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Preparation of Nickel Catalyst from Nickel Containing Chrysotile
B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
519
PREPARATION OF NICKEL CATALYST FROM NICKEL CONTAINING CHRYSOTILE Y. ONO, N. KIKUCHI and H. WATANABE Department of Chemical Engineering, Meguro-ku, Tokyo 152 (Japan)
Tokyo Institute of Technology,
Ookayama,
SUMMARY Nickel-containing chrysotiles (NixM93_x(OH)4Si205) were prepared under hydrothermal conditions. They showed the catalytic activity for ethylene dimerization after their treatment under vacuum at high temperatures, which leads to the dehydroxylation of the chrysotiles. The active sites for the dimerization are plausibly nickel cations exposed to the surface. Nickel metal catalysts were prepared by reducing the chrysotiles with hydrogen. The catalytic activity for ethylene hydrogenation at -50 0C increased with the reduction temperature up to 450 0C, and decreased by further increase of the reduction temperatures, though the extent of Ni (II) reduction monotonically increased with the temperature. Electron micrograph study revealed that the particle size of Ni metal increased with the reduction temperature, indicating that the lowering of the hydrogenation activity at higher reduction temperature can be ascribed to the lowering of the metal surface area due to the growth of the metal particles. Very uniform nickel particles can be prepared especially from the chrysotile of x = 0.6.
INTRODUCTION Chrysotile M93(OH)4Si05 is a layered magnesium silicate and can be synthesized under hydrothermal conditions (ref.
1).
Because of its defined crystal
structure together with its high surface area, chrysotile may be useful as a catalyst or as a support for a catalytically active species. Suzuki and Ono (ref. 2) studied the reaction of 2-propanol over chrysotile and discussed on the acid-base property of the surface. Catalytically active materials could also be obtained by incorporating metal ions like Ni(II), Co(II) and Al(III) in place of all or part of the Mg(II) ions in the structure. Such applications has been reviewed by Swift (ref. 3). Kibby et a L, (ref. 4) reported on the preparation of nickel metal catalysts by reducing nickel containing chrysotile. Jacobs et al. (ref. 5) studied the catalytic property of the reduced garnierite (a nickel-containing mineral with chrysotile structure). In this work, (1) chrysotiles in which part or all of the Mg(II) ions were replaced by Ni(II) ions were prepared and their catalytic activities for the dimerization of ethylene were studied, and (2) nickel metal catalysts were prepared by reducing Ni(II)containing chrysotiles with hydrogen and their catalytic activities for hydrogenation of ethylene and carbon monoxide were examined.
520 METHODS Synthesis of Ni-containing chrysotiles Nickel-chrysotile (Ni3(OH)4Si20S) was synthesized according to the method described by Noll (ref.
and Sholten et a L, (ref. 6). Sodium metasilicate
1)
(Na 2Si0 of 94.7 g and nickel sulfate (NiS0 46H 20) of 141 g was dissolved 39H20) into 600 cm 3 of water. To this solution, 19.1 g of sodium hydroxide was added and the mixture was stirred. The resulting gel of pH 11.8-12.0 with the composition of Ni/Si ~ loS and H20/Si ~ 118, was then placed in an autoclave. The temperature was raised to 295 0C in 4 h and kept at 295 0C for 8 h. After cooling, the insoluble product was washed and dried at l80 oC. The materials with both Ni and Mg cations (NixM93_x(OH)4Si20S) were synthesized similarly except that the part of nickel sulfate was replaced by magnesium chloride
Catalytic reactions The treatment of the chrysotile samples and the catalytic reactions were carried out with a conventional gas-recirculation system. During the reduction of the chrysotiles with hydrogen, the water produced was trapped in aU-shape tube. The extent of the reduction was estimated from the pressure drop of hydrogen. The rate of ethylene hydrogenation or carbon monoxide was determined also from the pressure drop of the system. RESULTS AND DISCUSSION Structure of the synthesized materials X-ray diffraction patterns confirmed that the synthesized materials had the chrysotile structure. Fig. lea) shows the electron transmission micropraph of Ni-chrysotile. The hallow tube form of the chrysotile is clearly seen in Fig. lea). The length of the tubes ranges from 650 to 11000 A, mainly ZOOO-3000 A.The outer and inner diameter of the tubes are 250-400 A and 70-110 A, respectively. Fig. lea) also shows the layer structure of the nickel-chrysotile. The synthesized materials with Ni/Mg
~
1 and Ni/Mg
~
1/4 showed essentially identical X-
ray diffraction patterns to Ni-chrysotile and had similar physical forms. The BET surface areas of Ni-chrysotile and the materials with Ni/Mg 63.2, 59.5 and 47.6 m2g- l, respectively.
