J. Phys. Chem. C 2008, 112, 3383-3391
3383
Alumina-Supported Trirhenium Clusters: Stable High-Temperature Catalysts for Methylcyclohexane Conversion Rodrigo J. Lobo-Lapidus,† Michael J. McCall,‡ Mary Lanuza,‡ Susan Tonnesen,‡ Simon R. Bare,‡ and Bruce C. Gates*,† †Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616, and UOP LLC, 25 East Algonquin Road, Des Plaines, Illinois 60017
ReceiVed: October 12, 2007; In Final Form: NoVember 20, 2007
Samples prepared from H3Re3(CO)12 adsorbed on porous γ-Al2O3 were decarbonylated at 773 K in flowing H2 and characterized by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge spectra show that rhenium in the treated sample was cationic, and extended X-ray absorption fine structure spectra show a Re-Re first-shell coordination number of approximately 2, consistent with trirhenium clusters bonded to the support. The samples were tested as catalysts for the conversion of methylcyclohexane in the presence of H2 at atmospheric pressure and at 723 and at 773 K in a flow reactor. A range of hydrocarbon products was observed, indicating the occurrence of dehydrogenation, isomerization, ring opening, and hydrocracking reactions. The catalyst used at 723 K underwent deactivation over a period of several hours, during which the selectivity for the major dehydrogenation product (toluene) increased significantly. At 773 K, the catalyst underwent activation, during which the product distribution changed. This increase in activity was retained when the temperature was reduced to 723 K, resulting in higher activity and different selectivity relative to what had been observed before at this temperature. The fresh and used catalyst samples were characterized by X-ray absorption spectroscopy, which showed that the trirhenium framework remained intact after catalysis, although changes in the rhenium coordination were observed. The catalytically active species are inferred to be trirhenium.
Introduction Metal particles on high-surface-area inorganic supports are widely used as catalysts for hydrocarbon conversions, and, to a first approximation, these particles are often well represented as small crystallites of bulk metal. However, when the particles become so small that they no longer have bulk properties, they offer new reactivities and catalytic properties.1 The most commonly used supported metals are of group 8, but the group 7 metal rhenium finds important applications in combination with platinum for catalytic reforming of naphtha.2 Rhenium on oxide supports has also been reported to catalyze conversion of cyclopropane3 and conversion of polycyclic aromatics into xylenes.4 Being more oxophilic than group 8 metals, rhenium in small clusters on oxide supports has a tendency to be present in a cationic form. Fung et al.5 found that γ-Al2O3-supported samples made from H3Re3(CO)12 were converted into supported clusters that were well approximated as Re3; the rhenium was inferred to be present in a positive oxidation state. Fung’s samples were not tested as catalysts. Our goal was to investigate such samples as catalysts for conversion of hydrocarbons under conditions similar to those of potentially useful processes. Methylcyclohexane conversion in the presence of H2 was used to probe the catalyst performance. The catalysts were characterized by X-ray absorption spectroscopy, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EX* To whom correspondence should be addressed. E-mail: bcgates@ ucdavis.edu. † University of California. ‡ UOP LLC.
AFS) spectroscopy to determine the state of the rhenium before and after catalysis. Experimental Methods Materials. Catalysts were prepared from the precursor H3Re3(CO)12, which was synthesized from Re2(CO)10 (Pressure chemicals), NaBH4 (98+%, Acros), tetrahydrofuran (THF, 99+%, Sigma-Aldrich), phosphoric acid (85%, Sigma-Aldrich), and cyclohexane (99%, EM Science). H3Re3(CO)12 dissolved in n-pentane (redistilled over sodium and benzophenone) was brought into contact with the support γ-Al2O3 (Degussa, AluC) according to the method of Fung et al.5 Reference compounds for X-ray absorption spectroscopy (XAS) were mixed with boron nitride powder (98%, Acros) in order to produce uniform samples. The following compounds were used as received as references for characterization of the catalysts by XAS: CH3ReO3 (98%, Strem), ReCl3 (99.8+%, Alfa Aesar), ReCl4 (99.8%, Alfa Aesar), ReCl5 (99.9%, Alfa Aesar), ReO3 (99.9%, Alfa Aesar), ReO2 (99.9%, Alfa Aesar), Re2O7 (99.9%, Alfa Aesar), and KReO4 (99%, Alfa Aesar). Rhenium powder (99.99%, Strem) was also used as a reference after treatment in H2 at 873 K overnight and storage under argon. The following compounds were used in the catalytic reaction experiments: methylcyclohexane (99.9%, Sigma-Aldrich) and H2 and N2 (Praxair), each of the latter treated with high-surfacearea sodium to give 11 Å-1. The functions used for the error minimization in k-space and in R-space are stated elsewhere.8 Criteria used to judge the appropriateness of a model tested in the