Surface Structures and Catalytic Hydroformylation Activities of Rh

Aug 8, 1996 - Stable Rhodium Pair Sites on MgO: Influence of Ligands and Rhodium Nuclearity on Catalysis of Ethylene Hydrogenation and H–D Exchange ...
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13636

J. Phys. Chem. 1996, 100, 13636-13645

Surface Structures and Catalytic Hydroformylation Activities of Rh Dimers Attached on Various Inorganic Oxide Supports Kyoko Kitamura Bando,†,⊥ Kiyotaka Asakura,‡ Hironori Arakawa,§ Kiyoshi Isobe,| and Yasuhiro Iwasawa*,† Department of Chemistry, Graduate School of Science, UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan; Research Center for Spectrochemistry, Faculty of Science, UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan; National Institute for Material and Chemical Research, Higashi, Tsukuba, Ibaraki 305, Japan; and Department of Material Science, Faculty of Science, Osaka City UniVersity, Sugimoto, Sumiyoshi, Osaka 558, Japan ReceiVed: October 23, 1995; In Final Form: May 28, 1996X

trans-[(RhCp*CH3)2(µ-CH2)2] (Cp* ) pentamethylcyclopentadienyl) was chemically attached to inorganic oxides such as SiO2, Al2O3, TiO2, and MgO to prepare Rh dimer catalysts. The catalytic activities of the obtained supported Rh dimers for ethene hydroformylation depended on the kinds of supports. The SiO2attached Rh dimers showed highest activity and selectivity for the hydroformylation. Surface structure and behavior of the Rh sites during CO insertion process were examined by in situ FT-IR and EXAFS techniques. Similarly to the previous observation that Rh-Rh metal bonding in the Rh dimers on SiO2 alternately broken and formed in the course of the catalytic hydroformylation, Rh dimers on TiO2 showed a reversible bond break formation, but CO-bridged Rh dimers inactive for the hydroformylation were produced unlike the case of Rh dimer/SiO2. On Al2O3 most of Rh dimers were degraded to monomers which exhibited no activity for the hydroformylation. The Rh dimers on MgO were aggregated to metal clusters. These structural behaviors of Rh sites are discussed in terms of the interaction between the Rh dimer and the support.

1. Introduction In supported metal catalyst systems, reaction mechanisms generally involve more than one metal atom at catalyst surfaces. Ethene hydroformylation on supported Rh catalysts shows a maximum activity with an optimum particle size, suggesting contribution of metal ensemble to the catalytic performance, while Rh catalysis in homogeneous systems where alkene hydroformylation proceeds on mononuclear Rh complexes. It may be of importance to know the role of metal-metal bonding in metal catalysis. The role of the adjacent metal atom in catalytic reactions has also been reported in homogenous systems,1,2 though the homogeneous systems often deal with irreversible reactions, because irreversible decomposition of the complex occurs. To stabilize the metal centers to work catalytically and to prepare new active metal structures, there have been many attempts to attach di- and multinuclear metal complexes on oxide supports.3,4 trans-[(RhCp*CH3)2(µ-CH2)2] (1) (Cp* ) pentamethylcyclopentadienyl) has CH3 and µ-CH2 ligands which are regarded as important intermediates for C-C bond chain growth. From this point of view, the reactivity of complex 1 has been intensively studied in homogeneous systems.1 We succeeded in attaching complex 1 to SiO2 surface with retention of the Rh-Rh bond, which were characterized by EXAFS and FTIR.5 The obtained Rh dimer catalyst showed efficient catalytic performance for the catalytic hydroformylation of ethene. It was found that Rh-Rh bonds in the attached Rh dimers were reversibly broken by CO adsorption and reformed by subsequent * To whom correspondence should be addressed. † Department of Chemistry, University of Tokyo. ‡ Research Center for Spectrochemistry, University of Tokyo. § National Institute for Material and Chemical Research. | Osaka City University. ⊥ Present address: Institute for Research and Innovation, Takada, Kashiwa, Chiba 277, Japan. X Abstract published in AdVance ACS Abstracts, July 15, 1996.

S0022-3654(95)03124-8 CCC: $12.00

CO insertion during the course of the hydroformylation, demonstrating an important role of the adjacent Rh atoms in the CO insertion (ethyl migration) reaction. In this paper we have extended the above work5 to other oxide supports to examine metal support interaction or support effects on metal catalysis. TiO2, Al2O3, and MgO were used as supports, which are representative among oxide supports. As already reported, supported Ru catalysts derived from Ru3(CO)126 and [Ru6C(CO)16(CH3)]- 7 showed different structures on these supports, which caused the unique catalytic behaviors. The purpose of this work is to obtain information on essential factors which control surface structure and catalytic activity of Rh dimers chemically attached on the oxide supports. 2. Experimental Section trans-[(RhCp*CH3)2(µ-CH2)2] (1) was synthesized according to the literature.1 Two kinds of deuterium-labeled complexes, trans-[(Rh(C5(CD3)5)(CH3))2(µ-CH2)2] (2) and trans-[(Rh(C5(CD3)5)(CD3))2(µ-CH2)2] (3), were also synthesized for IR study. SiO2 (Aerosil 300, surface area ∼300 m2/g, Davison No. 57, surface area ∼280 m2/g), TiO2 (Aerosil titanium oxide P-25, surface area ∼50 m2/g) , Al2O3 (Aerosil aluminum oxide C, surface area ∼100 m2/g), and MgO (Ube High Purity 100A, surface area 100-170 m2/g) were used as supports. The inorganic oxides were pretreated in situ before use for the attachment of the Rh dimer complex; SiO2 was treated at 673 K for 1 h in vacuum to control the concentration of the surface OH groups at about 2 OH nm-2,3 TiO2 was treated at 673K for 1 h (1∼2 OH nm-2),3 Al2O3 was treated at 473K for 1 h (8∼10 OH nm-2),3 MgO was treated at 673K for 2 h (6∼8 OH nm-2).3 The loading of Rh was regulated to be 1-2 wt %. Thus the surface concentration of the Rh dimer was estimated to be 0.10.2 dimers nm-2 for SiO2, 0.6-1.2 dimers nm-2 for TiO2, 0.30.6 dimers nm-2 for Al2O3, and 0.2-0.6 dimers nm-2 for MgO, on average. © 1996 American Chemical Society

