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Isomerization of 2-methyl-1-butene on copper on-silica catalysts prepared by ion exchange
Journal of Molecular
Catalysis,
51 (1989)
361 - 367
361
ISOMERIZATION OF 2.METHYL-l-BUTENE ON COPPER-ON-SILICA CATALYSTS PREPARED BY ION EXCHANGE kRPAD
MOLNAR,
JANOS T. KISS and MIHliLY
Department of Organic Chemistry, H-6720 (Hungary)
Attila J&sef
BART6K
University,
Szeged,
D6m te’r 8,
Summary Copper-on-silica gel catalysts prepared by ion exchange exhibit a unique activity in the isomerization of alkenes that are able to form the most stable tertiary carbocations. Poisoning experiments with pyridine and 2,6dimethylpyridine, together with IR studies, indicate the presence of different surface acidic sites generated during the activation of the catalysts, inducing an ionic-type isomerization.
Introduction In a recent study, an unusually extensive isomerization (double bond migration) of 2-methyl-1-butene as compared to other isomeric pentenes was observed on certain copper catalysts [l].Under hydrogenation conditions, a 2.01% copper-on-silica catalyst (Cu/S-Xx) prepared by ion exchange brought about a 68% conversion of 2-methyl-1-butene to 2-methyl-2-butene at a temperature as low as 323 K in pulse experiments. In comparison, copper powder and catalysts prepared by precipitation exhibited negligible activity under identical conditions. The results of further studies to reveal the nature of this isomerization activity are reported here. 7H3
CH,=C-CH,--CH,
(73
-
CH,-C=CH-CH,
Experimental Materials Of the alkenes studied, 2-methyl-1-butene (2MelBu), 2-methyl-2butene (2Me2Bu), 3-methyl-l -butene (3MelBu) and 2,3dimethyl-1 -butene (2,3diMelBu) were GC grade products of Fluka, with purities better than 99.5%. Cis-2-pentene (cis2P, >98%), methylenecyclopentane (>9’7%) and 0304-5102/89/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
362
methylenecyclohexane (> 98%) were used as received. Pyridine (Fluka, puriss.) and 2,6_dimethylpyridine (Merck, for synthesis) were kept on potassium hydroxide and distilled before use. Heptane (Fluka, puriss., 100% pure by CC) was used without further purification. Oxygen-free hydrogen was prepared with a Matheson 8326 generator operating with a palladium membrane. Helium (99.99%) was further purified by passage through an Oxy-Trap and an indicating Oxy-Trap (Alltech). Catalysts Ion-exchanged copper catalysts were prepared by a literature method [2], using CU(NH,),~’ and silica gel (Strem, large pore, 120 - 230 mesh) or Cab-0-Sil@ M5 (BDH) supports (denoted Cu/S-X and Cu/CS-X, respectively). Catalysts were also prepared by precipitating basic copper carbonate on the above supports (Cu/S-P and Cu/CS-P) [3]. After preparation all catalyst precursors were treated at 773 K for 3 h in air. Before use the catalysts were reduced in hydrogen flow for 2 h at the temperature indicated in the Figures, then kept in helium flow for 1 h at the same temperature. Methods A pulse microreactor equipped with a Carlo Erba model GV chromatograph operating with a hot-wire detector was used with catalyst samples of 10 mg (flow rate of hydrogen or helium carrier gases = 20 ml min-' ). In poisoning experiments, 5 X 10F8 molar solutions of the bases in heptane were used. GC analyses were performed on a 10% ethyl iV,Ndimethyloxamate (Supelco)-on-Chromosorb P column (4 m) at room temperature. Infrared spectroscopic studies were carried out in a Specord 71 IR apparatus (Carl Zeiss, Jena) equipped with a static cell. Self-supporting wafers prepared from 24 f 0.5 mg catalyst were used in these studies.
