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J . Phys. Chem. 1990, 94, 4640-4644
Study of Paramagnetic Rh Species Formed in Rh-Exchanged Ca-Mordenite and Ca-L Channel Zeolites during Ethylene Dimerization by Electron Spin Resonance Spectroscopy J. Stephen Bass and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: November 6, 1989; In Final Form: December 14, 1989)
Electron spin resonance spectroscopy was used to characterize paramagnetic Rh species formed in RhCa-L and RhCa-M channel zeolites during ethylene dimerization. Activation of RhCa-M in the range 560-580 K generates species MI, assigned to Rh(I1) coordinated to four oxygens of a twisted eight-membered ring in the zeolite structure. Similar activation of RhCa-L generates very weak signals, indicating that only very small amounts of Rh(11) are formed after activation. Adsorption of ethylene onto RhCa-M at 77 K followed by warming to 195 and 296 K generates paramagnetic Rh(I1) species E2, which was observed previously in the cage zeolite RhCa-X. Two paramagnetic Rh(I1) species, El and E2, are formed in RhCa-L. I n contrast to the cage zeolite RhCa-X, where E2 has axial symmetry, species E2 in RhCa-L and RhCa-M has rhombic symmetry, resulting from the lower symmetry of the cation sites of these zeolite structures. E2 has previously been identified as a o-bonded Rh(II)-ethylene intermediate,and it is seen here to form in both cage and channel zeolites. Also, the mechanism for ethylene dimerization appears to be similar in both types of zeolites. Ethylene dimerization occurs in RhCa-X, RhCa-M, and RhCa-L zeolites only when species E2 is observed to form, indicating that it is necessary for the reaction. The cage zeolites are about an order of magnitude more reactive for ethylene dimerization than the channel zeolites. Including silica, the order of catalytic activity is Rh/Si02 > RhCa-X > RhCa-L 1 RhCa-M.
Introduction Most of the work to date on zeolite-supported rhodium catalysts has focused on the use of the cage type zeolites X, Y , and A as supports.'-7 More recently, studies of Rh supported on mordenite, which is a channel type zeolite, have appeared in the literature.s-10 Essentially these studies have concentrated on the ion-exchange properties of mordenite and the state of dispersion of Rh. Little has been reported about the catalytic properties of Rh-M in comparison to other zeolite supports.* It has been determined that Rh, incorporated as [Rh(NH3)5CI]C12,is easily ion-exchanged into the interior of the mordenite rather than only on its surface?JO Considering the large size of the main channels in mordenite, which are 0.67 X 0.70 nm and can easily admit the chloroaminorhodium complex, this result is not surprising. The locations of some extraframework cations, such as Na+ and Ca2+, before and after dehydration, are somewhat understood from X-ray diffraction Zeolite L has main channels of similar size (0.71 nm). Less work has been reported on Rh-L, but it is reported to ion-exchange Rh easily.16 There is no information available about the site locations of Rh within the frameworks of zeolite L and mordenite. Rh species are known to catalyze ethylene dimerization; thus ( I ) Takahashi, N.; Orikasa, Y; Yashima, T. J . Caral. 1979, 59, 61. (2) Yashima, T.; Ebisawa, M.; Hara, N. Chem. Lert. 1972, 6, 473. (3) Yashima, T.; Ushida, Y.; Ebisawa, M.; Hara, N. J. Caral. 1975, 36, 320. (4) Okamoto, Y.; Ishida, N.; Imanaka, T.; Teranishi, S. J . Catal. 1979, 58, 82. ( 5 ) Givens, K. E.; Dillard, J. G. J . Caral. 1984, 86, 108. (6) Davis, M. E.; Rode, E.; Taylor, D.; Hanson, B. E. J. Catal. 1984, 86, 67. (7) Davis, R. J.; Rossin, J. A,; Davis, M. E. J. Coral. 1986, 98, 477. (8) Anderson, S. L. T.; Scurrell, M. S. J . Caral. 1980, 71, 233. (9) Schoonheydt, R. A.; Van Brabant, H.; Pelgrims, J. Zeolites 1984, 4, 67. (IO) Shannon, R. D.; Vedrine, J. C.; Naccache, C.; Lefebvre, F. J. Catal. 1984, 88, 43 I . ( I I ) Meier, W. M. Z. Krisrallogr. 1961, 115, 439. (12) Mortier, W. J. J . Phys. Chem. 1977, 81, 1334. (13) Elsen, J.; King, C.S . D.; Mortier, W. J. J . Phys. Chem. 1987, 91, 5800. (14) Schelenker. J. L.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1979, 14, 751. (15) Breck, D. Zeolite Molecular Sieves; Wiley: New York, 1974; Chapter 2. (16) Sayari. A.; Morton, J. R.; Preston, K. F. J . Phys. Chem. 1989, 93, 2093.
