J . Phys. Chem. 1990, 94, 5317-5327
5317
Characterization of Nay-Entrapped Rh6(CO)16Cluster by FTIR and EXAFS Spectroscopies and the Catalytic Behavior in 13C0 Isotopic Exchange Reaction Ling-Fen Rao,+*SAtsushi Fukuoka,+ Nobuhiro Kosugi,t Haruo Kuroda,t:and Masaru Ichikawa**t Catalysis Research Center. Hokkaido University, Sapporo 060, Japan, and Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113. Japan (Received: October 6, 1989; In Final Form: February 7, 1990)
NaY zeolite entrapped Rh6(C0),6 and the product of its oxidation followed by reduction, [Rh6],/NaY, have been structurally characterized by EXAFS and FTIR spectroscopies and CO/H2 chemisorption. Rh6(CO),,(p3-CO),, [A], in NaY supercages has been synthesized with minor formation of Rhf(CO)2, [B], by carbonylation of Rh3+/NaY at 373-423 K. The coordination of Rh atoms (coordination number and bond length of Rh-Rh and Rh-CO bonds) and the C-0 stretching modes of [A] are similar to those of crystalline Rh6(CO),,. Oxidation and successive reduction of [A] result in [Rh61rd/NaY with high dispersion (less than seven Rh atoms). Admission of CO to [Rh61rd/NaY produces a new Rh carbonyl cluster [C], which resembles [A] in terms of the coordination number and bond length of Rh-Rh and Rh-CO bonds and differs from [A] in the C-O stretching modes ([A], v(C0) = 2098 and 1760 cm-I; [C], v(C0) = 2092 and 1830 cm-I. The new carbonyl cluster is suggested to be an isomer of [A], Rh6(CO)lz(p2-CO),, which is transformed into Rh6(CO),2(p3-CO), at temperatures higher than 423 K. [Rh61rd/NaY chemisorbs CO with the stoichiometry CO/Rh = 2.6, in accordance with that of the Rh6(C0)16molecule. The carbonyl activity of [A] in isotopic exchange with "CO(g) is lower than that of [B]. The carbonyl ligands of [A] are found to take part in an intermolecular scrambling reaction with coexisting [B]. The slow exchange reaction of Rh6(C0),2(p3-C0)4/NaYwith CO(g) takes place through intermolecular scrambling between Rh6(CO)12(p3-CO)4and Rh+(CO), inside N a y .
1. Introduction Metal carbonyl clusters grafted on supports such as oxides and zeolites offer a great advantage in the preparation of supported metal catalysts at molecular level. Removing the carbonyl ligands from the precursors may result in active metal centers having uniform sizes and metal compositions.'S2 Among inorganic supports, zeolites are most promising to accommodate metal clusters in cages without cluster aggregation. A catalyst of metal cluster/zeolite may combine the high activity of the metal cluster and the good shape selectivity of the ~ e o l i t e . ~In fact, such catalysts exhibited considerably improved selectivities over conventional amorphous oxide-supported catalysts in some catalytic
reaction^.^^ The concept of building metal clusters in zeolite cages has been coined as a "ship-in-a-bottle" synthesis3 In the past decade syntheses of carbonyl clusters such as Rh6(C0)16,7-9[HFe3(Co)ll]-,lo Ir6(C0)16," and Pd,3(C0),12 have been attempted in the supercages of N a y . Recently we synthesized intrazeolite bimetallic Clusters [FezRh4(C0)16]2-13,14 and Rh6-,IrX(CO)16 (X = 0-6).15 Since Mantovani et aL7 reported the formation of Nay-entrapped Rh6(C0),6 in hexane hydroformylation catalyzed by Rh3+-exchanged N a Y under a high CO H2pressure (80 atm), several papers have been reported on the synthesis and characterization of Rh6(CO),6/NaY. Formation of intrazeolite Rh6(CO),, is claimed on the basis of the characteristic IR bands at 2098 and 1760 cm-' 7-9 and the contribution of the Rh-Rh bond at a distance of 2.76 A in radial electron distribution from X-ray scattering (RED).16 Rode et aL9 found that these characteristic IR bands were not affected by exposure of Rh6(CO)l,/NaY to n-hexyldiphenylphosphine, which is larger than the N a Y neck aperture (about 7 A in diameter). This is evidence that the cluster is located in the supercage of N a y . Nevertheless, the detailed structure and the reactivity of Rhs(CO)@aY are still unclear. Van't Blik et aI.I7 found by EXAFS and IR studies that adsorption of CO on highly dispersed Rh/AI2O3significantly perturbs the coordination number of Rh-Rh, giving only two I R bands characteristic of isolated Rh+(CO), species at 2095 and 2023 cm-'.
+
'Hokkaido University.
tThe University of Tokyo. Permanent address: Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China. *To whom correspondence should be addressed.
0022-3654/90/2094-53 17$02.50/0
TABLE I: Preparation and Treatments of Nay-Entrapped Rh Clusters
sample treatment Rh'+/NaY NaY exchanged with RhCI, at 363 K for 12 h Rh6(C0),6/NaY Rh3+/NaY heated at 343-393 K in CO + H 2 0 or 343-423 K in CO + H, (1:l) [Rh6],,/NaY Rh6(C0)16/NaY heated from 300 to 473 K under flowing 0,; kept at 473 K for 2 h; cooled to 300 K under O2 [Rh,],,/NaY evacuated at 300 K; exposed to H2 [Rh&/NaY at 300 K and 600 Torr" for 30 min; evacuated at 343 K for 30 min; heated in H2 flow at 300 to 473 (or 673) K; kept at 473 (or 673) K for 2 h; cooled to 300 K under H2 COad-Rh6/NaY [Rh61rd/NaY evacuated at 300 K; exposed to CO at 300 K and 450 Torr "Note: 1 Torr = 133.3 N m-2. Smith et a1.18 proposed that the cleavage of Rh-Rh metal bonds in R h 2 occurring in CO chemisorption is associated with the (1) Metal Clusters in Catalysis (Studies in Surface Science and Caralysis. 29); Gates, B. C., Guczi, L., Knozinger, H., Eds.; Elsevier: Amstersam, 1986, and references therein. (2) Ichikawa, M. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; Reidel: Dordrecht, 1986; p 183. (3) Lunsford, J. H. ACS Symp. Ser. 1977, 40, 473. (4) Fraenkel, D.; Gates, B. C. J . Am. Cbem. SOC.1980, 102, 2748. (5) Lefebvre, F.; Gelin, P.; Elleuch, B.; Biab, Y.; Ben Taarit, Y. Structure and Reactivity of Modified Zeolites (Studies in Surface Science and Catalysis. 18); Jacobs, P. A., et al., Eds.; Elsevier: Amsterdam, 1984; p 257. (6) Yamaguchi, I.; Joh, T.; Takahashi, S . J . Cbem. Soc., Cbem. Commun. 1986, 1412. (7) Mantovani, E.; Palladino, N.; Zanobi, A. J . Mol. Catal. 1977/1978, 3, 285. (8) Gelin, P.; Ben Taarit. Y.; Naccache, C. J . Catal. 1979, 59, 357. (9) Rode, E. J.; Davis, M . E.; Hanson, B. E. J . Caral. 1985, 96, 574. (10) Iwamoto, M.; Kagawa, S. J . Pbys. Cbem. 1986, 90, 5244. ( 1 1) Bergeret, G.; Gallezot, P.; Lefebvre, F. New Deo. Zeolite Sci. Tecbnol. (Studies in Surface Science and Catalysis. 28) 1986, 401. (12) Sheu, L. L.; Knozinger, H.; Sachtler, W. M. H. Catal. Lett. 1989, 2, 129. (13) Rao, L.-F.; Fukuoka, A.; Ichikawa, M. J . Cbem. SOC.,Cbem. Commun. 1988,458. (14) (a) Rao, L.-F.; Fukuoka, A.; Ichikawa, M. In Acid-Base Catalysis; Tanabe, K., Hattori, H., Yamaguchi, T., Tanaka, T., Eds.; Elsevier: Amsterdam, 1989; p 341. (b) Fukuoka, A,; Rao, L.-F.; Kosugi, N.; Kuroda, H.; Ichikawa, M. Appl. Catal. 1989, 50, 295.
