Characterizing Surface Acidic Sites in Mesoporous-Silica-Supported

ARTICLE pubs.acs.org/JPCC

Characterizing Surface Acidic Sites in Mesoporous-Silica-Supported Tungsten Oxide Catalysts Using Solid-State NMR and Quantum Chemistry Calculations Jian Zhi Hu,* Ja Hun Kwak, Yong Wang, Mary Y. Hu, Romulus V. Turcu, and Charles H. F. Peden* Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-98, Richland, Washington 99352, United States

bS Supporting Information ABSTRACT: The acidic sites in dispersed tungsten oxide supported on SBA15 mesoporous silica were investigated using a combination of pyridine titration, both fast- and slow-MAS 15N NMR, static 2H NMR, and quantum chemistry calculations. It is found that the bridging acidic OH groups in surface-adsorbed tungsten dimers or multimers (i.e., W OH W) are the Brønsted acid sites. The unusually strong acidity of these Brønsted acid sites is confirmed by quantum chemistry calculations. In contrast, terminal W OH sites are very stable and only weakly acidic as are terminal Si OH sites. Furthermore, molecular interactions between pyridine molecules and the Brønsted and terminal W OH sites for dispersed tungsten oxide species are strong. This results in restricted molecular motion for the interacting pyridine molecules even at room temperature, that is, a reorientation mainly about the molecular C2 symmetry axis. The restricted reorientation results in efficient 1H 15N cross-polarization, making it possible to estimate the relative ratio of the Brønsted to the weakly acidic terminal W OH sites in the catalyst using the slow-MAS 1H 15N CP PASS method.

1. INTRODUCTION Supported metal oxide catalysts are widely used in many applications, such as ethylene polymerization, alkane oxidative dehydrogenation, isomerization, metathesis, and selective catalytic reduction of NOx with ammonia.1 7 Early transition-metal oxides with the metal in oxidation states of 5 or 6 have shown the highest activity. For some of these metal oxides, catalytic activities appear to be linked to the formation of strong Brønsted acid sites.8 10 In particular, tungsten oxide has perhaps the strongest Brønsted acid sites, either as a bulk oxide or when supported.11 17 Even for the same WOx/support system, the tungsten oxide species can adopt a variety of surface structures that are strongly dependent on the sample preparation, such as the metal oxide precursor, the loading amount of the catalytic oxide, and the thermal treatment.17 However, a fundamental understanding of the nature of active sites and how these sites are modified by chemical or physical treatments is still lacking. Nuclear magnetic resonance (NMR) spectroscopy, a quantitative, nondestructive, and element-specific probe of local structure with the capability of accessing buried surfaces/interphases, is an ideal tool for studying surface structures. In particular, magic-angle spinning (MAS) 15N NMR combined with 15Nenriched pyridine titration has been recognized as a powerful method to characterize the acid sites on solid surfaces by utilizing the large nuclear shielding associated with nitrogen.18 23 Although MAS at a sample spinning rate of several kilohertz or more offers the highest possible spectral resolution, the high r 2011 American Chemical Society

resolution is obtained at the expense of the chemical shift anisotropy (CSA) information that is closely related to subtle electronic structure effects near the nucleus. It is well known that the principal values of the CS tensor provide significantly more information compared with that of the isotropic shift obtained from fast-MAS and is, thus, more powerful for obtaining structural information.24 The principal values of the CS tensor can be obtained from the anisotropic powder spectrum acquired from either a static or slowly spinning sample. However, in a complex system, such as tungsten oxide, where there are several isotropic chemical shift peaks that coexist, the powder spectra associated with these various isotropic chemical shifts can significantly overlap, rendering the interpretation of the spectrum difficult. This is likely the primary reason that, although there have been several 15N MAS NMR studies on other solid surfaces, such as silica alumina,18 22 γ-alumina and mordenite,19,23 and mesoporous silica,21 there have been no prior studies of tungsten oxide catalysts using 15N MAS NMR combined with 15N-enriched pyridine titration. Fortunately, a number of chemical shift isotropic chemical shift anisotropic 2D correlation spectroscopy methods25 39 have been developed that allow the anisotropic powder pattern to be separated by isotropic chemical shifts. Among them, the phase-adjusted spinning sideband (PASS) Received: April 25, 2011 Revised: September 16, 2011 Published: October 18, 2011 23354

dx.doi.org/10.1021/jp203813f | J. Phys. Chem. C 2011, 115, 23354–23362

The Journal of Physical Chemistry C method37 offers both high resolution and high sensitivity and is a promising method for studying surface chemistry. However, to our knowledge, this method has never been applied for studying solid surfaces, including complex tungsten oxide catalysts. Recently, our group has reported40 that highly dispersed tungsten oxides on the surface of SBA-15 materials can be prepared utilizing an atomic layer deposition (ALD) synthesis strategy. Although these materials have been characterized by a variety of methods, such as UV vis DRS, X-ray diffraction, TEM, and MAS 1H NMR, the nature of the acid sites remains unclear. In this work, the techniques of 15N single-pulse (SP-MAS) and 1H 15N cross-polarization MAS (CP-MAS) combined with pyridine titration were used to characterize the acidic sites in highly dispersed tungsten oxide catalysts supported on SBA-15 mesoporous silica. To obtain the nitrogen CSA information corresponding to each type of surface acid site, 1H 15N CPPASS was utilized. Further, the origin of the nitrogen CS tensors is explained by quantum chemistry calculations using density functional theory. Collectively, these investigations provide important new insights into the acidic sites in these catalysts.

