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Oligomerization as an important step and side reaction for skeletal isomerization of linear butenes on H-ZSM-5
Applied Catalysis A: General 255 (2003) 349–354
Oligomerization as an important step and side reaction for skeletal isomerization of linear butenes on H-ZSM-5 Olaf Klepel a,∗ , Andrei Loubentsov a , Winfried Böhlmann b , Helmut Papp a b
a Fakultät für Chemie und Mineralogie, Universität Leipzig, Linnéstr. 3, D-04103 Leipzig, Germany Fakultät für Physik und Geowissenschaften, Universität Leipzig, Linnéstr. 5, D-04103 Leipzig, Germany
Received 3 April 2003; received in revised form 16 July 2003; accepted 17 July 2003
Abstract The skeletal isomerization of n-butenes on a H-ZSM-5 zeolite with high Si/Al ratio was investigated by using a gradientless recirculation batch reactor. It was concluded that at the very beginning of the reaction, the formation of C8 surface species should be the first step. These species decompose into propene and pentenes, one part of the just formed and still adsorbed C3 species immediately dimerizes to hexenes. In the further course of reaction, an oligomerization to C12 surface species (trimerization) occurs followed by decomposition as shown by the products pentenes, hexenes, and heptenes. The tendency to oligomerization decreased with increasing temperature. For the reaction of lower olefins like propene and ethene, the reaction course was the same as it was observed for n-butene: a gradual increase of the degree of oligomerization with reaction time followed by immediate decomposition. © 2003 Elsevier B.V. All rights reserved. Keywords: Butene; Isomerization; Gradientless recirculation batch reactor
1. Introduction In the last decade, the demand for isobutene overtook the rate of production because of its increasing importance as raw material for the production of methyl tert-butyl ether (MTBE). MTBE is used as a fuel additive. Because of the limited capacity of the traditional cracking processes, many efforts have been undertaken to develop new catalyst systems for skeletal isomerization of n-butenes to isobutene. Among other catalysts like modified alumina [1,2], a number of 10-membered ring molecular sieves [3–17] have been proven to be efficient catalysts for this reaction. Especially, ferrierite has been described ∗ Corresponding author. Tel.: +49-341-9736321; fax: +49-341-9736349. E-mail address: [email protected] (O. Klepel).
as a very selective catalyst for skeletal isomerization. To explain the high selectivity an intramolecular isomerization, the so-called monomolecular mechanism, and a pseudomolecular mechanism with carbenium ions as active sites have been proposed [3–11,16]. On the other side, the selectivity obtained with H-ZSM-5 was remarkably lower [3–15]. On H-ZSM-5, dimerization [5,7,9,18–20] or even trimerization [8,20] of n-butene takes place in the cavities at the intersections of the channels. This oligomerization is followed by non-selective cracking into short-chained olefins, among them isobutene. Because of its feature, this mechanism is called bimolecular or oligomerization–cracking mechanism. The aim of our work was to elucidate the reaction behavior of olefins on H-ZSM-5 concerning oligomerization and consecutive reactions by using a gradientless recirculation batch reactor. This reactor allows a
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00589-1
O. Klepel et al. / Applied Catalysis A: General 255 (2003) 349–354
