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selectivity and ligation of the Co ions in zeolites in ammoxidation of ethane to acetonitrile
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
869
Activity~selectivity
and iigation of the Co ions in zeolites in ammoxidation of ethane to acetonitrile
B. Wichteflovh a, Z. Sobalik', Y. Lib, and J.N. Armor c aj. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-182 23 Prague 8, Czech Republic* bEngelhard Corporation, Iselin, NJ 08820 USA CAir Products & Chemicals, Inc., Allentown, PA 1819-1501, U.S.A.
The reactivity of the Co ions exchanged in pentasil-ring zeolites, BEA, MFI and MOR and six-ring zeolites, Y and dealuminated USY differing in Si/A1 and Co ion coordination was studied with respect to the activity/selectivity in the ammoxidation of ethane to acetonitrile, and strength of bonding of the Co ions with reactants, intermediates and products. It has been shown that a decisive factor controlling the activity of the Co ions is negative charge of the zeolite framework, and the accessibility of the reactants to the Co ions. The strength of Coligand bonding was estimated from the extent of the shift of the antisymmetric vibration of the framework T-O bonds adjacent to the cation-ligand complexes vs. the dehydrated sample: ethylamine > acetonitrile > ammonia >> ethylene. This also correlated well with the temperature for the decomposition of the individual Co-ligand 1:1 complexes. This highlights the importance of stronger ammonia bonding compared to weakly bound ethylene and the stronger bonding of ethylamine for achieving high acetonitrile selectivity in the ethane/NH3/O2 reaction. 1. INTRODUCTION Co ions planted by ion exchange into BEA and MFI structures have been reported as highly active in selective ammoxidation of ethane to acetonitrile [ 1-3] The reaction pathway suggested [3] includes ethane oxidative dehydrogenation to ethylene, which in the presence of ammonia, is in further steps selectively oxidized into acetonitrile via ethylamine. However, if ammonia is not present in the gas phase, ethane is oxidized non-selectively to carbon dioxide. Therefore, the Co ion activity/selectivity has been assumed to be strongly modified by its ligation with ammonia. Recently, we have shown for Co-FER [4,5], that antisymmetric T-O vibrations of the framework together with the characteristic vibrations of the adsorbate molecules can be used to monitor ligand bonding and its strength to the exchanged metal ions.
Acknowledgement. B. W. and Z. S. appreciate financial support for this study from Air Products & Chemicals, Inc, and J.N.A. thanks to the same company for allowance to publish these results.
870 This study deals with a comparison of the function of the Co ions in BEA, MFI, MOR and in Y-USY zeolites in ammoxidation of ethane to acetonitrile, i.e. those with different topology and content of aluminum in their framework. Bonding of the Co ions in the ligand field of polyoxoanion-cation-extraframework ligand complexes, relevant to the ethane/NH3/O2 reaction, at a temperature close to that of the catalytic reaction, is described. The effect of AI related acidic Broensted and Lewis sites on the reactant complexation and reaction performance is given as well. 2. E X P E R I M E N T A L NH4-BEA, NHn-MFI, NH4-MOR, NH4-Y, dealuminated NHa-USY, highly differing in content of aluminum in their framework and topology, were Co(II) ion exchanged with 0.01 M Co acetate solutions refluxing at 80~ to reach near complete ion exchange, i.e. Co/AI = 0.5; Co levels, and bulk and framework Si/A1 molar ratios are given in Table 1. Some of dealuminated USY zeolites were treated in 0.3 M HCI solution to remove extraframework aluminum, and then transferred into their NHa-ion forms. Dehydration of Co-zeolites took place in situ in an oxygen stream or under vacuum at 480~ before catalytic activity or IR spectra measurements. It has been proven elsewhere [6] that divalent Co ions, if placed at cationic sites of zeolites, preserve their divalent state at < 500~ treatment in both oxygen and vacuum. Catalytic activity of ethane ammoxidation to acetonitrile was carried out in a throughflow microreactor at steady-state conditions at 450 or 475~ Reactant feed consisted of 10 % ethylene, 5 -10 % NH3 and 6.5 % 02, balance as helium, at total flow-rate of 100 ml/min. Catalyst weight was 0.2 - 0.3 g. Turn over frequency, TOF (sec'~), represents number of acetonitrile molecules formed per Co atom per second. The data come from Refs. 1-3. IR