J. Phys. Chem. 1988, 92, 4439-4446
of R. The slope of the curve, always positive, indicates that as R increases (i.e., the water content increases), the number of H20 molecules dynamically seen as bulk water increases. In the range 20 < R < 40 a different slope is observed. This Occurrence seems to be indicative of a mechanism (droplets aggregation?) in the growth process of the reversed micelles. On this basis, by following the evolution of the (1 - a)/. ratio against R , the filling mechanism of AOT reversed micelles can be explained in terms of an initial hydration of the head groups of the surfactant followed by the formation of a water pool where a continuous equilibrium between bonded and bulk water can exist.
4439
Conclusively, we can assert that a boundary layer of water (bonded water), with a structure different from that of bulk water, is evidenced through the Raman scattering experiment. At any value of R, the 0-H stretching contribution is interpreted as originating from two coexisting spatially separated structural arrangements for the water: the first one is surfactant imposed, and the second one is the usual water structure. The relative amount of the bonded water is estimated as a function of the molar ratio R . Registry No. AOT, 577-11-7; H20, 7732-18-5.
Catalytic and ESR Studies of Ethylene Dimerization on Palladium-Exchanged Na-X and Ca-X Zeolites Ashim K. Ghosh and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: October 15, 1987)
Oxygen-pretreated, palladium-exchanged Na-X and Ca-X zeolites are catalytically active for ethylene dimerization in both static and flow reactors. The catalytic activity of the zeolites for this reaction is shown to be due to palladium species and is greatly dependent on the type of major cocation (Na' or CaZ+)in the zeolites. The cocations influence the location of the active palladium species. In NaPd-X zeolites, palladium cations occupy sites (SI or SI') which are inaccessibleto ethylene, while in CaPd-X zeolites palladium cations occupy sites (SIP or SII) which are relatively accessible to ethylene. As a result, the reaction occurs after a longer induction period in NaPd-X zeolites due to migration of palladium species toward more accessible locations. This induction period decreases with an increase of reaction temperature. Paramagnetic species, giving ESR signals at g,, = 2.53 and g,, = 2.33-2.34, both with g1 at 2.10, are assigned to Pd' coordinated to ethylene and are detected prior to butene formation. Consequently, monovalent palladium cations are considered to be catalytically active sites for ethylene dimerization.
Introduction
Several papers'-4 have treated ethylene dimerization on Pdexchanged zeolites. It is generally accepted that palladium species directly participate in the formation of catalytically active sites. However, the oxidation and coordination states of the palladium species are uncertain. It has been speculated that monovalent2 or divalent4 palladium cations are catalytically active in promoting ethylene dimerization. , The catalytic efficiency of a cation-exchanged zeolite for nonacid catalysis is dependent on the type, amount, and location of active cations in the zeolite Recent work9 in this laboratory has shown that transition-metal-cation location in a zeolite can be controlled by a number of factors including (a) the charge and size of the more abundant cocation, (b) the Si/Al ratio, (c) the structural type of zeolite, (d) the thermal pretreatment, and (e) the presence of various adsorbates. Of these factors the use of cocation variation as a control method seems particularly promising. In preliminary work,4 it was shown that, irrespective of palladium concentration, CaPd-X zeolites are active for ethylene (1) Lapidus, A. L.; Mal'tsev, V. V.; Garanin, V. 1.; Minachev, Kh. M.; Eidus, Ya. T Izv. Akad. Nauk SSSR,Ser. Khim. 1975, 2819. (2) Lapidus, A. L.; Mal'tsev, V. V.; Shpiro. E. S.;Antoshin, G. V.; Garanin, G. V.; Minachev, Kh. M. Izv. Akad. Nauk SSSR,Ser. Khim. 1977, 2454. (3) US.Patent 3 738 977, 1973. (4) Michalik, J.; Lee, H.; Kevan, L. J . Phys. Chem. 1985, 89, 4282. (5) Maxwell, I. E. Adv. Catal. 1982, 31, 1. (6) Lunsford, J. H. Catal. Reu. 1975, 12, 137. (7) Ben Taarit, Y.; Che, M. In Caralysis by Zeolites; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 67. (8). Smith, J. V. In Zeolite Chemistry and Catalysis; Rabo, J. A,, Ed.; American Chemical Society: Washington, D.C., 1976; Chapter 1. (9) Kevan, L. Acc. Chem. Res., 1987, 20, 1.