~
1 and 1/4 are
Heat treatment of nickel crysotile To know the thermal stability of the prepared nickel chrysotile, the thermal analysis was made. The weight loss of the sample with an endothermic peak was observed in the temperature range of 500-6000C. The weight loss corresponded with the loss of all the hydroxyl groups as water. The X-ray diffraction pattern was examined for the samples heated under vacuum at various tempera-
521
Fig. 1. Electron micrograph of nickel-chrysotile. (a) as prepared. (b) after heating under vacuum for 2 h at 700°C.
522 tures for 2 h. The sample treated at 400 0C gave similar pattern as the original one,
while the pattern due to chrysotile structure was completely lost for the
samples treated above 500 0C, Fig.
in conformity with result of the thermal analysis.
l(b) shows the electron micrograph of nickel chrysotile after heat treat-
ment at 700 0C for 2 h under vacuum. As mentioned above, this sample showed no X-ray diffraction lines due to the chrysotile structure. However, Fig. l(b) clearly shows that the tubular form of the material was mostly retained and even the layer structure of the material was also kept although it was more or less disordered. The samples with Ni/Mg
1 and Ni/Mg
1/4 behaved very similarly with
nickel chrysotile. Dimerization of ethylene The reaction of ethylene over Ni-chrysotile was carried out at SOoC in a conventional gas-recirculation system with initial pressure of 47.6 kPa.
Prior
to the reaction, Ni-chrysotile was heated under vacuum for 2 h at varying temperatures. The main reaction is the dimerization of ethylene,
hexenes being
also formed over the samples heated over 450 0C. Among the butenes produced, trans-2-butene is most predominant over the samples treated at 300 and 400 0C. On the other hand, 450 0C,
I-butene was most predominant over the samples treated over
indicating that the mechanism of the dimerization depends on the heat-
treatment temperature of Ni-chrysotile. Over Ni-chrysotile treated at 500 0C, the butene distribution changed with reaction time. Thus, the fraction I-butene decreased gradually with reaction time and that of 2-butenes increased instead. The activation energy of the dimerization was estimated to be 20.0 kJ mol- l from the temperature dependence of the initial rate over the sample treated at 500 0C. Fraction of I-butene among butenes decreased with the reaction temperature, indicating the increase in the rate of butene isomerization with temperature. The initial rates of the reaction calculated from the pressure drop-time curves were plotted as a function of the temperature of the heat treatment in Fig. 2. The rate increased sharply in the temperature range of 450-600 0C. Since the surface area did not change appreciably by the heat treatment (Fig. 2),
the
change in the activity is not caused by the change in the surface area. The fact that the temperature range of the activity rise agrees with the temperature of the dehydroxylation of the chrysotile, indicates that the activity rise may be caused by the exposure of nickel cations with unsaturated coordination. It is well known that nickel species in zeolites or on silica surface are active catalysts for the dimerization of ethylene (refs.
7,S).
In order to obtain the information on the active centers for the dimerization,
several experiments were performed. The reaction of ethylene was carried
523 out at so?c with ethylene containing varying amount of carbon monoxide. The rate of dimerization decreased as the increase in the amount of carbon monoxide introduced into the reaction system. The activity for the dimerization was almost completely lost with the introduction of 3 x 10- 6 mol of carbon monoxide to 1 g of Ni-chrysotile. Provided that one molecule of carbon monoxide poisons one active center,
the ratio of the number of active centers to the number of
nickel atoms is 3 x 10- 4, indicating that the very small fraction of nickel cations located on the surface of the chrysotile can be active centers for the dimerization. As for the active species, Yashima et al. (ref. 7) reported that highly dispersed nickel metal gives active centers in nickel(II)-exchanged Y-zeolites, while Bonneviot et al. (ref. 8) reported that Ni(I) cations are responsible for the dimerization. The effect of the reduction of the catalyst was examined. Thus,
Ni -chrysotile pretreated at 600 0 e was exposed to hydrogen at 3000C for
varying periods. From the hydrogen consumed, the extent of the reduction of nickel(II) cations was estimated. Fig. 3 shows that the rate of ethylene dimerization at SOOC decreased only slightly with the extent of the reduction. On the other hand,
the rate of hydrogenation of ethylene at OOC increased sharply
with the extent of the reduction, indicating that the active centers for the dimerization and those for the hydrogenation are entirely different. Since the active centers for the hydrogenation are plausibly metallic nickel,
'0'1
15
-'
'e
E oE 10
!D'
-..... 5
I
0 .-
e (»
....
f -o-o-i-o-t::o-. /
,
0'1
100 r--r
-m E
•
(» L
50
•
ra.-
'c
.
0
_I
the possi-
0 300 400 500 600 700 Evacuation Temperature I °C
(»
u
m =' If) L
Fig. 2. Change in the initial rate of ethylene dimerization and the surface area of nickel-chrysotile with the temperature of the heat treatment.
