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Zeolite-catalyzed low-temperature combustion of ecologically harmful chlorobenzene
H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.)
Catalysis by Microporous Materials Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.
627
Zeolite-catalyzed Low-temperature Combustion of Ecologically Hannfid Chlorobenzene L. Becker, U. Hatje and H. Frrster Institute of Physical Chemistry, University of Hamburg, D-20146 Hamburg, Germany
The low-temperature combustion of chlorobenzene over Cu- and Pd-containing faujasites has been investigated applying reactor experiments and X-ray absorption spectroscopy. Although being active catalysts, the samples revealed deactivation, which could be protracted by increasing the Pd content and protonation. Formation of an intermediate Pd-C1 complex can be proven for the unprotonated catalysts but does not seem to be responsible for deactivation. Protonation also reduces the chlorine incorporation into the catalyst and stabilizes the oxidation state of Pd. Chlorine can be removed by hydrogen treatment which simultaneously gives rise to the formation of small palladium clusters.
1. INTRODUCTION Halogenated aromatics such as chlorobenzene are most frequently applied as solvents and source materials for the synthesis of a number of valuable chemical compounds so that at present a renouncement of them does not seem possible. On the other hand, their toxicity and carcinogenic potential have raised public awareness to find a suitable way for annihilation of the residues of these harmful materials. In the case of low contaminant concentration where a recycling does not pay, stripping and combustion at very high temperatures are the preferred method of disposal. But the avoidance of the much more toxic dioxins formed as by-products and the inefficiency of this process have called for the development of alternate low-temperature processes committed to the use of catalysts. Therefore, the design of catalysts suitable for decomposition of halogenated waste has become a task of high importance and calls for more fundamental research. For this purpose the application of reactor experiments combined with standard EXAFS (Emended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure) [1] as well as time-resolved m situ DEXAFS (Dispersive EXAFS) [ 1-3] studies on Pd-exchanged zeolites Y seemed to be useful tools for finding potential catalysts.
2. EXPERIMENTAL
Transition metal ion-exchanged faujasites CuY, PdY and HPdY (metal content in weight% indicated as suffix), activated at 623 K (CuY) or 723 K (PdY, HPdY), respectively, in an oxygen flow of ca. 20 ml/min were used as catalysts. Details about preparation of the zeolite samples as well as experimental procedures are reported elsewhere [4]. The home-made flow apparatus was equipped with an analytical tube for chlorine detection from Draeger, Ltibeck,
628 (detection limit 0.2ppm) and two wash-bottles each containing 200 ml of bidistilled water, working as HC1 absorption traps. Thus, the overall pressure within the flow apparatus was slightly above atmospheric. Generally, a catalytic run lasted 6 h. The concentration of the HC1 liberated in that period of time was determined by conventional volumetric analysis. For the detection of organic by-products a HRC~/MS combination from Hewlett Packard was used (column = 50 m CP-SiI8CB; linear velocity = 35cm/s; sample volume -- 1 1; injection: 0.5 min splitless; injector temperature = 523 K). For separation and identification of the products, a temperature program had been worked out: Initial temperature = 333 K (3 minutes); heating rate = 6 K/min; final temperature = 573 K. For the XAS experiments at the CI K- and the Pd Lm-edges samples were taken out of the reactor, ground and pressed into polyethylene. The XAS spectra at these absorption edges were recorded at beamline E4 at HASYLAB. Formation, interaction as well as reaction of chloro- and bromobenzene on Pd zeolites have been studied in situ in a continuous flow reactor at the Pd and Br K-edges using the ROMO II and DEXAFS stations at beamline X at HASYLAB. The experimental set-up of this beamline as well as the reactor and gas flow set-up is described elsewhere [5].