2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. SBA-15 mesoporous silica was synthesized using a previously reported protocol.41 The resultant BET surface area of this material after calcination at 500 °C for 4 h is ∼860 m2/g, and the average pore size is ∼7 nm. Prior to the dispersion of tungsten oxide, mesoporous silica was suspended in anhydrous toluene and refluxed for 3 h under a N2 atmosphere to remove physically adsorbed water. Tungsten precursor solutions were prepared by first dissolving a predetermined amount of WCl6 (depending on the desired W loading) in approximately 150 mL of pure toluene at room temperature, followed by addition of 20 mL of ethanol. The solution was then refluxed. During reflux, N2 was bubbled through the liquid phase to prevent the contact by water, and the reflux continued until no HCl was detected in the nitrogen exhaust. The amount of added WCl6 was based on the assumption that three silanol groups on the mesoporous silica can hydrolyze one WCl6 molecule; that is, 1 mol of WCl6 per 3 mol of Si OH corresponds to a total tungsten oxide loading of one WOx monolayer (100% coverage) which, in turn, corresponds to ∼30 wt % WO3, or 1.33 WOx/nm2 on the SBA-15,40 since the corresponding theoretical density of Si OH groups on the mesoporous silica was about 4 1018 Si OH per m2.42 After cooling the tungsten precursor solution to room temperature, the dehydrated mesoporous silica in toluene solution was added and the mixture was refluxed overnight under a N2 atmosphere. After cooling to room temperature, the reaction mixture was filtered and washed with toluene several times until no tungsten precursors could be detected in the washing solvent. The solid was collected and dried in an oven at 120 °C for 30 min and then calcined in flowing dry air at 400 °C prior to the pyridine titration experiments. Using the same procedure, a 50% W coverage/SBA-15 sample, corresponding to ∼15 wt % WO3, or 0.67 WOx/nm2 on the SBA-15,40 was also prepared. 2.2. Surface Titration. Samples with 50% and 100% theoretical surface coverage of tungsten oxide were activated by calcining in dry air for 1 h at 400 °C and used for pyridine titration. Following activation, each sample was cooled to room temperature and exposed to a known quantity of 99.9% 15Nenriched pyridine vapor to give a hypothetical saturation loading level of (1.0 pyridine)/(1.0 W site). Given the surface area of

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860 m2 per 1.0 g of SBA-15 and the surface density of 1.0 Si OH per 0.5 nm 0.5 nm,42 0.0057 mol of Si OH could be available per 1 g of SBA-15. Because three Si OH groups are required for anchoring one W site, about 0.0019 mol of pyridine is required for the titration experiment on 1 g of SBA-15 with 100% W. For a 150 mL adsorption volume at 293 K, the corresponding pyridine pressure is 0.30 atm based on the ideal gas law PV = nRT. The amount of pyridine used for both the SBA-15 with 50% W coverage and the pure SBA-15 samples was the same as that used for the sample with 100% WOx coverage, that is, approximately one pyridine for every three Si OH sites in the pure SBA-15, or one pyridine per W site plus one pyridine for every three Si OH in the sample with 50% W coverage. We refer to such a level of titration as saturation with pyridine. 2.3. NMR Experiments. The static 2H NMR experiments were performed on a Varian 500 MHz (11.7T) NMR spectrometer with a 2H Larmor frequency of 76.784 MHz using a homemade low-temperature 2H NMR probe with a 5 mm internal diameter RF coil that holds about 80 mg of sample.43,44 A solid (45°-τ-45°-τ-acq, with a 45° pulse width of 3.2 μs and τ = 50 μs) echo pulse sequence was used for the measurement. The 15N SP-MAS, 1H 15N CP-MAS, and the slow-MAS 1 H 15N CP-PASS experiments were performed on a VarianChemagnetics 300 MHz (7.05 T) Infinity spectrometer with 1H and 15N Larmor frequencies of 299.98 and 30.402 MHz, respectively, using a 7.5 mm MAS probe. A commercial crosspolarization/MAS probe with a 7.5 mm outside diameter and a 6.0 mm internal diameter pencil-type spinner system was used. The 15N π/2 pulse width in this case was 5.3 μs. High-power 1H decoupling was applied during data acquisition for all the 15N measurements with the strengths of the decoupling fields of approximately 60 kHz. All 15N chemical shifts are referenced to nitromethane (CH3NO2) at 0 ppm according to the IUPAC recommendations 2008.45 Because our 15N NMR data will be compared heavily with those reported in ref 21, where pyridine titration studies on pure SBA-15 and MCM-41 were carried out and where the 15N NMR was referenced to solid NH4Cl. We measured the 15N NMR chemical shift of solid NH4Cl and found that it was at 341.17 ppm relative to CH3NO2 (0 ppm). Thus, to convert the NH4Cl scale to the recommended nitromethane scale, 341.17 ppm should be added. Using nitromethane as the 15 N NMR reference, some useful findings from ref 21 can be summarized as follows: 65 ppm for bulk pyridine, 82 ppm for pyridine hydrogen-bonded to some water molecules, and 90 ppm for pyridine hydrogen-bonded to surface Si OH groups. 2.4. Quantum Chemistry Calculations. Density functional calculations were performed using the Amsterdam Density Functional (ADF) program.46 48 Geometries were optimized and the chemical shielding values of the chemical shielding tensor were calculated at the BLYP/QZ4P level of theory. Under these conditions, the calculated absolute 15N isotropic chemical shielding value with respect to a bare nucleus for CH3NO2 was 160 ppm. Because chemical shifts and associated chemical shift anisotropy are experimentally measured quantities with respect to a standard, whereas chemical shieldings are theoretically derived or calculated with respect to a bare nucleus, the calculated chemical shielding values were converted to the experimental chemical shift scale relative to a standard according to δ(15N)sample = σ(15N)standard σ(15N)sample, where δ and σ are the chemical shift and nuclear shielding, respectively. Using CH3NO2 (0 ppm) as the reference standard, the conversion equation becomes δ (ppm) = 160 ppm σcalculated ppm. 23355

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The Journal of Physical Chemistry C

Figure 1. Static 2H NMR spectra of samples titrated with pyridine-d5: (a) SBA-15 acquired at 293 K, (b) WOx/SBA-15 samples with 50% theoretical W coverage acquired at 293 K, and (c) WOx/SBA-15 samples with 50% theoretical W coverage acquired at 50 K. The recycle delay time and the number of accumulation numbers were 0.2 and 64 scans (a), 0.2 and 83 304 scans (b), and 5 and 4820 scans (c), respectively.

