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Characterization and catalytic activity of nickel-zeolite catalysts. IV. Effects of support on dispersion of nickel and catalytic activity
Characterization and catalytic activity of nickel-zeolite catalysts. IV. Effects of support on dispersion of nickel and catalytic activity Minoru Suzuki, Kazuo Tsutsumi and Hiroshi Takahashi Institute of Industrial Science, The University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan (Received 23 November 1981) The dispersion states of reduced nickel on zeolites and their activities were studied on a series of nickel-zeolites A, X, Y and nickel-mordenites. The dispersion of nickel was affected by the types of zeolite support, and increased in the order; NiNa-Y < NiNa-X < NiNa-M NiNa-A. The reduced nickel was highly dispersed in NiSr-A, NiCa-A and NiBa-A. All sites active to hydrogen chemisorption act as sites for the hydrogenolysis of ethane in nickelzeolites A, X and Y. Keywords" Catalysis; nickel-zeolites; dispersion
INTRODUCTION
EXPERIMENTAL
Once metal catalysts supported on zeolites had been prepared by the reduction of transition-mctal ion-exchanged zeolites with hydrogen, many studies were performed on the dispersions of metal and their catalytic activities, mainly in Pt- and Pdzeolites l-s. These catalysts have been characterized by high dispersions and high activities. For example, the specific activities for the hydrogenation of ethylene in Pt-zeolites are higher than in Pt-SiO2 and Pt-AI203 catalysts 6, and Pt-zeolites possess high resistance to sulphur compounds 7. The unusual properties of these metal-zeolites are caused by the electron transfer between metal and zeolite_support 4,6,7. However, the effects of the zeolite-support on the dispersion of nickel and on the catalytic activity have not been described though a few papers 8-1° have been published on this topic.
Materials Nickel-zeolites were prepared by the methods of our previous paper 12. Nickel-silica-alumina was prepared by means of consecutive ion-exchange at 80°C with a buffer solution (pH --~ 8) consisting of ammonium acetate and ammonium hydroxide containing 0.1 N nickel nitrate. After ion-exchange, the samples were washed several times with a nickel-free buffer solution, and dried at room temperature in vacuo. The compositions of these samples are summarized in Table 1. Nickel-zeolites and nickel-silica-alumina were calcined in flowing nitrogen followed by reduction in a hydrogen stream 11.
In the previous paper n, a series of nickel-zeolite Y with alkaline earth, lanthanum and hydrogen ions as parent-cations in zeolites were prepared. The dispersion of reduced nickel was affected by the types of parent-cations in zeolites, while the turnover frequencies for benzene hydrogenation were nearly identical in these zeolites regardless of the types of parent-cations and the dispersions of reduced nickel. It has been suggested that the reduced nickel is distributed b o t h inside and outsid~the zeolite cavities. In the present paper, the effects of zeolite-support on the dispersions of nickel and their activities have been investigated on a series of nickel-zeolites of type A, X, Y and nickel-mordernites. The activities of these catalysts have been evaluated for benzene hydrogenation, cyclohexane dehydrogenation and ethane hydrogenolysis respectively. 0144-2449/82/020193-07503.00 © 1982 Butterworth & Co. (Publishers) Ltd.
Dispersion The dispersion states of reduced nickel on zeolites were evaluated by means of hydrogen chemisorption, electron micrographs and X-ray line broadening u.
Catalytic activity The hydrogenation of benzene was carried out by the method described earlier u. The dehydrogenation of cyclohexane was performed using a fixedbed continuous flow reactor at 280°C, at a feed rate of 4.44 × 10 -s mol min -1 and at W/F of 41 (g-cat' h mo1-1. The hydrogenolysis of ethane was carried out at temperatures ranging from 320 ° to 350°C using a pulse reactor. The catalyst (0.15 g) fixed in a quartz tube (6 mm (~) was calcined in flowing nitrogen at 370°C for 3 h followed by reduction in a hydrogen stream at 370°C for 6 h. 21/amol of ethane were injected into a hydrogen carrier gas (50 ml min-1). The activities were evaluated in terms of the extent of conversion of ethane at a first pulse because the extent of conversion was ZEOL/TES, 1982, Vol 2, July' 193
Characterization and catalytic activity o f nickel-zeolite catalysts. I V: M. Suzuki et al. Table 1
Chemical compositions, degrees of reduction and surface areas of nickel-zeolites
Zeolite
Ni2+(wt %)
Sc (mag-1)
Exchange (%) Ni 2+
M(n +)a
o~b
1
2
--
--
519
--
100
0.72 0.77
590 586
574 565
100 87
Na-X
0
--
NiNa-X-6
1.16 4.64
6.2 24.9
0
--
--
--
NiNa-A-37 NiCa-A-7 NiSr-A-13 NiSr-A-22 N i Ba-A-6
1.09 4.58 7.47 1.50 2.43 4.21 0.93
5.3 22.4 36.7 7.2 13.1 22.2 5.6
---75.5 73.7 55.8 75.4
0.84 0.63 0.34 0.61 0.62 0.65 0.56
Na-M
0
--
--
N i Na-M-36 NiNa-M-54 Ni H - M - 2 8 Ni-SiO 2 - AI203
2.71 4.10 2.03 4.24
35.5 53.8 27.6 --
--69.1
Na-X-25
Ni
Na-A
Na-A-5
Ni
NiNa-A-22
a b c d
/~0±20 d
- -
--
--
100
23 45 95 483 365 161 10
9
24 58 72 300 125 35 4
100 86 87 50 20 0 0
--
254
--
100
1.00 1.00 0.10 0.62
252 250 260 313
271 254 255 277
100 100 90 --
The e x t e n t of exchange (%) o f the cations other than Na + and Ni 2+ The degree of nickel ion reduction B E T areas of the catalysts before (1) and after (2) reduction The relative intensity of X-ray d i f f r a c t i o n
independent of pulse number from 1 to 10. The products were analysed using a gas chromatograph with a column of active carbon (6 mm @ × 1 m) at i 00°C. Benzene and cyclohexane adsorption After the reduced catalysts were calcined in nitrogen stream at 370°C for 3 h, benzene was injected under the same conditions as the hydrogenation; at 150°C and at a partial pressure of benzene of 0.017. Similarly, cyclohexane was adsorbed on these zeolites at 280°C and at a partial pressure of 0.02. The amounts of adsorbed benzene and cyclohexane were determined from the increasing weight of the samples with an electrobalance (Shimazu Co. Ltd, TG-20 type) equipped with a gas flow system. RESULTS Characterization The degrees of nickel ion reduction were determined by means of hydrogen consumption (Table 1). The degree of nickel reduction was independent of the extent o f nickel-exchange in NiNa-X; this differs from the results observed in NiNa-Y a2. In NiNa-A, the reduction-degree decreased as the nickelexchange level increased. The nickel ions in NiH-M28 were hardly reduced in contrast to NiNa±M in which all of the exchanged nickel ions were rapidly reduced to metal at 370°C. The BET areas and the relative intensities of X-ray diffraction were determined in nickel-zeolites before and after hydrogen reduction (Table 1). The BET areas were nearly identical in NiNa-X-6, NiNa-X-25, NiNa-M-36 and NiNa-M-54 before and after reduction; these areas agreed with those of parent Na-X and Na-M respectively. The BET areas in NiNa-A, NiSr-A and NiCa-A were larger than in parent Na-A because of the increase of free apertures of the cavities, which was caused by the
194
ZEOLITES, 1982, Vol 2, July
exchange of sodium ions with divalent ions t3. When these zeolites were reduced by hydrogen, the BET areas were unchanged in NiNa-A, but decreased in NiCa-A and NiSr-A. The relative intensities of X-ray diffraction were determined for the reduced nickel-zeolites when the respective intensities of the following two peaks in sodium-zeolites were defined as 100; (5 3 3) and (6 4 2) diffraction in Na-X, (3 1 1) and (3 2 1) in Na-A, and (5 3 0) and (4 0 2) in Na-M. The relative intensities agreed within the experimental error (~ -+ 20%) in NiNa-X, NiNa-A and NiNa-M before and after reduction, whereas the intensities decreased in NiSr-A-13 and NiCa-A-7. No diffraction peaks were detected in NiSr-A-22 and NiBa-A-6. These results suggest that the zeolite crystals are partially collapsed in NiCa-A-7 and NiSr-A-13, and almost completely in NiSr-A-22 and NiBa-A-6 after hydrogen reduction. In NiBa-A, the zeolite crystals can be destroyed by ionexchange with barium ions 14. Selenina et al. 8 reported that NiNa-A up to 14% exchange and NiNa-X up to 27% exchange did not show any change in structure, however the crystal structure was retained even in NiNa-A-37 of 37% exchange in the present study.
