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Langmuir 2006, 22, 490-496
Rhodium Complex with Ethylene Ligands Supported on Highly Dehydroxylated MgO: Synthesis, Characterization, and Reactivity Vinesh A. Bhirud,† Justin O. Ehresmann,‡ Philip W. Kletnieks,‡ James F. Haw,*,‡ and Bruce C. Gates*,† Department of Chemical Engineering and Materials Science, UniVersity of California at DaVis, DaVis, California 95616, and Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089 ReceiVed August 19, 2005. In Final Form: October 11, 2005 Mononuclear rhodium complexes with reactive olefin ligands, supported on MgO powder, were synthesized by chemisorption of Rh(C2H4)2(C5H7O2) and characterized by infrared (IR), 13C MAS NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopies. IR spectra show that the precursor adsorbed on MgO with dissociation of acetylacetonate ligand from rhodium, with the ethylene ligands remaining bound to the rhodium, as confirmed by the NMR spectra. EXAFS spectra give no evidence of Rh-Rh contributions, indicating that site-isolated mononuclear rhodium species formed on the support. The EXAFS data also show that the mononuclear complex was bonded to the support by two Rh-O bonds, at a distance of 2.18 Å, which is typical of group 8 metals bonded to oxide supports. This is the first simple and nearly uniform supported mononuclear rhodium-olefin complex, and it appears to be a close analogue of molecular catalysts for olefin hydrogenation in solution. Correspondingly, the ethylene ligands bonded to rhodium in the supported complex were observed to react with H2 to form ethane, and the supported complex was catalytically active for the ethylene hydrogenation at 298 K. The ethylene ligands also underwent facile exchange with C2D4, and exposure of the sample to carbon monoxide led to the formation of rhodium gem dicarbonyls.
Introduction Determination of accurate structure-property relationships of supported catalysts requires that they be synthesized precisely and that the ligands on the metals be reactive enough to allow the start of a catalytic cycle under conditions mild enough to minimize reactions leading to the loss of structural uniformity of the catalyst. Molecular organometallic precursors that react simply with supports appear to offer the best opportunities for such precise synthesis.1,2 Typically, supported metal complexes are synthesized from organometallic precursors having reactive ligands (such as alkyl, allyl, carbene, etc.) reacting with surface functional groups of the support, such as oxygen atoms or OH groups. Successful syntheses usually involve chemistry that is closely analogous to known solution chemistry. For example, Goellner et al.3 reported the synthesis and characterization of site-isolated Rh+(CO)2 complexes on dealuminated Y zeolite. The presence of relatively nonreactive CO ligands bonded to the rhodium center made this sample appealing for characterization [e.g., by the νCO bands in the infrared (IR) spectrum], but the sample is quite stable and relatively unreactive as a catalyst. Our goal was to prepare a comparable well-defined supported metal complex from a precursor with ligands more reactive than CO and to investigate it as an entry point into a catalytic cycle. We chose a rhodium complex with olefin ligands as the precursor [Rh(C2H4)2(acac)], with the goal of preparing a supported complex retaining the olefin ligands, because they are present in reaction intermediates in the catalytic cycles for olefin hydrogenation. π-Bonded ethylene has also been shown * To whom correspondence should be addressed. Telephone: (530) 752-3953. Fax: (530) 752-1031. E-mail:
[email protected]. † University of California at Davis. ‡ University of Southern California. (1) Coperet, C.; Chabanas, M.; Saint-Arroman, R. P.; Basset, J.-M. Angew. Chem. Int. Ed. 2003, 42, 156. (2) Guzman, J.; Gates, B. C. Dalton Trans. 2003, 3303. (3) Goellner, J. F.; Gates, B. C.; Vayssilov, G. N.; Ro¨sch, N. J. Am. Chem. Soc. 2000, 122, 8056.