524 bility of the participation of metallic nickel to the dimerization can be eliminated. The catalytic activities for ethylene dimerization over chrysotiles with Ni/Mg = 1 and 1/4 were also studied. The conversions of ethylene were 2.8, and 9.3% for chrysotiles of Ni/Mg
=
1 and Ni/Mg
=
5.5
1/4 and Ni-chrysotile,
respectively, after 1 h reaction at 50 0 e with the initial ethylene pressure of 40.7 kPa and 1 g of catalyst. Reduction of nickel chrysotile Nickel chrysotile dehydroxylated at 600 0e was reduced at varying temperatures with hydrogen (initial pressure of 15.1 kPa) in a gas-recirculation system with a cold-trap,
which trapped water formed by the reduction. The rate
of the reduction was fast at the beginning and it slowed down at the later stage. In Fig. 4 was shown the temperature dependence of the degree of the reduction after 2 h-exposure to hydrogen. The higher the reduction temperature, the higher was the degree of the reduction. Thus, at 300 o e, the rate of the reduction was very slow. The degrees of the reduction at 400, 480 and 600
0e
were 20, 63 and 94%, respectively. The rate of the reduction depended very much on the content of nickel in the chrysotile structure. Thus, the rate of the Ni(II) reduction was much slower on Ni-Mg-chrysotile (Ni/Mg the degree of the reduction was 66.5% after
=
1/4),
for which
2 h-reduction at 600 o e. This
indicates that magnesium(II) cations in the chrysotile structure stabilize the nickel cations or the chrysotile structure and serve to inhibit the reduction of nickel (II) cations in the structure. Hydrogenation of ethylene Hydrogenation of ethylene over reduced Ni-chrysotile (50 mg) was carried out at -50 o e with an equimolar mixture of hydrogen and ethylene (4.0 kPa each). Nichrysotile was reduced at various temperatures for 2 h with the initial hydrogen pressure of 15.1 kPa. The initial rate of the hydrogenation was plotted as a function of the temperature of the reduction in Fig. 4, where the degree of the reduction was also shown as described above. Though the degree of the reduction increased with the temperature of the reduction, the rate of ethylene hydrogenation went through a maximum around 4800 e of the reduction temperature, where the degree of the reduction was 63%. Fig. 5 shows the change in the catalytic activity per reduced nickel as a function of the reduction temperature. It decreased monotonically with the reduction temperature, indicating that the growth of nickel metal particles with the reduction temperature. Similar experiments were carried out over Ni-Mg-chrysotile (Ni/Mg = 1/4). this case, the activity for
hydrog~nation
In
increased with the reduction tempera-
ture. The activity per reduced nickel for Ni-Mg chrysotile was shown also in
525 Fig. 4. The activity per reduced nickel for Ni-Mg-chrysotile does not differ greatly from that for Ni-chrysotile, indicating that the state of the metal particles does not depend greatly on the starting material, but depends more on the temperature of the reduction. Hydrogenation of carbon monoxide Hydrogenation of carbon monoxide over reduced nickel chrysotile was carried out in a gas-recirculation system. Nickel chrysotile was reduced with hydrogen (initial pressure,
26.7 kPa) at varying temperatures. The reaction temperature
was 300 oC, and the initial pressures of carbon monoxide and hydrogen were 6.7
and 13.3 kPa, respectively. Water produced was removed from the system with a cold trap. In contrast to the hydrogenation of ethylene, the activity for carbon monoxide hydrogenation increased monotonically with the reduction temperature. The ratio of the activity per reduced nickel for the samples reduced at 600 0C and for those at 400 0C was 0.6, much larger than the ratio of 0.15 in ethylene hydrogenation. This indicates that the active centers for the hydrogenation of carbon monoxide was different from those for the hydrogenation of ethylene. It has been reported that the hydrogenation of ethylene is a structure-insensitive reaction and that the hydrogenation of carbon monoxide is structure-sensitive.
10
10'5
-- E4 ~, Ie
C
.
0(5
.- E
CUlO. 3
I-
......
of
Nb ~
~
E
2
0
'0 ~
m 0 0:: ~
I-
_.~o
->: -==t-
0 ......
8
e_
C
g~o
4
0
0
--o t:E.
6 ~§ Ci·
I
I
b
~
1J
>-
::r:
2 '0 ~
m 0 0:: ~
I
4 1 2 3 Ni (II) Reduced I %
5
Fig. 3. The effect of nickel(II) reduction on the catalytic activities of nickel-chrysotile for the dimerization and the hydrogenation of ethylene. Dimerization: SOoC, C2H4 = 40.7 kPa. Hydrogenation: OOC, C2H4 = H2 = 13.3 kPa.
526
~
o
4
100 §
E
M'
'a
--
~ 2 ......
