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Hydrocracking activity of NiMo-USY zeolite hydrotreating catalysts
® 1997 Elsevier Science B. V. All rights reserved. Hydrotreatment and hydrocracking of oil fractions G,F. Froment, B. Delmon and P. Grange, editors
567
Hydrocracking activity of NiMo - USY zeolite hydrotreating catalysts B. Egla'', J.F. Cambra'', B. Guemez^ P.L Arias'', B. Paweiec^ and J.LG.Fierro^. 1 Departamento de Ingenieria Quimica y del Medio Ambiente. Escuela de Ingenieros. Alda Urquijo s/n. 48013 Bilbao. Spain. 2 Instituto de Catalisis y Petroleoquimica (CSIC). Campus U.A.M. Cantoblanco. 28049 Madrid. Spain. 1.
INTRODUCTION
Hydrotreating reactions are usually carried out with bimetallic catalysts containing Mo or W and Ni or Co depending on the function to be promoted [1,2]. As hydrogenation seems to be an important step In heteroatom removal from polyaromatic compounds the use of Ni as promoter is a growing tendency. As the carrier plays an important role on the activity of the final sulfided catalyst [2], the behaviour of different supports is an important field of research. For example, alumina is the most extended carrier for this kind of reactions, but it has a moderate to low hydrocracking activity when severe hydrotreatment of the feedstock is needed. Research on the application of zeolites in heteroatom removal processes is fairly recent [3], being their exceptional properties as catalityc activity and resistance to poissoning by sulfur and nitrogen containing organic compounds an incentive for its use as hydrotreating catalyst support. In previous works [4-6] the HDS and HDN activities of Ni, Mo and MoNi zeolite supported catalysts were studied, and a remarkable initial deactivation by cocking was detected. Thus in the present work the hydrocracking activity of these catalysts in the sulfided state has been studied. 2. EXPERIMENTAL 2.1. Catalysts The catalyst compositions used in this work are shown in table 1, as measured by atomic absortion. The Ni loaded ultraestable Y (USY) zeolite catalysts were prepared by ion exchange with Ni(N03)2.4H20. These catalysts will be refered to as Nix, where x is the theoretic content of Ni atoms per unit cell, (x = 2,5,9,14,23). The three molibdena loaded ultraestable Y zeolite catalysts were prepared using various procedures and different precursors. One catalyst was prepared by solid ion exchange by mixing the USY zeolite with M0CI5 [7-11]. These catalyst will be referred to hereafter as MoCI. Catalyst MoCO was prepared by wet impregnation of USY zeolite with neutral Mo(CO)6 complex [12-17]. Catalyst 4MoA was prepared by conventional aqueous impregnation of USY zeolite with (NH4)6Mo7024.H20 (HMA) followed by removal of water in a rotary evaporator
568 [18-20]. The Impregnates were dried in vacuum for 8 h and in air at 383 K. Calcination was achieved at 773 K, and for the 4MoA catalyst at low and constant pressure of water extended over a period of time of 800 h [19]. Table 1 Chemical composition of the calcined catalysts and HC activity. Total conversion (wt %) Catalysts
M0O3 (wt %)
NiO (wt %)
Ni2 Ni14 4MoA MoCI MoCO 4MoNi2 4MoNi5 4MoNi9 4MoNi14 4MoNi23
-
0.8 5.0 0.8 1.5 2.3 5.3 10.2
3.9 6.3 3.1 5.2 5.3 5.6 6.0 6.3
548 K 14.3 2.5 7.6 8.2 11.6 19.7 11.5 18.9 13.0
598 K 15.9 35.1 12.8 23.9 30.5 34.2 28.6 25.4 46.4 21.5
648 52.8 21.7 45.7 48.4 66.5 63.1 55.6 65.4 43.4
The five binary MoNi catalysts that will be refered to as 4MoNix, where x is the theoretic content of Ni atoms per unit cell, were prepared by incorporating Ni first by ion exchange following impregnation from aqueous solutions of Ni(N03)2.4H20, being the procedure the one decribed for Ni/USY catalysts. Mo was subsequently incorporated in a second step by solid-solid ion exchange with M0CI5 precursor using the same method described previously for MoCI catalyst. These catalysts were characterised by atomic absortion, nitric oxide and pyridine adsorption and X-ray photoelectron spectroscopy, and results are included in previus works [4-6]. 