0 1990 American Chemical Society
5318
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990
reduction of surface OH groups. Recently, Basu et aI.l9 obtained direct IR evidence that isolated OH groups on A1203were responsible for the oxidation of Rh; to Rh'. Therefore, it is interesting to study the C O chemisorption behavior of Rh particles in NaY compared to that of those impregnated on amorphous oxides such as AI2O3 and SO2. In the present work, we conducted FTIR and EXAFS studies to characterize Rh6(C0)16/NaY, [Rh61rd/NaY, and C0,dRh6/NaY. The catalytic performance in the I3CO(g) exchange reaction is investigated for the different intrazeolite carbonyl species Rh6(C0),6 and Rh+(C0)2. The mechanism of the I3CO exchange reaction between R h 6 ( C O ) , 2 ( ~ 3 - C Oand ) , c o ( g ) is discussed.
2. Experimental Section 2.1, Catalyst Preparation. The preparation and treatments of Nay-entrapped Rh cluster samples are presented in Table I . Rh3+-exchanged N a Y (Rh3+/NaY) was prepared by slow addition of aqueous solution of RhCI, (4 mM) to a NaY slurry (Toyo Soda Co., SiO2/AI2O3= 5.6, surface area = 910 m2/g) with rapid stirring for 3 h. The ion exchange was continued for another 15 h at 360-368 K. Then it was filtered, washed free of CI- ion with deionized H 2 0 , and dried in air at 393 K. The Rh loading determined by atomic absorption spectroscopy was 1.8 w t %. Rhb(C0)16/NaY was prepared by two methods: Method 1: Rh3+/NaY was exposed to 5 Torr of H 2 0 vapor and heated under 600 Torr of C O at 343-393 K with stirring, resulting in Rh6(C0)16according to the proposed successive carbonylation reaction (la-c), where v(C0) was carbonyl Rh3+/NaY + 3CO + H 2 0 R h + ( C 0 ) 2 / N a Y + CO, + 2H+-NaY
-
v(C0) = 2098, 2022; 21 14, 2048 cm 4Rh+(C0)2/NaY
+ 6CO + 2 H 2 0
(la)
I
+
+
Rh4(C0)12/NaY 2C02
+ 4H+-NaY
-+
v(C0) = 2082 s, 2032 w, 1876 w, 1835 s cm-' Rh,(CO),,/NaY
+ 2Rh+(CO)*+ CO + H 2 0 Rh6(CO)l6/NaY
+ Co2
(lb)
2H+-NaY
v(C0) = 2098 s. 2066 w, 1760 s cm-I (IC) stretching frequency of the intermediate carbonyl species at each step of synthesis. The apparent color of the sample changed from pale yellow of Rh3+/NaY to pale gray. Method 2: Rh3+/NaY was heated in a flow of mixed gases C O H2 (1 :1) at 343-423 K for 12 h.9 The color of the resultant sample was reddish yellow, and its IR spectrum was identical with the sample prepared by method 1 (Figure la): 6Rh3+/NaY + 16CO + 9H2 Rh6(cO),,/NaY + 18H+-NaY (Id)
+
-
In eqs la-d we assume that a proton is formed during the reductive carbonylation processes. Proton formation in H2 reduction of metal ions supported on NaY has been proved by Sachtler and co-workers.20 They found that the protons were attached to zeolite lattice oxygen, forming O H groups that were detected by FTIR. (15) Ichikawa, M.; Rao, L.-F.; lto, T.: Fukuoka, A. Faraday Discuss. Chem. SOC.1989, 87, 232. (16) Bergeret, G . ; Gallezot, P.; Gelin, P.; Ben Taarit, Y . ; Lefebvre, F.; Naccache, C.; Shannon, R. D. J . Carol. 1987, 104,279. (17) (a) Van7 Blik, H. F. J.; Van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J . Phys. Chem. 1983,87,2264. (b) Van't Blik, H. F. J.; Van Zon, J. 8. A. D.; Huizinga, T.; Vis, J. C . ; Koningsberger, D. C.; Prins, R . J . Am. Chem. Sot. 1985, 107, 3139. (18),Smith, A . K.; Hugues, F.; Theolier, A.; Basset, J. M.; Ugo, R.; Zanderighi, G . M.: Bilhou, J. L.; Bilhou-Bougnol, V.; Graydon, W. F. Inorg. Chem. 1979, 18. 3105. (19) (a) Basu, P.; Panayotov, D.; Yates, Jr. J . T. J . Phys. Chem. 1987, 91, 3133. (b) Basu. P.: Panayotov, D.: Yates. Jr., J . T. J . Am. Chem. Soc. 1988,
I 10, 2074. (20) Homeyer, S T.: Sachtler, W . M . H . Appl. Caral. 1989, 54. 189.
Rao et al. [ Rh6jd/NaY was prepared by evacuation of Rh,(C0),6/NaY at 343 K for 30 min to remove water on zeolite and then oxidation in an O2 flow at 473 K for 2 h. The oxidized sample, [Rh,],,/ N a y , was subsequently reduced in a H2 flow at 473 or 673 K for 2 h. 2.2. IR Spectroscopy. IR spectra were recorded by a Shimadzu FTIR-4100 double-beam spectrometer, coadding 25 scans at 2-cm-l resolution. Each sample was pressed into a disk of 8 mg/cm2 and mounted in an IR cell that was connected to a Torr). The IR cell was equipped with an electric vacuum line ( heater and a liquid N 2 reservoir for high- and low-temperature measurements. The contribution of the gas phase was compensated for by using a reference IR cell having the same optical length as the sample cell. I3CO isotopic exchange reactions were performed in the IR cell using enriched I3CO (98%) purchased from Merck Reagent Co., Ltd. 2.3. EXAFS EXAFS measurements were carried out at the Photon Factory in the National Laboratory for High Energy Physics (KEK-PF) with an electron energy of 2.5 GeV at currents of 100-280 mA. Spectra at the Rh K edge (23.2 keV) were measured at 293 K. EXAFS oscillations were extracted from the observed spectra by a cubic spline function, and the k-weighted EXAFS spectra (Figure 3a) were obtained at a k interval from 3 to 16 ,kl.Then Fourier transforms of k 3 x ( k )were calculated (Figure 3b). The peaks in Figure 3b were inverse Fourier transformed in the range 1-3 A, and curve-fitting analyses were carried out on the inverse Fourier transforms using empirical parameters (the phase shift and backscattering amplitude function) obtained from reference samples. The resultant optimized coordination numbers and bond lengths are summarized in Table 111. Using these parameters, we calculated an EXAFS function for each sample; they are in good agreement with the experimental data (dashed and solid curves in Figure 3c). The reference samples used were crystalline Rh6(C0)16,Rh203,and Rh foil. The detailed analytic procedure and the computer program used were described elsewhere.21 2.4. CO and H 2 Chemisorpfion. C O or H2 adsorption was carried out in a static volumetric system made of Pyrex glass, equipped with a 222AB-barotron pressure sensor. The in situ reduced sample [Rh61rd/NaY (0.2 g) was evacuated at 473 K and lo4 Torr for 2 h. Then the first C O or H2 adsorption isotherm was observed by admitting C O or H 2 at 10-80 Torr and 298 f 2 K. After that the sample was evacuated at 300 K and the second adsorption isotherm was determined by admission of C O or H2 again.
