cl,F, 1 3
to
(%I
Conversion C,Cl,F,
to
(%x)
0 0 0 0
1 0 0 0 0 0
Reaction conditions: CFCl13 1000 ppm, H,O 4000 ppm, balance air; WHSV: 30 1 h-’ g-cat-’ a Measured at 2 h of time-on-stream.
changed at initial stage of reaction, the stabilized activity was obtained 2 h of time-on-stream. H-type zeolite, y-Al,O,, and SiO,-TiO,, which were known as typical acid catalysts, exhibited high activity. The CFCl13 was decomposed to selectively form equimolar amounts of CO and CO,, and no other carbon-containing compounds were detected. Furthermore, HCI and HF were detected in a KOH solution trap in the exhaust gas, but these amounts were not determined quantitatively. H-type zeolites exhibited the highest activity among the catalysts examined, and the activity was in the order H-mordenite > H-ZSM-5 > H-Y. Only on the y-Al,O,, the decomposition produced by-products, 16% of C,Cl,F, and 1% of C2C1,F,, while on other catalysts, less than 3% of C,Cl,F, was formed. On the other hand, NaY, Fe,O,, and SiO, showed low activity, and ZrO, and TiO, were found to be inactive. Fig. 1 shows the temperature dependence of the decomposition of CFC 113 over the H-mordenite in the presence of water vapor. The conversion of CFCl13 increased with increasing temperature, and reached nearly 100% at 773 K. The temperature for decomposition of CFCl13 over the H-mordenite is thus obviously lower than that for the thermal decomposition without catalyst which requires above 1073 K; the potential priority of the decomposition is thereby noteworthy. Fig. 2 shows the influence of the addition of air on the reaction. There was no difference between reactions undertaken with and without air. It was found that the catalyst activity was enhanced at an early stage of the reaction. The conversion of CFCll3 was 85% at 30 min after the initiation of reaction, but it increased, and reached about 98% after 1 h. Variation of reactivity as a function of CFC molecules containing different ratios of Cl atom to F atom is depicted in Fig. 3. Ccl, was decomposed nearly 100% at 503 K, and CCl,F, CCl,F,, and CClF, were decomposed nearly 100% at the higher temperatures, 623 K, 723 K, and 803 K, respectively. However,
M. Tajima et al. /Applied
600
Catalysis
650
B: Enuironmental
9 (1996) 167-l
700
Reaction Temperatuer
750
77
171
800
/K
Fig. 1. Decomposition of CFCl13 over the H-mordenite under the following H20, 4000 ppm; balance air, 500 ml/min; WHSV = 30 1 h- ’ g-cat.
CF, was not decomposed at 873 K over the H-mordenite. CFCs produced CO,, but not CO, while the decomposition CO and CO,.
conditions:
CFCI 13, 1000 ppm:
These compounds of of CFCll3 produced
3.2. Life of catalytic actiuity Fig. 4 shows the life of catalyst activity over the H-mordenite with varying the H,O concentration from 3000 ppm to 16000 ppm at 773 K. Initial activity
60
’
0 0
1
2
3
4
Reaction Time / H Fig. 2. Decomposition of CFCl13 over the H-mordenite at 773 K in air (a) and Nz (b) under the conditions: CFCl13, 1000 ppm; H,O, 4000 ppm; balance, 500 ml/min; WHSV = 30 I hh’ g-cat-‘. (0) conversion of CFC 113, (a) conversion to C,Cl, F4, ( A) conversion to CO?, and ( A ) conversion to CO.
172
M. Tajima et al. /Applied Catalysis B: Environmental 9 (1996) 167-177
0 300
,/,/
,A’*
400
500
,/
600
700
800
900
Reaction Temperature/K Fig. 3. Catalytic decomposition of CFCs (0) CC],, (0) CCl,F, (A) CCl,F,, and (A) CClF,) over the H-mordenite under the conditions: CFCs, 1000 ppm; H,O, 4000 ppm; balance air, 500 ml/min; WHSV = 30 1 h-’ g-cat-‘.
was very high at any H,O concentration. However, it was dropped slowly at 3000 ppm of the H 2O concentration, and about 90% of CFC 113 decomposed at the run of 30 h. The deactivation was enhanced with increasing H,O concentration. At 16000 ppm of the H,O concentration, the activity was dropped rapidly, and CFCl13 did not decompose at the run of 6 h. 3.3. Characterization
The character of the fresh sample and deactivated sample were measured by several methods. The deactivated sample and used sample were reacted under the standard condition (CFC113, 1000 ppm; H,O, 4000 ppm; air, balance) at 773 K for 30 h of time-on-stream.
