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The Nature Of Active Sites On Fluorided Alumina Supported Cobalt-Molybdenum Catalysts
S. Kaliaguine and A. Mahay (Editors), Catalysis on the Energy Scene ~. 1984 Elsevier Science Publishers RV_, Amsterdam - Printed in The Netherlands
55
THE NATURE OF ACTIVE SITES ON FLUORIDED ALUMINA SUPPORTED COBALT-MOLYBDENUM CATALYSTS Z. SARBAK:, P.t1. BOORt1AN, R.A. KYDD* and A. SOtlOGYVARI Department of Chemistry, University of Calgary, Calgary, Alberta, Canada, T2N IN4
ABSTRACT The addition of fluoride ion to Co-Mo-Al~03 catalysts is found to increase their reactivity for cumene conversion. Infrared studies of pyridine and tbutyl nitrile adsorption on these catalysts show that the fluoride ions enhance the Bronsted acidity of the surface and also appear to strengthen the Lewis acid sites. The correlation of catalytic reactivity with acidity is investigated. INTRODUCTION A recent publication discussed the effect of the addition of fluoride ion to Co-Mo-A1 20 3 catalysts, as applied to the hydrocracking of Athabasca bitumen [1]. In developing hydrocracking catalysts which are resistant to deactivation, one approach is to enhance the surface acidity in the hope that adequate cracking will occur at a lower temperature, and that hydrogenation will still be sufficiently rapid to prevent polymerization and coking reactions. Fluoride ion has been shown by several groups of workers [2] to enhance the surface acidity of alumina. In order to investigate more fully the properties of the catalysts described in Ref. 1, we have undertaken an infrared spectral study of their surface acidities, and we have also examined their reactions with various model compounds. In this preliminary account we restrict our discussion to fluorided CO-Mo/A1 20 3 catalysts in their oxide form, and to their reactions with cumene. EXPERH1ENTAL Catalysts: The catalysts were prepared as described in Ref. 1. A summary of these catalysts is given in Table 1. Reagents: Cumene (Eastman Reagent Grade) was purified by passing through A1 20 3 column, and then distilled before use. The pyridine used for IR adsorption studies was A.C.S. grade, distilled and degassed by several freeze-pump-thaw cycles, and stored over molecular sieves before use. Similarly, the t-butylt
On leave from Adam Mickiewicz University, Poznan, Poland
* Author presenting the paper
56
nitrile, also used as a probe molecule, was multiply distilled under a nitrogen atmosphere and then stored over molecular sieves. TABLE 1 Catalyst composition and surface area
Catalyst Description MB MB MB MB
480 501 481 500
Composition (wt ) MoO l CoO 3
3 3 3
15 15 15 15
Number of F atoms per 20 atoms of Al
Surface area mC/g
0 1 2 4
187 197 175 147
Reactor Design: The testing of catalysts with respect to cumene conversion was carried out in a continuous flow, fixed bed reactor. The reactor itself was of stainless steel construction, and an inside diameter of 15 mm. It was packed with alternating layers of quartz wool, and between these layers the catalyst (0.1500 g, particle size 80-100 mesh) was placed. The cumene was contained in a presaturator, held at a suitable constant temperature and through which the carrier gas, helium, was passed at a measured flow rate. Catalyst activation was carried out by heating for 90 min. with a helium flow rate of 30 ml/min. The reaction was then carried out at the desired flow rate, partial pressure of helium, and temperature, over a four hour period. Samples of the feed and products were collected every 16 min. in a sample loop collector, which was in turn directly connected to the gc (Varian 3700, T.C. Detector, equipped with CDS-Ill data processor; column description: 6 ft. x 1/8 in. o.d. stainless steel, packed with 5% Bentone-34 + 5% Di-isodecylphthalate on 60-80 mesh Chromosorb W.) Definitions of Conversion (C) and Selectivity (S) are: C(%) Cumene (inlet) - Cumene (outlet) X 100 Cumene (inlet) S(%) = Product X 100 Cumene (inlet) - Cumene (outlet) G.C.-M.S. analysis of reaction products and unreacted cumene was carried out by collection of the outlet gases in a trap at liquid N2 temperature, followed by gc analysis (as above) and mass spectrometry, using a Hewlett Packard 5990 A. Infrared Spectroscopic Studies: Infrared spectra were obtained at 2 cm- 1 resolution on a Nicolet 8000 Fourier transform spectrometer. The greaseless cell used for these studies is similar to the IR cell described previously [3]. It holds four sample wafers at a time, so different samples can be subjected to identical treatment before their spectra are measured. Samples weighing ca. 35 mg were formed at low pressure into 13 mm diameter self-supporting wafers. These samples were activated by subjecting them to evacuation at 673 K, then cleaned
57
in 0 (at 673 K) before adsorbing either pyridine or TBN (t-butylnitrile), the probe molecules (at 293 K). Spectra were recorded after desorption at different temperatures (293, 393 and 493 K) and in the case of pyridine, H:O vapour was then added to enhance the Bronsted acidity. RESULTS 1. Catalytic Reactivity: The conversion of cumene over these catalysts gave C1-C 4 hydrocarbons, benzene and a-methylstyrene as major products and small amounts «1~) of toluene, ethyl benzene and styrene as by-products.