time-resolved observation of the reaction course and thus the investigation of consecutive reactions. 2. Experimental 2.1. Synthesis and characterization H-ZSM-5 with a Si/Al ratio of 90 was synthesized according to the following procedure [21,22]. A solution of 5.11 g tetrapropylammonium bromide (Aldrich) in 9.92 g of distilled water was added to 10.62 g of colloidal SiO2 (Ludox-AS 40, DuPont) and stirred for 15 min at room temperature. As aluminum source, 0.16 g aluminum triisopropylate (Aldrich) was dissolved in 24.75 ml of ammonia (25 wt.%) and added to the gel which was vigorously stirred for 5 min. The fluid gel was heated at 453 K in a Teflon-lined stainless steel autoclave for 7 days under autogenous pressure. After this procedure, the reaction product was washed, filtered and dried for 6 h at 90 ◦ C. To remove the organic template, the zeolite material was calcined for 12 h at 873 K. The obtained product was characterized by XRD and 27 Al and 29 Si MAS NMR spectroscopy [23]. 2.2. Catalysis
3. Results Figs. 1–3 show the product distribution of the conversion of 1-butene on H-ZSM-5 at different temperatures in the first 7 min of reaction time. In all cases, the main products were C7 , C6 , C5 and C3 alkenes and isobutene. Alkanes were not observed. The total conversion of n-butenes was between 60 and 70% (carbon atom based) and nearly independent of the reaction temperature. The main products were always pentenes, followed by hexenes at the lowest investigated reaction temperature of 548 K. However, at 598 K, the yield of hexenes decreased and was similar to that of isobutene and propene. A further decreasing of the yields of hexenes was observed at a temperature of 673 K, while the main products pentenes were followed by isobutene. The concentration of heptenes decreased also with increasing reaction temperature. At 548 K, after a reaction time of about 2 min, the amount of propene remained constant within the reaction time but the concentration of pentenes, hexenes and heptenes increased equally. At 673 K, besides pentenes and hexenes the yields of isobutene and
25
100
548 K i-C4 C3 C5 C6 C7
90 80
n-But
20
15
70 10
60 50
5
concentration products / %
Catalytic experiments were performed at temperatures from 548 to 673 K in a gradientless recirculation batch reactor at atmospheric pressure. The reactor volume was 236 ml. A mixture of 5% butene (99.0%, AGA) in nitrogen was recirculated by a brennschede-pump with a velocity of about 60 l/h. Prior to the experiments the catalyst (10 mg) was pressed, sieved (0.5–0.6 mm), and heated in situ under vacuum to 673 K. The volume of the catalyst filling was lower than 0.1 ml. A Hewlett-Packard HP GC 5890 Series II (HP MS 5971 Series) GC/MS equipment with a HP-PLOT/Al2 O3 /KCl (50 m × 0.32 mm × 5 m) capillary column was used for product analysis. For calculation purposes, the three n-butene isomers were grouped together, since it was established that under experimental conditions the double bond isomerization is much faster than the skeletal isomerization and all n-butenes are converted to isobutene via the same intermediate. The product distribution was estimated from the MS signals (total
ion current) and accords, except for aromatic products, to the molar product distribution. Aromatics appeared only as traces and so an additional calibration was not necessary. The selectivities were determined by division of the yields by the conversion of n-butenes whereas the conversion is defined as the molar percentage of the linear butenes which are consumed.
concentration n-butenes / %
350
40 0 0
2
4 reaction time / min
6
8
Fig. 1. Product distribution for n-butene isomerization at 548 K.
O. Klepel et al. / Applied Catalysis A: General 255 (2003) 349–354
351
598 K
25
i-C4 C3 C5 C6 C7
90 80 n-But 70
20 15
60 10
50 40
5
concentration products / %
concentration n-butenes / %
100
30 0 0
1
2
3
4
5
6
7
8
reaction time / min
Fig. 2. Product distribution for n-butene isomerization at 598 K.
i-C4 C3 C5 C6 C7
90 80
25
673 K n-But
20
15 70 10
60 50
5
concentration products / %
concentration n-butenes / %
100
40 0 0
1
2
3
4
5
6
7
8
reaction time / min
100
100
80
80 C2 C3 i-C4 C5
60
40
60
40
20
20
0
0 0
20
40
60
80
100
reaction time / min
Fig. 4. Conversion of ethene at 623 K.