spectra of self-supported pellets (5 mg.cm 2) were recorded at the 4000-400 cm-1 region at RT on a Magna-IR System 550 FTIR (Nicolet) spectrometer using MCT-B liquid nitrogen cooled detector and equipped with a heated cell up to 500~ connected to a vacuum/gas system. A single spectrum was obtained with resolution of 2 crn-~ and 200 scans. Time-resolved measurements with up to 15 spectra per second were done at 8 cm -1 resolution. Adsorption of ethylene (0.35 kPa), ammonia (3.3 kPa), ethylamine (3.3 kPa) and acetonitrile (0.6 kPa) took place for 30 min at RT on dehydrated Co-zeolites, followed by time-resolved desorption under evacuation (up to 102 kPa) for 30 min at temperature steps up to 480 ~
3. RESULTS and DISCUSSION 3.1 Catalytic activity of Co-zeolites in ethane/NH3/O2 to acetonitrile reaction Catalytic activity of Co-zeolites, with Co/AI values of about 0.5, but differing in topology and in a broad range of Si/A1 framework ratio are given in Table 1 and Figure 1. The yields of acetonitrile (per catalyst weight) increased with increasing Si/A1 values, although the content of Co decreased, being the highest with Co-BEA and Co-MFI. In contrast, the lowest activity exhibited Co-zeolites with high content of aluminum in their frameworks (Si/A1 < 4), and thus high cobalt concentration. TOF values (per Co ion) clearly indicates that with
decreasing content of aluminium in the zeolite framework, the activity~selectivity of the Co ions dramatically increases. However, in the series of the studied zeolites, the Co ions are located at different cationic sites, with different coordinations, as reported in the previous studies. In MFI, MOR and BEA [7-9] three typical Co coordinations were found, with most of the Co ions (60-70 %) bound to six framework oxygens of the deformed six-ring in BEA and
871 MFI, and of the twisted eight-ring in MOR. While this Co site is at the channel intersection in MFI framework and very open in BEA, in MOR it is accessible only from the inter-connected narrow channels. Moreover, all these zeolites exhibit Co ions (ca 30%) coordinated to four oxygens in their main channels, weakly bound to the framework, and therefore, potentially very reactive, because of the coordinative unsaturation (details on the coordination and reactivity see Ref. 8). A series of Co-Y and dealuminated Co-USY zeolites with the same -faujasitetopology, provides the Co ions with the same coordinations, i.e. at SII/SII'/SI' sites coordinated to three oxygens in C3v symmetry of the regular six-ring; SI site in the center of hexagonal prism is not occupied by the Co ions. At SII/SII' sites the Co ions are accessible to reactants, but SI' site Cannot be reached by ethane, oxygen and acetonitrile. Under the assumption that the population of the cationic sites by the Co ions does not differ dramatically for dealuminated zeolites, then the catalytic activity/selectivity of the dealuminated Co-Y zeolites (Table 1, Figure 1) indicates that a decisive effect controlling the activity/selectivity of the Co ions is the negative framework charge balancing the cobalt ions. 3.2 Co-ligand complexes relevant to ethane/NH3/O2 to acetonitrile reaction Complexation of the Co ions in a polyoxoframework-Co-extraframework ligand complexes, such as those assumed to take place during the catalytic reaction, is demonstrated for Co-BEA (Figure 2). Bare Co ions bound only to framework oxygens induce perturbation of the adjacent framework T-O bonds, yielding a characteristic shiit of the antisymmetric T-O vibration to lower frequency into a skeletal transmission window (BM band). This effect quantitatively mirrors the presence of the Co ions at cationic sites (see Refs. 4 and 9). By adsorption of ethylene, ammonia, ethylamine or acetonitrile on Co-BEA, 1:1 complexes were detected by a relaxation shiit of the BM band into a BML band (of higher frequency) with isosbestic points in the time-resolved spectra, as illustrated in Figure 2 for ammonia adsorption-desorption. The relaxation shifts characteristic for the individual adsorbates bound to the Co ion in 1:1 complexes, and temperatures for the existence of dehydrated Co ion and Co-ligand 1:1 complexes are given in Table 2. The values of relaxation shifts (Ar) together with the temperatures indicate weakening of the Co ion bonding to the framework oxygens due to the ligand, and thus are a measure of the strength of the Co-ligand bond. The strength of adsorbates bonding to the Co ion (Table 2) is following: ethylene << ammonia < acetonitrile < ethylamine.
