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Diffuse reflectance spectra and catalytic activity of Co- and Mo-containing zeolites
Short papers
Loewenstein's rule, then, is a constraint on the siting of aluminium atoms, but relates to the question of randomness only in that it dictates a specific value for a probability (q = 0). The consideration of the parameters p and q as probabilities has additional conceptual benefits. For example, i f p = q, equation (5) reduces to: 1
P=q-I+R This equation characterizes a structure in which it is equally probable that any ,given lattice silicon (or aluminium) has either all aluminium or silicon neighbour. Melchior et al. 8 have termed this structure 'random' but it represents merely one specific case of randomness - however, a most general case in which no distinction between silicon and aluminium atoms is drawn. Treating q as a probability with a functional dependence on R, as in equation (5), thus allows more general
deviations from Loewenstein's rule that remain compatible with the concept of randomness. This potential may be especially significant with zeolites other than faujasite 1' 4, s REFERENCES 1 Lippmaa, E., Magi, M., Samoson, A., Englehardt, G. and Grimmer, A. R. J. Am. Chem. Soc. 1980, 102, 4889; Lippmaa, E., Magi, M., Samoson,A., Tarmak, M. and Englehardt, G. J. Am. Chem. Soc. 1981,103, 4992 2 Freude, D. and Behrens, H.-J. Cryst. Res. Tech. 1981, 16, 3 3 Engelhardt, G., Ziegan, D., Lippmaa, E. and Magi, M. Z. Anorg. AIIg. Chem. 1980, 468, 35 4 Klinowski, J., Thomas, J. M., Fyfe, C. A. and Hartman, J. S. J. Phys. Chem. 1981,85, 2590 5 Bursill, L. A., Lodge, E. A., Thomas, J. M. and Cheetham, A. K. J. Phys. Chem. 1981,85, 2409 6 Englehardt, G., Lohse, U., Lippmaa, E., Tarmak, M. and Magi, M.Z. Anorg. AIIg. Chem. 1981,482, 49 7 Ramdas, S., Thomas, J. M., Klinowski, J., Fyfe, C. A. and Hartman, J. S. Nature 1981,292,228 8 Melchior, M. T., Vaughan, D. E. W. and Jacobson, A. J. J. Am. Chem. Soc. 1982, 104, 4859 9 Mikovsky, R. J. and Marshall, J. F.J. Catal. 1976, 44, 170
Diffuse reflectance spectra and catalytic activity of Co- and Mo-containing zeolites P. Kovacheva, N. Davidova and D. Shopov
Bulgarian Academy of Sciences, Institute of Organic Chemistry, 1040, Sofia, Bulgaria (Received 14 July 1982) Cobalt and/or m o l y b d e n u m containing catalysts, based on type Y zeolite have been used for thiophene conversion studies. It has been established by diffuse reflectance spectroscopy that under the studied conditions cobalt is not reduced and suppresses the reducibility o f molybdenum; and that some o f the Mo changes f r o m octahedral to tetrahedral coordination. Keywords: Thiophene; Co-zeolites; Mo-zeolites; u.v. spectroscopy
INTRODUCTION It is known that the surface and catalytic properties of alumina supported Co and Mo catalysts are improved by the addition of certain amounts of zeolites 1, 2. On the other hand to clarify the influence of the zeolite structure on the formation and the state of the active metal components it is important to study the properties of different zeolites, containing transitional elements - Co, Ni, Mo, Cu, etc. In the present work the changes in the state of the metals after H 2 pretreatment of zeolite type Y, containing Co and/or Mo have been investigated by means of diffuse reflectance spectroscopy. Catalytic activity was evaluated by the degree of thiophene conversion. 0144-2449/831030092--03 $03.00 © Butterworth & Co. (Publishers) Ltd.
