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Reactions of Highly Uniform Zeolite H-β-Supported Rhodium Complexes: Transient Characterization by Infrared and X-ray Absorption Spectroscopies Isao Ogino and Bruce C. Gates* Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: January 23, 2010; ReVised Manuscript ReceiVed: March 18, 2010
A zeolite H-β-supported mononuclear rhodium diethene complex (Rh(C2H4)2{O2Al}, where the braces indicate a part of the zeolite) was formed by the reaction of Rh(acac)(η2-C2H4)2 (acac ) acetylacetonate, C5H7O2-) with the zeolite. Transient characterization of the sample by X-ray absorption near edge structure (XANES) and infrared (IR) spectroscopies (combined with mass spectrometry of the effluent gas) while the sample was in contact with flowing CO indicates a simple stoichiometric conversion of the supported metal complex into another species, identified by the spectra as the zeolite-supported rhodium gem-dicarbonyl (Rh(CO)2{O2Al}). The sharpness of the νCO bands in the IR spectrum indicates a high degree of uniformity of the supported rhodium gem-dicarbonyl, and isosbestic points in the XANES spectra as the transformation was occurring imply that the rhodium diethene complex was also highly uniform. Spectra similarly show that treatment of the supported rhodium gem-dicarbonyl with flowing C2H4 resulted in another stoichiometrically simple transformation, giving a species suggested to be Rh(C2H4)(CO)2{O2Al}. The intermediate was ultimately transformed when the sample was purged with helium into another highly uniform supported species, inferred on the basis of IR spectra to be Rh(C2H4)(CO){O2Al}. Extended X-ray absorption fine structure spectra characterizing the supported rhodium diethene complex and the species formed from it show how the Rh-O bond distance at the Rh-support interface varied in response to the changes in the ligands bonded to the rhodium. Introduction Although oxide-supported metal complexes are important catalysts,1-7 understanding of their chemistry is hindered by the complexity of their structures, which in part is a consequence of the intrinsic nonuniformity of the support surfaces. In contrast, crystalline supports such as zeolites provide nearly uniform surface sites and therefore opportunities for the formation of nearly uniform supported metal complexes, which offer the prospect of detailed understanding of the structures and reactivities of the metal complexes. Examples of zeolite-supported metal complexes are those formed by the reactions of metal complexes with acetylacetonate (acac) ligands, including Rh(acac)(η2-C2H4)2 (I),8-12 Ir(acac)(η2-C2H4)2,13-15 and cis-Ru(acac)2(η2-C2H4)216-19 with dealuminated HY zeolite. Spectra and calculations at the level of density functional theory characterizing the dealuminated HY zeolite-supported rhodium diethene complex Rh(η2-C2H4)2+ formed from I showed good agreement with each other.12 Thus, the uniformity of the supported species allows the application of theory to guide the spectroscopic assignments and to make predictions of structures and reactivities of highly reactive species that may not be observable, including catalytic reaction intermediates.10 We also recognize an opportunity to compare various zeolites as ligands in these metal complexes and to investigate the chemistry of various ligand-exchange reactions by using gasphase reactants to eliminate the solvent effects that complicate matters in solution chemistry. Characterization of supported metal complexes can benefit from the application of transient spectroscopic techniques that allow elucidation of the pathways for transformation of one * To whom correspondence should be addressed. E-mail: bcgates@ ucdavis.edu.