3. RESULTS AND DISCUSSION 3.1. 2H NMR. The room-temperature (RT) 2H NMR spec-

trum of pyridine titrated onto pure SBA-15 (Figure 1a) is characterized by a single peak with a line width (defined as the width at the half peak height positions) of about 3.7 kHz. The lack of a quadrupolar line shape suggests that pyrdine molecules are undergoing almost isotropic reorientation at RT. In contrast, the spectrum of pyridine titrated onto either the 50% or the 100% WOx/SBA-15 sample at RT consists of two components, a narrow component with a line width of approximately 9.3 kHz and a broad component with a width of about 130 kHz. Figure 1b shows the RT 2H NMR spectrum of the 50% sample, whereas Figure 1c gives the 2H NMR spectrum of the 50% sample that was acquired at 50 K. The results from the 100% sample are not shown because they are similar to the results from the 50% sample. The line shape at 50 K is identical to that acquired at 9.8 K (not shown) for the same 50% W/Ox/SBA-15 sample, indicating that the molecular motion of pyridine is frozen at 50 K. A careful evaluation of the spectra in Figure 1b reveals that the width/span of the broad component is about 130 kHz, which is similar to the splitting of the two center horns of the spectrum acquired at 50 K (Figure 1c). The line shape associated with the broad component at 293 K for the 50% WOx/SBA-15 sample is a characteristic 2H NMR line shape of pyridine-d5 undergoing a 2-fold averaging about the pyridine C2 symmetric axis.49,50 Note that, in refs 49 and 50, 2H NMR spectra of pyridine-d5 at various temperatures ranging from 77 to 323 K have been analyzed, where the detailed molecular motion of pyridine correlated with a particular line shape is reported. On the basis of the results from refs 49 and 50, the existence of a broad component at RT (Figure 1b) indicates that a portion of the titrated pyridine undergoes a restricted 2-fold flip motion even at RT. Because a 2-fold flip motion of the pyridine molecule does not change the chemical shift anisotropy (CSA) line shape (refs 49 and 50), the CS tensor principal values obtained from 15N 2D-PASS

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Figure 2. Solid-state 15N NMR spectra of the pyridine molecules used to titrate acid sites of various samples: (a) SP-MAS of SBA-15 acquired at room temperature (RT), (b) RT SP-MAS of WOx/SBA-15 samples with 50% theoretical W coverage, (c) RT SP-MAS of WOx/SBA-15 samples with 100% theoretical W coverage, (d) RT SP-MAS of sample (c) after evacuation to 10 6 Torr at RT for about 20 min, and (e) 1 H 15N CP MAS of WOx/SBA-15 samples with 100% theoretical W coverage. For the CP experiment in (e), a contact time of 1 ms, a recycle delay time of 1 s, and a sample spinning rate of 5 kHz were used. For the SP-MAS experiments, a sample spinning rate of 5 kHz and a 45° pulse with a recycle delay time of 1.2 s were used. The number of accumulations were 584 (a), 13 521 (b), 71 176 (c), 43 391 (d), and 66 070 (e), respectively.

combined with quantum chemistry calculations can then be utilized in subsequent analyses to investigate the details regarding the nature of those surface sites where the titrated pyridines are undergoing a restricted 2-fold flip motion. 3.2. 15N SP-MAS and 1H 15N CP-MAS. Figure 2 shows the 15 N SP-MAS (spectra a d) and CP-MAS (spectrum e) spectra obtained on SBA-15 (spectrum a), WOx/SBA-15 with 50% W coverage (spectrum b), and WOx/SBA-15 with 100% W coverage (spectra c e) that were titrated to nominal saturation levels with pyridine (defined earlier). A relatively sharp single peak, centered at about 89 ppm, is observed for SBA-15 at room temperature (RT) (spectrum a, Figure 2). The corresponding line width is about 1.74 ppm. This peak is assigned to nitrogen in pyridine interacting with the surface silanol (i.e., Si OH) groups based on ref 21. The narrow line width indicates a fast and random molecular motion, consistent with the 2H NMR results in Figure 1a. It is also known21,51 that the position for nonhydrogen-bonded pyridine 15N is at ∼ 65 ppm. No 65 ppm peak is observed in spectrum a in Figure 2, indicating that all of the pyridine molecules have been successfully titrated by the SBA-15 surface. The 15N SP-MAS spectrum for the SBA-15 with 50% W coverage titrated with a saturation coverage of pyridine is presented in Figure 2, spectrum b. The center position of the most intense peak is found at about 89 ppm, that is, at the same chemical shift position of the Si OH peak (see Figure 2, spectrum a) and thus assigned to Si OH groups interacting with pyridine. The line width is approximately 15 ppm and is much broader than that in spectrum a (Figure 2). The broader line width can be due to either a distribution of the Si OH sites 23356



Figure 3. 2D 15N CP-PASS spectra of the 100% W coverage SBA-15 sample that was calcined at 400 °C. These spectra were acquired at a 7.05 T magnetic field using a sample spinning rate of 1000 ( 1 Hz and at room temperature. The parameter, n = ω1/ωr (where ω1 is the frequency along the evolution dimension of the 2D experiment and ωr is the sample spinning rate), denotes the nth sideband. n = 0 corresponds to the centerband. The spectra were acquired using a CP-PASS sequence similar to that reported previously37 except that a 96-phase cycling60 was used here. The contact time was 1 ms, and 16 evolution increments were used, each with 4416 accumulations. The recycle delay time was 1.2 s, resulting in an experimental data collection time of about 24 h. The 1H π/2 and 13C π pulse widths were 5.3 and 10.6 μs, respectively.