3. Results and Discussion 3.1. Characterization of R ~ ~ ( C O ) I ~ / N U FTIR Y . Study. Figure la shows the IR spectrum of intrazeolite Rh6(C0)16/NaY prepared in situ, which is in good agreement with the previous The spectrum is not affected by exposure of Rh6( C 0 ) 1 6 / N a Y to 450 Torr of o2at 300 K for 2 h. An ex situ prepared sample pelleted in air exhibits a similar IR spectrum after the adsorbed water has been evacuated at 343 K. This stability to O2 or air (for a short period) contrasts with that of Al20,-supported Rh6(C0)16,which is readily decomposed to Rh+(C0)2 upon air oxidation (more easily in the presence of water).8.18x22 On exposure O f Rh6(CO)l,/NaY to 10 Torr Of l 3 c 0 ( g ) at 170 K, four weak bands of linear C O (COJ at 21 14,2098,2048, and 2022 cm-' are immediately replaced by new bands at 2067, 2051, 2000, and 1975 cm-' (dotted line in Figure la,b), whereas the strong bands at 2098 and 1760 cm-I remain at the same positions as before the I3CO(g) admission, showing no obvious exchange at T < 293 K. Figure I C is the difference spectrum of after and before I3CO(g) admission at 293 K. Table I1 gives the activity and assignment of the carbonyl species on the sample of Rh6(CO)16/NaY. The unexchanged bands are (21) Kosugi, N.; Kuroda, H. Program EXAFS-I, Research Center for Spectrochemistry, The University of Tokyo, 1985. (22) Asakura. K.; Iwasawa, Y.; Kuroda, H. Bull. Chem. SOC.Jpn. 1986, 59. 647.
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5319
Nay-Entrapped Rh6(C0)16Cluster I
2.0
a
I .6
\
6 0 f
2200
2000 I800 1600 Wovenumber /cm-'
8 a 1.2 0
0.8
I
2; 5
I
I
I
2000
I
I800
I
I
1600
2200
2000
Wavenumber /cm-'
I800
1600
Wovenumber
Wavenumber /cm-'
c/cm-'
Figure 1. (a) IR spectrum of Nay-entrapped Rh6(C0)16/NaY. Solid bands are due to Rh6(CO)lz(g3-CO)4,and dotted bands are due to Rh+(CO)? (b) After exposure to IO Torr of I3CO(g) for 3 min at 293 K. (c) Difference spectrum (b) - (a). (d) Band distribution of g 3 - C 0 (fitted by Gaussian function). TABLE 11: Activity of Carbonyl Species on Rh6(CO),JNaY in ' C O IsotoDic Exchange Reaction species [A] species [B] 2098,2022;21 14,2048 v,,p(12CO)/cm-1 2098,2066, 1760 2022, 1720 2051, 1975;2067,2000 ~ , , ~ ( l ~ C O ) / c m - 2048, ' 2051, 2051, 1977,2067,2002 ~,~(l~CO)~/cm - ~ 2020, 1721 1.6:l IIIIP below 170 exch temp, K 333-373 abundance: % 80-85 20-1 5 abundance! % 78 22 assgnt Rh6(C0)12(fi3-C0)4in Rh+I(CO)*/NaY in two states: BI and B2 NaY supercage molar ratio 1 1 OCalculated according to the two-atoms model:26 Y , ~ ( ~ ~ C = O)
(12CO)(m/*m)1/2= 0.97777. m and *m are the reduced masses of and l I f are the integral areas of the IR bands for I2C0 and "CO. *I linear and face-bridging CO, respectively. CBasedon IR analysis, e.g., [species B]% = [total areas of the four COI bands of [B]]/[total areas of all the COI bands of [A] and [B]]. dBased on EXAFS analysis.
associated with the carbonyls of intrazeohte Rh6(C0)12(p3-C0)4 ([A]). The 2098 s and 2066 w cm-' bands are assigned to the symmetric and asymmetric stretching of the 12 twin CO1bound to each Rh atom in the hexanuclear Rh6(C0)16. The 1760-cm-' band corresponds to the 4 face-bridging COf bound to three Rh atoms of Rh,(CO)16/NaY (p3-CO).7-9 The ratio of the band intensities of COI:COffor Rh6(C0)16/NaY is about I l / I f = 1.6. Therefore, the relative extinction coefficients for COl and C o r can be roughly estimated as CI:tr = 11/32:If/4 = 0.53:1 The four exchanged bands are assigned to coexisting gem-dicarbonyl Rh+(C0)2([B]) in two different states, Rh+(CO)2(0,)2 ([B,]) with bands at 2098 and 2022 cm-' and Rh+(C0)2(0,)(H20) ([B2]) with bands at 21 14 and 2048 cm-l, where 0, denotes the lattice oxygen of zeolite as reported by Shannon et a~.*3
The relative abundances of [A] and [B] are calculated from the areas of the two CO, bands in [A] and the four COI bands in [B], where we assume the equal extinction coefficients of CO, bands for [A] and [B]. The abundance is 80-85% for [A] and 20-15% for [B]. The highest abundance of [A] we achieved was 87.5% of the total Rh in N a y , corresponding to a molar ratio of [A]:[B] = 1. On highly dispersed Rh/A1203,it has been reported that metallic Rh,O sites and cationic Rh+ sites coexist even after strong H2 r e d u ~ t i o n . ~ The ~ - ~coexisting ~ Rh> and Rh+ on NaY and A1203is possible due to the metal-oxygen interaction at the metal/support interface to stabilize Rh+. IR bands of [A] at 2098 and 1760 cm-' are sharper than those of Rh6(CO)16on the external surface of Nay2' or of crystalline Rh6(CO)16.28 The distribution of the band absorbance is fitted to the Gaussian function A = A. exp - (o.ii::m
)
(2)
where A and A. denote the band absorbance at frequency Y and vo (the center of the band), respectively. Fwhm is the full width at half-maximum. Figure Id shows an example for the curvefitting of the p3-CO band, where the parameters in eq 2 are found as vo = 1760 cm-l and fwhm = 23 cm-I. The narrow and symmetric Gaussian distribution of the IR bands, especially of the fi3-C0 band, implies that the four p3-C0 ligands in Rh6(C0),2(p3-C0)4are under the same chemical environment and every intrazeolite Rh6(CO)16 is uniformly accommodated inside NaY cavities in terms of the configuration and the electrostatic field. The clusters may be located at the center of the zeolite supercages. Given that there are 8 mol of supercages/mol of NaY zeolite, the occupation probability of a supercage with a R& cluster is about 5%. Statistically, clusters are isolated from each other by about two empty supercages. (23) Shannon, R. D.; Vedrine, J. C.; Naccache, C.; Lefebvre, F. J. Coral. 1984, 88, 43 1. (24) (a) Yates, Jr., J. T.; Duncan, T. M.; Worley, S. D.; Vaughen, R. M. J . Chem. Phys. 1979.70, 1219. (b) Yates, Jr., J. T.;Duncan, T. M.:Vaughan, R. M. J. Chem. Phys. 1979, 71, 15. (25) Wang, H. P.; Yates, Jr., J. T. J. Catal. 1984, 89, 79. (26) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London, 1975. (27) (a) Rao, L.-F.; Guo,X.-X.; Fukuoka, A.; Ichikawa, M. Acta Chim. Sin., Engl. Ed. 1989,477. (b) Rao, L.-F.; Guo, X.-X.: Fukuoka, A.; Ichikawa, M. Cuihua Xuebao (J. Caral.) 1989, 10, 143 (in Chinese). (28) The Aldrich Library of FT-IR Spectra; Pouchert, C. J., Ed.; Aldrich: Milwaukee, WI, 1985; p 1300.
