ZrO2

10662

J. Phys. Chem. B 2006, 110, 10662-10671

Acidity of Mesoporous MoOx/ZrO2 and WOx/ZrO2 Materials: A Combined Solid-State NMR and Theoretical Calculation Study Jun Xu,† Anmin Zheng,† Jun Yang,† Yongchao Su,† Jiqing Wang,† Danlin Zeng,† Mingjin Zhang,*,‡ Chaohui Ye,† and Feng Deng*,† State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Insitute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, P. R. China, and Department of Chemistry, Wuhan UniVersity of Science and Technology, Wuhan 430081, P. R. China ReceiVed: March 7, 2006; In Final Form: April 9, 2006

The acidity of mesoporous MoOx/ZrO2 and WOx/ZrO2 materials was studied in detail by multinuclear solidstate NMR techniques as well as DFT quantum chemical calculations. The 1H MAS NMR experiments clearly revealed the presence of two different types of strong Brønsted acid sites on both MoOx/ZrO2 and WOx/ZrO2 mesoporous materials, which were able to prontonate adsorbed pyrine-d5 (resulting in 1H NMR signals at chemical shifts in the range 16-19 ppm) as well as adsorbed trimethylphosphine (giving rise to 31P NMR signal at ca. 0 ppm). The 13C NMR of adsorbed 2-13C-acetone indicated that the average Brønsted acid strength of the two mesoporous materials was stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which was in good agreement with theoretical predictions. The quantum chemical calculations revealed the detailed structures of the two distinct types of Brønsted acid sites formed on the mesoporous MoOx/ZrO2 and WOx/ZrO2. The existence of both monomer and oligomer Mo (or W) species containing a Mo-OH-Zr (or W-OH-Zr) bridging OH group was confirmed with the former having an acid strength close to zeolite HZSM-5, with the latter having an acid strength similar to sulfated zirconia. On the basis of our NMR experimental and theoretical calculation results, a possible mechanism was proposed for the formation of acid sites on these mesoporous materials.

Introduction Because the environmentally friendly demands on the chemical and petroleum industries are becoming increasingly intense, the use of solid acid catalysts offers new alternatives to the highly corrosive, hazardous, and polluting liquid acids.1 The sulfated metal oxides (such as sulfated zirconia) showing strong acidity have found their potential applications in catalytic process such as isomerization, cracking, and alkylation, and these kinds of catalysts have been extensively studied in the past decades.2,3 However, the easy loss of the dopant, sulfur compound, during thermal treatment and regeneration of sulfate metal oxides may limit their applications. Complex oxide catalyst, which possesses many merits such as high thermal stability, no corrosion, and easy separation, has become an attractive candidate. Aratra et al.4 reported complex oxides as solid acid catalysts, which are superior to the sulfated solid catalysts in many catalytic reactions. ZrO2 has been found to be an excellent catalytic support due to its high thermal stability, more active when interacting with the dopant phase.5 Thus, considerable interest has been invested in the study of MoOx/ZrO2 and WOx/ZrO2 catalysts,6 including their physical and chemical properties and catalytic performance. However, similar to sulfated catalysts, low surface area and disappointing selectivity are still the drawbacks of the complex oxide catalyst. As pointed out by Weitkamp et al.,7 a promising * Corresponding authors. E-mail address: [email protected] (F.D.).Telephone: +86-27-87198820. Fax:+86-27-87199291. † State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Insitute of Physics and Mathematics. ‡ Department of Chemistry, Wuhan University of Science and Technology.

