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Effect of Pd addition on the catalytic performance of H-ZSM-5 zeolite in chlorinated VOCs combustion
Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
847
E f f e c t o f P d a d d i t i o n on th e c a t a l y t i c p e r f o r m a n c e o f H - Z S M - 5
z e o l i t e in c h l o r i n a t e d VOCs c o m b u s t i o n R. L6pez-Fonseca, S. Cibrihn, J.I. Guti6rrez-Ortiz and J.R. Gonzhlez-Velasco* Departamento de Ingenierla Quimica, Facultad de Ciencias, Universidad del Pals Vasco/EHU, P.O. Box 644, E-48080 Bilbao, Spain. Phone: +34-946012681; Fax: +34-944648500; E-mail: [email protected] The aim of this work was to evaluate the influence of the addition of palladium on the catalytic behaviour of H-ZSM-5 zeolite in the combustion of 1,2dichloroethane (DCE) and trichloroethylene (TCE). Both catalysts showed similar activity in the oxidation of DCE, by contrast, the metal loading led to a substantial improvement in TCE combustion. Vinyl chloride was detected as an intermediate in DCE conversion, and it was appreciably suppressed when adding Pd to the zeolite. In TCE oxidation trace amounts of tetrachloethylene were identified as a by-product, Pd/H-ZSM-5 showing larger quantities of this undesired by-product. Pd/H-ZSM-5 was more selective towards CO2 formation instead of CO, which was the major carboncontaining product formed over H-ZSM-5. However, H-ZSM-5 zeolite showed a lower selectivity to C12 generation while the metal loaded zeolite considerably promoted the formation of this toxic by-product by the Deacon reaction. 1. I N T R O D U C T I O N Emissions of Volatile Organic Compounds (VOCs) into the earth's atmosphere result from naturally occurring (biogenic) as well as human-made sources. In the latter, the use of solvents is the main source of the atmospheric pollution, followed by a variety of industrial processes. One important group of VOCs consists of halogenated organic compounds, where the halogen is generally chlorine. These compounds are usually toxic, particularly through their action on the liver function, and may be carcinogenic or mutagenic. Moreover, chlorinated VOCs are implicated in the destruction of the ozone layer. Because of their harmful properties, the release of halogenated organic compounds into the environment is being controlled by increasingly stringent regulations. 1,2-Dichloroethane (DCE) is the main component of the waste stream gases from the chemical plants that produce it as an intermediate to obtain the monomer vinyl chloride, which is used for the production of polyvinylchloride (1). Trichloroethylene (TCE) is a toxic solvent widely used in dry cleaning and degreasing processes and it is also present in air stripping and soil venting remediation off-gases (2).
848 The most commonly used catalysts for the catalytic oxidation of chlorinated VOCs are alumina supported noble metals and metal oxide catalysts. Recently, zeolites have gained interest as an effective and advantageous alternative, but there are only a few investigations into these reactions over metal-modified H-zeolites (3,4). The objective of this work is to evaluate the influence of the addition of palladium to H-ZSM-5 zeolite on the catalytic behaviour for the combustion of two common chlorinated compounds, between 200-550 ~ The concentration of the chlorocarbons was set at 1000 ppm.
2. EXPERIMENTAL 2.1. Catalyst preparation The zeolite NH4-ZSM-5 (CBV 5524 G) was supplied from Zeolyst Inc. The H-ZSM-5 form was obtained by calcining the NHn-ZSM-5 zeolite in air at 550 ~ for 3 h. The preparation of Pd/H-ZSM-5 zeolite catalysts can be divided into two steps: introduction of the metal palladium and activation, the latter including pre-treatment (or the calcination in an oxygen containing atmosphere), and reduction. The preferred way of introducing a low loading of palladium into a zeolite is to exchange H § ions for positively charged palladium complexes (5). In this case the precursor salt used was [Pd(NH3)4]C12. A dilute solution of this salt, which contained an excess of 15% of the palladium, was added to a stirred HZSM-5 slurry. After stirring for 24 h at room temperature, the slurry was filtered and washed with deionized water, and the sample was then dried overnight (12 h) at 110 ~ To obtain the desired particle size of the catalyst, the sample was pelletised and sieved between 0,3-0,5 mm. The second step of the preparation was the activation of the catalyst in a glass quartz tube reactor. First, a flow of air (21% 02) of 150 ml min -1 was passed, while the temperature was increased from room temperature to 550 ~ with a heating rate of 1 ~ min -1. This final temperature was maintained for 3 h. The sample was then purged in a nitrogen flow and cooled down to 300 ~ and this temperature was maintained for 3 h in an hydrogen/nitrogen flow (1/3). Finally, the sample was cooled down to room temperature in a nitrogen flow.