Dispersion The surface areas (SR) and the average particle sizes (dH) of nickel determined by means of hydrogen chemisorption are summarized in Table 2. The surface areas of reduced nickel decreased with the increase of exchanged nickel ions in these catalysts (Figure 1). They were affected by the types of zeolite-support, and increased in the order; NiNa-Y < NiNa-X < NiNa-M < NiNa-A In NiSr-A-13, NiSr-A-22, NiCa-A-7 and NiBa-A-6 where the crystal structure was considerably collapsed, the surface areas of nickel were large,
Characterization and catalytic activity of nickel-zeolite catalysts. I V: M. Suzuki
in these catalysts; dvM was smaller than dH. The average crystaIlite size of nickel (dx-ray) determined from X-ray line broadening was 21 nm in NiNa-X25, which was smaller than dH. No Ni (1 1 1) peak was detected in NiNa-X-6. Because the diffraction peak for Ni (1 1 1) overlapped with the diffractions of the zeolite crystals, dx-ray could not be determined in NiNa-A and NiNa-M. Similarly, for NiCa-A-7, NiSr-A-13, NiSr-A-22 and NiBa-A-6 because of a broad and a weak spectrum of their Ni (1 1 1) peaks, however, these nickel particles could be very small as indicated by a broad Ni (1 1 1) diffraction peak.
1 4 0 -z~
120 --
\
+\ 100
-
\
-
\ \ 80 Z i
6O
$
\ \
\ \ \ 0
4O
\ \
Catalytic activity
\
Benzene hydrogenation
\ X\qb
\\a x
20
* M , o °-I
I 2
]
I I i i I 4 6 8 Nickel content (wt°/o)
I
] lO
Figure 1 Surface areas of nickel supported on zeolites and silicaalumina, o, NiNa-X; a, NiNa-A; u, NiNa-M; ~, NiNa-Y (by ref 11); +. NiCa-A; {, NiSr-A;-~, N i B a - A , ~ , Ni-SiO 2 - AI203 Table 2 Surface areas and average particle sizes of nickel on zeolites and silica-alumina
Catalyst
VHa
SHb
(#mol (g-cat)-') (m =(g-Ni)-') N i N a - X -6 Ni Na-X-25 NiNa-A-5 NiNa-A-22 NiNa-A-37 NiCa-A-7 NiSr-A-13 NiSr-A-22 NiBa-A-6 NiNa-M-36 NiNa-M-54 NiH-M-28 Ni-SiO 2• AI20 ~
et al.
5.40 8.62 14.8 13.3 8.10 8.80 7.60 17.0 6.90 15.6 5.85 . 10.8
.
52.2 19.5 130 37.2 25.7 77.6 40.7 50.1 107 46.4 11.5 . 29.8
Average diameter (nm) dR dEM 10.8 29.0 4.3 15.2 21.9 7.3 13.9 11.3 5.3 12.2 49.1
-15,1 -9.0 14.6 --6.4 -9.9 --
19.0
9.7
.
a Chemisorbed hydrogen at 13.3 kPa b Surface area of the reduced nickel determined from hydrogen chemisorption
and nearly identical to those in NiNa-A. Hydrogen was only slightly chemisorbed on NiH-M-28 similarly to NiH-Y-33 xl. The particle size distributions of nickel derived from electron micrographs are shown in Figure 2. Many nickel particles were distributed in the narrow range of 4-6 nm in NiNa-A-22 and Ni-SiO2" A1203 where the nickeldispersions are higher than in NiNa-Y-11. The nickel particles were mainly distributed in the range 8-10 nm in NiNa-M-36. No nickel particles were observed in NiH-M-28 with electron microscopy. In Table 2, the average particle sizes (dEM) determined from electron micrographs are collected. Discrepancies between dEM and dn were observed
The extents of conversion for benzene hydrogenation decreased with time on stream, and both cyclohexane and methylcyclopentane were produced in NiNa-M-36 and NiNa-M-54. However, in the other nickel-catalysts, only cyclohexane was produced and the activities only decreased slightly with time on stream. As shown in Figure 3, the extent of conversion decreased with increasing nickel content in NiNa-Y, while the highest activity was observed at the nickel content of ~ 4 % in NiNa-A. NiNa-M showed low conversion despite the high dispersion of reduced nickel. NiCa-A-7, NiSr-A-13, NiSr-A-22 and NiBa-A-6 showed high activities. The order of activity was as follows: NiNa-M < NiNa-Y < NiNa-X < NiNa-A --~ NiCa-A, NiSr-A, NiBa-A The extent of conversion increased in proportion to the amount of chemisorbed hydrogen in these catalysts except NiNa-M as shown in Figure 4. This result suggests that the active sites for hydrogen chemisorption act as the hydrogenation sites. As shown in Figure 4, the relationship between the amount of chemisorbed hydrogen and the extent of conversion has been classified into three groups as follows: NiNa-Y, NiNa-X and NiNa-A belong 40
20
o
,
i
i
i
i p,
a. 40
20
i
i
4
8
12
16
20
,~1 O
4
8
12
16
dE. (rim)
Figure 2 Particle size distributions of nickel on zeolites and silicaalumina. A, NiNa-A-22; B, NiNa-M-36; C, NiNa-Y-11; D, Ni-SiO 2 . A1203
ZEOLITES, 1982, Vol 2, July
195
Characterization and catalytic activity o f nickel-zeolite catalysts. IV: M. Suzuki et al.
hydrogenation. Arrhenius plots deviated from the linear relationships above 160°C because the catalyst deactivation was pronounced and both hydrogenation and isomerization proceeded. The apparent activation energies were 64.8-71.5 kJ mo1-1 in the temperature range of 100°-150°C for catalysts 'A', 'B' and 'C'.
~oo
80
"2 v
60
+ u
It has been reported earlier u that the turnover frequencies are less than 400 h -1 in NiNa-Y-71, NiMg-Y- 19, NiMg-Y-52 and NiLa-Y-21, and these low activities are improved by poisoning the acidic sites on zeolites with ammonia. Therefore, the effects of acidic sites on the activities of the hydro-
(~
... - A . .