Rao et al.
5320 The Journal of Physical Chemistry, Vof.94,No. 13, 1990
TABLE 111: Results of the Curve-Fitting Analysis of Rh K Edge EXAFS Data Obtained at 300 K for Nay-Entrapped Rh Cluster Samples" Rh-Rh samples
N
RIA 2.74
Rh-COj N RIA 1.5 1.88
2.70 2.70 2.72
Rh-0
RIA
N 1.6
2.15
1.4
1.85
1.4
2.15
2.1 2.0
1.87 I .864
2.0 2.0
2.17 2.168
N 1.8 6.8 0.7 0.7 0.8
RIA
6.0
2.05
2.06 2.06 2.10 2.09 2.03
2.69 2.76 2.776
"Estimated exuerimental errors are f 0 . 0 2 A for atomic distance R and f 0 . 2 for coordination number N on the present EXAFS data evaluation. bThe results basdd on the X-ray diffraction analysis.'*
unsupported Rhs(CO)i6
.1 evacuation
T
340 K
Wavenumber /cm-'
Figure 2. Changes of v(COI)and u(COf) of Nay-entrapped Rh6(C0)12(~3-C0)., with adsorption of H 2 0 .
The stretching frequency of the COI bond, v(COI), is blue shifted by about 20 cm-l, while v(C0,) is red shifted by about 40 cm-l with respect to those of Rh6(C0)16on the external surface of r ~ ~ reverse shifts Nay2' or crystalline Rh6(C0)16.28S h r i ~ e reported of u(COI) and v(C0b) (bridging CO) in adduct formation between [(C5Hs)Fe(CO)J2and AIR, or BX,, which give a large red shift v(C0b) of 100-300 cm-I and a large blue shift v(coI) about 30-70 an-'. This reverse shift is due to the interaction between the oxygen end of C o b and the Lewis acid reagent as AIR, and BX,. Adsorption of HzO or pyridine on Rh6(C0),6/NaY gives only a small red shift of CO1bands and a blue shift of the COf band (see Figure 2). The shift amount is reversibly proportional to the coverage of H 2 0 or pyridine. The v(C0,) and v(COf) are 2098, 1760 2090, 1772 2098, 1760 cm-I in the cycle of evacuation saturation with H 2 0 evacuation. After H 2 0 or pyridine is adsorbed on NaY zeolite, the Lewis acidity of AI3+is expected to be weakened and a Bronsted acid site H+ is formed.30 This weakens the interaction between the oxygen end of p,-CO and AI3+. Since u ( c o I ) and v ( c o f ) of Rh6(C0),6/NaY do not completely revert to those of crystalline Rh6(C0),, (2076 and 1800 cm-I) with excess H 2 0 completely blocking AI3+ Lewis sites, the band shifts of u(CO1) and u(COr) compared to crystalline Rh6(CO),, might arise from the interaction of p 3 - C 0 with not only Lewis acid AI3+but also Bronsted acid H+ inside N a y . Perhaps Rh,(CO) 16 is not frozen configurationaly in the NaY supercage by the interaction with an electron acceptor such as AI3+ and H+. EXAFS Study. To obtain more insight into the structure of Rh6(CO)l,/NaY, especially into the metal framework, we have performed a Rh K edge EXAFS analysis. The smoothed EXAFS functions k x ( k ) of Rh6(cO)16/NaY and crystalline Rh6(C0)16 are significantly different, indicating that the EXAFS of Rh6(CO),6/NaY is caused by Rh-Rh and Rh-CO as well as Rh-0,
--
--
(29) Shriver, D. F. Orgunomet. Chem. 1975, 94, 259. (30) Tanabe, K. In Catalysis; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1981; Vol. 2, p 232.
scattering. In the k x ( k ) vs k spectrum of Rh6(CO)16/NaY (Figure 3al), the relatively large amplitude in the high-k region indicates that the Rh atom is surrounded by heavy atoms such as Rh. In fact, its Fourier transform of k 3 x ( k ) vs R shown in Figure 3bl exhibits a strong peak at 2.74 8, attributable to Rh-Rh. As shown in Table 111, the coordination of Rh-Rh and Rh-CO bonds agrees well with that of crystalline Rh6(CO),,, suggesting stoichiometric formation of Rh6(C0)16clusters. The coordination numbers N(Rh-Rh) of Rh6(C0)16/NaY and crystalline Rh6(C0)16 are in the ratio of 3.1:4. This could imply that about 78% of the Rh atoms are present in the Rh6(C0)16cluster state while the remaining Rh atoms (22%) are present in the mononuclear form such as Rh+(C0)2. This interpretation agrees with the abundance of [A] and [B] calculated on the basis of IR studies (Table 11). Additional contribution of Rh-0, may arise from the metal/support interaction of [B] with the oxygen of zeolite. 3.2. Characterization of [Rh6],,/NuY. As shown in Figure 4, on warming Rhb(cO)16/NaY in o2from 300 to 430 K the bands at 2098 and 1760 cm-I decrease while the other bands at 2 1 14, 2048, and 2022 cm-l increase. On raising the temperature to 430 K, the bands at 2098 and 1760 cm-I disappear, leaving four intense bands in v(C0,) region (Figure 4c). At that point when 13CO(g) is admitted at 170 K, these four CO, bands are replaced readily by new bands at 2066,2048,2000, and 1975 cm-l, showing a facile isotopic exchange reaction with 13CO(g). On the basis of their high activity in isotopic exchange reaction and their band positions, these four bands are attributed to newly formed Rh+(C0)2. Hence, in an O2 atmosphere Rh6(C0)12(p,-CO), is directly oxidized to the stable species Rh+(C0)2. In the difference spectra shown in Figure 4B, the parallel development of three negative bands at 2098, 2066, and 1760 cm-I supports the assignment of them to the carbonyls of the same species [A]. The positive bands should be two doublets at 21 14,2048 and 2098, 2022 cm-I due to the formation of [B]; here the positive band at 2098 cm-I is not observed in view of the strong negative band of [A] at the same wavenumber. The carbonyl bands of Rh+(C0)2 eventually vanished at T > 473 K, where the EXAFS data show the Rh atoms are surrounded by oxygen with N(Rh-0) = 6.8, indicative of the complete conversion of Rh+(C0)2 to highly dispersed Rh oxide such as R h 2 0 3after being oxidized at T > 473 K. lntrazeolite Rh,(C0)16 is oxidized in two steps, a s shown in eqs 3a,b. The protons, which are proposed to be created during Rh,(Co)l6/NaY 4-
+ -+
'f/Zoz4- 6H+
T 260 K. Further exchange does not proceed until 323 K. The remaining unexchanged bands at 2098, 2066, and 1760 cm-l are found to exchange with I3CO(g) at 340-400 K. They correspond to species [A]. At temperatures above 343 K the conversion of [C] to [A] also proceeds. The stepwise isotopic exchange reactions at 170-400 K reveal that there are at least three kinds of species on C0,d-Rh6/NaY. Given that the extinction coefficients of CO, bands for Rh"C(C0)2 ( n = 0, 1 ) (C2,).groups in [A], [B], and [C] are the same, irrespective of their residual structure of moieties, the relative abundances of these three species can be estimated from the intensities of the respective CO, bands. Table IV summarizes the exchange activity, abundance, and assignment of [A], [B], and [C]. The order of exchange activity is as follows: [B], Rh+(C0)2, abundance of 20-3076, exchanges readily with I3CO(g) at T C 170 K; [C], abundance of 60-70%, exchanges at T C 280 K and corresponding to the carbonyl cluster; [A], R h 6 ( C 0 ) 1 2 ( ~ 3 - C 0 ) 4 , abundance of IO%, exchanges at T > 340 K. Hence, after C O chemisorption on [Rh61rd/NaY almost 70-80% of the Rh atoms are still in cluster states. EXAFS Study. [ R h l r d / N a Y evacuated at 300 K for 30 min was exposed to CO(g) at 600 Torr and 300 K, and then the EXAFS spectrum was recorded. As shown in Table 111, the N and R of Rh-Rh, Rh-CO,, and R h - m b bonds are very close to those in Rh6(C0)16/NaY,indicating that the coordination of Rh atoms in COad-Rh6/NaY is similar to that in R ~ ~ ( C O ) I ~ / N ~ Y . Possibly, upon C O chemisorption the reduced Rh particles in [Rh6lrd/NaY reconstruct to give a species such as the Rh6(C0)16 cluster. After CO chemisorption the Rh-Rh bond is extended by 0.02 A. Thus relaxation of the Rh-Rh bonds is greater with chemisorbed CO than with H2. Castner et aLMfound that on a Rh( 11 1)
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5323
Nay-Entrapped Rh6(C0)16 Cluster
1
A
0
A
E,
Rh-Rh
Rh-Rh
li
1830 1760
e 420K 20mn 6 410K lomin
( b ) 423K 60 mir
c 360K 5mm
( a ) 298K I
!200
l
l
1
I
I
2000 IS00 Wovenumber (cm-l)
3 min
I
1600
I I
00
I
B
4’ 1.0
E?