0022-3654/88/2092-4439$01.50/0
dimerization. In contrast, only higher palladium loadings in PdNa-X zeolite were shown to be active. This was explained by the different locations of the active palladium cations dependent on the presence of Na+ versus CaZ+cocations. In NaPd-X the Pd cations occupy relatively inaccessible sites, while in CaPd-X the Pd cations occupy more accessible sites with respect to ethylene. The present work describes studies of ethylene dimerization on palladium-exchanged Na-X and Ca-X zeolites using both static and flow reactors. Using electron spin resonance (ESR) spectroscopy, the generation and migration of paramagnetic palladium species are monitored before and during the reaction. By comparison of catalytic and ESR results the formation and location of catalytically active palladium sites are assessed. The effect of using static versus flow reactors is also compared. Experimental Section
Linde Na-X zeolite was obtained from Alpha Chemicals. Ca-X zeolite was prepared from Na-X by ion exchange with 0.1 M CaC12solution at 80 O C for 1 week. Palladium was introduced into the zeolite as Pd(NH3)t' cation by ion exchange with various amounts of 0.01 M palladium tetraammine chloride (Alpha) solution a t room temperature for 24 h. Commercial atomic absorption was used to determine the palladium content. Most experiments were done with samples of CaPd,,,-X, CaPd,,,-X, and NaPdlL,5-X zeolites where the subscript refers to the number of palladium ions per unit cell. Some experiments were done with Ca-X and NaPd,,,-X zeolites. Premixed ethylene (4 wt %) in He, ethylene, 1-butene, cis-2-butene, and trans-2-butene were obtained from the Linde Division of Union Carbide Corporation. Experiments were carried out by using a fixed bed type reactor made of glass with either continuous gas flow at atmosphere pressure or a closed, static reactor system of total internal volume 0 1988 American Chemical Society
4440 The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
Ghosh and Kevan
TABLE I: Ethylene Dimerization on Various Palladium-Exchanged Ne- and Ca-X Zeolites Heated at 500 O C in Flowing 0, ethylene dimerization
zeolite
NaPd1.6-X NaPd I 2.5-X
reactor condition
reaction temD/OC
static
25, 50, 65 85 25
static
50 65 85
flow static flow
static
50 65 85 25
85 25
50
flow
85 25 50 65 85
induction period /min
% conversion‘
catalyst deactivation
0 90 240 60 15
25 0.1 95 100 14
0
3
30 10
9 11 48 13
0 30 10 15
I1
0 0
100 26
30
4
5 0 0
12.5 18
9
not obsd not obsd (1.5 h)b (3.0 h)‘ -0.042d -0.6d
-0.21d not obsd -0.3gd (2.5 h)b (50 min)b (1 h)‘ -0.006d
-0.3d -0.1 3d -0.43d
“Ethylene dimerization after 4 h of reaction in static reactor and at the time of maximum ethylene conversion in a flow reactor. bEthylene dimerization slowed down after the time specified. cEthylene dimerization stopped in the static system at the time specified. dRate of decrease of ethylene dimerization as percent of ethylene converted per minute. of about 58 cm3. A zeolite sample of 0.05 g was placed on a sintered glass disk inside the reactor. The zeolite was heated in oxygen flow (30 cm3/min) while the temperature was slowly increased to some maximum temperature at which heating was continued for 16 h. In one series of experiments, the oxygentreated sample was cooled to room temperature and briefly evacuated to a residual pressure of Torr. In another series of experiments, the oxygen-treated sample was evacuated at the maximum temperature for 16 h and the sample was cooled to room temperature. In some cases, the catalyst samples were pretreated by using N2 flow instead of O2flow. Ethylene dimerization was studied on variously pretreated zeolite samples by flowing premixed ethylene (4 wt %) in helium over the catalyst at a specific reaction temperature at a flow rate of 20 cm3/min. In the closed, static reactor system, a measured quantity of ethylene (0.65 mmol) was admitted into the reactor containing pretreated zeolite sample at a specific reaction temperature. The reaction products were analyzed in both flow and static systems at different reaction times by withdrawing aliquots for analysis by an on-line gas chromatograph (Varian Model 3300 equipped with an electronic integrator) with a thermal conductivity detector using a 6-ft column (i.d. 0.085 in.) packed with 0.19 wt % picric acid on 80/lOO mesh Graphic-GC support at 35 OC. The generation and migration of paramagnetic palladium species in the system were examined by ESR. The reactor was connected to a small quartz ESR tube (3-mm 0.d.) so that the ESR spectrum could be recorded without exposure to the sample to air. Both before and after various amounts of reaction, the sample was quenched to room temperature and transferred in situ to the ESR tube. ESR spectra were immediately recorded at 77 K with a Varian E-4 spectrometer.
Results A . Catalytic Studies of Ethylene Dimerization. In this work, most of the catalytic and ESR studies were done with samples of CaPd,,,-X, CaPd9,6-X, and NaPd,2,5-X zeolites. In addition, Na-X, Ca-X, and NaPdl,6-X were occasionally used. Ca-X and Na-X zeolites were found to be catalytically inactive while the palladium-exchanged zeolites were active for ethylene dimerization, indicating that palladium directly participates in the formation of catalytically active sites. A1 , Palladium-Exchanged Na-X Zeolites. A1 a. Static Reactor. Table I summarizes the results of catalytic studies of ethylene dimerization on various palladium-exchanged X zeolites using both static and flow reactors. Figure 1 shows catalytic runs of the ethylene dimerization reaction studied on pretreated samples of palladium-exchanged Na-X zeolites using a static reactor.