120
concentration products / %
propene increased within reaction time too. However, at that temperature there were only traces of heptenes. The conversions of ethene and propene at a reaction temperature of 623 K are shown in Figs. 4 and 5. In comparison to 1-butene, it is obvious that these substances were less reactive. The reactivity of the substrate depended on the length of its carbon chain: the longer the chain the higher the reactivity. For ethene, no conversion was observed in the first 7 min. After about 20 min, the first formed product was propene followed by butenes and pentenes after about 100 min reaction time. With regard to propene, the observed products were butenes, pentenes and hexenes.
concentration ethene / %
Fig. 3. Product distribution for n-butene isomerization at 673 K.
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80
80
concentration propene / %
100
60
60
C3 isoC4 C5 C6 n-C4
40 20
40 20
concentration products / %
100
0
0 0
1
2
3
4
5
6
7
8
reaction time / min
The product distribution of the n-butene conversion from our experiments points to the dominance of the oligomerization–cracking mechanism. However, at all reaction temperatures, the concentrations of pentenes and propene were not identical as would be expected for the bimolecular mechanism (Eq. (2)). The concentration of propene was up to three times lower than that of pentenes. Additionally, unexpected high amounts of hexenes and remarkable yields of heptenes appeared. One reason for that unusual behavior could be the dimerization of propene to hexene: (4) 2C= → C= 3
Fig. 5. Conversion of propene at 623 K.
4. Discussion Different mechanisms for the isomerization of n-butenes to isobutene have been proposed in the literature, the monomolecular, the pseudomolecular and the oligomerization–cracking mechanism [24,25]. The monomolecular and the pseudomolecular mechanism are described as selective to isobutene. On the other side, the oligomerization–cracking mechanism is not selective to isobutene because it includes at least a dimerization to octenes, followed by cracking to isobutene, n-butenes, pentenes and propene [26]: 2C= → [C= ] (s = surface) (1) 4
8 s
= [C8= ]s → C= 5 + C3
(2)
= [C8= ]s → C= 4 + iso-C4
(3)
The results of our investigations clearly show that the reaction on the used H-ZSM-5 zeolite was not very selective to isobutene. Therefore, a dominance of the monomolecular and/or the pseudomolecular mechanism can be excluded. However, the increase of the isobutene production with increasing reaction temperature points to a partial contribution of a selective isomerization mechanism at higher temperatures, because the oligomerization of butenes is suppressed at higher temperatures. This is in accordance to Rutenbeck et al. [15] who showed that for H-ZSM-5 with higher Si/Al ratios the mechanism depended on the reaction temperature. With increasing temperature the mechanism changed from a oligomerization to a more selective isomerization one.
6
In this case, the sum of the concentrations of propene and twice C6= (includes two C= 3 units) should be equal to the concentration of the pentenes. This is the case for the very beginning of the reaction at 548 K and for the whole reaction course at the highest investigated reaction temperature of 673 K. It is worth noting that from 548 to 673 K, the amount of the yield of propene increased at the expense of the hexenes. This points to a suppression of the propene dimerization with increasing temperature. It can be concluded that at 673 K the reaction behavior is dominated by a dimerization–cracking mechanism according to reactions (1)–(3) accompanied by a slight dimerization of propene to hexenes as shown by Eq. (4). However, with increasing reaction time at a temperature of 548 K (as early as 3 min) the sum of concentrations of propene and twice C6= was considerably getting higher than that of the pentenes. In the same way, the yields of propene and isobutene remained constant and that of pentenes and heptenes increased with increasing reaction time. This hints to a change of the mechanism depending on the reaction time and the reaction temperature. To elucidate this problem in more detail we investigated the reaction behavior of propene at 623 K (Fig. 5). Astonishingly, hexenes were not the dominating products. High yields of pentenes and butenes were observed. This shows that a simple dimerization from the gas phase C3 species should be not the predominant reaction path. The product distribution and equal amounts of isobutene and pentenes point rather to a dominating trimerization mechanism: 3C= → [C= ] (5) 3
9 s
