J. Phys. Chem. 1987, 91, 6389-6395
6389
Study of Rh(I1) Species in Ca-A Zeolite by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopies Daniella Goldfarb, Larry Kevan,* Department of Chemistry, University of Houston, Houston, Texas 77004
Mark E. Davis, Carlos Saldarriaga, and Joseph A. Rossin Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (Received: December 19, 1986)
Paramagnetic Rh(I1) species generated in RhCa-A after activation and adsorption of various molecules were characterized was incorporated by electron spin resonance (ESR)and electron spin echo modulation (ESEM) spectroscopies.. [Rh(NH3)SCl]C12 into the zeolite during synthesis. Activation of RhCa-A and RhNa-A at 400-500 OC either in flowing oxygen or under vacuum did not generate any substantial amount of Rh(I1). However, exposure of activated RhCa-A to molecules such as 02, CO, H20,NH3, CH30H, H2,and C2H4 at room temperature resulted in a considerable increase in the spin concentration which was assigned mainly to Rh(I1) species. No such changes were observed in RhNa-A under similar conditions. The formation of the various paramagnetic Rh(I1) species, their location within the zeolite structure, and their interaction with adsorbates in terms of the distance and number of interacting molecules as obtained from the ESEM data are discussed. The results are further compared with results previously obtained in Rh-exchanged X and Y zeolites.
Introduction Rhodium-exchanged zeolites have shown catalytic activity for a number of processes such as olefin hydroformylation,' methanol c a r b o n y l a t i ~ n ,olefin ~ . ~ dimerization," olefin hydrogenation? and thiophene hydrodesulfurization.6 The oxidation state, location, and interaction of the Rh cations with adsorbates within the zeolite structure are important for the understanding of the role of the Rh and the zeolite support in the catalytic process. In previous work7-Iowe studied the formation of paramagnetic Rh species in Rh-exchanged Na-X, Na-Y, Ca-X, and Ca-Y zeolites after activation, oxidation, reduction, and adsorption of various molecules. We found that the formation of some Rh species and their location are greatly affected by the cocation and the Si/A1 ratio, especially when the %/A1 ratio is low.'o The elucidation of the effect of zeolite structure on the location and generation of paramagnetic Rh species is studied here by comparing results in A zeolite with previous results in X and Y zeolite. The a-cage is smaller in A zeolite, and the hexagonal prisms of X and Y zeolite are replaced by double 4-rings in A zeolite which are too small to host any cation. The absence of the hexagonal prism containing cation site I in A zeolite is expected to be significant in view of the RhNa-X7 results which showed that Rh(I1) has a high tendency for occupying site I and the RhCa-X data which showed that the affinity of Ca2+ for site I considerably reduces the amount of Rh(I1) after activation. Accordingly, we might expect Rh in A zeolite to behave more like RhCa-X than like RhNa-X. Unfortunately, ion exchange of rhodium( 111) complexes into zeolite A locates the rhodium on the surface of the zeolite crystallites since the rhodium aquo or amino complexes are too large to penetrate the 0.5-nm pores in Ca-A." This is also illustrated ~~~
~~~~~~~~~
(1) Rode, E. J.; Davis, M. E.; Hanson, B. E. J . Catal. 1985, 96, 563. (2) Lars, S.; Anderson, T.; Scurrell, M. S. Zeolites 1983, 4, 261. (3) Scurrell, M. S.; Howe, R. F. J. Mol. Catal. 1980, 7, 535. (4) Takahashi, N.; Fujiwara, Y.; Mijin, A. Zeolites 1985, 5, 363. ( 5 ) Okamoto, Y.; Ishida, N.; Imanaka, T.; Teranishi, S. J . Catal. 1979, 58, 82. (6) Givens, K. E.; Dillard, J. G. J . Catal. 1984, 86, 108. (7) Goldfarb, D.; Kevan, L. J . Phys. Chem. 1986, 90, 264. (8) Goldfarb, D.; Kevan, L. J. Phys. Chem. 1986, 90, 2137. (9) Goldfarb, D.; Kevan, L. J . Phys. Chem. 1986, 90, 5787. (10) Goldfarb, D.; Kevan, L. J . Am. Chem. SOC.1987, 109, 2303. (1 1) Shannon, R. D.; Verdine, J. C.; Naccache, C.; Lefebvre, F. J . Catal. 1984, 88, 43 1.
0022-3654/87/2091-6389$01.50/0
in the work of Kuznicki and Eyring,I2 who found that intracrystalline Rh in Rh-exchanged Y zeolite is reduced to Rho while in Rh-exchanged A zeolite similar reduction did not occur for the same conditions. This was presumably related to lack of catalytic activity of Rh-A zeolite. Davis et al.I33l4showed that it is possible to obtain Rh(II1) incorporated into the A zeolite if the rhodium cations are introduced during synthesis. Such an A zeolite shows high selectivity for 1-hexane hydroformylation due to its small pore size.I5 In this work we report the generation and characterization of paramagnetic Rh species in Rh-containing A zeolite in which the rhodium is introduced during synthesis and is located inside the zeolite pores. These results are compared with the results obtained in X and Y zeolites7-I0 to demonstrate the effect of the zeolite structure and pore size on the location of the Rh cations and their interaction with adsorbates.
