Ind. E n g . C h e m . Res. 1990,29, 1758-1761
1758
Decomposition of Dichlorodifluoromethane on Ti02/Si02 Seiichiro Imamura,* Toshihiko Shiomi, Shingo Ishida, and Kazunori Utani Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, J a p a n
Hitoshi Jindai N i p p o n Fine Gas Co. Ltd., 1-4 Takasago, Takaishi, Osaka 592, J a p a n
Catalytic decomposition of dichlorodifluoromethane was carried out on various metal oxides. Acid catalysts such as zeolite, silica-alumina, and titania-silica exhibited high decomposition activities, whereas the activities of transition-metal oxides and CaO were low. Titania-silica exhibited the best performance among the acid catalysts investigated. Its acid property correlated well with its activity; the strong acid site with a Hammett acidity parameter Hoof less than -5.6 was effective in decomposing dichlorodifluoromethane. Deactivation, however, occurred rapidly due to an attack of fluorine on silicon in titania-silica. A combination of titania-silica with CaO, which reacted with fluorine more readily than did silicon, protected the titania-silica from corrosion by fluorine, and the lifetime of this catalyst was prolonged. There is an increasing concern about the depletion of the ozone layer in the stratosphere caused by chlorofluorocarbons (CFCs), which, because of their excellent characteristics, are used for many purposes such as solvents in electronic industries, refrigerants, and propellants. More than 1 million metric tons of CFCs is being produced annually all over the world, and an enormous amount of them has been preserved. Although their production will be diminished in the future owing to the United Nations Environmental Protection Protocol for CFC regulation adopted in Montreal, Canada, in 1987, the amount of preserved CFCs will inevitably continue to increase. Therefore, destruction of these preserved CFCs will be eventually needed to prevent their release, which leads to the further depletion of the ozone layer. CFCs can be destroyed by some techniques such as reduction with sodium naphthalenide (Oku et al., 1989) or hydrogen (Witt et al., 1981),decomposition by solar energy (Blake, 1988), and decomposition on activated charcoal (Colussi and Amorebieta, 1987). However, incineration seems to be the most practical technique when a large amount of CFCs must be destroyed. Although incineration can be successfully applied for chlorinated organic compounds such as 2,4,5-trichlorophenoxyaceticacid (Ackerman et al., 1978), incineration of CFCs has scarcely been investigated (Graham et al., 1986). As incineration requires a large amount of energy, the use of catalysts is energetically favorable. Previously, we investigated the catalytic combustion of 1,2-dichloroethane and proposed that the multiple-step catalytic combustion is promising for the treatment of chlorinated organics as described below (Imamura et al., 1989a,b). Chlorinated organics are first decomposed into inorganic chlorines (C12 and HC1) and carbonaceous moiety on acid catalysts that are resistant to the chlorines. The resultant inorganic chlorines are trapped by basic oxides such as CaO and MgO to prevent the formation of harmful chlorinated byproducts in a later stage. The residual carbonaceous moiety is completely oxidized on combustion catalysts in the final stage. The present report deals with the catalysts to be used in the first step of the multiple-step catalytic combustion of CFCs. Dichlorodifluoromethane (Freon 12) was used as a model compound.
Experimental Section Materials. Silica-alumina N633-L was obtained from Nikki Kagaku Co., mordenite with a silica-to-alumina weight ratio of 10 from Toso Co. (designated as mordenite A), mordenite with a silica-to-alumina weight ratio of 10 0888-5885/90/2629-1758$02.50/0
from Norton Co. (designated as mordenite B), zeolite Y TSZ-330HUA from Toso Co., and titania-zirconia from Nippon Shokubai Kagaku Kogyo Co. Zirconium phosphate (ZrP20,) was prepared from zirconium(1V) oxychloride and H3P04and was calcined at 500 "C for 3 h in air (Kagiya et al., 1963). Zirconia-molybdenum(V1) oxide with a Mo content of 5 wt % as metal was prepared by impregnation of molybdic acid (H,Mo04) into zirconium hydroxide, followed by calcination at 770 "C for 3 h in air (Arata, 1989). Titanium(1V) oxide was prepared by calcinating titanium hydroxide, which was obtained by hydrolysis of titanium(1V) isopropoxide, a t 550 "C for 3 h in air. Co304,Mnz03, and Cr203were obtained by precipitation from an aqueous solution of the corresponding metal nitrates (Imamura et al., 1989a). Titaniaailica was prepared as follows. Known amounts of titanium(1V) isopropoxide and ethyl silicate (about 30 g as a mixture) were dissolved in 80 mL of ethanol, and the solution was refluxed a t 80 "C for 30 min. Then a mixture of 100 mL of ethanol and 200 mL of aqueous acetic acid (0.01 N) was added to the solution dropwise to obtain a white precipitate. This precipitate was dried at 80 "C, followed by calcination at 550 "C for 3 h in air. Freon 12 in He (0.74%) was obtained from Nippon Fine Gas Co. Benzene was purified by refluxing overnight over sodium metal, followed by distillation. Commercial CaO and other reagents were used without further purification. Apparatus and Procedure. Reactions were carried out under atmospheric pressure by using a tubular flow reactor made of alumina (outer diameter, 10 mm; inner diameter, 6 mm). One milliliter of the catalyst was charged in the reactor, and the reactor was heated with an electric furnace. The helium containing Freon 12 was mixed with oxygen (Freon 12,0.60%; 02,21.2%; He, 78.2%) and was introduced into the reactor at a space velocity (SV) of 5900 h-l. The reacted gas was analyzed for the remaining Freon 12, CO, and C02. The reaction was not affected by any mass-transfer limitation; the particle size of titania-silica with a Ti-to-Si molar ratio of 1 did not affect the rate of the decomposition of Freon 12. Generally only one experiment was performed for each catalyst to evaluate its activity.