2)
0 .> <,
6
CH 2 II
+ CH I
Dealkylation of cumene
CH 3
Dehydrogenation of cumene
The typical pattern of cumene conversion and selectivity with time on stream is shown in Fig. 1, confirming that dealkylation and dehydrogenation are the major processes occurring. Isomerization 60 products (e.g. n-propylbenzene, ethyltoluene) and disproportionation products 50 (e.g. di-isopropylbenzene) were not ~ • • ••• • • formed. The conversion of cumene and ....>~ 40 the yield of products was found to ....u w depend on several parameters which are -J ~ 30 discussed below. z o a. Effect of contact time: The ~ 20 ratio W/F (in g hr/mole) which is a z> o measure of contact time is defined as 10 .......--....... W/F catalyst weight (g) feed rate of cumene (mole/hr) It was found that cumene conversion I 2 3 4 TIME IN HOURS reaches a maximum at W/F 450 and Fig. 1 Conversion of cumene and remains constant at ratios above selectivity of benzene and a-methylthat, indicating that at W/F > 450 styrene. • tota 1 cumene convers i on; • selectivity of benzene; • selectall catalytic sites are covered by ivity of a-methyl styrene. cumene molecules.
\
.
(f)
O
.&
..
58
b. Effect of activation temperature: Cumene conversion was found to increase with activation temperature up to 500°C, and then decrease at higher activation temperatures. For this reason all catalysts were activated at 500DC for subsequent studies of reactivity. c. Effect of reaction temperature: Catalyst deactivation occurs much more rapidly at higher reaction temperatures, as shown in Fig. 2. In addition, in separate experiments it was shown that at high temperatures, the reactor itself, with quartz wool in place of the catalyst, showed some activity; cumene conversion was linear with reaction temperature up to 4500C, but at higher temperatures thermal cracking started to playa role. Therefore all catalytic investigations were made at a reaction temperature of 4000C. The apparent activation energy for cumene conversion calculated from the Arrhenius plot shown as Fig. 3 for catalyst MB 481, at reaction temperatures
i! z
0
(f)
z 0·5
90
0
Vi OA a:: w > z 0·3
a:: 80 w
> z 70
0
0
o 60
u
z 50
z
::J U
::J U
w
w
w 0·2
w ::!: 40
::!:
\
TIME
2 IN
3
4
HOURS
Fig. 2 Deactivation of catalyst MB 481 during cumene conversion at different reaction temperatures: • 550°C; "'500 0C.