Results and discussion Detailed studies of the four different catalysts revealed that the copperon-silica catalyst prepared by precipitation (6.62% Cu/S-P), or catalysts prepared from Cab-0-Sil by either ion-exchange (3.66% Cu/CS-X) or precipitation (6.79% Cu/CS-P), were inactive in isomerization. After pretreatment in the same way as the loaded catalysts, the supports exhibited negligible activity. Consequently, only the CL@-X catalysts were used in further studies. These observations indicate that both the method of preparation and the nature of the support play an essential role in determining the activity of Cu/Si02 catalysts. Investigations in helium of the ion-exchanged, heat-treated precursors of the Cu/S-X catalysts led to the finding that the precursors themselves without reduction also exhibit an initial isomerizing activity. This rather low activity decreases with increasing pretreatment temperature and decays rapidly with increasing pulse number (Fig. 1). The initial activity is lost upon
363
._
pAA 8 u
A
A A
50-
A
A
q
A
A
0
’
A
0
A
0 m
A
I3
A
(3
5
J
.
No.ol
_10
pulses
No of pulses
Fig. 1. Isomerization of 2-methyl-1-butene on a 1.91% Cu/S-X catalyst (reaction temperature: 423 K; 10 mg catalyst, 1 I.tl pulses). Initial activities after pretreatment in helium for 3 h each at 0 473 K or A 673 K. Activities after a 2 h reduction followed by helium treatment for 1 h each at l 473 K or A 673 K. Fig. 2. Isomerization of 2-methyl-1-butene as a function of metal loading (pretreatment: 573 K, 2 h in hydrogen then 1 h in helium; reaction temperature: 423 K; 10 mg catalyst, 1~1 pulses). 0 6.36% Cu/S-X, A 3.45% Cu/S-X, 0 1.91% Cu/S-X.
low-temperature reduction, while a new, higher and more stable activity is generated during reduction at higher temperatures (Fig. 1). Experiments with the reduced Cu/S-X catalysts indicated that the isomerizing activity of the catalysts depended on the copper loading. The higher the loading, the higher and the more stable the activity (Fig. 2). It was also observed that the pretreatment conditions strongly affected the resulting activity. A higher temperature of activation in hydrogen (Fig. 3) and a higher temperature of the following treatment in helium led to more active catalysts with more stable activity. Further work with different olefins revealed that only compounds containing the CH,=C
/R \B’
-
A+
,B CH,-C 1 +‘B’
364
fi:’
7
2
m
C’
.o
E, z
AV
&7
x
0
s
x X
so-
V
o
*
v A
v V A
.o
.
o
0
Q
10.
0 Oo
iI??,...,,
0
10
5 No.
of pulses
No.
of pulses
Fig. 3. Isomerization of 2-methyl-1-butene as a function of the reduction temperature (catalyst: 1.91% Cu/S-X, 10 mg; reaction temperature: 423 K; 1 /Alpulses). Activations are 2 h in hydrogen then 1 h in helium at 0 473 K, 0 523 K, 0 573 K, A 623 K,v 673 K. Fig. 4. Comparison of the reactivity in isomerization of isomeric Cs alkenes on a 6.36% Cu/S-X catalyst (pretreatment and reaction conditions are the same as in Fig. 2). 0 2MelBu + 2Me2Bu; a 2Me2Bu + 2MelBu; + cis2P --f trans2P l 3MelBu --f 2Me2Bu; 0 3MelBu + 2MelBu.
The driving force to the more stable isomer can also explain the formation of the limited amount of truns2P from the cis compound (Fig. 4), though the latter can form only a secondary carbocation. The only exception is 3MelBu, which exhibits a relatively high initial reactivity yielding two isomeric alkenes (Fig. 4). However, the rearrangement of the initially formed secondary carbocation to the most stable tertiary one accounts for its unique behaviour. Poisoning experiments with different bases gave further evidence of the acidic nature of the isomerization. Alternating pulses of 2MelBu and dilute solutions of either pyridine or 2,6dimethylpyridine were injected onto the reduced 6.36% Cu/S--X catalyst. The results of the above experiments were used to calculate the decrease in activity relative to the activity in the unpoisoned reaction (Fig. 5). These results show that pyridine brings about a marked decrease in activity, while the effect of 2,6dimethylpyridine is much milder. The use of these two bases and the comparison of their effects in poisoning experiments were originally suggested by Benesi [4] to distinguish between different acidic centres. It was proved by Jacobs and Heylen [5] that 2,6_dimethylpyridine, which is sterically hindered by the two methyl groups, can interact only with Briinsted acid centres, despite its higher basicity. In contrast, pyridine can react with both BrGnsted and Lewis sites. Although Knbzinger and Stolz [6] found that 2,6_dimethylpyridine is not as selective as expected, our results clearly indicate a significant difference in the poisoning strengths of the two bases. In the present case, this difference can be taken as an indication of the presence of different acidic centres on the surfaces of the active catalysts, and their participation in isomerization.