IO0 2
0 v,
6
>
75
2
s W
5> 50 I
IW
I-
5
25
2W a
0 0
2 3 1 REACTION TIME, h
4
Figure 1. Ethylene dimerization, using a static reactor, on NaPdl,6-X and NaPd,,,5-X zeolites pretreated under 0,flow at 500 O C . Reaction conditions: catalyst 0.05 g, PoBH4= 400 Torr, reaction temperature 25 (A), 50 (o),65 (0),and 85 “C (v)on NaPd,2,5-Xand 85 OC ( 0 )on
NaPd 1,6-X. Ethylene dimerization did not occur to any significant extent at 25, 50, or 65 O C on low palladium content zeolite (NaPd,,,-X) during a 4-h reaction period (not illustrated). However, this zeolite did show activity for ethylene dimerization to isomers of butene (8.6 f 0.5% 1-butene, 27.2 f 0.6% cis-2-butene, and 64.2 f 0.6% trans-2-butene) at the higher temperature of 85 O C with an induction period of about 90 min. Na-X zeolite containing a higher content of palladium (NaPd,,,5-X) also did not show any catalytic activity at 25 OC but is shown to be active at higher temperatures of 50, 65, and 85 OC with decreasing induction periods of about 60, 15, and 0 min, respectively. These results indicate that the catalytic activity as well as the induction period is dependent on the amount of exchanged palladium and on the reaction temperature. On NaPd,2,5-X at 85 O C ethylene conversion initially increases sharply but then the reaction almost stopped in about 3 h, presumably due to deactivation of the active sites at this temperature. A l b . Flow Reactor. In the flow reactor, the conversion of ethylene to butenes, after an induction period, increases with time to a maximum and then declines as shown in Figure 2. This decrease of ethylene dimerization is ascribed to deactivation of catalytically active sites, a common interpretation in heterogeneous catalysis. The maximum percentage of ethylene converted is defined as the “activity” of the catalyst. The defined activities of NaPd,,,,-X zeolite are 3, 9, and 11% conversion at reaction
Ethylene Dimerization on Pd-Exchanged Zeolites
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4441
20
COPdg,g-X FLOW
w 2
' 0
I
201 NOPdl2,S-X FLOW
30
60
90
I
120
REACTION TIME, MIN
REACTION TIME, MIN
Figure 2. Ethylene dimerization, using a flow reactor, on NaPd!2.5-X zeolite samples pretreated under O2flow at 500 OC. Reaction conditions: catalyst 0.05 g, CzHI (4 wt % in He) flow rate 20 cm3/min, reaction temperature 50 (a), 65 (A), and 85 OC (0).The insert shows the
Figure 4. Ethylene dimerization, using a flow reactor, on CaPd9,6-X
zeolite pretreated under O2flow at 500 OC. Reaction conditions: catalyst 0.05 g, C2H4 (4 wt % in He) flow rate 20 cm3/min,reaction temperature 25 ( O ) , 50 ( O ) , 65 (A),and 85 "C (0).
expanded range for 50 OC. COPdse-X
a >-
REACTION TIME, h
Figure 3. Ethylene dimerization, using a static reactor, on CaPdl ,-X
and CaPdg6-X zeolites pretreated under O2flow at 500 "C (open sy.mbok) and on subsequent prolonged evacuation (solid symbols). Reaction conditions: catalyst 0.05 g, POczH4 = 400 Torr, reaction temperature 25 (0, 0 , A, A), 50 (a), and 85 O C (V). temperatures of 50,65, and 85 "C. The induction periods at 50, 65, and 85 "C are 30, 10, and 0 min, respectively, and seem to be slightly shorter in comparison to a static system. The catalyst deactivation rate increases with an increase of reaction temperature. A2. Palladium-Exchanged Ca-X Zeolites. A2a. Static Reactor. Figure 3 shows results of ethylene dimerization of CaPd-X zeolites using a static reactor. A shorter induction period and a higher ethylene conversion are observed in comparison to Pd-exchanged Na-X zeolites. For example, ethylene dimerization did not occur at 25 "C on NaPd-X zeolites, whereas both CaPd,,,-X and CaPd, 6-X zeolites showed high catalytic activity (see Table I). At 85 "C the dimerization reaction of CaPd9,6-X zeolite almost stopped after about 1 h, earlier than occurred on NaPd,, 5-X due to the location and/or activity of the palladium sites. As will be shown later, O2pretreatment of CaPdg6-X zeolite generates Pd3+ cations, and subsequent evacuation of the 02treated sample generates Pd+ cations. This variation of pretreatment conditions, however, did not result in any appreciable change in the catalytic activity; see Figure 3 and compare the solid and open symbols. A2b. Flow Reactor. Figure 4 shows the results of catalytic runs of ethylene dimerization of CaPd9,-X zeolite using a flow reactor. The maximum percent ethylene conversion, described as the "activity", increases with an increase of reaction temperature
Figure 5. Variation of formation of Pd3+ions and ethylene dimerization "activity" as a function of O2 pretreatment temperature of CaPd, 6-x zeolite. The maximum percent of ethylene converted in a flow reactor is defined as the dimerization "activity" (see text). Reaction conditions: catalyst 0.05 g, C2H4 (4 wt % in He) flow rate 20 cm3/min, reaction temperature 85 "C.