to be a reactive intermediate in catalysis by supported metal clusters and single crystals of metal,4-6 and therefore, the results were intended to provide a foundation for a comparison of rhodium catalysts in various forms. There are reports of the chemistry of rhodium allyl complexes on SiO2 supports. Adsorption of tris-allyl rhodium on SiO2 led to the formation of rhodium bis-allyl species grafted to the surface.7 These rhodium species, however, were found to undergo relatively facile reduction to give rhodium metal nanoclusters under conditions of catalytic interest.8-10 A consequence of the reduction and aggregation of the rhodium is the loss of uniformity and structural simplicity of the catalyst. Similarly, in an attempt to prepare a supported rhodium complex with olefin ligands, Vierko¨tter et al.11,12 used the precursor [Rh(C2H4)2Cl]2 to synthesize a catalyst on γ-Al2O3, but although they found evidence of ethylene bonded to rhodium in the adsorbed species, the presence of chloride led to nonuniform surface species, with some chloride remaining bonded to the rhodium. Werner et al.13 prepared various acetato and acetylacetonato rhodium olefin complexes on SiO2 and investigated them by Raman spectroscopy and calculations at the density functional level, concluding that the anionic ligands (acetate or acetyl acetonate) were not displaced (4) Argo, A. M.; Gates, B. C. J. Phys. Chem. B 2003, 107, 5519. (5) Ko, M. K.; Frei, H. J. Phys. Chem. B 2004, 108, 1805. (6) Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942. (7) Ward, M. D.; Harris, T. V.; Schwartz, J. J. Chem. Soc., Chem. Commun. 1980, 357. (8) Dufour, P.; Houtman, C.; Santini, C.; Nedez, C.; Basset, J.-M.; Hsu, L. Y.; Shore, S. G. J. Am. Chem. Soc. 1992, 114, 4248. (9) Dufour, P.; Houtman, C.; Santini, C.; Basset, J.-M. J. Mol. Catal. 1992, 77, 257. (10) Foley, H. C.; DeCanio, S. J.; Tau, K. D.; Chao, K. J.; Onuferko, J. H.; Dybowski, C.; Gates, B. C. J. Am. Chem. Soc. 1983, 105, 3074. (11) Vierko¨tter, S. A.; Barnes, C. E.; Garner, G. L.; Butler, L. G. J. Am. Chem. Soc. 1994, 116, 7445. (12) Vierko¨tter, S. A.; Barnes, C. E.; Hatmaker, T. L.; Penner-Hahn, J. E.; Stinson, C. M.; Huggins, B. A.; Benesi, A.; Ellis, P. D. Organometallics 1991, 10, 3803. (13) Werner, H.; Mo¨hring, U. J. Organomet. Chem. 1994, 475, 277.
10.1021/la052268f CCC: $33.50 © 2006 American Chemical Society Published on Web 12/01/2005
Rhodium Complex Supported on Highly Dehydroxylated MgO
by surface hydroxyl groups of the support. Thus, there are no reports of structurally simple supported rhodium complexes with olefin ligands. The primary goal of the work reported here was to prepare such well-defined samples. The plan was to take advantage of the reactivity of the acetylacetonate (acac) groups in Rh(C2H4)2(acac); MgO was chosen as the support because of indications [on the basis of evidence of its reaction with Au(CH3)2(acac)]14 that it might react with the support surface with dissociation of the acac ligand, leaving the ethylene ligands on the metal. Further goals were to identify the ligands remaining on the rhodium after its reaction with the support and to determine their reactivity and the catalytic activity of the complex for ethylene hydrogenation. Experimental Procedures Materials and Procedures. Sample syntheses and transfers were performed with the exclusion of moisture and air in a Braun MB150M glovebox (purged with argon recirculating through traps containing particles of Cu and of zeolite 4A for O2 and moisture, respectively) and a double manifold Schlenk vacuum line. H2, He, and ethene with purities of 99.999% (Matheson) were passed through similar traps to remove traces of O2 and water. CO (CP grade) was further purified by passage through a trap containing activated γ-Al2O3 particles and zeolite 4A, to remove any traces of metal carbonyls from the high-pressure gas cylinder and water, respectively. n-Pentane was dried over sodium benzophenone ketyl and deoxygenated with flowing N2 prior to use. Diethylene acetylacetonato rhodium(I), Rh(C2H4)2(acac) (Strem, 99%), was used as received. C2D4, 13C-labeled ethylene (99% 13C), and 13C-labeled CO were obtained from Cambridge Isotopes. The MgO support was obtained from EM Science. Deionized water was added to the MgO to form a paste, which was dried overnight in air at 393 K. The solid was ground and calcined as O2 flowed through a bed of the particles as the temperature was ramped linearly from room temperature to 973 K and held for 2 h. The O2 treatment was immediately followed by evacuation of the sample for 14 h at 973 K. The resultant MgO (BET surface area ) 18 m2/g) was then cooled to room temperature under vacuum, isolated, and stored in an argon-filled glovebox.15 Rh(C2H4)2(acac) was mixed with the support in the glovebox and placed in a Schlenk flask. The amounts were chosen to give a final, dried sample containing 1.0 wt % Rh. Dried and deoxygenated n-pentane was added to the mixture, and the resultant slurry was stirred at 200 K overnight, followed by removal of the solvent by evacuation. IR Spectroscopy. IR spectra were measured for samples pressed between KBr disks in a glovebox at a pressure of 1 bar. The powder samples were pressed into thin wafers in a glovebox and placed into an IR cell supplied by In-situ Research and Instruments, Granger, IN. Spectra were recorded as the sample was treated with various reactive gases flowing through the cell. The spectra were recorded with a Bruker IFS-66 v spectrometer with 4-cm-1 spectral resolution. Each spectrum represents an average of 64-128 scans. The outlet gas was sometimes sampled for on-line analysis by mass spectrometry. 13C NMR Spectroscopy. Samples were prepared for MAS NMR spectroscopy by using a shallow-bed CAVERN apparatus.16 Typically, 0.3 g of catalyst was loaded into the CAVERN in a N2-filled glovebox. The CAVERN was then connected to a vacuum line without contamination of the sample. The sample was evacuated to a final pressure of less than 5 × 10-5 Torr. Adsorption of ethylene was carried out at room temperature at pressures less than 1.5 Torr to minimize any side reactions that might have been catalyzed by the support. Each sample was loaded into a 7.5-mm zirconia rotor, which was capped within the CAVERN. Conventional 13C CP/MAS spectra were acquired with a Chemagnetics CMX 300 (7.05 T) spectrometer equipped with a modified Chemagnetics 7.5-mm MAS (14) Guzman, J.; Gates, B. C. Angew. Chem. Int. Ed. 2003, 42, 690. (15) Lai, F. S.; Gates, B. C. Nano Lett. 2001, 1, 583. (16) Xu, T.; Haw, J. F. Top. Catal. 1997, 4, 109.