Fig. 4. The dependence of the initial rate of ethylene hydrogenation and the degree of Ni(II) reduction on the reduction temperature of Ni-chrysotile. Reduction: initial hydrogen pressure 15.1 kPa, 2 h. Hydrogenation: -SOoC, ini tial pressures, C2H4 = H = 4.0 kPa. 2
o
-
-
I-
OL-------l...------JL..-.--~
300
400
500
600
Reduction Temperature foe
Fig. 5. The dependence of the initial rate of ethylene hydrogenation per reduced Ni(II) ions on the reduction temperature of Ni-chrysotile. Reduction and reaction conditions are described in Fig. 4.
527
Fig. 6. Electron micrographs of (a) Ni-chrysotile and (b) Ni-Mg-chrysotile (Ni/Mg = 1/4) reduced at 500 0C for 2 h.
528
Electron Micrograph of Nickel Particles The nickel-containing chrysotile samples after reduction was examined by their electron micrograph. Though the nickel-chrysotile reduced at 3000C showed the catalytic activity for ethylene hydrogenation (Fig. 3), nickel-metal particles were not observed in the electron micrograph. The nickel-chrysotile after reducion at 350°C for 2 h gave the nickel particles with the diameter of 15-20 A ,
the number of particles being very small. The size of nickel particles and
the number of particles increased with increasing reduction temperature. Thus, the average particle sizes of nickel metals formed after 2 h reduction were 34, 36 and 82 A for the reduction temperature of 400, 470 and 600 0C, respectively. Fig. 6(a) shows the electron micrograph of nickel chrysotile after 2 h-reduction at 500 0C. While a large number of nickel particles were observed, it is also clear that the physical form of the original chrysotile was not completely retained. Ni-Mg chrysotile (Ni/Mg = 1/4) after reduction at 500°C gave the nickel-particle size similar to Ni-chrysotile reduced at the same temperature (Fig. 6(b)). Here, however, the particle size distribution is more uniform than that of nickel-chrysotile reduced at 500 oC,
and the physical form of the
chrysotile was maintained. Thus, it is concluded that very uniform metal particles can be prepared on the support with a very uniform physical shape. It is no doubt that the size of nickel particles can be varied by changing the content of nickel in the chrysotile structure and the conditions of conducting reduction of nickel cations.
REFERENCES 1 2 3 4 5 6 7 8
W. Noll, H. Hirscher and W. Syberts, Kolloid Z., 157 (1958) 1-11. S. Suzuki and Y. On o , Applied Cata1., 10 (1984) 361-368. H. E. Swift, in J. J. Burton and R. L. Garten (Eds.), Advanced Materials in Catalysis, Academic Press, New York and London, 1977, Ch. 7, p.230. L. L. Kibby, F. E. Massoth and H. E. Swift, J. Cata1., 42 (1976) 350-359. P. A. Jacobs and H. H. Nijs, J. Catal., 64 (1980) 251-259. J. J. F. Scholten, A. M. Beers and A. M. Kie1, J. Catal., 36 (1975) 23-29. T. Yashima, Y. Ushida, M. Ebisawa and N. Hara, J. Catal., 36 (1975) 320-326. L. Bonneviot, D. Olivier and M. Che, J. Mol. Catal., 21 (1983) 415-430.
529
DISCUSSION J.A. SCHWARZ: You have used reduction temperature to correlate your activity data for ethylene hydrogenation. Most of your interpretation has been directed toward the structural changes that occur during the heat treatment. Could you comment on the possibility of compound formation during H2 treatment at elevated temperatures ? Y. ONO : The change in the catalytic activity for ethylene hydrogenation seems to be well correlated to the dispersion of the nickel particles formed. Since the starting material of our catalysts is nickel silicate, we simply presume that unreduced nickel cations still remain in some form of nickel silicates. G. BELLUSSI : During the reduction of Ni-chrysoti1e at a temperature higher than 400°C, probably a phase transformation or a loss of crystallinity of the Ni-chrysoti1e takes place. Have you tried to correlate these variations with the data of activity in the hydrogenation reaction? Y. ONO : Before the reduction, the Ni-chrysoti1e was pretreated at 600°C. At this stage, X-ray diffraction of the chrysotile was completely lost, although the shape of the original crystals was still retained (Fig. l(b)). By reducing the material at temperature above 500°C, the original shape was also lost as seen in Fig. 6(a). It seems, however, that the activity can be well correlated with the change in the particle size and the extent of Ni(II) reduction. We cannot find any positive evidence for correlating the structural change in the support material with the change in the catalytic activity for hydrogenation. D. REINALDA : By reduction of Ni, are there acid sites created in the lattice? Y. ONO : Though we have not measured the acidity directly, the data for the catalytic activity for isopropy1alcohol dehydration suggest that the material is not so acidic.