3. RESULTS AND DISCUSSION
Initial runs using Cu-containing Y zeolites yielded remarkable conversion levels (T = 523-623 K; residence time = 1-6 s; concentration c = 23 or 37 mg/1, respectively). During these runs, however, we observed deactivation of the catalyst and deposition of crystalline by-products at the exit of the fixed bed reactor. These deposits were identified as congeners of polychlorinated benzenes by means of HRC~/MS. The phenomenon of transchlorination is also known in connection with the thermal decomposition of 1,2-dichlorobenzene [6,7]. As we confirmed for catalytic experiments with unchlorinated aromatics carried out in our laboratory [8], no oxygenated products were released from the catalyst surface even in the case of chlorobenzene 18[~'" c]l]orobenzenemput. . . . ~
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Figure 1. Deactivation during CBz oxidation Figure 2. Conversion of chlorobenzene on (a) over PdY-14.6 at low reaction temperature PdY-14.6, (b) HPdY-2.0, (c) PdY-7.6, (d) (523 K); T(cat)=523 K; m(cat)=2.4 g; PdY-2.0; T(cat)=573 K; m(cat)-2.2-2.4 g; V(cat)=3 ml; F-120 ml/min; =1.5 s; c=19 V(cat)=3 ml; F-120 ml/min; =1.5 s; c=23 mg/1. mg/l.
629 (CBz). At low temperatures we observed deactivation also for PdY (see Fig. 1). However, compared to CuY a decisively decreased tendency for deactivation and formation of crystalline deposits could be established. Thus, further studies were focussed on the suppression of these undesired side-reaction (transchlorination) and deactivation processes on PdY samples. Conversion plots in Fig. 2 give a hint for a correlation between Pd content and the period of time until deactivation sets in. Under the adjusted standard reaction conditions PdY-2.0 deactivates already after 60 min time on stream (TOS). Increasing the Pd concentration in the zeolite matrix up to 7.6 wt.%, we found the active interval being prolonged to about 260 min. For a PdY-14.6 sample at least no deactivation occurred within a space of 360 min. It seems noteworthy that, obviously, protonation protects it from poisoning. HPdY-2.0 (Fig. 2b) shows a similar behaviour towards deactivation like a proton-free Y zeolite with a much higher Pd content (PdY-7.6 in Fig. 2c). Noble metal catalysts are known for their propensity to poisoning by chlorine released in the reaction. For enlightening the deactivation process we studied selected samples at the Pd Lmand the C1 K-edges. Due to the low photon energy (2.8-3.2 keV) these experiments cannot be carried out in situ. The amount of chlorine incorporated in the different samples can easily be calculated from the edge jump of the chlorine K-edge. Table 1 summarises the results of four different samples for 15 minutes or one hour on stream and after deactivation. Table 1 Incorporated chlorine of different catalysts for the indicated times on stream during the reaction of chlorobenzene with air after 15 min. after l h after deactivation sample metal contents massc~/masssample masscl/maSSsample massc~/mass=mole [mg/g] [mg/g] [mg/$] PdY 0.5 0.5 wt. % 150 240 324 PdY 4.0 4.0 wt. % 203 258 278 HPdY 0.5 0.5 wt % 115 214 241 HPdY 2.2 2.2 wt. % 20 34 51 In the case of the unprotonated PdY samples (PdY-0.5 and PdY-4.0) there is only a small difference in chlorine uptake. The protonated HPdY-2.2, however, reveals a significant lower (7 to 10 times) incorporation of chlorine in comparison to the unprotonated samples. This might be an explanation for the above mentioned behaviour of the HPd samples towards a far longer lasting activity compared to the unprotonated samples of the same metal content. The protonated and the unprotonated samples also differ with regard to the oxidation state of the palladium. This can be derived from the XANES of the Pd Lm-edge (Fig. 3), as oxidized palladium shows a so-called white line, i.e., a strong absorption precisely at the absorption edge due to unoccupied d-orbitals, whereas metallic palladium only reveals a small white line. While the HPdY samples remain in an oxidized palladium state for at least one hour time on stream, the proton-free PdY samples undergo an almost complete reduction of the palladium during the first 15 minutes. The XANES as well as the EXAFS at the chlorine K-edge give indications for a palladiumchlorine interaction. If a Pd-C1 complex is formed, a so-called pre-edge peak, i.e., an absorption prior to the chlorine edge itself, becomes visible. While chlorobenzene itself shows no absorption prior to ionization, a pre-edge peak at 2.822 keV appears during adsorption of chlorobenzene on PdY, indicating an 1s---~c* transition of Pd-Cl interaction [9,10].