2.2. Activity tests Hydrocracking of n-decane was performed in a stainless steel fixed bed catalytic reactor. This reactor was filled with glass beads (1 mm diameter) to the desired level for the catalytic bed. The bed, which was between two thin layers of SiC, was made of 0.3 g of catalyst diluted with SiC (1:3) of the same size (0.42-0.75 mm). The activation procedure consited of heating to the sulfidation temperature (673 K) in a nitrogen flow at atmospheric pressure. When this temperature was reached, a flow os H2:H2S (10:1 molar) mixture was passed through the catalyst bed for 4 h. The reactor was then purged in a nitrogen flow at 673 K for 0.5 h, and then cooled to room temperature. Finally, the nitrogen pressure increased up to that of the experiment and the catalytic bed was heated up to the temperature of experiment. Hydrogen and n-decane, with a 1% of dimethyl disulfure to mantain the sulfided state of the metals in the catalysts, were then passed through the reactor, with reaction products analyzed by a on-line HP 5890 A gas
569 chromatograph provided with FID detector. Reactions conditions were: P = 3 MPa, T = 548, 598 and 648 K and W/F = 19.5 Qcat-h/mol. 3. RESULTS The n-decane HC model reaction have been used to establish the perfomance of the Ni, Mo and MoNi zeolite supported catalysts. Results for Mo-zeolites tested in HC as a function of the time on-stream for a temperature of 598 K are shown in figure 1. Figure 1 shows a strong deactivation during the first hours on stream for all catalysts. The MoCO catalysts displays the best HC activity. The second one in activity is the MoCI catalyst and finally the 4MoA, where Mo is well dispersed in zeolite internal channels, but as Mo content is relatively low, and probably diffusional hinderance is present, the concentration of the active sites available for reaction is small and these could explain its low activity.
100
100 4M0-A
O 4MoNi2 ^ 4MoNi5
Mo-CO
80
801
Mo-CI
c o
• D
•
•(0
60
>
60
Bo
40
Q>
c
A
C
4MoNi9
^ 4MoNi14
o •
A
O
A
o 4MoNi23
A
B 40 Q
o I
20
-1—1
2
• •
« • •
• - _
1
1
1
4
1
6
O X
A D
D
4
« fi i I 20 H
r-
8
10
Time on-stream (h)
Figure 1. Mo/USY catalysts. HC activity vs time on stream (T=598 K)
2
4
6
8
10
Time on-stream (h)
Figure 2. MoNi/USY catalysts. HC activity vs time on stream (T=598 K)
For Ni/USY catalysts, as can be seen in table 1, the HC activity increases with increasing Ni content, in agreement with the results obtained in HDS and HDN reactions for these catalysts [5]. The n-decane HC results for MoNi/USY series as a function of the time onstream for a temperature of 598 K are shown in figure 2. These data show a continuous decrease in conversion during the first hours on stream. The most active catalyst is the 4MoNi14 and then the catalysts with the lower Ni content, 4MoNi2 and 4MoNi5. The catalyst that exhibits the lowest HC activity is the
570 4MoNi23, with the highest Ni content, this behaviour could be explained in terms of a blockage of the zeolite porous structure by Ni atoms. From these data, it appears that the Ni content is an important but not a determinant factor in HC activity. 4. DISCUSSION The comparison of the n-decane HC total conversion data for the most active MoNi/USY catalyst, 4MoNi14, with the monometalic precursors, MoCI and Nil4, indicates that, for all the temperatures studied HC activity is higher for the binary system than for the monometallic ones, but it is lower than the corresponding to the addition of their activities, suggesting that there is not a promotional effect between the Ni and the Mo, probably due to the blockage of Ni atoms by the Mo incorporated in a second step As can be seen in table 1, HC activity for Ni catalyst is higher than that for Mo catalyst when comparing catalyts with the same metal content, MoCI and Nil4. as distinguished from the alumina supported in which the Ni sulfided catalysts present a lower activity than the Mo sulfided ones. This finding is in agreement with the work of Welters [21], who stated that these differences in activity are influenced for the metal phases dispersion over the support due to the different metal-support interactions. -J!