data fitting included that both the magnitude and the imaginary part of the Fourier-transformed data fit well with both k1 and k3 weightings. The use of both weightings is required to minimize the correlations between the fitting parameters (e.g., the coordination number is correlated with the sigma-squared factor, and the distance is correlated with ∆E0).8 Amplitude- and phase-shift functions characterizing the various EXAFS contributions were obtained from theoretical calculations by use of the software FEFF7.10 The structures used as inputs for the calculations were those of compounds having known crystal structures.11,12 The data of Table 1 indicate the compounds used for each EXAFS contribution. To estimate the value of the amplitude reduction factor in the EXAFS equation (S02), the EXAFS spectra characterizing the precursor H3Re3(CO)12 were analyzed by fixing the values of interatomic distances and coordination numbers to those determined by X-ray diffraction crystallography data13 and allowing the values of S02 to vary. The values of the inner potential correction, ∆E0, and the sigma-squared factor, ∆σ,2 with respect to the reference material were also allowed to vary for each contribution in the determination of the value of S02. A satisfactory value of S02 of 0.91 was obtained and used for analysis of the data characterizing the catalyst samples.14
Alumina-Supported Trirhenium Clusters
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3385
Figure 1. IR spectra in the νCO region characterizing (A) a sample prepared from H3Re3(CO)12 and γ-Al2O3 (as prepared sample) and (B) a sample prepared from H3Re3(CO)12 and γ-Al2O3 treated for 2 h at 500 °C in flowing H2.
TABLE 2: List of Reference Compounds with Rhenium in Various Oxidation States and Their Re L3 Edge Shiftsa compound/sample KReO4 CH3ReO4 NH4ReO4 Re2O7 ReO2 ReO3 ReCl5 ReCl4 ReCl3 sample formed from H3Re3(CO)12 on γ-Al2O3 after H2 treatment at 723 K sample formed from H3Re3(CO)12 on γ-Al2O3 after H2 treatment at 773 K
formal oxidation state of Re
Re L3 edge shift (eV)
+7 +7 +7 +7 +4 +6 +5 +4 +3
6.5 ( 0.3 6.6 ( 0.5 5.9 ( 0.4 6.9 ( 0.5 6.0 ( 0.3 6.5 ( 0.5 5.6 ( 0.6 4.5 ( 0.6 4.5 ( 0.3 5.0 ( 0.8 5.0 ( 0.4b
a Error bars represent one standard deviation, calculated on the basis of the error determined by repeat measurements and the point spacing used. b Scanned with a resolution of 0.35 eV at SSRL.
Results Decarbonylated Trirhenium Clusters on a Support. Samples formed by adsorption of H3Re3(CO)12 on γ-Al2O3 were treated at temperatures up to 773 K in the presence of flowing H2 at 1 bar. IR spectra of the resulting samples (Figure 1) include no peaks in the νCO region (2200 to 1700 cm-1), indicating complete decarbonylation. This result is consistent with the observation of Fung et al.,5 whose samples were shown to have undergone complete decarbonylation after treatment at 673 K. X-ray absorption spectra characterizing the sample prepared from H3Re3(CO)12 supported on γ-Al2O3 after treatments at 723 and at 773 K show that the edge position was shifted with respect to that of rhenium metal by 5.3 ( 0.8 and 5.0 ( 0.4 eV, respectively, indicating that the species resulting from the decarbonylation were cationic (in contrast to what would be expected for supported metallic rhenium particles). The edge positions characterizing the samples treated at 723 and 773 K are compared in Table 2 and Figure 2 with those of references incorporating rhenium in known oxidation states.17 On the basis of the comparisons, it is estimated that the formal oxidation state of rhenium in the supported samples was in the range of
Figure 2. Edge shifts determined by XANES characterizing reference compounds. The points represent data determined for the reference materials; the lines represent the ranges of edge shifts observed for the sample incorporating clusters approximated as trirhenium on the γ-Al2O3 support; the dotted line represents the range of edge shifts for the supported sample treated in H2 at 723 K; and the full line represents the range of edge shifts for the supported sample treated in H2 at 773 K.
TABLE 3: EXAFS Fitting Parameters Characterizing the Sample Treated at 773 K in Flowing H2a,b shell
N
∆σ2 × 103 (Å2)
R (Å)
∆E0 (eV)
Re-O Re-Re Re-O1
0.9 ( 0.1 2.1 ( 0.1 0.7 ( 0.1
0.2 ( 0.1 7.4 ( 0.3 0.5 ( 0.3
2.01 ( 0.01 2.65 ( 0.01 2.48 ( 0.01
-0.9 ( 0.5 -0.9 ( 0.5 -8.8 ( 0.5
a N is the coordination number, R is the distance between absorber and backscatterer atoms, ∆σ2 is the sigma-squared value relative to the reference, and ∆E0 is the inner potential correction relative to the reference. The goodness of fit is 10.1; the value of (∆χ)2 is 18.1;24 the k range used in the fit was 4.3-13.65 Å-1. The mean free path of the ejected electron was taken to be 6 Å. b The estimated accuracies associated with the parameters representing the metal shell are as follows: N, (10%; R, (0.02 Å; ∆σ,2 (20%; ∆E0, (20%. Those representing the metal-light backscatterer combinations are as follows: N, (20%; R, (0.04 Å; ∆σ,2 (20%; ∆E0, (20%.