Experimental Section Four rhodium-containing zeolites were used. (a) RhCa-A( 1): Na-A was cation-exchanged with RhC13, and a portion of this material was placed in a zeolite A synthesis gel for seeding purpose^.'^ The RhNa-A product was then exchanged a t room temperature with CaC12. The rhodium loading as determined by chemical analysis was 0.96 wt %. (b) RhNa-A: In this case the Rh was introduced into zeolite A by using the pentaamino chloride ~ o m p l e x . ' ~A precursor solution with composition 4. 1Na20Al2O3.3.1Si02~100H20~0.07 [Rh(NH3)sCl]C12was mixed after heating to 50 OC with a second synthesis solution of composition 1.7Na20~A1203~0.2Si02~172H20. The Rh content of the product was 0.37 wt 7% as obtained by chemical analysis. This is about 1 Rh cation per 13 unit cells. (c) RhCa-A (2): This was obtained from the synthesized RhNa-A by cation exchange with CaCl,. The amount of Rh is the same as in RhNa-A. In these three zeolites X-ray photoelectron spectral data indicate that most of the rhodium-amine complexes are intra~eolitic.'~,'~ (d) RhCaA(3): This was obtained by exchanging Ca-A with a solution of -4 mM [Rh(NH3)sCl]C12 where the Ca-A was obtained from Na-A exchanged with CaC12. After the Rh exchange the zeolite (12) Kuznicki, S. M.; Eyring, E. M. J . Catal. 1980, 65, 227. (13) Davis, M. E.; Saldarriaga, C.; Rossin, J. A. J . Catal., in press. (14) Rossin, J. A.; Davis, M. A. J . Chem. SOC.,Chem. Commun. 1986, 234. (15) Davis, R. J.; Rossin, J. A,; Davis, M. E. J . Catal. 1986, 98, 477.
0 1987 American Chemical Society
6390 The Journal of Physical Chemistry, Vol. 91, No. 25, 1987 was washed several times with water. The Rh content as determined by atomic absorption analysis was only 0.1 wt %, which is only 10% of the total Rh which was present in the exchange solution. For comparison, exchanging Ca-X or Ca-Y9J0 with a solution of 4 mM [Rh(NH3)5CI]C12under the same conditions yields 1 wt % RhCa-X and RhCa-Y which indicates complete Rh exchange into these larger pore zeolites. Samples (- 50 mg) were heated under flowing oxygen up to 400 and 500 "C at a rate of 30 OC/30 min, evacuated overnight at the high temperature to a residual pressure of Torr, and then cooled to room temperature. Some samples were activated by heating under vacuum to 400 OC. All adsorptions were carried out at room temperature, and the samples were sealed at 77 K in 3-mm-0.d. by 2-mm4.d. Suprasil quartz tubes. Electron spin resonance (ESR) spectra were recorded at 77 K and at room temperature with a modified Varian E-4 spectrometer. Electron spin echo modulation (ESEM) spectra were recorded at 4.2 K with a home-built spectrometer described elsewhere.16 The spectrometer was interfaced with a Nicolet 1280 computer controlling a 293B Nicolet pulse ~ r 0 g r a m m e r . I ~The pulse sequence for a field-swept electron spin echo experiment is (271 3)-~-(2.rr/3),-recho, where 7 is held constant while the magnetic field is scanned. ESEM spectra were obtained with the sequence a/2-~-~/2-T-~/2-r-echo. The phases of the pulses were cycled as follows, (x, x, x) + (-x, -x, x) - (-x, -x, -x) - (x, x, -x), to eliminate interferences from two-pulse echo^'**'^ and base line drift. The modulation is observed by recording the stimulated echo amplitude as a function of T while T is maintained between 0.27 and 0.30 ks to suppress 27Al modulation.20 Calculated ESEM traces were obtained by using the spherical mode120 and neglecting the deuterium quadrupole interaction tensor.
Results Dehydration and Adsorption of CO, O,,and Hz.In fresh samples the Rh is present as [Rh(NH3)5Cl]Z+or as [Rh(NH3)50H]Z+"~21 due to hydrolysis. In these complexes the Rh is diamagnetic, low-spin d6. Upon activation the complex decomposes and Rh(II), which can be further reduced to Rh(1) at higher temperature, is formed.22 Activation in flowing oxygen should increase the relative amounts of Rh(II1) and Rh(II)7J023*24 while activation under vacuum should yield more Rh(1) and maybe ~h(01.5~23 In all the RhCa-A and RhNa-A samples prepared, the spin concentration after activation, either under vacuum or in flowing oxygen, at 400-500 OC was negligible. However, exposure of a RhCa-A(2) sample, synthesized with [Rh(NH3)5Cl]C1zin the reaction mixture, to various adsorbates produced a considerable increase in the amount of the paramagnetic species. The corresponding RhNa-A and the RhCa-A prepared with RhC13did not show any significant ESR signals after similar treatment or after oxidation or reduction at 200-500 OC. Henceforth, we shall refer to the RhCa-A(2) samples as RhCa-A. First we describe the results obtained after adsorption of soft ligands, such as CO, olefins, and Oz, which are expected not to replace the lattice oxygens in the first coordination sphere of the (16) Ichikawa, T.; Kevan, L.; Narayana, P. A. J . Phys. Chem. 1979,83,
_1378 _ _.
(17) Isoya, J.; Bowman, M. K.; Norris, J. R.; Weil, J . A. J . Chem. Phys.
1983, 78, 1785.
(18) Bowman, M. K. A footnote in Mins, W. B. J . M a p . Reson. 1984, 59, 293.
(19) Fauth, J. M.; Schweiger, A.; Braunschweiler, L.; Farrer, J.; Ernst, R. R. J . Magn. Reson. 1986, 66, 74. (20) Kevan. L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R., Eds.; Wiley-Interscience: New York, 1979; Chapter 8. (21) Schoonheydt, R. A,; Van Brabant, H.; Pelgrim, J. Zeolites 1984, 4 , 67. (22) Naccache, C.; Ben-Taarit, Y.;Boudart, M. In Molecular Sieues II; Katzer, Z. R., Ed.;American Chemical Society: Washington, DC, 1977; ACS Symp. Ser. No.40, p 15. (23) Anderson, S.; Scurrel, M. S. J . Catal. 1981, 71, 233. (24) Van Bradant, H.; Schoonheydt, R. A,; Pilgrims, J. In Meral Micro Structures in Zeolites; Jacobs, P. A,, et al., Eds.; Elsevier: Amsterdam, 1982; Vol. 12, p 61
Goldfarb et al. RhCa-A/CO
2,165-1 Q.