solid acid catalyst should have the following features: (1) high specific surface area (SSA), (2) more and strong Brønsted acid sites, and (3) zeolite-like micropores or mesopores, etc. To enhance surface area and pore size, attempts to support sulfated zirconia on mesoporous materials such as MCM-41 have been done, whereas their catalytic performance was still disappointing due to the weaker acid strength. Also, the mesostructured sulfated zirconia has been reported while suffering the thermal stability problem. On the basis of above considerations, porous zirconia with a high surface area and large pores may be an excellent catalyst support. In addition, the successful preparations of mesoporous zirconia in the recent years may satisfy the above requirements.8,9 Melezhyk10 reported the preparation of mesoporous WO3/ZrO2 by using poly(vinyl alcohol) as a template. Compared with the conventional catalysts, the resultant mesoporous material exhibited much better activity and selectivity in the catalytic reactions with large molecules.11 To the best of our knowledge, characterizations of the conventional MoOx/ZrO2 and WOx/ZrO2 materials were well documented. However, intensive investigation on the acidity of mesoporous MoOx/ZrO2 and WOx/ZrO2 as potential solid acid for catalytic reactions has never been found in the previous literature. Owing to its importance, the acidity of various solid catalysts has been extensively studied using various analytic techniques such as Hammett indicator, FT-IR, calorimetry, temperature-programmed desorption (TPD), and NMR. Among them, NMR techniques play an important role in the description of the detailed nature and acid strength of solid catalysts.12 1H NMR can be used to identify different surface OH groups,12d,13 while according to the 13C chemical shift of 2-13C-acetone adsorbed on solid acids, one can quantitatively measure the

10.1021/jp0614087 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/13/2006

Acidity of Mesoporous MoOx/ZrO2 and WOx/ZrO2

J. Phys. Chem. B, Vol. 110, No. 22, 2006 10663

TABLE 1: Physical Characteristics of Mesoporous Zirconia, MoOx/ZrO2, and WOx/ZrO2 Materials

zirconia MoOx/ZrO2 WOx/ZrO2

surface area (m2/g)

pore diameter (nm)

pore volume (ml/g)

32 134 154

5.6 3.4 3.8

0.073 0.14 0.19

Brønsted acid strength. In comparison to 13C or 15N NMR, 31P NMR of a probe molecule is more suitable for solid acid characterization due to the relatively high NMR sensitivity of 31P. Trimethylphosphine (TMP) is one of the probe molecules widely used for this purpose. 31P NMR has a large chemical shift range for TMP bound to different acid sites, and Brønsted and Lewis acid sites can be easily distinguished. In this paper, we employed various solid-state NMR techniques including 1H, 13C, and 31P MAS NMR to elucidate the detailed nature of acid sites present on mesoporous ZrO2, MoOx/ZrO2, and WOx/ZrO2 materials. Theoretical calculation can reveal the acid strength, the interaction between probe molecules and acid centers as well as the reaction transition state formed on solid acids.14,15 Haw et al. calculated the proton affinity and 13C chemical shift of adsorbed 2-13C-acetone in order to measure the acid strength of various solid acids.15 Our previous studies also have demonstrated that the theoretical method was an efficient method to predicted chemical shift, such as supramolecular assembly of amino acids and organic compounds adsorbed on zeolites.16 Therefore, in the present work, theoretical calculations were performed as well on several selected models that were proposed based on our NMR experimental results to study the acid strength of mesoporous MoOx/ZrO2 and WOx/ZrO2 systems. The good agreement between the theoretical calculations and the experimental observations revealed the detailed structures of the acid sites formed on the mesoporous materials and their relative acid strengths. Experimental Section Sample Preparation. Poly(ethylene glycol) (PEG) is an innocuous and lower-price polymer and successfully employed in the preparation of mesoporuos materials.17 The mesoporous MoOx/ZrO2 and WOx/ZrO2 materials were prepared using PEG as the template with a procedure similar to that used by Melezhyk.10 To a 20% aqueous solution of ZrOCl2‚8H2O, an aqueous solution of ammonium heptamolybdate was added while stirring. The resulting mixture was kept at 100 °C in a water bath for 1 h. Then a 10% aqueous solution of PEG (polymer unit is 68-84, Mw ) 3000-3700 g/mol) was added to the above mixed solution while stirring. Subsequently, the prepared aqueous ammonia was added dropwise under stirring. Precipitated gel was obtained (pH ) 10) and washed with distilled water until the filtrate showed a negative test for Cl-, and then it was dried in a drying oven at 120 °C overnight. The resultant sample was placed into a quartz tube and gradually heated to 600 °C at rate of 1 °C/min in a pure nitrogen flow and kept at the final temperature for 2 h. Then, the sample was calcined at 600 °C for 5 h under the flow of oxygen. A MoOx/ ZrO2 sample with a loading of 15% (wt/wt) was obtained. A WOx/ZrO2 sample with a loading of 15% with ammonium metatungstate as the source and a pure zirconia sample was prepared in a similar way. The physical characteristics of these materials were listed in Table 1. Characterization. The crystal structures of our prepared samples were determined by X-ray diffraction on a Phillips