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
519
PREPARATION OF NICKEL CATALYST FROM NICKEL CONTAINING CHRYSOTILE Y. ONO, N. KIKUCHI and H. WATANABE Department of Chemical Engineering, Meguro-ku, Tokyo 152 (Japan)
Tokyo Institute of Technology,
Ookayama,
SUMMARY Nickel-containing chrysotiles (NixM93_x(OH)4Si205) were prepared under hydrothermal conditions. They showed the catalytic activity for ethylene dimerization after their treatment under vacuum at high temperatures, which leads to the dehydroxylation of the chrysotiles. The active sites for the dimerization are plausibly nickel cations exposed to the surface. Nickel metal catalysts were prepared by reducing the chrysotiles with hydrogen. The catalytic activity for ethylene hydrogenation at -50 0C increased with the reduction temperature up to 450 0C, and decreased by further increase of the reduction temperatures, though the extent of Ni (II) reduction monotonically increased with the temperature. Electron micrograph study revealed that the particle size of Ni metal increased with the reduction temperature, indicating that the lowering of the hydrogenation activity at higher reduction temperature can be ascribed to the lowering of the metal surface area due to the growth of the metal particles. Very uniform nickel particles can be prepared especially from the chrysotile of x = 0.6.
INTRODUCTION Chrysotile M93(OH)4Si05 is a layered magnesium silicate and can be synthesized under hydrothermal conditions (ref.
1).
Because of its defined crystal
structure together with its high surface area, chrysotile may be useful as a catalyst or as a support for a catalytically active species. Suzuki and Ono (ref. 2) studied the reaction of 2-propanol over chrysotile and discussed on the acid-base property of the surface. Catalytically active materials could also be obtained by incorporating metal ions like Ni(II), Co(II) and Al(III) in place of all or part of the Mg(II) ions in the structure. Such applications has been reviewed by Swift (ref. 3). Kibby et a L, (ref. 4) reported on the preparation of nickel metal catalysts by reducing nickel containing chrysotile. Jacobs et al. (ref. 5) studied the catalytic property of the reduced garnierite (a nickel-containing mineral with chrysotile structure). In this work, (1) chrysotiles in which part or all of the Mg(II) ions were replaced by Ni(II) ions were prepared and their catalytic activities for the dimerization of ethylene were studied, and (2) nickel metal catalysts were prepared by reducing Ni(II)containing chrysotiles with hydrogen and their catalytic activities for hydrogenation of ethylene and carbon monoxide were examined.
520 METHODS Synthesis of Ni-containing chrysotiles Nickel-chrysotile (Ni3(OH)4Si20S) was synthesized according to the method described by Noll (ref.
and Sholten et a L, (ref. 6). Sodium metasilicate
1)
(Na 2Si0 of 94.7 g and nickel sulfate (NiS0 46H 20) of 141 g was dissolved 39H20) into 600 cm 3 of water. To this solution, 19.1 g of sodium hydroxide was added and the mixture was stirred. The resulting gel of pH 11.8-12.0 with the composition of Ni/Si ~ loS and H20/Si ~ 118, was then placed in an autoclave. The temperature was raised to 295 0C in 4 h and kept at 295 0C for 8 h. After cooling, the insoluble product was washed and dried at l80 oC. The materials with both Ni and Mg cations (NixM93_x(OH)4Si20S) were synthesized similarly except that the part of nickel sulfate was replaced by magnesium chloride
Catalytic reactions The treatment of the chrysotile samples and the catalytic reactions were carried out with a conventional gas-recirculation system. During the reduction of the chrysotiles with hydrogen, the water produced was trapped in aU-shape tube. The extent of the reduction was estimated from the pressure drop of hydrogen. The rate of ethylene hydrogenation or carbon monoxide was determined also from the pressure drop of the system. RESULTS AND DISCUSSION Structure of the synthesized materials X-ray diffraction patterns confirmed that the synthesized materials had the chrysotile structure. Fig. lea) shows the electron transmission micropraph of Ni-chrysotile. The hallow tube form of the chrysotile is clearly seen in Fig. lea). The length of the tubes ranges from 650 to 11000 A, mainly ZOOO-3000 A.The outer and inner diameter of the tubes are 250-400 A and 70-110 A, respectively. Fig. lea) also shows the layer structure of the nickel-chrysotile. The synthesized materials with Ni/Mg
~
1 and Ni/Mg
~
1/4 showed essentially identical X-
ray diffraction patterns to Ni-chrysotile and had similar physical forms. The BET surface areas of Ni-chrysotile and the materials with Ni/Mg 63.2, 59.5 and 47.6 m2g- l, respectively.