630
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HPdY 15 min. time on stream
PdY 15 min. time on stream
HPdY or PdY after deactivation
11
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Figure 3. XANES of PdY and HPdY after 15 minutes time on stream and after deactivation.
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2.81
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2.87
2.88
2.89
Figure 4. C1 K-edge XANES of different catalysts after 15 min time on stream and after deactivation.
631 The XANES of the CI K-edge of different samples after 15 min. time on stream and after deactivation are shown in Fig. 4. A direct Pd-C1 interaction is perceptible only for the PdY-0.5 and 4.0 after 15 min TOS by the occurrence of the pre-edge peak, while it is missing for the protonated HPdY sample, thus disproving direct Pd-C1 coordination. After deactivation neither of the catalysts show a direct interaction of chlorine with Pd. This means that no formation of a palladium chloro complex takes place which might lead to deactivation. The possible generation of NaCI (in the case of PdY samples) can also be excluded. Therefore, we assume the formation of chlorinated coke being responsible for the deactivation. This also explains the temperature dependence of the deactivation [4] as well as the later shown possibility of reactivation by hydrogen at room temperature. i
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Fig. 5. FT[~(k)-k] at the CI K-edge of PdY-4.0 (left) and protonated HPdY-2.2 (fight) during reaction with chlorobenzene. For comparison the radial distribution functions of chlorobenzene and PdC12 are included. The Pd-C1 interaction of the PdY-4.0 catalyst during reaction is further demonstrated by the Fourier transform of the EXAFS at the C1 K-edge seen in Figure 5 (left): The shell at 2.3/~ can be ascribed to a Pd backscatterer (refer to the PdC12 spectrum given for reference). This shell emerges after 15 minutes time on stream and vanishes after deactivation. The shell at 1.2 A represents backscattering from the carbon-nearest neighbors as in chlorobenzene, the other shells at 2/1~ and near 3/~ are either C1-C interactions as well or C1-C1 interactions visible in highly chlorinated aromatic systems. In case of the protonated samples (HPdY-2.2, Fig. 5, right.) a palladium shell is discernible neither during reaction nor after deactivation. Altogether this is in good correlation with the C1-K-edge XANES results mentioned above. The interaction of chlorobenzene with the catalyst PdY-4.0 has also been studied in situ at the palladium K-edge. The emergence of a chlorine shell at 1.9A after one hour reaction time can be seen in Figure 6. Part of the chlorine could be removed by hydrogen treatment at approximately 50-80 ~ under development of hydrogen chloride, and further chlorine was removed by an oxidative treatment at 450~ in air. The hydrogen treatment leads to palladium clusters (shell at 2.25 A) with a coordination number being about 3A of that of bulk palladium, i.e., of a cluster size somewhat like 13 A. By subsequent reoxidation the clusters could only partly be reoxidized.
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Figure 7. Effect of metal loading and protonation on the bromine uptake of three PdY catalysts.
633 As it seemed impossible to carry out in situ DEXAFS studies at the CI-K edge due to its low energy, we applied bromobenzene in exchange for chlorobenzene. The uptake of bromine during combustion of bromobenzene with air and interrupted by 5 and 3 min conducting pure air over the catalysts (Fig. 7) monitored from the viewpoint of the halogen gives no indication for a steady state. The bromine concentration only drops slightly during admission of air and rises again after switching back to the bromobenzene/air mixture. The bromine concentration in PdY-4.0 is largest, followed by about 60% in case of PdY-0.5 and 30% for the protonated HPdY-0.5, i.e. bromine incorporation is lowered with decreasing palladium content and to a further extent by protonation. As for chlorobenzene, it was possible to remove part of the bromine with hydrogen at room temperature.