= [C9= ]s → iso-C= 4 + C5
(6)
O. Klepel et al. / Applied Catalysis A: General 255 (2003) 349–354
Even the formation of C12 surface species is possible as shown by the appearance of hexenes and linear butenes from propene: 4C= → [C= ] (7) 3
=] → [C12 s
12 s
3C= 4 2C= 6
(8)
This experiment emphasizes that in the case of n-butene conversion consecutive reactions of formed and desorbed (gas phase) propene are not dominating, perhaps they are initiated by still adsorbed surface species. A trimerization and even further oligomerization of the educt seem to be possible on the investigated catalyst. To elucidate the tendency to oligomerization further, we also investigated the conversion of ethene. The reaction of this simple molecule should cause a product distribution which clearly points to the character of the reaction mechanism. Despite of the very low activity, the formation of propene points to trimerization followed by decomposition according to: 3C= → [C= ] (9) 2
6 s
[C6= ]s → 2C= 3
(10)
The appearance of butenes and pentenes in our experiment indicates an even four-fold oligomerization with increasing reaction time: 4C= → [C= ] (11) 2
[C8= ]s →
8 s
2C= 4 = C= 5 + C3
(12)
The oligomerization of lower olefins is described as an important step for aromatization of lower hydrocarbons on acid catalysts like zeolites [27,28]. Due to the moderate temperature of our investigation, we did not find aromatic products. From our observations in case of propene one can conclude that for the n-butene conversion the dimerization of gas phase propene is not responsible for hexene formation at reaction temperatures lower than 673 K. For these temperatures, a trimerization– cracking mechanism according to Eq. (13) is conceivable [8,20]: 3C= → [C= ] (13) 4
12 s
[C= 12 ]s →
= C5 + C= 7 2C= 6
353
(14)
At a temperature of 673 K, only traces of heptenes were found. Obviously, at this high reaction temper= was supature the trimerization of butenes to C12 pressed. The hexenes were possibly formed by still adsorbed C3= species which are the products of C= 8 decomposition according to reaction (2). Based on these considerations, it is concluded that only the simultaneous appearance of hexenes and heptenes can be attributed to the decomposition of adsorbed C12 species which are formed by trimerization of butenes. Consecutive reactions either of gas phase = propene and butenes or adsorbed C= 3 and C4 species to heptenes should be not dominating. The appearance of propene should be attributed to the decomposition of adsorbed C8 species. Theoretically, the formation of propene by decomposition of C12 species is possible too. But in this case the increase of propene concentration with reaction time would be accompanied by an increasing concentration of heptenes, since, as it is shown by reaction (14), hep= surface species. In tenes indicate the existence of C12 our experiments, it was demonstrated that an increase in propene concentration with reaction time was not connected with a heptene formation. A remarkable increase of heptenes was only observed in the case of a constant propene concentration as shown in Fig. 1. This points to an independent formation mechanism of heptenes and propene. Therefore, an increasing concentration of propene with reaction time can be considered as an indicator for a bimolecular mechanism. In case the concentration of propene remains constant, the contribution of the bimolecular mechanism to the overall reaction is small and a trimerization mechanism will be dominating, as shown by the further simultaneous increase of the yields of pentenes, hexenes and heptenes.
5. Conclusion The overall reaction behavior of n-butene isomerization on H-ZSM-5 is dominated by oligomerization– cracking steps. At the very beginning of the reaction, the formation of C8 surface species should be the first step. These species decompose into propene and
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pentenes. One part of the just formed and still adsorbed C3 species dimerizes immediately to hexenes. In the further course of reaction, an oligomerization to C12 surface species (trimerization) and following decomposition occurs as it is shown by the products pentenes, hexenes and heptenes. Furthermore, the tendency to oligomerization decrease with increasing temperature. The pathway for the reaction of lower olefins is the same as it was observed for n-butene, i.e. a gradual increase of the degree of oligomerization with reaction time.
[9]
[10] [11] [12] [13] [14] [15]
Acknowledgements This work was supported by the DFG within the Graduiertenkolleg “Physikalische Chemie der Grenzflächen”.