92 ZEOLITES 1983, Vo13, April
EXPERIMENTAL The starting material was zeolite Ca-Y with molar ratio SIO2/A1203 = 5.1 (Ca ion-exchange degree about 80%). The sample of CoCa-Y, containing 2.2 wt.% Co, was prepared by ion-exchange with Co-ions. For this purpose Ca-Y was treated with 0.1 N Co(NO3)2.6H20, washed and dried for 3 h at 120°C. The sample of MoCa-Y, containing 7.4 wt.% Mo was prepared by treating the starting Ca-Y zeolite with 10% (NH4)6MoTO24" 4H20, drying at 120°C for 3 h and calcining in air at 450°C for 3 h prior to use. The sample of CoMoCa-Y, with atomic ratio Co/Mo = 0.3, and content of Co and Mo 2.2 and 7.4 wt.% respectively, was prepared from CoCa-Y which was then impregnated with Mo as described for MoCa-Y.
Short papers
tween 250 and 300 nm is observed. Bands of Mo(VI) oxide species in tetrahedral (250-280 nm) and octahedral (290-330 nm) symmetry have been reported in Ref. 4. The absorption band observed at 295 nm for the MoCa-Y sample in the air-dried state (Figure 1 ), strongly suggests the existence of octahedral Mo(VI). After reduction at 350°C a shoulder at 275 nm is observed, which is assigned to the presence of tetrahedral Mo(VI). This shows that one part of the Mo has altered its environment and the other part has been reduced. The reduction is confirmed by the band in the 930-950 nm range which is most probably caused by d-d transitions in octahedral Mo(IV). The reduction of the sample at 450°C causes a strong decrease in the band intensity of Mo and a high background absorbance. Ni in NiCa-X shows similar behaviour after reduction above 250°C as observed by other authors s.
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5O0
Figure 1 Diffuse reflectance spectra of the MoCa-Y sample before reduction (1), after reduction at 350°C (2) and after reduction at 450°C (3)
The state of Co and Mo after reduction of the samples was studied by diffuse reflectance spectroscopy in the 250-2500 nm region. Reduction was carried out by hydrogen increasing the temperature to 350°C and 450°C respectively and keeping the samples for 2 h at the given temperatures. The spectra were taken with a Beckman recording spectrophotometer (Model 52700). Zeolite Na-Y was used as the reference. For comparison, the spectra of the samples dried in air have been used.
A sample containing only Co(CoCa-Y) gives rise to a broad triple band in the 5 2 0 - 6 2 0 nm range. It is due to a Co(II) surrounded tetrahedrally by oxygen ions 6. Figure 2 shows that the intensity of this band is not changed after H2 pretreatment which indicates that Co(II) reduction is not observed under the described conditions. This fact can be connected with the migration of Co(II)-ions to inaccessible sites. A complex of the type Co(H20) 2+ and its possibility for location in the zeolite ~- and/3-cages (S~, Six and S~) has been reported 6. In our case the formation of a triplet at 5 2 0 - 6 2 0 nm can be ascribed to Co(II) interaction with oxygen from the zeolite crystal lattice. A similar band has been observed for pure COA12044. Alumina supported Co samples with similar metal content also exhibit a lower reducibility and this is explained with the interaction between Co(II) and the lattice 7. The spectra of CoMoCa-Y show some features characteristic of both Co and Mo. A partial masking of the Co(II) absorption band is connected with the mode of sample preparation. The spectra in the u.v. region (Figure 3) indicate that the reduction at 350°C leads to decreased amounts of
The catalytic experiments were carried out in a flow apparatus at atmospheric pressure in the temperature range 300°-400°C using 1 ml of catalyst. Hydrogen, saturated with thiophenc at 0~C was fed into the reactor. The feed composition was "~5 vol.% thiophene in hydrogen. The flow rate was 40 ml min -1. Pretreatment of the samples, as well as reaction product analysis, are described elsewhere 3. I
400
RESULTS AND DISCUSSION In the diffuse reflectance spectra of Mo-containing zeolites an absorption band with a maximum be-
I
500
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600 X Cnm)
I
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Figure 2 Diffuse reflectance spectra of the CoCa-Y sample before reduction (1 }, after reduction at 350°C (2) and after reduction at 450°C (3)
ZEOLITES 1983, Vo13, April
93
Short papers
tetrahedral Mo(VI) -- 275 nm. In contrast, with MoCa-Y, this band is still intensive at 450°C in spite of the increased background absorbance. Most probably, Co(II) suppresses the reducibility of Mo(VI) s. This effect is revealed more distinctly after reduction at 450°C in this work.