0022-36S4/9Q/2094-4640$02.S0/0
it is of interest to study the paramagnetic Rh species formed in zeolite L and mordenite following ethylene adsorption. Both these zeolites have unit cells of lower symmetry than X zeolite. The unit cells for mordenite and zeolite L have orthorhombic and hexagonal symmetry, respectively, as compared with the cubic symmetry of the X zeolite unit cell." In mordenite the site locations capable of coordinating transition-metal ions are known to be of low symmetry and involve coordination to four lattice oxygens of a twisted eight-membered ring.I7 The coordination sites within L zeolite are similarly less symmetrical than in X zeolite. In this work the effect of zeolite structures, in which transition-metal ions are not expected to be symmetrically coordinated with the lattice,I8 on the formation of paramagnetic Rh intermediates during ethylene dimerization is studied. The electron spin resonance (ESR) spectra of intermediate paramagnetic Rh species during ethylene dimerization are investigated for the first time in zeolite L and mordenite, and these results are compared with the results previously reported with RhCa-X zeoliteIg and Rh/Si02.20 In the two latter catalysts, a a-bonded Rh(I1)ethylene complex, produced by a one-electron reduction of Rh(II1) by ethylene during an induction period, was shown to form as a necessary intermediate species before a second one-electron reduction by ethylene to a catalytically active Rh(1) complex occurs. Experimental Section Synthetic K-L (K6Na3A19Si27072-21 H 2 0 , type ELZ-L, lot no. 41 40-08B) and Na-M (Na8A@i&,6.24H20) were obtained from Union Carbide Corp. Both zeolites contained iron impurities easily detectable by an ESR signal near g = 4.3 for Fe(II1). Each was washed with a 0.1 M solution of sodium acetate at 340 K for 24 h in an attempt to remove some of this iron. This was repeated five or six times, after which the g 4.3 ESR signal intensity was reduced severalfold. The sodium or potassium ions were then exchanged by calcium ions by washing the zeolites three times with a 0.1 M CaC12solution at 340 K. Rhodium-exchanged Ca-M and Ca-L was prepared by dropwise addition of a solution
-
(17) Mortier, W. J.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1975, IO, 1037. ( I 8) Barbowski, E. D.; Mirodatos, C; Primet, M.; Mathieu, M. V. J . Phys. Chem. 1983, 87, 303. (19) Bass, J. S.;Kevan, L. J . Phys. Chem. 1990, 94, 1483. (20) Bass, J. S.; Kevan, L. J . Chem. Soc., Faraday Trans. I . , submitted.
0 1990 American Chemical Society
ESR Study of Paramagnetic Rhodium Species
The Journal of Physical Chemistry, Vol. 94, No. 1 I, 1990 4641
of [Rh(NH3)SCI]C12to a slurry of 1.00 g of the zeolite in -900 mL of triply distilled water. The mixture was stirred at 296 K for 24 h, filtered, washed, and dried in air. Both zeolite L and mordenite were prepared to contain 0.5% Rh by weight, assuming complete exchange. This is a good assumption at this low exchange level. For RhCa-L. this loading corresponds to 1 Rh per 6.7 unit cells, and for RhCa-M to 1 Rh per 6 unit cells. In one set of experiments, Rh was exchanged into Ca-M at a loading of 0.2 wt%. All catalysts were activated by heating 60 mg of the sample in flowing oxygen at a rate of 30 K/30 min to the activation temperature. The samples were activated in the range 400-770 K. The samples were then evacuated to a pressure of > Torr at the activation temperature for 16 h and then sealed and cooled to 296 K. Adsorptions were carried out as described p r e v i ~ u s l y . ' In ~~~~ all cases an evacuated flask (69 f 2 mL) was filled with an adsorbate to a pressure of 100 Torr. The activated sample was transferred in situ into a 0.2-cm i.d. by 0.3-cm 0.d. Suprasil quartz tube and immersed in liquid nitrogen. The adsorbate was then transferred semiquantitatively through an evacuated vacuum line manifold to the sample. The samples were allowed to warm to 195 K in a dry ice-acetone bath or to 296 K in air for a certain length of time, after which time the reaction was quenched by immersing in liquid nitrogen. The samples containing C2H4, adsorbed as described above, were periodically analyzed to determine the amount of ethylene conversion. This was done by withdrawing an aliquot of the reaction mixture above the catalyst into a Varian Model 3300 gas chromatograph equipped with a thermal conductivity detector. A 6-ft column with a 0.085-in. i.d. packed with 0.19 wt % picric acid supported a 80/ 100 mesh graphiffic support was used, and all runs were conducted isothermally at 308 K. All ESR spectra were recorded at 77 K on a modified Varian E-4, spectrometer. Electron spin echo (ESE) measurements at 4.5 K were attempted on the home-built spectrometer described C2D4 (99.2 at. % D) was obtained from MSD Isotopes. CzH4 was obtained from Union Carbide, Linde Division. All gases were purified before use by repeated freeze-pump-thaw cycles.