or restricted motion, or a combination of both. However, restricted motion for pyridine interacting with Si OH group can be ruled out based on the 15N CP-PASS results in Figure 3, where no SSBs are observed at 1 kHz. A weak shoulder peak located at about 118 ppm and a low intensity peak located at ∼ 181 ppm are also observed. Figure 2, spectrum c, shows the 15N SP-MAS spectrum of the 100% W coverage SBA-15 titrated to saturation with pyridine. Clearly, the relative intensities corresponding to both the 118 and the 181 ppm peaks increase. Furthermore, the peak center for the most intense peak is shifted from 89 ppm (Figure 1a) to about 96 ppm, while the center position for the peak of 118 ppm is essentially unchanged. The reason for this 7 ppm shift associated with the most intense peak in spectrum c (Figure 2), relative to that in spectrum b (Figure 2), is explained by the simulated/deconvoluted spectra at the right side of Figure 2, where a total of four peaks located at 181, 118, 103, and 89 ppm were used for fitting the entire spectrum. The most intense peak is a superposition of the 89 and the 103 ppm peaks. In spectrum c (Figure 2), the relative amount of the 89 ppm peak is decreased relative to that in spectrum b (Figure 2), resulting in an apparent 7 ppm peak shift for the most intense peak in the spectrum. To further clarify the 7 ppm shift for the most intense peak, the sample in spectrum c (Figure 2) was evacuated for about 20 min and a 15N SP-MAS spectrum was acquired under the same experimental conditions. The resultant spectrum (Figure 2, spectrum d) along with its peak fitted spectrum again shows that the peak positions for both the 181 and the 118 ppm peaks remain unchanged, but the peak center for the most intense peak is further shifted to about 103.2 ppm due to the significantly reduced intensity from the 89 ppm component. To validate the peak fitting, an attempt has been made to fit the 103 ppm peak in spectrum d (Figure 2) using a single peak (see the bottom trace at the right of Figure 2). Clearly, a single peak does not generate a good fit. The relative intensity of the 181 ppm peak is higher than that of the other peak(s) in the 1H 15N CP-MAS spectrum (Figure 2, spectrum e), indicating that the 181 ppm peak is

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associated with pyridine molecules that have strong 1H 15N dipolar interactions. A peak centered at approximately 107 ppm is also observed. Because this sample is the same as that of spectrum c (Figure 2), the relatively reduced peak intensity of the 107 ppm peak in spectrum e (Figure 2) compared with that of the most intense peak in spectrum c (i.e., the 96 ppm peak) indicates that a major part of the pyridine molecules contributing to the 96 ppm peak in spectrum c arises from pyridine molecules having weak 1H 15N dipolar interactions, that is, from pyridine interacting with surface silanol (Si OH) groups based on the results from Figure 1 and the fitted spectra in Figure 2. Because of the inefficient CP mechanisms associated with very weak 1H 15N dipolar interactions, the pyridine molecules that are weakly interacting with protons of surface Si OH groups are not detected or their corresponding signals are significantly reduced. Therefore, the major components that contribute to the 107 ppm peak in spectrum e (Figure 2) come from the 103 and the 118 ppm peaks. It can also be seen that the 103 ppm peak has a reduced relative peak intensity in spectrum e compared with that in spectrum c or spectrum d, indicating a weak 1H 15N dipolar interaction and thus a low CP efficiency. Summarizing the results from Figure 2, four types of pyridine peaks can be suggested in spectra of WOx/SBA-15 samples titrated to the assumed saturation level of pyridine. These include the 181 ppm peak having strong 1H 15N dipolar interactions, the 118 ppm peak having relatively strong 1H 15N dipolar interactions, the 103 ppm peak having weak 1H 15N dipolar interactions, and the 89 ppm peak arising from pyridine interacting with Si OH groups and having very weak 1H 15N dipolar interactions and, thus, contributing little/no CP signal. Despite these interesting findings, the exact nature of the 181, 118, and the 103 ppm peaks cannot be further defined based on the isotropic shift results from the 1D measurement in Figure 2 alone. In particular, the 118, 103, and 89 ppm peaks are overlapped, making quantitative interpretation impossible. To solve the spectral overlap problem and to further define the nature of the peaks, the chemical shift tensor was measured using 2D 1H 15N CP-PASS and the results interpreted using quantum chemistry calculations. 3.3. 15N CP-PASS. Figure 3 presents a stacked plot of the 2D 15 N CP-PASS spectra for the SBA-15 sample with 100% W coverage acquired at a 7.05 T magnetic field using a sample spinning rate of 1000 ( 1 Hz. In the 2D PASS spectra, the spinning sidebands arising from the CSA are separated by sideband order, where “n = 0” denotes the centerband that gives the isotropic chemical shift positions similar to those observed in the SP-MAS and CP-MAS experiments at a sample spinning rate of 5 kHz (Figure 2). The sideband patterns associated with different isotropic chemical shift peaks are clearly separated in the 2D plot. For example, up to 12 orders of SSBs associated with an isotropic shift of 181 ppm (peak “1”) were clearly observed, while up to 14 SSBs were observed for peak “2” at an isotropic shift of 118 ppm. In contrast, only the centerband was observed for peak “3”, at an isotropic shift of 103 ppm, indicating an apparently very small chemical shift anisotropy. It is possible that the pyridine molecules interacting with the Si OH sites experience fast molecular motion, which serves to average the nitrogen CSA. Note that no SSBs associated with peak 3 are observed by lowering the temperature to 65 °C (Figure S1 in the Supporting Information). The molecules are moving isotropically even at low temperatures, fast enough to average the nitrogen CS tensor 23357



Figure 4. 2D 15N CP-PASS spectra of the 50% W coverage SBA-15 sample that was calcined at 400 °C. These spectra were acquired using the same experimental parameters as those given in Figure 3 except that 7200 accumulations were obtained for each of the 16 evolution steps. The total experimental time was about 38.5 h.