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2'$8 A
"
1998
r
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Figure 1. ESR specta (a) of activated RhCa-A followed by 30-min exposure to 57 Torr of CO at room temperature recorded at 77 K, (b) of activated RhCa-A after exposure to 57 Torr of CO recorded at room temperature, and (c) of activated RhCa-A followed by exposure to 57 Torr of CO and evacuation at room temperature recorded at 77 K. The gains are 1.4 X lo2, 6.3 X lo2, and 5.0 X lo2, respectively. 2.018-1
RhCa-A/02
Y8 1.974
v7
1.949
r
2.006
Figure 2. ESR spectra at 77 K of (a) RhCa-A activated in flowing 0, followed by IO-min exposure to 14 Torr of 0,; (b) same as (a) with subsequent evacuation at room temperature. (c) RhCa-A activated under vacuum followed by 10-min exposure to 14 Torr of 0,; (d) same as (c) with subsequent room-temperature evacuation. The gains are 1.4 X lo2, 6.3 X lo2, 9.0 X lo', and 2.0 X lo2, respectively.
Rh cation,25though they could coordinate to the cation if it is accessible and cause some location displacement. The interaction with these adsorbates should provide information about the Rh cation distribution following activation. Introduction of C O (57 Torr) generated two major species as shown in the ESR spectra in Figure 1. One is a Rh(I1)-CO adduct with g, = 2.165 and gll = 1.994. The gll was assigned by comparison with a reported Rh(I1)-CO adduct in RhNa-Yz2 with a similar g, and by comparing samples with different relative amounts of both species. The spectrum of the Rh(II)-CO adduct was not observable at room temperature (Figure lb), and brief evacuation at room temperature caused decomposition of the adduct (Figure 2c). The second species, termed C1, with g, = 2.268 and gll g2.0 was not affected by outgassing, and its ESR spectrum was observable at room temperature. The adduct and species C1 were generated in samples activated under vacuum as well. Immediately after O2(1-100 Torr) was introduced into samples activated either under vacuum or in flowing oxygen, a strong ESR signal, shown in Figure 2, appeared showing the presence of two major species. Outgassing at room temperature caused their decomposition, and hence their assignment, to Rh-oxygen adducts. (25) Mortier, W. J.; Schoonheydt, R. A. Prog. Solid Stare Chem. 1985, 16, 1-125.
The Journal of Physical Chemistry, Vol. 91, No. 25, 1987
Rh(I1) Species in Ca-A Zeolite
A
6391
RhCa-A/Hp I t
jll ll ll JI II 2 3 4 5 I
OO
T (ps)
b
Figure 3. ESR spectra at 77 K of (a) RhCa-A activated in flowing oxygen followed by exposure to 200 Torr of D2for 1 h. (b) RhCa-A activated under vacuum followed by exposure to 230 Torr of H2for 10 min at room temperature. (c) Same as (b) but for 3-h H2exposure at room temperature. (d) Same as (c) with subsequent evacuation at room temperature. (e) Calculated trace. The gains are 1.25 X lo3, 4.0 X lo’, 3.2 X lo3,and 3.2 X lo’, respectively.
Samples activated in O2generated adduct 1 with g, = 1.949, gll = 2.018 and adduct (2) with gll = 2.018, g, = 1.964. Adduct 1 has the same g values as the samples activated under vacuum, but the second adduct has gvalues somewhat shifted, g, = 2.018 and gll 1.974. A weak signal at g = 2.120 corresponding to the gIlof a minor species was also observed. Upon evacuation at room temperature this minor species became the only species present, termed 0 1 , as shown in Figure 2, traces b and d, with g,, = 2.120, g, = 2.000, and gVu= 1.954. Most intriguing is the appearance of ESR signals after exposure to 200 Torr of H2or D2at room temperature. In samples activated in oxygen only one major species, appearing already after 10 min, termed Ha, with g, = 2.505 and gll= 1.983, was generated (Figure 3). Evacuation at room temperature caused the disappearance of the signal which was restored after reintroduction of hydrogen. Exposure of a sample activated under vacuum to H2 generated several species with g, = 2.680, 2.555, and 2.46 and gll 1.95-1.98 as shown in Figure 3b,c. After 3-h exposure, the signals at 2.679 and 2.555 along with the signal at 1.954 increased. Upon evacuation at room temperature the shoulder at g = 2.46 along with the signal at g = 1.974 disappeared (Figure 3d), suggesting that these lines correspond to the Ha species observed in the sample activated in flowing oxygen. We assign the signals at g = 2.68 and 2.553 to one species, Hb,with a nonaxial g tensor; g,, = 1.95, ,g = 2.555, and gV = 2.680. The bottom trace (Figure 3e) shows a calculated powder pattern obtained by using the above tensor and a 35-G Lorentzian line width. The narrow line at g = 2 appeared in most samples. We do not know its origin; its relative intensity is very weak, and we choose to ignore it. The spectrum of Hbat room temperature was very weak and did not show any resolved lines at g = 2.68 or 2.55. Subsequent exposure to 28 Torr of O2 generated the oxygen adduct mentioned above. The Hb signal was somewhat reduced but not significantly broadened. The appearance of the Rh-oxygen adducts along with the Hb species suggests their different origins. Species Ha, unlike species Hb, is stabilized by the presence of hydrogen. To get some information about the interaction between the paramagnetic Rh center and the hydrogen molecules, ESEM measurements were performed. The ESEM pattern obtained from species Ha in a sample activated in oxygen after introduction of 200 Torr of D2is shown in Figure 4. The experiment was performed at the g, position. We had difficulties in simulating the modulation pattern using only one shell of interacting deuteriums. Using a two-shell model with one deuterium at a distance of 0.28 nm and a isotropic hyperfine coupling of 0.4 M H z and
Figure 4. (a) ESEM of RhCa-A activated in flowing oxygen followed by exposure to 200 Torr of D2 at H = 2703 G and T = 0.29 ,us. The calculated trace was obtained by using two shells: one with N = 1, R = 0.28 nm, and A = 0.4 MHz and the second with N = 6, R = 0.47 nm, and A = 0.0 MHz. (b) The corresponding Fourier transform.