1·5 \·6 1'7 1 3 liT X 10 (K- )
Fig. 3 Arrhenius plot for cumene conversion on catalyst MB 481 activated at 500°C.
between 300°C and 400°C, is 7.0 kcal/mole. For comparison, Corma and Wojciechowski reported 11 to 47 kcal/mole for amorphous and crystalline silicaalumina catalysts [4]. Since our experiments were run under such conditions that diffusional limitations were absent, it appears that alumina supported cobalt-molybdenum catalysts modified by fluoride are significantly better catalysts. d. Effect of fluoride content: The catalytic reactivity of the various samp16 was studied under the optimal conditions chosen as described above, viz. activation temperature = 500°C, reaction temperature = 4000C, W/F = 450 g hr mole-I. The total activity of the catalysts was found to increase with fluoride ion content, i.e. MB 480 < MB 501 < MB 481 < MB 500. Preliminary results (not accurately calibrated for different gases) for the different samples are summarized
59
in Table 2, which also includes some IR results (see next section). results are also shown in Fig. 4.
These
TABLE 2 Effect of fluoride addition Sample
Conversion C (%)
y-ALO J MB 480 MB 501 MB 481 MB 500
29 33 43 47 59
Selectivity, S (%) Dealkylation Dehydrogenation 28.4 32.3 33.5 33.8 30
0.6 0.7 9.5
13
29
')CN (TBN)
[B]/[L] (pyridine)
2294.0 2293.6 2295.3 2296.1 2296.2
0 0.210 0.225 0.256 0.278
Fig. 4 Cumene conversion as a function of fluoride content.
z 60
o
(Jl
0:: W
> 50 z o o
~ 40
w
:!: ::J
o
o I 2 3 ATOMIC RATIO F/20 AI
4
2; Infrared Studies: The infrared spectra of various probe molecules adsorbed on catalyst surfaces are commonly used to study surface acidity. In this work pyridine and 2,2-dimethylpropanenitrile (t-butylnitrile, TBN) were used as probe molecules. Pyridine is the standard molecule used for such studies [5] and can reveal the presence of both lewis and Bronsted sites. TBN, which is a weaker base, has been used to study Lewis sites [2,6]. The pyridine vibrational mode assignments employed in this section are well established. lewis-pyridine (L-Py) gives rise to a band near 1450 em-I, Bronsted-pyridine (B-Py) a band near 1545 em-I, and a band near 1490 cm- l contains both Bronsted and Lewis components. For TBN adsorbed on oxide surfaces, the position of the band near 2300 cm- l has been used as an indicator of lewis acid strength [2,6]. a. Pyridine adsorption: Alumina itself exhibits no Bronsted acidity, and the characteristic B-Py band at 1545 cm- l is not found on such samples, as can be seen in Fig. 5. As also can be seen, the presence of cobalt and molybdenum produces a weak Bronsted-pyridine peak at 1545 cm- l (MB 480, spectrum B) and the
60
additional presence of fluoride ion intensifies this adsorption signif-cantly (~B 481, spectrum C). These spectra were obtained in the absence of H:O vapour, showing that the Bronsted acidity does not arise from H20 adsorbed on Lewis sites on the alumina surface, but is associated with the cobaltmolybdenum. The Bronsted acidity reported in Table 2 (as [B]/[LJ) was w u measured after addition of H,O Z
61
molybdenum/alumina catalysts studied by Segawa and Hall in that the Bronsted sites are associated with the added transition metal oxides while the strength of the Lewis acid sites is not affected by their presence. Bronsted acidity certainly increases with fluoride content (see Table 2), but our results are somewhat at variance with those of Tejuca ~ al. in that the strength of the Lewis sites (although not their concentration) also appears to increase slightly with fluoride concentration. Catalyst reactivity for cumene conversion for our Co-Mo/alumina catalysts was found to be related to fluoride content, as shown in Fig. 4 and Table 2. Whether cumene conversion depends on Bronsted or Lewis sites is still an open question. As shown in Fig. 6, there is a good relationship between the Bronsted to Lewis ratio and total cumene conversion, which could be interpreted as indicating the importance of Bronsted sites. 028
027 026 0·25
Fig. 6 Relationship between surface acidity and cumene conversion.