HC
I
I
I
S HDS
L
Q HDN |
zzzzzzzzzzzz o o o o o o o o o o o o o t o
Figure 3. Mo/USY catalysts. Comparison of HDS, HDN and HC activities
Figure 4. MoNi/USY catalysts. Comparison of HDS, HDN and HC activities
In order to compare activities in heteroatom removal (HDS and HDN) and HC reactions, the normalized steady state conversions corresponding to the different model compounds for the Mo/USY and MoNi/USY catalysts series are plotted in figures 3 and 4, respectively. Run conditions are different for each reaction, so data
571 are normalized to the total conversion of the MoCO catalyst in Mo series catalysts and to the 4MoNi14 catalyst for the binary systems. For Mo/USY catalysts, figure 3 shows that the activity trend in HDS and HC reactions is the same, that is: MoCO < MoCI < 4MoA, while in HDN reaction, the best catalyst was MoCI, being the catalyst which shows the highest acidity. For MoNi catalysts, the most active catalyst in HDS and HC reactions was 4MoNi14 catalyst with a high Ni content. HDS activity increased with increasing Ni content although for the catalyst with the highest Ni content, 4MoNi23, the conversion is lower, probably due to a blockage of the porous structure with metal atoms. The explanation for the difference in the order of activities in HDS, HDN and HC reactions is based on the fact that the active sites for these reactions are not the same, being the NiMoS phases active for the first two reactions, while the balance between hydrogenating (metal) and acid (support) functions is critical for HC reactions. 5.
CONCLUSIONS
From the results of this work, it may be concluded that: The bimetallic system supported on HY zeolite is more active in HC reaction than the monometallic Ni catalysts and these more than the Mo catalysts. The lack of sinergy in bimetallic catalysts may be due to the blockage of Ni atoms by Mo that is added in the second step. There is an initial deactivation of all the catalysts tested due to cocking reactions. For the binary systems there are not a correlation between metal content and HC activity, although apparently a maximum is found for the 4MoNi14 catalyst. Other factors as residual acidity after deactivation or pore blockage seem to play a significant role. ACKNOWLEDGMENTS This work was supported by the EC- Research Programme JOULE-2 (Contract 0049) and the University of the Basque Country.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
Gates, B.C., Katzer, J.R. and Schuit, G.C.A., "Chemistry of Catalytic Processes", McGraw-Hill, London, (1979). Pratt K.C., Sanders, J.V. and Christov, V., J.Catal., 124, 416 (1990). Vasudevan, P.T. and Fierro, J.L.G., Catal. Rev. Scl Eng., 38(2), 161 (1996). Anderson, J.A., Pawelec, B. y Fierro, J.L.G., Applied Catalysis, 99, 37 (1993). Pawelec, B. "Reacciones de Conversion de Hidrocarburos Sobre Catalizadores de Ni y Mo Depositados en Zeolitas Ultraestables", Tesis Doctoral, Universidad Complutense, Madrid (1994). Arias, P.L, Cambra, J.F., Guemez, M.B., Legarreta J.A., Pawelec, B., y Fierro, J.L.G., Bull. Soc. Chim. Belg. ,104, 197 (1995). Dai, P.E. y Lunsford, J.H., J. Catal., 64,173 (1980). Ward, M.B. y Lunsford, J.H., "Proceeding of 6th International Conference on Zeolites", Butterworths, Guildford, 405 (1984). Johns, J.R. y Howe. R.F., Zeolites, 5, 251 (1985).
572 10. Min-Ming, H. y Howe, R.F., J. Catal.. 108, 283 (1987). 11. Beyer, H.K., Karge, H.G. y Borbely, G., Zeolites, 8,79 (1988). 12. Gallezot, P., Coudurier, G., Primet, M y Imelik, B., ACS Symp. Ser, 40, 144 (1977). 13. Vrinat, M.L, Gachet, C.G. and de Mourgues, L, "Catalysis by Zeolites", Elsevier, Amsterdam, 219 (1980). 14. Balakrishnan, I., Hedge, S.G., Rao, B.S., Kulkarni, S.B. y Ratnasamy, P., "Proceedings of the 4th International Conference of the Chemistry and Uses of Molybdenum", Climax Molybdenum Co., London, 331 (1982). 15. Nambar, S., Komatsu, T. y Yashima, T., Chem. Lett., 115 (1982). 16. Abdo, S. y Howe, R.F., J Phys. Chem., 87,1713 (1983). 17. Kovacheva, P., Davidova, N y Shopov, D., Zeolites, 6, 92 (1983). 18. Cid, R., Gil Llambias, F.J., Fierro, J.L.G., Lopez Agudo, A. y Villasenor, J., J. Catal. 89, 478(1984). 19. Lopez Agudo, A., Cid, R., Orellana, F. y Fierro, J.L G., Polyhedron, 5, 187 (1986). 20. Fierro, J.LG., Conesa, J.C. y Lopez Agudo, A., J. Catal., 108, 334 (1987). 21. Welters, W.J.J., van der Waerden, O.H., Zandbergen, H.W., Ind. Eng. Chem. Res., 34, 1156(1995).