(e.& 4% at 25 "C to 18% at 85 "C). Concurrently, the catalyst deactivates at a faster rate at the higher temperature, as was seen on NaPd12,5-X. The induction time also decreases with increasing temperature. The time at which the maximum conversion is observed is also dependent on the temperature. At 25 "C the maximum conversion of 4% is observed after about 100 min while at 85 "C the maximum conversion of 18% is observed after about 10 min. These results imply that the migration and/or formation of catalytically active sites as well as their deactivation is enhanced by temperature. B. Effect of Pd3+ Cations. In order to examine the influence of Pd3+cations on the catalytic activity for ethylene dimerization, the reaction was studied by using a flow reactor on samples of CaPdg6-X zeolite pretreated in O2 at various temperatures between 250 and 500 "C. As shown in Figure 5, no appreciable formation of Pd3+, as determined by ESR at g,,, = 2.23,4 was observed after pretreating the catalyst at 250 or 300 "C. However, the catalyst showed catalytic activity for ethylene dimerization. This suggests that Pd2+ and/or Pd+ cations, formed by reduction of Pd2+ by ethylene, are catalytically active. In agreement, the catalyst pretreated in N, flow at 300 and 400 "C, producing no paramagnetic species, is shown to dimerize ethylene after a relatively longer induction period and is relatively less active by comparison to O2 pretreatment (see Table 11). It is noted that an increase in pretreatment temperature in flowing O2 for the catalyst from 250 to 300 "C increases the catalytic "activity". This may be attributed to the decomposition
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The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
Ghosh and Kevan
TABLE II: Ethylene Dimerization in CaPd9,6-X Zeolite Pretreated under Flowing N2 ethylene dimerization induction catalyst pretreatment Pd+(G)’ time, min activityb N,/3OO OC/16 h 40 10 8 N2/4O0 OC/16 h N,/5OO OC/16 h 1.73 10 10 N2/300 OC/16 h, evac/300 OC/16 h 2.16 40 1 N2/400 OC/16 h, evac/400 OC/16 h 4.11 20 6 N2/500 OC/16 h, evac/500 OC/16 h 6.25 10 11 ‘Relative yield of the Pd+(G) species. bActivity is defined as the maximum percent conversion versus reaction time in a flow system; see Figure 2.
E l I
I
-r“ 0
200 300 400 500 PRETREATMENT TEMPERATURE,’C
Figure 7. (a) Production of Pd+ and Pd” species in CaPd9,6-Xzeolite measured by ESR as a function of pretreatment temperature in a flow reactor (0,pretreatment and subsequent evacuation) and (b) ethylene dimerization “activity” in percent on the pretreated zeolite samples. Reaction conditions are the same as in Figure 6.
1
0
30 60 90 REACTION TIME, MIN
120
.
gl, = 2 79
Figure 6. Variation of ethylene conversion as a function of reaction time in a flow reactor on CaPd9,6-Xpretreated at 300 OC under 0,flow (0) and on subsequent evacuation of the 02-treated sample (A). Reaction
conditions: catalyst 0.05 g, C2H4(4 wt 7%in He) flow rate 20 cm3/min, reaction temperature 85 OC. of Pd(NH3)42+,the initially exchanged ions, to Pd2+ and NH3. The formation of Pd3+ cations was found to increase with an increase in the pretreatment temperature above about 350 OC. As will be discussed below, the ESR assigned to Pd3+concentration did not correlate with the dimerization “activity”. This was also investigated in a static reactor. The results showed that the amount of ethylene converted on samples pretreated in O2 flow at 300 and 500 O C is almost the same, confirming that Pd3+cations do not correlate with the dimerization activity. C . Effect of Pd+ Cations. The influence of Pd+ cations determined by ESR4 on ethylene dimerization was also investigated. Figure 6 shows the results of ethylene dimerization, using a flow reactor, on CaPd9,6-X zeolite activated by (a) heating in o2flow at 300 OC and (b) subsequently evacuating the 02-treated sample at 300 OC. From an ESR study, it was shown that pretreatment a produced no paramagnetic palladium species, whereas in pretreatment b a part of the Pd2+cations were reduced to Pd+ cations. In the case of pretreatment a, the ethylene dimerization “activity” (maximum percent conversion) is slightly higher than that in the case of