J. Phys. Chem. 1990, 94, 1483-1489
1483
Electron Spin Resonance and Electron Spin Echo Spectroscopic Studies of Paramagnetic Rhodium Species Produced in RhCa-X Zeolite during Ethylene Dimerization: Evidence for a a-Bonded Intermediate J. Stephen Bass and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: December 12, 1988; In Final Form: April 28, 1989) Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopieswere used to characterize paramagnetic active sites formed in RhCa-X zeolite during ethylene dimerization and the structure of a key reaction intermediate for the first time. Adsorption of ethylene at 195 K onto RhCa-X activated in flowing O2at -673 or -773 K produces two new Rh(I1) ESR signals, El and E2, which decay as a function of time. The rate of decay in samples activated at -673 K is faster compared to the decay in samples activated at -773 K, and this correlates with the percent ethylene conversion observed after 18-20 h at 296 K under static reaction conditions. Species E2 formed with deuterated ethylene could be analyzed by ESEM to give the number and distance of the nearest deuteriums. The structural result for this reaction intermediate indicates a Rh(I1) o-bonded to an ethylene carbon that has rehybridized toward sp3 in contrast to *-bonded intermediates observed for Pd(1). Species E2 is suggested to result from a reductive coordination of Rh(II1) with ethylene to produce Rh(I1). The subsequent decay results from a further reductive coordination to Rh(I), which is suggested to be the active species for the ethylene dimerization. On the basis of the available data, a suggested reaction mechanism is proposed in which the reaction proceeds by a mechanism similar to that for dimerization by a homogeneous RhC13 catalyst.
Introduction It has been established that rhodium supported on zeolites forms an active site for the selective dimerization of ethylene.’+ The valence and coordination of the active site has not been determined, however. Using infrared (IR) spectroscopy in correlation with
catalytic studies, Yashima et al. proposed highly dispersed Rh(0) atoms as the active site.2 Using X-ray photoelectron spectroscopy (XPS), Okamoto et al. conducted a more complete study of the total oxidation-state distribution and presented evidence for Rh(1) as an active site.3 This is consistent with the generally accepted mechanism proposed by Cramer for ethylene dimerization in the presence of a homogeneous RhC13 catalyst in which the Rh(II1) complex is initially reduced in the presence of ethylene to produce an active Rh(1) complex.s Immobilizing a catalytically active metal species on a support may alter its heterogeneous catalytic properties from the homogeneous conditions. Thus, it is of interest to compare heterogeneous and homogeneous reaction conditions. In addition to the disagreement regarding the active oxidation state of rhodium, the local geometry and location of these active sites within the zeolite are uncertain. A number of factors affect the location of a transition-metal ion in cation-exchanged zeolites, including the (a) major cocation: (b) Si/Al ratio,’ (c) degree of exchange,” and (d) pretreatment technique^.^ In general, these factors influence the formation and accessibility of the active sites. In previous work1&13conducted in this laboratory, paramagnetic Rh(I1) species have been found and well characterized in zeolites A, X, and Y by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. In particular, with RhCa-X zeolite, the activation temperature was found to have a pronounced effect on the location of Rh(I1) within the 1attice.I’ When the sample was heated to 623-673 K and then evacuated, uncomplexed Rh(I1) cations were shown to form and migrate to sites I1 or 11’ in the B-cage of the zeolite structure. This Rh(I1) ( I ) Yashima, T.; Ebisawa, M.; Hara, N. Chem. Lerr. 1972, 6, 473. (2) Yashima, T.; Ushida, Y.; Ebisawa, M.; Hara, N. J . Caral. 1975, 36, 320. (3) Okamoto, Y.; Ishida, N.; Imanaka, T.; Teranishi, S.J . Caral. 1979, 38, 82. (4) Takahashi, N.; Fujiwara, Y.; Mijin, A. Zeolites 1985, 5, 363. (5) Cramer, R. J . Am. Chem. 1965, 87, 4717. (6) Michalik, J.; Lee, H.; Kevan, L. J. Phys. Chem. 1985, 89, 4282. (7) Lee, H.; Kevan, L. J . Phys. Chem. 1986, 90,5781. (8) Andersson, S. L. T.; Scurrell, M. Zeolires 1933, 4, 261. (9) Ghosh, A.; Kevan, L. J . Phys. Chem. 1988, 92,4439. (IO) Goldfarb, D.; Kevan, L. J . Phys. Chem. 1986, 90, 264. ( I I ) Goldfarb, D.; Kevan, L. J . Phys. Chem. 1986, 90, 5787. (12) Goldfarb, D.; Kevan, L.J . Phys. Chem. 1987, 91, 6389. (13) Goldfarb, D.; Kevan, L. J . Am. Chem. SOC.1987, 109, 2303.
0022-3654/90/2094-1483$02.50/0
species, called species A, has a g tensor with g, = 2.532 and g,, = 1.891. When the sample was heated to 673-723 K, the overall spin concentration decreased, and this was attributed to the formation of diamagnetic Rh(I1) dimers probably within the P-cage. At an activation temperature of 723-773 K, another Rh(I1) species (species C) appeared with g, = 2.61 and gll = 2.041. This was attributed to the thermally induced dissociation of some of the dimers into Rh(I1) monomers and subsequent migration into site I in the hexagonal prism of the zeolite structure. When ethylene was adsorbed onto RhCa-XI1 and RhCa-A,I2 an ESR signal appeared and then decayed in 15-30 min at room temperature, implicating Rh(I1) in the reaction of ethylene. These signals were not observed after adsorption onto a RhNa-X catalyst, demonstrating a cocation effect on the Rh(I1) reactivity.I0 In this work, we study the relationship between the catalytic activity for ethylene dimerization and the local structure of the reactive and precursor species in rhodium-exchanged zeolites for the first time. ESEM is used to provide quantitative information about the local structure of a paramagnetic rhodium species formed in RhCa-X zeolite during the dimerization of ethylene. A rather unique o-bonded species is found for the first time. On the basis of structural information obtained concerning the initial reaction steps, a suggested reaction mechanism is proposed that is consistent with the mechanism proposed for homogeneous catalysis.