4
RhCa-A/ethylene 2.2L4
a. 4min.
2.180
\4 /EYE//
/fJjr? 2.427
b. IO min.
1
’/
Ir
c. 24min.
+ 200 G
-
Figure 5. ESR spectra at 77 K of activated RhCa-A followed by adsorption of ethylene for (a) 4 min, (b) 10 min, and (c) 24 min at room temperature.
six other deuteriums at a distance of 0.47 nm, we obtained a reasonable fit (dashed trace Figure 4a). This is not a unique fit, however. Fourier transformation was done to gain additional information. Prior to Fourier transformation the missing data up to T 7 = 0 were reconstructed by using the linear prediction method.26 The frequency domain spectrum obtained is shown in Figure 4b. Unfortunately, it consists of a single peak at 1.72 MHz which is the Larmor frequency of deuterium at the observing magnetic field of 2700 G. Diffuse reflectance infrared spectra taken from samples activated in flowing oxygen after introduction of H2 or D2 did not show any vibrations which could be attributed to rhodium hydrides. Interaction w i t h Ethylene. Adsorption of ethylene resulted in an intense ESR spectrum, appearing as soon as 2 min after exposure to 94 Torr of ethylene. Figure 5 shows the ESR spectra recorded a t 77 K as a function of exposure time. The lines at g = 2.254, 2.180, and 2.097 decreased considerably as a function of time whereas the signal at g = 2 increased. We assign the signals decreasing with time to a Rh cation interacting with
+
( 2 6 ) Barkhuijsen, H.; de Beer, R.; Bovee, W.M.M.J.;van Ormondt,D. J . Magn. Reson. 1985, 61, 465.
Goldfarb et al.
6392 The Journal of Physical Chemistry, Vol. 91, No. 25, 1987 RhCa-A/D20
RhCa-A /CzD4
t
-EXP
- EXP --- CAL
H=3008G
A.035 MHz
H.2604 G
R = 0 28 nm A=025MHz R = 0 29 nm
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/
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T(ps)
ESEM of activated RhCa-A followed by adsorption of ethylene for 2 min at room temperature recorded at H = 3008 G and T = 0.28 ps. The parameters used for the simulations are indicated in the figure. Figure 6.
Figure 8. ESEM of activated RhCa-A followed by adsorption of D20 recorded at H = 2.604 G with T = 0.30 ps. The parameters used for the
8\!a
simulations are shown in the figure. RhCa-A/CD30H
IOm
RhCa-A
RhCa-A/0z+CD30H
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2
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Figure 9. ESEM of activated RhCa-A (a) with adsorption of CD,OH recorded at H = 2890 G and T = 0.29 ps. (b) Same as (a) recorded at
H = 2484 G and T = 0.32 ps. (c) With adsorption of methanol in the presence of oxygen recorded at H = 2855 and T = 0.29 ps. (d) Same as (c) but recorded at H = 2653 G and T = 0.30 ps. The parameters used for the simulations are indicated in the figure; the numbers in parentheses indicate another set of parameters which gave a good fit. Figure 7. ESR spectra at 77 K of RhCa-A activated in flowing oxygen
(a) followed by water adsorption, (b) followed by methanol adsorption, and (c) followed by oxygen and methanol adsorption. (d) Calculated ESR spectrum with g,, = 2.407, g, = 2.250, and a line width of 30 G. The relative gains are 1.25 X lo3, 2.0 X lo3,and 1.6 X lo3, respectively. The arrows not indicating g values point at the position at which ESEM experiments were performed. ethylene, termed species El, with a nonaxial g tensor with g,, = 2.254, g,, = 2.097, and gyy = 2.180. This assignment may be ambiguous, since the change in the relative intensity of the peaks as a function of time may indicate the presence of a minor species in addition to species E l . The signals at g -2, particularly in the 10- and 24-min traces, resemble the spectrum of 02-in zeolites?' Intensity considerations suggest that the E l species does not convert into the species at g z2.0. ESEM measurements (shown in Figure 6) were carried out at two different field positions indicated by arrows in Figure 5a. ESEM obtained at positions g = 2.18 and 2.097 had the same pattern, which supports their assignment to a single species. Good agreement between the calculated and experimental traces could be obtained with the parameters A = 0.28 MHz, R = 0.31 nm, and N = 4 which correspond to one ethylene molecule. Adsorption of acetylene on RhCa-A generated rather weak signals near g =2. Adsorption of Water and Methanol. Hydration of samples activated in flowing oxygen at 400 OC generated paramagnetic species for which the ESR spectrum is shown in Figure 7a. The asymmetric line shape of the g, feature is indicative of the presence of two species with g, 2.66 and -2.56 rather than of a single species with a nonaxial g tensor. The gl,features appear
-
(27) Che, M.;Tench, A. J. Ado. Catal. 1983, 32, I.