This sequence indicates the importance of the presence of ammonia for selective transformation of ethylene on the Co ions. Ammonia is much more strongly bound to the Co ions compared to ethylene, resulting in limited adsorption of ethylene on the Co ions. C2H4 adsorption might induce its oligomerization and easy oxidation to carbon dioxide. Instead, ethylene interacts with Co-NH3 complex with formation of the Co-ethylamine complex, which was suggested to be a reaction intermediate. As a ligand, ethylamine is strongly held with the highest stability among possible ligands for the Co ions in the ethane/NH3/O2 reaction, while acetonitrile is less strongly bound. It implies that once formed, acetonitrile preferably desorbs from the Co ions as a final product. If low Co loaded Co-BEA also contained Al-Lewis and Broensted sites, then oligomerization of ethylene on these acidic sites could take place as documented by IR studies (10). At reaction temperature (450~ these sites are not covered by ammonia, like Co ions, and represent highly exposed strong electron acceptor and proton donor sites appropriate for catalyzing ethylene oligomerization, instead of its reaction with the Co-NH3 complex. This results in decreasing acetonitrile yields as well as deactivation of the zeolite.
872 10
20
18
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8 16
-q
o
"13 X
14
.
.....-'" "5, a) e-o
.e""
12
O O
4
.......:]il]ol]i ...o ~ , /
"to
tD o
......o
6 --" O
/ - ii-i//
8
.o :;:""" 7"
i
2
'
I
4
~
i
'
1'
6
'
'
8
1;
'
lJ2
14
Si/A]
Fig. 1. Dependence ofacetonitrile yield and TOF on Si/A1 framework ratio for Y - USY (o and o, resp.) and zeolites of various topologies (O and I, resp.), see Table 1.
f___ 3 - - ' - - '~ - - ' g 0 a0. ~
3(X~ '
o5 '
950 '
S '
g30 '
'
850 '
wavenumber, cm I Fig. 2. FTIR spectra of desorption of ammonia from Co-BEA (Co/A1 0.16) with Co-NH3 complex at RT (a), 100 (b), 250 (c), 300 (d), 350 (e) and 480 ~ (f).
ol r t">.
874
4. CONCLUSIONS The following factors have been indicated to control the ethane/NH3/O2 reaction over Cozeolites (i)
(ii)
(iii)
the cation should be accessible to reactants, with a possibility for an optimum approach of reactants to the cation, controlled by the cation coordination and the environment of the cationic site, given by the framework topology; a decisive effect of negative framework charge for the Co ions activity~selectivity was shown; low negative framework charge given by low content of aluminum in the zeolite framework leads to a high Co ion reactivity. Generally, weak bonding of the Co ions to framework oxygens controlled by their low negative charges and cation coordination provide high activity of the Co ions; stronger bonding of ammonia compared to ethylene avoids ethylene oligomerization on the Co ions and enables interaction of ethylene with Co-NH3 complex to form ethylamine, an intermediate for acetonitrile formation. If alumina originated acid sites (protonic or Lewis) are present in the zeolite besides the Co ions, ethylene oligomerization occurs, and decreases performance of the Co zeolites in acetonitrile formation.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
Y. Li and J.N. Armor, J. Chem. Sot., Chem. Commun. 1997, 2013. Y. Li and J.N. Armor, J. Catal. 173 (1998) 511. Y. Li and J.N. Armor, J. Catal. 176 (1998) 495. Z. Sobalik, Z. Tvarfi~kovh and B. Wichterlov/l, J. Phys. Chem. 102 (1998) 1077. Z. Sobalik, Z. Tvarfi~kov/L and B. Wichterlov/l, Microp. Mesop. Mater. 25 (1998) 525. J. D~de6ek, D. Kauck~, and B. Wichterlovb., Microp. Mesop. Mater. (1999) in press. J. D6de6ek and B. Wichterlov/t, J. Phys. Chem. 103 (1999) 1462. B. Wichterlovfi, J. D6de6ek and Z. Sobalik, Proc. 12th Int. Zeol. Conf., Baltimore, 1998, Eds. M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins, Material Research Society, 1998, p.941. 9. Z. Sobalik, Z. Tvarfi~kov/l and B. Wichterlovb., Microp. Mesop.Mater. 25 (1998) 225. 10. Z. Sobalik, A.A. Belhekar, Z. Tvarfi~kov/l and B. Wichterlov/l, Appl. Catal.A: 188 (1999) 175.
