Studies of the Active Sites for Methane Dehydroaromatization Using

Jan 26, 2009 - Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-98, Richland, Washington 99352, U.S.A.,...
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Studies of the Active Sites for Methane Dehydroaromatization Using Ultrahigh-Field Solid-State 95Mo NMR Spectroscopy Jian Zhi Hu,*,† Ja Hun Kwak,† Yong Wang,† Charles H. F. Peden,*,† Heng Zheng,‡,§ Ding Ma, and Xinhe Bao‡ Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-98, Richland, Washington 99352, U.S.A., State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China, and Southwest Research & Design Institute of Chemical Industry in Chengdu, China ReceiVed: December 8, 2008

In this contribution, we show that the spin-lattice relaxation time, T1, corresponding to zeolite exchanged molybdenum species in Mo/HZSM-5 catalysts is about 2 orders of magnitude shorter than the corresponding T1 for small MoO3 crystallites. Such a difference is utilized to differentiate the exchanged Mo species from MoO3 agglomerates in Mo/HZSM-5 catalysts and to readily estimate their relative fractions present in catalysts with varying Mo loading. A good linear correlation between the amount of zeolite exchanged species and the aromatics formation rate during catalytic methane dehydroaromatization is obtained. This result significantly strengthens our prior conclusion that the exchanged Mo species are the active centers for this reaction on Mo/HZSM-5 catalysts (J. Am. Chem. Soc. 2008, 130, 3722-3723). Of more general interest for Mo-exchanged zeolites, the results may provide useful data for analyzing the binding of exchanged Mo species in zeolite cages. In particular, the NMR data suggest a possible saturation loading for the exchanged Mo species at a Mo/Al ratio of approximately 0.5 for the ZSM-5 zeolite used in this study (Si/Al ) 25). Furthermore, for polycrystalline MoO3 powder samples, the parameters related to the electric field gradient (EFG) tensor, the chemical shift anisotropy (CSA), and the three Euler angles required to align the CSA principal axis system with the quadrupolar principal axis system are determined by analyzing both the magic angle spinning (MAS) and static 95Mo spectra. The new results obtained from this study on MoO3 powders should help to clarify some of the contradictions in prior literature reports of studies of Mo-containing solids by 95Mo NMR. Introduction Direct conversion of methane to value-added chemicals and fuels remains a significant technical challenge. Since the first report that methane can be transformed into aromatics via the methane dehydroaromatization (MDA) reaction on Mo/HZSM-5 catalysts under nonoxidative conditions by Wang, et al. in 1993,1 considerable progress has been made in the understanding and further development of this process. However, the reaction mechanism is still being debated,2-5 mainly due to an inadequate understanding of the active centers on Mo/HZSM-5 catalysts.6-12 In particular, a direct atomic-level measurement of molybdenum is needed to unambiguously determine the nature of these active centers. Nuclear magnetic resonance (NMR) spectroscopy, a quantitative, nondestructive, element-specific probe of local structure with the capability of accessing buried surfaces/interphases is, in principle, an ideal tool for investigating different types of molybdenum species in Mo/zeolite catalysts. However, the use of solid-state 95Mo (I ) 5/2) NMR is hindered by a combination of the relatively high quadrupolar moment, low gyromagnetic ratio (γ), and low natural abundance of the NMR active isotope. Because of the challenges associated with the low sensitivity * Authors to whom correspondence may be sent. E-mail: Jianzhi.Hu@ pnl.gov; Telephone: (509) 371-6544; Fax: (509) 371-6546. E-mail: [email protected]; Telephone: (509)371-6501. † Institute for Interfacial Catalysis, Pacific Northwest National Laboratory. ‡ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. § Southwest Research & Design Institute of Chemical Industry.

of 95Mo NMR, only a few studies using solid-state molybdenum NMR have been reported to date.13-20 The investigation of surface Mo species using solid-state 95Mo NMR is even more challenging since such samples contain reduced amounts of Mo by weight. Recently, advancements of “ultrahigh” field NMR spectrometers combined with high-speed magic angle spinning (MAS) have made the observation of a wide range of low-γ quadrupolar nuclei, including molybdenum, feasible due to the significantly improved sensitivity and spectral resolution afforded by these high fields and spinning rates.18-20 On the basis of an ultrahigh-field static solid-state 95Mo NMR study combined with line shape analysis,21 we have recently suggested that molybdenum anchors on Brønsted acid sites during catalyst synthesis, and that such exchanged Mo species are the active centers for the MDA reaction. However, the relative amount of the exchanged Mo obtained via a line shape simulation, key information needed to verify such a proposal, remains to be performed. This validation is necessary due to the severe overlap of the static line shapes from both the exchanged and agglomerate/crystallite Mo species, making it difficult to obtain a unique solution in line shape simulations. Furthermore, the chemical nature and molecular dynamics associated with the exchanged Mo, information of direct importance for understanding the properties of the active centers, have not yet been been explored. Finally, there are contradictory reports in the literature14,17 regarding parameters related to the electric field gradient (EFG) tensor of the bulk MoO3 materials and the corresponding parameters related to the chemical shift anisotropy (CSA) tensors. These apparent discrepancies need