-
at g 2 and are superimposed on a narrow line which was discussed previously. An ESEM spectrum recorded at H = 2604 G (gl) is shown in Figure 8. Its deep modulation indicates a short interaction distance, and the best-fit parameters obtained are A = 0.35 MHz, R = 0.28 nm, and N = 3, though a reasonable fit could also be obtained with A = 0.25 MHz, R = 0.29 nm, and N = 4. Both fits indicate direct coordination to D20. Adsorption of D 2 0 in the presence of O2 (-20 Torr) generated additional species in the g = 2 region with the major species (W3) having the following gvalues: g, = 2.012, gyy= 1.978, and g,, = 2.125. These are similar to the species generated after adsorption of methanol in the presence of O2 (see Figure 7c). ESEM performed at the gvv position showed shallow modulation which could be well-simulated with A = 0.0 MHz, R = 0.36 nm, and N = 2, indicating nondirect coordination. Similar ESR signals were obtained under the same conditions in RhCa-X,g RhCa-Y,Io and RhNa-Y,10122but there the modulation was deeper and the interaction distance shorter (0.29-0.32 nm). Adsorption of methanol yielded a multitude of paramagnetic species as seen in Figure 7b. The signals in the g = 2 region are similar to those obtained when methanol is adsorbed in the presence of oxygen as shown in Figure 7c, although their relative intensities are weaker. This signal appeared in all methanol samples studied and did not result from an accidental exposure to 02.The relative intensities of the lines at g = 2.407 and 2.235 in several samples studied recorded both at 77 K and at room temperature suggest that they correspond to one species with gll 2.407 and g, = 2.235. For comparison note the calculated powder pattern with these g values and a line width of 30 G. We term this species M1. The lines appearing at g = 2.68 and 2.57 are similar to those observed for species Hb. ESEM experiments were performed on two samples, one on which only CD30H was ad-
The Journal of Physical Chemistry, Vola91, No. 25, 1987
Rh(I1) Species in Ca-A Zeolite RhCa-A/NH3
6393
RhCa-A/ND3
A.O.05 MHz
0
Figure 10. ESR spectra of RhCa-A activated under vacuum followed by adsorption of NH, (a) (final pressure 44 Torr). (b) Same as (a) with subsequent evacuation at room temperature for 1 h. The gains are 3.6 X lo2 and 2.0 X lo3, respectively.
1
2
3 T(p)
4
5
Figure 11. ESR spectrum at 77 K and ESEM recorded at g = 2.007 of RhCa-A activated in flowing oxygen followed by ND, adsorption. The parameters used for the simulation are indicated in the figure.
the separate N 1 and N 2 contributions to the echo. Accordingly, we can only state that the relatively deep modulation suggests a rather close proximity of the ammonia deuteriums. sorbed (Figure 7b) and one on which C D 3 0 H was adsorbed in Exchanged RhCa-A. Samples in which Rh was introduced the presence of oxygen (Figure 7d). Besides the different intensity by exchange in contrast to synthesis were activated in flowing of the lines in the g 2 region which could be attributed to 0 ~ : ~oxygen at 400 OC. Following activation no ESR signal appeared. the ESR spectra are similar. The ESEM spectra corresponding Adsorption of CO, 02,and ammonia yielded signals identical with to species M1 are shown in Figure 9a,c. At this field position those observed in the synthesized RhCa-A sample described the major contribution to the echo is from species M1. The higher previously. However, the signals were about 2 orders of magnitude field lines of other species do not overlap with the M1 lines, but weaker. The relative amount of Rh in the samples is 0.1 and 0.37 the lower field lines of other species may contribute to the echo wt. %, respectively, which does not account for this large difference to a certain extent. The trace in Figure 9a could be reasonably in intensity. We attribute this large difference in intensity to the simulated with A = 0.05 MHz, R = 0.39 nm, and N = 6 which fact that in the exchanged RhCa-A the Rh is mainly on the corresponds to two C D 3 0 H molecules. The trace in Figure 9c surface," whereas in the synthesized samples the rhodium is inside could be simulated either with A = 0.05 MHz, R = 0.42 nm, and the zeolite cavities. The small amount of paramagnetic species N = 9 or with A = 0.05 MHz, R = 0.39 nm, and N = 6, which which are formed in the exchanged sample is probably due to is in good agreement with the previous one. ESEM data obtained migration of some Rh cations into the zeolite cavities during from similar samples after adsorption of C H 3 0 D gave the best-fit activation after decomposition of the pentaamine complex. parameters A = 0.0 MHz, R = 0.40 nm, and N = 2. Assuming Discussion that the methanol molecule remained intact, this suggests that the methanol molecule is not oriented with the oxygen pointing The absence of significant ESR signals in RhCa-A and toward the Rh and that Rh is not directly coordinated to methanol. RhNa-A after activation indicates that the Rh species present Figure 9b shows the modulation obtained with the field position are diamagnetic. Such species could be Rh(III), Rh(I), or Rh(I1) at g = 2.65. The best-fit parameters are A = 0.05 MHz, R = dimers. We rule out the possibility of the presence of nonob0.41 nm, and N = 6. ESEM obtained at g = 2.47 (Figure 9d) servable Rh(I1) at 77 K due to short relaxation times on the basis could be simulated with two sets of parameters, A = 0.05 MHz, that in RhNa-X7 and RhNa-Y22 signals attributed to Rh(I1) R = 0.41 nm, and N = 6 or A = 0.0 MHz, R = 0.44 nm, and could always be observed at 77 K. N = 9. A similar sample with adsorbed C H 3 0 D gave A = 0.0 In RhNa-X7 and RhNa-Y'o*22activated in flowing oxygen at MHz, R = 0.42 nm, and N = 3, again indicating nondirect 500 OC Rh(I1) was found to be stabilized in the hexagonal prism coordination assuming that the methanol molecule remained intact. position of the zeolite structure. Thus, it is not surprising that Adsorption of Ammonia. Adsorption of ammonia was perno such species was found in zeolite A which does not possess an formed on two types of samples, one activated in flowing oxygen analogous cation site. However, it is not clear why a species at 400 O C and the other activated at 400 "C under vacuum. The analogous to species Rh2+(A), found in RhNa-X7 and in ESR spectrum shown in Figure 10 was obtained after adsorption RhCa-X'O after activation in the range of 320-400 OC located of 40 Torr of ND3 on a sample