8 <
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I 300
I 400
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7, (nm) Figure 3 Diffuse reflectance spectra of the CoMoCa-Y sample before reduction (1), after reduction at 350°C (2) and after reduction at 450°C (3)
100
o
The samples under investigation show high catalytic activity for thiophene conversion. The curves in Figure 4 indicate that the common conversion is above 80% at the beginning, and the most stable sample is CoMoCa-Y. Thiophcne conversion products are: tetrahydrothiophene and C r C 4 hydrocarbons. Tetrahydrothiophene is an intermediate product in thiophene conversion to C 1-C4 hydrocarbons. Its amount is always reduced with time and by increasing the temperature. We have already observed such an effect for Nicontaining zeolite catalysts 3. The smallest amount of tetrahydrothiophene is obtained for the CoCa-Y sample. Also, the hydrogenation of butenes yields the smallest butane amount with the same sample (Figure 5a). Cracking of the butane and the mixture of butenes to C1-C3 hydrocarbons is another process that occurs on the zeolite support (Figure 5b). Cracking is observed mainly on CoCa-Y and it is insignificant on MoCa-Y. The reason ['or the above behaviour is that the Co-ions increase the acidity of Ca-Y. The reduction of Mo(VI) in MoCa-Y results in ml increased hydrogenation activity. The reduction of Mo(VI) in the CoMoCa-Y sample, as indicated by the spectra, is difficult. Therefore this sample will manifest decreased activity for cracking but will maintain the hydrogenation activity typical for MoCa-Y. The present results indicate that the diffuse reflectance spectra of zeolite catalysts, containing transition elements, give significant information about the state of the ions in the zeolite thus contributing to the better understanding of the possible paths in thiophene conversion.
50
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ACKNOWLEDGEMENT 0
1
2
3
Time (h) Figure 4 Total thiophene conversion (X) at 300°C as a function of run time: MoCa-Y (1), CoCa-Y (2) and CoMoCa-Y (3)
REFERENCES
40
~b
a
3O
1
I
I
300
350
I 400
300
350
400
T(°C) Figure5 Dependence of amount ofC4H~0(a) andCz-C 3hydrocarbons (b) on the temperature: MoCa-Y (1), CoCa-Y (2) and CoMoCa-Y (3)
Z E O L I T E S 1983, Vo13, A p r i l
Vysotskii, A. V., Chuikova, N. A. and Lipovich, V. G. Kinet.
CataL 1977, 18, 1345
3
o E 2O v o "110 (.9
94
The authors are indebted to Dr. R. Kardjieva for taking the spectra and for helpful discussions.