[16] [17] [18] [19]
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Oligomerization as an important step and side reaction for skeletal isomerization of linear butenes on H-ZSM-5 Olaf Klepel a,∗ , Andrei Loubentsov a , Winfried Böhlmann b , Helmut Papp a b
a Fakultät für Chemie und Mineralogie, Universität Leipzig, Linnéstr. 3, D-04103 Leipzig, Germany Fakultät für Physik und Geowissenschaften, Universität Leipzig, Linnéstr. 5, D-04103 Leipzig, Germany
Received 3 April 2003; received in revised form 16 July 2003; accepted 17 July 2003
Abstract The skeletal isomerization of n-butenes on a H-ZSM-5 zeolite with high Si/Al ratio was investigated by using a gradientless recirculation batch reactor. It was concluded that at the very beginning of the reaction, the formation of C8 surface species should be the first step. These species decompose into propene and pentenes, one part of the just formed and still adsorbed C3 species immediately dimerizes to hexenes. In the further course of reaction, an oligomerization to C12 surface species (trimerization) occurs followed by decomposition as shown by the products pentenes, hexenes, and heptenes. The tendency to oligomerization decreased with increasing temperature. For the reaction of lower olefins like propene and ethene, the reaction course was the same as it was observed for n-butene: a gradual increase of the degree of oligomerization with reaction time followed by immediate decomposition. © 2003 Elsevier B.V. All rights reserved. Keywords: Butene; Isomerization; Gradientless recirculation batch reactor
1. Introduction In the last decade, the demand for isobutene overtook the rate of production because of its increasing importance as raw material for the production of methyl tert-butyl ether (MTBE). MTBE is used as a fuel additive. Because of the limited capacity of the traditional cracking processes, many efforts have been undertaken to develop new catalyst systems for skeletal isomerization of n-butenes to isobutene. Among other catalysts like modified alumina [1,2], a number of 10-membered ring molecular sieves [3–17] have been proven to be efficient catalysts for this reaction. Especially, ferrierite has been described ∗ Corresponding author. Tel.: +49-341-9736321; fax: +49-341-9736349. E-mail address: [email protected] (O. Klepel).
as a very selective catalyst for skeletal isomerization. To explain the high selectivity an intramolecular isomerization, the so-called monomolecular mechanism, and a pseudomolecular mechanism with carbenium ions as active sites have been proposed [3–11,16]. On the other side, the selectivity obtained with H-ZSM-5 was remarkably lower [3–15]. On H-ZSM-5, dimerization [5,7,9,18–20] or even trimerization [8,20] of n-butene takes place in the cavities at the intersections of the channels. This oligomerization is followed by non-selective cracking into short-chained olefins, among them isobutene. Because of its feature, this mechanism is called bimolecular or oligomerization–cracking mechanism. The aim of our work was to elucidate the reaction behavior of olefins on H-ZSM-5 concerning oligomerization and consecutive reactions by using a gradientless recirculation batch reactor. This reactor allows a
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00589-1
O. Klepel et al. / Applied Catalysis A: General 255 (2003) 349–354
time-resolved observation of the reaction course and thus the investigation of consecutive reactions. 2. Experimental 2.1. Synthesis and characterization H-ZSM-5 with a Si/Al ratio of 90 was synthesized according to the following procedure [21,22]. A solution of 5.11 g tetrapropylammonium bromide (Aldrich) in 9.92 g of distilled water was added to 10.62 g of colloidal SiO2 (Ludox-AS 40, DuPont) and stirred for 15 min at room temperature. As aluminum source, 0.16 g aluminum triisopropylate (Aldrich) was dissolved in 24.75 ml of ammonia (25 wt.%) and added to the gel which was vigorously stirred for 5 min. The fluid gel was heated at 453 K in a Teflon-lined stainless steel autoclave for 7 days under autogenous pressure. After this procedure, the reaction product was washed, filtered and dried for 6 h at 90 ◦ C. To remove the organic template, the zeolite material was calcined for 12 h at 873 K. The obtained product was characterized by XRD and 27 Al and 29 Si MAS NMR spectroscopy [23]. 2.2. Catalysis