10.1021/jp8107914 CCC: $40.75  2009 American Chemical Society Published on Web 01/26/2009

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to be clarified. The goals of this work are to address the above outlined issues. To achieve these research objectives, static and magic angle sample spinning (MAS) spectra, and 95Mo spin-lattice relaxation times were acquired at an ultrahigh magnetic field strength of 21 T for Mo/HZSM-5 catalysts with varying Mo loadings. Experimental Section Synthesis of Mo/Zeolite Catalysts. A controlled amount of Mo isotope-enriched MoO3 was refluxed in an excess of a saturated acetic acid solution at 363 K for 4 h to form 95Mo isotope-enriched molybdenum acetate (MoAC). Variously loaded Mo-ZSM5 catalysts were then prepared by MoAc ion exchange and then dried at room temperature for 24 h. After further drying at 373 K for 8 h, the catalysts were calcined in air at 753 K for 5 h. Catalytic Evaluation. Methane dehydroaromatization reactions were carried out in a continuous flow reactor system equipped with a quartz tube (10 mm i.d.) packed with 1.5 mL of catalyst particle agglomerates of 20-40 mesh. A feed gas mixture of 90% CH4 with 10% N2 was purified and then introduced into the reactor at a flow rate of 1500 mL/g · h. The reaction was conducted at 973 K under a pressure of 1 atm, and an online gas chromatograph (Varian CP-3800) equipped with a flame ionization detector (FID) was used for the analysis of CH4, C6H6, C7H8, and C10H12. A thermal conductivity detector (TCD) was used for the analysis of H2, N2, CH4, CO, C2H4, and C2H6. N2 (10%) in the feed was used as an internal standard for analyzing all products, with carbonaceous deposition determined by carbon balance on the basis of converted methane molecules. 95 Mo NMR Experiments. All of the 95Mo NMR experiments were performed on a Varian-Oxford Inova 63 mm widebore 900 MHz NMR spectrometer. The main magnetic field was 21.1 T, and the corresponding 95Mo Larmor frequency was 58.666 MHz. A homemade large-sample-volume static probe was used for acquiring the static 95Mo NMR spectra. The RF coil is a solenoid consisting of five turns with an internal diameter (ID) of 10 mm and coil length of about 12 mm. The axis of the solenoid coil is perpendicular to the main magnetic field. The sample container is a specially designed sealed NMR rotor made of Kel-F with outside diameter (OD) of 10 mm, ID of 8.5 mm and effective length of 10 mm. For each measurement, about 0.668 cm3 of tightly packed sample was used. For the 95Mo MAS experiments, a homemade 3.2 mm probe with an effective sample volume of 11 µL was used, with a sample spinning rate of about 15 kHz. A standard solid Hahn echo sequence, i.e., (π/2-τ/2-π-τ/2-acq)-d1 with special phase cycling designed for quadrupolar nuclei, was employed, where “d1” denotes the recycle delay time.22 For both the static and MAS measurements, the solid π/2 and π pulse widths were 2 and 4 µs, respectively, and were calibrated as 1/3 of the corresponding pulse widths determined using 2 M Na2MoO4 solution. The value of τ/2 was 20 µs for acquiring the static spectra, while the value of τ/2 (approximately 66 µs) was synchronized to the sample spinning rate of 15 kHz for the MAS measurements. The recycle delay and the number of scans for recording each spectrum vary depending on the experiments and are detailed in the captions of each figure. Chemical shifts are referenced to 2 M Na2MoO4 in D2O at 0 ppm. For measuring spin-lattice relaxation times, T1, the saturation-recovery method with the detection segment replaced by the solid Hahn echo sequences were used. 95

Figure 1. 95Mo MAS spectra of solid MoO3 powders: (a) experimental spectrum acquired using 1680 scans with a recycle delay time of 100s; (b) simulated line shape with CQ ) 2.76 ( 0.04 MHz, ηQ ) 0.30 ( 0.02, and δiso ) -73 ppm.