Langmuir, Vol. 22, No. 1, 2006 491 probe. The sample was referenced to the secondary standard hexamethylbenzene (methyl signal of 17.35 ppm relative to TMS), and the hexamethylbenzene sample was also used to calibrate the 90° flip for both carbon and proton. The data from 2000 scans were accumulated using a 0.5-s pulse delay. The MAS spinning speed was 5.0 kHz. EXAFS Spectroscopy. EXAFS spectra were collected at beamline X-18B of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, Upton, NY. The storage ring electron energy was 2.8 GeV; the beam current ranged from 140 to 240 mA. In a nitrogen-filled glovebox at the synchrotron, each powder sample was pressed into a self-supporting wafer. The mass was chosen to give an X-ray absorbance of approximately 2.5 at the Rh K edge (23 220 eV). The wafer was loaded into an EXAFS cell, sealed under a positive N2 pressure, and removed from the glovebox. The cell was then evacuated (pressure ≈ 10-5 Torr), and the sample was aligned in the X-ray beam and cooled to a nearly liquid nitrogen temperature. EXAFS spectra were then collected in transmission mode. A Si(111) double-crystal monochromator was detuned by 20-25% at the Rh K edge to suppress higher harmonics in the X-ray beam. The data represent the average of 5 scans for each sample. Catalytic Activity Measurements. Catalytic activity measurements were carried out with a once-through plug-flow reactor that was nearly isothermal. Reactant gases (He, H2, and C2H4) flowed through a bed of catalyst of known mass (usually 20-30 mg); the total flow rate was 100 mL (NTP)/min. The reported catalytic reaction rates were calculated from conversions of less than 5%, which were shown to be differential. Analysis of X-ray Absorption Spectra. Analysis of the data was based on experimentally determined reference files prepared from EXAFS data representing materials of known structure. EXAFS data characterizing rhodium foil and Rh2O3 were used for the phase shifts and backscattering amplitudes of the first-shell Rh-Rh and Rh-Osupport interactions. Ru3(CO)12 mixed with X-ray-transparent BN was used to obtain the phase shifts and backscattering amplitudes used in analyzing the Rh-C interactions. (The transferability of the phase shifts and backscattering amplitudes for neighboring atoms in the periodic table has been justified experimentally.)17 Details of the preparation of the reference files are presented elsewhere.18-20 A summary of the parameters used to construct the reference files from the EXAFS data is presented in the Supporting Information. Analysis of the EXAFS data was carried out with a difference file technique21,22 and the software XDAP.23 The main contributions to the spectra were isolated by inverse Fourier transformation of the final EXAFS function, and the analysis was done with the Fourierfiltered data. Iterative fitting was carried out until satisfactory agreement was attained between the calculated k0-, k1-, and k2weighted data and the postulated model (k is the wave vector). The number of parameters used in fitting the data to each model was always less than the number justified statistically according to the Nyquist theorem.24 The fitting ranges in both momentum and real space were determined by the data quality.
Results Spectroscopic Evidence of the MgO-Supported RhodiumDiethylene Complex. After the solution containing the precursor Rh(C2H4)2(acac) was contacted with the MgO support, the solution was clear, indicative of nearly the complete uptake of (17) Duivenvoorden, F. B. M.; Koningsberger, D. C.; Uh, Y. S.; Gates, B. C. J. Am. Chem. Soc. 1986, 108, 6254. (18) Coey, J. M. D. Acta Crystallogr., Sect. B: Struct. Sci. 1970, 26, 1876. (19) Crystal Structures, 2nd ed.; Wycoff, R. W. G., Ed.; Wiley: New York, 1963; Vol. 1. (20) Mason, R.; Rae, A. I. M. J. Chem. Soc. A 1968, 778. (21) van Zon, J. B. A. D.; Koningsberger, D. C.; van’t Blik, H. F. J.; Sayers, D. E. J. Chem. Phys. 1985, 82, 5742. (22) Kirlin, P. S.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. J. Phys. Chem. 1990, 94, 8439. (23) Vaarkamp, M.; Linders, J. C.; Koningsberger, D. C. Physica B 1995, 209, 159. (24) Lytle, F. W.; Sayers, D. E.; Stern, E. A. Physica B 1989, 158, 701.