40
/
A/
\
Table 3 Activities o f benzene hydrogenation on the nickel supported on zeolites and silica-alumina
20 - .~.,£~
Catalyst I
0
p-o-r
2
-- P--
I
T-
J
J
4 6 Nickel content ( w t %)
~
I
8
/
c/ / / / /
8o
Activation energy
Ea (kJ mo1-1)
10
Figure 3 Activities o f benzene hydrogenation f o r nickel-catalysts. Reaction temperature, 150°0; W/F, 33 (g-cat) h m o I - L Symbols as in Figure I
lO0
Turnover frequency TF (10=h -I)
_
NiNa-X-6 Ni Na-X-25 NiNa-A-5 NiNa-A-22 NiNa-A-37 NiCa-A-7 NiSr-A-13 NiSr-A-22 NiBa-A-6 NiNa-M-36 NiNa-M-54 NiH-M-28 N i-SiO 2 . AI203
10.8 10.2 6.5 10.7 11.1 24.5 24.1 21.5 21.3 0.22 1.2 -20.3
64.8 64.9 69.3 67.4 70.0 71.0 68.5 66.7 69.8 71.4 70.2 71.5
/
A/
/
/ ~//
60
//
/
100
/
/
/ O
/
40/
L)
/
f 50
\ o \ \,
/
20 --
of f
/ 0
"T- D T {5 10 VH ( k m o l (g-cet)-~)
TO ~ ~ 15
~
B "120
Figure 4 Relationship between the amount of chemisorbed hydrogen and the activity of benzene hydrogenation. Symbols as in
Figure I
to the first group (catalyst 'A') as well as NiCa-Y, NiSr-Y and NiBa-Y reported in the earlier I~. However, only NiNa-A-5 deviated from this relation. Catalyst 'C', to which NiCa-A-7, NiSr-A13, NiSr-A-22, NiBa-A-6 and Ni-SiO2" A1203 belong, shows higher activities than 'A'. In catalyst 'B', to which NiNa-M-36 and NiNa-M-54 belong, the extent of conversion is low and independent of the amount of chemisorbed hydrogen. For benzene hydrogenation, the turnover frequencies (TF) in the range of 1000-1100 h -x and of 2030-2450 h -1 were determined in catalyst 'A' and 'C' respectively (Table 3). They were also 22 and 120 h -1 in NiNa-M-36 and NiNa-M-54. Figure 5 shows the effect of reaction temperature on the rates of
196 ZEOLITES, 1982, Vol 2, July
z '
o~
lO
"7 $z
E
5
o v t~
I 2.2
I 2.3
I 2.4
25
2.6
2.7
l l T (kK} Figure 5 Arrhenius plots f o r benzene hydrogenation over nickelcatalysts, o, NiNa-X-25; % NiNa-A-22; )~,'NiNa-Y-31 ; o, NiNa~M-36 (r X 1 0 ) ; ~ , Ni-SiO 2 • AI=O3
Characterization and catalytic activity of nickel-zeolite catalysts. I V: M. Suzuki e t al.
genation were investigated on NiNa-X-25, NiNa-A22 and NiNa-M-36 of which the turnover frequencies were smaller than catalyst 'C'. The extents of conversion (CN) determined after the adsorption of ammonia increased only slightly as compared with those (Co) for the catalysts merely reduced by hydrogen as follows: CN/Co ratios were 1.09, 1.05 and 1.83 in NiNa-X-25, NiNa-A-22 and NiNa-M-36 respectively. Therefore, the lower turnover frequencies in catalysts 'A' and 'B' than in catalyst 'C' suggest that all of the active sites determined by means of hydrogen chemisorption do not act as hydrogenation sites, rather that this is an effect of the acidic sites and/or a characteristic of hydrogenation sites on nickel in these catalysts. Cyclohexane d e h y d r o g e n a t i o n A relationship between the extent of conversion for cyclohexane dehydrogenation and the amount of chemisorbed hydrogen is shown in Figure 6. The activities increased with increased chemisorbed hydrogen. The turnover frequencies for NiNa-X, NaBa-A, and NaSr-A, NiNa-Y-11 and NiNa-Y-31 were nearly identical. In NiNa-Y-71, NiCa-Y-29 and NiSr-Y-21, the extents of conversion decreased with time on stream, and b o t h benzene and methylcyclopentane were produced. Ethane
hydrolysis
The extents of conversion for ethane hydrogenolysis in relation to the amount of chemisorbed hydrogen are shown in Figure 7. The activities increased in proportion to the chemisorbed hydrogen in these catalysts except NiNa-M-36 and NiNa-M-54. In NiNa-M-36 and NiNa-M-54, the extent of conversion for ethane hydrogenolysis was slight despite the large amount of chemisorbed hydrogen as well as benzene hydrogenation and cyclohexane dehydrogenation. It has been reported that the orders of reaction in ethane and hydrogen are respectively 1 and --2 on Ni-SiO2 catalysO s. Under the conditions of this study, the apparent activation energies have been determined on the
50
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2/
40
30
y"
/
/
", (D
>
A
20-
¢0//
U 10
/ I
5
i
[] I
15 lo vH (Ixmol(g-cot) -~)
20
Figure 6 Relationship between the amount of chemisorbed hydrogen and the activity of cyclohexane dehydrogenation. Reaction temperature, 280°C; W/F, 41 (g-cat) h tool-l; o, NiNa-X; A, NiNa-A; u, NiNa-M; ~, NiNa-Y; e, NiCa-Y; % NiSr-Y; {, NiSr-A,
-~, N i B a - A ; - ~ ,
Ni-SiO 2 • AI=O a
loo
/ 8o
+ ~ 60 o
/
/
/
/
/
/
/
+
¢ /zx /
e/ /
~ 4o o u
/ /
//+ ~/
2o _ / 0
/~
t•
5
[]
I 10 VH (p,mol ( g - c o t )
1
15
1
20
"1 )
Figure 7 Relationship between the amount of chemisorbed hydrogen and the activity of ethane hydrogenolysis. Reaction temperature, 350°C; catalyst charged, 0.15 g; ethane injection, 21 #tool, H2 flow rate, 50 ml rain-l; [], NiMg-Y; e, NiBa-Y. Other symbols are shown in Figure 6
basis of the assumption that the hydrogenolysis rates are of first order in ethane partial pressure and of quasi-zero order in hydrogen pressurO 6. The activation energies determined in the range of 320 °350°C were nearly identical in these catalysts, and were 170-180 kJ tool-a; these agreed fairly well with the result reported by Yates et al. is Tayler et al. 17 reported that Ni-SiO2, Ni-A1203 and NiSiO2" A1203 catalysts gave different specific activities for ethane hydrogenolysis. However, Richardson 1s,19 reported that the specific activities were nearly identical in NiNa-Y, NiMg-Y, NiCa-Y and NiLi-Y catalysts, and the activities were unaffected by the acidity of supports in a series of NiSiO2" A1203 catalysts of which the acidic sites were progressively poisoned by sodium ions. The results reported by Richardson were supported by this study; the turnover frequencies and the apparent activation energies for ethane hydrogenolysis were independent of the support structures, their acidities and the dispersion states of nickel on zeolites in these nickel-zeolites except NiNa-M. Benzene and c y c l o h e x a n e adsorption The amount of adsorbed benzene and the free apertures of main cavities (or channels) 2° for zeolites are shown in Table 4. Almost all of the adsorbed benzene was desorbed when the catalysts were calcined at 370°C in flowing nitrogen. Because the crystallographic size of entrance of the cavity in zeolite A is 0.5 nm 2°, the benzene molecule of critical dimension of 0.68 nm 21 cannot penetrate the cavity. Actually, only small amounts of benzene were adsorbed on Na-A and NiNa-A-22. However, a large amount of benzene was adsorbed on NiNa-Y-13, NiNa-X-25 and NiNa-M-36 as compared with NiNa-A-22 because the benzene molecule can diffuse into the cavities of zeolites X, Y and mordenite. The amounts of adsorbed cyclo-
Z E O L I T E & 1982, Vol 2, July
197
Characterization and catalytic activity of nickel-zeolite catalysts. IV: M. Suzuki et al. Table 4
Adsorption of benzene on zeolites
Zeolite
VB a (mmol (g-zeol) -1)