[BJ 0·24
ill
0.23 0·22 0·21
40
C UMENE
50
60
CONVERSION ("10)
However, the data in Table 2, while not conclusive, also suggest that cumene conversion increased with Lewis acid strength (i.e. with vCN of TBN). Further work is underway to resolve the question of which of these sites plays a primary role in cumene conversion. As can also be seen from Table 2, the deakylation selectivity of the reaction apparently is influenced by the same factors as the total conversion, i.e. fluoride content via either Bronsted or Lewis sites. The dehydrogenation selectivity however, is determined by different factors. Dehydrogenation increases from 28.4 to 32.3% in the presence of Co-Mo. However, adding increasing amounts of F- to the Co-Mo/A1 20 3 samples has no consistent influence on the dehydrogenation. ACKNOWLEDGEMENTS The continued assistance and encouragement given to us by Drs. J. Kriz and M. Ternan is gratefully acknowledged. This work was supported by an NSERC Strategic Grant (to RAK and PMB).
62
REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
P.M. Boorman, J.F. Kriz, J.R. Brown and M. Ternan, Proc. Fourth Climax Inter. Conf. Chemistry and Uses of Molybdenum (H.F. Barry and P.C.H. Mitchell eds.) Climax Molydbenum Co., Golden, Colorado. 1982, p. 192-196. See, for example, P.O. Scokart, S.A. Selim, J.P. Damon and P.G. Rouxhet, J. Colloid Interface Sci. 70 (1979) 209. M.B. Sayed, R.A. Kydd and R.P. Cooney, J. Catal. in press. A. Carma and B.W. Wojciechowski, Catal. Rev. Sci. Eng. 24 (1982) 1-65. H. Kniizinger, Adv. Catal. 25 (1976) 184. P.O. Scokart and P.G. Rouxhet, J. Colloid Interface Sci., 86 (1982) 96. Y. Amenomiya, J. Catal. 46 (1977) 326. H.G. Karge, Z. Sarbak, K. Hatada, J. Weitkamp and P. Jacobs, J. Catal. 82 (1983) 236-239, and references therein. T. Tagawa, T. Hattori and Y. Murakami, J. Catal. 75 (1982) 56. K. Segawa and ~J. K. Hall, J. Catal. 76 (1982) 133. L.G. Tejuca, C.H. Rochester, A.L. Agudo and J.L.G. Fierro, J. Chem. Soc. Faraday Trans. I, 9 (1983) 2543.
55
THE NATURE OF ACTIVE SITES ON FLUORIDED ALUMINA SUPPORTED COBALT-MOLYBDENUM CATALYSTS Z. SARBAK:, P.t1. BOORt1AN, R.A. KYDD* and A. SOtlOGYVARI Department of Chemistry, University of Calgary, Calgary, Alberta, Canada, T2N IN4
ABSTRACT The addition of fluoride ion to Co-Mo-Al~03 catalysts is found to increase their reactivity for cumene conversion. Infrared studies of pyridine and tbutyl nitrile adsorption on these catalysts show that the fluoride ions enhance the Bronsted acidity of the surface and also appear to strengthen the Lewis acid sites. The correlation of catalytic reactivity with acidity is investigated. INTRODUCTION A recent publication discussed the effect of the addition of fluoride ion to Co-Mo-A1 20 3 catalysts, as applied to the hydrocracking of Athabasca bitumen [1]. In developing hydrocracking catalysts which are resistant to deactivation, one approach is to enhance the surface acidity in the hope that adequate cracking will occur at a lower temperature, and that hydrogenation will still be sufficiently rapid to prevent polymerization and coking reactions. Fluoride ion has been shown by several groups of workers [2] to enhance the surface acidity of alumina. In order to investigate more fully the properties of the catalysts described in Ref. 1, we have undertaken an infrared spectral study of their surface acidities, and we have also examined their reactions with various model compounds. In this preliminary account we restrict our discussion to fluorided CO-Mo/A1 20 3 catalysts in their oxide form, and to their reactions with cumene. EXPERH1ENTAL Catalysts: The catalysts were prepared as described in Ref. 1. A summary of these catalysts is given in Table 1. Reagents: Cumene (Eastman Reagent Grade) was purified by passing through A1 20 3 column, and then distilled before use. The pyridine used for IR adsorption studies was A.C.S. grade, distilled and degassed by several freeze-pump-thaw cycles, and stored over molecular sieves before use. Similarly, the t-butylt
On leave from Adam Mickiewicz University, Poznan, Poland