2 Surin, S. A., Aliev, R. R., Nefedov, B. K., Sidelkovskaya, V. G., Turovskaya, L. V. and Gullyev, Ch. Kinet. CataL 1981, 22, 1327 3 Davidova, N. and Kovacheva, P. Neftechimia 1982, 22, 93 4 Gajardo, P.,Grange, P. andDelmon, B.J. CataL 1980,63,201 5 Olivier, D., Richard, M., Che, M., Bozon-Verduraz, F. and Clarkson, R. B. J. Phys. Chem. 1980, 84, 420 6 Dyakonov, S. S., Kiselev, A. V. and Lygin, V. I. 'Application of Zeolites in Catalysis', Novosibirsk, 1976, p. 106 7 Vagin, A. I., Erofeev, V. I., An, V. V. and Kalechits, I. V. Kinet. Catal. 1 9 8 1 , 2 2 , 1411 8 Loktev, M. I. and Slinkin, A. A. 'ltogi neuki i techniki Kinet. Cata/. 1980, 7, 39
Loewenstein's rule, then, is a constraint on the siting of aluminium atoms, but relates to the question of randomness only in that it dictates a specific value for a probability (q = 0). The consideration of the parameters p and q as probabilities has additional conceptual benefits. For example, i f p = q, equation (5) reduces to: 1
P=q-I+R This equation characterizes a structure in which it is equally probable that any ,given lattice silicon (or aluminium) has either all aluminium or silicon neighbour. Melchior et al. 8 have termed this structure 'random' but it represents merely one specific case of randomness - however, a most general case in which no distinction between silicon and aluminium atoms is drawn. Treating q as a probability with a functional dependence on R, as in equation (5), thus allows more general
deviations from Loewenstein's rule that remain compatible with the concept of randomness. This potential may be especially significant with zeolites other than faujasite 1' 4, s REFERENCES 1 Lippmaa, E., Magi, M., Samoson, A., Englehardt, G. and Grimmer, A. R. J. Am. Chem. Soc. 1980, 102, 4889; Lippmaa, E., Magi, M., Samoson,A., Tarmak, M. and Englehardt, G. J. Am. Chem. Soc. 1981,103, 4992 2 Freude, D. and Behrens, H.-J. Cryst. Res. Tech. 1981, 16, 3 3 Engelhardt, G., Ziegan, D., Lippmaa, E. and Magi, M. Z. Anorg. AIIg. Chem. 1980, 468, 35 4 Klinowski, J., Thomas, J. M., Fyfe, C. A. and Hartman, J. S. J. Phys. Chem. 1981,85, 2590 5 Bursill, L. A., Lodge, E. A., Thomas, J. M. and Cheetham, A. K. J. Phys. Chem. 1981,85, 2409 6 Englehardt, G., Lohse, U., Lippmaa, E., Tarmak, M. and Magi, M.Z. Anorg. AIIg. Chem. 1981,482, 49 7 Ramdas, S., Thomas, J. M., Klinowski, J., Fyfe, C. A. and Hartman, J. S. Nature 1981,292,228 8 Melchior, M. T., Vaughan, D. E. W. and Jacobson, A. J. J. Am. Chem. Soc. 1982, 104, 4859 9 Mikovsky, R. J. and Marshall, J. F.J. Catal. 1976, 44, 170
Diffuse reflectance spectra and catalytic activity of Co- and Mo-containing zeolites P. Kovacheva, N. Davidova and D. Shopov
Bulgarian Academy of Sciences, Institute of Organic Chemistry, 1040, Sofia, Bulgaria (Received 14 July 1982) Cobalt and/or m o l y b d e n u m containing catalysts, based on type Y zeolite have been used for thiophene conversion studies. It has been established by diffuse reflectance spectroscopy that under the studied conditions cobalt is not reduced and suppresses the reducibility o f molybdenum; and that some o f the Mo changes f r o m octahedral to tetrahedral coordination. Keywords: Thiophene; Co-zeolites; Mo-zeolites; u.v. spectroscopy
INTRODUCTION It is known that the surface and catalytic properties of alumina supported Co and Mo catalysts are improved by the addition of certain amounts of zeolites 1, 2. On the other hand to clarify the influence of the zeolite structure on the formation and the state of the active metal components it is important to study the properties of different zeolites, containing transitional elements - Co, Ni, Mo, Cu, etc. In the present work the changes in the state of the metals after H 2 pretreatment of zeolite type Y, containing Co and/or Mo have been investigated by means of diffuse reflectance spectroscopy. Catalytic activity was evaluated by the degree of thiophene conversion. 0144-2449/831030092--03 $03.00 © Butterworth & Co. (Publishers) Ltd.