2.2. Catalysts characterisation The BET surface areas of the catalysts samples were determined by nitrogen adsorption-desorption a t - 1 9 6 ~ in a Micromeritics ASAP 2010 equipment. The composition was determined using a Philips PW 1480 X-ray fluorescence (XRF) spectrometer. The X-ray powder diffraction (XRD) patterns were recorded on a Philips PW 1710 X-ray diffractometer with C u I ~ radiation. The metal content of the catalyst was measured by atomic absorption spectroscopy (AAS) in a Perkin Elmer 1100 B equipment. 2.2.a. Temperature programme desorption (TPD) of ammonia
849 T e m p e r a t u r e programme desorption (TPD) of a m m o n i a was performed on a Micromeritics AutoChem 2910 instrument. The overall process involved heating of the sample at 550 ~ in a nitrogen flow with a heating rate of 20 ~ min -1, after this the sample was cooled down to 100 ~ in a helium flow and then pulses of a m m o n i a were introduced until the sample was saturated. Finally, desorption of chemisorbed ammonia from the sample was carried out by heating from 100 ~ to 550 ~ at a rate of 10 ~ min -1. 2.2.b. T e m p e r a t u r e p r o g r a m m e d reduction (TPR) T e m p e r a t u r e p r o g r a m m e d reduction experiments were also performed in a Micromeritics AutoChem 2910 equipment. Prior to the reduction, the [Pd(NH3)4]2+/H-ZSM-5 catalyst was activated in 5% oxygen/helium flow for 1 h at 550 ~ (heating rate of 1 ~ minl), and purged in a nitrogen flow while cooling down to - 5 0 ~ TPR experiments were carried out in a 5% hydrogen/argon flow, increasing the t e m p e r a t u r e from - 5 0 ~ to 550 ~ at a rate of 10 ~ min 1.
2.3. Catalytic a c t i v i t y m e a s u r e m e n t s Oxidation reactions were carried out under atmospheric pressure in a fixed bed tubular reactor. Liquid reactants were injected into a dry, oil-free compressed air stream by a syringe pump. The flow rate through the reactor was set at 500 cm 3 min 1 and the gas hourly space velocity was maintained at 15000 h -1. Reactor effluent was analysed on line by a Hewlett Packard 5890 Series II gas chromatograph equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD). Operation conditions and reaction product analysis were described in detail elsewhere (6). 3. R E S U L T S AND D I S C U S S I O N
3.1. C h a r a c t e r i s a t i o n r e s u l t s The Si/A1 atomic ratio of the zeolite measured by XRF was found to be 27.3. The BET areas obtained from nitrogen adsorption-desorption isotherms for the H-ZSM-5 and Pd/H-ZSM-5 zeolites were 425 and 380 m 2 g-l, respectively. The content of the metal exchanged in the zeolite resulted 0,32% wt. of Pd. XRD analysis of the catalysts indicated t h a t no appreciable differences were found between Pd/H-ZSM-5 and H-ZSM-5 zeolites. TPD of a m m o n i a profiles (Figure 1) indicated t h a t the total n u m b e r of acid sites was larger for H-ZSM-5 compared to Pd/H-ZSM-5, however, the ratio of strong acid sites was higher for the containing palladium zeolite, being these 56,2% and 65,6% for H-ZSM-5 and Pd/H-ZSM-5 zeolites respectively. It is well-known t h a t calcination/reduction conditions of the catalysts have a strong influence on the final location of metal particles (7). The purpose of a slow heating ramp for the calcination step is to minimize the autoreduction of the metal and its aglomeration into larger particles (8).
850
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Temperature,~ F i g u r e 1. NHa-TPD profiles from HZSM-5 and Pd/H-ZSM-5 zeolites.