4. CONCLUSIONS Although at first being catalytically active in chlorobenzene combustion zeolite CuY disclosed a rapid deactivation under simultaneous formation of polychlorinated by-products. Both tendencies were less pronounced with zeolites PdY. Increasing the noble metal content as well as protonation prolonged the active interval. Protonation also significantly reduced the chlorine incorporation into the catalyst and ensures an oxidized palladium state to remain while the proton-flee samples undergo almost complete reduction to metallic state. At the beginning of the reaction for the unprotonated catalysts a direct Pd-C1 interaction is discernible which vanishes after deactivation so that no direct blocking of the noble metal by chlorine seems to be responsible for poisoning. In the case of protonated zeolites a Pd-CI coordination has not been observed at all. As a hypothesis for deactivation, the formation of a chlorinated coke has to be assumed. Hydrogen treatment removes the chlorine and gives rise to the formation of palladium clusters of ca. 13 ~ diameter, which could be only partly reoxidized. The halogen uptake was studied time-resolved applying bromobenzene in exchange for chlorobenzene and proved to be lowered with declining palladium concentration and to a further extent by protonation.
ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support by Deutsche Bundesstiftung Umwelt.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
H. Bertagnolli and T.S. Ertel, Angew. Chem., 106 (1994) 15. M. Hagelstein, S. Cunis, R. Frahm, W. Niemann and P. Rabe, Physica B, 158 (1989) 324. M. Hagelstein, PhD-Thesis, 1991. L. Becker and H. FOrster, in preparation. H. FOrster, M. Hagelstein, U. Hatje, W. Metz, and T. ReBler, J. Mol. Struct., in press. M. Kluwe, B. Kalmann, K.E. Lorber and H. Meier zu K6cker, Organohalogen Compounds, 30 (1990) 97. C.M. Young and K.J. Voorhees, Organohalogen Compounds, 3 (1990) 203. L. Becker and H. FOrster, in preparation.
634 9. C. Sugiura, M. Kitamura and S. Muramatsu, J. Chem. Phys., 85 (1986) 5269. 10. U. Hatje, M. Hagelstein and H. F6rster, in 'Zeolites and Related Microporous Materials: State of the Art 1994, Proceedings of the 10th International Zeolite Conference, Garmisch-Partenkirchen, 1994', J. Weitkamp et al. (eds.), Stud. Surf. Sci. Catal., 84, Elsevier, Amsterdam, 1994, pp. 773-780.
Catalysis by Microporous Materials Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.
627
Zeolite-catalyzed Low-temperature Combustion of Ecologically Hannfid Chlorobenzene L. Becker, U. Hatje and H. Frrster Institute of Physical Chemistry, University of Hamburg, D-20146 Hamburg, Germany
The low-temperature combustion of chlorobenzene over Cu- and Pd-containing faujasites has been investigated applying reactor experiments and X-ray absorption spectroscopy. Although being active catalysts, the samples revealed deactivation, which could be protracted by increasing the Pd content and protonation. Formation of an intermediate Pd-C1 complex can be proven for the unprotonated catalysts but does not seem to be responsible for deactivation. Protonation also reduces the chlorine incorporation into the catalyst and stabilizes the oxidation state of Pd. Chlorine can be removed by hydrogen treatment which simultaneously gives rise to the formation of small palladium clusters.
1. INTRODUCTION Halogenated aromatics such as chlorobenzene are most frequently applied as solvents and source materials for the synthesis of a number of valuable chemical compounds so that at present a renouncement of them does not seem possible. On the other hand, their toxicity and carcinogenic potential have raised public awareness to find a suitable way for annihilation of the residues of these harmful materials. In the case of low contaminant concentration where a recycling does not pay, stripping and combustion at very high temperatures are the preferred method of disposal. But the avoidance of the much more toxic dioxins formed as by-products and the inefficiency of this process have called for the development of alternate low-temperature processes committed to the use of catalysts. Therefore, the design of catalysts suitable for decomposition of halogenated waste has become a task of high importance and calls for more fundamental research. For this purpose the application of reactor experiments combined with standard EXAFS (Emended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure) [1] as well as time-resolved m situ DEXAFS (Dispersive EXAFS) [ 1-3] studies on Pd-exchanged zeolites Y seemed to be useful tools for finding potential catalysts.