492 Langmuir, Vol. 22, No. 1, 2006
Figure 1. IR spectra of the sample formed from Rh(C2H4)2(acac) and MgO that was dehydroxylated at 973 K.
the precursor. IR spectra characterizing the C-H stretching region (3200-2800 cm-1) and the fingerprint region characterizing the acac groups (1800-1200 cm-1) of the sample formed by adsorption of the precursor are shown in parts A and B of Figure 1, respectively. The band positions are summarized in parts A and B of Table 1, respectively. These spectra demonstrate the presence of surface species formed from the precursor. The band at 3064 cm-1 is assigned to the stretching vibration belonging to an olefinic CH2 moiety bonded to a rhodium center. The band position essentially matches those of others observed for ethylene π-bonded to transition-metal atoms in mononuclear metal complexes.4,25,26 This band indicates the retention of ethylene ligands bonded to the rhodium after adsorption of the precursor on MgO. The 13CP MAS NMR spectrum of this sample (Figure 2) includes resonances at 57 and 27 ppm. The former matches well with the resonances observed for complexes containing ethylene ligands π-bonded to rhodium in Rh(C2H4)2(acac) in solution27 and in the solid state.12 The latter resonance suggests natural abundance signals from CH3 groups from the acac ligand retained on the support; Vierko¨tter et al.11 made a similar observation and a similar suggestion for the precursor Rh(C2H4)2(acac). The rhodium is formally in the +1 oxidation state in this compound, as we infer it is in our supported analogue. If, in (25) Cramer, R. Inorg. Synth. 1974, 15, 14. (26) Guzman, J.; Gates, B. C. J. Catal. 2004, 226, 111. (27) Jesse, A. C.; Gijben, H. P.; Stufkens, D. J.; Vrieze, K. Inorg. Chim. Acta 1978, 31, 203.
Bhirud et al.
contrast, rhodium had been in the zero-valent state, the rhodium would have had an odd number of electrons, and we would anticipate that the NMR spectrum shown in Figure 2 would not have been observed. The important result for our purposes is that π-bonded ethylene was retained on the rhodium after adsorption of the precursor. The IR spectra of the supported rhodium ethylene complex include additional bands at 3080, 2997, 2963, 2924, and 2854 cm-1. A comparison of these bands with those of reference materials is shown in parts A and B of Table 1, where the corresponding assignments are indicated. The spectrum representing the sample formed by contacting Rh(C2H4)2(acac) with MgO is almost identical to those characterizing Mg(acac)2 and Hacac adsorbed on MgO, suggesting that in the reaction of Rh(C2H4)2(acac) with MgO, Rh-acac groups were converted and Mg-acac and/or Hacac surface species were formed. Thus, we infer that acac ligands dissociated from the rhodium and were present in some form on the support. The parameters corresponding to the best fit of the EXAFS data (Table 2; the corresponding fits are shown in parts A and B of Figure 3) show no observable Rh-Rh contributions, demonstrating that the rhodium complex was still mononuclear after adsorption. The metal-support interface is characterized by a Rh-O contribution with a coordination number of 1.4. The error in the coordination number is rather large (approximately ( 30%), in part because of the correlation between the Rh-O and Rh-C contributions. The Rh-O distance (2.18 Å) is typical of bonds between group 8 metals and surface oxygen atoms in complexes bonded to metal oxide supports.28 Thus, each rhodium atom is inferred to have been bonded to oxygen atoms of the MgO support, with the number of oxygen atoms per rhodium atom being approximately 1 or 2; we cannot rule out mixtures with different coordinations to the surface. Rh-C contributions were also identified, with a coordination number of about 4 (again, with an error of approximately (30%), consistent with the presence of ethylene ligands bonded to the rhodium. Thus, the EXAFS data confirm the formation of rhodium ethylene complexes on the support, consistent with the presence of two ethylene ligands per rhodium atom, although this number is not determined with confidence.29 The Rh-C bond distance determined by the EXAFS data is 2.04 Å; for a comparison, the Rh-C bond distance determined by X-ray diffraction crystallography for the precursor Rh(C2H4)2(acac) is 2.14 Å.30 The EXAFS data also indicate a Rh-O contribution at a longer distance than the bonding Rh-O distance; such longer metaloxygen contributions are expected, having been observed for numerous metal complexes (and metal clusters) on oxide supports.4,28 Reactivity of the Supported Rhodium Complex in Various Gas Atmospheres (C2D4, CO, and H2). The reactivities of the surface rhodium complex in various atmospheres were characterized by observations of the surface species by IR and EXAFS spectroscopies and by analysis of gas-phase products by mass spectrometry. C2D4 Exchange with C2H4 of the Supported Rhodium Complex. Upon exposure of the sample to perdeuterated ethylene at 298 K, the 3060-cm-1 band in the C-H stretching region of the IR spectrum disappeared and ethylene appeared in the gas phase, as shown by the mass spectrometric analysis. Simultaneously, (28) Alexeev, O.; Gates, B. C. Top. Catal. 2000, 10, 273. (29) The total number of Rh(C2H4)2 moieties present per gram of sample was 6.0 × 1018. Considering that the surface area of MgO calcined at 973 K is 18 m2/g, there was one Rh(C2H4)2 moiety on average per 3 nm2 of the support. (30) Price, D. W.; Drew, M. G. B.; Hii, K. K.; Brown, J. M. Chem.sEur. J. 2000, 6, 4587.