-50
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,
,
,
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Temperature,~ F i g u r e 2. TPR profile of the catalyst Pd/H-ZSM-5.
In the TPR profile (Figure 2) two zones could be clearly identified: the broad peak that appeared at the beginning of the temperature programme (0-150 ~ was due to the reduction of more accessible metal. The peak appearing at approximately 400-450 ~ was related to particles located in hidden positions, more difficult to be reduced, but the proportion of these particles was very low in comparison with the total metal content in the zeolite (9,10). The results from TPR experiments revealed that a reduction temperature of 300 ~ was necessary to activate the Pd/H-ZSM-5 catalyst.
3.2. A c t i v i t y results DCE and TCE were chosen due to their different H:C1 ratio and chemical structure. The light-off curves of the combustion of these CVOCs are shown in Figure 3. Both zeolite catalysts showed a noticeable activity for the destruction of both chlorinated compounds. It is reported that H-type zeolites exhibit high activity for oxidation of chlorinated VOCs such as DCE and TCE (11-13). It was noted that DCE was converted at significantly lower temperatures t h a n TCE. This behaviour can be attributed to the relatively large size and high electronegativity of the chlorine atom that can produce severe steric and electronic hindrances to the adsorption of chlorinated ethylene molecules (14,15). Windawi y cols, (16,17) also reported that catalytic oxidation of saturated hydrocarbons is easier than that of the u n s a t u r a t e d ones. The activity of Pd/H-ZSM-5 for DCE conversion was almost identical to that observed for H-ZSM-5, showing a slightly higher Ts0 (temperature at 50% conversion was attained) value of 280 ~ compared to 270 ~ over H-ZSM-5 and obtaining a complete conversion (>95%) with both catalysts at 350 ~
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F i g u r e 3. Light-off curves of DCE and TCE. By contrast, the metal loading led to a substantial improvement in TCE catalytic activity. Hence, T~0 decreased from 440 ~ over metal-free zeolite to 360 ~ over Pd/H-ZSM-5. It was established that strong Bronsted acidity played a major role in determining the catalytic activity of H-zeolites since these sites are believed to act as chemisorption sites for chlorocarbons (18-21). The major oxidation products during chlorinated VOCs decomposition were CO, CO2, HC1 and C12. Added to this, in the case of DCE, as soon as conversion began to be noticeable vinyl chloride was detected; however, this intermediate disappeared at elevated temperatures (>450 ~ (22). The presence of vinyl chloride indicated that the abstraction of HC1 was the first step in the reaction scheme. The formation of this intermediate was appreciably suppressed when using Pd/H-ZSM-5. The maximum concentration of vinyl chloride obtained was 735 ppm with H-ZSM-5, but when adding Pd to the zeolite the peak amount was reduced to 100 ppm. As regards TCE, trace amounts of perchloroethylene were observed as a by-product. This compound was generated by chlorination of the TCE and was partially destroyed at higher temperature (23). Pd/H-ZSM-5 zeolite led to larger quantities of this undesired by-product t h a n H-ZSM-5, since this concentration increased from 120 ppm to 355 ppm. This increase is due to the known activity of noble metals in chlorination reactions (24).