Xpert Pro instrument using Cu KR radiation (40 kv, 40 kmA). A scan range of 1° < 2θ < 90° with a step of 0.02°/s for 0.5° < 2θ < 10° and a step of 0.2°/s for 10° < 2θ < 90° was used. Monoclinic silicon was used as an internal standard for quantitative analysis. Specific surface areas of the samples were measured by using the BET method with nitrogen adsorption at 77 K, and the corresponding pore sizes were calculated using the Barret-Joyner-Halenda (BJH) method. Sample Preparation for MAS NMR. A known amount of the sample was placed in a glass tube to a bed length of about 10 mm. The tube was connected to a vacuum line. The temperature was gradually increased at a rate of 1 °C/min, and the sample was kept at a final temperature of 400 °C at a pressure below 10-3 Pa over a period of 12 h and then cooled. After the sample cooled to ambient temperature, a measured volume of adsorbed molecules (TMP, 2-13C-acetone, and pyridine-d5) with a known pressure was condensed and frozen inside the sample by cooling the sample region of the NMR tube with liquid nitrogen. In some cases, the physisorbed molecules were removed under vacuum at a temperature ca. 80 °C for 0.5 h. Finally, the NMR tube was flame sealed. Prior to NMR measurements, the samples were filled into NMR rotors with a Kel-F endcap (cut with 20 grooves) under a dry nitrogen atmosphere in a glovebox. NMR Measurements. All the NMR experiments were carried out at 9.4 T on a Varian Infinity Plus-400 spectrometer, equipped with a Chemagnetic triple-resonance 7.5 mm probe, with resonance frequencies of 400.1, 100.6, and 161.9 MHz for 1H, 13C, and 31P, respectively. The π/2 pulse lengths for 1H, 13C, and 31P were typically 3.7, 4.2, and 4.9 µs, respectively. 1H MAS NMR experiments were recorded with a composite pulse sequence capable of suppressing the 1H signals from the spinning module.18 A recycle delay of 4 s was used in the 1H NMR experiments. Single-pulse 31P MAS experiments with 1H decoupling were performed with a 10 s recycle delay. For the 1H f 13C CP/MAS NMR experiments, the Hartmann-Hahn condition was achieved by using hexamethylbenzene (HMB), with a contact time of 2.0 ms and a repetition time of 2.0 s. The Hartmann-Hahn condition for 1H f 31P CP/MAS NMR experiment was optimized by using (NH4)2HPO4, and the contact time and repetition time were 1.0 ms and 2.0 s, respectively. The number of scans acquired for each spectrum was varied between 500 and 5000, and the MAS spinning speed ranged from 5 to 6.5 kHz. The chemical shifts were referenced to tetramethylsilane (TMS) for 1H, HMB for 13C, and 85% H3PO4 solution for 31P, respectively. Computational Models and Methods. The optimizations and chemical shift predictions were carried out by using the Amsterdam density functional (ADF) package,19 with the local spin density approximation for exchange and correlation potential employing the Vosko-Wilk-Nusair generalized gradient approximations (GGA) used for the density gradient correction to the exchange and correlation function.20,21 The triple-ζ basis set, including two polarization functions (TZ2P) based on the Slater-type orbital (STO), was chosen. 1s-4f orbitals for W and 1s-4p orbitals for Mo and Zr atoms were kept frozen with the frozen core approximation, while all electron basis sets for the O, N, C, and H atoms were used in the structure optimizations and NMR calculations. It was demonstrated that the relativistic effects are important for Mo, Zr, and W atoms in defining the geometry of structure, therefore, the calculations were performed within the relativistic scheme with the zeroorder regular approximation (ZORA)22 including the scalar effects. Three possible cluster models (I, II, and III)

10664 J. Phys. Chem. B, Vol. 110, No. 22, 2006

were proposed for the Brønsted acid sites formed on the mesoporous ZrO2, MoOx/ZrO2, and WOx/ZrO2 materials. No atoms in pyridine and acetone molecules were constrained during all the configuration optimizations of adsorption complexes. The NMR calculations were performed using the GIAO method at the same level of structure optimization. The calculated 13C NMR isotropic chemical shift of the carbonyl carbon of acetone absorption complexes was referenced to the NMR experimental value of the gas-state acetone (208 ppm), and 1H chemical shift values were obtained relative to the isotropic shielding tensors of TMS, which were calculated by the same theoretical method.