567
Hydrocracking activity of NiMo - USY zeolite hydrotreating catalysts B. Egla'', J.F. Cambra'', B. Guemez^ P.L Arias'', B. Paweiec^ and J.LG.Fierro^. 1 Departamento de Ingenieria Quimica y del Medio Ambiente. Escuela de Ingenieros. Alda Urquijo s/n. 48013 Bilbao. Spain. 2 Instituto de Catalisis y Petroleoquimica (CSIC). Campus U.A.M. Cantoblanco. 28049 Madrid. Spain. 1.
INTRODUCTION
Hydrotreating reactions are usually carried out with bimetallic catalysts containing Mo or W and Ni or Co depending on the function to be promoted [1,2]. As hydrogenation seems to be an important step In heteroatom removal from polyaromatic compounds the use of Ni as promoter is a growing tendency. As the carrier plays an important role on the activity of the final sulfided catalyst [2], the behaviour of different supports is an important field of research. For example, alumina is the most extended carrier for this kind of reactions, but it has a moderate to low hydrocracking activity when severe hydrotreatment of the feedstock is needed. Research on the application of zeolites in heteroatom removal processes is fairly recent [3], being their exceptional properties as catalityc activity and resistance to poissoning by sulfur and nitrogen containing organic compounds an incentive for its use as hydrotreating catalyst support. In previous works [4-6] the HDS and HDN activities of Ni, Mo and MoNi zeolite supported catalysts were studied, and a remarkable initial deactivation by cocking was detected. Thus in the present work the hydrocracking activity of these catalysts in the sulfided state has been studied. 2. EXPERIMENTAL 2.1. Catalysts The catalyst compositions used in this work are shown in table 1, as measured by atomic absortion. The Ni loaded ultraestable Y (USY) zeolite catalysts were prepared by ion exchange with Ni(N03)2.4H20. These catalysts will be refered to as Nix, where x is the theoretic content of Ni atoms per unit cell, (x = 2,5,9,14,23). The three molibdena loaded ultraestable Y zeolite catalysts were prepared using various procedures and different precursors. One catalyst was prepared by solid ion exchange by mixing the USY zeolite with M0CI5 [7-11]. These catalyst will be referred to hereafter as MoCI. Catalyst MoCO was prepared by wet impregnation of USY zeolite with neutral Mo(CO)6 complex [12-17]. Catalyst 4MoA was prepared by conventional aqueous impregnation of USY zeolite with (NH4)6Mo7024.H20 (HMA) followed by removal of water in a rotary evaporator
568 [18-20]. The Impregnates were dried in vacuum for 8 h and in air at 383 K. Calcination was achieved at 773 K, and for the 4MoA catalyst at low and constant pressure of water extended over a period of time of 800 h [19]. Table 1 Chemical composition of the calcined catalysts and HC activity. Total conversion (wt %) Catalysts
M0O3 (wt %)
NiO (wt %)
Ni2 Ni14 4MoA MoCI MoCO 4MoNi2 4MoNi5 4MoNi9 4MoNi14 4MoNi23
-
0.8 5.0 0.8 1.5 2.3 5.3 10.2
3.9 6.3 3.1 5.2 5.3 5.6 6.0 6.3
548 K 14.3 2.5 7.6 8.2 11.6 19.7 11.5 18.9 13.0
598 K 15.9 35.1 12.8 23.9 30.5 34.2 28.6 25.4 46.4 21.5
648 52.8 21.7 45.7 48.4 66.5 63.1 55.6 65.4 43.4
The five binary MoNi catalysts that will be refered to as 4MoNix, where x is the theoretic content of Ni atoms per unit cell, were prepared by incorporating Ni first by ion exchange following impregnation from aqueous solutions of Ni(N03)2.4H20, being the procedure the one decribed for Ni/USY catalysts. Mo was subsequently incorporated in a second step by solid-solid ion exchange with M0CI5 precursor using the same method described previously for MoCI catalyst. These catalysts were characterised by atomic absortion, nitric oxide and pyridine adsorption and X-ray photoelectron spectroscopy, and results are included in previus works [4-6]. 