Analyses. Freon 12, CO, and C02were determined with a Shimadzu GC-12A gas chromatograph equipped with a flame ionization detector. The column packing for the analysis of Freon 12 was Chromosorb 101 (1 m), and the column temperature was 120 "C. CO and CO, were converted to methane with a Shimadzu MTN-1 methanizer and were analyzed with the gas chromatograph at 120 "C (C 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1759 Table I. DecomDosition of Freon 12 at 500 O c a 5 minb 120 minb conv: CO + COzd C02/(C0 conv; CO + C o t catalvst % select, % + CO,), % % select, % _ _ ~ SiOz/ 99.2 100 82.8 98.3 62.6 ~~
A1203
mordenite A mordenite B zeolite Y ZrP207 ZrOJ M~o, Ti/Si(50150)' Ti02/ Zr02 TiOz Cr203
Mn203 c0304
Fez03 CaO
100
100
98.5
84.3
60.4
92.2
86.4
98.4
75.0
89.7
86.4 95.3 95.7
100 91.0 97.4
100 98.6 97.5
37.2 81.7
47.5 56.2 68.4
95.7
100
94.7
90.3
96.0
80.2
93.0
91.0
27.1
83.8
34.4 1.2 17.2 16.3 33.4 4.2
88.2 92.5 100 95.0
96.6 100 89.2 65.4
17.1 1.4 10.0 13.1
100 100 100 84.9 1008
f Oh
f
1.2
0.98
3.1
C
.-
'? > 0
0
Temp ("C)
Figure 1. Decomposition of Freon 12 on titania-silica. Titaniasilica = 1 mL; SV of reaction gas (0.60% Freon 12 t 21.2% O2 + 78.2% He) = 5900 h-l; Ti (mol % ) = (A)5, ( 0 )10, (V)20, (0) 30, (0) 40, (V)50, (0)60, (A)70, (W) 100. 600
1
[Freon 121 = 0.60% in O2 t He; catalyst, 1 mL; SV = 5900 hA1. bReaction time. cFreon 12 decomposed. dSelectivity of CO + COP 'Titania-silica with a molar ratio of 50/50. 'Deactivation was too rapid to determine CO and C02. #After 20 min. h C 0 2 and/or CO was trapped by CaO.
using an activated charcoal column (1 m). The acid strength of titania-silica was measured in benzene by the use of the following Hammett indicators: methyl yellow (pK, = 3.3), benzeneazodiphenylamine (pK, = 1.5), dicinnamalacetone (pK, = -3.0),and benzalacetophenone (pK, = -5.6). The amount of the acid site was determined by titration with n-butylamine in the presence of these Hammett indicators. 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.
Results and Discussion Decomposition of Freon 12 on Various Catalysts. Table I shows the results of the decomposition of Freon 12 a t 500 "C. Transition-metal oxides, which are typical combustion catalysts, basic oxides such as CaO and MgO, and acid catalysts such as silica-alumina and zeolite were active in decomposing 1,2-dichloroethane without exception (Imamura et al., 1989b). However, transition-metal oxides had only low activities in the decomposition of Freon 12, and CaO was almost inactive. As 1,2-dichloroethane has two hydrogen atoms, oxidative decomposition (combustion) by the transition-metal oxides would have occurred. Freon 12 has no hydrogen atom, and therefore, transition-metal oxides could not catalyze the oxidative decomposition of Freon 12. All acid catalysts exhibited high activities. More than 90% Freon 12 was decomposed, and the selectivity to CO plus COPwas 100% for silicaalumina, mordenite A, zeolite Y, and titania-silica in the initial stage of the reaction (5 min). Formation of COP predominated over that of CO in all cases. The inorganic halogens produced under the condition where 100% selectivity to CO and COz was attained seemed to be molecular chlorine and fluorine because no hydrogen source was present. However, they were not determined because they, especially fluorine, were trapped by the catalysts during the reaction as mentioned later. Different from the results on the decomposition of 1,2-dichloroethane, the deactivation of the catalysts was remarkable. The percentage decomposition of Freon 12 dropped sharply on
T i (mol %)
Figure 2. Effect of Ti content on the acidity and activity of titania-silica. (a) Temperature a t which the rate of the decomposition of Freon 12 reached 6.0 X lo4 mol/(min m2). (b) Acid amount per unit surface area of titania-silica. Acid strength (ITo):( 0 )