Xu et al.

Figure 1. X-ray diffractograms of mesoporous materials: (a) zirconia, (b) MoOx/ZrO2, and (c) WOx/ZrO2.

Results and Discussion XRD Analysis. Figure 1 shows wide-angle XRD patterns of the zirconia, MoOx/ZrO2, and WOx/ZrO2 samples calcined at 600 °C. It can be seen from Figure 1a that the tetragonal ZrO2 phase is predominant in the zirconia sample accompanying a small amount of monoclinic phase that results from crystalline transition during high-temperature calcinations. In contrast, only the tetragonal phase is present in the MoOx/ZrO2 sample, and no diffraction lines corresponding to crystalline MoO3 are observed (Figure 1b), indicative of a high dispersion of molybdenum oxide on the material. Supported molybdenum oxides can exist as MoOx monomers, two-dimensional oligomers, and bulk MoO3.23 The polymolybdate saturation capacity on zirconia has been reported to be ∼5 Mo/nm2,24 and the Mo surface density study here is about 4.7 Mo/nm2, close to the saturation value. Thus, the molybdenum oxide probably exits as either isolated MoOx monomers or two-dimensional oligomers instead of bulk MoO3. A similar result is obtained for the WOx/ZrO2 material (Figure 1c). Tungsten oxide (i.e., as monomers or oligomers) is also probably highly dispersed on the tetragonal zirconia. In addition, the suppression of sintering and phase transformation of zirconia on the tetragonal phase by the loaded molybdenum and tungsten is of essential importance for the catalytic performance of the materials, which has been confirmed by many previous investigations.25 Although there is no evidence for the formation of other new phases (i.e., Zr(MoO4)2 or Zr(WO4)2), the existence of Mo-O-Zr and W-O-Zr bonds or other surface species is highly possible. Unlike other reported mesoporous ZrO2,8,9 the XRD patterns of our samples in the small-angle region (not shown) do not give rise to some discernible diffraction lines that characterize ordered mesoporous structure of the materials, implying that less-ordered pore systems are present in the materials. For assynthesized ZrO2-related mesoporous materials, ordered mesopores (characterized by low-angle XRD peaks) are usually observed, while no crystalline ZrO2 particles (characterized by high-angle XRD peaks) are present.8,26 After sample calcination, the ordering of mesopores is largely decreased, and the lowangle XRD peaks become less resolved or even disappeared, depending on the calcination temperature. In addition, a phase transition from amorphous phase to crystalline phase usually occurs for the ZrO2 particles, corresponding to the appearance of the high-angle XRD peaks. The existence of the crystalline phase (such as the tetragonal ZrO2 phase) is essential for the

Figure 2. Nitrogen adsorption-desorption isotherms of mesoporous zirconia, MoOx/ZrO2, and WOx/ZrO2 at 77 K. The inset shows the pore size distribution of the samples.

catalytic activity of the mesoporous materials.11 Most likely, the phase transition leads to the somewhat collapsing of mesopores present inside the ZrO2 particles, which eventually decreases the ordering of the mesopore system as well as the surface area. The existence of mesopores in our materials can be confirmed by the N2 adsorption-desorption measurements. N2 Adsorption-Desorption Analysis. Figure 2 shows adsorption-desorption isotherms of nitrogen in zircona, MoOx/ ZrO2, and WOx/ZrO2 materials. According to the BrunauerDeming-Deming-Teller (BDDT) classification, all of them show a typical type IV isotherm with an E-type hysteresis loop at relative pressures between 0.5 and 0.7, which can be considered as a H2 type in the IUPAC classification, indicative of the presence of “wormlike” mesoporousity. The quite steep desorption step in the isotherm is an indication of a narrow distribution of pore size in the range of 3-6 nm. The pore size distributions calculated from the desorption branch by using the BJH method are shown in Figure 2 as well (inset). The pore sizes of 3.8, 3.4, and 5.6 nm are obtained for MoOx/ZrO2, WOx/ ZrO2, and zircona materials, respectively, confirming the presence of uniform mesopores. The previous studies demonstrated that the pore size uniformity of mesoprous zirconia materials does not necessarily imply bi- or three-dimensional XRD detectable ordering, indicative of a pore formation mechanism different from that of silicate-based mesopostructured materials.26 Obviously, introduction of the metal oxides into zirconia results in a decrease in the pore size as well as its distribution. These mesoporous materials with relatively larger pore size and surface area (see Table 1) would be advantageous to the catalytic reactions of large molecules. Although the resultant mesopores are long-range disordered, they are believed to be more active than their ordered analogues (such as MCM41S) due to the more easy accessibility of the guest molecules