100
0 0
10
20
30
40
Reaction Time / H Fig. 4. Decomposition of CFCl13 over the H-mordenite at 3000 ppm (01, and 16000 ppm (0) of the H,O concentration at 773 K under the conditions: 500 ml/min; WHSV = 30 1 h-’ g-cat-‘.
4000 ppm (A), 8000 ppm (U), CFCl13, 1000 ppm; balance air,
M. Tajima et al. /Applied Catalysis B: Environmental 9 (1996) 167-177
173
r-
(b)
4
10 2 R (degree)
Fig. 5. X-ray diffraction
patterns of the fresh (a) and deactivated
(b) H-mordenite
Fig. 5 shows the XRD analysis of the H-mordenite before and after the reaction. Because there was no difference between them, the zeolite was not destructed.
i
m
d
A& 1542
(b)
(a) I
I
1680
1
1600
I
I
1520
1440
Wavenumber / cm _ ’ Fig. 6. IR spectra of adsorbed pyridine on the H-mordenite fresh sample, (c) deactivated sample.
after the evacuation
at 423 K; (a) background,
(b)
M. Tajima et al/Applied
174 Table 2 Atomic composition
of H-mordenite
Catalyst
Catalysis B: Environmental 9 (1996) 167-177
as revealed by XPS
Atomic ratio
Fresh sample Used sample
Si
Al
F
Cl
1
0.1 0.03
0 0.2
0 0
1
Fig. 6 shows the IR spectra of pyridine adsorbed on the fresh and used H-mordenite at 423 K. The 1542 cm- ’ band, identified as the Brijnsted acid, was observed on the fresh sample, while it was not on the used sample. Table 2 shows the surface composition observed by XPS on the fresh and used H-mordenite. Si, Al, and F were detected, but Cl was not. It was found that 70% of surface Al atom was eliminated on the deactivated sample, and the surface of the zeolite was fluorinated.
4. Discussion
4.1. Decomposition
of CFCl13
ouer several catalysts
Because the acid catalysts exhibit the high activity, it is suggested that CFCl13 is decomposed on the acid sites. H-mordenite has the highest activity among the H-type zeolites. It is known that the H-mordenite has the stronger acid strength rather than H-ZSM-5 and H-Y [9,10]. Therefore, it is suggested that the activity of the decomposition of CFCs is determined by the acid strength of the catalysts. H-type zeolites have the Brijnsted acid site, but y-Al,O, has only the Lewis acid site; therefore, CFCs seemed to be decomposed not only on the Brijnsted acid site but also on the Lewis acid site. In the presence of water, the decomposition of CFC 113 formed equimolar amounts of CO and CO, with HCl and HF, as shown in Fig. 2. It may be thereby deduced that the decomposition reaction can be described as in the equilibrium (5). C,CI,F,
+ 3H,O
= CO, + CO + 3HCl+
3HF
(5)
Because no influence by the addition of air upon the decomposition of CFCl13 was observed, oxygen had no effect on the decomposition of CFCl13 in the presence of water vapor. It is considered that C,Cl,F, is a by-product arising from the disproportionation between two molecules of CFC113, as shown in the equilibrium (6). 2C,Cl,F,
+ C,Cl,F,
+ C,Cl,F,
M. Tajima et al/Applied
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175
Okazaki studied the disproportionation reaction of CClF, over various metal salts [ll]. He found that the relative catalytic activities were given by the sequence, AlCl, > FeCl, > CaCl, > CrCl,, PbCl, > HgCl,, and the results were discussed in terms of the acidity of metal chlorides. He described that the disproportionation of CFCs occurred on the acid site, and the catalytic activities depended on the amount of acid site. Based on Okazaki’s study, it is suggested that the disproportionation of CFCs occurred on the Lewis acid sites of metal salts. In this study, only y-Al,O, produced C ,Cl,F, in the decomposition of CFCll3. Because y-Al,O, and such metal salts as AlCl, have only the Lewis acid, it is suggested that the disproportionation of CFCl13 occurred on the Lewis acid site. The disproportionation of CFC 113 produced equal amounts of C,Cl,F, and C,Cl,F,, but no or a few of C,C1,F2 were detected by the reaction over H-mordenite and y-Al,O,. This will be mentioned in the pathway of decomposition reaction. 4.2. Deactivation
of H-mordenite
Although the H-mordenite had the highest activity among the tested catalysts, it was deactivated during the prolonged uses, as observed in Fig. 4. The deactivation of the catalyst is associated with the decrease in the number of acid site as well as the Al atoms, as found from IR and XPS measurements. The Briinsted acid site was decreased during the reaction, and there were only a few of the Brijnsted acid sites left on the deactivated H-mordenite, as shown in Fig. 6. It seemed that the decreasing of the Briinsted acid sites was caused by the decreasing of Al atoms in the zeolite framework. Here the H,O in the gas phase played an important role in the deactivation of the H-mordenite, as shown in Fig. 4. At 3000 ppm of the H,O concentration, which was required stoichiometrically to decompose 1000 ppm of the CFC113, the activity of the H-mordenite dropped very slowly. But the activity dropped faster as the H,O concentration was increased. Therefore, it seems that Al atoms were removed from the zeolite framework by the accompanied H,O, since the dealumination of the zeolite was reported to occur easily in moisture conditions
[la. In addition, the presence of HCl and HF was undesirable for the zeolite in moisture conditions. Because acid leaching is a tool for the dealumination of the zeolite, it seems that the Al atoms were removed easily from the zeolite framework by the reaction of HCl and HF which were simultaneously produced in the moisture condition. The removed Al atoms from the zeolite framework disappeared from the zeolite surface, as shown in Table 2. Probably, Al atoms are chlorinated and vaporized, because the boiling point of AlCl, is 450.8 K while that of AlF, is above 1473 K [ 131, and the gaseous products are removed from the zeolite crystal.