Results RhCu-M. Activation of RhCa-M produces only one paramagnetic species, in contrast to the two major Rh(I1) species A and C produced under identical conditions in RhCa-X zeolites.22 This species is called M1 and has an axial g tensor characterized by g, = 2.73 and gll = 1.95 (Figure la). This signal is strongest in samples activated at 560-580 K and is present but very weak in samples activated at -670 K. It is not present after activation above -670 K or below -500 K. Reducing the concentration of Rh in Ca-M from 0.5 to 0.2 wt % stabilizes species M1 somewhat at the higher activation temperature of -670 K. This is consistent with reports that decreasing the Rh loading increases the relative proportions of intermediate oxidation states in Y Adsorption of ethylene at 77 K onto samples activated in the range 560-600 K followed by warming to 195 K generates ESR signals that change with time upon further warming to 296 K. A rhombic signal with g, = 2.42, gz= 2.26, and g3= 2.00 appears at 195 K (Figure Ib) and does not change in intensity to an appreciable degree at 195 K. This ESR signal then decays in 10-20 min upon warming the sample to 296 K. This behavior is identical with that observed for species E2 in RhCa-X ze01ite.I~ The species in RhCa-M shows rhombic symmetry, in contrast to the axial symmetry of species E2 in RhCa-X. On the basis of the identical temperature behavior, this ESR signal in RhCa-M is assigned to species E2. No species analogous to E l , which is formed at 195 K and decays within seconds upon warming to 296 (21) Ichikawa, T.; Kevan, L.; Narayana, P. A. J . Phys. Chem. 1979,83, 3378. (22) Goldfarb, D.: Kevan. L. J . Phys. Chem. 1986, 90, 5787.
RhCa-M/C,H4
dr-
ADSORPTION 77 K
b/ 3 MIN 195 K
A
2.42
2.26\
dpT;l IO MIN 296 K
0.4 Yo CONVERSION
zoo
Figure 1. ESR spectra (77 K) showing g values of RhCa-M (a) activated at 567 K, (b) same as (a) after C2H4 adsorption at 77 K and warming to 195 K for 3 min, (c) same as (b) after warming to 296 K for 1 min, (d) same as (c) after 10 min, and (e) same as (d) after 23 h. In (e) GC analysis revealed 0.4%ethylene conversion.
K in RhCa-X,19 is observed in RhCa-M. After the adsorbed RhCa-M sample is left at 296 K, an axial ESR signal with g, = 2.16 and g,,= 2.0 appears, as shown in Figure 1. This signal increases in intensity until reaching a maximum after about 20 min and then begins to decay. The behavior of this species is similar to species B1' observed in both RhCa-X(A) and RhCa-X(C).I9 During the time signal B1' in RhCa-M decays, the signal at g = 2.00 grows in intensity at 296 K. This signal is reduced in intensity by brief evacuation at 296 K. This g = 2.00 species is assigned to a similar species B2 observed in RhCa-X(A), which has been tentatively assigned to Rh(I1) or Rh(0) weakly interacting with n - b ~ t e n e s . ' ~ Signals M I , E2, B l', and B2 also appear in samples activated at -680-700 K, but all signals are very weak. These signals do not appear in samples activated above -700 or below -500 K. Note that in this temperature range species M1 also does not appear. Note in Figure l b that after warming the sample to 195 K after ethylene adsorption at 77 K, the ESR signal at g = 2.73 (associated with species M1) disappears immediately and is replaced by a signal a t g = 2.63. Similar behavior is observed in X zeolites, in which ESR signal C shifts in g value when ethylene is added to the sample and warmed to 195 K.I9 This has also been reported after adsorption of other adsorbates and has been attributed to a slight displacement of Rh(I1) within the same framework site.22-24 The shift in the g value of signal C is not as large as the shift observed for signal M1, however. Signal M1 then decays slightly, in contrast to signal C, which grows at 296 K. Attempts were made to detect species E2 formed with C2D4 in RhCa-M by ESE. No echoes were detected in samples warmed to 195 K. In addition no echoes were seen in adsorbed samples warmed to 296 K for 10 min (containing species Bl') or for 20 h (containing species B2). The ESR spectra of these samples,are identical to those shown in Figure 1. The lack of electron spin echoes is attributed to the iron impurities present, resulting in a short phase memory time at 4.2 K. Fe(lI1) has an ESR signal at g = 4.3, and despite repeated washing with sodium acetate, (23) Goldfarb, D.; Kevan, L. J . Phys. Chem. 1986, 90, 264. (24) Goldfarb. D.; Kevan, L. J . Am. Chem. SOC.1987, 109. 2303.