852 T a b l e 1. Selectivities towards desired by-products. DCE
H-ZSM-5 Pd/H-ZSM-5
TCE
CO2,%
HCI,%
CO2, % HC1, %
54 100
96.6 91.3
63 73
57.0 42.5
As far as CO and CO2 formation was concerned, the formation of CO2 was relatively favoured as temperature increased with both catalysts, but Pd/H-ZSM5 zeolite was more selective towards CO2 formation instead of CO, which was the major product formed over H-ZSM-5. The high activity of Pd for CO oxidation is the cause of this beneficial effect (25). When decomposing DCE, the selectivity to CO2 obtained with the metal loaded zeolite at 550 ~ was 100% but this improvement in CO2 selectivity was less noticeable with TCE, as only 73% selectivity was achieved (Table 1). The H-form zeolite showed a lower selectivity to C12 generation. On the contrary, the metal loaded zeolite considerably promoted the formation of this toxic by-product by the Deacon reaction (2HCl+89 (6,26). The selectivity towards C12 was higher for the case of TCE rather t h a n for DCE due to the H:C1 ratio greater t h a n 1. Table 1 sumarises the selectivities obtained to the desired oxidation products (CO2 and HC1) in the decomposition of the chlorinated compounds with both catalysts. It must be pointed out that the combustion of DCE over H-ZSM-5 and Pd/H-ZSM-5 was accompanied by the formation of coke due to the polymerisation of vinyl chloride (27). By contrast, no coke deposition was observed during TCE combustion with any of the catalysts. Carbon balances closed above 95-100% when decomposing TCE, but they were found to be higher than 100% at elevated temperatures in the DCE reaction due to the combustion of the coke formed during the reaction. Chlorine balance was in the range 65-85% in the oxidation of both chlorinated compounds. It is known that AI-O bonds in the zeolite framework can be easily attacked by the HC1 formed during reaction leading to the formation of volatile A1C13 which causes the partial collapse of the framework and the blockage of the porous structure (28). Furthermore, a change in colour of the Pd/H-ZSM-5 catalyst from grey to orange yellowish was observed at the end of the activity test with both compounds. This colour fitted quite well with that of metal chloride complexes (PdC12), thus the interaction of chlorine with the metal in presence of halocarbons could also explain the unfitted chlorine balance (6).
853 4. CONCLUSIONS Both catalysts, H-ZSM-5 and Pd/H-ZSM-5, studied in this work showed a high activity in the chlorinated VOCs decomposition. Pd/H-ZSM-5 zeolite was found to be most active in the oxidation of TCE, while no noticiable difference between both catalysts was noticed for DCE combustion. The main oxidation products were CO, CO2, HC1 and C12. Additionally, vinyl chloride was detected in DCE reaction, indicating that the first step in the mechanism is the dehydrochlorination of the feed. On the other hand, perchloroethylene was also observed in TCE conversion as a result of the chlorination of the feed molecule. The addition of the metal to the zeolite improved both the activity and the selectivity towards CO2, but this also involved a significant increase in undesired by-products formation such as chlorine and highly chlorinated hydrocarbons. ACKNOWLEDGEMENTS The authors whis to thank Universidad del Pals Vasco/EHU (9/UPV 0069.310-13517/2001) and Ministerio de Ciencia y Tecnologla (PPQ2001-1364) for the financial support. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
H. Muller, K. Deller, B. Despeyroux, E. Peldszus, P. Kammerhofer, W. Kuhn, R. Spielmannleitner and M. Stoger, Catal. Today 17 (1993) 383. A.R. Gavaskar, B.C. Kim, S.H. Rosansky, S.K. Ong and E.G. Marchand, Environ. Prot. 14 (1995) 33. S. Chatterje and H.L. Greene, J. Catal. 130 (1991) 76. S. Chatterjee, H.L. Greene and Y.J. Park, J. Catal. 138 (1992) 179. C. Gerard-Gomez, M. Dufaux, J. Morel, C. Naccache and Y.B.Taarit, Appl. Catal. A 165 (1997) 371. J.R. Gonz~lez-Velasco, A. Aranzabal, J.I. Guti6rrez-Ortiz and R. L6pezFonseca, Appl. Catal. B 19 (1998) 189. P. Cafiizares, A. de Lucas, F. Dorado and J. Aguirre, Microporous Mesoporous Mater. 42 (2001) 245. W.M.H. Sachtler, Catal. Today 15 (1992) 419. V.Z. Radkevich, M.F. Savchits and Y.G. Egiazarov, Russian J. Appl. Chem. 7 (1997) 759. S.T. Homeyer and W.M.H. Sachtler, J. Catal. 117 (1989) 91. M. Tajima, M. Niwa, Y. Fujii, Y. Koinuma, R. Aizawa, S. Kushiyama, S. Kobayashi, K. Mizuno and H. Ohuchi, Appl. Catal. B 9 (1996) 167. B. Ramachandran, H.L. Greene and S. Chatterjee, Appl. Catal. B 8 (1996) 157. R. L6pez-Fonseca, A. Aranzabal, J.I. Guti6rrez-Ortiz, J.I..s and J.R. Gonz~lez-Velasco, Appl. Catal B 30 (2001) 303. G.C. Bond and N. Sadeghi, J. Appl. Chem. Biotechnol. 25 (1975) 241.