365
Additional studies of the catalysts were carried out by means of IR spectroscopy. Activation in deuterium resulted in characteristic and informative absorption patterns in the range 2400 - 2800 cm-‘. The silica and Cab0-Sil supports (Fig. 6, A and B), and all the inactive catalysts (for example, Cu/S-P, Fig. 6C) except Cu/CS-X, gave ,a band of weak intensity at around 2730 - 2750 cm-’ after pretreatment in deuterium. In contrast, the spectrum of Cu/CS-X consisted of a strong, sharp maximum at 2740 cm-’ (Fig. 6D),
5
25
50 *lO-%i base
Fig. 5. Relative decreases in activities in poisoning experiments by pyridine (v) or 2,6dimethylpyridine (0). 1 /.d pulses of 2MelBu and a 5 x lo-* molar solution of the base were alternately used. (Catalyst: 6.36% Cu/S--X, 10 mg; pretreatment: 573 K, 2 h in hydrogen, then 1 h in helium; reaction temperature: 423 K.)
2800
cm-’
Fig. 6. Absorption patterns of the supports and the catalysts after pretreatment in deuterium for 2 h at 673 K, followed by a 2 h evacuation (residual pressure: 10U2 Pa). A: silica; B: Cab-0-Sil; C: 6.62% Cu/S-P; D: 3.66% Cu/CS--X; E: 6.36% Cu/S-X.
366
while in the spectrum of Cu/S-X the absorption was much broader towards lower wavenumbers (Fig. 6E). It follows from a comparison of the spectra of the supports, the precursor, the inactive catalysts and the active Cu/S-X specimens that a large number of hydroxyl (deuteroxyl) groups are generated on the active catalysts during reduction. Their formation can be attributed to the effect of H,O (DzO) formed as a result of the reduction of CuO. It also follows that all these newly developing OD bands must correspond to surface deuteroxyl groups. Earlier observations [7 - lo] had already demonstrated that the strong, sharp maximum corresponds to isolated unperturbed OD groups, while the broad absorption feature results from mutually interacting OD groups. In our case, it is conceivable that the possible interaction of the OD groups with certain surface copper species may also induce the appearance of the broad IR absorption.
Conclusions The results presented clearly demonstrate that new acidic-type surface species are generated during the reduction of ion-exchanged copper-on-silica catalysts. On the other hand, the poisoning experiments indicate the presence of two different active sites, participating in parallel in the isomerization. These two types of active sites can be tentatively assigned as Briinsted and Lewis acidic centres. The IR data indicate that the ion-exchange and subsequent reduction greatly enhance the formation of surface hydroxyl groups on silica gel (Fig. 6). The increase in activity and the significant enhancement of the stability upon high-temperature reduction, as compared to the urn-educed precursor (Fig. l), also indicate a decisive role of reduction in the formation of the active catalysts. All these observations and the fact that alkenes with increased proton affinity exhibit outstanding reactivity, point to the presence of weak protonic centres acting as active sites. With regard to recent observations [ll - 131 indicating the difficulty of the total reduction of ion-exchanged copper catalyst precursors, it is conceivable to propose the role of non-reduced Cu(1) ions in the isomerization. These ions, coordinated with the support through oxygens [14], might constitute the other Lewis acid-type active sites. Further investigations (IR spectroscopy, temperature-programmed desorption and kinetic studies) are under way with the aim of acquiring further data for a more exact characterization of the active sites.