activated under vacuum. It shows near a 6-ring site, was not generated in the corresponding A zeolites since A zeolite does possess this site. two main groups of signals, one in the g = 2 region which corresponds to species N 1 (gxx= 2.01 1, gYY= 1.993, and g,, = 2.104) A common feature to the RhCa-A samples was that exposure of the activated zeolite to various molecules such as 02,H2, CO, and N 2 (gxx= 2.003, gyy = 2.008, and g,, = 2.085), similar to those observed in RhCa-X,9 and the other at g = 2.324. In the H 2 0 , NH3, C H 3 0 H , and C2D4, which have quite different latter, the g,,and g, features are unresolved but the asymmetrical chemical natures, generated a significant amount of paramagnetic species which are summarized in Table I. line shape indicates g anisotropy. We term this species N3. The ESR spectra of species N 2 and N 3 are observable at room temWith the exception of a few signals which could be assigned perature. Evacuation at room temperature for 1 h reduced we attribute most of the ESR signals observed to Rh(I1) to 02-, considerably the intensity of species N3; N 2 was also affected but species due to the large anisotropy and the large deviation from g: and on the basis of comparison with previous r e s ~ l t s .The ~ to a lesser extent. Samples activated in oxygen showed similar spectra, but the relative amount of species N 3 was much smaller. Rh(I1) species which are generated after interaction with various adsorbates could be formed either by some oxidation or reduction ESEM obtained from such a sample is shown in Figure 11 and process involving Rh(1) or Rh(II1) and the adsorbate or by diswas best simulated with A = 0.05 MHz, R = 0.34 nm, and N = 9. At the field position measured for the maximum echo sociation of Rh(1I) dimers induced by coordination of the adintensity ( g = 2.007), both N 1 and N 2 should contribute to the sorbate to Rh(I1). Since ESR signals with comparable intensities echo; unfortunately, the echo intensity was rather poor and we were generated upon exposure to molecules with diverse chemical could not obtain good quality field-swept ESE in order to discern properties, the latter possibility seems more plausible. Unfortu-
-
-
6394 The Journal of Physical Chemistry, Vol. 91, No. 25, 1987
Goldfarb et al.
TABLE I: Summary of Identified Paramagnetic Rh Species Observed in RhCa-A Zeolite
ESEM~
species Rh(IIbC0 , ,
adsorbate
a. b a, b a, b
co co
a b
0 2 0 2
01
a, b
02,vac
Ha
a, b
H2
Hb
b
c1
Rh(II)-02 Rh(II)-02 Rh(II)-O,
W1 w2 MI M2 El N1 N2 N3 w3 a
activation
0 2
a
a a a a
a, b a, b
b a
gxx
gYY
2.165 2.268 1.949 2.018 2.018 2.00 2.505
2.165 2.268 1.949 2.018 2.018 1.954 2.505
2.680 2.560 2.660 2.235 2.647 2.097 2.01 1 2.003 2.324 2.012
2.555 2.560 2.660 2.235 2.647 2.180 1.993 2.008 1.978
gzz
N
R,nm
A , MHz
1 6
0.28 0.47
0.4' 0.0
6 6 4
0.39 0.41 0.31
0.05 0.05 0.28
2
0.36
0.0
1.994 -2 2.018 1.964 1.974 2.120 1.983 1.954 -2 -2 2.407 2.254 2.104 2.085 2.115
Activation in flowing 02.Activation under vacuum. CTwo-shellmodel. dThe estimated error in R is *0.01 nm and in A , *0.05 MHz.
nately, X-ray photoelectron spectroscopic measurements which could give some information about the oxidation-state distribution after activation did not give sufficient signal-to-noise due to the relatively low Rh ~ 0 n t e n t . l ~ We calculated the number of spins in RhCa-A samples after hydration and adsorption of ethylene, oxygen, and ammonia (using double integration) and found that the Rh(I1) species comprised 20-30% of the total Rh, which is a significant amount. In the following discussion we shall attempt to account for the formation of the various paramagnetic species as well as for their location within the zeolite structure. The two species generated after C O adsorption are rather different. In one of them the C O is weakly bound, its ESR spectrum cannot be observed at room temperature, and its signal disappears upon evacuation. The formation of this species may be attributed to dissociation of Rh(I1)-Rh(I1) species upon interaction with C O as given by reaction 1 where 0, represents a 2CO
+ (0,)3Rh11- Rh11(0,)3 F? 2C0.Rh11-(0,)3
(1)
zeolite lattice oxygen. If the reaction is sufficiently reversible at room temperature, it could cause extreme broadening of the ESR lines so that no spectrum would be observed, whereas at 77 K a slower reverse rate could allow ESR observation of the Rh(I1) species. In the second species, C1, the C O is more strongly bound, evacuation does not affect the signal intensity, and it can be observed both at room temperature and at 77 K. Species C1 could be generated from more accessible Rh(I1) dimers which can produce Rh(I1)-(CO), complexes. Another possibility is that C1 is formed through reduction of rhodium(II1) oxides, which are formed during the activation in flowing oxygenz4according to the reaction
+
+
2Rh111 02- (2n
+ l)CO
-
2Rh" - (CO),
+ C02
(2)
The first suggestion seems more reasonable since these two species are also formed in samples activated under vacuum where no rhodium(II1) oxides should be p r e ~ e n t . ~ The P-cage could host the Rh(I1) dimers after activation. There is sufficient room for two Rh(I1) to be situated in the vicinity of two 11' sites, coordinated to three oxygens and forming a weak Rh(I1)-Rh(I1) bond. Upon adsorption of CO, which is too big to enter the 8-cage, the Rh(I1) could be attracted toward site I1 in the center of the 6-ring to weakly coordinate to a C O molecule forming a Rh(I1)-CO adduct. A possible site for Rh(I1) dimers in the a-cage could be adjacent to sites II* and 111. In this location one Rh(I1) is coordinated to two oxygens and the other Rh(I1) is coordinated to three oxygens; thus, coordination of C O is expected to yield two species with similar intensities, one being the Rh(I1)-CO adduct and the other C1 species. The fact that the