2.2. Activity tests Hydrocracking of n-decane was performed in a stainless steel fixed bed catalytic reactor. This reactor was filled with glass beads (1 mm diameter) to the desired level for the catalytic bed. The bed, which was between two thin layers of SiC, was made of 0.3 g of catalyst diluted with SiC (1:3) of the same size (0.42-0.75 mm). The activation procedure consited of heating to the sulfidation temperature (673 K) in a nitrogen flow at atmospheric pressure. When this temperature was reached, a flow os H2:H2S (10:1 molar) mixture was passed through the catalyst bed for 4 h. The reactor was then purged in a nitrogen flow at 673 K for 0.5 h, and then cooled to room temperature. Finally, the nitrogen pressure increased up to that of the experiment and the catalytic bed was heated up to the temperature of experiment. Hydrogen and n-decane, with a 1% of dimethyl disulfure to mantain the sulfided state of the metals in the catalysts, were then passed through the reactor, with reaction products analyzed by a on-line HP 5890 A gas
569 chromatograph provided with FID detector. Reactions conditions were: P = 3 MPa, T = 548, 598 and 648 K and W/F = 19.5 Qcat-h/mol. 3. RESULTS The n-decane HC model reaction have been used to establish the perfomance of the Ni, Mo and MoNi zeolite supported catalysts. Results for Mo-zeolites tested in HC as a function of the time on-stream for a temperature of 598 K are shown in figure 1. Figure 1 shows a strong deactivation during the first hours on stream for all catalysts. The MoCO catalysts displays the best HC activity. The second one in activity is the MoCI catalyst and finally the 4MoA, where Mo is well dispersed in zeolite internal channels, but as Mo content is relatively low, and probably diffusional hinderance is present, the concentration of the active sites available for reaction is small and these could explain its low activity.
100
100 4M0-A
O 4MoNi2 ^ 4MoNi5
Mo-CO
80
801
Mo-CI
c o
• D
•
•(0
60
>
60
Bo
40
Q>
c
A
C
4MoNi9
^ 4MoNi14
o •
A
O
A
o 4MoNi23
A
B 40 Q
o I
20
-1—1
2
• •
« • •
• - _
1
1
1
4
1
6
O X
A D
D
4
« fi i I 20 H
r-
8
10
Time on-stream (h)
Figure 1. Mo/USY catalysts. HC activity vs time on stream (T=598 K)
2
4
6
8
10
Time on-stream (h)
Figure 2. MoNi/USY catalysts. HC activity vs time on stream (T=598 K)
For Ni/USY catalysts, as can be seen in table 1, the HC activity increases with increasing Ni content, in agreement with the results obtained in HDS and HDN reactions for these catalysts [5]. The n-decane HC results for MoNi/USY series as a function of the time onstream for a temperature of 598 K are shown in figure 2. These data show a continuous decrease in conversion during the first hours on stream. The most active catalyst is the 4MoNi14 and then the catalysts with the lower Ni content, 4MoNi2 and 4MoNi5. The catalyst that exhibits the lowest HC activity is the
570 4MoNi23, with the highest Ni content, this behaviour could be explained in terms of a blockage of the zeolite porous structure by Ni atoms. From these data, it appears that the Ni content is an important but not a determinant factor in HC activity. 4. DISCUSSION The comparison of the n-decane HC total conversion data for the most active MoNi/USY catalyst, 4MoNi14, with the monometalic precursors, MoCI and Nil4, indicates that, for all the temperatures studied HC activity is higher for the binary system than for the monometallic ones, but it is lower than the corresponding to the addition of their activities, suggesting that there is not a promotional effect between the Ni and the Mo, probably due to the blockage of Ni atoms by the Mo incorporated in a second step As can be seen in table 1, HC activity for Ni catalyst is higher than that for Mo catalyst when comparing catalyts with the same metal content, MoCI and Nil4. as distinguished from the alumina supported in which the Ni sulfided catalysts present a lower activity than the Mo sulfided ones. This finding is in agreement with the work of Welters [21], who stated that these differences in activity are influenced for the metal phases dispersion over the support due to the different metal-support interactions. -J!