3. Results Figs. 1–3 show the product distribution of the conversion of 1-butene on H-ZSM-5 at different temperatures in the first 7 min of reaction time. In all cases, the main products were C7 , C6 , C5 and C3 alkenes and isobutene. Alkanes were not observed. The total conversion of n-butenes was between 60 and 70% (carbon atom based) and nearly independent of the reaction temperature. The main products were always pentenes, followed by hexenes at the lowest investigated reaction temperature of 548 K. However, at 598 K, the yield of hexenes decreased and was similar to that of isobutene and propene. A further decreasing of the yields of hexenes was observed at a temperature of 673 K, while the main products pentenes were followed by isobutene. The concentration of heptenes decreased also with increasing reaction temperature. At 548 K, after a reaction time of about 2 min, the amount of propene remained constant within the reaction time but the concentration of pentenes, hexenes and heptenes increased equally. At 673 K, besides pentenes and hexenes the yields of isobutene and
25
100
548 K i-C4 C3 C5 C6 C7
90 80
n-But
20
15
70 10
60 50
5
concentration products / %
Catalytic experiments were performed at temperatures from 548 to 673 K in a gradientless recirculation batch reactor at atmospheric pressure. The reactor volume was 236 ml. A mixture of 5% butene (99.0%, AGA) in nitrogen was recirculated by a brennschede-pump with a velocity of about 60 l/h. Prior to the experiments the catalyst (10 mg) was pressed, sieved (0.5–0.6 mm), and heated in situ under vacuum to 673 K. The volume of the catalyst filling was lower than 0.1 ml. A Hewlett-Packard HP GC 5890 Series II (HP MS 5971 Series) GC/MS equipment with a HP-PLOT/Al2 O3 /KCl (50 m × 0.32 mm × 5 m) capillary column was used for product analysis. For calculation purposes, the three n-butene isomers were grouped together, since it was established that under experimental conditions the double bond isomerization is much faster than the skeletal isomerization and all n-butenes are converted to isobutene via the same intermediate. The product distribution was estimated from the MS signals (total
ion current) and accords, except for aromatic products, to the molar product distribution. Aromatics appeared only as traces and so an additional calibration was not necessary. The selectivities were determined by division of the yields by the conversion of n-butenes whereas the conversion is defined as the molar percentage of the linear butenes which are consumed.
concentration n-butenes / %
350
40 0 0
2
4 reaction time / min
6
8
Fig. 1. Product distribution for n-butene isomerization at 548 K.
O. Klepel et al. / Applied Catalysis A: General 255 (2003) 349–354
351
598 K
25
i-C4 C3 C5 C6 C7
90 80 n-But 70
20 15
60 10
50 40
5
concentration products / %
concentration n-butenes / %
100
30 0 0
1
2
3
4
5
6
7
8
reaction time / min
Fig. 2. Product distribution for n-butene isomerization at 598 K.
i-C4 C3 C5 C6 C7
90 80
25
673 K n-But
20
15 70 10
60 50
5
concentration products / %
concentration n-butenes / %
100
40 0 0
1
2
3
4
5
6
7
8
reaction time / min
100
100
80
80 C2 C3 i-C4 C5
60
40
60
40
20
20
0
0 0
20
40
60
80
100
reaction time / min
Fig. 4. Conversion of ethene at 623 K.
120
concentration products / %
propene increased within reaction time too. However, at that temperature there were only traces of heptenes. The conversions of ethene and propene at a reaction temperature of 623 K are shown in Figs. 4 and 5. In comparison to 1-butene, it is obvious that these substances were less reactive. The reactivity of the substrate depended on the length of its carbon chain: the longer the chain the higher the reactivity. For ethene, no conversion was observed in the first 7 min. After about 20 min, the first formed product was propene followed by butenes and pentenes after about 100 min reaction time. With regard to propene, the observed products were butenes, pentenes and hexenes.
concentration ethene / %
Fig. 3. Product distribution for n-butene isomerization at 673 K.