Rhodium Complex Supported on Highly Dehydroxylated MgO
Langmuir, Vol. 22, No. 1, 2006 493
Table 1. Frequencies of IR Bands (cm-1) Observed in the C-H (A) and Fingerprint (B) Regions Characterizing Reference Compounds and the Sample Formed by Adsorption of Rh(C2H4)2(acac) on MgO (A) sample sample formed from Rh(C2H4)2(acac) and MgO
Rh(C2H4)2(acac)25
3080 3064 2997 2963 2924 2854
Hacac/MgO35
MgII(acac)235
assignments
3080
3128 3070
na ν(C3-H) (acac) ν(CH2) (ethylene) ν(CH3) (acac) ν(CH3) (acac) ν(CH3) (acac) combination band combination band
3064 2993 2965 2920
2993 2970 2925 2866 2836
(B) sample sample formed from Rh(C2H4)2(acac) and MgO
Rh(C2H4)2(acac) 25
Hacac/MgO35
MgII(acac)235
assignments
1575 1558 1524
1623
1620
1528 1489 1423 1371 1263
1521 1465 1418 1367 1265
ν(C-C) {or ν(C-O)} combination band ν(C-O) {or ν(C-C)} νs(C-C) + δ(C-H) δd(CH3) δs(CH3) νs(C-C) + ν(C-CH3)
1621 1520 1466 1412 1370 1259
1425 1372 1267
Table 2. EXAFS Fit Parameters Characterizing the Sample Made by Bringing Rh(C2H4)2(acac) in Contact with MgOa shell
N
R (Å) ∆σ2 × 103 (Å2) ∆E0 (eV) EXAFS reference
Rh-Rh Rh-Os Rh-C Rh-Ol
b 1.4 4.0 1.7
2.18 2.04 2.79
0.86 0.68 6.62
-12.8 11.2 -0.2
Rh-Rh Rh-O Ru-C Rh-O
a Notation: N, coordination number; R, distance between absorber and backscatterer atoms; ∆σ2, Debye-Waller factor; ∆E0, inner potential correction. Expected errors: N, (30%; R, (0.02 Å; ∆σ2, (20%; ∆E0, (20%. The subscripts s and l refer to short and long, respectively. b Undetectable.
Figure 2. 13C CP/MAS (75.4 MHz) spectra of the sample formed from Rh(C2H4)2(acac) and MgO, after exchange of ethylene ligands with 13C-labeled ethylene. The spectrum was measured at 153 K.
new IR bands grew in at 2213 and 2176 cm-1. These match well with the IR bands observed for π-bonded C2D4 species formed during adsorption of C2D4 on Pt/SiO231 or Pt/γ-Al2O3.32 Thus, the ethylene ligands π-bonded to the rhodium in the supported complex are inferred to have undergone facile and clean exchange with C2D4. Moreover, there were no observed bands corresponding to the formation of di-σ-bonded ethylene or ethylidyne, (31) Chesters, M. A.; De La Cruz, C.; Gardner, P.; McCash, E. M.; Prentice, J. D.; Sheppard, N. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 739. (32) Mohsin, S. B.; Trenary, M.; Robota, H. J. J. Phys. Chem. 1988, 92, 5229.
bolstering the conclusion that the rhodium complexes were siteisolated. After this exchange, when a pulse of C2H4 gas was introduced into the IR cell, the band at 3064 cm-1 reappeared and that at 2176 cm-1 disappeared. Thus, the exchange was reversible. In summary, the ethylene bonded to the rhodium center underwent facile, reversible ligand exchange at room temperature and 1 bar. Replacement of Ethylene Ligands with CO. CO reacted with the supported rhodium ethylene complex, resulting in the disappearance of the 3064-cm-1 band as ethylene was removed; correspondingly, the appearance of ethylene in the gas phase was detected with mass spectrometry. Simultaneously, new bands appeared rapidly in the IR spectrum, at 2082 and 2004 cm-1. These are assigned, on the basis of a comparison with reported data,33 to isolated Rh+(CO)2 (rhodium gem dicarbonyl) complexes on the support. When the supported rhodium gem dicarbonyl was treated in flowing ethylene at 298 K and at increasing temperatures up to 353 K, no changes were observed in the νCO bands, indicating the irreversibility of the carbonylation process. The possibility that some ethylene was bonded to the rhodium gem dicarbonyl cannot be ruled out, because the corresponding bands in the IR spectrum would have been difficult to distinguish from those indicating gas-phase ethylene. Reaction of Ethylene Ligands with H2 To Form Ethane. Treatment of the supported rhodium ethylene complex with H2 led to the disappearance of the 3064-cm-1 band representing π-bonded ethylene. Simultaneously, the formation of gas-phase ethane was observed by mass spectrometry, indicating that the ethylene ligands had been hydrogenated. During this period, the IR bands at 3734, 3694, and 3600 cm-1, corresponding to isolated and hydrogen-bonded support OH groups of the support, increased in intensity.34,35 Such bands are expected to form as a result of replenishment of support OH groups during hydrogen treatment and point to the spillover of hydrogen.36-38 (33) Hadjiivanov, K.; Vayssilov, G. AdV. Catal. 2002, 47, 307. (34) Diwald, O.; Sterrer, M.; Kno¨zinger, E. Phys. Chem. Chem. Phys. 2002, 4, 2811. (35) Guzman, J.; Anderson, B. G.; Vinod, C. P.; Ramesh, K.; Niemantsverdriet, J. W.; Gates, B. C. Langmuir 2005, 21, 3675. (36) Sermon, P. A.; Bond, G. C. Catal. ReV. 1973, 8, 211. (37) Conner, W. C., Jr.; Pajonk, G. M.; Teichner, S. J. AdV. Catal. 1986, 34, 1.