Surface Structures of Rh Dimers

J. Phys. Chem., Vol. 100, No. 32, 1996 13637

TABLE 1: Curve-Fitting Analysis for the EXAFS Data of Reference Rh Complexes by Use of Theoretical Parameters5 distance/nm sample

bond

CN

EXAFS

X-ray

Rh metal Rh2O3 Rh2(CO)4Cl2

Rh-Rh Ru-O Rh-C Rh-Cl Rh---O

12 6 2 2 2

0.266 ( 0.002 0.203 ( 0.003 0.183 ( 0.002 0.233 ( 0.003 0.298 ( 0.002

0.268 0.205 0.181 0.2355 0.298

Incipient attached Rh dimers were prepared by reaction of trans-[(RhCp*CH3)2(µ-CH2)2] (1) with the surface OH groups of oxide supports in pentane or deuterium-labeled benzene at ambient temperature under high-purity inert gas (99.9999% He or 99.9999% Ar). Handling of the sample in the inert gas was necessary because the obtained attached Rh dimer species were air sensitive unlike the original Rh dimer complex (1). Pentane used as the solvent for the complex 1 was purified before use by distillation over Na wire or CaH2. The solvent was removed by evacuation or under a flow of high-purity He. Commercially available deuterium-labeled benzene (Aldrich, 99.5 atom % D) was also used as the solvent without further purification for IR study. FT-IR spectra were measured by JEOL JIR-100 spectrometer. trans-[(Rh(C5(CD3)5)(CH3))2(µ-CH2)2] (2) and trans-[(Rh(C5(CD3)5)(CD3))2(µ-CH2)2] (3) were employed to separate the IR absorption peaks for methyl and methylene ligands from those of the pentamethylcyclopentadienyl (Cp*) ligand in the ν(CH) region. The oxide powder was pressed into a form of a disk (60 mg/20 mm diameter), and was pretreated in an IR cell. Then the dimer solution was added dropwise to the disk under the flow of high-purity He. The IR cell used in this work was a stainless steel IR cell connected to a flow system. Rh K-edge EXAFS measurements were conducted under in situ conditions in a transmission mode at BL 10B of Photon Factory in the National Laboratory for High-Energy Physics (KEK-PF) with a Si(311) channel-cut monochromator. The estimated energy resolution is 7 eV. The spectra were measured at room temperature. To measure the in situ EXAFS spectra, we used a Pyrex glass EXAFS cell with two thin glass windows in which no Rh impurity was detected. The in situ EXAFS cell was connected to a closed circulating system through a stop valve. The following formula was used for the curve-fitting analysis of EXAFS data8,9

χ(k′j) ) ∑SjNjFj(k′j) exp(-2k′2jσj2) sin(2k′jrj + φj(k′j))/k′jrj2 (1) k′j ) xk2 - 2m∆Ej/p where k is the photoelectron wave number, F(k) is the back scattering amplitude function, and φ(k) is the phase shift function. For φ(k) and F(k), either theoretical or empirical form was chosen depending on the kind of system. Model compounds for Rh-Rh and Rh---O (carbonyl) were Rh metal and [Rh(CO)2]2(µ-Cl)2, respectively. Theoretical parameters determined by Teo and Lee were used for the analysis of the first shell.9,10 These theoretical parameters are not recommended by the international XAFS committee now. So we carefully checked the validity of these parameters using reference compounds as shown in Table 1.5 The interatomic distances could be determined within the error of 0.003 nm. Thus, the theoretical parameters derived from Teo and Lee did not seem to cause serious problems in our system. However, we will only discuss the change of the structure but not the absolute structure for safety. Moreover, we have paid much attention to the independent number of fitting parameters which will be

discussed later. Sj in eq 1 is the amplitude reduction parameter that arises from many-body effects and inelastic losses in the scattering process. This value can be regarded as independent of k because the two factors have an opposite k dependency.10 Sj for each scatterer was determined by the model compound when a theoretical amplitude function was used. Sj is unity when an empirically derived amplitude function is used. Nj, σj, and rj are coordination number, Debye-Waller factor, and interatomic distance, respectively. The fitting parameters are usually Nj, rj, ∆Ej (difference between the origin of the photoelectron wave vector and that conventionally determined), and σj. Nidp (independent number of fitting parameters) given by 2∆k∆r/π + 211 is nearly 8 in this work. We used four independent parameters for one-shell fitting. But we reduced them to half by fixing ∆E and σ to the values of reference compounds when we carried out two- and three-shell fitting. Because structures derived from EXAFS are reduced to be of one dimension from three-dimensional real structures, the curve fitting often converges in completely different structures from the real ones at the same level of fitting accuracy. Therefore, in this work, we first gave the possible structures from TPD and IR data which could provide quantitative information about ligands and homogeneity of a sample. Based on these inputs, we conducted the curve-fitting analysis to determine bond distances and coordination numbers. The errors were estimated according to the Report on Standards and Criteria in XAFS Spectroscopy.12 We carried out two- or threeshell fitting on a single Fourier transform peak because other techniques indicate the presence of two or more bondings and because we did not fit well with one shell. In these cases, due to the correlation problem between fitting parameters, the determined parameters had a little larger error bars. We additionally compared the fitting results such as bond length with those of the compounds with similar structures in the literature and confirmed that the determined structures were chemically reasonable. By doing the procedure of EXAFS analysis explained above, we could deduce a surface structure with less ambiguity. The catalytic hydroformylation reaction was conducted in a closed circulating system. The products were collected by a dry ice-acetone trap (179 K) and detected by a gas chromatograph using VZ-10 and DOS columns. VZ-10 column was used for the detection of ethane untrapped and DOS column was used for the detection of oxo compounds collected in the trap. The components of reactants were CO:C2H4:H2 ) 1:1:1 (total pressure ) 40.0 kPa) and CO:C2H4:H2 ) 0.2:1:1 (total pressure ) 33.3 kPa). An impregnated Rh/SiO2 catalyst with 30% dispersion conventionally prepared from RhCl3 was used as reference. It showed the highest activity among series of impregnation catalysts for propanal formation in ethene hydroformylation. The impregnation Rh/SiO2 catalyst was calcined at 637 K for 1 h followed by reduction with H2 at 773 K for 1 h. The reactivity was compared by the turnover frequencies (product molecules/Rh atom/min). The number of active Rh sites in attached Rh dimer catalysts was assumed to be equal to the number of the Rh dimers deposited. 3. Results and Discussion 3.1. Catalytic Hydroformylation of Ethene. In general, Rh catalysts are known to have high activity for CO insertion (alkyl migration). Alkene hydroformylation reaction can be sometimes regarded as a reaction to reflect the ability of CO insertion for metal catalysts. We conducted hydroformylation of ethene by the attached Rh dimer catalysts prepared by using various supports. First the reaction was carried out in a closed