176
M. Tajima et al./Applied
Catalysis B: Environmental 9 (1996) 167-177
Table 3 Bond energy of CFCs [14] Compound
Bond energy, kJ mol-’ C-Cl
ccl, CCl,F CCI,F, CClF,
C-F _
298+5 305+8 318+8 360.2+5
42719 460 f 25 490 f 25 543*4
CF,
4.3. Pathway of the decomposition
reaction
The CFC molecules had the following sequence of reactivity: Ccl, > CC1,F > CCl,F2 > CClF,. On the other hand, the bond energies of C-Cl and C-F are shown in Table 3 [14], and the order of bond energy of C-Cl in these compounds, Ccl, < CCl,F < CCl,F, < CClF,, is in agreement with the reverse order of reactivity in the decomposition. Hence, CFCs become to decompose easily with increasing number of Cl atoms in the molecule. Furthermore, it is justified that the C,CI,F, is formed more than C,Cl,F,, because C,Cl,F, is easily decomposed rather than C,Cl,F,. The first step of the reaction is the decomposition of CFC113, and its disproportionation occurs to form C,Cl,F, and C,Cl,F,; these are followed by subsequent decomposition of C,Cl,F,. In addition, it can be presumed that the reaction is initiated with the rupture of C-Cl bond, because the C-Cl bond energy is lower than that of C-F bond, as shown in Table 3. Fig. 7 shows the correlation between the reactivity and the bond energy; there is a good relationship between the C-Cl bond energy and the reactivity. This correlation supports the consideration that the rate controlling step is the cleavage of the C-Cl bond.
REACTION
“;”
C2C13F3 I
decomposition disproportionstian
OF C&1$3
CO+COa+HCl+HF
C2C12F4 + C2C14F2
H2O
Fig. 7. Correlation between decomposition reactivity of CFCs and C-Cl bond energy. The activity measured at the condition of CFCs, 1000 ppm; H,O, 4000 ppm; balance air, 500 ml/min; WHSV = 30
g-cat-‘.
was
I h-’
M. Tajima et al./AppEied Catalysis B: Enuironmental9 (1996) 167-177
177
Okazaki reported that the order of reactivity of CFCs with H,O was CCl,F,, CCl,FCClF, (CFC113) > CClF, > CF,, and the low activity of CF, was not explained merely by the strong C-F bond [3]. His result agrees with this study, and it is suggested that the rupture of C-F bond does not contribute to the initiation of the reaction, and the order of reactivity of CFCs is determined by the CC1 bond energy, as shown in Fig. 7. The existence of the C-Cl bond, therefore, is essential in the decomposition of CFCs.
Acknowledgements This research was supported by TOSOH Corp. and National Institute for Resourced and Environment. We thank TOSOH Corp. and Asahi Glass Co. Ltd. for supplying the zeolite samples and CFCs.
References [II J.C. Dickerman, T.E. Emmel, G.E. Harris and K.E. Hummel, Technologies for CFC/Halon Destruction, EPA report, Washington DC, 1989. [2] D.R. Stull et al., JANAF Thermochemical tables, 1970. [3] S. Okazaki and A. Kurosaki, Chem. Lett., (1898) 1901. [4] S. Karmakar and H.L. Greene, J. Catal., 138 (1992) 364. [Sl S. Okazaki and H. Habutsu, J. Fluorine Chem., 57 (1992) 191. 161 S.C. Fung and J.H. Sinfelt, J. Catal., 103 (1987) 220. [7] J.S. Campbell and C. Kernball, Trans. Faraday Sot., 57 (1961) 809. 181 J.H. Scofield, J. Electron Spectrosc. Relat. Phenom., 8 (1976) 129. [9] K. Tsutsumi and K. Nishiyama, Thermochim. Acta, 143 (1989) 299. [IO] M. Niwa, N. Katada, M. Sawa and Y Murakami, J. Phys. Chem., 99 (1995) 8812. 1111 S. Okazaki, Shokubai, 10 (1968) 242. [12] M. Briend-Faure, 0. Comu, D. Delafosse, R. Monque and M.J. Pelter, Appl. Catal., 38 (1988) 71. 1131 K. Shudou et al., Table of Chemical Constants for Laboratory Use, Vol. 219, Hirokawa Publishing Co., Tokyo, 1989. [14] R.C. Weast and M.J. Astle, CRC Handbook of Chemistry and Physics, 63rd ed., CRC Press, Boca Raton, FL, 1978.