854 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
P.S. Chintawar and H.L. Greene, Appl. Catal. B 13 (1997) 81. H. Windawi and M. Wyatt, Platinum Metals Rev. 37 (1993) 186. H. Windawi and Z.C. Zhang, Catal. Today 30 (1996) 99. S. Karmakar and H.L. Greene, J. Catal. 138 (1992) 364. M. Tajima, M. Niwa, Y. Fujii, Y. Koinuma, R. Aizawa, S. Kushiyama, S. Kobayashi, K. Mizuno and H. Ohuchi, Appl. Catal. B 9 (1996) 167. S. Chatterjee, H.L. Greene and Y.J. Park, J. Catal. 138 (1992) 179. R. L6pez-Fonseca, A. Aranzabal, P. Steltenpohl, J.I. Guti~rrez-Ortiz and J.R. Gonz~lez-Velasco, Catal. Today 62 (2000) 367. G. Sinquin, J.P. Hinderman, C. Petit and A. Kiennemann, Catal. Today 54 (1999) 107. H. Shaw, Y. Wang, T.C. Yu and A.E. Cerkanowicz, ACS Syrup. Ser. 495 (1992) 358. J.R. Gonz~lez-Velasco, A. Aranzabal, R. L6pez-Fonseca, R. Ferret and J.A. Gonz~lez-Marcos, Appl. Catal. B 24 (2000) 33. S. Chatterjee and H. Greene, Appl. Catal. A 98 (1993) 139. J.C. Lou and S.S. Lee, Appl. Catal. B 12 (1997) 111. S. Imamura, H. Tarumoto and S. Ishida, Ind. Eng. Chem. Res. 28 (1989) 1449. Z. Konya, I. Hannus and I. Kiricsi, Appl. Catal. B 8 (1996) 391.
847
E f f e c t o f P d a d d i t i o n on th e c a t a l y t i c p e r f o r m a n c e o f H - Z S M - 5
z e o l i t e in c h l o r i n a t e d VOCs c o m b u s t i o n R. L6pez-Fonseca, S. Cibrihn, J.I. Guti6rrez-Ortiz and J.R. Gonzhlez-Velasco* Departamento de Ingenierla Quimica, Facultad de Ciencias, Universidad del Pals Vasco/EHU, P.O. Box 644, E-48080 Bilbao, Spain. Phone: +34-946012681; Fax: +34-944648500; E-mail: [email protected] The aim of this work was to evaluate the influence of the addition of palladium on the catalytic behaviour of H-ZSM-5 zeolite in the combustion of 1,2dichloroethane (DCE) and trichloroethylene (TCE). Both catalysts showed similar activity in the oxidation of DCE, by contrast, the metal loading led to a substantial improvement in TCE combustion. Vinyl chloride was detected as an intermediate in DCE conversion, and it was appreciably suppressed when adding Pd to the zeolite. In TCE oxidation trace amounts of tetrachloethylene were identified as a by-product, Pd/H-ZSM-5 showing larger quantities of this undesired by-product. Pd/H-ZSM-5 was more selective towards CO2 formation instead of CO, which was the major carboncontaining product formed over H-ZSM-5. However, H-ZSM-5 zeolite showed a lower selectivity to C12 generation while the metal loaded zeolite considerably promoted the formation of this toxic by-product by the Deacon reaction. 1. I N T R O D U C T I O N Emissions of Volatile Organic Compounds (VOCs) into the earth's atmosphere result from naturally occurring (biogenic) as well as human-made sources. In the latter, the use of solvents is the main source of the atmospheric pollution, followed by a variety of industrial processes. One important group of VOCs consists of halogenated organic compounds, where the halogen is generally chlorine. These compounds are usually toxic, particularly through their action on the liver function, and may be carcinogenic or mutagenic. Moreover, chlorinated VOCs are implicated in the destruction of the ozone layer. Because of their harmful properties, the release of halogenated organic compounds into the environment is being controlled by increasingly stringent regulations. 1,2-Dichloroethane (DCE) is the main component of the waste stream gases from the chemical plants that produce it as an intermediate to obtain the monomer vinyl chloride, which is used for the production of polyvinylchloride (1). Trichloroethylene (TCE) is a toxic solvent widely used in dry cleaning and degreasing processes and it is also present in air stripping and soil venting remediation off-gases (2).