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TABLE 2: TOF and Selectivity of Ethene Hydroformylation at 413 Ka TOF/10-4 min-1 catalyst

total

ethane

propanal

selectivity (%)

impreg Rh/SiO2 Rh2/SiO2 Rh2/TiO2 Rh2/Al2O3 Rh2/MgO

22.8 36.9 6.3 6.8 56

21.5 4.1 6.3 6.8 56

1.3 32.8

5.6 88.9

a

CO:H2:C2H4 ) 1:1:1; total pressure ) 40.0 kPa.

circulating system (CO:H2:C2H4 ) 1:1:1, total pressure ) 40.0 kPa). The results obtained for the attached Rh dimer catalysts and a conventional SiO2-supported Rh catalyst are shown in Table 2. The Rh dimer catalysts were all inactive except for the SiO2-attached Rh dimer catalyst (Rh2/SiO2), which exhibited much higher activity and selectivity for propanal formation than the impregnated Rh/SiO2 catalyst.5 This result is opposite to the supported Rh clusters catalysts which showed higher activity when Rh4(CO)12 and Rh6(CO)16 were deposited on basic supports like MgO, ZnO, and La2O3 compared with the ones supported on acidic supports like SiO2 and Al2O3.13 Thus, the possibility of another mechanism and support effect which works in the Rh dimer catalyst system is suggested. The catalytic ethene hydroformylation reaction will be discussed again in section 3.6 after the surface Rh structures are described. 3.2. Transformation of the Surface Structure. To elucidate the mechanism which controls performance of the attached Rh dimer catalysts, we investigated the transformation of the surface structure of the attached Rh dimers under various conditions. 3.2.1. SiO2-Attached Rh Dimers (Rh2/SiO2). In our previous work, we investigated the transformation of surface structure of the SiO2-attached Rh dimers by means of in situ EXAFS, in situ FT-IR, and TPD.5 Figure 1 partly reproduced the IR spectra for the transformation of the SiO2-attached Rh dimer complex.5 In order to clarify the discussion, we briefly summarize the results obtained in our previous work. The assignment of ν(CH) peaks was based on the IR spectra for the deuterated Rh dimer complexes. A peak at 2935 cm-1 is assigned to the antisymmetric ν(CH) of µ-CH2. Both symmetric ν(CH) of µ-CH2 and asymmetric ν(CH) of Rh-CH3 appear at 2879 cm-1. A peak at 2790 cm-1 is assigned to symmetric ν(CH) of Rh-CH3. When the Rh dimer complex (2) was attached to the SiO2 surface and treated at 373 K, the peak of 2879 cm-1 and the peak of Cp* around 2200 cm-1 decreased to half of the original ones and the peak at 2790 cm-1 almost disappeared as shown in Figure 1b. The surface OH peak at 3740 cm-1 reduced in intensity upon supporting complex 2 on SiO2 at 313 K and treating the species at 373 K. Thus, it was suggested that a methyl ligand and a Cp* ligand reacted with the surface OH groups, making Rh-O bonds which were characterized separately by EXAFS as shown in Scheme 1.5 At this stage a Rh-Rh bond was observed by EXAFS analysis. By exposure to CO, dicarbonyls were formed, which appeared at 2032 and 1969 cm-1 in Figure 1c. From both volumetric analysis and IR spectra (12CO-12CO, 12CO-13CO, 13CO-13CO), it was concluded that the dicarbonyl was attached on the Rh atom without Cp* ligand as shown in Scheme 1, species 5. At the same time, the cleavage of Rh-Rh bond was observed by EXAFS. Although the Rh-Rh bond cannot be seen due to the effect of static or thermal disorder, two Rh atoms may still be located nearby similar to the case of Rh2(CO)4Cl2.14 However, there should be a kind of Rh-Rh interaction between two Rh atoms because the frequencies of two CO adsorbates were lower than those observed in conventional SiO2-supported

Figure 1. IR spectra of the SiO2-attached Rh dimers (2): (a) after attachment at 313 K, (b) treatment at 373 K, (c) CO adsorption at 313 K, (d) treatment at 473 K, and (e) hydrogenation at 423 K.

Rh catalysts. We designated this Rh-Rh interaction by a broken line, to represent noncovalent bonding interaction. By subsequent treatment of species 5 at 473 K, the peaks of the dicarbonyls almost disappeared, new peaks at 1710 and 1394 cm-1 (Figure 1d) appeared, and the ν(CH) peaks changed, which implies that CO insertion into an ethyl ligand occurred (species 6). The peak corresponding to ν(CH) of µ-CH2 group disappeared possibly by reaction with surface OH groups. Interestingly, Rh-Rh bond was regenerated at this stage. When species 6 was exposed to CO again at 313 K, dicarbonyl species was formed, while Rh-Rh bond was cleaved (species 7). Transformation between 6 and 7 was found to reversibly occur by both IR and EXAFS. CO insertion from 7 to 6 proceeded under CO-deficient conditions. Ordinarily in homogeneous systems, CO insertion proceeds more favorably under CO-rich (highpressure CO) conditions or in the coexistence of cocatalysts like PPh3 because excess CO or PPh3 occupies the vacant site on the Rh atom to stabilize the produced acyl species.17 On Rh2/SiO2 the regenerated Rh-Rh bond played the similar role as an additional ligand. We proposed a novel CO insertion mechanism which was promoted by the formation of metalmetal bonding.5 This mechanism was also rationalized by kinetic data of CO hydroformylation.15 Anderson and co-workers carried out theoretical calculation on this Rh dimer system and found that a stable Rh-Rh bond is present in the Rh acyl species (6).16 They also calculated decarbonylation mechanism. The Rh-Rh bond in species 6 was stretched to 0.566 nm by the adsorption of CO to make CO bridging species (C2H5)(CO)RhCO---RhCp*. But IR spectra did not reveal any lowfrequency peak assignable to bridging CO. Furthermore, if species 7 really had a side-on CO ligand, four independent peaks