Acknowledgement We acknowledge support for this research provided by the Hungarian Academy of Sciences (Grant 320/1986).
367
References 1 A. Molnfir, G. Sirokmin and M. Bartok, 11 th Conf. Organic Reactions Catalysis Society, Savannah, GA, April, 1986, Abstracts, p. 14. 2 H. Kobayashi, N. Takezawa, C. Minochi and K. Takahashi, Chem. Lett., (1980) 1197. 3 (a) Czech. Pat. 91 868 (1958) to V. Ruzicka and J. Soukup; Chem. Abstr., 54 (1960) 14506g; (b) M. Hajek and K. Koechloefl, Coil. Czech. Chem. Commun., 34 (1969) 2739. 4 H. A. Benesi, J. Catal., 28 (1973) 176. 5 P. A. Jacobs and C. F. Heylen, J. Catal., 34 (1974) 267. 6 H. KnSzinger and H. Stolz, Ber. Bunsenges. Phys. Chem., 75 (1971) 1055. 7 C. G. Armistead, A. J. Tyler, F. H. Hambleton, S. A. Mitchell and J. A. Hockey, J. Phys. Chem., 73 (1969) 3947. 8 G. Ghiotti, E. Garrone, C. Morterra and F. Boccuzzi, J. Phys. Chem., 83 (1979) 2863. 9 A. A. Tsyganenko and V. N. Filimonov, J. Mol. Structure, 19 (1973) 579. 10 A. J. van Rosmalen and J. C. Mol, J. Phys. Chem., 82 (1978) 2748; 83 (1979) 2485. 11 M. Shimokawabe, N. Takezawa and H. Kobayashi, Bull. Chem. Sot. Jpn., 56 (1983) 1337. 12 M. A. Kohler, H. E. Curry-Hyde, A. E. Hughes, B. A. Sexton and N. W. Cant, J. Catal., 108 (1987) 323. 13 A. MolnQ, I. Palink and M. Bartok, J. Catal., 114 (1988) 47.8. 14 M. Amara, M. Bettahar, L. Gengembre and D. Olivier, Appl. Catal., 35 (1988) 153.
Catalysis,
51 (1989)
361 - 367
361
ISOMERIZATION OF 2.METHYL-l-BUTENE ON COPPER-ON-SILICA CATALYSTS PREPARED BY ION EXCHANGE kRPAD
MOLNAR,
JANOS T. KISS and MIHliLY
Department of Organic Chemistry, H-6720 (Hungary)
Attila J&sef
BART6K
University,
Szeged,
D6m te’r 8,
Summary Copper-on-silica gel catalysts prepared by ion exchange exhibit a unique activity in the isomerization of alkenes that are able to form the most stable tertiary carbocations. Poisoning experiments with pyridine and 2,6dimethylpyridine, together with IR studies, indicate the presence of different surface acidic sites generated during the activation of the catalysts, inducing an ionic-type isomerization.