92 ZEOLITES 1983, Vo13, April
EXPERIMENTAL The starting material was zeolite Ca-Y with molar ratio SIO2/A1203 = 5.1 (Ca ion-exchange degree about 80%). The sample of CoCa-Y, containing 2.2 wt.% Co, was prepared by ion-exchange with Co-ions. For this purpose Ca-Y was treated with 0.1 N Co(NO3)2.6H20, washed and dried for 3 h at 120°C. The sample of MoCa-Y, containing 7.4 wt.% Mo was prepared by treating the starting Ca-Y zeolite with 10% (NH4)6MoTO24" 4H20, drying at 120°C for 3 h and calcining in air at 450°C for 3 h prior to use. The sample of CoMoCa-Y, with atomic ratio Co/Mo = 0.3, and content of Co and Mo 2.2 and 7.4 wt.% respectively, was prepared from CoCa-Y which was then impregnated with Mo as described for MoCa-Y.
Short papers
tween 250 and 300 nm is observed. Bands of Mo(VI) oxide species in tetrahedral (250-280 nm) and octahedral (290-330 nm) symmetry have been reported in Ref. 4. The absorption band observed at 295 nm for the MoCa-Y sample in the air-dried state (Figure 1 ), strongly suggests the existence of octahedral Mo(VI). After reduction at 350°C a shoulder at 275 nm is observed, which is assigned to the presence of tetrahedral Mo(VI). This shows that one part of the Mo has altered its environment and the other part has been reduced. The reduction is confirmed by the band in the 930-950 nm range which is most probably caused by d-d transitions in octahedral Mo(IV). The reduction of the sample at 450°C causes a strong decrease in the band intensity of Mo and a high background absorbance. Ni in NiCa-X shows similar behaviour after reduction above 250°C as observed by other authors s.
\ o
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300
X(nm)
I
I
400
5O0
Figure 1 Diffuse reflectance spectra of the MoCa-Y sample before reduction (1), after reduction at 350°C (2) and after reduction at 450°C (3)
The state of Co and Mo after reduction of the samples was studied by diffuse reflectance spectroscopy in the 250-2500 nm region. Reduction was carried out by hydrogen increasing the temperature to 350°C and 450°C respectively and keeping the samples for 2 h at the given temperatures. The spectra were taken with a Beckman recording spectrophotometer (Model 52700). Zeolite Na-Y was used as the reference. For comparison, the spectra of the samples dried in air have been used.
A sample containing only Co(CoCa-Y) gives rise to a broad triple band in the 5 2 0 - 6 2 0 nm range. It is due to a Co(II) surrounded tetrahedrally by oxygen ions 6. Figure 2 shows that the intensity of this band is not changed after H2 pretreatment which indicates that Co(II) reduction is not observed under the described conditions. This fact can be connected with the migration of Co(II)-ions to inaccessible sites. A complex of the type Co(H20) 2+ and its possibility for location in the zeolite ~- and/3-cages (S~, Six and S~) has been reported 6. In our case the formation of a triplet at 5 2 0 - 6 2 0 nm can be ascribed to Co(II) interaction with oxygen from the zeolite crystal lattice. A similar band has been observed for pure COA12044. Alumina supported Co samples with similar metal content also exhibit a lower reducibility and this is explained with the interaction between Co(II) and the lattice 7. The spectra of CoMoCa-Y show some features characteristic of both Co and Mo. A partial masking of the Co(II) absorption band is connected with the mode of sample preparation. The spectra in the u.v. region (Figure 3) indicate that the reduction at 350°C leads to decreased amounts of
The catalytic experiments were carried out in a flow apparatus at atmospheric pressure in the temperature range 300°-400°C using 1 ml of catalyst. Hydrogen, saturated with thiophenc at 0~C was fed into the reactor. The feed composition was "~5 vol.% thiophene in hydrogen. The flow rate was 40 ml min -1. Pretreatment of the samples, as well as reaction product analysis, are described elsewhere 3. I
400
RESULTS AND DISCUSSION In the diffuse reflectance spectra of Mo-containing zeolites an absorption band with a maximum be-
I
500
[
600 X Cnm)
I
700
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Figure 2 Diffuse reflectance spectra of the CoCa-Y sample before reduction (1 }, after reduction at 350°C (2) and after reduction at 450°C (3)
ZEOLITES 1983, Vo13, April
93
Short papers
tetrahedral Mo(VI) -- 275 nm. In contrast, with MoCa-Y, this band is still intensive at 450°C in spite of the increased background absorbance. Most probably, Co(II) suppresses the reducibility of Mo(VI) s. This effect is revealed more distinctly after reduction at 450°C in this work.