* Author presenting the paper
56
nitrile, also used as a probe molecule, was multiply distilled under a nitrogen atmosphere and then stored over molecular sieves. TABLE 1 Catalyst composition and surface area
Catalyst Description MB MB MB MB
480 501 481 500
Composition (wt ) MoO l CoO 3
3 3 3
15 15 15 15
Number of F atoms per 20 atoms of Al
Surface area mC/g
0 1 2 4
187 197 175 147
Reactor Design: The testing of catalysts with respect to cumene conversion was carried out in a continuous flow, fixed bed reactor. The reactor itself was of stainless steel construction, and an inside diameter of 15 mm. It was packed with alternating layers of quartz wool, and between these layers the catalyst (0.1500 g, particle size 80-100 mesh) was placed. The cumene was contained in a presaturator, held at a suitable constant temperature and through which the carrier gas, helium, was passed at a measured flow rate. Catalyst activation was carried out by heating for 90 min. with a helium flow rate of 30 ml/min. The reaction was then carried out at the desired flow rate, partial pressure of helium, and temperature, over a four hour period. Samples of the feed and products were collected every 16 min. in a sample loop collector, which was in turn directly connected to the gc (Varian 3700, T.C. Detector, equipped with CDS-Ill data processor; column description: 6 ft. x 1/8 in. o.d. stainless steel, packed with 5% Bentone-34 + 5% Di-isodecylphthalate on 60-80 mesh Chromosorb W.) Definitions of Conversion (C) and Selectivity (S) are: C(%) Cumene (inlet) - Cumene (outlet) X 100 Cumene (inlet) S(%) = Product X 100 Cumene (inlet) - Cumene (outlet) G.C.-M.S. analysis of reaction products and unreacted cumene was carried out by collection of the outlet gases in a trap at liquid N2 temperature, followed by gc analysis (as above) and mass spectrometry, using a Hewlett Packard 5990 A. Infrared Spectroscopic Studies: Infrared spectra were obtained at 2 cm- 1 resolution on a Nicolet 8000 Fourier transform spectrometer. The greaseless cell used for these studies is similar to the IR cell described previously [3]. It holds four sample wafers at a time, so different samples can be subjected to identical treatment before their spectra are measured. Samples weighing ca. 35 mg were formed at low pressure into 13 mm diameter self-supporting wafers. These samples were activated by subjecting them to evacuation at 673 K, then cleaned
57
in 0 (at 673 K) before adsorbing either pyridine or TBN (t-butylnitrile), the probe molecules (at 293 K). Spectra were recorded after desorption at different temperatures (293, 393 and 493 K) and in the case of pyridine, H:O vapour was then added to enhance the Bronsted acidity. RESULTS 1. Catalytic Reactivity: The conversion of cumene over these catalysts gave C1-C 4 hydrocarbons, benzene and a-methylstyrene as major products and small amounts «1~) of toluene, ethyl benzene and styrene as by-products.
2)
0 .> <,
6
CH 2 II
+ CH I
Dealkylation of cumene
CH 3
Dehydrogenation of cumene
The typical pattern of cumene conversion and selectivity with time on stream is shown in Fig. 1, confirming that dealkylation and dehydrogenation are the major processes occurring. Isomerization 60 products (e.g. n-propylbenzene, ethyltoluene) and disproportionation products 50 (e.g. di-isopropylbenzene) were not ~ • • ••• • • formed. The conversion of cumene and ....>~ 40 the yield of products was found to ....u w depend on several parameters which are -J ~ 30 discussed below. z o a. Effect of contact time: The ~ 20 ratio W/F (in g hr/mole) which is a z> o measure of contact time is defined as 10 .......--....... W/F catalyst weight (g) feed rate of cumene (mole/hr) It was found that cumene conversion I 2 3 4 TIME IN HOURS reaches a maximum at W/F 450 and Fig. 1 Conversion of cumene and remains constant at ratios above selectivity of benzene and a-methylthat, indicating that at W/F > 450 styrene. • tota 1 cumene convers i on; • selectivity of benzene; • selectall catalytic sites are covered by ivity of a-methyl styrene. cumene molecules.