869
Activity~selectivity
and iigation of the Co ions in zeolites in ammoxidation of ethane to acetonitrile
B. Wichteflovh a, Z. Sobalik', Y. Lib, and J.N. Armor c aj. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-182 23 Prague 8, Czech Republic* bEngelhard Corporation, Iselin, NJ 08820 USA CAir Products & Chemicals, Inc., Allentown, PA 1819-1501, U.S.A.
The reactivity of the Co ions exchanged in pentasil-ring zeolites, BEA, MFI and MOR and six-ring zeolites, Y and dealuminated USY differing in Si/A1 and Co ion coordination was studied with respect to the activity/selectivity in the ammoxidation of ethane to acetonitrile, and strength of bonding of the Co ions with reactants, intermediates and products. It has been shown that a decisive factor controlling the activity of the Co ions is negative charge of the zeolite framework, and the accessibility of the reactants to the Co ions. The strength of Coligand bonding was estimated from the extent of the shift of the antisymmetric vibration of the framework T-O bonds adjacent to the cation-ligand complexes vs. the dehydrated sample: ethylamine > acetonitrile > ammonia >> ethylene. This also correlated well with the temperature for the decomposition of the individual Co-ligand 1:1 complexes. This highlights the importance of stronger ammonia bonding compared to weakly bound ethylene and the stronger bonding of ethylamine for achieving high acetonitrile selectivity in the ethane/NH3/O2 reaction. 1. INTRODUCTION Co ions planted by ion exchange into BEA and MFI structures have been reported as highly active in selective ammoxidation of ethane to acetonitrile [ 1-3] The reaction pathway suggested [3] includes ethane oxidative dehydrogenation to ethylene, which in the presence of ammonia, is in further steps selectively oxidized into acetonitrile via ethylamine. However, if ammonia is not present in the gas phase, ethane is oxidized non-selectively to carbon dioxide. Therefore, the Co ion activity/selectivity has been assumed to be strongly modified by its ligation with ammonia. Recently, we have shown for Co-FER [4,5], that antisymmetric T-O vibrations of the framework together with the characteristic vibrations of the adsorbate molecules can be used to monitor ligand bonding and its strength to the exchanged metal ions.
Acknowledgement. B. W. and Z. S. appreciate financial support for this study from Air Products & Chemicals, Inc, and J.N.A. thanks to the same company for allowance to publish these results.
870 This study deals with a comparison of the function of the Co ions in BEA, MFI, MOR and in Y-USY zeolites in ammoxidation of ethane to acetonitrile, i.e. those with different topology and content of aluminum in their framework. Bonding of the Co ions in the ligand field of polyoxoanion-cation-extraframework ligand complexes, relevant to the ethane/NH3/O2 reaction, at a temperature close to that of the catalytic reaction, is described. The effect of AI related acidic Broensted and Lewis sites on the reactant complexation and reaction performance is given as well. 2. E X P E R I M E N T A L NH4-BEA, NHn-MFI, NH4-MOR, NH4-Y, dealuminated NHa-USY, highly differing in content of aluminum in their framework and topology, were Co(II) ion exchanged with 0.01 M Co acetate solutions refluxing at 80~ to reach near complete ion exchange, i.e. Co/AI = 0.5; Co levels, and bulk and framework Si/A1 molar ratios are given in Table 1. Some of dealuminated USY zeolites were treated in 0.3 M HCI solution to remove extraframework aluminum, and then transferred into their NHa-ion forms. Dehydration of Co-zeolites took place in situ in an oxygen stream or under vacuum at 480~ before catalytic activity or IR spectra measurements. It has been proven elsewhere [6] that divalent Co ions, if placed at cationic sites of zeolites, preserve their divalent state at < 500~ treatment in both oxygen and vacuum. Catalytic activity of ethane ammoxidation to acetonitrile was carried out in a throughflow microreactor at steady-state conditions at 450 or 475~ Reactant feed consisted of 10 % ethylene, 5 -10 % NH3 and 6.5 % 02, balance as helium, at total flow-rate of 100 ml/min. Catalyst weight was 0.2 - 0.3 g. Turn over frequency, TOF (sec'~), represents number of acetonitrile molecules formed per Co atom per second. The data come from Refs. 1-3. IR spectra of self-supported pellets (5 mg.cm 2) were recorded at the 4000-400 cm-1 region at RT on a Magna-IR System 550 FTIR (Nicolet) spectrometer using MCT-B liquid nitrogen cooled detector and equipped with a heated cell up to 500~ connected to a vacuum/gas system. A single spectrum was obtained with resolution of 2 crn-~ and 200 scans. Time-resolved measurements with up to 15 spectra per second were done at 8 cm -1 resolution. Adsorption of ethylene (0.35 kPa), ammonia (3.3 kPa), ethylamine (3.3 kPa) and acetonitrile (0.6 kPa) took place for 30 min at RT on dehydrated Co-zeolites, followed by time-resolved desorption under evacuation (up to 102 kPa) for 30 min at temperature steps up to 480 ~