494 Langmuir, Vol. 22, No. 1, 2006
Bhirud et al.
Figure 4. IR spectrum of the sample formed by bringing CO in contact with the MgO-supported rhodium ethylene complex at 298 K. The solid line represents the sample after exposure to an atmosphere with PCO ) 10 Torr and PHe ) 750 Torr. The dotted line represents the sample prior to coming in contact with CO.
Figure 3. (A) Results of EXAFS analysis characterizing the sample formed by adsorption of Rh(C2H4)2(acac) on MgO that had been calcined at 973 K: experimental EXAFS function (χ, s) and the χ functions calculated (‚‚‚) on the basis of the fit with the contributions shown in Table 2. (B) Results of EXAFS analysis characterizing the sample formed by adsorption of Rh(C2H4)2(acac) on MgO that had been calcined at 973 K: imaginary part and magnitude of the Fourier transform of the experimental EXAFS (χ) function (s) and calculated (‚‚‚) on the basis of the fit with the contributions shown in Table 2.
During this period of hydrogen spillover on the support, a weak band appeared at 2000 cm-1. This band could be attributed to either small amounts of CO bonded to rhodium species or to rhodium hydride.39 To test the latter possibility, a flow of D2 over the sample was started, and the spectrum was monitored; the band did not change, as might have been expected if it had represented rhodium hydrides. Furthermore, the 2000-cm-1 band also formed during a period of deuterium spillover (during D2 flow), just as it had during hydrogen spillover during H2 flow. Thus, the attribution of the band to surface CO is considered likely. Consistent with this inference, when O2 flowed over the sample, the band immediately disappeared, presumably because of oxidation to form CO2. We suggest that the OH groups produced by hydrogen spillover might have reacted with the surface acac species, leading to the formation of acetates on the support and acetone in the gas phase. The small amount of acetone supposedly generated in situ could then undergo decomposition on the rhodium to form a small amount of CO bonded to the rhodium.40 (38) Conner, W. C., Jr.; Falconer, J. L. Chem. ReV. 1995, 95, 759. (39) Vayssilov, G.; Ro¨sch, N. J. Am. Chem. Soc. 2002, 124, 3783.
Simultaneously with the hydrogen flow, there were changes in the acac fingerprint region of the IR spectrum; the band at 1621 cm-1, assigned to ν(C-C) [or ν(C-O)], disappeared, and a new band grew in at 1600 cm-1. We infer that these changes point to the formation of magnesium acetate groups on the support, because such bands have been observed for similar reactions of acac ligands on MgO and as a result of the adsorption of acetic acid on MgO.41 Acetate formation has been associated with surface reactions with OH groups, leading to the release of acetone in the gas phase.42 During the hydrogen treatment, surface OH groups were replenished partially (via hydrogen spillover, as indicated by IR spectra), possibly facilitating this reaction. We attribute the formation of these surface species to the reaction of acac derived from Rh(C2H4)(acac)2 on the support.35,43 Ethylene Hydrogenation Catalysis. The supported rhodium complex initially incorporating ethylene ligands was tested as a catalyst for ethylene hydrogenation in a tubular plug flow reactor at 298 K. The total feed flow rate was 100 mL (NTP)/min, with partial pressures of PH2 ) 50 Torr and PC2H4 ) 40 Torr and the balance He. After the catalyst was placed on stream, its activity declined until a steady-state conversion was attained after 45 min. The catalyst did not show significant deactivation during a subsequent 12-h period of continuous operation. Under these conditions, the conversions were differential (less than 5%), determining reaction rates directly. The steady-state turnover frequency was 1.6 × 10-2 s-1. There was no detectable activity in the absence of the catalyst, and the MgO support alone was inactive; thus, the catalytic activity is attributed to the supported rhodium complexes.