Figure 4. 13C CP/MAS NMR spectra of 2-13C-acetone loaded on (a) MoOx/ZrO2 and (b) WOx/ZrO2.

Figure 3. 1H MAS NMR spectra of mesoporous materials: (a) zirconia, (b) zirconia loaded with pyridine-d5, (c) MoOx/ZrO2, (d) MoOx/ZrO2 loaded with pyridine-d5, (e) WOx/ZrO2, and (f) WOx/ZrO2 loaded with pyridine-d5. Asterisks denote spinning sidebands.

to active sites in the multidimensional framework.27 In addition, the narrow pore size distributions of the materials would be much more important for the selectivity in catalytic applications. 1H MAS NMR Spectra. 1H MAS NMR spectrum of the dehydrated zirconia shown in Figure 3a consists of an intense signal at 4.4 ppm and two weak shoulder peaks at 1.8 and 0.4 ppm. The 4.4 ppm signal may be ascribed to acidic hydroxyl groups, and the other two peaks to weak or nonacidic hydroxyl groups present on ZrO2.28,29 After the introduction of Mo species, there are some significant changes in the corresponding spectrum (Figure 3c). First, the peak at 4.4 ppm completely disappears and two new signals at 7.2 and 5.6 ppm are present, indicating that reaction (or interaction) between the acidic OH groups of ZrO2 and Mo species occurs and two types of new acidic OH groups are likely formed. Second, an intense peak at 1.7 ppm with a shoulder peak at 0.6 ppm appears. Generally, the chemical shift of hydroxyl groups increases with increasing

acidity if hydrogen bonding is absent. The correlation between them was well established for all types of surface hydroxyl groups of zeolites and other oxide catalysts by 1H MAS NMR as well as quantum chemical calculation.12d,13,30 We tentatively assign the 7.5 and 5.6 ppm signals to the strong acidic OH groups, while those at 1.7 and 0.6 ppm are assigned to weak or nonacidic OH groups. It is noteworthy that the concentration of the nonacidic OH groups considerably increases after the addition of Mo species. Deuterated pyridine31 is one of probe molecules widely used for the determination of the acid strength of surface OH groups. For zeolites, the formation of a hydrogen bond between pyridine and a nonacidic silanol group (SiOH) shifts the 1H MAS NMR signal position from 2 to ca. 10 ppm. In the case of acidic OH groups (Brønsted acid sites), the adsorption of pyridine results in 1H NMR signals at chemical shifts in the range of 12-19 ppm. The downfield signals result from a proton transfer to the probe molecule, forming pyridine ions. The adsorption of pyridine-d5 onto ZrO2 results in three new signals at 8.7, 7.5, and 7.2 ppm at the expense of the signal at 4.4 ppm (Figure 3b), implying that only hydrogen-bonded pyridines are formed, and thus the acid strength of the acidic OH groups present on ZrO2 is not strong enough to protonate the adsorbed pyridine molecules. In contrast, after the adsorption of pyridine-d5 onto the MoOx/ZrO2 material (Figure 3d), two signals appear at 19.3 and 16.6 ppm, resulting from the formation of pyridine ions between the adsorbed pyridine molecules and the two distinct kinds of acidic OH groups (at 7.2 and 5.6 ppm). The signals at 8.7 and 7.5 ppm are probably due to the H/D exchange between deuterons bound to the ring of pyridine and the acidic proton.32 The relatively narrow line width of the signals is probably due to the high mobility of adsorbed pyridine molecules. 1H MAS NMR spectra (Figure 3e and f) of the dehydrated WOx/ZrO2 and pyridine-loaded WOx/ZrO2 resemble those of MoOx/ZrO2. Therefore, our 1H MAS NMR results indicates that the introduction of Mo or W species leads to the formation of two different types of acidic OH groups (Brønsted acid sites), with acid strength much stronger than that of ZrO2. 13C NMR of Adsorbed 2-13C-Acetone. As demonstrated by earlier studies,33,34 the 13C isotropic chemical shift of the carbonyl carbon of 2-13C-acetone can be used as a mark to evaluate the relative acid strength of various solid acids. The formation of a hydrogen bond between the acidic proton and the carbonyl oxygen of adsorbed 2-13C-acetone will cause a downfield shift of the carbonyl carbon. Generally, the stronger the Brønsted acidity, the more downfield of the 13C isotropic chemical shift. Figure 4a shows the 13C NMR spectrum of 2-13C-