~
1 and 1/4 are
Heat treatment of nickel crysotile To know the thermal stability of the prepared nickel chrysotile, the thermal analysis was made. The weight loss of the sample with an endothermic peak was observed in the temperature range of 500-6000C. The weight loss corresponded with the loss of all the hydroxyl groups as water. The X-ray diffraction pattern was examined for the samples heated under vacuum at various tempera-
521
Fig. 1. Electron micrograph of nickel-chrysotile. (a) as prepared. (b) after heating under vacuum for 2 h at 700°C.
522 tures for 2 h. The sample treated at 400 0C gave similar pattern as the original one,
while the pattern due to chrysotile structure was completely lost for the
samples treated above 500 0C, Fig.
in conformity with result of the thermal analysis.
l(b) shows the electron micrograph of nickel chrysotile after heat treat-
ment at 700 0C for 2 h under vacuum. As mentioned above, this sample showed no X-ray diffraction lines due to the chrysotile structure. However, Fig. l(b) clearly shows that the tubular form of the material was mostly retained and even the layer structure of the material was also kept although it was more or less disordered. The samples with Ni/Mg
1 and Ni/Mg
1/4 behaved very similarly with
nickel chrysotile. Dimerization of ethylene The reaction of ethylene over Ni-chrysotile was carried out at SOoC in a conventional gas-recirculation system with initial pressure of 47.6 kPa.
Prior
to the reaction, Ni-chrysotile was heated under vacuum for 2 h at varying temperatures. The main reaction is the dimerization of ethylene,
hexenes being
also formed over the samples heated over 450 0C. Among the butenes produced, trans-2-butene is most predominant over the samples treated at 300 and 400 0C. On the other hand, 450 0C,
I-butene was most predominant over the samples treated over
indicating that the mechanism of the dimerization depends on the heat-
treatment temperature of Ni-chrysotile. Over Ni-chrysotile treated at 500 0C, the butene distribution changed with reaction time. Thus, the fraction I-butene decreased gradually with reaction time and that of 2-butenes increased instead. The activation energy of the dimerization was estimated to be 20.0 kJ mol- l from the temperature dependence of the initial rate over the sample treated at 500 0C. Fraction of I-butene among butenes decreased with the reaction temperature, indicating the increase in the rate of butene isomerization with temperature. The initial rates of the reaction calculated from the pressure drop-time curves were plotted as a function of the temperature of the heat treatment in Fig. 2. The rate increased sharply in the temperature range of 450-600 0C. Since the surface area did not change appreciably by the heat treatment (Fig. 2),
the
change in the activity is not caused by the change in the surface area. The fact that the temperature range of the activity rise agrees with the temperature of the dehydroxylation of the chrysotile, indicates that the activity rise may be caused by the exposure of nickel cations with unsaturated coordination. It is well known that nickel species in zeolites or on silica surface are active catalysts for the dimerization of ethylene (refs.
7,S).
In order to obtain the information on the active centers for the dimerization,
several experiments were performed. The reaction of ethylene was carried
523 out at so?c with ethylene containing varying amount of carbon monoxide. The rate of dimerization decreased as the increase in the amount of carbon monoxide introduced into the reaction system. The activity for the dimerization was almost completely lost with the introduction of 3 x 10- 6 mol of carbon monoxide to 1 g of Ni-chrysotile. Provided that one molecule of carbon monoxide poisons one active center,
the ratio of the number of active centers to the number of
nickel atoms is 3 x 10- 4, indicating that the very small fraction of nickel cations located on the surface of the chrysotile can be active centers for the dimerization. As for the active species, Yashima et al. (ref. 7) reported that highly dispersed nickel metal gives active centers in nickel(II)-exchanged Y-zeolites, while Bonneviot et al. (ref. 8) reported that Ni(I) cations are responsible for the dimerization. The effect of the reduction of the catalyst was examined. Thus,
Ni -chrysotile pretreated at 600 0 e was exposed to hydrogen at 3000C for
varying periods. From the hydrogen consumed, the extent of the reduction of nickel(II) cations was estimated. Fig. 3 shows that the rate of ethylene dimerization at SOOC decreased only slightly with the extent of the reduction. On the other hand,
the rate of hydrogenation of ethylene at OOC increased sharply
with the extent of the reduction, indicating that the active centers for the dimerization and those for the hydrogenation are entirely different. Since the active centers for the hydrogenation are plausibly metallic nickel,
'0'1
15
-'
'e
E oE 10
!D'
-..... 5
I
0 .-
e (»
....
f -o-o-i-o-t::o-. /
,
0'1
100 r--r
-m E
•
(» L
50
•
ra.-
'c
.
0
_I
the possi-
0 300 400 500 600 700 Evacuation Temperature I °C
(»
u
m =' If) L
Fig. 2. Change in the initial rate of ethylene dimerization and the surface area of nickel-chrysotile with the temperature of the heat treatment.