metal complex into another and any accompanying changes in the metal-support interface. The usefulness of such transient methods was illustrated in recent investigations of the formation of metal clusters from mononuclear metal complexes (and the reverse);11,14,15 the spectroscopic methods used in these experiments were time-resolved X-ray absorption near edge structure (XANES) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and infrared (IR) spectroscopy. There are only few data characterizing the conversions of highly uniform supported metal complexes. Here we report the conversion of the rhodium diethene complex Rh(η2-C2H4)2+ formed from I and supported on zeolite H-β. This rhodium complex was chosen because it incorporates highly reactive ethene ligands and thereby facilitates investigations of reactions at low enough temperatures to prevent the complication of rhodium cluster formation.11 Zeolite β was chosen as the crystalline support because (a) it has large three-dimensional pores for easy access of the precursor I into the interior pore structure for formation of the supported rhodium complex and (b) it is available with low Al-Si ratios to allow formation of cationic rhodium complexes at the Al sites with a wide separation of these species from each other to maximize the likelihood that that they will act independently of each other and in a sense constitute nearly ideal site-isolated supported species. The reactions of the supported rhodium diethene complexes include the conversion with CO to form rhodium carbonyls, which are readily characterized by νCO IR spectra; we investigated this conversion and the conversion of the resultant rhodium carbonyls with ethene to test the reversibility of the reaction and to establish patterns of reactivity that might be relevant to catalytic reactions involving both ethene and CO ligands, such
10.1021/jp100673y 2010 American Chemical Society Published on Web 04/02/2010
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as alkene hydroformylation, which has been shown to be catalyzed by rhodium complexes supported on hydroxyapatite.20 The reactions were investigated with the spectroscopic methods mentioned above to provide the following insights into the chemistry: The IR spectra of the rhodium carbonyls demonstrate a high degree of uniformity of the supported species; transient XANES spectra demonstrate that the conversions of the supported species are stoichiometrically simple (as shown by the presence of isosbestic points); and EXAFS spectra demonstrate how the Rh-O bond length at the rhodium-support interface varies in response to changes in the ligands on the rhodium. Experimental Methods Materials and Procedures. Samples were handled with airexclusion techniques, including Schlenk techniques, and stored in a glovebox under dry argon. The O2 and moisture contents in the glovebox atmosphere were each less than 0.1 ppm. Glassware was dried overnight at 393 K prior to use. n-Pentane solvent (Fisher Scientific) was purified in chromatographic columns containing particles of activated Al2O3 and Al2O3supported activated copper (MBraun, MB-SPS). The rhodium complex Rh(acac)(η2-C2H4)2 (I) (Strem, 99%) was used as received. The zeolite β (CP814C, Zeolyst International, Si/Al atomic ratio )19) was used as a powder (without binders) in the NH4+ form and converted to the H+ form as described in the following section. Helium, CO, and C2H4 (all from Airgas, UHP grade) were purified by passage through traps containing reduced Cu/Al2O3 and activated zeolite 4A to remove traces of O2 and moisture, respectively. Synthesis of Zeolite-Supported Rhodium Complexes. The zeolite was calcined by heating in flowing dry O2 from room temperature to 773 K over a period of 3 h, held in flowing O2 at 773 K for 4 h, and then held under vacuum at 773 K for an additional 16 h. The calcined zeolite under vacuum was cooled to room temperature and used immediately for sample preparation, as follows: to the zeolite (1.0 g) in a 100 mL Schlenk flask with a stir bar was added complex I (0.025 g). To this mixture was added freshly distilled n-pentane (ca. 20 mL). The mixture was stirred at room temperature, and after 24 h the n-pentane solvent was removed by evacuation, giving a powder with a light-yellow color, containing 1.0 wt % Rh (Rh/Al atomic ratio ≈ 1/8). Characterization of Supported Metal Complexes. IR Spectroscopy. A Bruker IFS 66v/S spectrometer with a spectral resolution of 2 cm-1 was used to collect transmission IR spectra of zeolite-supported rhodium samples. In an argon-filled glovebox, the supported sample (approximately 30 mg) was pressed into a thin wafer and loaded into the cell (In-situ Research Institute, South Bend, IN), through which reactive gases flowed. The cell was sealed and connected to a flow system without exposure of the sample to air, so that spectra could be recorded while gases (helium, CO, C2H4, and/or mixtures thereof) flowed through the cell at 298 K and atmospheric pressure. Most experiments were done with a DTGS detector that allowed recording of one set of 64 scans in approximately 2 min; the corresponding spectra reported here are the averages of 64 scans. Alternatively, some experiments were done with an MCT detector that allowed recording of one set of 6 scans in approximately 10 s; the corresponding spectra reported here are the averages of 6 scans. The experiments were started with the supported sample in flowing helium. Then, the inlet flow was switched to 0.3 mol % CO in helium for 60 min, followed by a purge of the cell with helium for 20 min. Then the feed was switched to 60 mol % C2H4 in helium for 60 min, followed by 20 min of helium