848 The most commonly used catalysts for the catalytic oxidation of chlorinated VOCs are alumina supported noble metals and metal oxide catalysts. Recently, zeolites have gained interest as an effective and advantageous alternative, but there are only a few investigations into these reactions over metal-modified H-zeolites (3,4). The objective of this work is to evaluate the influence of the addition of palladium to H-ZSM-5 zeolite on the catalytic behaviour for the combustion of two common chlorinated compounds, between 200-550 ~ The concentration of the chlorocarbons was set at 1000 ppm.
2. EXPERIMENTAL 2.1. Catalyst preparation The zeolite NH4-ZSM-5 (CBV 5524 G) was supplied from Zeolyst Inc. The H-ZSM-5 form was obtained by calcining the NHn-ZSM-5 zeolite in air at 550 ~ for 3 h. The preparation of Pd/H-ZSM-5 zeolite catalysts can be divided into two steps: introduction of the metal palladium and activation, the latter including pre-treatment (or the calcination in an oxygen containing atmosphere), and reduction. The preferred way of introducing a low loading of palladium into a zeolite is to exchange H § ions for positively charged palladium complexes (5). In this case the precursor salt used was [Pd(NH3)4]C12. A dilute solution of this salt, which contained an excess of 15% of the palladium, was added to a stirred HZSM-5 slurry. After stirring for 24 h at room temperature, the slurry was filtered and washed with deionized water, and the sample was then dried overnight (12 h) at 110 ~ To obtain the desired particle size of the catalyst, the sample was pelletised and sieved between 0,3-0,5 mm. The second step of the preparation was the activation of the catalyst in a glass quartz tube reactor. First, a flow of air (21% 02) of 150 ml min -1 was passed, while the temperature was increased from room temperature to 550 ~ with a heating rate of 1 ~ min -1. This final temperature was maintained for 3 h. The sample was then purged in a nitrogen flow and cooled down to 300 ~ and this temperature was maintained for 3 h in an hydrogen/nitrogen flow (1/3). Finally, the sample was cooled down to room temperature in a nitrogen flow.
2.2. Catalysts characterisation The BET surface areas of the catalysts samples were determined by nitrogen adsorption-desorption a t - 1 9 6 ~ in a Micromeritics ASAP 2010 equipment. The composition was determined using a Philips PW 1480 X-ray fluorescence (XRF) spectrometer. The X-ray powder diffraction (XRD) patterns were recorded on a Philips PW 1710 X-ray diffractometer with C u I ~ radiation. The metal content of the catalyst was measured by atomic absorption spectroscopy (AAS) in a Perkin Elmer 1100 B equipment. 2.2.a. Temperature programme desorption (TPD) of ammonia
849 T e m p e r a t u r e programme desorption (TPD) of a m m o n i a was performed on a Micromeritics AutoChem 2910 instrument. The overall process involved heating of the sample at 550 ~ in a nitrogen flow with a heating rate of 20 ~ min -1, after this the sample was cooled down to 100 ~ in a helium flow and then pulses of a m m o n i a were introduced until the sample was saturated. Finally, desorption of chemisorbed ammonia from the sample was carried out by heating from 100 ~ to 550 ~ at a rate of 10 ~ min -1. 2.2.b. T e m p e r a t u r e p r o g r a m m e d reduction (TPR) T e m p e r a t u r e p r o g r a m m e d reduction experiments were also performed in a Micromeritics AutoChem 2910 equipment. Prior to the reduction, the [Pd(NH3)4]2+/H-ZSM-5 catalyst was activated in 5% oxygen/helium flow for 1 h at 550 ~ (heating rate of 1 ~ minl), and purged in a nitrogen flow while cooling down to - 5 0 ~ TPR experiments were carried out in a 5% hydrogen/argon flow, increasing the t e m p e r a t u r e from - 5 0 ~ to 550 ~ at a rate of 10 ~ min 1.