2. EXPERIMENTAL
Transition metal ion-exchanged faujasites CuY, PdY and HPdY (metal content in weight% indicated as suffix), activated at 623 K (CuY) or 723 K (PdY, HPdY), respectively, in an oxygen flow of ca. 20 ml/min were used as catalysts. Details about preparation of the zeolite samples as well as experimental procedures are reported elsewhere [4]. The home-made flow apparatus was equipped with an analytical tube for chlorine detection from Draeger, Ltibeck,
628 (detection limit 0.2ppm) and two wash-bottles each containing 200 ml of bidistilled water, working as HC1 absorption traps. Thus, the overall pressure within the flow apparatus was slightly above atmospheric. Generally, a catalytic run lasted 6 h. The concentration of the HC1 liberated in that period of time was determined by conventional volumetric analysis. For the detection of organic by-products a HRC~/MS combination from Hewlett Packard was used (column = 50 m CP-SiI8CB; linear velocity = 35cm/s; sample volume -- 1 1; injection: 0.5 min splitless; injector temperature = 523 K). For separation and identification of the products, a temperature program had been worked out: Initial temperature = 333 K (3 minutes); heating rate = 6 K/min; final temperature = 573 K. For the XAS experiments at the CI K- and the Pd Lm-edges samples were taken out of the reactor, ground and pressed into polyethylene. The XAS spectra at these absorption edges were recorded at beamline E4 at HASYLAB. Formation, interaction as well as reaction of chloro- and bromobenzene on Pd zeolites have been studied in situ in a continuous flow reactor at the Pd and Br K-edges using the ROMO II and DEXAFS stations at beamline X at HASYLAB. The experimental set-up of this beamline as well as the reactor and gas flow set-up is described elsewhere [5].
3. RESULTS AND DISCUSSION
Initial runs using Cu-containing Y zeolites yielded remarkable conversion levels (T = 523-623 K; residence time = 1-6 s; concentration c = 23 or 37 mg/1, respectively). During these runs, however, we observed deactivation of the catalyst and deposition of crystalline by-products at the exit of the fixed bed reactor. These deposits were identified as congeners of polychlorinated benzenes by means of HRC~/MS. The phenomenon of transchlorination is also known in connection with the thermal decomposition of 1,2-dichlorobenzene [6,7]. As we confirmed for catalytic experiments with unchlorinated aromatics carried out in our laboratory [8], no oxygenated products were released from the catalyst surface even in the case of chlorobenzene 18[~'" c]l]orobenzenemput. . . . ~
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Figure 1. Deactivation during CBz oxidation Figure 2. Conversion of chlorobenzene on (a) over PdY-14.6 at low reaction temperature PdY-14.6, (b) HPdY-2.0, (c) PdY-7.6, (d) (523 K); T(cat)=523 K; m(cat)=2.4 g; PdY-2.0; T(cat)=573 K; m(cat)-2.2-2.4 g; V(cat)=3 ml; F-120 ml/min; =1.5 s; c=19 V(cat)=3 ml; F-120 ml/min; =1.5 s; c=23 mg/1. mg/l.
629 (CBz). At low temperatures we observed deactivation also for PdY (see Fig. 1). However, compared to CuY a decisively decreased tendency for deactivation and formation of crystalline deposits could be established. Thus, further studies were focussed on the suppression of these undesired side-reaction (transchlorination) and deactivation processes on PdY samples. Conversion plots in Fig. 2 give a hint for a correlation between Pd content and the period of time until deactivation sets in. Under the adjusted standard reaction conditions PdY-2.0 deactivates already after 60 min time on stream (TOS). Increasing the Pd concentration in the zeolite matrix up to 7.6 wt.%, we found the active interval being prolonged to about 260 min. For a PdY-14.6 sample at least no deactivation occurred within a space of 360 min. It seems noteworthy that, obviously, protonation protects it from poisoning. HPdY-2.0 (Fig. 2b) shows a similar behaviour towards deactivation like a proton-free Y zeolite with a much higher Pd content (PdY-7.6 in Fig. 2c). Noble metal catalysts are known for their propensity to poisoning by chlorine released in the reaction. For enlightening the deactivation process we studied selected samples at the Pd Lmand the C1 K-edges. Due to the low photon energy (2.8-3.2 keV) these experiments cannot be carried out in situ. The amount of chlorine incorporated in the different samples can easily be calculated from the edge jump of the chlorine K-edge. Table 1 summarises the results of four different samples for 15 minutes or one hour on stream and after deactivation. Table 1 Incorporated chlorine of different catalysts for the indicated times on stream during the reaction of chlorobenzene with air after 15 min. after l h after deactivation sample metal contents massc~/masssample masscl/maSSsample massc~/mass=mole [mg/g] [mg/g] [mg/$] PdY 0.5 0.5 wt. % 150 240 324 PdY 4.0 4.0 wt. % 203 258 278 HPdY 0.5 0.5 wt % 115 214 241 HPdY 2.2 2.2 wt. % 20 34 51 In the case of the unprotonated PdY samples (PdY-0.5 and PdY-4.0) there is only a small difference in chlorine uptake. The protonated HPdY-2.2, however, reveals a significant lower (7 to 10 times) incorporation of chlorine in comparison to the unprotonated samples. This might be an explanation for the above mentioned behaviour of the HPd samples towards a far longer lasting activity compared to the unprotonated samples of the same metal content. The protonated and the unprotonated samples also differ with regard to the oxidation state of the palladium. This can be derived from the XANES of the Pd Lm-edge (Fig. 3), as oxidized palladium shows a so-called white line, i.e., a strong absorption precisely at the absorption edge due to unoccupied d-orbitals, whereas metallic palladium only reveals a small white line. While the HPdY samples remain in an oxidized palladium state for at least one hour time on stream, the proton-free PdY samples undergo an almost complete reduction of the palladium during the first 15 minutes. The XANES as well as the EXAFS at the chlorine K-edge give indications for a palladiumchlorine interaction. If a Pd-C1 complex is formed, a so-called pre-edge peak, i.e., an absorption prior to the chlorine edge itself, becomes visible. While chlorobenzene itself shows no absorption prior to ionization, a pre-edge peak at 2.822 keV appears during adsorption of chlorobenzene on PdY, indicating an 1s---~c* transition of Pd-Cl interaction [9,10].
630
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PdY 15 min. time on stream
HPdY or PdY after deactivation
11
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l 3.22
3.24
Figure 3. XANES of PdY and HPdY after 15 minutes time on stream and after deactivation.
6.25
5
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9
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2.81
2.82
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2.87
2.88
2.89
Figure 4. C1 K-edge XANES of different catalysts after 15 min time on stream and after deactivation.
631 The XANES of the CI K-edge of different samples after 15 min. time on stream and after deactivation are shown in Fig. 4. A direct Pd-C1 interaction is perceptible only for the PdY-0.5 and 4.0 after 15 min TOS by the occurrence of the pre-edge peak, while it is missing for the protonated HPdY sample, thus disproving direct Pd-C1 coordination. After deactivation neither of the catalysts show a direct interaction of chlorine with Pd. This means that no formation of a palladium chloro complex takes place which might lead to deactivation. The possible generation of NaCI (in the case of PdY samples) can also be excluded. Therefore, we assume the formation of chlorinated coke being responsible for the deactivation. This also explains the temperature dependence of the deactivation [4] as well as the later shown possibility of reactivation by hydrogen at room temperature. i
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6
distance R [A]
Fig. 5. FT[~(k)-k] at the CI K-edge of PdY-4.0 (left) and protonated HPdY-2.2 (fight) during reaction with chlorobenzene. For comparison the radial distribution functions of chlorobenzene and PdC12 are included. The Pd-C1 interaction of the PdY-4.0 catalyst during reaction is further demonstrated by the Fourier transform of the EXAFS at the C1 K-edge seen in Figure 5 (left): The shell at 2.3/~ can be ascribed to a Pd backscatterer (refer to the PdC12 spectrum given for reference). This shell emerges after 15 minutes time on stream and vanishes after deactivation. The shell at 1.2 A represents backscattering from the carbon-nearest neighbors as in chlorobenzene, the other shells at 2/1~ and near 3/~ are either C1-C interactions as well or C1-C1 interactions visible in highly chlorinated aromatic systems. In case of the protonated samples (HPdY-2.2, Fig. 5, right.) a palladium shell is discernible neither during reaction nor after deactivation. Altogether this is in good correlation with the C1-K-edge XANES results mentioned above. The interaction of chlorobenzene with the catalyst PdY-4.0 has also been studied in situ at the palladium K-edge. The emergence of a chlorine shell at 1.9A after one hour reaction time can be seen in Figure 6. Part of the chlorine could be removed by hydrogen treatment at approximately 50-80 ~ under development of hydrogen chloride, and further chlorine was removed by an oxidative treatment at 450~ in air. The hydrogen treatment leads to palladium clusters (shell at 2.25 A) with a coordination number being about 3A of that of bulk palladium, i.e., of a cluster size somewhat like 13 A. By subsequent reoxidation the clusters could only partly be reoxidized.