\
.
(f)
O
.&
..
58
b. Effect of activation temperature: Cumene conversion was found to increase with activation temperature up to 500°C, and then decrease at higher activation temperatures. For this reason all catalysts were activated at 500DC for subsequent studies of reactivity. c. Effect of reaction temperature: Catalyst deactivation occurs much more rapidly at higher reaction temperatures, as shown in Fig. 2. In addition, in separate experiments it was shown that at high temperatures, the reactor itself, with quartz wool in place of the catalyst, showed some activity; cumene conversion was linear with reaction temperature up to 4500C, but at higher temperatures thermal cracking started to playa role. Therefore all catalytic investigations were made at a reaction temperature of 4000C. The apparent activation energy for cumene conversion calculated from the Arrhenius plot shown as Fig. 3 for catalyst MB 481, at reaction temperatures
i! z
0
(f)
z 0·5
90
0
Vi OA a:: w > z 0·3
a:: 80 w
> z 70
0
0
o 60
u
z 50
z
::J U
::J U
w
w
w 0·2
w ::!: 40
::!:
\
TIME
2 IN
3
4
HOURS
Fig. 2 Deactivation of catalyst MB 481 during cumene conversion at different reaction temperatures: • 550°C; "'500 0C.
1·5 \·6 1'7 1 3 liT X 10 (K- )
Fig. 3 Arrhenius plot for cumene conversion on catalyst MB 481 activated at 500°C.
between 300°C and 400°C, is 7.0 kcal/mole. For comparison, Corma and Wojciechowski reported 11 to 47 kcal/mole for amorphous and crystalline silicaalumina catalysts [4]. Since our experiments were run under such conditions that diffusional limitations were absent, it appears that alumina supported cobalt-molybdenum catalysts modified by fluoride are significantly better catalysts. d. Effect of fluoride content: The catalytic reactivity of the various samp16 was studied under the optimal conditions chosen as described above, viz. activation temperature = 500°C, reaction temperature = 4000C, W/F = 450 g hr mole-I. The total activity of the catalysts was found to increase with fluoride ion content, i.e. MB 480 < MB 501 < MB 481 < MB 500. Preliminary results (not accurately calibrated for different gases) for the different samples are summarized
59
in Table 2, which also includes some IR results (see next section). results are also shown in Fig. 4.
These
TABLE 2 Effect of fluoride addition Sample
Conversion C (%)
y-ALO J MB 480 MB 501 MB 481 MB 500
29 33 43 47 59
Selectivity, S (%) Dealkylation Dehydrogenation 28.4 32.3 33.5 33.8 30
0.6 0.7 9.5
13
29
')CN (TBN)
[B]/[L] (pyridine)
2294.0 2293.6 2295.3 2296.1 2296.2
0 0.210 0.225 0.256 0.278
Fig. 4 Cumene conversion as a function of fluoride content.