so that the averaged CS tensor has a span (δ33 δ11) much smaller than 1 kHz. However, this motion is not fast enough to average the 15N 1H dipolar couplings. As a result, a CP signal is still observed for peak 3 in the 2D-PASS spectrum (Figure 3), albeit with a reduced spectral intensity due to an inefficient crosspolarization between 1H and 15N, enforcing the result obtained from Figure 2, spectrum e. The ability to observe many SSBs associated with both the 181 and the 118 ppm peaks indicates that the molecular motion of the corresponding pyridine molecules is highly restricted. It has been established in Figure 1b that a portion of the pyridines interacting with the surface undergoes a restricted 2-fold flip motion about the pyridine C2 symmetry axis. It is then reasonable to link the 2-fold flip motion to both the 181.2 and the 118.2 ppm peaks. As a result of the restricted 2-fold flip motion, the 1H 15N cross-polarization (CP) is still efficient via both the intra- and inter-1H 15N dipolar interactions at a sample spinning rate of 1 kHz. Thus, it is reasonable to assume that the integrated intensities associated with these two sites acquired using a contact time of 1 ms and a sample spinning rate of 1 kHz are quantitative relative to each other; that is, their CP efficiencies are similar. The integrated spectral intensities can then be used to estimate their relative abundance. The total spectral intensity of each peak can be obtained by adding the integrated spectral intensities corresponding to all orders of SSBs associated with a specified isotropic chemical shift. Although this can be done easily for the 181 ppm peak because of the absence of spectral overlap in the 2D CP-PASS spectrum, the centerband for the 118 ppm peak is severely overlapped with that of the 103 ppm peak. In this case, the 15N pyridine centerband intensity is predicted by fitting the SSB patterns, as will be detailed shortly. Using this fitting procedure, we obtain an integrated area ratio of the 181 ppm peak to the 118 ppm peak to be approximately 1.1 ( 0.1:1.0 in the 400 °C calcined SBA-15 sample with 100% W coverage. It is also found from Figure 3 that the integrated spectral intensity for the 103 ppm peak is approximately 40% that of the 181 ppm peak. However, it should be pointed out that, in the CP-PASS spectra of Figure 3, the intensity of the 103 ppm peak is not quantitative due to inefficient cross-polarization arising from significant molecular motion. Attempts have been made to use SP-PASS to acquire quantitative spectra for all of the peaks. Unfortunately, the experimental time to obtain such spectra with a good signal-to-noise ratio was prohibitively long (Figure S2 in the Supporting Information).

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The 15N CP-PASS spectra for the 50% W coverage SBA-15 sample using a sample spinning rate of 1000 ( 1 Hz are illustrated in Figure 4. The centerband peak position labeled as “3” is at 103 ppm, consistent with the corresponding results given in Figure 3. The centerband peak position for the 118 ppm peak (peak “2”) is visible as a shoulder peak to peak “3”. Similar to that of the sample with 100% W coverage, the sideband families for both the 181 and the 118 ppm peaks are clearly observed in Figure 4, albeit with a decreased signal-to-noise ratio due to the lower amount of W coverage. By adding the sideband intensities belonging to each family, the relative ratio of the 181 ppm peak to the 118 ppm peak is determined to be 0.5 ( 0.1:1.0. The principal values of the 15N CS tensor for the pyridine molecules that are associated with the 181 and 118 ppm peaks were determined by fitting the corresponding SSB pattern with a single tensor using the Herzfeld Berger method,52 and the results at both RT and 50 °C are listed in Table 1. The quality of the SSB fitting is provided in Figure S3 in the Supporting Information. Because each order of SSBs associated with the 181 ppm peak is well separated from the other peaks in the 2D PASS spectrum of Figure 3, the integrated SSB intensity was directly fit. As indicated in the top trace of Figure S3 (Supporting Information), the SSBs can be fit well using a single tensor with 44 ppm (δ11), 148 ppm (δ22), and 351 pm (δ33), respectively. For the 118 ppm peak, the centerband is severely overlapped with the centerband peak from the 103 ppm peak, whereas all other SSB orders in the spectra are well separated from other components. By adjusting the integrated intensity of the centerband for the 118 ppm peak, the entire SSB family can be fit well using a single tensor with 43 ppm (δ11), 30 ppm (δ22), and 368 ppm (δ33), respectively. This adjusted centerband intensity is taken as the isolated intensity for the centerband of the 118 ppm peak, which was then used to calculate the total sideband intensities above for determining the ratio of the 181 ppm peak to the 118 ppm peak. Table 1 also gives the experimental values of the CS tensor associated with both the 181 and the 118 ppm peaks at a temperature of 50 °C. Compared with the values obtained at RT, the differences are within 10 ppm, which are within the experimental error range. It is known (ref 56) that δ22 is directed along the lone pair, and δ33 is perpendicular to the ring plane. Hence, if there was some sort of 2-fold rotation at RT, we might expect to see a reduction in the overall span as a result of the averaging of δ33 and δ11 components (and not δ22). However, if the motion is a restricted 2-fold flip motion, δ11 and δ33 consistently point in the same directions, and no real averaging will be observed. Because there is no difference in the nitrogen CS tensors measured at RT and 50 °C, it seems that there is no mode of motion that serves to average any of their principal components (i.e., rotational motion about the 2-fold symmetry axis). Therefore, except for the 2-fold flip motion about the pyridine C2 symmetry axis, other modes of motion are not likely. 3.4. Quantum Chemistry Calculations. Because a 2-fold flip motion of the pyridine molecules about the molecule’s C2 symmetry axis does not change the powder line shape of the 15 N CS tensor, or the principal values of the CS tensor, quantum chemistry calculation of the CS tensor can then be utilized for explaining the experimental results. Prior to NMR calculations, the geometry of each model was optimized. The NMR parameters were then calculated based on these optimized geometries. The calculated principal values for nitrogen in an isolated 23358



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Table 1. Experimental 15N Principal Values of the Chemical Shift Tensors of Pyridine Interacting with Supported Tungsten Oxide on SBA-15b δ11 (ppm)

15

N isotropic chemical shift/model compound

δ22 (ppm)

δ33 (ppm)

δiso (ppm)