Surface Structures of Rh Dimers

J. Phys. Chem., Vol. 100, No. 32, 1996 13639

SCHEME 1: Transformation of the Surface Rh Dimers Attached on SiO25

TABLE 3: Temperature-Programmed Desorption Data for the TiO2-Attached Rh Dimers quantity of evolved gas per Rh dimer temp/K

CH4

C2H6

C2H4

totala

293-303 303-343 343-383 383-423 423-463

0.35 0.44 0.10 0.06 0.02

0.00 0.00 0.01 0.06 0.00

0.00 0.00 0.11 0.07 0.01

0.35 0.44 0.34 0.29 0.04

a

Total ) CH4 + 2(C2H6) + 2(C2H4).

should be observed by 13CO substitution, but we observed three couples of six peaks as reported previously,5 which indicated the existence of dicarbonyl species, where the two equivalent CO ligands were strongly coupled. If RhsCO---Rh was arranged in a collinear form, we could observe the Rh-Rh distance by EXAFS due to the strong multiple scattering effect but it was not the case. The discrepancy between the experimental and calculated structures might occur from the fact that they used the mobile OH groups to optimize the structure though the position and the orientation of OH groups on a rigid SiO2 surface cannot freely be changed. 3.3. TiO2-Attached Rh Dimers (Rh2/TiO2). 3.3.1. IR and TPD Analyses. Table 3 shows the integrated temperatureprogrammed desorption (TPD) products, and Figure 2 shows FT-IR spectra for the TiO2-attached deuterated Rh dimer (2) catalyst under various conditions. Figure 2, a and a′, shows the spectra observed right after attachment at 313 K, and they were almost the same as the spectra observed for the authentic Rh dimer complex 2 itself. The catalyst was subsequently treated at 373 K under He to complete the attachment reaction. At this stage, the intensity of the peak at 2879 cm-1 for both the methyl ligand and the methylene ligand and that at 2065 cm-1 for ν(CD) of d-Cp* decreased to half of the original peak intensity (Figure 2, b and b′). The shape of the peak at 2935 cm-1 attributed to antisymmetric ν(CH) became broad, and the top of the peak red-shifted about 14 cm-1. These results suggest that the original Rh dimer structure changed at 373 K. The decrease in intensity of the absorption peaks for the methyl and

d-Cp* ligands took place in conjunction with decrease of the peak for the surface OH groups of TiO2. Accordingly, it is most likely that the methyl and Cp* ligands reacted with the surface OH groups to form Rh-O bonds. According to the TPD measurement in Table 3, one methane molecule per Rh dimer was desorbed at 293-343 K, which is in agreement with the IR results. The methylene-methyl recombination to form an ethyl ligand on the Rh dimer was also suggested by appearance of the peak at 2917 cm-1 similarly to the case of SiO2 support. After the chemical attachment was completed at 373 K, CO was admitted to the catalyst at 313 K. New peaks attributed to Rh dicarbonyls appeared at 2021 and 1953 cm-1 (Figure 2c′). According to the volumetric measurement, 1.9 molecules of CO per Rh dimer adsorbed at 313 K. They disappeared together by the following treatment at 473 K and reappeared together by CO admission under ambient pressure. The ν(CH) for the remaining methylene completely disappeared at 473 K in Figure 2d. Comparing the peak at 1814 cm-1 (Figure 2d′) with bridged CO peaks found in the literature listed in Table 4, we can assign the peak at 1814 cm-1 as a bridged carbonyl. The volumetric analysis revealed that about 0.6 CO per Rh dimer desorbed by treatment at 473K, which means that the formation of bridged CO species at 473 K was accompanied with desorption of about one molecule of CO for the TiO2-attached Rh dimer species. There was another weak peak at 1737 cm-1, and it was assigned to acyl, judging from its position. But the intensity was about 13% of that of the acyl observed on Rh2/SiO2. Nearly 90% of the Rh2/TiO2 were converted to the species with bridged CO unlike the case of Rh2/SiO2. Other peaks marked with + in Figure 2d′ are regarded as the adsorbed species on TiO2 support judging from their intensity change during CO adsorption at 313 K, the treatment at 473 K, and the reduction by H2 at 573 K. It was observed that the transformation between the Rh dicarbonyl species and the CO-bridged Rh species occurred reversibly as proved by FT-IR. 3.3.2. EXAFS Analysis. Figure 3 shows the Fourier transform of the k3-weighted EXAFS data for the Rh dimers

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Bando et al.

Figure 2. IR spectra of the TiO2-attached Rh dimers (2): (a, a′) after attachment at 313 K; (b, b′) treatment at 373 K; (c, c′) CO adsorption at 313 K; and (d, d′) treatment at 473 K.

TABLE 4: C-O Stretching Frequencies for Adsorbed CO Species sample Rh2/SiO2 Rh2/TiO2 Rh2/Al2O3 Rh2/MgO [Rh(Cp*)(CO)]2(µ-CH2) [Rh(Cp)(CO)]2(µ-CH2) Cp*Rh(CO)2 [RhCO(PPh3)2Cl] [Rh(Cp*)(CO)]2(µ-CO) Rh/Al2O3