Discussion Mononuclearity (Site Isolation) of the Rhodium Complex on the Support. The reaction between an organometallic complex containing a basic anionic ligand and an oxide support depends upon the reactivity of the support, as influenced by its acid/base properties and the density of surface OH groups, which can be (40) Anderson, J. A.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1989, 85, 1117. (41) Foster, M.; Passno, D.; Rudberg, J. J. Vac. Sci. Technol., A 2004, 22, 1640. (42) Kyto¨kivi, A.; Rautiainen, A.; Root, A. J. Chem. Soc., Faraday Trans. 1997, 93, 4079. (43) Serp, P.; Kalck, P.; Feurer, R. Chem. ReV. 2002, 102, 3085.
Rhodium Complex Supported on Highly Dehydroxylated MgO
removed by treatment at high temperatures to remove water (dehydroxylation).44 Higher degrees of surface hydroxylation facilitate the migration of supported species and hence the tendency of the metal to form aggregates and ultimately (reduced) metal particles.45 The group 8 metal rhodium is easily reduced, and attempts to prepare isolated rhodium complexes with allyl ligands on SiO2 readily led to the formation of rhodium nanoparticles.8-10 The results presented here show that dehydroxylation of the MgO at the relatively high temperature of 973 K gave a support that allows the grafting reaction of our precursor to proceed cleanly, as shown by the EXAFS data demonstrating that the complexes formed on MgO remained mononuclear and did not undergo aggregation. The results thus indicate the first evidence of a rhodium olefin complex bonded directly to an oxide support through metal-oxygen bonds. Ethylene Bonded to Rhodium Complexes. The IR and NMR spectra demonstrate the presence of π-bonded ethylene ligands on the supported rhodium complexes. The ethylene ligands in the precursor and in related rhodium complexes are thermodynamically rather stable. The complex Rh(C2H4)2(acac) is stable in glycol solution up to 353 K without evolution of ethylene; it decomposes at 417 K.25,46 Vierko¨tter et al.12 found that [RhClL]2 supported on γ-Al2O3 (where L ) (C2H4)2, (CO)2, and (COD) and COD ) cycloocta-1,5-diene) underwent dissociation of the chloride ligands, with the ethylene remaining bonded to the rhodium. Our results are broadly consistent with these observations. The EXAFS data show that the supported mononuclear rhodium complex was bonded to approximately one or two oxygen atoms of the support. We approximate it as Rh(C2H4)2{OMg}2 (where the braces denote groups that terminate the solid support), with two oxygen atoms bonded to the rhodium, because this 16-electron complex is expected to be much more stable than a 14-electron complex with one fewer oxygen ligand of the support. Structural Model of the Supported Rhodium Ethylene Complex. A structural model that is consistent with all of the data and characterized by interatomic distances determined by the EXAFS spectra is shown in Figure 5. We emphasize that the model is schematic with respect to the bonding of the rhodium to the support, because the exact positions on the MgO surface where the rhodium is bonded are unknown, corresponding to the intrinsic nonuniformity of the surface and the expected presence of defect sites. We expect that the complexes would be bonded at more than one type of site. Reaction of acac Ligands. As stated above, we infer that the acac ligands reacted in the process of anchoring the rhodium complex to the support, forming new surface species derived from the acac. IR results indicate the formation of Mg(acac) and Hacac/MgO species formed on the support. A review of the chemistry of reactions of various metal acac complexes with metal oxide surfaces35,47,48 indicates the formation of a variety of surface species derived from acac, depending upon the other ligands present in the complex and the metal oxide. For example, it has been reported that tertiary amines can displace the β-diketonate ligand from (1,5-COD)Rh(β-diketonate) in prefer(44) Dun˜ski, H.; Jo´zwiak, W. K.; Sugier, H. J. Catal. 1994, 146, 166. (45) Basset, J.-M.; Lefebvre, F.; Santini, C. Coord. Chem. ReV. 1998, 178180, 1703. (46) Cramer, R. J. Am. Chem. Soc. 1964, 86, 217. (47) Van Der Voort, P.; van Welzenis, R.; de Ridder, M.; Brongersma, H. H.; Baltes, M.; Mathieu, M.; van de Ven, P. C.; Vansant, E. F. Langmuir 2002, 18, 4420. (48) Van Der Voort, P.; Mathieu, M.; Vansant, E. F.; Rao, S. N. R.; White, M. G. J. Porous Mater. 1998, 5, 305.
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Figure 5. Schematic representation of the rhodium ethylene complex bonded to MgO. This depiction represents one of several coordination environments that are consistent with the EXAFS data characterizing the supported complex. Mg and O atoms are shown at unrelaxed positions in the bulk crystal structure of MgO, and the Rh atom is bound to two O atoms at a corner defect site. Two ethylene ligands are π-bonded to the metal ion.