3 to 7 and 3 to 5, respectively. This estimate is rough because of the uncertainty associated with the determination of the edge position. The results of the analysis of the EXAFS region of the spectrum characterizing the sample decarbonylated at 773 K are shown in Table 3. Plots of the fits in k-space and in R-space are shown in Figure 3. The data indicate a Re-Re coordination number of approximately 2, with an internuclear distance of 2.67 Å, which is significantly shorter than that characteristic of metallic rhenium (2.74 Å). The short distance between the Re atoms indicates that the rhenium was not present in a metallic form after the high-temperature treatment in H2; rather, the trirhenium framework was maintained during the treatment. Two Re-O shells were found in the fits (Table 3), the shorter one at a distance consistent with bonding of the rhenium to an oxygen of the support,18 a result that further supports the inference that the rhenium in the sample was cationic and not metallic in nature. Results of the EXAFS analysis characterizing the sample decarbonylated at 723 K in H2 are shown in Table 4. The corresponding plots of the fits in k-space and in R-space are shown in Figure 4. The fit parameters, within error, are the same as those obtained for the sample treated at 773 K, but the quality of the data is significantly less than that of the data obtained at 773 K, and consequently, the fits are less reliable.19 Furthermore,
3386 J. Phys. Chem. C, Vol. 112, No. 9, 2008
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Figure 3. EXAFS data characterizing a supported rhenium cluster sample that had been decarbonylated at 773 K (data, continuous line; best-fit model, broken line): (a) k1-weighted EXAFS function in k-space; (b) imaginary part and magnitude of the k1-weighted EXAFS function in R-space; and (c) imaginary part and magnitude of the k3weighted EXAFS function in R-space.
TABLE 4: EXAFS Fitting Parameters Characterizing the Sample Treated at 723 K in Flowing H2a shell
N
Re-O Re-Re Re-O1
0.5 ( 0.1 2.4 ( 0.1 1.1 ( 0.1
∆σ2
× 10 (Å ) 3
0.1 ( 1.0 2.3 ( 0.3 3.1 ( 0.8
2
R (Å)
∆E0 (eV)
2.04 ( 0.01 2.71 ( 0.01 2.51 ( 0.01
-6.3 ( 0.3 -6.3 ( 0.3 -6.1 ( 0.1
a The goodness of fit was found to be 3.3. The value of (∆χ)2 was found to be 5.6. The k range for the fit was found to be 2.58-12.45 Å-1.
the Re-Re distance characterizing the sample treated at 723 K is also, within error, indistinguishable from that of metallic rhenium; however, because the Re-O and Re-Ol coordination
Figure 4. EXAFS data characterizing a supported rhenium sample that had been decarbonylated at 723 K (data, continuous line; best-fit model, broken line): (a) k1-weighted EXAFS function in k-space; (b) imaginary part and magnitude of the k1-weighted EXAFS function in R-space; and (c) imaginary part and magnitude of the k3-weighted EXAFS function in R-space.
numbers remain indistinguishable (within error) from those of the sample treated at 773 K and the XANES spectrum indicates that the rhenium clusters were cationic, we infer that the rhenium clusters were not metallic. This inference is further supported by the EXAFS results of Fung et al.5 and those shown here for the same sample treated in H2 at 673 and at 773 K, respectively, which suggest the presence of cationic trirhenium clusters bound to the support. Catalysis of Methylcyclohexane Conversion. Catalytic ActiVity. Figure 5 shows the conversion profiles (determined from the disappearance of reactant methylcyclohexane), representing the catalyst samples used at 723 K, at 773 K, and at
Alumina-Supported Trirhenium Clusters
Figure 5. Catalytic activity of γ-Al2O3-supported rhenium clusters for the conversion of methylcyclohexane under various conditions. Temperature: ×, 723 K; [, 773 K; O, 723 K after use for 4.75 h at 773 K. The mass of the catalyst was 0.25 g; the feed flow rate was 50 mL/ min, and the feed composition was 1.6 mol % of methylcyclohexane (balance H2).
723 K after use at 773 K (as described in experimental section) plotted against the time after isothermal conditions had been attained. In the experiments carried out at 723 K, the conversions of methylcyclohexane were