2.3. Catalytic a c t i v i t y m e a s u r e m e n t s Oxidation reactions were carried out under atmospheric pressure in a fixed bed tubular reactor. Liquid reactants were injected into a dry, oil-free compressed air stream by a syringe pump. The flow rate through the reactor was set at 500 cm 3 min 1 and the gas hourly space velocity was maintained at 15000 h -1. Reactor effluent was analysed on line by a Hewlett Packard 5890 Series II gas chromatograph equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD). Operation conditions and reaction product analysis were described in detail elsewhere (6). 3. R E S U L T S AND D I S C U S S I O N
3.1. C h a r a c t e r i s a t i o n r e s u l t s The Si/A1 atomic ratio of the zeolite measured by XRF was found to be 27.3. The BET areas obtained from nitrogen adsorption-desorption isotherms for the H-ZSM-5 and Pd/H-ZSM-5 zeolites were 425 and 380 m 2 g-l, respectively. The content of the metal exchanged in the zeolite resulted 0,32% wt. of Pd. XRD analysis of the catalysts indicated t h a t no appreciable differences were found between Pd/H-ZSM-5 and H-ZSM-5 zeolites. TPD of a m m o n i a profiles (Figure 1) indicated t h a t the total n u m b e r of acid sites was larger for H-ZSM-5 compared to Pd/H-ZSM-5, however, the ratio of strong acid sites was higher for the containing palladium zeolite, being these 56,2% and 65,6% for H-ZSM-5 and Pd/H-ZSM-5 zeolites respectively. It is well-known t h a t calcination/reduction conditions of the catalysts have a strong influence on the final location of metal particles (7). The purpose of a slow heating ramp for the calcination step is to minimize the autoreduction of the metal and its aglomeration into larger particles (8).
850
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Temperature,~ F i g u r e 1. NHa-TPD profiles from HZSM-5 and Pd/H-ZSM-5 zeolites.
-50
i
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,
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,
,
,
350
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Temperature,~ F i g u r e 2. TPR profile of the catalyst Pd/H-ZSM-5.
In the TPR profile (Figure 2) two zones could be clearly identified: the broad peak that appeared at the beginning of the temperature programme (0-150 ~ was due to the reduction of more accessible metal. The peak appearing at approximately 400-450 ~ was related to particles located in hidden positions, more difficult to be reduced, but the proportion of these particles was very low in comparison with the total metal content in the zeolite (9,10). The results from TPR experiments revealed that a reduction temperature of 300 ~ was necessary to activate the Pd/H-ZSM-5 catalyst.
3.2. A c t i v i t y results DCE and TCE were chosen due to their different H:C1 ratio and chemical structure. The light-off curves of the combustion of these CVOCs are shown in Figure 3. Both zeolite catalysts showed a noticeable activity for the destruction of both chlorinated compounds. It is reported that H-type zeolites exhibit high activity for oxidation of chlorinated VOCs such as DCE and TCE (11-13). It was noted that DCE was converted at significantly lower temperatures t h a n TCE. This behaviour can be attributed to the relatively large size and high electronegativity of the chlorine atom that can produce severe steric and electronic hindrances to the adsorption of chlorinated ethylene molecules (14,15). Windawi y cols, (16,17) also reported that catalytic oxidation of saturated hydrocarbons is easier than that of the u n s a t u r a t e d ones. The activity of Pd/H-ZSM-5 for DCE conversion was almost identical to that observed for H-ZSM-5, showing a slightly higher Ts0 (temperature at 50% conversion was attained) value of 280 ~ compared to 270 ~ over H-ZSM-5 and obtaining a complete conversion (>95%) with both catalysts at 350 ~
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i /
/" / i I //./ 300
=
j~/
./
"ii 8
2.-- ~ =
/[
350
-400
~,,.-zs.-~
450
500
550
Temperature, ~
F i g u r e 3. Light-off curves of DCE and TCE. By contrast, the metal loading led to a substantial improvement in TCE catalytic activity. Hence, T~0 decreased from 440 ~ over metal-free zeolite to 360 ~ over Pd/H-ZSM-5. It was established that strong Bronsted acidity played a major role in determining the catalytic activity of H-zeolites since these sites are believed to act as chemisorption sites for chlorocarbons (18-21). The major oxidation products during chlorinated VOCs decomposition were CO, CO2, HC1 and C12. Added to this, in the case of DCE, as soon as conversion began to be noticeable vinyl chloride was detected; however, this intermediate disappeared at elevated temperatures (>450 ~ (22). The presence of vinyl chloride indicated that the abstraction of HC1 was the first step in the reaction scheme. The formation of this intermediate was appreciably suppressed when using Pd/H-ZSM-5. The maximum concentration of vinyl chloride obtained was 735 ppm with H-ZSM-5, but when adding Pd to the zeolite the peak amount was reduced to 100 ppm. As regards TCE, trace amounts of perchloroethylene were observed as a by-product. This compound was generated by chlorination of the TCE and was partially destroyed at higher temperature (23). Pd/H-ZSM-5 zeolite led to larger quantities of this undesired by-product t h a n H-ZSM-5, since this concentration increased from 120 ppm to 355 ppm. This increase is due to the known activity of noble metals in chlorination reactions (24).