632 0.12
t
, ~ ,
I
,
,
,
,
,
,
,
,
,
I
,
,
,
,
I
,
,
,
,~
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~0.08
,~"-CII I / ~/'/
006
~ a~~er H2 at RT
i
w.
0.04 0.02 i i i I l'l
0
i Ill
1
i I i I
2
i I II
3 R[A]4 distance
5
6
IIII
7
Figure 6. FT[~(k).k2] of the Pd K-edge EXAFS of PdY 4.0 during reaction with chlorobenzene.
0.1
0.05
0 0
5
10 15 time[mini
20
25
Figure 7. Effect of metal loading and protonation on the bromine uptake of three PdY catalysts.
633 As it seemed impossible to carry out in situ DEXAFS studies at the CI-K edge due to its low energy, we applied bromobenzene in exchange for chlorobenzene. The uptake of bromine during combustion of bromobenzene with air and interrupted by 5 and 3 min conducting pure air over the catalysts (Fig. 7) monitored from the viewpoint of the halogen gives no indication for a steady state. The bromine concentration only drops slightly during admission of air and rises again after switching back to the bromobenzene/air mixture. The bromine concentration in PdY-4.0 is largest, followed by about 60% in case of PdY-0.5 and 30% for the protonated HPdY-0.5, i.e. bromine incorporation is lowered with decreasing palladium content and to a further extent by protonation. As for chlorobenzene, it was possible to remove part of the bromine with hydrogen at room temperature.
4. CONCLUSIONS Although at first being catalytically active in chlorobenzene combustion zeolite CuY disclosed a rapid deactivation under simultaneous formation of polychlorinated by-products. Both tendencies were less pronounced with zeolites PdY. Increasing the noble metal content as well as protonation prolonged the active interval. Protonation also significantly reduced the chlorine incorporation into the catalyst and ensures an oxidized palladium state to remain while the proton-flee samples undergo almost complete reduction to metallic state. At the beginning of the reaction for the unprotonated catalysts a direct Pd-C1 interaction is discernible which vanishes after deactivation so that no direct blocking of the noble metal by chlorine seems to be responsible for poisoning. In the case of protonated zeolites a Pd-CI coordination has not been observed at all. As a hypothesis for deactivation, the formation of a chlorinated coke has to be assumed. Hydrogen treatment removes the chlorine and gives rise to the formation of palladium clusters of ca. 13 ~ diameter, which could be only partly reoxidized. The halogen uptake was studied time-resolved applying bromobenzene in exchange for chlorobenzene and proved to be lowered with declining palladium concentration and to a further extent by protonation.
ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support by Deutsche Bundesstiftung Umwelt.
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H. Bertagnolli and T.S. Ertel, Angew. Chem., 106 (1994) 15. M. Hagelstein, S. Cunis, R. Frahm, W. Niemann and P. Rabe, Physica B, 158 (1989) 324. M. Hagelstein, PhD-Thesis, 1991. L. Becker and H. FOrster, in preparation. H. FOrster, M. Hagelstein, U. Hatje, W. Metz, and T. ReBler, J. Mol. Struct., in press. M. Kluwe, B. Kalmann, K.E. Lorber and H. Meier zu K6cker, Organohalogen Compounds, 30 (1990) 97. C.M. Young and K.J. Voorhees, Organohalogen Compounds, 3 (1990) 203. L. Becker and H. FOrster, in preparation.
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