Results and Discussion MoO3. The 95Mo MAS spectrum of a solid MoO3 powder is given in Figure 1a. The five distinct break points associated with the typical line shape of the central transition of a halfinteger quadrupolar nucleus under MAS conditions are clearly observed. Note that the most shielded break point at about -108 ppm was hardly seen in prior literature reports.14,17 The theoretical line shape simulated using either the SIMPSON Program23 or the STARS software package24,25 as implemented on a Varian spectrometer is presented in Figure 1b. Both programs generate identical results with a quadrupolar coupling constant, CQ ) 2.76 ( 0.04 MHz, quadrupolar asymmetry parameter, ηQ ) 0.30 ( 0.02, and an isotropic chemical shift value, δiso ) -73 ppm. The isotropic chemical shift position is, in fact, located outside of the overall line shape. This is because the observed peak position is a summation of the chemical shift isotropic value and the isotropic shift due to the second-order quadrupolar interaction. The latter is always negative, and its value in ppm is given by:26 (2Q) δiso )-

( )

3 CQ 40 νL

2

(I(I + 1) - 43 ) (1 + η ) × 10 Q

I (2I - 1) 2

2

3

6

(1)

where νL is the Larmor frequency and I ) 5/2 for 95Mo. The (2Q) predicted value for δiso at 21 T field would be -13.7 ppm. In Figure 2, a and b show the static spectra of MoO3 at 21.1 T field, in which four break points can be identified at 158, 48, -6, and -369 ppm, respectively. The average of 158, -6, and -369 ppm gives rise to (158 - 6 -369)/3 ) -72.3 ppm, which is close to the isotropic chemical shift value of δiso ) -73 ppm found from the MAS data simulation. These results suggests that the three principal values of chemical shift tensor are approximately located at δ11 ) 158, δ22 ) -6, and δ33 ) -369 ppm, respectively. To avoid confusion associated with the use of different reference strategies between this work and prior publications,14,17 it is useful to convert the principle values into the anisotropy and the asymmetry parameters. Using the convention |δzz - δiso| g |δxx - δiso| g |δyy - δiso|, csa ) |δzz δiso|, and ηcsa ) (δyy - δxx)/csa,24,25 we get δxx ) δ11, δyy ) δ22, and δzz ) δ33, and obtain csa ) 296 ppm and ηcsa ) (- 6 158)/(- 296) ) 0.55. Compared with the two prior literature reports,14,17 the CQ and ηQ values obtained from this work are similar to those reported in ref 17 and can be considered as refined measurements at ultrahigh field. However, the ηcsa ) 0.55 obtained from this work is significantly different from that obtained from ref 17, ηcsa ) 0.78. This discrepancy can be explained qualitatively

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Figure 2. 95Mo static spectra of solid MoO3 powders. (a) Experimental spectrum obtained at 21.1 T field, acquired using a recycle delay time of 100 s and 1984 scans. A 200 Hz Lorentzian line broadening was applied prior to Fourier transformation. (b) The spectrum in (a), vertically expanded by 3 times to highlight the breakpoints. (c) Simulated spectrum at 21.1 T field using ψ ) 13°, χ ) 70°, ξ ) 90°, csa ) 310 ppm, ηcsa ) 0.55, CQ ) 2.76 MHz, ηQ ) 0.30, and δiso ) -73 ppm, respectively. (d) Experimental spectrum at 9.4 T field (from ref 17). Note that the span of the spectrum in ppm, i.e., the difference between the most upfield and the most downfield breakpoints, is essentially the same as that at 21.1 T (see text). (e) Simulated 9.4 T spectrum using ψ ) 13°, χ ) 70°, ξ ) 90°, csa ) 310 ppm, ηcsa ) 0.55, CQ ) 2.76 MHz, ηQ ) 0.30, and δiso ) -73 ppm, respectively, the same parameters as those in (c).

by comparing the static spectrum in ref 17 acquired at 9.4 T field with that in Figure 2. A careful evaluation of the static 95 Mo MoO3 spectrum in ref 17 gives a difference of 13.75 kHz between the most deshielded and the most shielded breakpoints. The corresponding difference at 21 T from this work is 30.937 kHz, approximately 2.23 times the value at 9.4 T field. The ratio of 2.23 is very close to the ratio of the magnetic field strength (i.e., 2.245), between the 21.1 and 9.4 T fields, indicating that the span of the static pattern is determined by the CSA rather than the second-order quadrupolar interactions. For the purpose of interpreting ηcsa, it is reasonable to align the span of the spectrum at 9.4 T with that of the spectrum at 21.1 T, since the span of the spectrum in ppm is nearly constant at both magnetic fields. This comparison is illustrated in Figure 2d. It is found that due to the second-order quadrupolar interaction, the major sharp middle peak is shifted to higher shielding at 9.4 T (Figure 2d) compared with the corresponding spectra obtained at 21 T field (Figure 2a). If the three break points in Figure 2d are used directly as the principle values of CSA, one would obtain ηcsa ≈ 0.74, very close to 0.78 in ref 17. Thus, this result indicates that the quadrupolar contribution to the overall static line shape was not appropriately addressed in ref 17. Assuming that the values of csa ) 296 ppm and ηcsa ) 0.55 obtained at 21.1 T field from this work are approximately correct, the three Euler angles (i.e., ψ, χ, and ξ) that are required to align the CSA principal axis system with the quadrupolar principal axis system can be calculated by manually simulating the static experimental line shape and by using the fixed values of CQ ) 2.76 MHz and ηQ ) 0.30 obtained from MAS experiment at 21.1 T. Since the static line shape is distorted due to a variety of factors such as the use of a finite recycle delay (i.e., a recycle delay time of 100 s is still not long enough

Hu et al.