ELSEVIER
B: Environmental
9 (1996) 167-177
Decomposition of chlorofluorocarbons in the presence of water over zeolite catalyst ’ Masahiro Tajima a, Miki Niwa a’*, Yasushi Fujii b, Yutaka Koinuma ‘, Reiji Aizawa ‘, Satoshi Kushiyama ‘, Satoru Kobayashi ‘, Koichi Mizuno ‘, Hideo Ohachi ’ a Department
ofMaterials Science, Facul@ of Engineering, Tottori Unimv-sir)?,Koyama, Tottori, 680, Japan
’ Tosoh Corp., Shin-nanyou, Yamaguchi 746, Japan ’ National Institute for Resources and Environment, Agency of Industrial Science and Technology, Onogawa, Tsukuba, Tharaki 305, Japan Received
18 July 1995; revised 28 November
1995; accepted
11 December
1995
Abstract The decomposition of chlorofluorocarbons (CFCs) in the presence of water was examined over a variety of solid acid catalysts. More than 40% of the conversion of CFC was observed on HY zeolite, H-mordenite, H-ZSM-5, y-Al,O,, and SiO,-TiO, catalysts, and the selectivity to CO and CO, was nearly 100% except on y-Al,O,. Although the H-mordenite had the highest activity among the tested catalysts, it was gradually deactivated during the reaction due to the elimination of Al atoms from the zeolite framework. A good relationship was found between the reactivity on H-mordenite and the bond energy of C-Cl in compounds of Ccl,, CCl,F, CCl,F,, and CClF,, suggesting that the rate controlling step was the cleavage of the C-Cl bond.
1. Introduction The decomposition of chlorofluorocarbons (CFCs) is an important reaction in order to protect the ozone layer surrounding the earth. However, it is difficult due to the extremely high stability of CFC molecules [l]. Although CFCs as detergents and solvents have been replaced by hydrochrolofluorocarbon (HCFC) or hydrofluorocarbon (HFC), remaining stocks of CFCs must be converted into harmless compounds.
* Corresponding author. Tel.: (+ 81-857) 3 15256; fax: (+ 8 l-857) 3 10881; e-mail: [email protected] ’Previously submitted on 8 March 1995. 0926.3373/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO926-3373(96)0001 1-2
168
M. Tajima et al/Applied
Catalysis B: Enoironmental9 (1996) 167-I 77
The thermal decomposition of CFCs is impractical, thermodynamic equilibrium, as shown in Eq. (1).
because of unfavorable
CC1,F + C + Cl, + CIF AG = 650kJ/mol
(1)
However, it is thermodynamically possible to decompose CFCs in the presence of such compounds as H,O, 02 and H,, because the change of free energy (AG) of the scheme (21, (31, and (4) at 1000 K show negative values. Values of the free energy of CCl,Fs were obtained from ref. [2]. CC1,F + 2H,O CC1,F + 0,
+ CO, + 3HCl+
HF AG = -441.37kJ/mol
+ CO, + Cl, + CIF AG = - 306.67 kJ/mol
CC1,F + 4H, =+ CH, + 3HCl+
HF AG = - 411.78 kJ/mol
(2) (3) (4)
Okazaki et al. studied the decomposition of CFC13 (CClF,) with H,O over various metal oxides unsupported and supported on activated carbon, and found that Fe,O,/carbon was an active catalyst at temperatures above 723 K; however, the gradual deactivation was also reported [3]. Karmakar and Greene studied the oxidation of CFC12 (CCl,F,) over Yzeolites (H-Y and Cr-Y) [4]. The initial activities of these catalysts were high, but the long term deactivation tests exhibited a serious drop in the catalytic activity because of the partial destruction of the zeolite crystalline structure. The decomposition of CFCs in the presence of H, also has been reported [5-71. Okazaki et al. studied the hydrodechlorination of CFCl13 (C,Cl,F,) over the NiO-Cr,O, catalysts which contained more than 70% Cr, and observed nearly 100% of the conversion even after a running time of 5.25 h [5]. In this study, H,O was used for the decomposition reaction, because H,O is easily available, and HCI and HF stoichiometrically produced by the decomposition are easily removed by neutralization. We examined several solid catalysts for the reaction, and found the H-type zeolite as an active catalyst. Moreover, we studied its possibility as an industrial catalyst. The purpose of this investigation is therefore to reveal the active catalyst in the decomposition of CFCs in moisture conditions.