I
I
1
2000 1800 1600 Wovenumber /cm-‘
Figure 8. IR spectrum of COad-Rh6/NaY before (a) and after (b) heating in CO at 423 K for 60 min.
any shift of C O bands are observed. The IR spectrum of CO,d-Rh,/NaY is not affected by exposure to H 2 or co H2 at 300 K, either. These indicate that C0-H2 coadsorption does not occur and H, is adsorbed very weakly compared to CO. A similar behavior has been reported on a Rh( 11 1) single-crystal surface, where there is no interference to C O chemisorption by coadsorption of C O + H2 gases, and adsorption of hydrogen is weaker than that of C0.34 When the temperature of [Rhslr4/NaY is raised in a C O H, flow, the 1760-cm-I band develops gradually at the expense of the 1830-cm-I band (Figure 7). Up to 398 K, the bands of [C] (2092 and 1830 cm-I) are replaced by the bands of [A] (2098, 1760 cm-I). Figure 7B shows the intensity change for the 1830and 1760-cm-I bands during the CO H2 treatment. The decrease of the 1830-cm-’ band well corresponds to an increase of the 1760-cm-I band, and the total intensity of both the 1830- and the 1760-cm-’ bands remains constant. Hence the treatment of COad-Rh,/NaY at 350-423 K in C O H2 results in the conversion of [C] into [A]. The conversion of p 2 - C 0 into p 3 - C 0 is also observed when COad-Rh6/NaY is heated 423 K under c o ( g ) , as shown in Figure 8 where the IR band at 1830 cm-I is replaced by the 1760-cm-I band. It has been reported previously that two isomers of h 6 ( c o ) l 6 have been synthesized in s o l ~ t i o n . ~One ~ , ~(the ~ red isomer) is Ir6(CO)12(p3-C0)4 with 4 face-bridging co groups and 12 terminal co groups;3Sthe other (the black isomer) is Ir6(C0)’2(pz-CO), with 4 edge-bridging carbonyl ligands.36 The stretching frequency of v(p2-CO) is ca. 70 cm-l higher than u(p3-CO) in the two isomers. Although the C - 0 bridging mode of [C] and [A] are very different (the former has a p 2 - C 0 mode and the latter p 3 - C 0 mode, respectively), the coordination circumstances around Rh atoms in [C] and [A] are essentially equivalent in terms of EXAFS data. These suggest that [C] and [A] are structural isomers of the hexarhodium cluster framework having different bridging CO coordination, edge- and face-bridging C O ligands, respectively. Recall the two facts that [C] can be thermally converted into [A] and the frequency difference Av between p2-C0 in [C] and p3-C0 in [A] is also 70 cm-l, almost the same as that between the isomers of Ir6(C0)’6 (Chart I). These further support the suggestion that species [C] is a “Rh6(CO)12(p2-C0)4n cluster in N a y , although Rh6(C0)12(p2-CO)4 has not been synthesized in solution or isolated yet.
+
1 ;
0.7 2
ii 0.5
e ce c
,z
10.4 0
0.2 0.3 0.4 0.5 0.6 Three centered bridged-CO, (+AA17601
0.1
Figure 7. (A) IR spectral change of CO,,.,-Rh,/NaY during heating in CO + H2( 1 : l ) at 300-420 K. (B) Intensity dependency of the IR band of p,-CO on pz-CO.
single-crystal surface, the desorption temperature of CO(ad) was higher than that of H(ad) in TPD experiments. Therefore, the higher the binding energy of adsorbate-Rh, the more the Rh-Rh bond relaxed. When C O molecules interact with the reduced Rh particles, the electrons of Rh-Rh orbital may partially delocalize due to the formation of the new bonds Rh-Rh-CO,. This results in an expansion of the Rh-Rh bond to the distance of a new carbonyl cluster. The N(Rh-Rh) in [Rh,],,d/NaY is not appreciably changed after CO chemisorption. The CO-induced fragmentation does not proceed on [Rh61rd/NaY in C O chemisorption, in contrast to the results of Bergeret et a1.16 on Rh/NaY and Van’t Blik et al.” on Rh/AI2O3. The latter authors have found that the amplitude of the EXAFS oscillation typical for the Rh-Rh metal coordination greatly decreased on C O adsorption on the highly dispersed Rh/AI2O3 after H 2 reduction. This leads to a complete cleavage of Rh-Rh bond, ultimately to form Rh+(C0)2. What is the reason for such a different behavior of small Rh particles between in N a Y and on A1203? The answer seems to be the different acidity (or oxidation ability) of O H groups on these two supports. Another evidence of their different oxidation ability is that intrazeohte R h 6 ( C 0 ) , 6 is stable in 0, while A1203-supported Rh6(CO),, is unstable in o2(see section 3.1). 3.5. Conversion of [qinto [A]. [Rh61rd/NaY sample has been treated in a H 2 + C O flow at 300-423 K, and in situ IR spectra have been recorded. After admission of H2 + C O at 300 K, the IR spectrum is almost the same as that after mere C O chemisorption (see Figures 7A and 5b). Neither new bands nor (34) Castner, D. G.; Sexton, B. A.; Somorjai, G. A. Surf. Sci. 1978, 71, 519.
+
+
+
(35) Malatesta, L.; Caglio, G.; Angoletta, M. Chem. Commun. 1970, 532. ( 3 6 ) Garlaschelli, L.; Martinengo, S.;Bellon. P. L.; Demartin, F.; MaBau, R. J . Am. Chem. SOC.1984, nasseno, M.; Chaing, M. y.;Wei, c.-y.; 106. 6664.
5324
Rao et al.
The Journal of Physical Chemistry, Vol. 94, No. 13. 1990
CHART I
F
r
1
r
unsupported Irs(C0)1~ (p3 - COLI
?,.
? C
>Rh-Rh< t(p3-CO)=1770cm-'(ref 36)
'
t(p~-C0)=1760cm-'
I
O
j 0
Cl
SA0
20 40 6b Equilibrium pressure (Torr)
:
Figure 9. Adsorption isotherms of (a) C O and (b) H, on [Rh61rd/NaY at 300 K. A is the first isotherm while B is the second after evacuating the sample to about lo4 Torr subsequent to the completion of A. The difference isotherm (A) - (B) is obtained by substracting B from A and represents the strongly chemisorbed fraction.