181.2 ppm obtained at RT

44

148

351

181

181.2 ppm obtained at 50 °C 118.2 ppm obtained at RT

48 43

139 30

357 368

181 118

53

29

378

118

200

33

422

63

50 °C

118.2 ppm obtained at pure pyridine a

From ref 56. The estimated error for the principal values (i.e., δ11, δ22, and δ33) from sideband fitting is (5 to (10 ppm. δiso is measured directly from the spectrum with an uncertainty of approximately 0.5 ppm. b Previously reported values for pure pyridine are also included for comparison. The chemical shift reference is CH3NO2 (0 ppm). a

Table 2. Principal Values of 15N Chemical Shift Tensors of Various Models Obtained by Quantum Chemistry Calculationsb namea Brønsted acid site

δ11 (ppm) δ22 (ppm) δ33 (ppm) δiso (ppm) 22

134

375

177

40

187

363

197

W OH site (model D) Si OH site (model E)

133 184

21 29

410 420

85 69

H2O site (model F)

208

33

423

61

tungsten Lewis site

161

30

415

75

K pyridine (model H)

129

13

418

92

pyridine

263

42

433

43

CH3NO2

137

39

176

0

(model B) protonated pyridine (model C)

(model G)

a

Note: the models given in the parentheses correspond to those in Figure 5. b The calculated results obtained from nitromethane and pyridine interacting with a positively charged sodium are also included for comparison. The shifts are referenced to CH3NO2 at 0 ppm.

pyridine are 263, 42, and 433 ppm for δ11, δ22, and δ33, respectively, with reference to CH3NO2 (0 ppm) (Table 2). The experimental values from the prior literature53 56 vary significantly. Probably, the most reliable values are those from Solum et al.56 with 200 ppm (δ11), 33 ppm (δ22), and 422 ppm (δ33), respectively. Thus, the agreement between the theoretical and the experimental values is reasonably good, demonstrating the validity of the calculations. On the basis of our prior report (ref 40), the possible surface sites in SBA-15 supported tungsten oxides include Brønsted acid sites, Lewis acid sites, W OH, Si OH, and physisorbed H2O. The titrated pyridine could interact with all of these sites. In a study of support effects on Brønsted acid site densities and alcohol dehydration turnover rates over tungsten oxide domains, Macht et al.57 proposed the formation of acidic Hδ+(WO3)nδ species as the Brønsted acid site. In another paper, where the mechanistic consequences of composition in acid catalysis by polyoxometalate Keggin clusters were studied, the bridging OH groups between tungsten atoms58 are proposed as the Brønsted acid site. However, the exact nature of the Brønsted acid sites in supported tungsten oxides remains unclear. Because supported catalysts with very low WOx coverage were relatively inactive, we propose an acidic OH group bridged between the two tungsten atoms, depicted in Figure 5A, as the Brønsted acid site. In this model, carrying 1 net positive charge, each W has six bonds, that

Figure 5. Theoretical models used for quantum chemistry calculations: (A) the model of the tungsten Brønsted acid site carrying 1 net positive charge, (B) a pyridine molecule interacting with a tungsten Brønsted acid site, (C) protonated pyridine carrying 1 net positive charge, (D) pyridine interacting with a W OH site, (E) model of pyridine interacting with a Si OH site, (F) pyridine interacting with a H2O molecule, (G) pyridine interacting with a tungsten Lewis acid site, and (H) pyridine interacting with a K+ cation. The bond lengths shown were obtained after being geometry-optimized. In models D G, the net charge is zero.

is, three single bonds to three isolated OH groups to simulate the anchoring of W at SBA-15 surface, one double bond to oxygen, and one single bond to the bridging OH group. The quantum-chemistry-optimized geometry indicates that the bond length between O and H in the acidic OH group is 0.973 Å. However, it is found (Figure 5B) that, when pyridine is interacting with this proton, the bond length between the O and the H increases to 1.726 Å. Concurrently, a strong bond between the H and the nitrogen in pyridine, with a bond length of 1.054 Å, is formed. The calculated 15N CSA principal values and the isotropic chemical shift are δ11 = 22 ppm, δ22 = 134 ppm, δ33 = 375 ppm, and δiso = 177 ppm, respectively (Table 2). These data are in good agreement with experimental data (δ11 = 44 to 48 ppm, δ22 = 139 to 148 ppm, δ33 = 351 to 357 ppm, and δiso = 181 ppm) (Table 1). This result indicates that the bridged tungsten OH group is highly acidic and, thus, an excellent candidate for the Brønsted acid site. To reinforce this conclusion, the 15N NMR parameters are calculated for an isolated pyridine molecule carrying 1 net positive charge (Figure 5C), where δ11 = 40 ppm, δ22 = 187 ppm, 23359