Figure 3. 95Mo MAS spectra of Mo/HZSM-5 with varying Mo loadings: (a) 2Mo/HZSM-5; (b) 6Mo/HZSM-5; (c) 10Mo/HZSM-5; and (d) MoO3. The following Lorentzian line broadenings were used prior to FT: 1000 Hz (a), 500 Hz (b), 350 Hz (c), and 50 Hz (d), respectively. The recycle delay time (in seconds)/total spectrum accumulation numbers are: 0.5 s/110000 (a); 0.5 s/180000 (b); 3 s/90000 (c); and 100 s/1680 (d).

to generate an ideal line shape for polycrystalline MoO3 powder), the focus of the simulation is on the frequencies/ chemical shifts of the break points. By varying the angles of ψ, χ, and ξ, a simultaneous fit for the static line shapes at both the 21.1 T and the 9.4 T fields can be obtained, and the resulting best fit spectra are given in Figure 2, c and e, using ψ ) 13°, χ ) 70°, ξ ) 90°, csa ) 310 ppm, ηcsa ) 0.55, CQ ) 2.76 MHz, ηQ ) 0.30, and δiso ) -73 ppm. Note that during the fitting, the value of csa has been adjusted to 310 ppm instead of 296 ppm to produce the best-fit spectra shown in Figure 2, c and e. Clearly, the chemical shifts of the experimental breakpoints at both fields are fitted appropriately, indicating that the CSA parameters qualitatively determined at an ultrahigh field of 21.1 T are quite reliable. It should be pointed out that both the quadrupolar and CSA parameters obtained from this work are different from those reported in ref 14 due to an error associated with the simulation software used in ref 14, as confirmed by the primary author of this prior study. Mo/HZSM-5 with Varying Mo Loadings. 95Mo MAS Spectra. The 95Mo MAS spectra of Mo/HZSM-5 catalysts with varying Mo loadings are given in Figure 3. Compared with the MAS spectrum from bulk MoO3 powders (Figure 3d), the spectra of the 2% (2Mo/HZSM-5) and 6% (6Mo/HZSM-5) Moloaded HZSM-5 samples (Figure 2, a and b) are much broader, covering a chemical shift range from about 0 ppm to about -200 ppm. The half-line width, defined as the line width at the halfheight positions of the peak, is about 7300 Hz that is about a factor of approximately 6 times broader than that of MoO3 (1230 Hz). The spectrum of the 10% Mo-loaded HZSM-5 sample (10Mo/HZSM-5) consists of two components, i.e., a narrow and a broader component. The narrow component is essentially the same as that of MoO3 in Figure 3d if a 350 Hz Lorentz line broadening was used prior to FT. A careful evaluation of Figure 3c but with a 1000 Hz Lorentz line broadening prior to FT reveals that the line shape of the broader component resembles that given in Figure 3, a and b. These results indicate that the narrow component in the 10Mo/HZSM-5 sample is from polycrystalline MoO3 agglomerates, while the broader component is from the dispersed Mo, i.e., zeolite exchanged Mo species. This result is consistent with our prior report using only static 95Mo NMR spectroscopy.21 The significantly increased line width associated with the zeolite exchanged Mo species may be explained by one of two mechanisms, i.e., heterogeneous

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Figure 4. (a) Static 95Mo NMR spectra of 4Mo and 10Mo catalysts as a function of saturation-recovery time (“τ”) for the values of τ between 15 ms and 0.5 s. Note that the vertical scale of the two sets of spectra are plotted using the same reference, allowing spectral intensities between these two samples to be directly compared. The spectrum at a given τ value was acquired using 30,000 accumulations. (b) Integrated spectral intensities in (a) as a function of recovery time, τ. (c) An expanded plot from (a) corresponding to two recovery times, i.e., 15 ms and 0.5 s, respectively, for both the 4Mo and the 10Mo catalysts.