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The Journal of Physical Chemistry, Vol. 94, No. 1 I , 1990
Bass and Kevan RhCa-L/C,H,
RhCa-L/C,H,
x1.25
x1.25 1.88 2.42
XI
I88 XI
XI
15 MIN 296 K XI
XI
2.00
v
Figure 2. ESR spectra (77 K) showing g values of RhCa-L (a) activated at 572 K, (b) same as (a) after C2H4adsorption at 77 K and warming to 195 K for 3 min, (c) same as (b) after warming to 296 K for 1 min, (d) same as (c) after 15 min, and (e) same as (d) after 18 h. In (e) GC
0.9 % CONVERSION
Figure 3. ESR spectra (77 K) showing gvalues of RhCa-L (a) activated at 666 K, (b) same as (a) after C2H4adsorption at 77 K and warming to 195 K for 3 min, (c) same as (b) after warming to 296 K for 1 min, (d) same as (c) after 15 min, and (e) same as (d) after 18 h. In (e) GC analysis revealed 0.9% ethylene conversion.
analysis revealed 1.2% ethylene conversion. this signal was much more intense than the g = 4.3 ESR signals remaining in X zeolite after such washing. Gas chromatographic analysis of the reactant/product mixture over RhCa-M activated at 561 K and adsorbed with ethylene at 296 K for RhCa-X > RhCa-L I RhCa-M. Considering the zeolites only, this order is not likely a result of differences in the rate of diffusion of ethylene throughout the zeolite framework since the free entrance to the a-cage in RhCa-X is comparable to that of the main channels in RhCa-M and RhCa-L. In both the channel zeolites, exchangeable cations seem to have a greater preference for sites located within the smaller channels, which are inaccessible to ethylene, resulting in a lower catalytic activity. Conclusions
Only one Rh(I1) species is produced in RhCa-M as a function of activation temperature. An activation temperature of 560-580 K generates the largest amounts of it. This species has an axial g tensor and is designated as M I , Because of the axial g, species M1 is in a symmetrical site within the zeolite framework and is assigned to be in the small channels. Activation of RhCa-L generates only very small amounts of Rh(ll), which is designated
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J . Phys. Chem. 1990, 94, 4644-4648
as LI and has a rhombic g tensor. Adsorption of ethylene onto RhCa-M and RhCa-L produces one or two ESR signals similar in behavior to those produced in RhCa-X which are assigned to Rh(1I) coordinated to ethylene. Species E l , which has a rhombic g tensor in RhCa-L and disappears at temperatures above 195 K, is produced in RhCa-X and RhCa-L but not in RhCa-M. Species E2 is produced in all three zeolites. In RhCa-X it has an axial g tensor, and in RhCa-M and RhCa-L, it has a rhombic g tensor. It can be observed at 296 K in most instances, and its rate of decay is correlated with the catalytic activity for ethylene dimerization in all three zeolites. It is suggested that the g tensor for species E2 in RhCa-M and RhCa-L has rhombic symmetry because most of the cation sites in these zeolites have lower symmetry than in RhCa-X. As ethylene dimerization proceeds another species, B2, appears in both RhCa-M and RhCa-L and is assigned to a Rh(I1)
or Rh(0) species coordinated to n-butenes. The appearance of B2 correlates with the appearance of n-butenes measured by gas chromatography. The similarity of the ESR signals produced in both channel zeolites, like M and L, and cage zeolites like X with different structures indicates that dimerization occurs by the same mechanism in all these zeolites. The formation of an intermediate u-bonded Rh(I1)-ethylene complex was directly detected in RhCa-X as species E2. The same structure for E2 is implied in RhCa-M and RhCa-L also. The formation of this o-bonded intermediate seems necessary for dimerization. Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Technology Research Program. Registry No. R h, 7440- 16-6; C2H4. 74-85- 1.