TABLE V: Parameters of Adsorption Equation" for CO and H2 Adsorption on IRh,ld/NaY at 300 K C O isotherm
0.01 12 0.0067
Yn(CO/Rh) 2.67 0.1 1
0.0045
2.56
(Y
first second diff
-
H, isotherm LY
0.0070 0.0030 0.0039
YdHIRh) 1.42 0.64
0.78
-
"Adsorption equation: Y = Yo+ aP (P in Torr). Y and Yo: ratios of CO/Rh or H / R h at P and Po (P 0), respectively.
The assignment of the 1830-cm-' band has not been well characterized in previous literatures. On the basis of IR bands of Rh,(CO),2/A1203 (2068, 1840 cm-I) and R h 4 ( C 0 ) 1 2 / S i 0 2 (2076, 2046, and 1881-1870 cm-l), Rode et aL9 attributed bands at 2086 and 1834 cm-l to Rh4(CO)12/NaY. Takahashi et al.37 assigned bands at 2092 and 1834 cm-' to the carbonyls of Rh6(CO),, on the external surface of N a y . The v(COb) in Rh4(C0),, and Rh6(CO)I,(p,-CO)4 is expected to be very close since their chemical environment is very similar and v(C0,) in Rh6(CO)12(p2-CO)4 may be higher than that in Rh4(C0)!* since v(C0,) generally increases with the number of Rh atoms in the cluster.3* On the basis of results of EXAFS and IR studies, we assign the bands at 2092 and 1830 cm-l to the terminal and edge-bridging C O Of Rh6(CO)12(~2-C0)4. In conclusion, [Rh6Jrdderived from Rh,(C0)16/NaY by oxidation and reduction still retains the Rh cluster framework and is converted into the unstable carbonyl cluster Rh6(CO)12(p2-CO)4 on C O chemisorption at 300 K. It is thermally transformed into the stable isomer Rh6(C0)12(p'3-C0)4 (see eq 4). [Rh6/NaYIied
+ CO
=OK --+
Rhs(COln)(p2-C0)4/NaY+ Rh'(C0h
,1 420 K
Rh6
( c o )(~p ~3- C o ) q / NoY
Cot Hz or CO
(4)
3.6. CO and H2 Adsorption Isotherms of [Rh,ld/NaY. Figure 9 shows CO and H2 adsorption isotherms at 300 K and 5-80 Torr. C O / R h and H / R h ratios present the number of adsorbed C O molecules and hydrogen atoms on per Rh atom, respectively. CO/Rh6 and H/Rh6 are those adsorbed on a cluster of six Rh atoms. For both CO and H2 adsorptions, the first and second isotherms are linear over the range 5-80 Torr. They are fitted to linear adsorption equations: Y = Yo+aP
where Y and Yo are the ratios of C O / R h or H / R h at pressure P and Po ( P 0), and (Y is a constant. Table V presents the parameters Yo and a calculated by the least-squares method. Two methods are commonly used to obtain the chemisorption ratio of C O / M and H / M . Method 1 assumes that extrapolation of the first isotherm to Po gives the chemisorbed amount^,^^-^' i.e., Yoof the first isotherm. On the other hand, method 2 assumes that Yoof the first isotherm is the sum of the gas adsorbed weakly on the support and strongly on the metal. The Yoof the difference isotherm is the amount strongly adsorbed on metal.39 Here C O / R h (2.6) is determined by method 2 and H / R h (1.4) by method 1, as was done by the same methods reported earlier by Tauster et al.39and Garten and S i n f e l ~ ~ ~ The CO/Rh ratio on [Rh61d/NaY is 2.6, which is almost equal to the stoichiometric CO/Rh ratio (16/6 = 2.7) in the Rh6(C0)16 cluster. The adsorbed CO/Rh ratio estimated from the EXAFS analysis of COad-Rh6/NaY is N(Rh-CO,) 0.5N(Rh-COb) = 1.4 1.4/2 = 2.1, which agrees with that determined by adsorption isotherms within an error of 19%. The H/Rh ratio has been reported to vary for Rh of different particle sizes and support^.^' The smaller the Rh particles, the higher the H / R h ratio. A ratio of 1.5-2 was found on atomically dispersed Rh/A1203. On Rh/SiOz the ratio did not exceeded 1. On Rh black H/Rh was equal to 1. The H / R h ratio of 1.4 on [Rh61rd/NaY suggests that the Rh is highly dispersed. It is clear from the C O and H, isotherms that about 40% of the total adsorbed H2 is removed by evacuation at 300 K, while only 4% of adsorbed CO is removed. Hence, hydrogen adsorption is weaker than CO adsorption. This is in consistent with the IR study on CO + H, coadsorption (section 3.5). In a previous communication,13we reported that [Rh61rd/NaY leads to higher selectivity toward C2-C4 alkenes in C O hydrogenation than conventional R h / S i 0 2 catalysts. That hydrogen adsorption on highly dispersed Rh particles is weaker than C O might be a reason [Rh,],&/NaY gives higher selectivity for unsaturated hydrocarbons. 3.7. Intermolecular Carbonyl Scrambling between Intrazeolite Rh6(CO)12(p3-C0)4and Rh+(CO),. Exchange Reaction of [ A ] and [ B ] with I3CO(g). Different carbonyl activities for the coexisting two species [A] and [B] in isotopic exchange reaction with 13CO(g) have been observed. With exposure of Rh6( C 0 ) 1 6 / N a Y to 10 Torr of l3C0(g) at temperatures as low as 170 K , only the carbonyls of the minor species Rh+(CO), ex-
+
+
(5) (39) Tauster, S. J.; Fung, S. C.; Garten, R. L. J . Am. Chem. SOC.1978,
100, 170.
(37) Takahashi, N.; Mijin, A,; Ishikawa, T.; Nebuka, K.; Suematsu, H . J . Chem. SOC.,Faraday Trans. I 1987, 83, 2605. (38) Hanlan, L. A,: Orin, G. A . .r. Am. Chem. SOC.1974, 96, 6324.
(40) Garten, R. L.; Sinfelt, J. H. J . Catal. 1980, 62, 127. (41) Scholten, J. J. F.; Pijpers, A. P.; Hustings, A. M. L. Carol. Rev.-Sci. Engl. 1985, 27, 15 I . and references therein
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5325
TABLE VI: Frequencies and Force Constants for the Carbonyl of the Local Species Rh(CO)2 (C,) in RbJCO),dNaY twin carbonyl ligands local C, Y , ~ ~ , v , , ~ ~ . kl,' k2,b cluster/NaY species em-' cm-I N m-I N m-I 2098 2066 1751 26.9 Rh6(C0)16 CO-Rh-CO Rh6(13C0)16 "CO-Rh-13C0 2048 2022 1750 22.4 Rh6(C0)16,(13CO), CO-Rh-'3C0 2085 2031 1750 24.7 (theor calc) Rh6(CO)l~,(13CO), CO-Rh-I'CO 2085 (exptl) kl: force constant of the C-O bond. * k7: force constant of COCO coupling.
0-30Omin
0-p
I 2100
2000 I800 Wavenumber / c m - l
1600
c P,I
0
= 4
.-E
2
I
A"
0 . 810-3 ~ b/,,,In
I
0 ;
b
IOtorr
-A
r-$+-P
I
IO0 200 Exchange timelmin
I
,
1
2100
300
1
1
2000
z I;;]
,
1
I800
1600
Wavenumber /cm-'
Figure 10. (a) IR spectral change of intrazeolite Rh6(C0)16/NaY during isotopic exchange with "CO(g) at 350 K (intensity of the carbonyl bands due to mononuclear of Rh+(C0)2/NaY kept constant). (b) Time dependence of band intensity for AI7m and A*l72t3. A1760 and A*l7n, are the maximum absorbances of the bands at 1760 and 1720 cm-l, respectively.
