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Ind. Eng. Chem. Res. 2002, 41, 32-36
Metal Sulfate Catalyst for CCl2F2 Decomposition in the Presence of H2O Junichi Moriyama, Hiroyasu Nishiguchi, Tatsumi Ishihara, and Yusaku Takita* Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700, Oita 870-1192, Japan
Activities for the hydrolysis of CCl2F2 (CFC-12) on various metal sulfates were investigated in this study. It was found that Zr(SO4)2 exhibits a high activity for CFC-12 decomposition and a high selectivity to CO2. On the other hand, the metal sulfates Al2(SO4)3, La2(SO4)3, Ce2(SO4)3, and Cr2(SO4)3 were also active for the hydrolysis of CFC-12. However, the activities for CFC-12 decomposition of MnSO4, CoSO4, and MgSO4 were low, and decomposition of CFC-12 hardly occurred on the metal sulfates CaSO4, SrSO4, and BaSO4. Among the investigated metal sulfates, Zr(SO4)2 was the most active, and the complete decomposition of CFC-12 was achieved at 598 K. The CFC-12 decomposition rate monotonically increased with increasing CFC-12 partial pressure. On the other hand, CFC-12 conversion became highest at an oxygen content of 18.9 mol % and a H2O content of 5 mol %. Therefore, the main reaction on Zr(SO4)2 for CFC-12 decomposition into CO2 and HCl and HF is the hydrolysis reaction. The high activity to CFC-12 decomposition on Zr(SO4)2 was stably sustained over the 40-h period examined. Therefore, the metal sulfate Zr(SO4)2 is highly active for the hydrolysis of CFC-12 and has sufficient stability against fluorination and/or chlorination. Introduction
Table 1. BET Surface Area of the Used Metal Sulfate Catalyst
Because chlorofluorocarbons (CFCs) are considered to be the main cause of the ozone hole1 and to have a high greenhouse effect, a simple, low-energy CFC decomposition technique is strongly required from an environmental viewpoint. Although various methods such as plasma treatment or reaction in alkaline liquid have been proposed for the decomposition of CFCs,2-4 catalytic decomposition methods have the advantages of mild reaction conditions, high reaction rates, continuous operation, and a simple and compact apparatus. Many catalysts and catalytic reactions have been proposed for the decomposition of CFCs5-9 or Cl-containing compounds such as CCl4.10 However, in the decomposition of CFCs, products usually contain the strong acid HF and/or HCl, and a significant deactivation of catalyst generally occurs. Therefore, not only high activity but also long life is required for a CFC decomposition catalyst. In our previous study, AlPO4 was found to be highly active for the decomposition of CFC-12 in the presence of H2O. In addition, the high activity of this catalyst was stably sustained over the 1000-h period examined.11 The addition of a small amount of CePO4 to AlPO4 is effective in increasing the activity of the AlPO4 catalyst for CFC-12 decomposition. This catalyst is also active for the decomposition of CF4, which is an extremely stable molecule.12 On the other hand, considering the Gibbs free energy change, metal sulfate is also stable against fluorination or chlorination. Therefore, it is expected that metal sulfates could also be used as stable catalysts for the decomposition of CFCs. However, there have been no * Correspondence should be sent to: Dr. Tatsumi Ishihara, Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700, Oita 870-1192, Japan. Tel.: +81-97-554-7895. Fax: +81-97-554-7979. E-mail: isihara@ cc.oita-u.ac.jp.