pretreatment b. However, the former shows faster catalyst deactivation. Using a static reactor, no significant difference in activity was observed with pretreatment a or b except for a slightly higher conversion in the initial stages after pretreatment a. The results imply that O2pretreatment may cause the Pd2+ cations to migrate to a more accessible location at which dimerization can initially occur more rapidly. Figure 7 shows the generation of Pd+ and Pd3+ cations and catalytic “activity” for ethylene dimerization on CaPd96-x zeolite as a function of pretreatment (02treatment and subsequent evacuation) temperature. At an activation temperature below about 350 OC,only Pd+ was generated and its formation increased
g,,=2 50
Figure 8. ESR spectra at 77 K of CaPdg6-X zeolite pretreated under 0, flow at 500 O C : (a) prior to butene adsorption, ( b e ) at various times
after cis-2-butene adsorption at room temperature with temperature. For activation above 350 “C, both Pd+ and Pd3+ were observed. There seems to be no direct correlation between the formation of these paramagnetic species and the dimerization “activity”. However, both the “activity” and the Pd+ ions increased with the pretreatment temperature up to a temperature of about 300 OC, presumably due to the decomposition of Pd(NH3)42+ions into Pd2+ and NH3. D. EIectron Spin Resonance Studies. The generation of paramagnetic species in palladium-exchanged Na-X and Ca-X zeolites was found to be very sensitive to the concentration of Pd2+ cations and the pretreatment conditions of the catalysts. In previous worklo an isotropic ESR signal at g = 2.23 has been assigned to Pd3+ and anisotropic ESR signals with g,,= 3.0-2.3 and g, = 2.10 have been assigned to Pd+ species. The ESR signal at g,,, = 2.23, designated as A (Figures 8a, 9a, and loa), was generated in CaPdg6-X and NaPd,, 5-X zeolites after pretreating the zeolites in O2 flow at a temperature above about 350 OC. On subsequent evacuation, the peak at g,, = 2.23 decreased significantly in the case of CaPd, 6-x and decreased slightly in case (10) Michalik, J.; Heming, M.; Kevan, L. J . Phys. Chem. 1986, 90, 2132.
Ethylene Dimerization on Pd-Exchanged Zeolites NaPdI2,,-X/C2H4/Static
.
The Journal of Physical Chemistry, Vol. 92, No. 15, I988 4443 TABLE IIk Assignment of Paramagnetic Species in Various Palladium-Exchanged Na-X and Ca-X Zeolites after Pretreatment and during Ethylene Dimerization
. . (b)x200 B
C2H4/5O0C/5min
species A
B
C D E F G H
x2000 gll= 2 . 4 0
Figure 9. ESR spectra of NaPd,2,5-Xat 77 K: (a) after pretreatment under O2flow at 500 "C, (b-f) at different reaction times in the presence of ethylene in a static reactor at 50 OC. CaPdS,6-X/CzH4/Static
(b)x2
L
G
g,, 3.0 I (c) x 2
g,, =2.65 9,',=240
1
Figure 10. ESR spectra of CaPd9,6-Xat 77 K: (a) after pretreatment under O2flow at 500 "C, (b) on subsequent evacuation at 500 "C, and (c-g) at different reaction times in the presence of ethylene in a static reactor at 25 OC.
of NaPd12,rX,and subsequently species G, showing an ESR signal at gll = 3.01 with g, = 2.10 (Figure lob), was formed. In the case of low palladium content samples, CaPd,,,-X generated weak signals due to species A and G, but NaPdl,6-X zeolite did not. ESR studies of the adsorption of I-butene, cis-2-butene, and trans-2-butene were done with samples of CaPd9.6-X zeolite pretreated in O2flow at 500 OC. The ESR spectra of the zeolite samples on which the butene isomers were adsorbed are essentially the same. As shown in Figure 8, the isotropic signal at gh = 2.23, attributed to Pd3+ ions, disappeared completely in less than 5 min, and a new peak appeared at gll = 2.79 with g, = 2.10 (Figure 8b) whose intensity slowly increased with time (Figure 8c,d). In addition to the peak a t gll = 2.79, four new peaks developed at gll = 2.88, gll = 2.66, gll= 2.50, and gll = 2.40, all with the same g, = 2.10 component (Figure 8e). DI. Static Reactor. D l a . NaPd-X Zeolites. The paramagnetic species generated in palladium-exchanged X zeolites