10666 J. Phys. Chem. B, Vol. 110, No. 22, 2006 acetone adsorbed on the surface of MoOx/ZrO2 material, and besides signals at 212, 173, 79, and 31 ppm arising from the products of bimolecular and trimolecular reactions (aldol reaction) of 2-13C-acetone, we can observe a signal at 226 ppm due to unreacted 2-13C-acetone adsorbed on the Brønsted acid sites in the 13C CP/MAS NMR spectrum. The large chemical shift indicates that the acid strength of MoOx/ZrO2 material is slightly stronger than that of the bridging OH group (SiOHAl) in zeolite HZSM-5, where adsorbed 2-13C-acetone gives rise to a 13C resonance at 223 ppm, but still weaker than that of 100% H2SO4, in which the isotropic 13C shift of 2-13C-acetone is 245 ppm.35 For acetone-2-13C adsorbed on WOx/ZrO2 material, a similar feature can be observed in the corresponding 13C NMR spectrum (Figure 4b), except that the unreacted 2-13C-acetone gives rise to a chemical shift of 228 ppm, indicating that the WOx/ZrO2 material has a slightly stronger acid strength compared with the MoOx/ZrO2 material. In the 13C NMR spectrum of acetone adsorbed on mesoporous ZrO2 (not shown), we only observed peaks associated with condensation products of acetone (such as diacetone alcohol at 222 and 75 ppm), and no signal of unreacted acetone adsorbed on the acid site could be observed even at a temperature of 200 K. For solid acids, the aldol reaction of acetone can occur on either Lewis or Brønsted acid sites.34,36 Lewis sites seem to be much more active compared with Brønsted sites,36a and all adsorbed acetone molecules are converted into condensation products. Because only Lewis sites are present on the mesoporous ZrO2, as evidenced by 31P NMR of adsorbed TMP (see the following), no unreacted acetone adsorbed on the acid sites could be detectable. Therefore, our 13C NMR results demonstrate that both the mesoporous MoOx/ZrO2 and WOx/ZrO2 materials have an acid strength stronger than that of zeolite HZSM-5. This is inconsistent with IR results, where IR spectroscopy of adsorbed CO as a probe revealed that the Brønsted acid strength of WOx/ ZrO2 was comparable to that of sulfated zirconia but still remained significantly below that of HZSM-5.25c It is, however, generally believed that HZSM-5 is not a superacid, and the acid strength of sulfated zirconia characterized by adsorbed 2-13Cacetone produces a chemical shift at 230 ppm,37 which is obviously larger than that (223 ppm) of HZSM-5, but close to the samples studied here. No matter whether the term “superacid” is proper or not for sulfated zirconia, its activity in certain catalytic reactions (e.g., the low-temperature isomerization of light n-alkenes) is extremely high due to its strong Brønsted acid sites.38 The comparable Brønsted acid strength of the MoOx/ ZrO2 and WOx/ZrO2 to sulfated zirconia probably reflects their similar high acid-catalyzed activity. 31P NMR Spectra of Adsorbed TMP. TMP is an extensively used probe molecule for discriminating Brønsted and Lewis acid sites and measure their relative concentrations in various solid acids, including zeolites.39,40 It is well accepted that the formation of a TMPH+ ion due to the interaction of TMP with Brønsted acid sites of zeolite will give rise to a 31P resonance at about 0 ppm, while TMP molecules bound to Lewis acid sites will result in resonance in the shift range from -32 to -58 ppm, and physisorbed TMP molecules will give rise to 31P NMR signal at about -68 ppm. In the 31P MAS NMR spectrum (Figure 5a) of TMP adsorbed on ZrO2, three signals at -30.3, -42.2, and -50.1 ppm are observed. The former two signals can be ascribed to TMP adsorbed on Lewis acid sites, while the latter is ascribed to physisorbed or weakly bound TMP. However, no signal at ca. 0 ppm due to TMPH+ is observable, indicative of the low acid strength of the hydroxyl groups present

Xu et al.