Catalyst
BET surface area (m2/g)
Zr(SO4)2 Al2(SO4)3 La2(SO4)3 Ce2(SO4)3 MnSO4 CoSO4 AlPO4
7.4 20.0 2.1 0.1 6.0 12.3 147.4
reports on metal sulfates for the decomposition of CFCs up to now. In the present study, the catalytic activities of various metal sulfates for the decomposition of CCl2F2 (CFC-12) in the presence of H2O were investigated. Experimental Section Commercial metal sulfates of reagent grade were used as catalysts without any further purification. The BET surface areas of the used metal sulfates are listed in Table 1. Before reaction, catalyst powders were pressed into disks, crushed, and sieved to 14-32 mesh. After calcination at 773 K for 5 h in air, hydrolysis of CFC12 was performed with a conventional fixed-bed microflow reactor under atmospheric pressure. Tubular stainless steel with a diameter of 16 mm was used for the reactor. Unless otherwise noted, a gaseous mixture of 0.5 mol % CFC-12, 57.6 mol % water, and air as the balance gas was fed to the catalyst bed at W/F ) 6.73 gcat‚s/cm3, where W and F represent the catalyst weight and the flow rate, respectively. Water was fed with a micropump and evaporated just ahead of the catalyst bed. Generally, 5 g of catalyst was loaded into the tubular reactor with stainless mesh. CFC-12 and gaseous products were analyzed by gas chromatograph with a thermal conductivity detector (TCD) and mass spectrometer. The reacted gas was exhausted after passing through NaOH aqueous solution, and the HCl and HF
10.1021/ie010334c CCC: $22.00 © 2002 American Chemical Society Published on Web 12/08/2001
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Figure 1. Temperature dependence of CFC-12 conversion on various metal sulfates (CFC-12, 0.5 mol %; O2, 7.5 mol %; H2O, 57.6 mol %; N2, 34.4 mol %; W/F ) 67.3 gcat‚h/mol). O, Zr(SO4)2; 3, Cr2(SO4)3; 0, Al2(SO4)3; 4, La2(SO4)3; ], Ce(SO4)2; 9, MnSO4; b, CoSO4; [, MgSO4; 1, SrSO4; 2, BaSO4; ×, CaSO4.
formed were concentrated in the NaOH trap. It is noted that the formation of HCl and HF was also confirmed by analysis of the NaOH aqueous trap. Results and Discussion Figure 1 shows the temperature dependence of the activities of the examined metal sulfates for CFC-12 decomposition. Among the examined metal sulfates, it is obvious that Zr(SO4)2 exhibited the highest activity for decomposition of CFC-12 and that complete decomposition was achieved at 598 K. The activity for CFC12 decomposition decreased in the order Al2(SO4)3 > La2(SO4)3 > Ce2(SO4)3 > Cr2(SO4)3. Although Cr2(SO4)3 was relatively active at low temperature, a 90% conversion of CFC-12 was attained at 773 K, but it became constant even withdespite increasing reaction temperature. Because the conversion should increase with increasing temperature, the constant conversion at temperatures higher than 773 K indicates deactivation of the catalyst. That is, if the reaction were performed at a constant temperature higher than 773 K, the activity of the Cr2(SO4)3 catalyst would decrease with time. This deactivation can be assigned to the sintering of the catalyst, because precalcination of the catalyst in this study was performed at 773 K, and no change in the XRD pattern of Cr2(SO4)3 was observed after reaction. On the other hand, it is obvious from Figure 1 that the activities of MnSO4, CoSO4 and MgSO4 for CFC-12 decomposition were low and that the conversions for these catalysts were ca. 50% at 773 K. In contrast to these metal sulfates, on the metal sulfates CaSO4, SrSO4, and BaSO4, decomposition of CFC-12 hardly occurred. It is well-known that the catalyst activity strongly depends on the surface area. However, considering the surface area of the metal sulfates listed in Table 1, it seems that there is no apparent relationship between the BET surface area and the decomposition activity. This suggests that some chemical property of the metal sulfates has more significant influence on the CFC-12 decomposition activity. In our previous study on AlPO4, the active sites for the decomposition of CFC-12 are considered to be acid sites.9 Therefore,
Figure 2. Decomposition rate of CFC-12 on Zr(SO4)2 as a function of CFC-12 concentration (O2, 18.9 mol %; H2O, 5.0 mol %; N2, balance; W/F ) 67.3 gcat‚h/mol).