Introduction In a recent study, an unusually extensive isomerization (double bond migration) of 2-methyl-1-butene as compared to other isomeric pentenes was observed on certain copper catalysts [l].Under hydrogenation conditions, a 2.01% copper-on-silica catalyst (Cu/S-Xx) prepared by ion exchange brought about a 68% conversion of 2-methyl-1-butene to 2-methyl-2-butene at a temperature as low as 323 K in pulse experiments. In comparison, copper powder and catalysts prepared by precipitation exhibited negligible activity under identical conditions. The results of further studies to reveal the nature of this isomerization activity are reported here. 7H3
CH,=C-CH,--CH,
(73
-
CH,-C=CH-CH,
Experimental Materials Of the alkenes studied, 2-methyl-1-butene (2MelBu), 2-methyl-2butene (2Me2Bu), 3-methyl-l -butene (3MelBu) and 2,3dimethyl-1 -butene (2,3diMelBu) were GC grade products of Fluka, with purities better than 99.5%. Cis-2-pentene (cis2P, >98%), methylenecyclopentane (>9’7%) and 0304-5102/89/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
362
methylenecyclohexane (> 98%) were used as received. Pyridine (Fluka, puriss.) and 2,6_dimethylpyridine (Merck, for synthesis) were kept on potassium hydroxide and distilled before use. Heptane (Fluka, puriss., 100% pure by CC) was used without further purification. Oxygen-free hydrogen was prepared with a Matheson 8326 generator operating with a palladium membrane. Helium (99.99%) was further purified by passage through an Oxy-Trap and an indicating Oxy-Trap (Alltech). Catalysts Ion-exchanged copper catalysts were prepared by a literature method [2], using CU(NH,),~’ and silica gel (Strem, large pore, 120 - 230 mesh) or Cab-0-Sil@ M5 (BDH) supports (denoted Cu/S-X and Cu/CS-X, respectively). Catalysts were also prepared by precipitating basic copper carbonate on the above supports (Cu/S-P and Cu/CS-P) [3]. After preparation all catalyst precursors were treated at 773 K for 3 h in air. Before use the catalysts were reduced in hydrogen flow for 2 h at the temperature indicated in the Figures, then kept in helium flow for 1 h at the same temperature. Methods A pulse microreactor equipped with a Carlo Erba model GV chromatograph operating with a hot-wire detector was used with catalyst samples of 10 mg (flow rate of hydrogen or helium carrier gases = 20 ml min-' ). In poisoning experiments, 5 X 10F8 molar solutions of the bases in heptane were used. GC analyses were performed on a 10% ethyl iV,Ndimethyloxamate (Supelco)-on-Chromosorb P column (4 m) at room temperature. Infrared spectroscopic studies were carried out in a Specord 71 IR apparatus (Carl Zeiss, Jena) equipped with a static cell. Self-supporting wafers prepared from 24 f 0.5 mg catalyst were used in these studies.
Results and discussion Detailed studies of the four different catalysts revealed that the copperon-silica catalyst prepared by precipitation (6.62% Cu/S-P), or catalysts prepared from Cab-0-Sil by either ion-exchange (3.66% Cu/CS-X) or precipitation (6.79% Cu/CS-P), were inactive in isomerization. After pretreatment in the same way as the loaded catalysts, the supports exhibited negligible activity. Consequently, only the CL@-X catalysts were used in further studies. These observations indicate that both the method of preparation and the nature of the support play an essential role in determining the activity of Cu/Si02 catalysts. Investigations in helium of the ion-exchanged, heat-treated precursors of the Cu/S-X catalysts led to the finding that the precursors themselves without reduction also exhibit an initial isomerizing activity. This rather low activity decreases with increasing pretreatment temperature and decays rapidly with increasing pulse number (Fig. 1). The initial activity is lost upon
363
._
pAA 8 u
A
A A
50-
A
A
q
A
A
0
’
A
0
A
0 m
A
I3
A
(3
5
J
.
No.ol
_10
pulses
No of pulses
Fig. 1. Isomerization of 2-methyl-1-butene on a 1.91% Cu/S-X catalyst (reaction temperature: 423 K; 10 mg catalyst, 1 I.tl pulses). Initial activities after pretreatment in helium for 3 h each at 0 473 K or A 673 K. Activities after a 2 h reduction followed by helium treatment for 1 h each at l 473 K or A 673 K. Fig. 2. Isomerization of 2-methyl-1-butene as a function of metal loading (pretreatment: 573 K, 2 h in hydrogen then 1 h in helium; reaction temperature: 423 K; 10 mg catalyst, 1~1 pulses). 0 6.36% Cu/S-X, A 3.45% Cu/S-X, 0 1.91% Cu/S-X.
low-temperature reduction, while a new, higher and more stable activity is generated during reduction at higher temperatures (Fig. 1). Experiments with the reduced Cu/S-X catalysts indicated that the isomerizing activity of the catalysts depended on the copper loading. The higher the loading, the higher and the more stable the activity (Fig. 2). It was also observed that the pretreatment conditions strongly affected the resulting activity. A higher temperature of activation in hydrogen (Fig. 3) and a higher temperature of the following treatment in helium led to more active catalysts with more stable activity. Further work with different olefins revealed that only compounds containing the CH,=C
/R \B’
-
A+
,B CH,-C 1 +‘B’
364
fi:’
7
2
m
C’
.o
E, z
AV
&7
x
0
s
x X
so-
V
o
*
v A
v V A
.o
.
o
0
Q
10.