Ogino and Gates purge of the cell. IR spectra were recorded continuously; separate experiments were carried out with each of the detectors. Mass Spectrometry of Effluent Gases. As the IR spectra were being recorded, the effluent gases flowing from the IR cell were monitored periodically by mass spectrometry with an online Balzers OmniStar mass spectrometer. X-ray Absorption Spectroscopy (XAS). X-ray absorption spectra of the supported samples were recorded at X-ray beamline X18-B at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The storage ring electron energy and ring currents were 2.8 GeV and 200-300 mA, respectively. A Si(111) double-crystal monochromator was used. In an N2-filled glovebox at the NSLS, the zeolite-supported rhodium sample was packed into a slit (approximately 4 mm × 20 mm × 10 mm) in a flow-through cell, described elsewhere;21 the gas flowed into the 4 mm × 20 mm face. Gas flow through this bed of powder was not plug flow (there was a substantial distribution of residence times).22 The cell was sealed in the glovebox and connected to a flow system without exposure of the sample to air. The mass of each powder sample (approximately 0.5 g) was chosen to yield optimal absorption measurements at the Rh K edge (23 220 eV) (giving an X-ray absorbance of approximately 2.0 calculated at an energy 50 eV greater than the absorption edge). The beam exiting the synchrotron ring passed first through an ion chamber containing a mixture of 80% N2 and 20% argon by volume, then through the sample cell, an ion chamber containing a mixture of 30% krypton and 70% argon by volume, a cell consisting of two plates containing the reference rhodium foil and essentially no gas, and an ion chamber containing argon. The cross-sectional area of the X-ray beam was approximately 1 mm × 10 mm, and it was positioned at the center of the sample. X-ray absorption spectra were recorded with a rhodium foil and with the supported sample at 298 K and atmospheric pressure. XANES spectra were recorded approximately once every 2 min, and EXAFS spectra were recorded approximately once every 15 min. At the start of the XAS experiments, four EXAFS spectra were recorded as the initially prepared sample was in contact with flowing helium in the XAS cell. Then, the feed stream was switched to 0.3 mol % CO in helium for 80 min as XANES spectra were recorded as the sample reacted with the CO. This sequence of experiments was followed by a purge of the cell with helium for 60 min. Then, four EXAFS scans were recorded. The feed stream was then switched to 60 mol % C2H4 in helium for 80 min, and XANES spectra were recorded to characterize the changes in the sample. Then the cell was purged with helium for 60 min, and four EXAFS scans were recorded as the helium flow continued. EXAFS Data Analysis. The X-ray absorption edge energy was calibrated with the measured signal of a rhodium foil (scanned simultaneously with the sample) at the Rh K edge, which was taken to be the inflection point at 23220 eV. The data were normalized by dividing the absorption intensity by the height of the absorption edge. Analysis of the EXAFS data was carried out with the software XDAP,23,24 which allowed the efficient application of a difference-file technique24 for determination of optimized fit parameters. Structural models postulated for the supported rhodium species were compared with the EXAFS data; each of the candidate models included the plausible contributions Rh-Rh, Rh-C, Rh-O, and Rh-Al. Reference files, with backscattering amplitudes and phase shifts for these contributions, were calculated with the software FEFF7.025 on the basis of crystal-
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TABLE 1: EXAFS Results at the Rh K Edge Characterizing the Zeolite-Supported Rhodium Complexes in Helium Flow and after CO Treatment and after C2H4 Treatmenta gas stream in contact with sample He 0.3 mol % CO in He 60 mol % C2H4 in He
Rh-O contribution N
R/ Å
2.1 2.1
2.19 2.10
3.4b 2.15b
Rh-C contribution
∆σ2 × 103/ ∆E0 / Å2 eV 9.9 5.0
6.4 3.4
8.7b
-1.5b
Rh-O* contribution
∆σ2 × 103/ ∆E0 / Å2 eV
N
R/ Å
4.4 1.9
2.11 1.85
5.9 8.5
0.88 1.79
3.0
∆σ2 × 103/ ∆E0/ Å2 eV
Rh-Al contribution
N
R/ Å
0.13 8.8
1.8
2.98
4.6
1.2 -0.93 1.1
8.5
0.92 2.95
3.6
-1.3
N
R/ Å
∆σ2 × 103/ ∆E0 / Å2 eV
3.00 3.10
7.4 13
-0.33 -9.8
0.82 3.06
5.4
4.1
a Estimated errors: coordination number N, (20%; interatomic distance R, (0.02 Å; sigma-squared value (Debye-Waller factor) ∆σ2, (20%; inner potential correction ∆E0, (10%. b This contribution averaged over the Rh-Ozeolite and Rh-Cethene contributions. Fit ranges: 3.63 < k < 14.96 Å-1 and 0.8 < R < 3.0 Å for the initially prepared sample, 2.14 < k < 14.95 Å-1 and 0.5 < R < 3.0 Å for the supported sample treated with CO; 2.48 < k < 14.88 Å-1 and 1.0 < R < 3.0 Å for the supported sample treated with C2H4. The coordination number and distance of the Rh-Al contribution were not determined with as much confidence as the other parameters. The Rh-O* contribution was characterized by colinear multiple scattering in the group Rh-C-O.