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80
80
concentration propene / %
100
60
60
C3 isoC4 C5 C6 n-C4
40 20
40 20
concentration products / %
100
0
0 0
1
2
3
4
5
6
7
8
reaction time / min
The product distribution of the n-butene conversion from our experiments points to the dominance of the oligomerization–cracking mechanism. However, at all reaction temperatures, the concentrations of pentenes and propene were not identical as would be expected for the bimolecular mechanism (Eq. (2)). The concentration of propene was up to three times lower than that of pentenes. Additionally, unexpected high amounts of hexenes and remarkable yields of heptenes appeared. One reason for that unusual behavior could be the dimerization of propene to hexene: (4) 2C= → C= 3
Fig. 5. Conversion of propene at 623 K.
4. Discussion Different mechanisms for the isomerization of n-butenes to isobutene have been proposed in the literature, the monomolecular, the pseudomolecular and the oligomerization–cracking mechanism [24,25]. The monomolecular and the pseudomolecular mechanism are described as selective to isobutene. On the other side, the oligomerization–cracking mechanism is not selective to isobutene because it includes at least a dimerization to octenes, followed by cracking to isobutene, n-butenes, pentenes and propene [26]: 2C= → [C= ] (s = surface) (1) 4
8 s
= [C8= ]s → C= 5 + C3
(2)
= [C8= ]s → C= 4 + iso-C4
(3)
The results of our investigations clearly show that the reaction on the used H-ZSM-5 zeolite was not very selective to isobutene. Therefore, a dominance of the monomolecular and/or the pseudomolecular mechanism can be excluded. However, the increase of the isobutene production with increasing reaction temperature points to a partial contribution of a selective isomerization mechanism at higher temperatures, because the oligomerization of butenes is suppressed at higher temperatures. This is in accordance to Rutenbeck et al. [15] who showed that for H-ZSM-5 with higher Si/Al ratios the mechanism depended on the reaction temperature. With increasing temperature the mechanism changed from a oligomerization to a more selective isomerization one.
6
In this case, the sum of the concentrations of propene and twice C6= (includes two C= 3 units) should be equal to the concentration of the pentenes. This is the case for the very beginning of the reaction at 548 K and for the whole reaction course at the highest investigated reaction temperature of 673 K. It is worth noting that from 548 to 673 K, the amount of the yield of propene increased at the expense of the hexenes. This points to a suppression of the propene dimerization with increasing temperature. It can be concluded that at 673 K the reaction behavior is dominated by a dimerization–cracking mechanism according to reactions (1)–(3) accompanied by a slight dimerization of propene to hexenes as shown by Eq. (4). However, with increasing reaction time at a temperature of 548 K (as early as 3 min) the sum of concentrations of propene and twice C6= was considerably getting higher than that of the pentenes. In the same way, the yields of propene and isobutene remained constant and that of pentenes and heptenes increased with increasing reaction time. This hints to a change of the mechanism depending on the reaction time and the reaction temperature. To elucidate this problem in more detail we investigated the reaction behavior of propene at 623 K (Fig. 5). Astonishingly, hexenes were not the dominating products. High yields of pentenes and butenes were observed. This shows that a simple dimerization from the gas phase C3 species should be not the predominant reaction path. The product distribution and equal amounts of isobutene and pentenes point rather to a dominating trimerization mechanism: 3C= → [C= ] (5) 3
9 s
= [C9= ]s → iso-C= 4 + C5
(6)
O. Klepel et al. / Applied Catalysis A: General 255 (2003) 349–354
Even the formation of C12 surface species is possible as shown by the appearance of hexenes and linear butenes from propene: 4C= → [C= ] (7) 3
=] → [C12 s
12 s
3C= 4 2C= 6
(8)