or homogeneous line broadening, or a combination of these two processes. Heterogeneous line broadening arises from the superposition of many MAS spectra with relatively narrow line width such as that in Figure 3d for the MoO3 powder but with different peak centers (static disorder). In contrast, the homogeneous line broadening is caused by a single peak with a peak center at a fixed position and the line is broadened due to, e.g., enhanced spin-spin relaxation, etc. It will be made clear later that, although both heterogeneous and homogeneous processes are involved, heterogeneous line broadening is the dominant mechanism. 95 Mo Spin-Lattice Relaxation Times. Static 95Mo NMR spectra for the 4Mo/HZSM-5 and 10Mo/HZSM-5 catalysts as a function of the saturation-recovery time (τ), between 0.015 and 0.5s, are given in Figure 4a. The corresponding integrated spectral intensities as a function of saturation-recovery time are plotted in Figure 4b. Figure 4c highlights the nearly identical line shapes and spectral intensities for the 4Mo and the 10Mo catalysts at two recovery times, i.e., 15 ms and 0.5 s, respectively. We also found that for the 4Mo catalyst, the integrated spectral intensity is constant for recovery time greater than about 0.25 s (data not shown from 0.5 to 100 s), indicating that essentially all of the molybdenum are zeolite exchanged species in this catalyst. Since the experimental conditions are identical for acquiring both the 4Mo and 10Mo spectra (including the amount of sample used), the essentially identical integrated spectral intensities versus recovery time suggests that the 10Mo catalyst contains an identical amount of exchanged Mo species. The data presented in Figure 4b for both 4Mo and 10Mo catalysts can be fit well using double exponential functions with T1f ) 7.9 ( 0.4 ms (67 ( 1%) and T1s ) 92 ( 4 ms (33 ( 1%), where the percentile given in the parenthesis denotes the relative ratio of the component. This result indicates that the spin-lattice relaxation time, T1, for the zeolite exchanged Mo species is short. The mechanisms that contribute to this fast relaxation may include paramagnetic relaxation due to surface radicals, a significantly increased molecular/segmental motion together with

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Figure 5. (a) Static 95Mo NMR spectra from the 10Mo catalyst as a function of saturation recovery time (τ) for values of τ between 0.5 s and 100 s. The spectrum at each τ value was acquired using 1000 accumulations. (b) Static 95Mo spectra acquired at τ ) 0.5 s but with 30,000 accumulations (black trace), and τ ) 100 s with 1000 accumulations, respectively to highlight the line shape changes.

distorted electric field gradient (EFG) tensors, and chemical shift anisotropies. These mechanisms will be further discussed shortly. Figure 5a shows static 95Mo NMR spectra of the 10Mo catalyst as a function of recovery time for values of τ between 0.5 and 100 s. It can be seen from Figure 5a that the integrated spectral intensity does not change until the value of τ is greater than about 1.5 s. At longer τ values, a relatively sharp spectral feature slowly develops that is characterized by a narrow peak at about -6.0 ppm. At τ ) 100 s, break points at 158 and -369 ppm are clearly observed. The spectrum at τ ) 100 s is highlighted in Figure 5b together with the spectrum acquired at τ ) 0.5 s. Recalling that the major breakpoints associated with a polycrystalline MoO3 sample are 158, -6, and -369 ppm (see Figure 2b), we conclude that the 10Mo catalyst contains a significant portion of MoO3 agglomerates that resemble a polycrystalline MoO3 structure. By subtracting the integrated spectral intensity of τ ) 0.5 s spectrum from the rest of the spectra in Figure 5a, it is possible to obtain the T1 of MoO3 clusters and the result is T1 ≈ 31 s. Thus, the 95Mo NMR peaks of MoO3 agglomerates relax very slowly compared with the zeolite exchanged Mo species, where the value of T1 is less than about 100 ms. The estimated relative ratio of the zeolite exchanged species to the overall Mo in the 10Mo catalyst is 39 ( 2%, consistent with a conclusion obtained from Figure 4 that the 4Mo sample contains only exchanged Mo species. The following additional observations can be made from the static 95Mo line shape of the zeolite exchanged Mo species that are highlighted in Figure 4c and Figure 5b. The tails of the spectra extend from about -500 to about 200 ppm. Although the difference, i.e., about 700 ppm, is larger than that (about 530 ppm) of the static MoO3 spectra, the chemical shift range for the major portion of the peak is quite similar to that of the polycrystalline MoO3 agglomerates present in the 10Mo 100 s spectrum of Figure 5b. Since the spectra appear symmetric about the spectral center for the exchanged Mo species, it is impossible to create a line shape that is similar to the zeolite exchanged Mo species by only applying line broadening to the static 95Mo line shape of MoO3. This is because the sharp break point at ∼-6 ppm is not at the center of the spectrum. Such a result indicates that the mechanisms that contribute to the broad line

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Figure 6. Calculated theoretical static 95Mo line shapes at a magnetic field of 21.1 T: (a) and (b) obtained using fixed values of csa ) 310 ppm, ηcsa ) 0.55, CQ ) 2.76 MHz, ηQ ) 0.30, and δiso ) -73 ppm but different values of ψ, χ, and ξ; (c) obtained with fixed values of ψ ) 13°, χ ) 70°, ξ ) 90°, csa ) 310 ppm, ηcsa ) 0.55, and δiso ) -73 ppm, but with varying values of CQ and ηQ.