probable assignt Pd3+at SI in NaPd-X and at g,, = 2.23 SI1 in CaPd-X 2.81 2.10 (NaPd-X) Pd+ (C,H4) at SI' 2.10 (CaPd-X) Pd+ (C2H4) at SI1 or SII' 2.79 Pd+ (C2H4)at SI1 or SII* 2.53 2.10 2.10 Pd+ (C2H4) at SI1 or SII* 2.33 2.10 Pd+ (C,H,) at SI1 2.65 2.10 Pd' (C4H8)at SI1 or SII* 2.40 Pd+ at SI or SI' in NaPd-X 3.01 2.10 and at SII' or SI in CaPd-X 2.88 2.10 (CaPd-X) Pd' (C4Hs)at SI or SI' gII
IZI
during pretreatment and ethylene dimerization are summarized in Table 111. At reaction temperatures of 50, 65, and 85 OC, on admission of ethylene into a static reactor containing pretreated NaPd1,,-X zeolite, species A (gh = 2.23) disappeared completely in less than 5 min and a new species, designated as B (gll= 2.81, g, = 2.10), appeared immediately whose growth increased with time and was enhanced by temperature (Figure 9). In addition to species B, two more species, designated as C (gll = 2.53) and D (gll = 2.33) with the same g, value at 2.10 appeared. Simultaneously, ethylene dimerization products were detected in the gas phase in the reaction effluent. It is worth emphasizing that at the reaction temperature of 25 OC species B appeared on ethylene admission but no other species except a very weak signal at gll = 2.53 (species C) appeared during a 4-h reaction period. As was seen earlier, only about 0.1% ethylene dimerized at 25 O C during the 4-h period. It seems that species B, attributed to Pd+, is situated in a location inaccessible to ethylene in the zeolite structure and is not directly coordinated to ethylene. However, the Pd+ cations may subsequently migrate to a location accessible to ethylene where ethylene may coordinate with them to result in species C and D. Subsequently, ethylene is dimerized to butenes. Species C and D were observed to decrease with a decrease of ethylene in the reactor, further supporting that C and D are due to Pd+ ions coordinated with ethylene. These results suggest that Pd+ cations are involved in the dimerization reaction. With an increase in butene formation, species E (gll = 2.65, g, = 2.10) and F (gll = 2.40, g, = 2.10) are observed and assigned to Pd+ coordinated with butenes. As mentioned earlier, ip the case of a sample of low Pd content (NaPdl 6-x) no paramagnetic species were observed after O2pretreatment. However, ESR signals due to species B-F were observed during the dimerization reaction. It has been mentioned earlier that at 85 "C the dimerization reaction stopped in about 3 h due to deactivation of the catalyst. The ESR spectra recorded at this time showed that the Pd+ cations, whether coordinated or not to ethylene and butenes, all decreased substantially suggesting further reduction of Pd+ to Pdo species. Dlb. CaPd-X Zeolites. As mentioned earlier, species A (gi, = 2-23), attributed to Pd3+ ions, was generated on pretreating Pd-exchanged zeolites in O2flow, while species G (gll = 3.01, g, = 2.10), attributed to Pd+ ions, was produced on prolonged evacuation of the 02-treated zeolite with a decrease of species A. For samples exposed to ethylene, species A and G disappeared completely with the simultaneous appearance of species B (gll = 2.79, g, = 2.10). However, gas-phase analysis indicated that ethylene was not dimerized to butenes. The spectra recorded at longer reaction times of 30 min and 1 h at 25 OC showed species C (gll = 2.53, g, = 2.10) and D (gll = 2.33, g, = 2.10) with simultaneous formation of butene isomers. This occurs even at 25 OC for CaPd-X zeolites in contrast to NaPd-X zeolites. Species C and D are assigned to an ethylene complex for Pd+ which is involved in the dimerization reaction. As the dimerization reaction proceeds, additional species designated as E (gll= 2.65, g, = 2.10), F (gll = 2.40, g, = 2.10), and H (gll= 2.88, g, = 2.10) are observed which may be Pd+ complexed to butenes. Note that H was not observed in NaPd-X.
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The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
Ghosh and Kevan
COP~~~-X/CZH~/FIOW
(a) x4
t'"
Figure 12. Cation sites and their designations in X zeolite.
TABLE I V Palladium Species in Pd-Exchanged Na-X Zeolite under Figure 11. ESR spectra of CaPd9g X at 77 K: (a) after pretreatment under O2flow at 500 OC, (W)at different reaction times in the presence of ethylene in a flow reactor at 50 OC. Reaction conditions: catalyst 0.05 g, C2H4(4 wt % in He) flow rate 20 cm3/min.