852 T a b l e 1. Selectivities towards desired by-products. DCE
H-ZSM-5 Pd/H-ZSM-5
TCE
CO2,%
HCI,%
CO2, % HC1, %
54 100
96.6 91.3
63 73
57.0 42.5
As far as CO and CO2 formation was concerned, the formation of CO2 was relatively favoured as temperature increased with both catalysts, but Pd/H-ZSM5 zeolite was more selective towards CO2 formation instead of CO, which was the major product formed over H-ZSM-5. The high activity of Pd for CO oxidation is the cause of this beneficial effect (25). When decomposing DCE, the selectivity to CO2 obtained with the metal loaded zeolite at 550 ~ was 100% but this improvement in CO2 selectivity was less noticeable with TCE, as only 73% selectivity was achieved (Table 1). The H-form zeolite showed a lower selectivity to C12 generation. On the contrary, the metal loaded zeolite considerably promoted the formation of this toxic by-product by the Deacon reaction (2HCl+89 (6,26). The selectivity towards C12 was higher for the case of TCE rather t h a n for DCE due to the H:C1 ratio greater t h a n 1. Table 1 sumarises the selectivities obtained to the desired oxidation products (CO2 and HC1) in the decomposition of the chlorinated compounds with both catalysts. It must be pointed out that the combustion of DCE over H-ZSM-5 and Pd/H-ZSM-5 was accompanied by the formation of coke due to the polymerisation of vinyl chloride (27). By contrast, no coke deposition was observed during TCE combustion with any of the catalysts. Carbon balances closed above 95-100% when decomposing TCE, but they were found to be higher than 100% at elevated temperatures in the DCE reaction due to the combustion of the coke formed during the reaction. Chlorine balance was in the range 65-85% in the oxidation of both chlorinated compounds. It is known that AI-O bonds in the zeolite framework can be easily attacked by the HC1 formed during reaction leading to the formation of volatile A1C13 which causes the partial collapse of the framework and the blockage of the porous structure (28). Furthermore, a change in colour of the Pd/H-ZSM-5 catalyst from grey to orange yellowish was observed at the end of the activity test with both compounds. This colour fitted quite well with that of metal chloride complexes (PdC12), thus the interaction of chlorine with the metal in presence of halocarbons could also explain the unfitted chlorine balance (6).
853 4. CONCLUSIONS Both catalysts, H-ZSM-5 and Pd/H-ZSM-5, studied in this work showed a high activity in the chlorinated VOCs decomposition. Pd/H-ZSM-5 zeolite was found to be most active in the oxidation of TCE, while no noticiable difference between both catalysts was noticed for DCE combustion. The main oxidation products were CO, CO2, HC1 and C12. Additionally, vinyl chloride was detected in DCE reaction, indicating that the first step in the mechanism is the dehydrochlorination of the feed. On the other hand, perchloroethylene was also observed in TCE conversion as a result of the chlorination of the feed molecule. The addition of the metal to the zeolite improved both the activity and the selectivity towards CO2, but this also involved a significant increase in undesired by-products formation such as chlorine and highly chlorinated hydrocarbons. ACKNOWLEDGEMENTS The authors whis to thank Universidad del Pals Vasco/EHU (9/UPV 0069.310-13517/2001) and Ministerio de Ciencia y Tecnologla (PPQ2001-1364) for the financial support. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
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