This experiment emphasizes that in the case of n-butene conversion consecutive reactions of formed and desorbed (gas phase) propene are not dominating, perhaps they are initiated by still adsorbed surface species. A trimerization and even further oligomerization of the educt seem to be possible on the investigated catalyst. To elucidate the tendency to oligomerization further, we also investigated the conversion of ethene. The reaction of this simple molecule should cause a product distribution which clearly points to the character of the reaction mechanism. Despite of the very low activity, the formation of propene points to trimerization followed by decomposition according to: 3C= → [C= ] (9) 2
6 s
[C6= ]s → 2C= 3
(10)
The appearance of butenes and pentenes in our experiment indicates an even four-fold oligomerization with increasing reaction time: 4C= → [C= ] (11) 2
[C8= ]s →
8 s
2C= 4 = C= 5 + C3
(12)
The oligomerization of lower olefins is described as an important step for aromatization of lower hydrocarbons on acid catalysts like zeolites [27,28]. Due to the moderate temperature of our investigation, we did not find aromatic products. From our observations in case of propene one can conclude that for the n-butene conversion the dimerization of gas phase propene is not responsible for hexene formation at reaction temperatures lower than 673 K. For these temperatures, a trimerization– cracking mechanism according to Eq. (13) is conceivable [8,20]: 3C= → [C= ] (13) 4
12 s
[C= 12 ]s →
= C5 + C= 7 2C= 6
353
(14)
At a temperature of 673 K, only traces of heptenes were found. Obviously, at this high reaction temper= was supature the trimerization of butenes to C12 pressed. The hexenes were possibly formed by still adsorbed C3= species which are the products of C= 8 decomposition according to reaction (2). Based on these considerations, it is concluded that only the simultaneous appearance of hexenes and heptenes can be attributed to the decomposition of adsorbed C12 species which are formed by trimerization of butenes. Consecutive reactions either of gas phase = propene and butenes or adsorbed C= 3 and C4 species to heptenes should be not dominating. The appearance of propene should be attributed to the decomposition of adsorbed C8 species. Theoretically, the formation of propene by decomposition of C12 species is possible too. But in this case the increase of propene concentration with reaction time would be accompanied by an increasing concentration of heptenes, since, as it is shown by reaction (14), hep= surface species. In tenes indicate the existence of C12 our experiments, it was demonstrated that an increase in propene concentration with reaction time was not connected with a heptene formation. A remarkable increase of heptenes was only observed in the case of a constant propene concentration as shown in Fig. 1. This points to an independent formation mechanism of heptenes and propene. Therefore, an increasing concentration of propene with reaction time can be considered as an indicator for a bimolecular mechanism. In case the concentration of propene remains constant, the contribution of the bimolecular mechanism to the overall reaction is small and a trimerization mechanism will be dominating, as shown by the further simultaneous increase of the yields of pentenes, hexenes and heptenes.
5. Conclusion The overall reaction behavior of n-butene isomerization on H-ZSM-5 is dominated by oligomerization– cracking steps. At the very beginning of the reaction, the formation of C8 surface species should be the first step. These species decompose into propene and
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pentenes. One part of the just formed and still adsorbed C3 species dimerizes immediately to hexenes. In the further course of reaction, an oligomerization to C12 surface species (trimerization) and following decomposition occurs as it is shown by the products pentenes, hexenes and heptenes. Furthermore, the tendency to oligomerization decrease with increasing temperature. The pathway for the reaction of lower olefins is the same as it was observed for n-butene, i.e. a gradual increase of the degree of oligomerization with reaction time.
[9]
[10] [11] [12] [13] [14] [15]
Acknowledgements This work was supported by the DFG within the Graduiertenkolleg “Physikalische Chemie der Grenzflächen”.
[16] [17] [18] [19]
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