HC
I
I
I
S HDS
L
Q HDN |
zzzzzzzzzzzz o o o o o o o o o o o o o t o
Figure 3. Mo/USY catalysts. Comparison of HDS, HDN and HC activities
Figure 4. MoNi/USY catalysts. Comparison of HDS, HDN and HC activities
In order to compare activities in heteroatom removal (HDS and HDN) and HC reactions, the normalized steady state conversions corresponding to the different model compounds for the Mo/USY and MoNi/USY catalysts series are plotted in figures 3 and 4, respectively. Run conditions are different for each reaction, so data
571 are normalized to the total conversion of the MoCO catalyst in Mo series catalysts and to the 4MoNi14 catalyst for the binary systems. For Mo/USY catalysts, figure 3 shows that the activity trend in HDS and HC reactions is the same, that is: MoCO < MoCI < 4MoA, while in HDN reaction, the best catalyst was MoCI, being the catalyst which shows the highest acidity. For MoNi catalysts, the most active catalyst in HDS and HC reactions was 4MoNi14 catalyst with a high Ni content. HDS activity increased with increasing Ni content although for the catalyst with the highest Ni content, 4MoNi23, the conversion is lower, probably due to a blockage of the porous structure with metal atoms. The explanation for the difference in the order of activities in HDS, HDN and HC reactions is based on the fact that the active sites for these reactions are not the same, being the NiMoS phases active for the first two reactions, while the balance between hydrogenating (metal) and acid (support) functions is critical for HC reactions. 5.
CONCLUSIONS
From the results of this work, it may be concluded that: The bimetallic system supported on HY zeolite is more active in HC reaction than the monometallic Ni catalysts and these more than the Mo catalysts. The lack of sinergy in bimetallic catalysts may be due to the blockage of Ni atoms by Mo that is added in the second step. There is an initial deactivation of all the catalysts tested due to cocking reactions. For the binary systems there are not a correlation between metal content and HC activity, although apparently a maximum is found for the 4MoNi14 catalyst. Other factors as residual acidity after deactivation or pore blockage seem to play a significant role. ACKNOWLEDGMENTS This work was supported by the EC- Research Programme JOULE-2 (Contract 0049) and the University of the Basque Country.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
Gates, B.C., Katzer, J.R. and Schuit, G.C.A., "Chemistry of Catalytic Processes", McGraw-Hill, London, (1979). Pratt K.C., Sanders, J.V. and Christov, V., J.Catal., 124, 416 (1990). Vasudevan, P.T. and Fierro, J.L.G., Catal. Rev. Scl Eng., 38(2), 161 (1996). Anderson, J.A., Pawelec, B. y Fierro, J.L.G., Applied Catalysis, 99, 37 (1993). Pawelec, B. "Reacciones de Conversion de Hidrocarburos Sobre Catalizadores de Ni y Mo Depositados en Zeolitas Ultraestables", Tesis Doctoral, Universidad Complutense, Madrid (1994). Arias, P.L, Cambra, J.F., Guemez, M.B., Legarreta J.A., Pawelec, B., y Fierro, J.L.G., Bull. Soc. Chim. Belg. ,104, 197 (1995). Dai, P.E. y Lunsford, J.H., J. Catal., 64,173 (1980). Ward, M.B. y Lunsford, J.H., "Proceeding of 6th International Conference on Zeolites", Butterworths, Guildford, 405 (1984). Johns, J.R. y Howe. R.F., Zeolites, 5, 251 (1985).
572 10. Min-Ming, H. y Howe, R.F., J. Catal.. 108, 283 (1987). 11. Beyer, H.K., Karge, H.G. y Borbely, G., Zeolites, 8,79 (1988). 12. Gallezot, P., Coudurier, G., Primet, M y Imelik, B., ACS Symp. Ser, 40, 144 (1977). 13. Vrinat, M.L, Gachet, C.G. and de Mourgues, L, "Catalysis by Zeolites", Elsevier, Amsterdam, 219 (1980). 14. Balakrishnan, I., Hedge, S.G., Rao, B.S., Kulkarni, S.B. y Ratnasamy, P., "Proceedings of the 4th International Conference of the Chemistry and Uses of Molybdenum", Climax Molybdenum Co., London, 331 (1982). 15. Nambar, S., Komatsu, T. y Yashima, T., Chem. Lett., 115 (1982). 16. Abdo, S. y Howe, R.F., J Phys. Chem., 87,1713 (1983). 17. Kovacheva, P., Davidova, N y Shopov, D., Zeolites, 6, 92 (1983). 18. Cid, R., Gil Llambias, F.J., Fierro, J.L.G., Lopez Agudo, A. y Villasenor, J., J. Catal. 89, 478(1984). 19. Lopez Agudo, A., Cid, R., Orellana, F. y Fierro, J.L G., Polyhedron, 5, 187 (1986). 20. Fierro, J.LG., Conesa, J.C. y Lopez Agudo, A., J. Catal., 108, 334 (1987). 21. Welters, W.J.J., van der Waerden, O.H., Zandbergen, H.W., Ind. Eng. Chem. Res., 34, 1156(1995).