0 2 . Flow Reactor. Essentially, no difference is observed in the ESR spectra recorded during ethylene dimerization using a flow reactor or a static reactor. Figure 11 shows ESR spectra of CaPd9g X zeolite recorded during ethylene dimerization at 50 "C.Species A (gb = 2.23), attributed to Pd3+cations, disappeared after flowing ethylene over the catalyst, and a new peak appeared at gll = 2.79 and gi = 2.10 (Figure l l b ) which is species B. No reaction products are detected after 5 or 10 min of reaction. However, at longer periods, when reaction products are detected, additional ESR peaks at g,, = 2.53 (species C) and gll = 2.34 (species D) with the same g, = 2.10 appear (Figure llc). This result supports the assignment that species C and D are precursors for butene formation. With an increase of reaction time, species C and D decreased substantially while ethylene dimerization also declines sharply. With the formation of butenes, additional peaks at gll = 2.65 (species E) and gl,= 2.88 (species H ) appear. All the Pd+ species decreased substantially after 2 h of reaction presumably due to further reduction to Pdo species. As mentioned earlier, the generation of species B and subsequent production of species C and D (butene precursors) was enhanced by temperature. For example, at 85 "C, species C and D were observed after less than 10 min of reaction. However, the catalyst is also deactivated much faster at higher temperature. Discussion Zeolite X is composed of alternating A102 and Si02tetrahedra with a Si/Al ratio of 1.25. These units are linked to form truncated octahedra called sodalite cages or @-cageswhich are tetrahedrally bonded together through a double six ring unit called a hexagonal prism to form larger supercages called a-cages. The free apertures to the a-cage and @-cageare 0.74 and 0.22 nm, respectively. Due to the excess negative charge present on the A102 units, compensating cations are present to balance the charge in the zeolite framework. Figure 12 shows various cation sites identified by X-crystallography.* SU and SV are near the center of the @-cageand cy-cage, respectively, SI is the center of the hexagonal prism, SI' is a site displaced into the @-cage,SI1 is the center of the hexagonal window between the @-cage and the a-cage, and SII' and SII* correspond to displacement from site SI1 into the @-cage and into the a-cage, respectively, along an axis perpendicular to the hexagonal window. Finally, SI11 is used in a broad sense to cover sites in the a-cage near a four ring. Generation and Location of Palladium Species The complex Pd(NH3)42+ions, exchanged into Na-X and Ca-X zeolites, occupy site S V in the cy-cage due to size considerations. In the course of thermal pretreatment of the zeolite under vacuum or gas flow, some rearrangement of the cations in the zeolite cages can occur. The Pd complex also decomposes ther-
Various Conditions NaPd(NH3)42+ - X . H 2 0
Slow Heating to
Pd(NH&+
P
ions in a-cage
Pd3+(A) at
500 OC. 0 2 flow
0 2 briefly
I
pumped off
Pd3+(A) at SI
Pdf-(C2H4(B) at SI' Pd+-C2H4(D) at SU or
I
Pd+-CzHq(B) at
Evac. at
'
500 OW16 h
Pd3+(A) at SI Pd+(G) at SI or
Pd+ CzH4(B) at
Pd+-C2H4(C) at SI1 or SII'
SI
S1'
SI'
c- No Butene Detected
SI
g, or to an isotropic signal with g,, = 2.23. In this laboratory, previous studies using ESR and X-ray photoelectron ~pectroscopy'~ have shown that the signal A is due to Pd3+. Prolonged evacuation of an 02-pretreated zeolite generates a second paramagnetic species G showing an ESR signal at gll = 3.01 with g, = 2.10 which is assigned to Pd+ c a t i ~ n s . ' ~Pd+ J~ (1 1) Nacchache, C.; Primet, M.; Mathieu, bf. V.Adu. Chem. Ser. 1973, 121, 266. (12) Nacchache, C.; Dutel, J. F.; Che, M. J . Catal. 1973, 29, 179. (13) Narayana, M.; Michalik, J.; Contarini, S.; Kevan, L. J. Phys. Chem. 1985,89, 3985. (14) Michalik, J.; Narayana, M.; Kevan, L. J . Phys. Chem. 1985,89,4553.
Ethylene Dimerization on Pd-Exchanged Zeolites
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4445
TABLE V Palladium Specie8 in Pd-Exchanged Ca-X Zeolite under Various Conditions
-
C ~ P ~ ( N H S- )X~, ~H20 + Slow Heating to Pd(NH3)d2+ ions in a-cage 500 oc, 0 2 flow
Pd3+(A) at SII
I
I
0 2 briefly
Evac. at
. .
500 OC/t6 h
w m m i off
1
Pd3+(A)
at SI
1
Pd3+(A) at
sn
Pd+(G) at
SIT or SI
C2H4, Flow or Static
-
Pd+-C2H4(B) at SI1 or SIP Pdt-C2H4(C) at SII or SII' Pdt-C2H4(D) at SI1 or Sll'
C2H4. Flow or Static
Pd+-C2H4(B) at SII or SIP
No Butene Detected
1
Pd+-C2H4(B) at SIIor SF Pd+-C2H4(C) at SIIor SII' Pdt-C2H4(D) at SI1 or SII' Pd+-C4H8(F) at SI1 or Sll' Pdt-C4H8(E) at SII Pd+-C4H8(H) at >SIor SI' Butene Detected
-
Pd
Catalyst Deadivated
Migration of Palladium Species during Ethylene Dimerization
complexes in various monocrystals15-" and in frozen solutions of mixed-ligand palladium complexes18 give rise to g tensors with g,, > g, where g, = 2.10 and gll max = 2.646. In Pd-Y zeolite,l'.l2 the g anisotropy of Pd+ is in the same range as in other matrices. However, in mordenite the anisotropy of the Pd+ signal (gll = 2.97, g:, = 2.146) is greater.Ig On the basis of previous ~ o r k , ' ~the , ' ~following sequence of reactions can be given for the formation of the various palladium species. Pd2+ 2Pd2+