3
1.01
changed readily with I3CO(g) (see section 3.1 and Figure la-c), i.e., eq 6. With increase of the temperature to 350 K, only the Rh+(C0)2/NaY
+ 213CO(g) & Rh+('3C0)2/NaY
Rh6(C0)16/NaY
+ 2CO(g)
(6)
+ 16I3C0(g)& Rh6(lSC0)16/NaY 16CO(g) (7)
carbonyls of species [A] exchanged (Figure lo), Le., eq 7. Complete exchange was achieved by increasing temperature or exchange time. The resulting Rh6(13C0),6/NaYreexchanges with I2CO(g) at T > 363 K, giving the original IR carbonyl spectrum as shown in Figure l a . This suggests no by-reaction occurred during the exchange process. As shown in Figure 10a the spectra systematically shift from the Rh6(C0)16/NaYcase to the Rh6(I3CO),6/NaY case. As the intensity of the 2098- and 1760-cm-I bands decreases gradually, new bands at 2048 and 1720 cm-l increase. In the u(COI) region, the peak pair of Rh(CO), (one of the six local species of Rh6(CO)16/NaYcontains the twin COI!igands) at 2098 s and 2066 w cm-I continuously disappears, while the Rh(13C0)2peak pair at 2048 s and 2022 w continuously appears as the reaction progress. The center of the 2098-cm-' band shifts to lower frequency continuously, and a new band at 2085 cm-l appears and increases at first and then decreases, which is evidence for the presence of the Rh(CO)(I3CO) C,,species in partially isotopic exchanged Rh,(CO) I,+,( I3CO),/NaY. Following the beautiful analysis of IR spectra for Rh+(COk on S O 2or A1203done by Knozinger et al.42and Yates et al.,
0 Inter-exchange time /min
Figure 11. (a) IR spectral change of Rh6(CO),6/NaY sample during the
intermolecular carbonyl scrambling between NaY-entrapped cluster and mononuclear Rh+(C0)2/NaY. (b) Time Rh6(CO)12(r3-CO),/NaY dependence of IR band intensities for ,417607 A*,720r B2022, and B*1975. Al7& A*l720, B2~2~, and B*l975 are the maximum absorbances of bands at 1760, 1720, 2022, and 1975 cm-I, respectively. we have calculated the values of k , (force constant of the C - 0 bond) and k2 (force constant of CO-CO coupling) from the observed symmetric and asymmetric wave numbers of Rh(CO), local species. Table VI presents the calculated k l and k2 for Rh(CO), and Rh(13C0), two species. The agreement between k , and k , for these two independent measurements is excellent. It is then possible to calculate vSymand vaSymfor Rh(CO)(I3CO) local species. They are found to be 2085 and 2031 cm-I. The symmetric band at 2085 cm-l is observable in the spectra of Figure loa. The asymmetric band at 2031 cm-' is not resolved here due (42) Knozinger, H.; Thornton, E. W.; Wolf, M. J . Chem. SOC.Faraday Trans. 1 1979, 75, 1888. (43) Yates, Jr., J. T.; Kolasinski, K. J . Chem. Phys. 1983, 79, 1026.
Rao et al.
5326 The Journal of Physical Chemistry, Vol. 94, No. I S , 1990 RhC13+ NaY
--+
Rh3+-NaY
T>330 K 13~0
Rh6(I3C0)16
‘2co
gem-dicarbonyls
/ \L
Jco
face-bridging tco=2098, 2066, Rh-Rh: C.N.=3.1
co co co
?
x
OZ OH2
gem-dicarbonyls b,=2114,2048; 2 0 9 8 , 2022 cm-l
q”c>/r-
v
edge- bridging Rh&0)16 ~ ~ 0 ’ 2 0 9 22, 0 7 2 , 2 0 6 0 , 1 8 3 0 ~ m - ~ Rh-Rh : C. N. = 3 . 2 , R = 2 . 7 2 %
“2
473/673 K
Rh-0 : C. N . = 6 . 8 R= 2.068
spherical, compact particle Rh-Rh: C . N. =4.6, R = 2.70%
Figure 12. Proposed surface chemistry of Rh,(CO),,/NaY under the treatment of oxidation-reduction and CO chemisorption.
to its weak absorbance. Thereby the shift of the 2098 cm-I band in Figure 10a is originate from the partially isotopic exchanged Rh6(CO)l6-,( i3CO),/NaY. Figure 10b shows the time dependence of the intensity of the 2098- and 1760-cm-‘ bands. The exchange of CO, and CO, ligands proceeds at about the same extent. Thus the intensity change of both the CO, and COf bands can express the rate of reaction 7. The initial exchange rate, r7, estimated from the slope 0, is about (4.1 f 0.8) X IO-jAlmin of the A*i720 curve at t at 350 K, where A is the absorbance. The rate of (6), r6,is larger than IO-lA/min at 170-350 K. The different activity in exchange reaction of these two carbonyl species has also been reported by Takahashi et al.37on Rh/NaY. This behavior is also very similar to that of Rh/A120j, where the carbonyls of Rh+(CO)* are much more active than C O bound to Rh: particles as reported by Yates et al.24 Intermolecular Carbonyl Scrambling between [A] and [ B ] . [A] and [B] coexist on a sample of Rh6(C0)i6/NaY. How do they interact? For the first time we have obtained direct spectroscopic evidence of intermolecular carbonyl scrambling between [A] and [B].A sample with labeled Rh6(i’C0)12(llj-i3C0)4 ([A*]) and unlabeled Rh+(C0)2 ([B]) was prepared by exposure of Rh6( ’ 3 C 0 ) i 6 / N a Yto I2CO(g) at 300 K. After removal of CO(g) by evacuating the IR cell to Torr at 300 K, the temperature was increased to 350 K and the intensity change of the IR bands was measured. The spectra shift from the Rh6(i3CO)iz(p3-CO)4 and R h + ( C 0 ) 2 case to the R h 6 ( C O ) i z ( ~ j - C O ) 4 / N a Yand Rh+(i3C0)2/NaYcase. In Figure 1 la, the four carbonyl bands of [B] showed their red shifts due to the isotopic effect with simultaneous blue shifts of the three bands of [A*]. We have used the peak heights of well-resolved bands at 1760, 1720, 2022, and 1975 cm-‘ to represent the relative abundance of species [A], [A*], [B], and [B*] (Rh+(I3CO),), respectively. Figure 1 I b shows the time dependence of the intensities for these
-
four species. Here it can be clearly seen that as [A*] and [B] are consumed, [A] and [B*] are produced. This strongly suggests the reaction A * + B A + B* (eq 8) has occurred. The initial
-
Rh6(i3CO)i6 + (x/2)Rh+(CO), Rh6( l3c0) i6-,( c o ) , 4- (x/2)Rhf( I3C0)2 (8) interexchange rate, f8, based on the 1720-cm-’ band is (3.7 f 0.4) X 10-3A/min at 350 K. By using the isotopic and IR technique, we have proved the intermolecular carbonyl scrambling between the two coexisting carbonyl species of Rh6(CO) , 2 ( ~ j - C O ) 4and mononuclear Rh+(CO)*. The reaction rate of (8) is high enough to be measured by IR technique at 350 K. As suggested in section 3.1, [A] and [B] coexist in NaY in a molar ratio of about 1. We propose that the intermolecular carbonyl scrambling reaction proceeds through an unstable hypothetical carbonyl species such as “ [ R ~ . I ( C ~ ) ~ which ~ ] + ” may , be formed inside NaY zeolite, as shown in the following equilibrium: Rh+(C0)2 + Rh6(”C0),6 + Rh+(’’C0)2
+
[Rh7(i3CO)16(CO)2]+
+ Rh6(’3cO)14(CO)2
(9)