2. Experimental 2.1. Materials Na-Y zeolite (atomic ratio of Si/Al = 2.81, H-Y (10) and H-ZSM5 (11) were obtained from Tosoh obtained from Nihon Ketzen Co. SiO, and TiO, Kagaku Co. SiO,-TiO, (Si/Ti = 1) was purchased Fe,O, was prepared by calcining iron(II1) hydroxide, Kanto Kagaku Co., at 723 K for 2 h in air. ZrO,
zeolite (2.81, H-mordenite Corp. y-Al,O, pellet was was obtained from Kanto from Nakarai Tesque Co. which was obtained from was prepared by calcining
M. Tajima et al. /Applied Catalysis B: Environmental 9 (1996) 167-177
169
zirconium hydroxide, which was obtained by hydrolysis of ZrOCl,, at 723 K for 2 h in air. These particles of catalysts were grained and sieved in lo-20 mesh. 1,2-Trichloro- 1,2,2-trifluoroetane (CFC113) and carbontetrachloride were obtained from Nakarai Tesque Co. Chlorotrifluoromethane (CFC 131, dichlorodifluoromethane (CFC 12) and trichlorofluoromethane (CFC 11) were supplied from Asahi Glass Co. 2.2. Apparatus
and procedure
The decomposition reactions were carried out at atmospheric pressure using a tubular flow reactor made of quartz (12 mm O.D., 9 mm I.D.>. 1 g of catalyst was charged in the reactor which was heated with an electric furnace. A gas mixture (CFCs, 1000 ppm; H,O, 3000-16000 ppm; N, or dry air, balance) was flown into the reactor at 30 1 hh ’g-cat- ’ of WHSV; unless otherwise described, air and 4000 ppm of the H,O were flown into the reactor. The exhaust gas was then dissolved in a KOH solution to remove HCl and HF produced in the reaction. 1 ml of the exhaust gas was collected before the KOH solution trap, and unconverted CFCs, and produced CO and CO, were analyzed respectively by gas chromatographs equipped with a flame ionization detector (FID) as follows. The CFCs were separated with a chromosorb 101 (2 m) column at 373 K, and then detected by an FID detector. CO and CO, were separated with an activated charcoal column (1 m) at 373 K, converted into methane with H, on a ruthenium catalyst, and detected by an FID detector. The X-ray diffraction was measured with a Rigaku Denki Geigerflex RAD-II A X-ray analyzer. The X-ray photoelectron spectroscopy (XPS) was measured with a Shimazu ESCA-750 spectrophotometer. Samples were mounted with Scotch double-stick tape. Elemental atomic ratios were calculated from integrated peak intensities of A12p, Si2p, and Fls which were corrected based on the reported cross-sectional area [S]. Transmission infrared spectra of pyridine adsorbed on H-mordenite were measured with a JEOL JIR-40 spectrophotometer. A thin wafer of H-mordenite was mounted in a vacuum-tight infrared cell. After the sample was evacuated at 773 K for 1 h, pyridine was adsorbed at 573 K for 1 h at the equilibrium pressure of 10 ton-, followed by evacuation at 423 K; the spectra for adsorbed pyridine were then measured at room temperature.
3. Results 3.1. Decomposition Table CFCl13
of CFCs over several catalysts
1 shows the activity of several catalysts for the decomposition of in the presence of water vapor at 773 K. Because the conversion
170
kf. Tajima et al./Applied
Table 1 Catalytic activities of CFCl13 Catalyst
HY NaY H-mordenite H-ZSM-5 -w&O, SiO, -TiO, FeA SiO, ZQ TiO,
decomposition
Conversion of CFCI 13 (%I 85 15 98 88 85 40 16 8 0 0
Catalysis B: Environmental 9 (1996) 167-I 77
at 773 K a Conversion C,Cl,F, 1 3
to
(%I
Conversion C,Cl,F,
to
(%x)
0 0 0 0
1 0 0 0 0 0
Reaction conditions: CFCl13 1000 ppm, H,O 4000 ppm, balance air; WHSV: 30 1 h-’ g-cat-’ a Measured at 2 h of time-on-stream.