The Journal of Physical Chemistry C δ33 = 363 ppm, and δiso = 197 ppm are obtained with an optimized N H bond distance of 1.021 Å. This result confirms that the bridged tungsten OH group in Figure 5A is highly acidic, as is evident by the almost complete donation of the proton to the pyridine to form a strong H N bond. Such a short N H bond length is also consistent with the very efficient crosspolarization in the 1H 15N CP experiments (Figure 5B). Therefore, it can be unambiguously concluded that the peak at an isotropic chemical shift of 181 ppm is due to pyridine molecules interacting with the tungsten Brønsted acid sites. For peaks with chemical shift values greater than 120 ppm, it has been determined from the results in Figure 2 that the isotropic chemical shift value for pyridine interacting with Si OH groups is 89 ppm. It is also known from ref 21 that the value of δiso for pyridine interacting with surface H2O molecules is located at about 82 ppm. The lack of observable 82 ppm peaks in our investigation indicates that our samples do not contain significant amounts of H2O that could directly coordinate with pyridine. This leaves only the 118 and the 103 ppm peaks in Figures 2 4 unassigned. The possibilities are limited to pyridines interacting with W OH groups, and pyridines interacting with the Lewis acid sites where a pyridine is directly interacting with a W atom. The efficient cross-polarization from the 1H 15N CP experiment at 5 kHz (Figure 2, spectrum e) and CP-PASS at 1 kHz (Figures 3 and 4) suggests that the 118 ppm peak arises from pyridine interacting with W OH sites, whereas the 103 ppm peak likely comes from pyridines interacting with tungsten Lewis sites. To clarify these assignments, quantum chemistry calculations were carried out on model systems consisting of pyridine molecules interacting with W OH, and tungsten Lewis acid sites, separately. In our calculations, models of pyridine interacting with Si OH, or H2O, are also included for comparison. The predicted trends for 15N CSA principal values, and, in particular, the isotropic chemical shifts, are then used to verify the above-described interpretation of the experimental results. Although the difference between the predicted shifts and the experimental values can be as large as 30 40 ppm even with the use of sophisticated methods and large basis sets,59 the predicted chemical shift trends in various systems can still be useful for interpreting the experimental results with relatively subtle chemical shift changes. The models are summarized in Figure 5D H, where the optimized bond distances (in units of Å) that are relevant to our discussions are also highlighted. Figure 5D is used to simulate the pyridine molecule interacting with the W OH group. In this model, W forms six bonds, four of them to four isolated OH groups and one double-bonded to an oxygen atom. The nitrogen in pyridine is interacting with one of the W OH groups. In the optimized geometry, the O H bond distance in the free W OH groups, that is, the W OH that are not interacting with pyridine, is 0.973 Å. In the pyridine-titrated W OH group, the corresponding O H bond distance only slightly increased to 1.041 Å. The corresponding N H bond distance is 1.571 Å, and the predicted 15N isotropic chemical shift value (δiso) is 85 ppm. When pyridine is interacting with Si OH groups (Figure 5E), the N H bond distance increases to 1.784 Å and the O H bond length becomes 1.001 Å, only very slightly longer than that of the uncoordinated Si OH groups (0.968 Å). This very weak interaction gives a predicted 15N δiso of 69 ppm. If a pyridine is hydrogen-bonded to a H2O molecule (Figure 5F), the N H bond distance further increases to 1.938 Å and the predicted 15N δiso further increases to 61 ppm. The

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long N H bond distances in Si OH coordinated pyridine (1.784 Å in Figure 5E) probably explains why we did not observe its corresponding 89 ppm peak in the 1H 15N CP PASS experiments (Figures 3 and 4), presumably due to a almost complete average of 1H 15N dipolar interactions by the fast random molecular motion of pyridines. The model in Figure 5G was used for gaining insights into the pyridine molecules interacting with tungsten Lewis acid sites. In this model, a pyridine molecule is directly interacting with the W atom (four single bonds to four isolated OH groups and one double bond to an oxygen atom). In the optimized geometry, the distance between N in pyridine and W is ∼2.669 Å, and the resulting 15N δiso is 75 ppm. We recognize that this may not be an ideal model for a tungsten Lewis acid site. As such, we also performed calculations on the model in Figure 5H, where pyridine is interacting with a potassium cation (K+) as additional information about pyridine interacting with Lewis acid sites. The optimized N K distance is 2.708 Å, which is very close to the N W distance in Figure 5G, and the predicted 15N δiso for the pyridine K+ structure is 92 ppm. The similarity of these values for the two pyridine Lewis acid site models gives us additional confidence that the model in Figure 5G is a reasonable one. Excluding model H, based on the quantum chemistry predictions just described, the isotropic chemical shift values of 15N in pyridine interacting with surface sites follows the sequence δiso (W OH) ( 85 ppm) < δiso (tungsten Lewis acid site) ( 75 ppm) < δiso (Si OH) ( 69 ppm) < δiso (H2O) ( 61 ppm). The experimental values are δiso ( 118.2 ppm) < δiso ( 103.2 ppm) < δiso (Si OH) ( 89 ppm) < δiso (H2O) ( 65 ppm). Because pyridine molecules interacting with either the Si OH groups (or H2O molecules) are not observed in the CP experiments due to the reasons discussed earlier, the quantum chemistry calculated isotropic chemical shifts suggest that the 118 ppm is caused by pyridine interacting with the W OH site, whereas the 103 ppm arises from the tungsten Lewis acid sites, consistent with the results inferred by 1H 15N CP experiments at both the 1 and the 5 kHz sample spinning rates above. With the chemical identities of the 15N chemical shifts assigned, the surface chemistry obtained from Figures 3 and 4 can be summarized as follows. The ratio of the Brønsted acid ( 181 ppm, peak 1 in Figures 3 and 4) to the W OH sites ( 103 ppm, peak 2) is approximately 1.1:1.0 in the 400 °C calcined SBA-15 sample with a theoretical 100% W coverage. This ratio drops to about 0.5:1.0 in the 400 °C calcined SBA-15 sample with a theoretical 50% W coverage. We assume that three of the six bonds for each surface adsorbed tungsten atom are used for anchoring one W to the SBA-15 surface and there is also one WdO bond associated with each tungsten species. With such assumed structures, this leaves only one W OH group for isolated (monomeric) surface tungsten species. To form a tungsten dimer structure on the SBA-15 surface, one of these OH groups from two monomers must be eliminated, with the other one forming the bridged tungsten Brønsted acid site, as discussed above. Our results clearly show that, in the 50% theoretical W coverage SBA-15 sample, the number of surface tungsten species associated with monomers is approximately the same as that associated with the dimer structure, considering that each surface W dimer structure contains two tungsten atoms. This result confirms that, even at low W coverage, dimeric (may include multimeric) tungsten species have already formed. However, in the 100% theoretical W coverage sample, the number of W associated with such dimeric species is significantly 23360


The Journal of Physical Chemistry C increased, to at least 2 times that associated with the monomers, indicating that dimeric (or multimeric) tungsten species are the preferred structure at high W loading on the SBA-15 surface. The main point to be made here is that formation of dimeric (or multimeric) tungsten structures should give rise to an increase in highly acidic OH structures that bridge two tungsten atoms relative to those attached to a single tungsten atom. Furthermore, on the basis of quantum chemistry calculations, the Brønsted acid sites associated with OH groups that bridge two tungsten atoms are highly acidic because the proton can be almost completely donated to pyridine. Such results are consistent with prior work (refs 11 17) showing that tungsten oxide has the strongest Brønsted acid sites among all the early transition-metal oxides. On the other hand, terminal W OH sites are very stable as are the terminal Si OH sites. Thus, the W OH and Si OH sites should have a much lower catalytic activity than the bridging tungsten Brønsted acid sites.