ν(C-O)/cm-1 2032, 1969 (dicarbonyl) 2021, 1953 (dicarbonyl) 1814 (bridged CO) 2017, 1953 (dicarbonyl) 2013 (linear CO) 1938 (linear CO) 1884 (bridged CO) 1933 (linear CO) 1901 (linear CO) 1984 (linear CO) 2000, 1950 (dicarbonyl) 1966 (linear CO) 1947 (linear CO) 1794 (bridged CO) 2101, 2031 (dicarbonyl) 1855-1870 (bridged CO)

ref 5 this work this work this work 18 19 20 21 22 23

on TiO2 under various conditions. Table 5 shows the results of curve fitting for the k3-weighted Fourier filtered EXAFS oscillations. Figure 3a was observed with treatment of the incipient attached Rh dimers at 373 K in vacuum. It shows no significant peak at 0.22-0.29 nm. Therefore, at this stage the adjacent two Rh atoms were not definitely bonded each other at fixed bond distance though the two Rh atoms had a bridged methylene ligand. This may be due to a large thermal disorder

Figure 3. Fourier transform of k3-weighted EXAFS oscillations for the TiO2-attached Rh dimers: (a) after attachment at 373 K, (b) CO adsorption at 313 K, (c) treatment at 473 K, (d) CO adsorption at 313 K, and (e) treatment at 473 K.

in Rh dimer species 8. The intensity of the first shell in Figure 3a increased as compared with that for the unsupported Rh dimer complex.5 This is well explained by the Rh-O (surface) formation by the reaction between the surface OH group and the Rh dimer complex as in the case of Rh2/SiO2. From the IR study, an ethyl ligand, a methylene ligand, and a Cp* ligand were suggested to be present in the surface species (8), but fourshell fitting did not give any reasonable result because of correlation problem between the parameters for EXAFS analysis. So two-shell fitting (Rh-O, Rh-C) was carried out instead and gave the bondings at 0.220 ( 0.003 nm and 0.207 ( 0.006 nm for Rh-O (surface) and Rh-C (µ-CH2), respectively, as in the case of Rh2/SiO2.

Surface Structures of Rh Dimers

J. Phys. Chem., Vol. 100, No. 32, 1996 13641

TABLE 5: Curve-Fitting Results of the EXAFS Data for the TiO2-Attached Rh Dimers sample 8 9

10 11 10a

a

assigned bond

CN

r/nm

σ/nm

Ru-O Rh-C(µ-CH2) Ru-O(dicarbonyl) Ru-O(surface) Rh-C(µ-CH2) Rh-C(dicarbonyl) Rh-Rh Ru-O(surface) Rh-C(µ-CO) Ru-O(dicarbonyl) Ru-O(surface) Rh-C(dicarbonyl) Rh-Rh Ru-O(surface) Rh-C(µ-CO)

1.8 ( 0.7 1.5 ( 1.2 0.8 ( 0.3 1.7 ( 0.3 1.5 ( 1.2 1.2 ( 0.8 1.0 ( 0.6 1.4 ( 0.4 1.2 ( 0.8 1.2 ( 0.4 1.4 ( 0.3 0.8 ( 0.6 1.0 ( 0.6 1.4 ( 0.4 1.2 ( 0.8

0.220 ( 0.003 0.207 ( 0.006 0.300 ( 0.002 0.220 ( 0.002 0.206 ( 0.007 0.188 ( 0.005 0.267 ( 0.002 0.218 ( 0.002 0.205 ( 0.006 0.300 ( 0.003 0.219 ( 0.002 0.185 ( 0.005 0.267 ( 0.003 0.218 ( 0.002 0.205 ( 0.006

0.005 ( 0.003 0.005 ( 0.003 0.005 ( 0.002 0.005 ( 0.003 0.005 ( 0.003 0.005 ( 0.003 0.007 ( 0.003 0.005 ( 0.003 0.005 ( 0.003 0.007 ( 0.002 0.005 ( 0.003 0.005 ( 0.003 0.007 ( 0.003 0.005 ( 0.003 0.005 ( 0.003

After heating species 11 at 473 K again.

Upon CO adsorption on the Rh species treated at 373 K, a second peak in Figure 3b appeared at 0.22-0.29 nm. Curvefitting analysis for the second peak was carried out for two models for bonding as Rh-Rh or Rh---O (carbonyl). The best fit was obtained by Rh---O rather than Rh-Rh, and the distance and the coordination number were determined to be 0.300 ( 0.002 nm and 0.8 ( 0.3, respectively, in Table 5. There was no Rh-Rh bond when CO was adsorbed on the Rh dimers. Combining the EXAFS data with the IR data and the volumetric analyses, it is concluded that two CO molecules per Rh dimer were adsorbed. For the first shell of Figure 3b, Rh-C(alkyl) and Rh-C(Cp*) were neglected and three-shell fitting was performed. The characterized bonds at 0.220 ( 0.002, 0.188 ( 0.005, and 0.206 ( 0.007 nm are attributable to Rh-O (surface), Rh-C (carbonyl), and Rh-C (µ-CH2), respectively, from the comparison with species 8 as shown in Table 5. Thus, the first shell of species 9 with the dicarbonyls has a similar local structure to species 5 in Rh2/SiO2. Based on the IR and EXAFS analyses, structure 9 was proposed as shown in Scheme 2 (9). Species 9 was subsequently treated at 473 K. The second peak still appeared in the Fourier transform as shown in Figure 3c. The second peak was curve-fitted on the assumption of Rh-Rh bonding. The curve-fitting result is shown in Figure 4a, and the obtained distance and coordination number were 0.267 ( 0.002 nm and 1.0 ( 0.6, respectively, as shown in Table 5. This result implies that Rh-Rh bond was reproduced again at the surface like Rh2/SiO2.5 Two-shell analysis for species 10 treated at 473 K gave good fitting results for the first shell as 0.218 ( 0.002 and 0.205 ( 0.006 nm for Rh-O (surface) and Rh-C (bridged CO), respectively (Table 5). After CO adsorption on species 10 at 313 K again, curvefitting analysis of the second peak of the Fourier transformed spectrum was performed. We obtained the best fitting result when Rh---O (carbonyl) is assumed as shown in Figure 4b, indicating breakage of the Rh-Rh bond by dicarbonyl formation (Rh---O (carbonyl) at 0.300 nm). By subsequent treatment at 473 K, Rh-Rh bond was reproduced accompanied by µ-CO formation again as shown in Scheme 2 (10). In summary, reversible formation-breaking of Rh-Rh bond accompanied by reversible µ-CO formation-dicarbonyl formation was observed on Rh2/TiO2 by EXAFS. 3.4. Al2O3-Attached Rh Dimers (Rh2/Al2O3). 3.4.1. IR Study. Figure 5 shows in situ FT-IR spectra observed for the Al2O3-attached Rh dimers (3) . After treatment at 373 K, the intensity of ν(CH) peaks at 2937 and 2869 cm-1 for the methylene ligand and ν(CD) peaks for Cp* decreased and a