relative intensity of C1 is usually weaker than that of the C O adduct indicates that this suggested dimer location is not the sole dimer location. The formation of the Rh(I1)-CO adduct in RhCa-X: RhCa-Y,lo and RhNa-Y10*22can be explained similarly. The formation of species El after adsorption of ethylene can be understood similarly. However, the situation is more complicated since the decay of the El signal indicates further reaction. Ethylene, like, CO, is too large to enter the @-cageand is not a strong enough ligand to replace the framework oxygens as liga n d ~ Ethylene .~~ could induce dissociation of the Rh(I1) dimers in the @cage followed by displacement of Rh(I1) toward the 6-rings to coordinate one ethylene molecule as indicated by the ESEM measurements. In the case of ethylene, since Rh-exchanged zeolites are catalysts for ethylene dimerization, we cannot rule out the possibility that reduction of Rh(II1) or Rh(1) occurred to produce Rh(I1) or Rh(O), respectively. Similar behavior has been observed in RhCa-X.28 RhCa-A activated either in oxygen or under vacuum interacts with oxygen to give three paramagnetic species, two adducts and species 0 1 . Similar species were generated in RhCa-X,9 RhCa-Y,l0 and RhNa-Y.10,22 As in the case of C0,9310the species formed can be divided into two types. In one type, species 0 1 , oxygen is strongly bound and is not affected by room-temperature evacuation whereas in the other type the oxygen is rather loosely bound. Pyrolysis of [Rh4(CO)12]-A1203with subsequent oxygen exposure29produced two species with g,, = 2.09, gxx= 2.03, and g,,,,= 1.95 and g,, = 2.06, g,, = 2.02, and g,,,,= 1.97 which were stable toward evacuation. These species were assigned to a dimeric and monomeric form of Rh(III)-02-, respectively, where the unpaired electron is partly located on the Rh as well, due to the large deviation of g,,, from g,. These g values are close to those of species 0 1 . Adduct 1 with gll > g, was observed in RhNa-Y and assigned to a Rh(II)-O~--Rh(II) species formed by interaction of O2with two nearby Rh(1) species in the (r-cage.22 The g values of the second adduct with gil C g, are quite different from the g values observed for 0,- coordinated to cations in zeolites.27 Accordingly, in this species the unpaired electron is assigned to Rh(I1). Such an adduct could be formed from Rh(I1) dimers as in the case of CO. In the species formed upon interaction with 02,the oxygen moiety should be located in the a-cage since O2is too large to enter the @-cagea t room temperature. Room-temperature reduction of samples activated under vacuum generated species Hb The g values of Hb are close to those of Rh2+(A),g, = 2.516, g,,,, = 2.56, and g,, = 1.883, formed after activation of RhNa-A to 400 OC in flowing oxygen.' If we (28) Goldfarb, D.; Kevan, L., unpublished results. (29),Gervasimi, A.; Marazzoni, F.; Stremolo, D.; Fina, F.; Strukul, Zanderighi, L. J . Chem. SOC.,Faraday Trans. 1 1986, 82, 1795.
G.;
Rh(I1) Species in Ca-A Zeolite attribute species Hb to Rh(II), then it must have been produced via the reduction of Rh(II1) which is unlikely to be present since the sample was activated under vacuum.5 A better explanation for the generation of species Hb is the reduction of Rh(1) to Rh(0). Rh(0) with g values g,, = 2.522, gyy = 2.585, and g,, = 1.989 was reported for [Rh°C14-H20]4-in NH4C1.30 Activation in flowing oxygen should reduce the relative concentration of Rh(1) and accordingly should affect the formation of species Hb, which indeed is not generated in these samples. The formation of species Ha is not as clear. Unlike Hb, Ha decomposes upon evacuation at room temperature. The ESEM data indicate that at least part of the hydrogens are directly coordinated to the Rh. In solution chemistry, [Rh11(OEP)]2 dimers, where OEP stands for octaethylporphyrin, are known to dissociate very easily upon interaction with hydrogen to form hydride^.^' Removal of the hydrogen restores the dimers. It is possible that in RhCa-A the effect of H2 is similar, and it dissociates the Rh(I1) dimers into monomers stabilized by molecular hydrogen. Hydride formation does not seem reasonable since one would expect the hydrides to be diamagnetic. In RhNa-X,7 after reduction at 200 OC, a species H1 was generated and was found to be stabilized by molecular hydrogen. Its g value, giso= 2.165, however, greatly differs from that of species Ha. Water molecules are small enough to penetrate into the @-cage at room temperature. Thus, following hydration one expects all cations in A zeolites to be coordinated to some extent to water molecules since there are no inaccessible cation sites to water. As with the other adsorbates, we attribute the signal to Rh(I1) generated from dissociation of Rh(I1) dimers. The g values are typical for Rh(I1) in a dZ2-r2ground state which in a d7 cation is caused by an elongated tetragonally distorted field.32 Accordingly, the Rh(I1) cation should be coordinated to six ligands. One possibility is to three framework oxygens and three water molecules or hydroxyl groups in site 11' or 11* as observed in RhCa-X and RhCa-Y.Io The ESEM data indicate three directly coordinated deuteriums which suggests three O D groups. However, uncertainties in the ESEM results may be larger in this case due to the possibility of overlapping species. Adsorption of ammonia generates different species from water. Species N 1 and N 2 have been observed in RhCa-X9 as well. The g values of N 1 are close to those observed for a complex of Rh(I1) with tetraphenylporphyrin (gZz= 2.089, gyy = 2.029, and g,, = 1.990).33 One can visualize such a geometry by having Rh(I1) either coordinated to four ammonia molecules in the a-cage or coordinated to two ammonia molecules and two framework oxygens in sites II*, 111, or 11' if ammonia can penetrate the @-cage. The ESEM data show deep modulation; however, the interaction distance of 0.34 nm is too long for direct coordination. Furthermore, the number of interacting deuteriums is 9 rather than 12 or 6 as suggested by the g values. Unfortunately, the ESEM data were recorded at a position where N 1 and N 2 overlap, and the result corresponds to some average of the two species and cannot give a clear-cut picture of the immediate environment of N 1 and N2. Species N3, which is generated only in samples activated under vacuum, involves weakly coordinated ammonia since its intensity decreases considerably upon evacuation. Species M1, which is generated upon adsorption of methanol, showed a large interaction distance of 0.42 nm with two deuteriums when C H 3 0 D was adsorbed and 0.39 nm with six deuteriums when C D 3 0 H was adsorbed. Considering that the methanol molecule was not involved in any reaction and remained intact, the 0.42-nm distance from the hydroxyl deuteron indicates nondirect coordination. Since methanol is too large to enter the (30) Sastry, M. D.; Savitri, K.; Joshi, B. D. J. Chem. Phys. 1980, 73,5568. (31) Farnos, D. M.; Woods, B. A.; Wayland, B. B. J . Am. Chem. SOC. 1986, 108, 3659.