4. CONCLUSIONS In summary, the surface acid sites in highly dispersed tungsten oxide catalysts supported on SBA-15 mesoporous silica have been investigated using a combination of 2H NMR, 15N fastspinning SP-MAS and CP-MAS, slow-spinning 1H 15N 2D PASS spectroscopy, and quantum chemistry calculations, where the acid sites are probed with pyridine titration. We find that the 15N MAS NMR peak corresponding to pyridine titrated onto the tungsten Brønsted acid sites is well separated from the rest of the peaks, whereas the peaks corresponding to pyridine interacting with W OH, the tungsten Lewis acid, and silanol (Si OH) sites severely overlap in the fast-MAS spectra, making their quantification difficult. This problem has been solved, in part, with slowMAS 1H 15N 2D CP-PASS applied at a sample spinning rate of 1 kHz. In the 2D PASS spectrum, the spinning sideband families corresponding to both the Brønsted acid sites and the W OH sites are uniquely separated, from which the principal values of 15 N chemical shift tensors are determined. By comparing experimental results with quantum chemistry calculations, we find that the bridged acidic OH group in the surface adsorbed tungsten dimers (i.e., W OH W) is the Brønsted acid site. The unusually strong acidity of this Brønsted acid site is also confirmed by the quantum chemistry calculations. In contrast, the W OH and Si OH sites are very stable, with minimal Brønsted acidity. The molecular interactions between pyridine molecules and both the Brønsted and the W OH sites in dispersed tungsten oxide species are strong, resulting in highly restricted molecular motion, that is, 2-fold flip motion about the molecule’s C2 symmetry axis, even at room temperature. Such a restricted motion makes it possible to quantitatively determine the relative ratio of the Brønsted (tungsten dimer) to the W OH (tungsten monomer) sites in the catalyst using the slow-MAS 1H 15N CP PASS method. We find that, in the 50% theoretical W coverage SBA-15 sample, the number of surface tungsten atoms associated with monomers is approximately the same as that associated with the dimer structure, whereas in the 100% theoretical W coverage sample, the number of W associated with dimers is significantly increased, to about 2.2 times that associated with the monomers. This result indicates that, at high W coverage, the major adsorbed surface W structure is the dimer structure. On the other hand, the pyridine molecules that are interacting with both the tungsten Lewis acid sites and the silanol sites experience fast and random molecular motion. As a result, no sidebands are observed for the

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corresponding tungsten Lewis acid site and no signal is observed for the silanol sites in the 1H 15N CP PASS spectra at temperatures as low as 50 °C and at a sample spinning rate of 1 kHz.

’ ASSOCIATED CONTENT Supporting Information. 2D 15N CP-PASS spectra and 2D 15N SP-PASS spectra of the 100% W coverage SBA-15 sample that was calcined at 400 °C and experimental and fit 15 N chemical shift anisotropy SSB patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (J.Z.H.), [email protected] (C.H.F.P.). Phone: (509) 371-6544 (J.Z.H.), (509) 371-6501 (C.H.F.P.). Fax: (509) 371-6546 (J.Z.H.).

’ ACKNOWLEDGMENT This research was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. All of the NMR experiments were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research, located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for the DOE by Battelle Memorial Institute under Contract DEAC06-76RLO 1830. The anonymous reviewer is acknowledged for his/her constructive critiques and suggestions. ’ REFERENCES (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) Thomas, C. L. Catalytic Processes and Proven Catalysts; Academic Press: New York, 1970. (3) Pasel, J.; Kassner, P.; Montanari, B.; Gazzano, M.; Vaccari, A.; Makowski, W.; Lojewski, T.; Dziembaj, R.; Papp, H. Appl. Catal., B 1998, 18, 199–213. (4) Butler, A.; Nicolaides, C. Catal. Today 1993, 18, 443–471. (5) Cheng, Z. X.; Ponec, V. Catal. Lett. 1994, 25, 337–349. (6) Alvarez-Merino, M. A.; Carrasco-Marín, F.; Moreno-Castilla, C. J. Catal. 2000, 192, 363–373. (7) Alvarez-Merino, M. A.; Carrasco-Marín, F.; Moreno-Castilla, C. J. Catal. 2000, 192, 374–380. (8) Barton, D. G.; Soled, S. L.; Iglesia, E. Top. Catal. 1998, 6, 87–99. (9) Barton, G. G.; Soled, S. L.; Meitzner, G. D.; Fuentes, G. A.; Iglesia, E. J. Catal. 1999, 181, 57–72. (10) Barton, D. G.; Shtein, M.; Wilson, R. D.; Soled, S. L.; Iglesia, E. J. Phys. Chem. B 1999, 103, 630–640. (11) Busca, G. Phys. Chem. Chem. Phys. 1999, 1, 723–736. (12) Carniti, P.; Gervasini, A.; Auroux, A. J. Catal. 1994, 150, 274–283. (13) Bernholc, J.; Horsley, J. A.; Murrell, L. L.; Sherman, L. G.; Soled, S. J. Phys. Chem. 1987, 91, 1526–1530. (14) Benitez, V. M.; Querini, C. A.; Figoli, N. S.; Comelli, R. A. Appl. Catal., A 1999, 178, 205–218. (15) Martin, C.; Malet, O.; Solana, G.; Rives, V. J. Phys. Chem. B 1998, 102, 2759–2768. (16) Thomas, R.; Van Oers, E. M.; De Beer, V. H. J.; Medema, J.; Moulijin, J. A. J. Catal. 1982, 76, 241. 23361


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Characterizing Surface Acidic Sites in Mesoporous-Silica-Supported

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