Experimental Section Linde 13X (Na-X) zeolite was repeatedly washed with a 0.1 M solution of sodium acetate to remove excess iron impurities. The sodium ions were then exchanged for calcium ions by washing the zeolite three times with a CaCl, solution at 70 O C . Rhodium-exchanged Ca-X zeolite was prepared by dropwise addition of a solution of [Rh(NH3)sCl]C12(Strem Chemical, Inc.) in triply distilled water to a slurry of 1.00 g of Ca-X in -900 mL of water. This mixture was stirred at 296 K for 24 h, filtered, washed, and dried in air. The samples studied contained 0.3, 0.9, or 4 wt % rhodium as determined by commercial atomic absorption analysis. The samples will be referred to as Rh,,Ca-X, Rh2Ca-X, and Rh,Ca-X where the subscript refers to the number of rhodium cations per unit cell. Most of the samples (60 mg) were activated by heating in flowing oxygen at a rate of 30 K/30 min to a certain temperature range (623-773 K), called the activation temperature. The samples were then evacuated to a pressure of > 10” Torr for 16 h at the activation temperature, after which time they were cooled to room temperature. In one set of experiments, the samples were 0 1990 American Chemical Society
1484 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
n
0
Bass and Kevan
Rh,Ca-X(C)/C,H,
J 'i
RhpCa-X (C)/C&14
ACTIVATED 773 K ADSORPTION 77 K [225
ACTIVATED
XI
x2.5
c
V
IO MIN
x32
\
e---" 268K6MIN 0%CONVERSION 1- 200
G
d
A r x 4
CONVERSION
''J'"6/yL 2'9
201
2.42 199
Figure 1. ESR spectra (77 K) showing g values of Rh2Ca-X (a) activated at 758 K, (b) same as a after C2H4 adsorption at 77 K and warming to 195 K for 16 min, (c) same as b after warming to 268 K for 2 min, (d) same as c after 4 min, and (e) same as c after 6 min. No dimerization products were detected by GC.
heated similarly under vacuum instead of flowing oxygen and then left at the activation temperature for 16 h before cooling. Catalysts activated in the range 623-673 K (where Rh(I1) species A is predominant) are referred to as RhCa-X(A) and those activated at 723-773 K (containing mostly Rh(I1) species C) are called RhCa-X(C). Reduction was performed by heating the activated samples in 150 Torr of H2at 473 K for 1.5 h. They were then evacuated at room temperature for 1.5 h. Adsorptions were carried out as follows. A 69" flask was evacuated and then filled with an adsorbate to a given pressure, typically at a pressure of 30-200 Torr. The activated sample was transferred in situ into a 0.2-cm4.d. by 0.3-cm-0.d. Suprasil quartz tube and cooled to 77 K in liquid nitrogen. The gas was then transferred semiquantitatively through an evacuated vacuum-line manifold onto the RhCa-X sample held at 77 K. The sample was then allowed to warm to a given temperature for a period of time, after which time the reaction was quenched by immersing in liquid nitrogen. The samples were allowed to warm to (1) 195 K in a dry iceacetone bath, (2) 268 K in a saturated NaCI-H,O bath, or (3) 296 K in air. At certain times during the reaction, the reactant/product mixture was desorbed by evacuating the sample tube through a liquid-nitrogen trap for 15 min. The trapped mixture was then injected 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 on 80/ 100 mesh graphic-GC support was used, and all runs were conducted isothermally at 308 K. C2D, (99.2 atom % D) was obtained from MSD Isotopes. 1-Butene and ethylene were obtained from Union Carbide Corp.-Linde Division. All gases were purified by repeated freeze-pump-thaw cycles. ESR spectra were recorded at 77 K on a modified Varian Model E-4 spectrometer. ESEM measurements were recorded at 4.2 K on a home-built spectrometer described previo~sly.'~ Results Adsorption of C2H4at 77 K onto RhCa-X activated in the range 623-773 K generates rather complex ESR signals that change as a function of time. The rate of change is determined both by the temperature after desorption and by the temperature at which the sample was activated prior to adsorption. Accord(14) Narayana,
18 h
P.A.; Kevan, L. Magn. Reson. Rev. 1983, I , 234
I89
Figure 2. ESR spectra (77 K) showing g values of RhzCa-X activated at 773 K followed by C2H4 adsorption at 77 K and warming to 296 K for (a) 2 min, (b) 6 min, (c) 10 min, and (d) 18 h. In d, GC analysis revealed 0.1% ethylene conversion.
ingly, the results from RhCa-X activated at -773 and -673 K will be presented separately. A. Activation at 723-773 K. Figure 1 shows the change in the ESR spectrum following ethylene adsorption at 77 K followed by warming to 195 and 268 K on Rh2Ca-X activated at 758 K. This particular activated sample contained a small amount of species A in addition to the predominantly formed species C. Note the rapid disappearance of signal A at 268 K (Figure 1C) and the slight growth of species C. This trend has been reported before," following adsorption of a wide variety of adsorbates at room temperature. Two major ESR signals with a spectral intensity more than 1 order of magnitude higher than the signal for species C appear. The first, species E l , is present mainly at low temperatures. It is characterized by a rhombic g tensor with gxx= 2.34, gVu= 2.25, and g, = 2.01. As the temperature is warmed to 268 K, species E l begins to disappear and in