Effect of Counterion Complexation on Micellar Structure and Dynamics: A NMR Relaxation and Self-Dtffusion Study Molly Cinley, Ulf Henriksson,* and Puyong Li Department of Physical Chemistry, Royal Institute of Technology, S- I00 44 Stockholm, Sweden (Received: November 7 , 1989)
The change in the micellar structure for sodium dodecyl sulfate micelles when the sodium counterions are complexed by the cryptand C222 has been monitored by multifield 2H and I3C relaxation, pulsed field gradient self-diffusion, and I3C-shielding measurements. The NMR data show that the complexation of the sodium ions is accompanied by a slowing down of both the reorientational and translational motions of the micelles, i.e., an increased hydrodynamic radius. The 13C shieldings as well as the order parameters along the dodecyl chain indicate that there is a change in the conformational equilibria toward an increased population of trans conformers in the part of the dcdecyl chain that is closest to the sulfate group. These observations, together with the decrease in the number of SDS molecules per micelle reported from fluorescence quenching measurements (Evans, D. F.; et al. J . Phys. Chem. 1988, 92, 784), can be reconciled if the hydrophobic complexed counterions that are associated with the micelle contribute to the volume of the hydrophobic core.
well as pulsed field gradient spin-echo (FT-PGSE) measurements Introduction of self-diffusion coefficients. The properties of spherical micelles formed by ionic surfactants and the conditions for their formation are by now ~ e l l - k n o w n . ~ * ~ NMR spectroscopy has given a detailed picture of the reorientational dynamics of surfactant molecules in micellar aggreRecently, it has been reported that there is a lowering of the cmc gates.’ Multifield ZHand I3C relaxation for surfactants with and a reduction in the aggregation number of SDS micelles when different chain has shown that the overall motion of the sodium counterions are complexed by macrocyclic compounds spherical micelles can be rationalized from the assumption that like crown ethers and cryptands.)+ In micelles with this small the micelle radius is close to the length of a surfactant molecule aggregation number the motional state of the alkyl chains should with a fully extended alkyl chain and that the correlation time be different from ordinary micelles. In order to investigate this for the aggregate motion is determined by the hydrodynamic we have studied the effect of counterion complexation by the cryptand C222 (4,7,13,16,21,24-hexaoxa-l,l0-diazabicyclo- resistance to the rotational diffusion according to the StokesEinstein equation and by the lateral diffusion of individual sur[8.8.8]hexacosane) on SDS micelles using 2Hand I3C NMR as factant molecules over the curved surface of the micelle. From the I3C relaxation for the carbons along the alkyl chain it is possible to obtain information about both the correlation times and the amplitude (as reflected by the order parameters) of the internal motions at different positions in the alkyl chain. Self-diffusion constants for individual components measured with the FT-PGSE methodlo can give detailed information about solubilizate and counterior binding to ionic micelles.”-13 c222 ( I ) Lindman, B.; Wennerstrom, H . Top. Curr. Cfiem. 1980, 87. (2) Eriksson, J. C.; Ljunggren, S.; Henriksson, U. J . Cfiem.Soc., Faraday Trans. 2 1985. 81, 833. (3) Evans, D. F.;Sen, R.; Warr, G. G. J. f f i y s . Chem. 1986, 90, 5500. (4) Quintela, P. A.; Reno, R. C. S.; Kaifer, A. E. J . Pfiys. Cfiem. 1987, 91, 3582. (5) Evans, D. F.; Evans, J. B.; Sen, R.; Warr, G.G. J. fhys. Cfiem. 1988, 92, 784. ( 6 ) Payne, K. A.; Magid, L. J.; Evans, D. F. Prog. Colloid Polym. Sci. 1987, 73, IO.
(7) Chachaty, C. frog. Nucl. Magn. Reson. 1987, 19, 183. (8) Siiderman, 0.;Walderhaug, H.; Henriksson, U.; Stilbs, P. J . ffiys. Cfiem. 1985, 89, 3693. (9) Siiderman, 0.;Henriksson, U.; Olsson, U.J . ffiys. Cfiem. 1987, 91. 116.
( I O ) Stilbs, P. frog. Nucl. Magn. Reson. 1987, 19, I . ( 1 I ) Stilbs, P. J. Colloid Interface Sci. 1982, 87, 385. (12) Stilbs, P. J. Colloid Interface Sci. 1983, 94, 463. (13) Lindman, B.;Puyal, M.-C.; Kamenka, N.; Rymdtn, R.; Stilbs, P. J . Phys. Cfiem. 1984, 88, 5048.
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