new peak at 2919 cm-1 appeared similarly to the cases of Rh2/ SiO2 and Rh2/TiO2. It is likely that the Rh dimer complex reacted with the surface OH groups on the methyl ligand and the Cp* on Al2O3 surface. By introduction of CO at 313 K, intense carbonyl peaks appeared at 2017 and 1953 cm-1 (Figure 5c). These two peaks behaved together under various conditions like those of dicarbonyls on Rh2/SiO2 and Rh2/TiO2. The low frequencies of the carbonyl peak suggested that there should be a kind of interaction between two Rh atoms. Besides the dicarbonyl peak, a peak at 2084 cm-1 was also observed, suggesting a heterogeneous property of the Rh sites. The antisymmetric stretching peak of dicarbonyls on Rh(I) monomers supported on Al2O3 has been reported to appear at similar wavenumbers as shown in Table 4.23 The higher frequency peak of 2084 cm-1 may be due to isolated Rh sites which were formed by decomposition of the Rh dimer complex at the surface. By treatment at 453 K, the peak at 2017 and 1953 cm-1 decreased in intensity (Figure 5d). A weak peak appeared at 1714 cm-1, which was assigned to the acyl species, compared with the absorption of the acyl observed on Rh2/SiO2. Acyl species formed on Rh2/Al2O3 was less than 23% of total Rh sites, estimated from its intensity. Unlike Rh2/SiO2, it seems that the majority of the adsorbed CO desorbed without acyl formation on Rh2/Al2O3. The adsorption and desorption of adsorbed CO occurred reversibly. 3.4.2. EXAFS Analysis. Figure 6a-c shows the k-weighted EXAFS oscillations for the Al2O3-attached Rh dimers, after treatment at 373 K, exposure to CO at 313 K, and treatment at 473K under vacuum, respectively. The EXAFS oscillation in the high-energy region was weak for all the samples on Al2O3. This may be ascribed to the fact that the Rh-Rh was not retained to Rh2/SiO2 and Rh2/TiO2. The curve fitting analysis was carried out and the results were given in Table 6. No RhRh distance was found in 0.2-0.3 nm. The proposed transformation of the Rh dimers on Al2O3 is depicted in Scheme 3. The species for Rh2/Al2O3 are proposed mostly to be monomer pairs without Rh-Rh bonding where there should be interaction between with two Rh atoms because the dicarbonyl species showed low-frequency peaks in IR than those of the dicarbonyls on Rh(I) monomers often reported in the literature.23 3.5. MgO-Attached Rh Dimers (Rh2/MgO). MgO did not easily react with the Rh dimer complex, and the characteristic IR absorption bands of the Rh dimer complex did not change until 473 K as shown in Figure 7d. The species obtained at 473 K was exposed to CO at 313 K, and three kinds of ν(CO) at 2013, 1938, and 1884 cm-1 were observed by IR (Figure 7e). The behaviors of these peaks were independent of one another because the 1884 cm-1 peak disappeared by treatment at 373 K and then the 2013 cm-1 peak disappeared at 473 K, while the 1938 cm-1 peak remained. This behavior of the adsorbed carbonyls is entirely different from that of the dicarbonyls adsorbed on the Rh dimers discussed above. But these peaks appear at low frequencies like the dicarbonyls on Rh2/SiO2, Rh2/TiO2, and Rh2/Al2O3. Acyl formation was not observed under any conditions. Two kinds of peaks around 2600-2800 cm-1 appeared at 473 K. They were assigned to Mg-OD groups generated by HD exchange between d-Cp* and Mg-OH groups. Figure 8a shows k-weighted EXAFS oscillation observed after treatment at 473 K. Amplitude of the oscillation did not decrease till the high-energy region. Fourier transform for k3weighted EXAFS oscillation is shown in Figure 8b. The peak at 0.25 nm is attributed to Rh-Rh bonding which is stronger as compared with other attached Rh dimers. Curve-fitting

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Bando et al.

SCHEME 2: Transformation of the Surface Rh Dimers Attached on TiO2

Figure 4. Curve-fitting results of the second peak of Figure 5. The solid curves represent observed oscillations and the dashed curves represents calculated ones: (a) treatment at 473 K; (b) CO adsorption at 313 K.

analysis of the Fourier-filtered spectra (0.20-0.27 nm) revealed that the bond length and coordination number for Rh-Rh did not change significantly under the examined conditions as shown in Table 7. The coordination number was around 1.3 ( 0.5, and the bond length was around 0.264 ( 0.002 nm. From these results we speculate an ensemble of Rh sites (Rh4 like species), but its structure cannot be drawn. It is concluded that the dimer structure was decomposed and clustered to species with Cp* ligands on MgO. CO insertion did not take place on this catalyst. 3.6. Catalytic Performance Related to Structure of Rh Species. Ethene hydroformylation activity was compared among Rh2/SiO2, Rh2/TiO2, Rh2/Al2O3, and Rh/MgO. Compared with a conventional impregnated catalyst, only Rh2/SiO2

Figure 5. IR spectra of the Al2O3-attached Rh dimers (3); (a) after attachment at 313 K, (b) treatment at 373 K, (c) CO adsorption at 313 K, and (d) treatment at 453 K.

showed activity and selectivity for hydroformylation at reduced pressures, and other catalysts were all inactive. It corresponds well to the CO insertion reaction promoted by Rh-Rh bond observed by IR and EXAFS. On Rh2/SiO2, CO insertion proceeded in conjunction with formation of the Rh-Rh bond as shown in Scheme 1. On Rh2/TiO2 the Rh-Rh bond was also regenerated by heating the dicarbonyl species, but it was accompanied by formation of the CO-bridged species, where one of the dicarbonyl ligands was merely desorbed. Thus CO insertion promoted by Rh-Rh bond formation did not take place on Rh2/ TiO2 as shown in Scheme 2. We could not observe Rh dimer structure by EXAFS as main species on Rh2/Al2O3. Rh atoms were attached as monomers

Surface Structures of Rh Dimers

J. Phys. Chem., Vol. 100, No. 32, 1996 13643 SCHEME 3: Transformation of the Surface Rh Dimers Attached on Al2O3

Figure 6. k-weighted EXAFS oscillations for the Al2O3-attached Rh dimers: (a) after attachment at 373 K, (b) CO adsorption at 313 K, and (c) treatment at 473 K.