3. RESULTS and DISCUSSION 3.1 Catalytic activity of Co-zeolites in ethane/NH3/O2 to acetonitrile reaction Catalytic activity of Co-zeolites, with Co/AI values of about 0.5, but differing in topology and in a broad range of Si/A1 framework ratio are given in Table 1 and Figure 1. The yields of acetonitrile (per catalyst weight) increased with increasing Si/A1 values, although the content of Co decreased, being the highest with Co-BEA and Co-MFI. In contrast, the lowest activity exhibited Co-zeolites with high content of aluminum in their frameworks (Si/A1 < 4), and thus high cobalt concentration. TOF values (per Co ion) clearly indicates that with
decreasing content of aluminium in the zeolite framework, the activity~selectivity of the Co ions dramatically increases. However, in the series of the studied zeolites, the Co ions are located at different cationic sites, with different coordinations, as reported in the previous studies. In MFI, MOR and BEA [7-9] three typical Co coordinations were found, with most of the Co ions (60-70 %) bound to six framework oxygens of the deformed six-ring in BEA and
871 MFI, and of the twisted eight-ring in MOR. While this Co site is at the channel intersection in MFI framework and very open in BEA, in MOR it is accessible only from the inter-connected narrow channels. Moreover, all these zeolites exhibit Co ions (ca 30%) coordinated to four oxygens in their main channels, weakly bound to the framework, and therefore, potentially very reactive, because of the coordinative unsaturation (details on the coordination and reactivity see Ref. 8). A series of Co-Y and dealuminated Co-USY zeolites with the same -faujasitetopology, provides the Co ions with the same coordinations, i.e. at SII/SII'/SI' sites coordinated to three oxygens in C3v symmetry of the regular six-ring; SI site in the center of hexagonal prism is not occupied by the Co ions. At SII/SII' sites the Co ions are accessible to reactants, but SI' site Cannot be reached by ethane, oxygen and acetonitrile. Under the assumption that the population of the cationic sites by the Co ions does not differ dramatically for dealuminated zeolites, then the catalytic activity/selectivity of the dealuminated Co-Y zeolites (Table 1, Figure 1) indicates that a decisive effect controlling the activity/selectivity of the Co ions is the negative framework charge balancing the cobalt ions. 3.2 Co-ligand complexes relevant to ethane/NH3/O2 to acetonitrile reaction Complexation of the Co ions in a polyoxoframework-Co-extraframework ligand complexes, such as those assumed to take place during the catalytic reaction, is demonstrated for Co-BEA (Figure 2). Bare Co ions bound only to framework oxygens induce perturbation of the adjacent framework T-O bonds, yielding a characteristic shiit of the antisymmetric T-O vibration to lower frequency into a skeletal transmission window (BM band). This effect quantitatively mirrors the presence of the Co ions at cationic sites (see Refs. 4 and 9). By adsorption of ethylene, ammonia, ethylamine or acetonitrile on Co-BEA, 1:1 complexes were detected by a relaxation shiit of the BM band into a BML band (of higher frequency) with isosbestic points in the time-resolved spectra, as illustrated in Figure 2 for ammonia adsorption-desorption. The relaxation shifts characteristic for the individual adsorbates bound to the Co ion in 1:1 complexes, and temperatures for the existence of dehydrated Co ion and Co-ligand 1:1 complexes are given in Table 2. The values of relaxation shifts (Ar) together with the temperatures indicate weakening of the Co ion bonding to the framework oxygens due to the ligand, and thus are a measure of the strength of the Co-ligand bond. The strength of adsorbates bonding to the Co ion (Table 2) is following: ethylene << ammonia < acetonitrile < ethylamine.