Na-Y NiNa-Y-31 c NiNa-X-25 c Na-A N i Na-A-22 c Na-M H-M d NiNa-M-36 c
2.02 1.95 2.14 0.15 0.17 0.51 0.60 0.58
Free aperture of main cages b (nm) 0.74-0.80 0.74-0.80 0.90 0.41 ~ 0.50 -0.67 X 0.70 --
a The a m o u n t of adsorbed benzene at 150°C b By ref 20 c The catalysts reduced by hydrogen at 370°C d H - M was prepared by treating N a - M w i t h 2 N hydrochloric acid at room temperature
hexane at 280°C were below ~ 0 . 0 2 mmot (g-zeol) -j on Na-A, Na-X, Na-Y and Na-M. DISCUSSION
In the previous paper 11, the average particle sizes of nickel on zeolite Y were determined by means of hydrogen chemisorption, electron microscopy and X-ray line broadening. Only nickel particles which have formed metal crystallites can be detected by means of X-ray diffraction. Particles of larger diameter than ~ 2 nm were observed by electron microscopy in this study. Because the size of cavity in zeolite Y is 1.3 nm 2°, only nickel particles situated outside the zeolite cavities are determined from X-ray line broadening and electron micrographs. Therefore, the average particle sizes of nickel (dn), determined by means of hydrogen chemisorption, could be smaller than the sizes determined from electron micrographs (dEM) and X-ray line broadening (dx-ray). However, dH was larger than dx-ra.yand dEM in our nickel-zeolite Y. These discrepancles were marked in NiNa-Y-71, NiMg-Y-52 and NiLa-Y-21 which showed low degrees of nickel ion reduction. These results were interpreted as follows: If the nickel species not detected by X-ray diffraction and electron microscopy are also inactive to the hydrogen chemisorption, the small amount of reduced nickel would lead to an overestimate of dH when these nickel species are present in cavities. Brooks et al. 22 reported similar results. Carbon monoxide was adsorbed on nickel in several forms, while hydrogen was adsorbed only on the surface of nickel crystallites. Kubo et al. 2 have also reported that atomically dispersed platinum is inactive to hydrogen chemisorption. As shown in Table 2, d n was larger than dEM in NiNa-A, NiNa-X and NiNa-M, whereas the discrepancies between dn and dEM in these catalysts except NiH-M-28 were small as compared with NiNa-Y-71, NiMg-Y-52 and NiLa-Y-2111. The dispersion of nickel was affected by the types of zeolite supports, and increased in the order: NiNa-Y < NiNa-X < NiNa-M < NiNa-A (Though the term 'dispersion' is defined as the ratio of the number of surface metal atoms to the
198
ZEOLITES, 1982, Vol 2, July
-
total ones, it corresponds to the amount of chemisorbed hydrogen because the number of surface metal atoms are generally determined by means of hydrogen chemisorption. Therefore, dispersion is also comparable to the surface area of metal or particle size derived from the chemisorption.) The above order is consistent with the result reported by Bager et al. 1° However, Coughlan et al. 23 reported that the dispersion of ruthenium determined by means of hydrogen chemisorption increased in the following order in contrast to nickel-ze olites: RuNa-A < RuNa-X < RuNa-Y Such high dispersions of ruthenium (H/Ru = 0.51.0) in RuNa-Y suggest that atomically dispersed ruthenium in zeolite cavities chemisorbs hydrogen if the spillover phenomena z4 are inconsiderable. If the low dispersions of nickel observed markedly in NiNa-Y-71, NiMg-Y-52, NiLa-Y-21, NiH-Y-33 and NiH-M-28 are caused by the presence of the highly dispersed nickel (probably, atomically dispersed nickel) and/or Ni ÷ species inactive to hydrogen chemisorption as supposed above, the true dispersion states of nickel on zeolites could be different from the dispersions determined by means of hydrogen chemisorption. Because the activities increase in proportion to the amount of chemisorbed hydrogen in these nickelcatalysts except NiNa-M, the active sites for hydrogen chemisorption act as the sites for benzene hydrogenation, cyclohexane dehydrogenation and ethane hydrogenolysis (Figures 4, 6 and 7). Table 5 shows the effects of zeolite-support on the activities of these reactions. In nickel supported on zeolite crystals (type A, X and Y), the turnover frequencies for benzene hydrogenation were less than one-half of those in nickel on amorphous supports. The small turnover frequencies in nickel-zeolite crystals are based on the fact that some of the reduced nickel is inactive to the hydrogenation of benzene though active to hydrogen chemisorption, rather than the effects of the acidic sites on zeolites, because the activities hardly increase after the adsorption of ammonia. Because benzene is sorbed in the cavities of zeolites X and Y up to about three-fifths of the maximum occlusion 2s for their cavities at 150°C as shown in Table 4, the hydrogenation of benzene on the active sites situated in zeolite cavities is limited, probably because of the diffusion limitation of hydrogen molecules and/or lower partial pressure of hydrogen in the cavities Table 5 Classification of the effect of supports on hydrocarbon reaction over nickel-zeolites Benzene hydrogenation Support effect: Zeolite-A, X, Y - A m o r p h o u s - Mordenite. TF (h-l) 1000-1100 2030-2450 22-120 Cyclohexane dehydrogenation Support effect: Z e o l i t e - A ~ Zeolite-X, Y ~ Mordenite TF (h-l): 400-500 750-850 ~45 Ethane hydrogenolysis Support effect: Zeolite-A, X, Y , A m o r p h o u s ~ Mordenite
Characterization and catalytic activity of nickel-zeolite catalysts. I V: M, Suzuki et al.
than outside the cavities. For cyclohexane dehydrogenation in NiNa-A, turnover frequencies smaller than in N i N a - ¥ and NiNa-X indicate that nickel, situated at the inside of zeolite A cavities, is inactive to the dehydrogenation. Figure 7 suggests that the turnover frequencies for ethane hydrogenolysis are independent of the pore sizes of these supports except mordenite. This is due to the fact that ethane molecules can diffuse into the cavities of zeolites A, X and Y. All sites determined from hydrogen chemisorption are also active for the hydrogenolysis of ethane. The low activity in NiNa-M is related to the channel structure of mordenite; marked diffusion limitation takes place in the channels in which a number of nickel particles active to hydrogen chemisorption are probably present.
2 3 4 5 6 7 8 9 10 11 12
CONCLUSION
13
The dispersion was affected by the extent of nickel-exchange, the types of parent-cations and the pore structure (or crystal structure) of zeolitesupports. The reduced nickel is highly dispersed in cavities and coarsely dispersed at the outside of the zeolite cavities. Such bidispersion is more marked for the catalysts of lower degree of nickel reduction. The sites active to hydrogen chemisorption on nickel particles supported at the inside and the outside of cavities of zeolites A, X and Y act as sites for the hydrogenolysis of ethane.