Figure 5. 31P single-pulse MAS NMR spectra (with proton decoupling) of TMP loaded on mesoporous materials: (a) zirconia, (b) MoOx/ZrO2, and (c) WOx/ZrO2. Asterisks denote spinning sidebands.

on the surface of ZrO2, consistent with our above 1H MAS NMR results. The 31P MAS NMR spectra of MoOx/ZrO2 and WOx/ ZrO2 materials (Figure 5b and c) show similar features, consisting of two major peaks at -3.3 and 57.8 ppm and a weak peak around -48 ppm. The appearance of the -3.3 ppm signal indicates that the introduction of the active components MoOx and WOx generates new acidic hydroxyl groups (Brønsted acid sites) that are able to protonate adsorbed TMP, forming TMPH+. The signal at ca. -48 ppm is probably due to TMP adsorbed on Lewis acid sites. It is noteworthy that the concentrations of Lewis acid sites present on both MoOx/ZrO2 and WOx/ZrO2 materials are considerably reduced compared with that of the parent ZrO2. This is in accordance to the previous results.41,42 The 57.8 ppm signal can be assigned to TMPO adsorbed on Brønsted acid sites, probably due to the oxidation of TMP. As is well-known, the 31P chemical shift of adsorbed TMPO can be used to probe the relative acid strength of various solid acids.43 For TMPO adsorbed on the Brønsted acid sites of the two mesoporous materials, a much smaller 31P chemical shift (57.8 ppm) was observed compared with TMPO-adsorbed HZSM-5 zeolite (86 ppm), indicative of a much weaker acid strength. This result is contrary to that obtained by the 13C NMR experiments of acetone and the following theoretical calculations. When a probe molecule is adsorbed on a Brønsted acid site, the acidity depends not only on the relative strengths of the O-H bond (which can be characterized by proton affinity), but also on the accessibility of the acidic OH group that will be controlled by both pore size and local steric constraint. Trout et al.44 found that four basic probe molecules (acetonitrile, methanol, ammonia, and pyridine) reveal different acid strength orders for the different acid sites inside chabazite zeolite by the theoretical calculation. Our previous investigation of HMCM22 zeolite45 also indicated that the larger the local steric hindrance around the adsorbed TMPO, the smaller the adsorption energy as well as the 31P chemical shift of TMPO, corresponding to a weaker acid strength. Because the molecular size of acetone is relatively small compared with TMPO, the steric constraint imposed by the meoporous surface for the adsorption of acetone is not so pronounced as in the case of TMPO. Therefore, it is likely that the local steric hindrance results in the difference of acid strength order. Because our sample preparation and sample transfer procedure can completely avoid the presence of O2 (no TMPO signal is present in the 31P MAS NMR spectrum of TMP adsorbed on ZrO2), it is doubtless that the formation of TMPO signifies the