The cationic carbonyl cluster [Rh,(CO),,]+ may be accommodated with the negatively charged zeolite wall. The gem-dicarbonyl may exist near by Rh6(CO),, cluster, or eke it is highly mobile through the zeolite channels. It has been reported that in MeCN solution partially decarbonylated [ R ~ ( C O ) , , ] ”reacts readily with [Rh(CO)2(MeCN)2]+ to give [Rh7(CO),6]3-according to eq The carbonyls of (44) Martinengo, S.; Fumagalli, A.; Chini, P. J . Organomet. Chem. 198$, 184 215
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5321
NaY -Entrapped Rh6(c o ) 16 Cluster this capped Rh7 anion cluster shows fast site exchange on the N M R time scale.4s
-
+
[Rh6(CO),4]4- [Rh(CO),(MeCN),]+ [Rh7(CO)1613- 2MeCN (10)
+
Mechanism of the Carbonyl Exchange Reaction between Rh6(CO)12(pJ-C0)4 and 13CO(g). The carbonyl desorption rates of species [A] and [B] have been measured. They are lower than the rates of the isotopic exchange reactions r, and r6, respectively. Therefore, the exchange reactions of (6) and (7) are not determined by the carbonyl desorption step and/or the carbonyl desorption is not involved in the exchange reaction. Comparison of r7 with r8 leads us to prediction that reaction 8 is the ratedetermining step of reaction 7. We suggest that the facile exchange of Rh+(C0)2with CO(g) proceeds through a tricarbonyl intermediate, Rh+(CO),, which has been previously proposed by for R h + ( C 0 ) 2on A1203. In contrast to this, the exchange of Rh6(C0)12(p3-C0)4/NaYwith CO(g) proceeds through the intermolecular carbonyl scrambling between the inactive Rh6(CO)12(pJ-CO)4and the active Rh+(C0)2. The coexisting Rh+(C0)2 acts as a catalyst that facilitates the gas-phase C O exchange with Rh6(C0)16inside N a y . Previously, we reported that Rh6(C0)16/NaYis a good catalyst for a CO-based reaction such as C O hydr~genationl~ and alkene hydr0formy1ation.l~The coexisting Rh+(C0)2 might be a promoter to transfer gas-phase co into the Rh6 site on which the catalytic reaction is occurring. NaY zeolite helps the isolated Rh3+ ions, which reside in the sodalite cages and the hexagonal prism of NaY formed during ion-exchange procedures, to migrate to the supercage under the carbonylation with C O H 2 0 or C O H 2 up to 423 K to build the larger clusters containing several Rh atoms up to a hexanuclear cluster framework. Another example for metal ions migrating through the channels of zeolite by the induce of CO(g) is that reported by Olivier et a1.,46in which Ni2+ ions in NiCaX migrates from sodalite cages to supercages in the presence of CO(g) to form mononuclear Ni(CO), species. We suggest that the driven force for the migration of metal ions to form metal clusters inside the zeolite is based on a great decrease in the free energy of a chemical system by formation of a stable carbonyl cluster. The formation entropy of Rh6(CO)16is as large as H = -2299 f 28 kJ/mol, which considerably reduces the free energy of the Rh-CO-zeolite system. Nevertheless, direct synthesis of Rh6(CO)12(p3-CO)4from dry RhCI, + C O in the presence of Ag or Cu catalyst requires ~ ~use of a proper solvent high C O pressure (ca. 200 a t ~ n ) . By like C H 3 0 H and the base O H - / H 2 0 , Rh6(C0)16cluster was prepared in 80-90% yield under milder conditions (333 K/50 a t ~ n ) . It ~ ~is interesting to find that N a Y zeolite favors the
+
+
(45) Brown, C.; Heaton, B. T.; Longhetti, L.; Smith, D. 0.; Chini, P.; Martinengo, S.J . Organomer. Chem. 1979, 169,309. (46) Olivier, D.: Richard, M.; Che, M. Chem. Phys. Left. 1978, 60, 77. (47)Corey, E. R.;Dahl, L. F.; Beck, W.J . Am. Chem. SOC.1963, 85, 1202.
formation of Rh6(CO)12(pJ-CO)4compared to the synthesis in solution. In this sense, NaY is a better solid “solvent” to synthesize Rh6(CO)12(p3-C0)4than organic solvent such as C H 3 0 H . However under similar conditions, Rh6(C0)12(p3-C0)4was hardly prepared in NaX zeolite, whose SiO2/AI20ratio is lower than N a y , but only Rh+(C0)2 was found.9 Since the geometric pore structure of NaX is equivalent to that of N a y , the favorable formation of Rh6(C0)12(p3-C0)4 in N a Y is due to not only the proper cage size but also the proper intrazeolite circumstance (for example, proper acidity) of N a y . The acidity of NaX may be too strong to form Rh6(C0)16cluster in its cage. On the other hand, NaY zeolite prevents aggregation of the already formed Rh6 clusters in its cages as well as the fragmentation of Rh6 particles during adsorption of CO. 4. Conclusion
The different carbonyl species are characterized by EXAFS, FTIR, and 13C0 FTIR studies as summarized in Figure 12 to depict the chemistry of zeolite entrapped Rh6(C0)16and Rh+(CO),. NaY zeolite entrapped Rh6(CO)12(p~-C0)4 cluster has been synthesized from RhCI, ion-exchanged N a y . About 8 0 4 5 % of Rh is transformed into the cluster along with mononuclear Rh+(C0)2 (20-15%) as revealed by IR and EXAFS analysis. The coordination of Rh atoms including Rh-Rh, Rh-COI, and Rh-COf bonds resembles that of crystalline Rh6(C0)16except that Rh-O, (zeolite) bond is formed due to the partial formation of Rh+( C O ) Z ( O ,and ) ~ Rh+(cO)AO,)(H20). The treatments of oxidation followed by reduction of Rh6(CO)16/NaYresult in small Rh particles showing weak interaction with zeolite oxygen. The coordination of Rh-Rh, Rh-COI, and Rh-COb bonds in COad-Rh6/NaY resembles that in Rh6(CO),,/NaY. However, the former consists of edge-bridging p2-C0 and the latter contains face-bridging pJ-CO. The CO adsorption on [Rh,],.&kY leads t o t h e formation of novel cluster “edge-bridging Rh6(C0)12(p2-CO)4”inside N a y . The carbonyl of Rh6(C0),2(p2-C0)4/NaYis more active toward isotopic exchange with CO(g) than is Rh6(CO)12(p3-CO)4. It is unstable and converted into the stable Rh6(CO)12(p3-CO)4 in a C O or C O H2 atmosphere at temperatures above 420 K. C O chemisorption on [Rh61rd/NaY gives a C O / R h ratio of 2.6, which is similar to the stoichiometry of Rh6(C0)16. The carbonyls of intrazeolite Rh6(CO)12(p3-Co)4are interscrambling with the carbonyls of the coexisting Rh+(C0)2. The isotopic exchange of Rh6(CO)12(p3-CO)4 with c o ( g ) is suggested to occur through the intermolecular carbonyl scrambling with Rh+(C0)2, which is more active for the exchange with CO(g).
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Acknowledgment. Part of this work was financially supported by a Grant-in-Aid for Scientific Research (No. 62470069) from the Ministry of Education, Science and Culture, Japan. (48)Chaston, S . H. H.; Stone, F. G. A. J . Chem. SOC.,Chem. Commun. 1967, 964.