changed at initial stage of reaction, the stabilized activity was obtained 2 h of time-on-stream. H-type zeolite, y-Al,O,, and SiO,-TiO,, which were known as typical acid catalysts, exhibited high activity. The CFCl13 was decomposed to selectively form equimolar amounts of CO and CO,, and no other carbon-containing compounds were detected. Furthermore, HCI and HF were detected in a KOH solution trap in the exhaust gas, but these amounts were not determined quantitatively. H-type zeolites exhibited the highest activity among the catalysts examined, and the activity was in the order H-mordenite > H-ZSM-5 > H-Y. Only on the y-Al,O,, the decomposition produced by-products, 16% of C,Cl,F, and 1% of C2C1,F,, while on other catalysts, less than 3% of C,Cl,F, was formed. On the other hand, NaY, Fe,O,, and SiO, showed low activity, and ZrO, and TiO, were found to be inactive. Fig. 1 shows the temperature dependence of the decomposition of CFC 113 over the H-mordenite in the presence of water vapor. The conversion of CFCl13 increased with increasing temperature, and reached nearly 100% at 773 K. The temperature for decomposition of CFCl13 over the H-mordenite is thus obviously lower than that for the thermal decomposition without catalyst which requires above 1073 K; the potential priority of the decomposition is thereby noteworthy. Fig. 2 shows the influence of the addition of air on the reaction. There was no difference between reactions undertaken with and without air. It was found that the catalyst activity was enhanced at an early stage of the reaction. The conversion of CFCll3 was 85% at 30 min after the initiation of reaction, but it increased, and reached about 98% after 1 h. Variation of reactivity as a function of CFC molecules containing different ratios of Cl atom to F atom is depicted in Fig. 3. Ccl, was decomposed nearly 100% at 503 K, and CCl,F, CCl,F,, and CClF, were decomposed nearly 100% at the higher temperatures, 623 K, 723 K, and 803 K, respectively. However,
M. Tajima et al. /Applied
600
Catalysis
650
B: Enuironmental
9 (1996) 167-l
700
Reaction Temperatuer
750
77
171
800
/K
Fig. 1. Decomposition of CFCl13 over the H-mordenite under the following H20, 4000 ppm; balance air, 500 ml/min; WHSV = 30 1 h- ’ g-cat.
CF, was not decomposed at 873 K over the H-mordenite. CFCs produced CO,, but not CO, while the decomposition CO and CO,.
conditions:
CFCI 13, 1000 ppm:
These compounds of of CFCll3 produced
3.2. Life of catalytic actiuity Fig. 4 shows the life of catalyst activity over the H-mordenite with varying the H,O concentration from 3000 ppm to 16000 ppm at 773 K. Initial activity
60
’
0 0
1
2
3
4
Reaction Time / H Fig. 2. Decomposition of CFCl13 over the H-mordenite at 773 K in air (a) and Nz (b) under the conditions: CFCl13, 1000 ppm; H,O, 4000 ppm; balance, 500 ml/min; WHSV = 30 I hh’ g-cat-‘. (0) conversion of CFC 113, (a) conversion to C,Cl, F4, ( A) conversion to CO?, and ( A ) conversion to CO.
172
M. Tajima et al. /Applied Catalysis B: Environmental 9 (1996) 167-177
0 300
,/,/
,A’*
400
500
,/
600
700
800
900
Reaction Temperature/K Fig. 3. Catalytic decomposition of CFCs (0) CC],, (0) CCl,F, (A) CCl,F,, and (A) CClF,) over the H-mordenite under the conditions: CFCs, 1000 ppm; H,O, 4000 ppm; balance air, 500 ml/min; WHSV = 30 1 h-’ g-cat-‘.
was very high at any H,O concentration. However, it was dropped slowly at 3000 ppm of the H 2O concentration, and about 90% of CFC 113 decomposed at the run of 30 h. The deactivation was enhanced with increasing H,O concentration. At 16000 ppm of the H,O concentration, the activity was dropped rapidly, and CFCl13 did not decompose at the run of 6 h. 3.3. Characterization
The character of the fresh sample and deactivated sample were measured by several methods. The deactivated sample and used sample were reacted under the standard condition (CFC113, 1000 ppm; H,O, 4000 ppm; air, balance) at 773 K for 30 h of time-on-stream.
100
0 0
10
20
30
40
Reaction Time / H Fig. 4. Decomposition of CFCl13 over the H-mordenite at 3000 ppm (01, and 16000 ppm (0) of the H,O concentration at 773 K under the conditions: 500 ml/min; WHSV = 30 1 h-’ g-cat-‘.