z 60
o
(Jl
0:: W
> 50 z o o
~ 40
w
:!: ::J
o
o I 2 3 ATOMIC RATIO F/20 AI
4
2; Infrared Studies: The infrared spectra of various probe molecules adsorbed on catalyst surfaces are commonly used to study surface acidity. In this work pyridine and 2,2-dimethylpropanenitrile (t-butylnitrile, TBN) were used as probe molecules. Pyridine is the standard molecule used for such studies [5] and can reveal the presence of both lewis and Bronsted sites. TBN, which is a weaker base, has been used to study Lewis sites [2,6]. The pyridine vibrational mode assignments employed in this section are well established. lewis-pyridine (L-Py) gives rise to a band near 1450 em-I, Bronsted-pyridine (B-Py) a band near 1545 em-I, and a band near 1490 cm- l contains both Bronsted and Lewis components. For TBN adsorbed on oxide surfaces, the position of the band near 2300 cm- l has been used as an indicator of lewis acid strength [2,6]. a. Pyridine adsorption: Alumina itself exhibits no Bronsted acidity, and the characteristic B-Py band at 1545 cm- l is not found on such samples, as can be seen in Fig. 5. As also can be seen, the presence of cobalt and molybdenum produces a weak Bronsted-pyridine peak at 1545 cm- l (MB 480, spectrum B) and the
60
additional presence of fluoride ion intensifies this adsorption signif-cantly (~B 481, spectrum C). These spectra were obtained in the absence of H:O vapour, showing that the Bronsted acidity does not arise from H20 adsorbed on Lewis sites on the alumina surface, but is associated with the cobaltmolybdenum. The Bronsted acidity reported in Table 2 (as [B]/[LJ) was w u measured after addition of H,O Z
61
molybdenum/alumina catalysts studied by Segawa and Hall in that the Bronsted sites are associated with the added transition metal oxides while the strength of the Lewis acid sites is not affected by their presence. Bronsted acidity certainly increases with fluoride content (see Table 2), but our results are somewhat at variance with those of Tejuca ~ al. in that the strength of the Lewis sites (although not their concentration) also appears to increase slightly with fluoride concentration. Catalyst reactivity for cumene conversion for our Co-Mo/alumina catalysts was found to be related to fluoride content, as shown in Fig. 4 and Table 2. Whether cumene conversion depends on Bronsted or Lewis sites is still an open question. As shown in Fig. 6, there is a good relationship between the Bronsted to Lewis ratio and total cumene conversion, which could be interpreted as indicating the importance of Bronsted sites. 028
027 026 0·25
Fig. 6 Relationship between surface acidity and cumene conversion.
[BJ 0·24
ill
0.23 0·22 0·21
40
C UMENE
50
60
CONVERSION ("10)
However, the data in Table 2, while not conclusive, also suggest that cumene conversion increased with Lewis acid strength (i.e. with vCN of TBN). Further work is underway to resolve the question of which of these sites plays a primary role in cumene conversion. As can also be seen from Table 2, the deakylation selectivity of the reaction apparently is influenced by the same factors as the total conversion, i.e. fluoride content via either Bronsted or Lewis sites. The dehydrogenation selectivity however, is determined by different factors. Dehydrogenation increases from 28.4 to 32.3% in the presence of Co-Mo. However, adding increasing amounts of F- to the Co-Mo/A1 20 3 samples has no consistent influence on the dehydrogenation. ACKNOWLEDGEMENTS The continued assistance and encouragement given to us by Drs. J. Kriz and M. Ternan is gratefully acknowledged. This work was supported by an NSERC Strategic Grant (to RAK and PMB).
62
REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
P.M. Boorman, J.F. Kriz, J.R. Brown and M. Ternan, Proc. Fourth Climax Inter. Conf. Chemistry and Uses of Molybdenum (H.F. Barry and P.C.H. Mitchell eds.) Climax Molydbenum Co., Golden, Colorado. 1982, p. 192-196. See, for example, P.O. Scokart, S.A. Selim, J.P. Damon and P.G. Rouxhet, J. Colloid Interface Sci. 70 (1979) 209. M.B. Sayed, R.A. Kydd and R.P. Cooney, J. Catal. in press. A. Carma and B.W. Wojciechowski, Catal. Rev. Sci. Eng. 24 (1982) 1-65. H. Kniizinger, Adv. Catal. 25 (1976) 184. P.O. Scokart and P.G. Rouxhet, J. Colloid Interface Sci., 86 (1982) 96. Y. Amenomiya, J. Catal. 46 (1977) 326. H.G. Karge, Z. Sarbak, K. Hatada, J. Weitkamp and P. Jacobs, J. Catal. 82 (1983) 236-239, and references therein. T. Tagawa, T. Hattori and Y. Murakami, J. Catal. 75 (1982) 56. K. Segawa and ~J. K. Hall, J. Catal. 76 (1982) 133. L.G. Tejuca, C.H. Rochester, A.L. Agudo and J.L.G. Fierro, J. Chem. Soc. Faraday Trans. I, 9 (1983) 2543.