0 Oo
iI??,...,,
0
10
5 No.
of pulses
No.
of pulses
Fig. 3. Isomerization of 2-methyl-1-butene as a function of the reduction temperature (catalyst: 1.91% Cu/S-X, 10 mg; reaction temperature: 423 K; 1 /Alpulses). Activations are 2 h in hydrogen then 1 h in helium at 0 473 K, 0 523 K, 0 573 K, A 623 K,v 673 K. Fig. 4. Comparison of the reactivity in isomerization of isomeric Cs alkenes on a 6.36% Cu/S-X catalyst (pretreatment and reaction conditions are the same as in Fig. 2). 0 2MelBu + 2Me2Bu; a 2Me2Bu + 2MelBu; + cis2P --f trans2P l 3MelBu --f 2Me2Bu; 0 3MelBu + 2MelBu.
The driving force to the more stable isomer can also explain the formation of the limited amount of truns2P from the cis compound (Fig. 4), though the latter can form only a secondary carbocation. The only exception is 3MelBu, which exhibits a relatively high initial reactivity yielding two isomeric alkenes (Fig. 4). However, the rearrangement of the initially formed secondary carbocation to the most stable tertiary one accounts for its unique behaviour. Poisoning experiments with different bases gave further evidence of the acidic nature of the isomerization. Alternating pulses of 2MelBu and dilute solutions of either pyridine or 2,6dimethylpyridine were injected onto the reduced 6.36% Cu/S--X catalyst. The results of the above experiments were used to calculate the decrease in activity relative to the activity in the unpoisoned reaction (Fig. 5). These results show that pyridine brings about a marked decrease in activity, while the effect of 2,6dimethylpyridine is much milder. The use of these two bases and the comparison of their effects in poisoning experiments were originally suggested by Benesi [4] to distinguish between different acidic centres. It was proved by Jacobs and Heylen [5] that 2,6_dimethylpyridine, which is sterically hindered by the two methyl groups, can interact only with Briinsted acid centres, despite its higher basicity. In contrast, pyridine can react with both BrGnsted and Lewis sites. Although Knbzinger and Stolz [6] found that 2,6_dimethylpyridine is not as selective as expected, our results clearly indicate a significant difference in the poisoning strengths of the two bases. In the present case, this difference can be taken as an indication of the presence of different acidic centres on the surfaces of the active catalysts, and their participation in isomerization.
365
Additional studies of the catalysts were carried out by means of IR spectroscopy. Activation in deuterium resulted in characteristic and informative absorption patterns in the range 2400 - 2800 cm-‘. The silica and Cab0-Sil supports (Fig. 6, A and B), and all the inactive catalysts (for example, Cu/S-P, Fig. 6C) except Cu/CS-X, gave ,a band of weak intensity at around 2730 - 2750 cm-’ after pretreatment in deuterium. In contrast, the spectrum of Cu/CS-X consisted of a strong, sharp maximum at 2740 cm-’ (Fig. 6D),
5
25
50 *lO-%i base
Fig. 5. Relative decreases in activities in poisoning experiments by pyridine (v) or 2,6dimethylpyridine (0). 1 /.d pulses of 2MelBu and a 5 x lo-* molar solution of the base were alternately used. (Catalyst: 6.36% Cu/S--X, 10 mg; pretreatment: 573 K, 2 h in hydrogen, then 1 h in helium; reaction temperature: 423 K.)
2800
cm-’
Fig. 6. Absorption patterns of the supports and the catalysts after pretreatment in deuterium for 2 h at 673 K, followed by a 2 h evacuation (residual pressure: 10U2 Pa). A: silica; B: Cab-0-Sil; C: 6.62% Cu/S-P; D: 3.66% Cu/CS--X; E: 6.36% Cu/S-X.