in the 95Mo MAS spectra of exchanged Mo species in Figure 3 cannot be due solely to homogeneous line broadening. Thus, heterogeneous line broadening must also be contributing to the line shape, where the broadened line shape is due to a superposition of many individual quadrupolar second-order MAS features whose centers have a distribution of chemical shifts. To account for both the chemical shift span of the static spectra of exchanged Mo and the distribution of the centers of the corresponding MAS spectra, one approach is to let the relatively sharp peak originally located at -6 ppm in the static spectrum of MoO3 to vary, e.g., between about 0 and about -300 ppm. For chemical shift anisotropic interactions, it has long been recognized,27,28 in the case of the chemical shift anisotropy pattern for the carbonyl carbon in CH3-CO-R, where R ) OH, OCH3, H, CH3, OAg, and O-CO-CH3, that the chemical shift principal values corresponding to δ33 and δ11 do not change appreciably in the series, but δ22 undergoes substantial shifts. The variations in isotropic chemical shift are thus due almost entirely to the variations in the δ22 component. For similar reasons, if the exchange sites of HZSM-5 are heterogeneous, the exchanged Mo species will experience a heterogeneous interaction with the surface. Indeed, there are a number of inequivalent T-sites in HZSM-5, differentiated by their location in the zeolite framework, giving rise to multiple Brønsted acid sites and correspondingly contributing to the heterogeneous anchoring of Mo species in the zeolite. Moreover, as revealed by 1H MAS NMR studies,29 in addition to the Brønsted acid sites, both surface Si-OH groups and Al-OH groups interacted with Mo species during the synthesis procedure, although the amounts of these latter two sites are relatively small. Thus, it is possible that the δ22 component of the chemical shift tensor is substantially changed, while the δ11 and δ33 components remain roughly the same. The summation over all the possibilities would generate a static 95Mo spectrum similar to that of the zeolite exchanged 95Mo spectra shown in Figure 4c and Figure 5b. Other possibilities that could contribute to the observed line shape for the zeolite exchanged Mo species include (i) variations in the three Euler angles (i.e., ψ, χ, and ξ) and (ii) changes in CQ and ηQ. A series of calculated theoretical line shapes at 21.1 T field are presented in Figure 6 to illustrate the possible impact

Hu et al. to the line shape. The spectra given in Figure 6, a and b, were obtained using fixed csa ) 310 ppm, ηcsa ) 0.55, CQ ) 2.76 MHz, ηQ ) 0.30, and δiso ) -73 ppm, while the values of ψ, χ, and ξ were allowed to vary. Although both the span and the middle break points can be shifted, the variation is not sufficient to generate a synthesized powder pattern similar to that of the zeolite exchanged Mo species since the middle breakpoints are shifted toward lower shielding. Furthermore, these results show that the span of the spectra is not sensitive to Euler angle changes. Similarly, as shown in Figure 6c, where ψ ) 13°, χ ) 70°, ξ ) 90°, csa ) 310 ppm, ηcsa ) 0.55, and δiso ) -73 ppm are fixed during calculation, variations in CQ and ηQ alone in a reasonable range cannot adequately account for the zeolite exchanged Mo spectra. Nevertheless, the combined changes in Euler angles and the values of CQ and ηQ can add sufficient complexities to the changes in the chemical shift anisotropy discussed above to produce the experimental line shape for the exchanged Mo species. Thus, although the data from this work alone cannot unambiguously determine the subtle mechanisms that are responsible for the observed experimental line shape of the exchanged Mo species, it is quite certain that the exchanged Mo species are heterogeneous due to the heterogeneous nature of the HZSM-5 exchanged sites. The short T1 associated with the zeolite exchanged Mo species can be rationalized by a combination of the following two possible mechanisms. (i) The value of T1 can be significantly reduced by an enhanced quadrupolar relaxation due to the distorted electric field gradient. At the zeolite exchange sites, an increased Mo-O bond vibration in both amplitude and frequency for the exchanged Mo species is expected. If the correlation time characterizing this vibration is close to the Larmor frequency, T1 can be significantly shortened. (ii) The presence of free radicals in Mo/HZSM-5 catalysts have been observed by ESR at room temperature.29 It is likely that these free radicals are the source of relaxation due to the well-known paramagnetic relaxation mechanism. Determination of the Amount of Exchanged Mo Species in Mo/HZSM-5 Catalysts. Si/Al ratios of the HZSM-5 zeolite and the actual Mo content in Mo/HZSM-5 catalysts were determined by X-ray fluorescence analysis. On this basis, the Si/Al ratio was around 24, resulting in an approximate formula for HZSM-5 zeolite used here of H4[Al4Si92O192], with a molecular weight of 5756/unit cell. The actual Mo content of 2Mo/HZSM-5, 4Mo/HZSM-5, 6Mo/HZSM-5, and 10Mo/ HZSM-5 materials determined by X-ray fluorescence analysis (XRF) were 1.7, 3.3, 4.2, and 7.7 wt %, respectively. The amount of the exchanged Mo species on Mo/HZSM-5 catalysts can then be calculated as follows. Defining “n” as the total number of Mo/unit cell, the following relationship is obtained:

n)