+ O2
+0 2
350 OC
500 o c
Pd3+
Pd3+
+ 02-
+ Pd2+-0y
+ + - +
(2)
500 OCfevac
Pd2+
Pdo (cluster)
500 T f e v a c
Pdo
Pd2+
2Pd+
500 OCfcvac
Pdo
Pd3+
Pd+
Pd2+
The Pd2+-02- and 02-species are found to be stable and have been observed by ESR.'0q'4 In pretreated NaPd-X zeolite, the Pd2+ ions occupy preferentially site SI' in the &cage. Only about 4% of the total Pd2+ ions are involved in the thermal oxidation process.I4 Using combined ESR and ESEM spectroscopy, previous worklo showed that the Pd3+ ions are located at site S I in the hexagonal prism in NaPd-X, since Pd3+ exposed to D 2 0 and ND3 gives distances for Pd3+-D(D20) and Pd3+-D(ND3) which are too long for direct coordination to a DzO or ND3 molecule. This indicates the location of Pd3+ in a protected site like SI a t which molecules of the size of D 2 0 or ND3 cannot approach closely. Thus, a fraction of the Pd2+ions occupy site SI and are oxidized to Pd3+ ions and remain in site SI and/or a fraction of the PdZ+ions occupy site SI', are oxidized to Pd3+, and then migrate to SI sites. Fujiwara, S.; Nakamura, M. J . Chem. Phys. 1971, 54, 3378. Krigas, T.; Rogers, M. T. J . Chem. Phys. 1971, 54, 4769. Sastry, M. D. J . Chem. Phys. 1976, 64, 3957. Nakamura, M.; Fujiwara, S. J. Phys. Chem. 1974, 78, 2136. Vedrine, J. C.; Dutaus, M.; Naccache, C.; Imelik, B. J. Chem. SOC., Faraday Trans. I 1978, 74, 440. (15) (16) (17) (18) (19)
It was shown, by a X-ray diffraction study,20that about 85% of the Pd2+ ions in activated CaPd-Y zeolite are located at site SII'. It is reasonable to assume that Pd2+ions in CaPd-X zeolites, in contrast to NaPd-X, occupy more accessible sites like SII' in the &cage due to the high affinity of Ca2+for site SI'. In CaPd-X, Pd3+ cations arise upon oxidation of PdZ+ions at site SII' and may partially migrate to site SII. When CaPd-X is pretreated in N2, it generates no Pd3+ species and is catalytically less active for ethylene dimerization than when it is pretreated in 02. This supports the argument that the Pd3+ ions are located at more accessible sites which promote the ethylene dimerization reaction. The isotropic character of signal A requires octahedral or trigonal coordination of the Pd3+ ion. In X zeolite, besides S I site with octahedral symmetry, the only other location of Pd3+ which is in accordance with the isotropic character of signal A is site SI1 with trigonal symmetry. Pd+ ions (signal G) are produced at the expense of Pd3+(A) ions. In NaPd-X zeolite, Pd3+ ions located at site S I are more stable since the decrease of signal A together with the appearance of signal G is significantly less than in CaPd-X zeolite. So, in NaPd-X zeolite, only a part of the Pd3+and Pd2+ions are reduced to Pd+ ions. However, in CaPd-X most of the Pd3+ at site SI1 are reduced to Pd2+ and Pd+ ions. The decreased ethylene dimerization (Figure 3) observed in the initial stages on a prereduced sample is associated with a decrease of the more accessible Pd3+ ions. The location of the catalytically active species in the zeolite structure is important for the occurrence of reaction. In the present work, pretreated palladium-exchanged zeolites are shown to dimerize ethylene to butene and it has been demonstrated that palladium directly participates in the formation of the active sites. In both static and flow reactor systems, the reaction products (butenes) are detected after a reaction induction period which is dependent on the reaction temperature and on the major cocations (Na+ or Ca2+) in the zeolites. In the case of CaPd-X zeolite, palladium cations occupy accessible sites like SI1 or SII'. In contrast, palladium cations in NaPd-X zeolite occupy relatively inaccessible sites (SI' or SI). The ethylene molecule (kinetic diameter of 0.39 nm) is too bulky to enter the 0-cage or the hexagonal prism. Therefore, the formation of a palladiumethylene complex needs a migration of palladium cations to site SI1 or into the supercage in both Na-X and Ca-X zeolites. Apparently, the induction period for ethylene dimerization is due to the time needed for palladium cation migration toward the supercage. In the case of Ca-X zeolite, the migration of palladium cations, at sites SII' or SI1 in the 0-cage, to the a-cage occurs more readily than that in Na-X zeolite since palladium ions in the latter case occupy sites S I or SI' initially. Also, at higher temperature the migration of exchangeable cations in the zeolite structure is faster. This explains a shortening of the induction period at a higher temperature or in CaPd-X zeolite. In both NaPd-X and CaPd-X zeolites, signal A (Pd3+) disappears readily in the presence of ethylene with the simultaneous appearance of signal B. The very fast decay of signal A in the presence of ethylene cannot be explained in terms of direct coordination between the Pd3+ and ethylene, since ethylene is too big to enter the hexagonal prism or the 0-cage. In the presence of C1