524 bility of the participation of metallic nickel to the dimerization can be eliminated. The catalytic activities for ethylene dimerization over chrysotiles with Ni/Mg = 1 and 1/4 were also studied. The conversions of ethylene were 2.8, and 9.3% for chrysotiles of Ni/Mg
=
1 and Ni/Mg
=
5.5
1/4 and Ni-chrysotile,
respectively, after 1 h reaction at 50 0 e with the initial ethylene pressure of 40.7 kPa and 1 g of catalyst. Reduction of nickel chrysotile Nickel chrysotile dehydroxylated at 600 0e was reduced at varying temperatures with hydrogen (initial pressure of 15.1 kPa) in a gas-recirculation system with a cold-trap,
which trapped water formed by the reduction. The rate
of the reduction was fast at the beginning and it slowed down at the later stage. In Fig. 4 was shown the temperature dependence of the degree of the reduction after 2 h-exposure to hydrogen. The higher the reduction temperature, the higher was the degree of the reduction. Thus, at 300 o e, the rate of the reduction was very slow. The degrees of the reduction at 400, 480 and 600
0e
were 20, 63 and 94%, respectively. The rate of the reduction depended very much on the content of nickel in the chrysotile structure. Thus, the rate of the Ni(II) reduction was much slower on Ni-Mg-chrysotile (Ni/Mg the degree of the reduction was 66.5% after
=
1/4),
for which
2 h-reduction at 600 o e. This
indicates that magnesium(II) cations in the chrysotile structure stabilize the nickel cations or the chrysotile structure and serve to inhibit the reduction of nickel (II) cations in the structure. Hydrogenation of ethylene Hydrogenation of ethylene over reduced Ni-chrysotile (50 mg) was carried out at -50 o e with an equimolar mixture of hydrogen and ethylene (4.0 kPa each). Nichrysotile was reduced at various temperatures for 2 h with the initial hydrogen pressure of 15.1 kPa. The initial rate of the hydrogenation was plotted as a function of the temperature of the reduction in Fig. 4, where the degree of the reduction was also shown as described above. Though the degree of the reduction increased with the temperature of the reduction, the rate of ethylene hydrogenation went through a maximum around 4800 e of the reduction temperature, where the degree of the reduction was 63%. Fig. 5 shows the change in the catalytic activity per reduced nickel as a function of the reduction temperature. It decreased monotonically with the reduction temperature, indicating that the growth of nickel metal particles with the reduction temperature. Similar experiments were carried out over Ni-Mg-chrysotile (Ni/Mg = 1/4). this case, the activity for
hydrog~nation
In
increased with the reduction tempera-
ture. The activity per reduced nickel for Ni-Mg chrysotile was shown also in
525 Fig. 4. The activity per reduced nickel for Ni-Mg-chrysotile does not differ greatly from that for Ni-chrysotile, indicating that the state of the metal particles does not depend greatly on the starting material, but depends more on the temperature of the reduction. Hydrogenation of carbon monoxide Hydrogenation of carbon monoxide over reduced nickel chrysotile was carried out in a gas-recirculation system. Nickel chrysotile was reduced with hydrogen (initial pressure,
26.7 kPa) at varying temperatures. The reaction temperature
was 300 oC, and the initial pressures of carbon monoxide and hydrogen were 6.7
and 13.3 kPa, respectively. Water produced was removed from the system with a cold trap. In contrast to the hydrogenation of ethylene, the activity for carbon monoxide hydrogenation increased monotonically with the reduction temperature. The ratio of the activity per reduced nickel for the samples reduced at 600 0C and for those at 400 0C was 0.6, much larger than the ratio of 0.15 in ethylene hydrogenation. This indicates that the active centers for the hydrogenation of carbon monoxide was different from those for the hydrogenation of ethylene. It has been reported that the hydrogenation of ethylene is a structure-insensitive reaction and that the hydrogenation of carbon monoxide is structure-sensitive.
10
10'5
-- E4 ~, Ie
C
.
0(5
.- E
CUlO. 3
I-
......
of
Nb ~
~
E
2
0
'0 ~
m 0 0:: ~
I-
_.~o
->: -==t-
0 ......
8
e_
C
g~o
4
0
0
--o t:E.
6 ~§ Ci·
I
I
b
~
1J
>-
::r:
2 '0 ~
m 0 0:: ~
I
4 1 2 3 Ni (II) Reduced I %
5
Fig. 3. The effect of nickel(II) reduction on the catalytic activities of nickel-chrysotile for the dimerization and the hydrogenation of ethylene. Dimerization: SOoC, C2H4 = 40.7 kPa. Hydrogenation: OOC, C2H4 = H2 = 13.3 kPa.
526
~
o
4
100 §
E
M'
'a
--
~ 2 ......