8 <
1
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7, (nm) Figure 3 Diffuse reflectance spectra of the CoMoCa-Y sample before reduction (1), after reduction at 350°C (2) and after reduction at 450°C (3)
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The samples under investigation show high catalytic activity for thiophene conversion. The curves in Figure 4 indicate that the common conversion is above 80% at the beginning, and the most stable sample is CoMoCa-Y. Thiophcne conversion products are: tetrahydrothiophene and C r C 4 hydrocarbons. Tetrahydrothiophene is an intermediate product in thiophene conversion to C 1-C4 hydrocarbons. Its amount is always reduced with time and by increasing the temperature. We have already observed such an effect for Nicontaining zeolite catalysts 3. The smallest amount of tetrahydrothiophene is obtained for the CoCa-Y sample. Also, the hydrogenation of butenes yields the smallest butane amount with the same sample (Figure 5a). Cracking of the butane and the mixture of butenes to C1-C3 hydrocarbons is another process that occurs on the zeolite support (Figure 5b). Cracking is observed mainly on CoCa-Y and it is insignificant on MoCa-Y. The reason ['or the above behaviour is that the Co-ions increase the acidity of Ca-Y. The reduction of Mo(VI) in MoCa-Y results in ml increased hydrogenation activity. The reduction of Mo(VI) in the CoMoCa-Y sample, as indicated by the spectra, is difficult. Therefore this sample will manifest decreased activity for cracking but will maintain the hydrogenation activity typical for MoCa-Y. The present results indicate that the diffuse reflectance spectra of zeolite catalysts, containing transition elements, give significant information about the state of the ions in the zeolite thus contributing to the better understanding of the possible paths in thiophene conversion.
50
x
ACKNOWLEDGEMENT 0
1
2
3
Time (h) Figure 4 Total thiophene conversion (X) at 300°C as a function of run time: MoCa-Y (1), CoCa-Y (2) and CoMoCa-Y (3)
REFERENCES
40
~b
a
3O
1
I
I
300
350
I 400
300
350
400
T(°C) Figure5 Dependence of amount ofC4H~0(a) andCz-C 3hydrocarbons (b) on the temperature: MoCa-Y (1), CoCa-Y (2) and CoMoCa-Y (3)
Z E O L I T E S 1983, Vo13, A p r i l
Vysotskii, A. V., Chuikova, N. A. and Lipovich, V. G. Kinet.
CataL 1977, 18, 1345
3
o E 2O v o "110 (.9
94
The authors are indebted to Dr. R. Kardjieva for taking the spectra and for helpful discussions.
2 Surin, S. A., Aliev, R. R., Nefedov, B. K., Sidelkovskaya, V. G., Turovskaya, L. V. and Gullyev, Ch. Kinet. CataL 1981, 22, 1327 3 Davidova, N. and Kovacheva, P. Neftechimia 1982, 22, 93 4 Gajardo, P.,Grange, P. andDelmon, B.J. CataL 1980,63,201 5 Olivier, D., Richard, M., Che, M., Bozon-Verduraz, F. and Clarkson, R. B. J. Phys. Chem. 1980, 84, 420 6 Dyakonov, S. S., Kiselev, A. V. and Lygin, V. I. 'Application of Zeolites in Catalysis', Novosibirsk, 1976, p. 106 7 Vagin, A. I., Erofeev, V. I., An, V. V. and Kalechits, I. V. Kinet. Catal. 1 9 8 1 , 2 2 , 1411 8 Loktev, M. I. and Slinkin, A. A. 'ltogi neuki i techniki Kinet. Cata/. 1980, 7, 39