TABLE 6: Curve-Fitting Results of the EXAFS Data for Al2O3-Attached Rh Dimers sample

assigned bond

CN

r/nm

σ/nm

12 13

Ru-O (surface) Ru-O (dicarbonyl) Ru-O (surface) Ru-O (surface) Ru-O (dicarbonyl) Ru-O (surface)

1.7 ( 1.0 1.0 ( 0.3 1.9 ( 1.0 1.7 ( 1.0 1.1 ( 0.3 1.9 ( 1.0

0.215 ( 0.004 0.301 ( 0.004 0.215 ( 0.004 0.214 ( 0.003 0.300 ( 0.001 0.215 ( 0.004

0.005 ( 0.004 0.008 ( 0.002 0.006 ( 0.004 0.004 ( 0.003 0.006 ( 0.002 0.004 ( 0.004

14 15

shown in Scheme 3. The Rh monomer species showed little CO insertion ability, which agrees with the work of Ichikawa on Rhn/ZnO (n ) 1, 2, 4, 6, 7, 13, infinity) where n ) 1 is the least active.3,13 For Rh2 on both Al2O3 and TiO2, a small production of acyl was observed after desorption of CO from Rh species in IR spectrum. These acyl groups were produced on minor components of Rh species. The reason for little production of hydroformylated species in the gas phase on these supports may be due to the stronger CO adsorption on Rh species on Al2O3 and TiO2 through larger back-donation as was proved by the IR spectra discussed later. Acyl was formed accompanied by the desorption of CO. Thus the little CO insertion occurred during the reaction conditions probably because CO desorption was unlikely to occur. MgO-supported Rh species was aggregated to form a small Rh cluster with direct Rh-O bonding. The Rh-Rh bonding in the cluster was stable and not cleaved by exposure to CO. The metal-assisted CO insertion was not observed with the Rh2/MgO catalyst. In conventional supported Rh catalyst systems the basic supports promote the CO insertion, because electronic contribution from basic supports makes the acyl species stable.13 This support effect was not observed on Rh2/MgO.

3.7. Metal-Support Interactions. The different structures of Rh dimers on several kinds of support materials can be explained by the different reaction patterns between the Rh dimers and support surfaces. The acid-base property of the supports may be a key factor for the different reactivity of the support. Acidity/basicity depends on the electronegativity of support metal ions which is in the following order: Si4+ > Ti4+ > Al3+ > Mg2+. The basicity becomes stronger as the electronegativity of metal ions becomes weaker. For the oxides with basic property, as the electron density of surface oxygen becomes higher, the coordination ability of the oxygen atom increases, and therefore the metal-support interaction becomes stronger. The interaction between metal and support can be estimated by ν(CO) for adsorbed carbonyl species in similar structures. When the basicity of support is stronger, more electrons transfer to Rh through Rh-O (surface) bonding, and the electron back-donation from Rh to CO increases and ν(CO) shifts to lower frequency. As shown in Table 4, the order of frequency of the Rh dicarbonyls is SiO2 > TiO2 > Al2O3, which is the same order as the electronegativity of support metal ions. In other words, the order of interaction between Rh dimer and support can be represented by the ν(CO) order. The strength of Rh-O bonding may be estimated by the bond length as in the case of Ru on Al2O3 and K-Al2O3, and MgO where distances of Ru-O and basicity of supports have good correlation.6 In the present case the error bars for Rh-O bonding are too large to distinguish small differences in bond distance due to the multishell fitting. If the attachment reaction in principle occurs in the following manner the reaction easily proceeds on the support with CH2

Cp* CH3

Rh

CH3 Rh

CH2

Cp*

H

Cp*

M

O M

+ 2O

CH3 Rh

+ CH4 + HCp*

Rh CH2

O M

Brønsted acidity. Thus the attachment reaction occurs on SiO2,

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Bando et al.

Figure 8. (a) k-weighted EXAFS oscillation and (b) Fourier transform of the k3-weighted EXAFS oscillation for the MgO-attached Rh species treated at 473 K.

TABLE 7: Curve-Fitting Results of the EXAFS Data for the MgO-Attached Rh Dimersa sample

assigned bond

CN

r/nm

σ/nm

16 17 18

Rh-Rh Rh-Rh Rh-Rh

1.3 ( 0.5 1.2 ( 0.8 1.2 ( 0.5

0.264 ( 0.002 0.262 ( 0.003 0.265 ( 0.002

0.006 ( 0.002 0.006 ( 0.002 0.006 ( 0.002

a 16: after attachment at 473 K. 17: exposure to CO at 313 K; 18: subsequent heating at 473 K.

reactivity cannot be explained simply by difference of basicity. The reactivity is thought to be influenced by many factors like Ru-O bond strength, the spacing between oxygen atoms that fix Rh atoms, or electronic state of Rh atoms. However, we think that a main factor may arise from the strength and flexibility of Ru-O bonding. We are now conducting further experiments to clarify this point. 4. Conclusion

Figure 7. IR spectra of the MgO-attached Rh dimers (2): (a) treatment at 313 K, (b) treatment at 373 K, (c) treatment at 423 K, (d) treatment at 473 K, (e) CO adsorption at 313 K, (f) treatment at 473 K, and (g) CO adsorption at 313 K.