4000 ppm (A), 8000 ppm (U), CFCl13, 1000 ppm; balance air,
M. Tajima et al. /Applied Catalysis B: Environmental 9 (1996) 167-177
173
r-
(b)
4
10 2 R (degree)
Fig. 5. X-ray diffraction
patterns of the fresh (a) and deactivated
(b) H-mordenite
Fig. 5 shows the XRD analysis of the H-mordenite before and after the reaction. Because there was no difference between them, the zeolite was not destructed.
i
m
d
A& 1542
(b)
(a) I
I
1680
1
1600
I
I
1520
1440
Wavenumber / cm _ ’ Fig. 6. IR spectra of adsorbed pyridine on the H-mordenite fresh sample, (c) deactivated sample.
after the evacuation
at 423 K; (a) background,
(b)
M. Tajima et al/Applied
174 Table 2 Atomic composition
of H-mordenite
Catalyst
Catalysis B: Environmental 9 (1996) 167-177
as revealed by XPS
Atomic ratio
Fresh sample Used sample
Si
Al
F
Cl
1
0.1 0.03
0 0.2
0 0
1
Fig. 6 shows the IR spectra of pyridine adsorbed on the fresh and used H-mordenite at 423 K. The 1542 cm- ’ band, identified as the Brijnsted acid, was observed on the fresh sample, while it was not on the used sample. Table 2 shows the surface composition observed by XPS on the fresh and used H-mordenite. Si, Al, and F were detected, but Cl was not. It was found that 70% of surface Al atom was eliminated on the deactivated sample, and the surface of the zeolite was fluorinated.
4. Discussion
4.1. Decomposition
of CFCl13
ouer several catalysts
Because the acid catalysts exhibit the high activity, it is suggested that CFCl13 is decomposed on the acid sites. H-mordenite has the highest activity among the H-type zeolites. It is known that the H-mordenite has the stronger acid strength rather than H-ZSM-5 and H-Y [9,10]. Therefore, it is suggested that the activity of the decomposition of CFCs is determined by the acid strength of the catalysts. H-type zeolites have the Brijnsted acid site, but y-Al,O, has only the Lewis acid site; therefore, CFCs seemed to be decomposed not only on the Brijnsted acid site but also on the Lewis acid site. In the presence of water, the decomposition of CFC 113 formed equimolar amounts of CO and CO, with HCl and HF, as shown in Fig. 2. It may be thereby deduced that the decomposition reaction can be described as in the equilibrium (5). C,CI,F,
+ 3H,O
= CO, + CO + 3HCl+
3HF
(5)
Because no influence by the addition of air upon the decomposition of CFCl13 was observed, oxygen had no effect on the decomposition of CFCl13 in the presence of water vapor. It is considered that C,Cl,F, is a by-product arising from the disproportionation between two molecules of CFC113, as shown in the equilibrium (6). 2C,Cl,F,
+ C,Cl,F,
+ C,Cl,F,
M. Tajima et al/Applied
Catalysis B: Environmental 9 (19961 167-177
175
Okazaki studied the disproportionation reaction of CClF, over various metal salts [ll]. He found that the relative catalytic activities were given by the sequence, AlCl, > FeCl, > CaCl, > CrCl,, PbCl, > HgCl,, and the results were discussed in terms of the acidity of metal chlorides. He described that the disproportionation of CFCs occurred on the acid site, and the catalytic activities depended on the amount of acid site. Based on Okazaki’s study, it is suggested that the disproportionation of CFCs occurred on the Lewis acid sites of metal salts. In this study, only y-Al,O, produced C ,Cl,F, in the decomposition of CFCll3. Because y-Al,O, and such metal salts as AlCl, have only the Lewis acid, it is suggested that the disproportionation of CFCl13 occurred on the Lewis acid site. The disproportionation of CFC 113 produced equal amounts of C,Cl,F, and C,Cl,F,, but no or a few of C,C1,F2 were detected by the reaction over H-mordenite and y-Al,O,. This will be mentioned in the pathway of decomposition reaction. 4.2. Deactivation
of H-mordenite
Although the H-mordenite had the highest activity among the tested catalysts, it was deactivated during the prolonged uses, as observed in Fig. 4. The deactivation of the catalyst is associated with the decrease in the number of acid site as well as the Al atoms, as found from IR and XPS measurements. The Briinsted acid site was decreased during the reaction, and there were only a few of the Brijnsted acid sites left on the deactivated H-mordenite, as shown in Fig. 6. It seemed that the decreasing of the Briinsted acid sites was caused by the decreasing of Al atoms in the zeolite framework. Here the H,O in the gas phase played an important role in the deactivation of the H-mordenite, as shown in Fig. 4. At 3000 ppm of the H,O concentration, which was required stoichiometrically to decompose 1000 ppm of the CFC113, the activity of the H-mordenite dropped very slowly. But the activity dropped faster as the H,O concentration was increased. Therefore, it seems that Al atoms were removed from the zeolite framework by the accompanied H,O, since the dealumination of the zeolite was reported to occur easily in moisture conditions
[la. In addition, the presence of HCl and HF was undesirable for the zeolite in moisture conditions. Because acid leaching is a tool for the dealumination of the zeolite, it seems that the Al atoms were removed easily from the zeolite framework by the reaction of HCl and HF which were simultaneously produced in the moisture condition. The removed Al atoms from the zeolite framework disappeared from the zeolite surface, as shown in Table 2. Probably, Al atoms are chlorinated and vaporized, because the boiling point of AlCl, is 450.8 K while that of AlF, is above 1473 K [ 131, and the gaseous products are removed from the zeolite crystal.