14 15
REFERENCES 1
Gallezot, P., Diaz, A. A., Dalmon, J. A., Renouprez, A. J. and Imelik, B. J. Catal. 1975, 39, 334
16 17 18 19 20 21 22 23 24 25
Kubo, T.,Arai, H.,Tominaga, H. and Kunugi, T:Bull. Chem, Soc. Jpn 1972, 45, 607 Gallezot, P., Datka, J., Massardier, J., Primet, M. and Imelik, B. 'Proc. 5th Intern. Congr. Catal.', Amsterdam, 1972, preprint All Figueras, F., Gomez, R. and Primer, M. 'Molecular Sieves', ACS, 1972, p 480 Kubo, T.,Arai, H.,Tominaga, H. and Kunugi,T. Nippon Kagaku Kaishi 1974, 1199 Dalla Betta, R. A. and Boudart, M. "Proc. 5th Intern. Congr. Catal.', Amsterdam, preprint 100-1,1972 Rabo, J. A., Schomaker, V. and Pickert, P. E. "Proc. 3rd Intern. Congr. Catal.', 1965, 2, 1264 Selenina, V. M. Z. Anorg. AIIg. Chem. 1972, 387, 179 lone, K. G., Romammikov, V. N., Davydov, A. A. and Orlova, L. B.J. Catal. 1979,57, 126 Bager, K. H., Vogt, F. and Bremer, H. 'Molecular Sieves I1', ACS, 1977, p 528 Suzuki, M., Tsutsumi, K. and Takahashi, H. Zeolites 1982, 2, 000 Suzuki, M., Tsutsumi, K. and Takahashi, H. Zeofites 1982, 2, 87 Reed, T.B. andBreck, D.W.J. Am. Chem, Soc. 1956,78, 5963 Sherry, H. S. J. Phys. Chem. 1966, 70, 1332 Yates, D. J. C., Taylor, W. F. and Sinfelt, J. H. J. Am. Chem. Soc. 1964, 86, 2996 8assett, D. W. and Habgood, H. W. J. Phys. Chem. 1960, 64, 769 Taylor, W. F., Yates, D. J. C. and Sinfelt, J. H. J, Phys. Chem, 1964, 68, 2962 Richardson, J.T.J. CataL 1971,21,122 Huang, C. P. and Richardson, J. T.J. CataL 1978,52,332 Breck, D. W. 'Zeolite Molecular Sieves, Structure, Chemistry and Use', J. Wiley & Son Inc., 1974, p 633 Barrer, R. M. and Ibbitson, D. A. Trans. Faraday Soc. 1944, 40, 206 Brooks, C. S. and Christopher, G. L. M. J, CataL 1968, 10, 211 Coughlan, B., Narayanan, S., McCann, W. A. and Carroll, W. M. J. CataL 1977,49, 97 Sermon, P.A. andBond, G.C. CataLRev. 1974,8,211 Barrer, R. M., Bultitude, F. W. and Sutherland, J. W. Trans. Faraday Soc. 1957, 53, 1111
Z E O L I T E S , 1982, V o l 2, J u l y
199
INTRODUCTION
EXPERIMENTAL
Once metal catalysts supported on zeolites had been prepared by the reduction of transition-mctal ion-exchanged zeolites with hydrogen, many studies were performed on the dispersions of metal and their catalytic activities, mainly in Pt- and Pdzeolites l-s. These catalysts have been characterized by high dispersions and high activities. For example, the specific activities for the hydrogenation of ethylene in Pt-zeolites are higher than in Pt-SiO2 and Pt-AI203 catalysts 6, and Pt-zeolites possess high resistance to sulphur compounds 7. The unusual properties of these metal-zeolites are caused by the electron transfer between metal and zeolite_support 4,6,7. However, the effects of the zeolite-support on the dispersion of nickel and on the catalytic activity have not been described though a few papers 8-1° have been published on this topic.
Materials Nickel-zeolites were prepared by the methods of our previous paper 12. Nickel-silica-alumina was prepared by means of consecutive ion-exchange at 80°C with a buffer solution (pH --~ 8) consisting of ammonium acetate and ammonium hydroxide containing 0.1 N nickel nitrate. After ion-exchange, the samples were washed several times with a nickel-free buffer solution, and dried at room temperature in vacuo. The compositions of these samples are summarized in Table 1. Nickel-zeolites and nickel-silica-alumina were calcined in flowing nitrogen followed by reduction in a hydrogen stream 11.
In the previous paper n, a series of nickel-zeolite Y with alkaline earth, lanthanum and hydrogen ions as parent-cations in zeolites were prepared. The dispersion of reduced nickel was affected by the types of parent-cations in zeolites, while the turnover frequencies for benzene hydrogenation were nearly identical in these zeolites regardless of the types of parent-cations and the dispersions of reduced nickel. It has been suggested that the reduced nickel is distributed b o t h inside and outsid~the zeolite cavities. In the present paper, the effects of zeolite-support on the dispersions of nickel and their activities have been investigated on a series of nickel-zeolites of type A, X, Y and nickel-mordernites. The activities of these catalysts have been evaluated for benzene hydrogenation, cyclohexane dehydrogenation and ethane hydrogenolysis respectively. 0144-2449/82/020193-07503.00 © 1982 Butterworth & Co. (Publishers) Ltd.
Dispersion The dispersion states of reduced nickel on zeolites were evaluated by means of hydrogen chemisorption, electron micrographs and X-ray line broadening u.
Catalytic activity The hydrogenation of benzene was carried out by the method described earlier u. The dehydrogenation of cyclohexane was performed using a fixedbed continuous flow reactor at 280°C, at a feed rate of 4.44 × 10 -s mol min -1 and at W/F of 41 (g-cat' h mo1-1. The hydrogenolysis of ethane was carried out at temperatures ranging from 320 ° to 350°C using a pulse reactor. The catalyst (0.15 g) fixed in a quartz tube (6 mm (~) was calcined in flowing nitrogen at 370°C for 3 h followed by reduction in a hydrogen stream at 370°C for 6 h. 21/amol of ethane were injected into a hydrogen carrier gas (50 ml min-1). The activities were evaluated in terms of the extent of conversion of ethane at a first pulse because the extent of conversion was ZEOL/TES, 1982, Vol 2, July' 193
Characterization and catalytic activity o f nickel-zeolite catalysts. I V: M. Suzuki et al. Table 1
Chemical compositions, degrees of reduction and surface areas of nickel-zeolites
Zeolite
Ni2+(wt %)
Sc (mag-1)
Exchange (%) Ni 2+
M(n +)a
o~b
1
2
--
--
519
--
100
0.72 0.77
590 586
574 565
100 87
Na-X
0
--
NiNa-X-6
1.16 4.64
6.2 24.9
0
--
--
--
NiNa-A-37 NiCa-A-7 NiSr-A-13 NiSr-A-22 N i Ba-A-6
1.09 4.58 7.47 1.50 2.43 4.21 0.93
5.3 22.4 36.7 7.2 13.1 22.2 5.6
---75.5 73.7 55.8 75.4
0.84 0.63 0.34 0.61 0.62 0.65 0.56
Na-M
0
--
--
N i Na-M-36 NiNa-M-54 Ni H - M - 2 8 Ni-SiO 2 - AI203
2.71 4.10 2.03 4.24
35.5 53.8 27.6 --
--69.1
Na-X-25
Ni
Na-A
Na-A-5
Ni
NiNa-A-22
a b c d
/~0±20 d
- -
--
--
100
23 45 95 483 365 161 10
9
24 58 72 300 125 35 4
100 86 87 50 20 0 0
--
254
--
100
1.00 1.00 0.10 0.62
252 250 260 313
271 254 255 277
100 100 90 --
The e x t e n t of exchange (%) o f the cations other than Na + and Ni 2+ The degree of nickel ion reduction B E T areas of the catalysts before (1) and after (2) reduction The relative intensity of X-ray d i f f r a c t i o n
independent of pulse number from 1 to 10. The products were analysed using a gas chromatograph with a column of active carbon (6 mm @ × 1 m) at i 00°C. Benzene and cyclohexane adsorption After the reduced catalysts were calcined in nitrogen stream at 370°C for 3 h, benzene was injected under the same conditions as the hydrogenation; at 150°C and at a partial pressure of benzene of 0.017. Similarly, cyclohexane was adsorbed on these zeolites at 280°C and at a partial pressure of 0.02. The amounts of adsorbed benzene and cyclohexane were determined from the increasing weight of the samples with an electrobalance (Shimazu Co. Ltd, TG-20 type) equipped with a gas flow system. RESULTS Characterization The degrees of nickel ion reduction were determined by means of hydrogen consumption (Table 1). The degree of nickel reduction was independent of the extent o f nickel-exchange in NiNa-X; this differs from the results observed in NiNa-Y a2. In NiNa-A, the reduction-degree decreased as the nickelexchange level increased. The nickel ions in NiH-M28 were hardly reduced in contrast to NiNa±M in which all of the exchanged nickel ions were rapidly reduced to metal at 370°C. The BET areas and the relative intensities of X-ray diffraction were determined in nickel-zeolites before and after hydrogen reduction (Table 1). The BET areas were nearly identical in NiNa-X-6, NiNa-X-25, NiNa-M-36 and NiNa-M-54 before and after reduction; these areas agreed with those of parent Na-X and Na-M respectively. The BET areas in NiNa-A, NiSr-A and NiCa-A were larger than in parent Na-A because of the increase of free apertures of the cavities, which was caused by the
194
ZEOLITES, 1982, Vol 2, July
exchange of sodium ions with divalent ions t3. When these zeolites were reduced by hydrogen, the BET areas were unchanged in NiNa-A, but decreased in NiCa-A and NiSr-A. The relative intensities of X-ray diffraction were determined for the reduced nickel-zeolites when the respective intensities of the following two peaks in sodium-zeolites were defined as 100; (5 3 3) and (6 4 2) diffraction in Na-X, (3 1 1) and (3 2 1) in Na-A, and (5 3 0) and (4 0 2) in Na-M. The relative intensities agreed within the experimental error (~ -+ 20%) in NiNa-X, NiNa-A and NiNa-M before and after reduction, whereas the intensities decreased in NiSr-A-13 and NiCa-A-7. No diffraction peaks were detected in NiSr-A-22 and NiBa-A-6. These results suggest that the zeolite crystals are partially collapsed in NiCa-A-7 and NiSr-A-13, and almost completely in NiSr-A-22 and NiBa-A-6 after hydrogen reduction. In NiBa-A, the zeolite crystals can be destroyed by ionexchange with barium ions 14. Selenina et al. 8 reported that NiNa-A up to 14% exchange and NiNa-X up to 27% exchange did not show any change in structure, however the crystal structure was retained even in NiNa-A-37 of 37% exchange in the present study.