Al2O3, and TiO2 under mild conditions. On MgO, the Rh dimer reacted with the surface oxygen atoms or OH- groups under more severe conditions and the dimer structure was destroyed to form clusters with higher nuclearity. The property of the attached Rh species depend on the nature of Ru-O bonding. Since Si-O bonding was the strongest among three, Ru-O bonding on Rh2/SiO2 became weak and restriction of the motion of Rh at the surface weakened. Therefore, “Rh-Rh bond-assisted CO insertion” occurred. On the other hand, on Al2O3 Ru-O bonding became stronger due to the stronger basicity, and the Rh was tightly bound to the Al2O3 surface and behaved like a Rh monomer species. On TiO2 surface Rh showed intermediate behavior. Rh-Rh bond was cleaved with the CO adsorption and was regenerated by CO desorption. However, the Rh-Rh bond-assisted CO insertion did not take place. The acyl species was formed on Rh2/SiO2, while bridge CO was simply formed on Rh2/TiO2. The difference between the two catalysts in CO insertion

Ethene hydroformylation by attached Rh catalysts was investigated in relation to the surface structure of Rh sites. Rh2/ SiO2 had high activity and selectivity, but other catalysts did not show any activity for this reaction. The reason why the activity varies so much according to the kind of supports was that the surface structure properties with the kinds of supports which cause significant difference in reactivity. The Rh dimer complex was attached on SiO2 keeping its dimer structure. The Rh-Rh bond has such a flexible character which makes CO insertion prompted by Rh-Rh bond formation possible. On TiO2, the Rh dimer was attached, keeping its original structure to some extent. Rh-Rh bond was broken at once, but regenerated by treatment at 473 K. Unlike Rh2/SiO2, the formation of Rh-Rh bond did not promote CO insertion; instead µ-CO was merely formed. On Al2O3, metal-support interaction became stronger and the Rh dimer was attached as monomers, and formation of Rh-Rh bond was prohibited. Again, metalmetal assisted CO insertion did not occur on Al2O3 support. On MgO, the Rh dimer was converted to clusters during heating the surface Rh dimers. No hydroformylation reaction occurred. On Rh2/TiO2, Rh2/Al2O3, and Rh2/MgO promotion of CO insertion by the adjacent Rh atom was not observed possibly because the interaction between Rh dimers and supports were too strong.

Surface Structures of Rh Dimers References and Notes (1) Isobe, K.; Andrews, D. G.; Mann, B. E.; Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1981, 809, Vazquez de Miguel, A.; Isobe, K.; Taylor, B. F.; Nutton, A.; Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1982, 758. Isobe, K.; Vazquez de Miguel, A.; Bailey, P. M.; Okeya, S.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1983, 1441. (2) Muetterties, E. L. Stein, J. Chem. ReV. 1979, 79, 479. (3) Iwasawa, Y. Tailored Metal Catalysts; D. Reidel: Dordrecht, 1986. (4) Iwasawa, Y. Catal. Today 1993, 18, 21. (5) Asakura, K.; Kitamura-Bando, K.; Iwasawa, Y.; Arakawa, H.; Isobe, K., J. Am. Chem. Soc., 1990, 112, 9096. Kitamura-Bando, K.; Asakura, K.; Arakawa, H.; Sugi, Y.; Isobe, K.; Iwasawa, Y. J. Chem. Soc., Chem. Commun. 1990, 3, 253. Asakura, K.; Kitamura-Bando, K.; Isobe, K.; Arakawa, H.; Iwasawa, Y. J. Am. Chem. Soc. 1990, 112, 3242. (6) Asakura, K.; Yamada; M.; Iwasawa; Y.; Kuroda, H. J. Chem. Soc., Chem. Commun. 1985, 511. Asakura, K.; Kitamura-Bando, K.; Iwasawa, Y. J. Chem. Soc., Faraday Trans. 1990, 86, 2645. Asakura, K.; Iwasawa, Y. J. Chem. Soc., Faraday Trans. 1990, 86, 2657. Asakura, K.; Iwasawa, Y.; Kuroda, H.; Nikkashi, 1986, 1539. (7) Izumi, Y.; Chihara, T.; Yamazaki, H.; Iwasawa, Y. J. Am. Chem. Soc. 1993, 115, 6462. Izumi, Y.; Iwasawa, Y. Chemtech 1994, 24(7), 20. Izumi, Y.; Liu, T. H.; Asakura, K.; Chihara, T.; Yamazaki, H.; Iwasawa, Y. J. Chem. Soc., Dalton Trans. 1992, 14, 2287. (8) Teo, B. K., EXAFS: Basic Principles and Data Analysis, Inorganic Chemistry Concepts 9; Springer-Verlag: Berlin, 1986.

J. Phys. Chem., Vol. 100, No. 32, 1996 13645 (9) Kosugi, N.; Kuroda, H. EXAFS analysis Program, EXAFS2; Research Center for Spectrochemistry, University of Tokyo: Tokyo. (10) Teo, B. K.; Lee, P. A. J. Am.Chem.Soc. 1979, 101, 2815. (11) Stern, E. Phys. ReV. B 1993, 48, 9825. (12) Lytle, F. W.; Sayers, D. E.; Stern, E. A. Physica B (Amsterdam) 1989, 158, 701. (13) Ichikawa, M. J. Catal. 1979, 59, 67. (14) Barnes, C. E., Ralle, M., Vierkoetter, S. A., Penner-Harn, J. E. J. Am. Chem. Soc. 1995, 117, 5861. (15) Bando, K. Doctoral Thesis, The University of Tokyo, 1993. (16) Jen, S. F.; Anderson, A. B. Inorg. Chem. 1992, 31, 2651. (17) Calderazzo, F. Angew. Chem., Int. Ed. Engl. 1977, 16, 299. (18) Herrmann, W. A.; Bauer, C.; Plank, J.; Kalcher, W.; Speth, D.; Ziegler, M. L. Angew. Chem., Int. Ed. Engl. 1981, 20, 193. (19) Clauss, A. D.; Dimas, P. A.; Shapkey, J. R. J. Organomet. Chem. 1980, 201, C31. (20) Kang, J. W.; Maitlis, P. M. J. Organomet. Chem. 1971, 26, 393. (21) Baird, M. C.; Mague, J. T.; Osborn, J. A.; Willkinson, G. J. Chem. Soc. (A) 1967, 1347. (22) Plank, J.; Riedel, D.; Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1980, 19, 937. (23) Yates, Jr., J. T.; Duncan, T. M.; Worley, S. D.; Vaughan, R. W. J. Chem. Phys. 1979, 70, 1219.

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