176
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Catalysis B: Environmental 9 (1996) 167-177
Table 3 Bond energy of CFCs [14] Compound
Bond energy, kJ mol-’ C-Cl
ccl, CCl,F CCI,F, CClF,
C-F _
298+5 305+8 318+8 360.2+5
42719 460 f 25 490 f 25 543*4
CF,
4.3. Pathway of the decomposition
reaction
The CFC molecules had the following sequence of reactivity: Ccl, > CC1,F > CCl,F2 > CClF,. On the other hand, the bond energies of C-Cl and C-F are shown in Table 3 [14], and the order of bond energy of C-Cl in these compounds, Ccl, < CCl,F < CCl,F, < CClF,, is in agreement with the reverse order of reactivity in the decomposition. Hence, CFCs become to decompose easily with increasing number of Cl atoms in the molecule. Furthermore, it is justified that the C,CI,F, is formed more than C,Cl,F,, because C,Cl,F, is easily decomposed rather than C,Cl,F,. The first step of the reaction is the decomposition of CFC113, and its disproportionation occurs to form C,Cl,F, and C,Cl,F,; these are followed by subsequent decomposition of C,Cl,F,. In addition, it can be presumed that the reaction is initiated with the rupture of C-Cl bond, because the C-Cl bond energy is lower than that of C-F bond, as shown in Table 3. Fig. 7 shows the correlation between the reactivity and the bond energy; there is a good relationship between the C-Cl bond energy and the reactivity. This correlation supports the consideration that the rate controlling step is the cleavage of the C-Cl bond.
REACTION
“;”
C2C13F3 I
decomposition disproportionstian
OF C&1$3
CO+COa+HCl+HF
C2C12F4 + C2C14F2
H2O
Fig. 7. Correlation between decomposition reactivity of CFCs and C-Cl bond energy. The activity measured at the condition of CFCs, 1000 ppm; H,O, 4000 ppm; balance air, 500 ml/min; WHSV = 30
g-cat-‘.
was
I h-’
M. Tajima et al./AppEied Catalysis B: Enuironmental9 (1996) 167-177
177
Okazaki reported that the order of reactivity of CFCs with H,O was CCl,F,, CCl,FCClF, (CFC113) > CClF, > CF,, and the low activity of CF, was not explained merely by the strong C-F bond [3]. His result agrees with this study, and it is suggested that the rupture of C-F bond does not contribute to the initiation of the reaction, and the order of reactivity of CFCs is determined by the CC1 bond energy, as shown in Fig. 7. The existence of the C-Cl bond, therefore, is essential in the decomposition of CFCs.
Acknowledgements This research was supported by TOSOH Corp. and National Institute for Resourced and Environment. We thank TOSOH Corp. and Asahi Glass Co. Ltd. for supplying the zeolite samples and CFCs.
References [II J.C. Dickerman, T.E. Emmel, G.E. Harris and K.E. Hummel, Technologies for CFC/Halon Destruction, EPA report, Washington DC, 1989. [2] D.R. Stull et al., JANAF Thermochemical tables, 1970. [3] S. Okazaki and A. Kurosaki, Chem. Lett., (1898) 1901. [4] S. Karmakar and H.L. Greene, J. Catal., 138 (1992) 364. [Sl S. Okazaki and H. Habutsu, J. Fluorine Chem., 57 (1992) 191. 161 S.C. Fung and J.H. Sinfelt, J. Catal., 103 (1987) 220. [7] J.S. Campbell and C. Kernball, Trans. Faraday Sot., 57 (1961) 809. 181 J.H. Scofield, J. Electron Spectrosc. Relat. Phenom., 8 (1976) 129. [9] K. Tsutsumi and K. Nishiyama, Thermochim. Acta, 143 (1989) 299. [IO] M. Niwa, N. Katada, M. Sawa and Y Murakami, J. Phys. Chem., 99 (1995) 8812. 1111 S. Okazaki, Shokubai, 10 (1968) 242. [12] M. Briend-Faure, 0. Comu, D. Delafosse, R. Monque and M.J. Pelter, Appl. Catal., 38 (1988) 71. 1131 K. Shudou et al., Table of Chemical Constants for Laboratory Use, Vol. 219, Hirokawa Publishing Co., Tokyo, 1989. [14] R.C. Weast and M.J. Astle, CRC Handbook of Chemistry and Physics, 63rd ed., CRC Press, Boca Raton, FL, 1978.