Dispersion The surface areas (SR) and the average particle sizes (dH) of nickel determined by means of hydrogen chemisorption are summarized in Table 2. The surface areas of reduced nickel decreased with the increase of exchanged nickel ions in these catalysts (Figure 1). They were affected by the types of zeolite-support, and increased in the order; NiNa-Y < NiNa-X < NiNa-M < NiNa-A In NiSr-A-13, NiSr-A-22, NiCa-A-7 and NiBa-A-6 where the crystal structure was considerably collapsed, the surface areas of nickel were large,
Characterization and catalytic activity of nickel-zeolite catalysts. I V: M. Suzuki
in these catalysts; dvM was smaller than dH. The average crystaIlite size of nickel (dx-ray) determined from X-ray line broadening was 21 nm in NiNa-X25, which was smaller than dH. No Ni (1 1 1) peak was detected in NiNa-X-6. Because the diffraction peak for Ni (1 1 1) overlapped with the diffractions of the zeolite crystals, dx-ray could not be determined in NiNa-A and NiNa-M. Similarly, for NiCa-A-7, NiSr-A-13, NiSr-A-22 and NiBa-A-6 because of a broad and a weak spectrum of their Ni (1 1 1) peaks, however, these nickel particles could be very small as indicated by a broad Ni (1 1 1) diffraction peak.
1 4 0 -z~
120 --
\
+\ 100
-
\
-
\ \ 80 Z i
6O
$
\ \
\ \ \ 0
4O
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Catalytic activity
\
Benzene hydrogenation
\ X\qb
\\a x
20
* M , o °-I
I 2
]
I I i i I 4 6 8 Nickel content (wt°/o)
I
] lO
Figure 1 Surface areas of nickel supported on zeolites and silicaalumina, o, NiNa-X; a, NiNa-A; u, NiNa-M; ~, NiNa-Y (by ref 11); +. NiCa-A; {, NiSr-A;-~, N i B a - A , ~ , Ni-SiO 2 - AI203 Table 2 Surface areas and average particle sizes of nickel on zeolites and silica-alumina
Catalyst
VHa
SHb
(#mol (g-cat)-') (m =(g-Ni)-') N i N a - X -6 Ni Na-X-25 NiNa-A-5 NiNa-A-22 NiNa-A-37 NiCa-A-7 NiSr-A-13 NiSr-A-22 NiBa-A-6 NiNa-M-36 NiNa-M-54 NiH-M-28 Ni-SiO 2• AI20 ~
et al.
5.40 8.62 14.8 13.3 8.10 8.80 7.60 17.0 6.90 15.6 5.85 . 10.8
.
52.2 19.5 130 37.2 25.7 77.6 40.7 50.1 107 46.4 11.5 . 29.8
Average diameter (nm) dR dEM 10.8 29.0 4.3 15.2 21.9 7.3 13.9 11.3 5.3 12.2 49.1
-15,1 -9.0 14.6 --6.4 -9.9 --
19.0
9.7
.
a Chemisorbed hydrogen at 13.3 kPa b Surface area of the reduced nickel determined from hydrogen chemisorption
and nearly identical to those in NiNa-A. Hydrogen was only slightly chemisorbed on NiH-M-28 similarly to NiH-Y-33 xl. The particle size distributions of nickel derived from electron micrographs are shown in Figure 2. Many nickel particles were distributed in the narrow range of 4-6 nm in NiNa-A-22 and Ni-SiO2" A1203 where the nickeldispersions are higher than in NiNa-Y-11. The nickel particles were mainly distributed in the range 8-10 nm in NiNa-M-36. No nickel particles were observed in NiH-M-28 with electron microscopy. In Table 2, the average particle sizes (dEM) determined from electron micrographs are collected. Discrepancies between dEM and dn were observed
The extents of conversion for benzene hydrogenation decreased with time on stream, and both cyclohexane and methylcyclopentane were produced in NiNa-M-36 and NiNa-M-54. However, in the other nickel-catalysts, only cyclohexane was produced and the activities only decreased slightly with time on stream. As shown in Figure 3, the extent of conversion decreased with increasing nickel content in NiNa-Y, while the highest activity was observed at the nickel content of ~ 4 % in NiNa-A. NiNa-M showed low conversion despite the high dispersion of reduced nickel. NiCa-A-7, NiSr-A-13, NiSr-A-22 and NiBa-A-6 showed high activities. The order of activity was as follows: NiNa-M < NiNa-Y < NiNa-X < NiNa-A --~ NiCa-A, NiSr-A, NiBa-A The extent of conversion increased in proportion to the amount of chemisorbed hydrogen in these catalysts except NiNa-M as shown in Figure 4. This result suggests that the active sites for hydrogen chemisorption act as the hydrogenation sites. As shown in Figure 4, the relationship between the amount of chemisorbed hydrogen and the extent of conversion has been classified into three groups as follows: NiNa-Y, NiNa-X and NiNa-A belong 40
20
o
,
i
i
i
i p,
a. 40
20
i
i
4
8
12
16
20
,~1 O
4
8
12
16
dE. (rim)
Figure 2 Particle size distributions of nickel on zeolites and silicaalumina. A, NiNa-A-22; B, NiNa-M-36; C, NiNa-Y-11; D, Ni-SiO 2 . A1203
ZEOLITES, 1982, Vol 2, July
195
Characterization and catalytic activity o f nickel-zeolite catalysts. IV: M. Suzuki et al.
hydrogenation. Arrhenius plots deviated from the linear relationships above 160°C because the catalyst deactivation was pronounced and both hydrogenation and isomerization proceeded. The apparent activation energies were 64.8-71.5 kJ mo1-1 in the temperature range of 100°-150°C for catalysts 'A', 'B' and 'C'.