ence to the olefin ligand.49 On the basis of our evidence of the surface species, we suggest that the reactivity of the basic MgO surface is analogous to that of the tertiary amine. Reactivity. Exposure of the supported rhodium ethylene complex to CO led to the displacement of ethylene ligands from the rhodium and the formation of rhodium gem dicarbonyls bonded to the support. This reaction is as expected, because CO is a stronger bonding ligand than ethylene, and rhodium gem dicarbonyls are known to be stable on various metal oxides.33 In analogous solution chemistry, Rh(C2H4)2(acac) has been used to prepare Rh(CO)2(acac) by treatment with CO.46 The exchange of C2D4 with C2H4 from the gas phase is also analogous to the exchange reaction occurring in solution; although the C2H4 ligands in Rh(C2H4)2(acac) exchange with various olefins in solution, the complex Rh(C2H4)2(C5H5) does not similarly exchange its ethylene ligands. This result has been explained on the basis of the former being a 16-electron species and the latter being a coordinatively saturated 18-electron species.46 The exchange occurring in our supported species provides evidence confirming the EXAFS spectra and our postulate of a 16-electron species, which is expected to be nearly square planar. The reaction of the supported species with H2 led to the formation of ethane in the gas phase. This reaction likely proceeds through ethyl group formation on rhodium and subsequent hydrogenation to give ethane. Thus, we suggest that the supported rhodium diethylene species might be a reaction intermediate in catalytic ethylene hydrogenation, which took place at room temperature. The supported complex is active as a catalyst for ethylene hydrogenation at 298 K. Vizza et al.50 reported the synthesis and (49) Duan, Z.; Hampden-Smith, M. J.; Duesler, E. N.; Rheingold, A. L. Polyhedron 1994, 13, 609. (50) Bianchini, C.; Burnaby, D. G.; Evans, J.; Frediani, P.; Meli, A.; Oberhauser, W.; Psaro, R.; Sordelli, L.; Vizza, F. J. Am. Chem. Soc. 1999, 121, 5961.
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characterization of tripodal polyphosphine rhodium catalysts immobilized on silica via hydrogen bonding of the sulfonate tail to the surface silanol groups. However, because of the presence of unreactive ligands such as COD (COD ) cycloocta-1,5-diene) or CO, these materials were found to be inactive for ethylene or propylene hydrogenation, requiring a high-temperature treatment (at 423 K) for activation by removal of COD or CO. Remarkably, the rhodium atoms were still isolated under catalytic reaction conditions at 393 K, with no evidence of formation of rhodium aggregates by EXAFS spectroscopy. The authors attributed the hydrogenation activity to isolated rhodium species. No catalytic activity data were presented. This is one of a number of examples of catalysis attributed to mononuclear rhodium complexes on supports, but we emphasize that, in contrast to our catalyst, the ligands bonded to the rhodium evidently did not include oxygen atoms of the support. There are reports of supported rhodium complexes anchored via tethered functional groups on supports, some of which have been found to be active for catalytic hydrogenation of olefins, exemplified by n-pentene, 1-hexene, 1-octene, and cyclohexene,51-53 among others,13,54 and the anchoring of the rhodium species seems to have enhanced activities in most cases. However, because of the presence of the ligands usually associated with the precursor rhodium complexes in these catalysts, they are invariably complicated and lead to difficulties in determining (51) Sa´nchez, F.; Iglesias, M.; Corma, A.; del Pino, C. J. Mol. Catal. 1991, 70, 369. (52) Capka, M.; Czakoova, M.; Urbaniak, W.; Schubert, U. J. Mol. Catal. 1992, 74, 335. (53) Tada, M.; Sasaki, T.; Iwasawa, Y. Phys. Chem. Chem. Phys. 2002, 4, 4561. (54) Schneider, M. E.; Mo¨hring, U.; Werner, H. J. Organomet. Chem. 1996, 520, 181.
Bhirud et al.
relationships between the catalytic activity and structure. Our sample offers new opportunities for determining such relationships.
Conclusions IR, NMR, and EXAFS spectroscopies were used to characterize the supported rhodium complex formed by adsorption of Rh(C2H4)2(acac) on MgO. The results provide evidence of mononuclear rhodium species bonded to the support through Rh-O bonds, with ethylene ligands bonded to the rhodium. The supported complex undergoes facile ligand exchange with C2D4 and reacts with CO to form rhodium gem dicarbonyl complexes anchored to the MgO. The ethylene ligands on the rhodium react with H2 to give ethane, and the supported complex is active as a catalyst for hydrogenation of ethylene at room temperature. Thus, this sample is one of the more nearly uniform and structurally simple oxide-supported metal complex catalysts and is unique in having (ethylene) ligands that are possibly present as reaction intermediates in olefin hydrogenation catalysis. Acknowledgment. This research was supported by the DOEBES, contract numbers DE-FG02-04ER15600 and DE-FG0204ER15598. We acknowledge beam time and the support of the U.S. Department of Energy, Division of Materials Sciences, under contract number DE-FG05-89ER45384, for its role in the operation and development of beamline X-18B at the National Synchrotron Light Source, supported by the Department of Energy, Division of Materials Sciences and Division of Chemical Sciences (contract number DE-AC02-76CH00016). We are grateful to the staff of beamline X-18B for their assistance. Supporting Information Available: Crystallographic data characterizing the reference compounds and the Fourier Transform ranges used in EXAFS analysis. This material is available free of charge via the Internet at http://pubs.acs.org. LA052268F