(5760 + n × 95.5) × p 95.5

(2)

Thus, the total number of Mo species per unit cell is n ) (p/ 95.5)/[(1 - p)/5760], where “p” is a weight percentage of total Mo in the sample. By replacing “p” with 1.71%, 3.3%, 4.2%, or 7.7%, the total number of Mo/unit cell for the 2Mo, 4Mo, 6Mo, and 10Mo catalysts are 1.05, 2.06, 2.64, and 5.03, respectively. On the basis of our relaxation measurements, essentially all the Mo species are exchanged for the 2Mo and 4Mo catalysts, and approximately 67% for the 6Mo and 40% for the 10Mo catalysts. Thus, the exchanged Mo species are 1.05, 2.06, 1.77, and 2.01 exchanged Mo/unit cell for the 2Mo, 4Mo, 6Mo, and 10Mo catalysts, respectively. This result may

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Mo NMR Studies of Methane Dehydroaromatization

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indicate that, within experimental error, the saturation loading for the exchanged Mo species is approximately 2Mo/unit cell. Since one unit cell has 4Al, the ratio of the exchanged Mo to Al is thus approximately 1 to 2, or 0.5. We emphasize that this estimate is based on the analytical determination of Mo content in these catalysts, and from the ability (demonstrated in this paper) to accurately determine the relative fraction of exchanged to particle-like Mo species (the latter likely located on the external surface of the zeolite) by ultrahigh-field 95Mo NMR spectroscopy. However, it should be recognized that this “saturation” can be affected by both physical and chemical properties of the zeolite including the number, strength, and vicinity of unreacted Brønsted acid sites, and the access of migrating MoOx ions to these sites during synthesis. Recent work by Tessonnier et al.11 indicates that the number of Brønsted acid sites used to anchor MoOx cluster ions in HZSM-5 is sensitive to the Si/Al ratio, with two sites being used per exchanged Mo at low (Si/Al ) 15) and one site at high (Si/Al ) 40) ratios. (However, we note that the tabulated number of acid sites consumed during Mo loading for an HSZM-5 material with an intermediate Si/Al ratio of 25, published in a very recent paper (Table 2 in ref 12), does not seem to trend consistently with the data for the other two catalysts. In particular, our reading of the newly published data indicates that two exchanged Mo consume only one acid site for the HZSM-5 material with a Si/Al ratio of 25.) In their earlier publication,11 the authors suggest that for HZSM-5 catalysts with high concentrations of acid sites (i.e., with low Si/Al ratios), a monomeric (MoO2)2+ ion can anchor to two nearby acid sites. At high Si/Al ratios, the probability of finding two neighboring Brønsted acid sites rapidly decreases,30 preventing such a structure from forming. Instead, it is possible for two monomeric clusters, each anchored to a single acid site, to condense to form a (Mo2O5)2+ cluster that bridges between sites5 that are relatively farther apart.12 The zeolite used in the present study has an intermediate Si/Al ratio ()25) so both modes of binding (and perhaps others) might be expected to be present. Furthermore, it is well-known30 that there is a wide distribution of the distances between nearest neighbor Al and, therefore, ionexchange sites in HZSM-5, resulting in a distribution of O-Mo-O angles in the likely multiple anchored structures. Since both the EFG and CSA are closely related to the local electronic structure of the nucleus, such changes associated with bond angles and Al-Al distances would induce sufficient heterogeneity to explain the above observed changes associated with the EFG and CSA parameters. Correlation Between the Amount of Exchanged Mo Species and the Aromatics Formation Rate During MDA Reaction. The quantity of Mo species, in µmol/gram-catalyst (µmol/g · cat), can be calculated using the following formula:

χ × 106 (5760 + n × 95.5)

(3)

where n is the total number of Mo species per unit cell of Mo/ HZSM-5 catalyst and χ represents the amount of either the total, the exchanged, or the crystalline MoO3 species per unit cell. The resulting correlation between the amount of Mo species and the aromatics formation rate is plotted in Figure 7. It is clear from Figure 7 that there is no linear correlation between the initial formation rate of the product aromatics and the quantities of total molybdenum or MoO3 crystallites. However, there is an approximate correlation between the amount of the exchanged molybdenum species and the aromatics formation rate. The new results, based on the large difference