~oo
80
"2 v
60
+ u
It has been reported earlier u that the turnover frequencies are less than 400 h -1 in NiNa-Y-71, NiMg-Y- 19, NiMg-Y-52 and NiLa-Y-21, and these low activities are improved by poisoning the acidic sites on zeolites with ammonia. Therefore, the effects of acidic sites on the activities of the hydro-
(~
... - A . .
40
/
A/
\
Table 3 Activities o f benzene hydrogenation on the nickel supported on zeolites and silica-alumina
20 - .~.,£~
Catalyst I
0
p-o-r
2
-- P--
I
T-
J
J
4 6 Nickel content ( w t %)
~
I
8
/
c/ / / / /
8o
Activation energy
Ea (kJ mo1-1)
10
Figure 3 Activities o f benzene hydrogenation f o r nickel-catalysts. Reaction temperature, 150°0; W/F, 33 (g-cat) h m o I - L Symbols as in Figure I
lO0
Turnover frequency TF (10=h -I)
_
NiNa-X-6 Ni Na-X-25 NiNa-A-5 NiNa-A-22 NiNa-A-37 NiCa-A-7 NiSr-A-13 NiSr-A-22 NiBa-A-6 NiNa-M-36 NiNa-M-54 NiH-M-28 N i-SiO 2 . AI203
10.8 10.2 6.5 10.7 11.1 24.5 24.1 21.5 21.3 0.22 1.2 -20.3
64.8 64.9 69.3 67.4 70.0 71.0 68.5 66.7 69.8 71.4 70.2 71.5
/
A/
/
/ ~//
60
//
/
100
/
/
/ O
/
40/
L)
/
f 50
\ o \ \,
/
20 --
of f
/ 0
"T- D T {5 10 VH ( k m o l (g-cet)-~)
TO ~ ~ 15
~
B "120
Figure 4 Relationship between the amount of chemisorbed hydrogen and the activity of benzene hydrogenation. Symbols as in
Figure I
to the first group (catalyst 'A') as well as NiCa-Y, NiSr-Y and NiBa-Y reported in the earlier I~. However, only NiNa-A-5 deviated from this relation. Catalyst 'C', to which NiCa-A-7, NiSr-A13, NiSr-A-22, NiBa-A-6 and Ni-SiO2" A1203 belong, shows higher activities than 'A'. In catalyst 'B', to which NiNa-M-36 and NiNa-M-54 belong, the extent of conversion is low and independent of the amount of chemisorbed hydrogen. For benzene hydrogenation, the turnover frequencies (TF) in the range of 1000-1100 h -x and of 2030-2450 h -1 were determined in catalyst 'A' and 'C' respectively (Table 3). They were also 22 and 120 h -1 in NiNa-M-36 and NiNa-M-54. Figure 5 shows the effect of reaction temperature on the rates of
196 ZEOLITES, 1982, Vol 2, July
z '
o~
lO
"7 $z
E
5
o v t~
I 2.2
I 2.3
I 2.4
25
2.6
2.7
l l T (kK} Figure 5 Arrhenius plots f o r benzene hydrogenation over nickelcatalysts, o, NiNa-X-25; % NiNa-A-22; )~,'NiNa-Y-31 ; o, NiNa~M-36 (r X 1 0 ) ; ~ , Ni-SiO 2 • AI=O3
Characterization and catalytic activity of nickel-zeolite catalysts. I V: M. Suzuki e t al.
genation were investigated on NiNa-X-25, NiNa-A22 and NiNa-M-36 of which the turnover frequencies were smaller than catalyst 'C'. The extents of conversion (CN) determined after the adsorption of ammonia increased only slightly as compared with those (Co) for the catalysts merely reduced by hydrogen as follows: CN/Co ratios were 1.09, 1.05 and 1.83 in NiNa-X-25, NiNa-A-22 and NiNa-M-36 respectively. Therefore, the lower turnover frequencies in catalysts 'A' and 'B' than in catalyst 'C' suggest that all of the active sites determined by means of hydrogen chemisorption do not act as hydrogenation sites, rather that this is an effect of the acidic sites and/or a characteristic of hydrogenation sites on nickel in these catalysts. Cyclohexane d e h y d r o g e n a t i o n A relationship between the extent of conversion for cyclohexane dehydrogenation and the amount of chemisorbed hydrogen is shown in Figure 6. The activities increased with increased chemisorbed hydrogen. The turnover frequencies for NiNa-X, NaBa-A, and NaSr-A, NiNa-Y-11 and NiNa-Y-31 were nearly identical. In NiNa-Y-71, NiCa-Y-29 and NiSr-Y-21, the extents of conversion decreased with time on stream, and b o t h benzene and methylcyclopentane were produced. Ethane
hydrolysis
The extents of conversion for ethane hydrogenolysis in relation to the amount of chemisorbed hydrogen are shown in Figure 7. The activities increased in proportion to the chemisorbed hydrogen in these catalysts except NiNa-M-36 and NiNa-M-54. In NiNa-M-36 and NiNa-M-54, the extent of conversion for ethane hydrogenolysis was slight despite the large amount of chemisorbed hydrogen as well as benzene hydrogenation and cyclohexane dehydrogenation. It has been reported that the orders of reaction in ethane and hydrogen are respectively 1 and --2 on Ni-SiO2 catalysO s. Under the conditions of this study, the apparent activation energies have been determined on the
50
/ /
2/
40
30
y"
/
/
", (D
>
A
20-
¢0//
U 10
/ I
5
i
[] I
15 lo vH (Ixmol(g-cot) -~)
20
Figure 6 Relationship between the amount of chemisorbed hydrogen and the activity of cyclohexane dehydrogenation. Reaction temperature, 280°C; W/F, 41 (g-cat) h tool-l; o, NiNa-X; A, NiNa-A; u, NiNa-M; ~, NiNa-Y; e, NiCa-Y; % NiSr-Y; {, NiSr-A,
-~, N i B a - A ; - ~ ,
Ni-SiO 2 • AI=O a
loo
/ 8o
+ ~ 60 o
/
/
/
/
/
/
/
+
¢ /zx /
e/ /
~ 4o o u
/ /
//+ ~/
2o _ / 0
/~
t•
5
[]
I 10 VH (p,mol ( g - c o t )
1
15
1
20
"1 )
Figure 7 Relationship between the amount of chemisorbed hydrogen and the activity of ethane hydrogenolysis. Reaction temperature, 350°C; catalyst charged, 0.15 g; ethane injection, 21 #tool, H2 flow rate, 50 ml rain-l; [], NiMg-Y; e, NiBa-Y. Other symbols are shown in Figure 6
basis of the assumption that the hydrogenolysis rates are of first order in ethane partial pressure and of quasi-zero order in hydrogen pressurO 6. The activation energies determined in the range of 320 °350°C were nearly identical in these catalysts, and were 170-180 kJ tool-a; these agreed fairly well with the result reported by Yates et al. is Tayler et al. 17 reported that Ni-SiO2, Ni-A1203 and NiSiO2" A1203 catalysts gave different specific activities for ethane hydrogenolysis. However, Richardson 1s,19 reported that the specific activities were nearly identical in NiNa-Y, NiMg-Y, NiCa-Y and NiLi-Y catalysts, and the activities were unaffected by the acidity of supports in a series of NiSiO2" A1203 catalysts of which the acidic sites were progressively poisoned by sodium ions. The results reported by Richardson were supported by this study; the turnover frequencies and the apparent activation energies for ethane hydrogenolysis were independent of the support structures, their acidities and the dispersion states of nickel on zeolites in these nickel-zeolites except NiNa-M. Benzene and c y c l o h e x a n e adsorption The amount of adsorbed benzene and the free apertures of main cavities (or channels) 2° for zeolites are shown in Table 4. Almost all of the adsorbed benzene was desorbed when the catalysts were calcined at 370°C in flowing nitrogen. Because the crystallographic size of entrance of the cavity in zeolite A is 0.5 nm 2°, the benzene molecule of critical dimension of 0.68 nm 21 cannot penetrate the cavity. Actually, only small amounts of benzene were adsorbed on Na-A and NiNa-A-22. However, a large amount of benzene was adsorbed on NiNa-Y-13, NiNa-X-25 and NiNa-M-36 as compared with NiNa-A-22 because the benzene molecule can diffuse into the cavities of zeolites X, Y and mordenite. The amounts of adsorbed cyclo-
Z E O L I T E & 1982, Vol 2, July
197
Characterization and catalytic activity of nickel-zeolite catalysts. IV: M. Suzuki et al. Table 4