This sequence indicates the importance of the presence of ammonia for selective transformation of ethylene on the Co ions. Ammonia is much more strongly bound to the Co ions compared to ethylene, resulting in limited adsorption of ethylene on the Co ions. C2H4 adsorption might induce its oligomerization and easy oxidation to carbon dioxide. Instead, ethylene interacts with Co-NH3 complex with formation of the Co-ethylamine complex, which was suggested to be a reaction intermediate. As a ligand, ethylamine is strongly held with the highest stability among possible ligands for the Co ions in the ethane/NH3/O2 reaction, while acetonitrile is less strongly bound. It implies that once formed, acetonitrile preferably desorbs from the Co ions as a final product. If low Co loaded Co-BEA also contained Al-Lewis and Broensted sites, then oligomerization of ethylene on these acidic sites could take place as documented by IR studies (10). At reaction temperature (450~ these sites are not covered by ammonia, like Co ions, and represent highly exposed strong electron acceptor and proton donor sites appropriate for catalyzing ethylene oligomerization, instead of its reaction with the Co-NH3 complex. This results in decreasing acetonitrile yields as well as deactivation of the zeolite.
872 10
20
18
"-~-rl
8 16
-q
o
"13 X
14
.
.....-'" "5, a) e-o
.e""
12
O O
4
.......:]il]ol]i ...o ~ , /
"to
tD o
......o
6 --" O
/ - ii-i//
8
.o :;:""" 7"
i
2
'
I
4
~
i
'
1'
6
'
'
8
1;
'
lJ2
14
Si/A]
Fig. 1. Dependence ofacetonitrile yield and TOF on Si/A1 framework ratio for Y - USY (o and o, resp.) and zeolites of various topologies (O and I, resp.), see Table 1.
f___ 3 - - ' - - '~ - - ' g 0 a0. ~
3(X~ '
o5 '
950 '
S '
g30 '
'
850 '
wavenumber, cm I Fig. 2. FTIR spectra of desorption of ammonia from Co-BEA (Co/A1 0.16) with Co-NH3 complex at RT (a), 100 (b), 250 (c), 300 (d), 350 (e) and 480 ~ (f).
ol r t">.
874
4. CONCLUSIONS The following factors have been indicated to control the ethane/NH3/O2 reaction over Cozeolites (i)
(ii)
(iii)
the cation should be accessible to reactants, with a possibility for an optimum approach of reactants to the cation, controlled by the cation coordination and the environment of the cationic site, given by the framework topology; a decisive effect of negative framework charge for the Co ions activity~selectivity was shown; low negative framework charge given by low content of aluminum in the zeolite framework leads to a high Co ion reactivity. Generally, weak bonding of the Co ions to framework oxygens controlled by their low negative charges and cation coordination provide high activity of the Co ions; stronger bonding of ammonia compared to ethylene avoids ethylene oligomerization on the Co ions and enables interaction of ethylene with Co-NH3 complex to form ethylamine, an intermediate for acetonitrile formation. If alumina originated acid sites (protonic or Lewis) are present in the zeolite besides the Co ions, ethylene oligomerization occurs, and decreases performance of the Co zeolites in acetonitrile formation.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
Y. Li and J.N. Armor, J. Chem. Sot., Chem. Commun. 1997, 2013. Y. Li and J.N. Armor, J. Catal. 173 (1998) 511. Y. Li and J.N. Armor, J. Catal. 176 (1998) 495. Z. Sobalik, Z. Tvarfi~kovh and B. Wichterlov/l, J. Phys. Chem. 102 (1998) 1077. Z. Sobalik, Z. Tvarfi~kov/L and B. Wichterlov/l, Microp. Mesop. Mater. 25 (1998) 525. J. D~de6ek, D. Kauck~, and B. Wichterlovb., Microp. Mesop. Mater. (1999) in press. J. D6de6ek and B. Wichterlov/t, J. Phys. Chem. 103 (1999) 1462. B. Wichterlovfi, J. D6de6ek and Z. Sobalik, Proc. 12th Int. Zeol. Conf., Baltimore, 1998, Eds. M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins, Material Research Society, 1998, p.941. 9. Z. Sobalik, Z. Tvarfi~kov/l and B. Wichterlovb., Microp. Mesop.Mater. 25 (1998) 225. 10. Z. Sobalik, A.A. Belhekar, Z. Tvarfi~kov/l and B. Wichterlov/l, Appl. Catal.A: 188 (1999) 175.