366
while in the spectrum of Cu/S-X the absorption was much broader towards lower wavenumbers (Fig. 6E). It follows from a comparison of the spectra of the supports, the precursor, the inactive catalysts and the active Cu/S-X specimens that a large number of hydroxyl (deuteroxyl) groups are generated on the active catalysts during reduction. Their formation can be attributed to the effect of H,O (DzO) formed as a result of the reduction of CuO. It also follows that all these newly developing OD bands must correspond to surface deuteroxyl groups. Earlier observations [7 - lo] had already demonstrated that the strong, sharp maximum corresponds to isolated unperturbed OD groups, while the broad absorption feature results from mutually interacting OD groups. In our case, it is conceivable that the possible interaction of the OD groups with certain surface copper species may also induce the appearance of the broad IR absorption.
Conclusions The results presented clearly demonstrate that new acidic-type surface species are generated during the reduction of ion-exchanged copper-on-silica catalysts. On the other hand, the poisoning experiments indicate the presence of two different active sites, participating in parallel in the isomerization. These two types of active sites can be tentatively assigned as Briinsted and Lewis acidic centres. The IR data indicate that the ion-exchange and subsequent reduction greatly enhance the formation of surface hydroxyl groups on silica gel (Fig. 6). The increase in activity and the significant enhancement of the stability upon high-temperature reduction, as compared to the urn-educed precursor (Fig. l), also indicate a decisive role of reduction in the formation of the active catalysts. All these observations and the fact that alkenes with increased proton affinity exhibit outstanding reactivity, point to the presence of weak protonic centres acting as active sites. With regard to recent observations [ll - 131 indicating the difficulty of the total reduction of ion-exchanged copper catalyst precursors, it is conceivable to propose the role of non-reduced Cu(1) ions in the isomerization. These ions, coordinated with the support through oxygens [14], might constitute the other Lewis acid-type active sites. Further investigations (IR spectroscopy, temperature-programmed desorption and kinetic studies) are under way with the aim of acquiring further data for a more exact characterization of the active sites.
Acknowledgement We acknowledge support for this research provided by the Hungarian Academy of Sciences (Grant 320/1986).
367
References 1 A. Molnfir, G. Sirokmin and M. Bartok, 11 th Conf. Organic Reactions Catalysis Society, Savannah, GA, April, 1986, Abstracts, p. 14. 2 H. Kobayashi, N. Takezawa, C. Minochi and K. Takahashi, Chem. Lett., (1980) 1197. 3 (a) Czech. Pat. 91 868 (1958) to V. Ruzicka and J. Soukup; Chem. Abstr., 54 (1960) 14506g; (b) M. Hajek and K. Koechloefl, Coil. Czech. Chem. Commun., 34 (1969) 2739. 4 H. A. Benesi, J. Catal., 28 (1973) 176. 5 P. A. Jacobs and C. F. Heylen, J. Catal., 34 (1974) 267. 6 H. KnSzinger and H. Stolz, Ber. Bunsenges. Phys. Chem., 75 (1971) 1055. 7 C. G. Armistead, A. J. Tyler, F. H. Hambleton, S. A. Mitchell and J. A. Hockey, J. Phys. Chem., 73 (1969) 3947. 8 G. Ghiotti, E. Garrone, C. Morterra and F. Boccuzzi, J. Phys. Chem., 83 (1979) 2863. 9 A. A. Tsyganenko and V. N. Filimonov, J. Mol. Structure, 19 (1973) 579. 10 A. J. van Rosmalen and J. C. Mol, J. Phys. Chem., 82 (1978) 2748; 83 (1979) 2485. 11 M. Shimokawabe, N. Takezawa and H. Kobayashi, Bull. Chem. Sot. Jpn., 56 (1983) 1337. 12 M. A. Kohler, H. E. Curry-Hyde, A. E. Hughes, B. A. Sexton and N. W. Cant, J. Catal., 108 (1987) 323. 13 A. MolnQ, I. Palink and M. Bartok, J. Catal., 114 (1988) 47.8. 14 M. Amara, M. Bettahar, L. Gengembre and D. Olivier, Appl. Catal., 35 (1988) 153.