Fig. 4. The dependence of the initial rate of ethylene hydrogenation and the degree of Ni(II) reduction on the reduction temperature of Ni-chrysotile. Reduction: initial hydrogen pressure 15.1 kPa, 2 h. Hydrogenation: -SOoC, ini tial pressures, C2H4 = H = 4.0 kPa. 2
o
-
-
I-
OL-------l...------JL..-.--~
300
400
500
600
Reduction Temperature foe
Fig. 5. The dependence of the initial rate of ethylene hydrogenation per reduced Ni(II) ions on the reduction temperature of Ni-chrysotile. Reduction and reaction conditions are described in Fig. 4.
527
Fig. 6. Electron micrographs of (a) Ni-chrysotile and (b) Ni-Mg-chrysotile (Ni/Mg = 1/4) reduced at 500 0C for 2 h.
528
Electron Micrograph of Nickel Particles The nickel-containing chrysotile samples after reduction was examined by their electron micrograph. Though the nickel-chrysotile reduced at 3000C showed the catalytic activity for ethylene hydrogenation (Fig. 3), nickel-metal particles were not observed in the electron micrograph. The nickel-chrysotile after reducion at 350°C for 2 h gave the nickel particles with the diameter of 15-20 A ,
the number of particles being very small. The size of nickel particles and
the number of particles increased with increasing reduction temperature. Thus, the average particle sizes of nickel metals formed after 2 h reduction were 34, 36 and 82 A for the reduction temperature of 400, 470 and 600 0C, respectively. Fig. 6(a) shows the electron micrograph of nickel chrysotile after 2 h-reduction at 500 0C. While a large number of nickel particles were observed, it is also clear that the physical form of the original chrysotile was not completely retained. Ni-Mg chrysotile (Ni/Mg = 1/4) after reduction at 500°C gave the nickel-particle size similar to Ni-chrysotile reduced at the same temperature (Fig. 6(b)). Here, however, the particle size distribution is more uniform than that of nickel-chrysotile reduced at 500 oC,
and the physical form of the
chrysotile was maintained. Thus, it is concluded that very uniform metal particles can be prepared on the support with a very uniform physical shape. It is no doubt that the size of nickel particles can be varied by changing the content of nickel in the chrysotile structure and the conditions of conducting reduction of nickel cations.
REFERENCES 1 2 3 4 5 6 7 8
W. Noll, H. Hirscher and W. Syberts, Kolloid Z., 157 (1958) 1-11. S. Suzuki and Y. On o , Applied Cata1., 10 (1984) 361-368. H. E. Swift, in J. J. Burton and R. L. Garten (Eds.), Advanced Materials in Catalysis, Academic Press, New York and London, 1977, Ch. 7, p.230. L. L. Kibby, F. E. Massoth and H. E. Swift, J. Cata1., 42 (1976) 350-359. P. A. Jacobs and H. H. Nijs, J. Catal., 64 (1980) 251-259. J. J. F. Scholten, A. M. Beers and A. M. Kie1, J. Catal., 36 (1975) 23-29. T. Yashima, Y. Ushida, M. Ebisawa and N. Hara, J. Catal., 36 (1975) 320-326. L. Bonneviot, D. Olivier and M. Che, J. Mol. Catal., 21 (1983) 415-430.
529
DISCUSSION J.A. SCHWARZ: You have used reduction temperature to correlate your activity data for ethylene hydrogenation. Most of your interpretation has been directed toward the structural changes that occur during the heat treatment. Could you comment on the possibility of compound formation during H2 treatment at elevated temperatures ? Y. ONO : The change in the catalytic activity for ethylene hydrogenation seems to be well correlated to the dispersion of the nickel particles formed. Since the starting material of our catalysts is nickel silicate, we simply presume that unreduced nickel cations still remain in some form of nickel silicates. G. BELLUSSI : During the reduction of Ni-chrysoti1e at a temperature higher than 400°C, probably a phase transformation or a loss of crystallinity of the Ni-chrysoti1e takes place. Have you tried to correlate these variations with the data of activity in the hydrogenation reaction? Y. ONO : Before the reduction, the Ni-chrysoti1e was pretreated at 600°C. At this stage, X-ray diffraction of the chrysotile was completely lost, although the shape of the original crystals was still retained (Fig. l(b)). By reducing the material at temperature above 500°C, the original shape was also lost as seen in Fig. 6(a). It seems, however, that the activity can be well correlated with the change in the particle size and the extent of Ni(II) reduction. We cannot find any positive evidence for correlating the structural change in the support material with the change in the catalytic activity for hydrogenation. D. REINALDA : By reduction of Ni, are there acid sites created in the lattice? Y. ONO : Though we have not measured the acidity directly, the data for the catalytic activity for isopropy1alcohol dehydration suggest that the material is not so acidic.