lographic coordinates of the unit cells of the reference compounds Rh(acac)(η2-C2H4)226 (for the Rh-C and Rh-O contributions), Rh(acac)(CO)227 (for the Rh-O* contribution; O* is carbonyl oxygen), Rh-Al alloy23 (for the Rh-Al contribution), and rhodium metal28 (for the Rh-Rh contribution). Analysis was carried out with unfiltered data; iterative fitting was performed until optimum agreement was attained between the calculated k1-, k2-, and k3-weighted EXAFS data and each postulated model. The data were fitted in R-space with the Fouriertransformed χ-data (R is the distance from the absorbing Rh atom; χ is the EXAFS function). The number of parameters used in the fitting was always less than the statistically justified number, computed with the Nyquist theorem (17 for the sample in flowing helium and 18 for the sample after CO or C2H4 treatment).29 Details of the models and the fits are presented in Supporting Information, where the bases for discrimination among them are stated in detail. The recommended model includes a Rh-Al contribution. It is not surprising that the Rh-Al contribution is indicated for the supported mononuclear species, as the charge-compensating site for the cationic rhodium complexes is expected to be associated with the aluminum sites, as discussed elsewhere.8-10,12,16,30 Results Structural Characterization of Initially Prepared ZeoliteSupported Rhodium Diethene Complex. EXAFS data reported earlier (Table 1) characterize the H-β zeolite-supported rhodium ethene complex prepared by the reaction of I with the zeolite; these data were obtained with the sample under vacuum at 77 K.16 Our EXAFS data (Table 1 and Figure 1S in the Supporting Information) characterizing the same supported sample in flowing helium at 298 K essentially match those reported,16 confirming that the sample was stable at 298 K and confirming that (within error) each Rh atom was bonded on average to 2 ethene ligands and to 2 oxygen atoms of the zeolite, in a structure represented schematically as follows (where the oxygen atoms are part of the zeolite lattice):
Reaction of Supported Rhodium Ethene Complex with CO to form Rhodium gem-Dicarbonyl. IR and EXAFS Spectroscopies. When a steadily flowing stream of 0.3 mol % CO in helium was brought in contact with the supported rhodium
Figure 1. IR spectra in the νCO region characterizing the supported rhodium complex. Data represent the sample after treatment in (A) helium, (B) 0.3 mol % CO in helium for 60 min, followed by a purge of the cell with helium for 20 min.
diethene complex at 298 K and atmospheric pressure, rhodium gem-dicarbonyl species formed rapidly, as shown by the appearance of two IR bands, at 2115 and 2048 cm-1, assigned to νCO, consistent with our previous results16 (spectrum B, Figure 1). The frequencies of the νCO bands are similar to (but higher than) those characterizing Rh(acac)(CO)2 in THF solution (2081, 2010 cm-1),31 indicating that the supported rhodium species were rhodium gem-dicarbonyls. The satellite peak at 2019 cm-1 in the νCO spectrum (Figure 1) is attributed to rhodium gem-carbonyls incorporating both 13 CO and 12CO.30 The very weak band at 2079 cm-1 is tentatively assigned to a tiny amount of a rhodium tricarbonyl species, Rh(12CO)3.32 The supported rhodium dicarbonyls are formally cationic species incorporating Rh(I) (as is I); thus, they are inferred to have been bonded at aluminum sites of the zeolite (these are the cation exchange sites and sites where various cationic metal complexes are bonded30). The frequencies of the νCO bands show that the rhodium in the chemisorbed rhodium gem-dicarbonyl is more electron deficient than that in Rh(acac)(CO)2.18,30 These νCO bands are relatively narrow, with the fwhm (full width at half-maximum) of each being