the low activity of metal sulfates containing alkaline earth cations can be attributed to weak acidities, and the low activities of these catalyst also support the prediction that the acidity of the catalyst is an important factor in determining the CFC-12 decomposition activity. As for gaseous products, the formation of CO2 was only observed on all catalysts shown in Figure 1 when humidity was present, and no other products such as polymerized products were detected. It is also noted that no carbon deposition was detected by TG-DTA analysis and visual observation. Furthermore, the total amount of carbon in the fed gas was balanced with that in outlet gas within an experimental error on all catalysts. From the XRD measurements, no significant change in crystal structure was observed after reaction, except for MnSO4. Therefore, metal sulfates are highly stable under the decomposition of CFC-12 in the presence of H2O, as expected from the Gibbs free energy change calculation. The activity of Zr(SO4)2 is almost the same as that of AlPO4, which is the most active catalyst for CFC-12 decomposition according to our previous study. Therefore, Zr(SO4)2 is attractive as a catalyst for the decomposition of CFC-12 in the presence of H2O. The effects of the reaction conditions on the decomposition of CFC12 in the presence of H2O on Zr(SO4)2 were further studied in detail and are described below. Figure 2 shows the decomposition rate of CFC-12 as a function of the CFC-12 concentration. The conversion rate of CFC-12 increased with increasing CFC-12 partial pressure. The dependence of the CFC-12 decomposition rate on the CFC-12 partial pressure is estimated to be 0.75. Therefore, an increase in the CFC-12 partial pressure has positive effects on the decomposition reaction. This suggests that the adsorption and activation of CFC-12 on Zr(SO4)2 catalyst is rate-limiting for CFC-12 decomposition in the presence of water. Regardless, it is obvious that a higher decomposition rate of CFC-12 can be achieved at a higher CFC-12 partial pressure. Figure 3 shows the effects of the oxygen partial pressure on the conversion of CFC-12. In this experiment, PH2O and W/F were fixed at 5.0 mol % and 6.73 gcat‚s/cm3, respectively. Compared to the effects of the CFC-12 partial pressure, the effects of the oxygen partial pressure are relatively small, as shown in Figure 3. The CFC-12 conversion at 548 K increased with
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selectivity) and CClF3 (ca. 10% in selectivity) in addition to CO2 was observed. Therefore, the decomposition of CFC-12 by a simple oxidation reaction using gaseous O2 also occurs; however, CFC-12 is rather stable against oxidation. Therefore, a rather high reaction temperature is required. In addition, the undesirable byproducts CO and CClF3 form under the conditions of the simple oxidation reaction. On the other hand, the addition of 5.0 mol % H2O drastically increased the conversion of CFC-12, and only CO2 was observed among the gaseous products. Further increases in the H2O concentration resulted in a decrease in the CFC-12 conversion. Because the conversion of CFC-12 was not significantly affected by the partial pressure of O2 present in the reactant, as shown in Figure 3, the main reaction for the decomposition of CFC-12 over Zr(SO4)2 is a hydrolysis reaction, which is expressed by the equation Figure 3. Effects of oxygen partial pressure on the conversion of CFC-12 over Zr(SO4)2 (CFC-12, 0.5 mol %; H2O, 5.0 mol %; N2, balance; W/F ) 67.3 gcat‚h/mol).
Figure 4. Effects of H2O partial pressure on the activity of Zr(SO4)2 to CFC-12 decomposition (CFC-12, 0.5 mol %; O2, 7.5 mol %; W/F ) 6.73 gcat‚h/mol).
increasing oxygen partial pressure and attained a maximum at 18.9 mol %. Therefore, some amount of CFC-12 seems to be decomposed by oxidative reaction. However, the conversion of O2 was always smaller than 10% when H2O was present. Therefore, the contribution of the oxidation reaction to CFC-12 decomposition under the reaction conditions in this study was not large. On the other hand, it is clear that the excess amount of oxygen decreased the CFC-12 conversion, because the CFC-12 conversion decreased at 50 mol % O2. Because the decomposition rate is strongly affected by the adsorption and activation of CFC-12 on the catalyst, as indicated by the results shown in Figure 2, the excess oxygen might cover the adsorption sites and disturb the adsorption and activation of CFC-12. Consequently, an increase in oxygen partial pressure decreased the CFC12 conversion when the amount of oxygen reached an excess. It is obvious that the highest CFC-12 conversion was achieved when 18.9 mol % oxygen was present in the reactant. Figure 4 shows the conversion of CFC-12 on Zr(SO4)2 as a function of H2O partial pressure. In the case of a dry atmosphere, the conversion of CFC-12 was as low as 32% at 578 K, and the formation of CO (