(32) Ben-Taarit, Y.; Vedrine, J. C.; Dutel, K. J.; Naccache, C. J . Magn. Reson. 1978, 31, 251. (33) James, B. R.; Stymes, D. V . J . Am. Chem. SOC.1972, 94, 6226.
The Journal of Physical Chemistry, Vol. 91, No. 25, 1987 6395
@-cage,a Rh(I1) cation in site 11' could have a larger interaction distance from the hydroxyl deuterium than from the methyl deuterium because the oxygen is not necessarily oriented toward the Rh(I1). Similar results were obtained for species M2. However, adoption of the above interpretation introduces difficulties in accounting for the appearance of the ESR signals. If the Rh(I1) species do not interact with the methanol molecules, why would the Rh(I1) dimers dissociate? Another possibility is that the ESR signals could be generated through an oxidation/ reduction reaction with methanol, which products could in turn cause dissociation of Rh(I1) dimers. In this case, however, the methanol molecule should not remain intact. A distance of 0.39 nm from the methyl deuteriums corresponds to a Rh-0 distance of 0.27 nm if the Rh(I1) is coordinated through the oxygen and the hydroxyl proton has been abstracted. Though the ESR signals obtained after adsorption of water in the presence of oxygen in RhCa-A are similar to those observed in RhCa-X: RhCa-Y,lo and RhNa-Y,10*22,34 the ESEM exhibited shallower modulation indicating indirect coordination, as opposed to direct coordination in the zeolites. In X and Y zeolites these signals were assigned to a Rh(I1) complex either with OD groups or with O D and D 2 0 ligands. In RhCa-A, however, the long interaction distance may favor the assignment of these signals to in spite of the rather large deviation of gyy (1.978) from 2.0. The similarity in the g values may imply that also in the X and Y zeolites the signals correspond to 0, species which happen to be in close proximity to D 2 0or O D through hydrogen bonding. The absence of a significant amount of paramagnetic Rh(I1) species in RhNa-A could be attributed to the large number of sodium cations which either prevent Rh(I1) dimer formation and thus promote formation of Rh(1) and Rh(II1) or inhibit dimer dissociation. This is consistent with the absence of the O2 and C O adducts in RhNa-X79* and their appearance in RhCa-X,9 RhCa-Y,lo and RhNa-Y.'o-22 Conclusions Although generally the Rh(I1) species formed in RhCa-A are similar to those found in RhCa-X and RhCa-Y, some significant differences are observed. The Rh(I1) cation assigned to site I in X and Y zeolite did not appear in RhCa-A as expected. Paramagnetic Rh species could be readily obtained after interaction with H2 at room temperature in A zeolite while in the corresponding X and Y zeolites a higher temperature was required. Furthermore, a species analogous to the Rh-hydrogen complex seen in A zeolite was not observed in RhCa-X or RhCa-Y. The similarity of some of the species formed in RhCa-A and RhCa-X and RhCa-Y indicates that the Rh cations are indeed located within the zeolite cavities and not on the surface. The differences observed can be attributed to the different structure and/or the Si/A1 ratio. The presence of the Rh cations in the A zeolite internal cages suggests its potential as a shape-selective catalyst since its a-cage is smaller than the a-cage in X and Y zeolites. The absence of paramagnetic species after activation is attributed to the existence of Rh(I), Rh(III), and Rh(I1) dimers. As in X and Y zeolites, these dimers readily dissociate after adsorption of various molecules to form paramagnetic Rh(I1) species. Zeolite A containing Na+ suppressed the formation of paramagnetic Rh(I1) species compared to its Ca2+counterpart, probably due to crowding of the a- and 6-cages with Na+ cations. Acknowledgment. This research was supported by the National Science Foundation and the Texas Advanced Technology Research Program. We also acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. CO, 630-08-0; O, 7782-44-7; H,, 1333-74-0; H 2 0 , 7732-18-5; CHSOH, 67-56-1; C2H4, 74-85-1; NHS, 7664-41-7. (34) Narayana, M.; Kevan, L.; Naccache, C. J . Carol. 1984, 86, 413.