Adsorption of benzene on zeolites
Zeolite
VB a (mmol (g-zeol) -1)
Na-Y NiNa-Y-31 c NiNa-X-25 c Na-A N i Na-A-22 c Na-M H-M d NiNa-M-36 c
2.02 1.95 2.14 0.15 0.17 0.51 0.60 0.58
Free aperture of main cages b (nm) 0.74-0.80 0.74-0.80 0.90 0.41 ~ 0.50 -0.67 X 0.70 --
a The a m o u n t of adsorbed benzene at 150°C b By ref 20 c The catalysts reduced by hydrogen at 370°C d H - M was prepared by treating N a - M w i t h 2 N hydrochloric acid at room temperature
hexane at 280°C were below ~ 0 . 0 2 mmot (g-zeol) -j on Na-A, Na-X, Na-Y and Na-M. DISCUSSION
In the previous paper 11, the average particle sizes of nickel on zeolite Y were determined by means of hydrogen chemisorption, electron microscopy and X-ray line broadening. Only nickel particles which have formed metal crystallites can be detected by means of X-ray diffraction. Particles of larger diameter than ~ 2 nm were observed by electron microscopy in this study. Because the size of cavity in zeolite Y is 1.3 nm 2°, only nickel particles situated outside the zeolite cavities are determined from X-ray line broadening and electron micrographs. Therefore, the average particle sizes of nickel (dn), determined by means of hydrogen chemisorption, could be smaller than the sizes determined from electron micrographs (dEM) and X-ray line broadening (dx-ray). However, dH was larger than dx-ra.yand dEM in our nickel-zeolite Y. These discrepancles were marked in NiNa-Y-71, NiMg-Y-52 and NiLa-Y-21 which showed low degrees of nickel ion reduction. These results were interpreted as follows: If the nickel species not detected by X-ray diffraction and electron microscopy are also inactive to the hydrogen chemisorption, the small amount of reduced nickel would lead to an overestimate of dH when these nickel species are present in cavities. Brooks et al. 22 reported similar results. Carbon monoxide was adsorbed on nickel in several forms, while hydrogen was adsorbed only on the surface of nickel crystallites. Kubo et al. 2 have also reported that atomically dispersed platinum is inactive to hydrogen chemisorption. As shown in Table 2, d n was larger than dEM in NiNa-A, NiNa-X and NiNa-M, whereas the discrepancies between dn and dEM in these catalysts except NiH-M-28 were small as compared with NiNa-Y-71, NiMg-Y-52 and NiLa-Y-2111. The dispersion of nickel was affected by the types of zeolite supports, and increased in the order: NiNa-Y < NiNa-X < NiNa-M < NiNa-A (Though the term 'dispersion' is defined as the ratio of the number of surface metal atoms to the
198
ZEOLITES, 1982, Vol 2, July
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total ones, it corresponds to the amount of chemisorbed hydrogen because the number of surface metal atoms are generally determined by means of hydrogen chemisorption. Therefore, dispersion is also comparable to the surface area of metal or particle size derived from the chemisorption.) The above order is consistent with the result reported by Bager et al. 1° However, Coughlan et al. 23 reported that the dispersion of ruthenium determined by means of hydrogen chemisorption increased in the following order in contrast to nickel-ze olites: RuNa-A < RuNa-X < RuNa-Y Such high dispersions of ruthenium (H/Ru = 0.51.0) in RuNa-Y suggest that atomically dispersed ruthenium in zeolite cavities chemisorbs hydrogen if the spillover phenomena z4 are inconsiderable. If the low dispersions of nickel observed markedly in NiNa-Y-71, NiMg-Y-52, NiLa-Y-21, NiH-Y-33 and NiH-M-28 are caused by the presence of the highly dispersed nickel (probably, atomically dispersed nickel) and/or Ni ÷ species inactive to hydrogen chemisorption as supposed above, the true dispersion states of nickel on zeolites could be different from the dispersions determined by means of hydrogen chemisorption. Because the activities increase in proportion to the amount of chemisorbed hydrogen in these nickelcatalysts except NiNa-M, the active sites for hydrogen chemisorption act as the sites for benzene hydrogenation, cyclohexane dehydrogenation and ethane hydrogenolysis (Figures 4, 6 and 7). Table 5 shows the effects of zeolite-support on the activities of these reactions. In nickel supported on zeolite crystals (type A, X and Y), the turnover frequencies for benzene hydrogenation were less than one-half of those in nickel on amorphous supports. The small turnover frequencies in nickel-zeolite crystals are based on the fact that some of the reduced nickel is inactive to the hydrogenation of benzene though active to hydrogen chemisorption, rather than the effects of the acidic sites on zeolites, because the activities hardly increase after the adsorption of ammonia. Because benzene is sorbed in the cavities of zeolites X and Y up to about three-fifths of the maximum occlusion 2s for their cavities at 150°C as shown in Table 4, the hydrogenation of benzene on the active sites situated in zeolite cavities is limited, probably because of the diffusion limitation of hydrogen molecules and/or lower partial pressure of hydrogen in the cavities Table 5 Classification of the effect of supports on hydrocarbon reaction over nickel-zeolites Benzene hydrogenation Support effect: Zeolite-A, X, Y - A m o r p h o u s - Mordenite. TF (h-l) 1000-1100 2030-2450 22-120 Cyclohexane dehydrogenation Support effect: Z e o l i t e - A ~ Zeolite-X, Y ~ Mordenite TF (h-l): 400-500 750-850 ~45 Ethane hydrogenolysis Support effect: Zeolite-A, X, Y , A m o r p h o u s ~ Mordenite
Characterization and catalytic activity of nickel-zeolite catalysts. I V: M, Suzuki et al.
than outside the cavities. For cyclohexane dehydrogenation in NiNa-A, turnover frequencies smaller than in N i N a - ¥ and NiNa-X indicate that nickel, situated at the inside of zeolite A cavities, is inactive to the dehydrogenation. Figure 7 suggests that the turnover frequencies for ethane hydrogenolysis are independent of the pore sizes of these supports except mordenite. This is due to the fact that ethane molecules can diffuse into the cavities of zeolites A, X and Y. All sites determined from hydrogen chemisorption are also active for the hydrogenolysis of ethane. The low activity in NiNa-M is related to the channel structure of mordenite; marked diffusion limitation takes place in the channels in which a number of nickel particles active to hydrogen chemisorption are probably present.
2 3 4 5 6 7 8 9 10 11 12
CONCLUSION
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The dispersion was affected by the extent of nickel-exchange, the types of parent-cations and the pore structure (or crystal structure) of zeolitesupports. The reduced nickel is highly dispersed in cavities and coarsely dispersed at the outside of the zeolite cavities. Such bidispersion is more marked for the catalysts of lower degree of nickel reduction. The sites active to hydrogen chemisorption on nickel particles supported at the inside and the outside of cavities of zeolites A, X and Y act as sites for the hydrogenolysis of ethane.
14 15
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