Figure 7. Correlation of the aromatics formation rate during catalytic MDA reaction with the quantity of different molybdenum species in catalysts with varying Mo loading. The solid line is the best linear fit of the data for the exchanged Mo species, while the dashed lines are nonlinear, i.e., the best polynomial fit of the data, for the total Mo species (triangle), and MoO3 crystallite (circle), respectively.

in relaxation times between the surface exchanged Mo species and the crystallite MoO3 presented here, strongly support our prior conclusions that the exchanged Mo species are the active centers for the MDA reaction, where the amount of exchanged Mo species in our prior work was obtained by a much less reliable line shape simulation.21 Note that in Figure 7, the data point for the exchanged Mo of the 4Mo/HZSM-5 catalyst deviates somewhat from the linear correlation. We believe that this deviation is due to the fact that the 4Mo catalyst was prepared using a different zeolite batch several months later than other catalysts and the elemental analysis and the catalyst activity tests were also carried out at different times. Considering this likely source of experimental error, it is still evident that a linear correlation is appropriate for the exchanged Mo species, while total and crystalline MoO3 correlations are clearly not linear. Conclusions In this work, we find that the spin-lattice relaxation time, T1, corresponding to zeolite exchanged molybdenum species in Mo/HZSM-5 catalysts is relatively short, i.e., less than about 100 ms at a magnetic field of 21.1 T field, whereas the value of T1 for the polycrystalline MoO3 is much longer, i.e., about 30 s. Such a difference, more than 2 orders of magnitude, can be utilized to differentiate the exchanged Mo species from the agglomerate MoO3 in Mo/HZSM-5 catalysts. At low Mo loading, i.e., less than or equal to about 4 wt %, essentially all of the Mo exists as zeolite exchanged species. Using the new data obtained from these relaxation studies, an approximately linear correlation between the amount of exchanged species and the aromatics formation rate during methane dehydroaromatization reaction is obtained. This result significantly strengthens our prior conclusion that the exchanged Mo species are the active centers for the MDA reaction over Mo/HZSM-5 catalysts. In addition, the NMR data suggest a possible saturation loading for the exchanged Mo species at a Mo/Al ratio of approximately 0.5 for the ZSM-5 zeolite used in this study (Si/Al ) 25). Analyzing the line shapes obtained from both the 95Mo MAS and static NMR spectra indicates that the exchanged sites are heterogeneous, resulting in significantly broadened MAS spectra with essentially a featureless but nearly symmetric static line shape for the exchanged Mo species. A heterogeneous line broadening mechanism is shown to be primarily responsible for the observed MAS and static line shapes for the exchanged species.

2942 J. Phys. Chem. C, Vol. 113, No. 7, 2009 For polycrystalline MoO3 powders, the quadrupolar coupling constant, CQ ) 2.76 ( 0.04 MHz, and the quadrupolar asymmetry parameter, ηQ ) 0.30 ( 0.02, are determined by fitting the corresponding MAS spectrum at 21.1 T. These new values are consistent with a prior report by Bastow et al.17 within experimental error. Analyzing the static line shapes obtained at both 21.1 T and the 9.4 T magnetic field strengths reveals that the three Euler angles required to align the CSA principal axis system with the quadrupolar principal axis system are approximately ψ ) 13°, χ ) 70°, ξ ) 90°, respectively. The same analysis also determines the values of chemical shift anisotropy (csa ) 310 ppm), the chemical shift anisotropy asymmetry parameter (ηcsa ) 0.55 ppm), and the isotropic chemical shift value for polycrystalline MoO3 (δiso ) -73 ppm). The new results obtained from this study on MoO3 powders help clarify apparent contradictory prior literature reports.14,17 Acknowledgment. This research was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences. All of the NMR experiments were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory, U.S.A. The authors from DICP thank the National Natural Science Foundation of China and the Ministry of Science and Technology of China for financial support. The authors thank Dr. Paul Ellis for his valuable suggestions in interpreting the line shape and relaxation data for the exchanged Mo species, as well as reviewing the draft manuscript. The authors are also grateful to Dr. Andrew S. Lipton and Dr. Vijayakumar Murugesan for their kind assistance in setting up the simulation software. Mr. Jesse A. Sears is acknowledged for his assistance with the NMR probe setup. References and Notes (1) Wang, L. S.; Tao, L. X.; Xie, M. S.; Xu, G. F.; Huang, J. S.; Xu, Y. D. Catal. Lett. 1993, 21, 35–41. (2) Xu, Y. D.; Lin, L. W Appl. Catal., A 1999, 188, 53–67. (3) Liu, S. T.; Wang, L.; Ohnishi, R.; Ichikawa, M. J. Catal. 1999, 181, 175–188. (4) Shu, Y. Y.; Ichikawa, M. Catal. Today 2001, 71, 55–67.

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