Acidity of Mesoporous MoOx/ZrO2 and WOx/ZrO2 redox properties of the MoOx/ZrO2 and WOx/ZrO2 materials. In a previous investigation, Santiesteban et al. also demonstrated that the WOx/ZrO2 catalyst with a high W loading (ca. 16%) would possess a relatively strong redox property and a strong acidity in the isomerization of n-pentane.46 The Acid Sites on MoOx/ZrO2 and WOx/ZrO2. It is generally accepted that WOx/ZrO2 and MoOx/ZrO2 catalysts possess both Lewis and Brønsted acidity, although their detailed structures are still actively debated. Afanasiev et al.47 proposed a structural model for the WOx/ZrO2 catalyst in which hydrated zirconia oxide was coordinated to several tungsten oxo-anions, and the OH groups were solely attached to the Zr atom, generating the Brønsted acid sites. In other investigations,48 the generation of Brønsted sites was ascribed to H2 dissociation on Lewis sites in the form of W6+ centers and the H+δ is stabilized by WOx clusters through electron transfer and charge delocalization. Recently, a new model has been proposed for the acid sites present on WOx/ZrO2 catalyst,49 in which it was assumed that the unsaturated W center was able to complex a Lewis base, namely a Lewis site, and the Brønsted acid site originated from that Lewis site bonding with water. Raman spectroscopic investigation24 indicated that Mo anchored on the surface of ZrO2 by interacting with its hydroxyl groups, leading to the presence of Mo-O-Mo and ModO bonds in octahedral polymolybdates and an OdModO bond in isolated MoO4 tetrahedra after high-temperature treatment. Similar results were found for WOx species supported on ZrO2.25c Zhao et al.50 deduced that Mo-O-Zr species was responsible for the strong acidity of MoO3/ZrO2 in the cumen cracking that was only catalyzed by solid acid with very strong strength. According to our NMR experimental results, although there are Zr-connecting hydroxyl groups present on the surface of ZrO2, their acid strengths are not strong enough to protonate either TMP or pyridine, and thus no signal corresponding to either TMPH+ (at ca. 0 ppm in the 31P MAS spectrum) or pyridine ion (at 16-19 ppm in the 1H MAS spectrum) was observable. Two distinct types of Lewis acid sites can be detected on the surface of ZrO2 by 31P MAS NMR of adsorbed TMP. After the introduction of Mo or W species, two kinds of new Brønsted acid sites are generated on both the MoOx/ZrO2 and the WOx/ZrO2 materials (as revealed by 1H MAS NMR of adsorbed pyridine-d5). The acid strengths of the two newly formed Brønsted acid sites are considerably increased compared with that of parent ZrO2, which is evidenced by the fact that the acid sites are able to protonate both TMP and pyridine molecules. In addition, 13C MAS NMR of adsorbed 2-13Cacetone suggests that their acid strengths are stronger than that of zeolite HZSM-5, similar to that of sulfated zirconia, but still weaker than that of 100% H2SO4. The introduction of Mo or W also causes the appearance of large amounts of a nonacidic hydroxyl group. It is noteworthy that the Lewis acid sites almost disappear after the introduction of Mo or W species. On the basis of our experimental observation as well as previous results reported by other researchers, a possible mechanism for the formation of acid sites is illustrated in Scheme 1. First, a large amount of hydroxyl groups form in the zirconium hydroxide gel, and then the Mo or W species interact with parts of the surface ZrOH groups during the coprecipitation, and finally MoOx or WOx species tightly anchor on the surface of zirconia by eliminating water molecules or coordinating to the unsaturated Zr4+ sites through their hydroxyl groups during the sample calcination. The coordination of Mo-OH or W-OH to the unsaturated Zr4+ sites leads to the appearance of bridging MoOH-Zr (or W-OH-Zr) hydroxyl groups that act as Brønsted

J. Phys. Chem. B, Vol. 110, No. 22, 2006 10667 SCHEME 1

acid sites and a remarkable decrease in the concentration of Lewis acid sites present on the surface of ZrO2 (consistent with 31P NMR observation). For the pure zirconia, after calcination of the zirconium hydroxide gel and, subsequently, dehydration, some weak or nonacidic ZrOH groups and some coordinatively unsaturated Zr4+ sites (as Lewis sites) are generated. It can be expected that the bridging Mo-OH-Zr (or W-OH-Zr) groups are responsible for the stronger Brønsted acidity of the mesoporous MoOx/ZrO2 and WOx/ZrO2 materials. This is likely created by electron-deficient regions resulting from the anionic dopants (MoO42- or WO42-) that attract electrons and increase the polarity of the acidic O-δ-H+δ, as in the case of the bridging Al-OH-Si groups in microprous zeolites. As shown in Scheme 1, two types of Brønsted acid centers with different acid strength may exist on the surface of the mesoporous MoOx/ZrO2 and WOx/ZrO2 materials, namely monomer or oligomer Mo (or W) species, which were unambiguously distinguishable in our 1H MAS NMR experiments. Computational Calculations We used the quantum chemical calculation method to obtain detailed information about our proposed structures and nature of the Brønsted acid sites formed on the mesoporous materials. The formation of a bridging hydroxyl group is assumed by the coordination of a Mo-OH or W-OH group to the coordinatively unsaturated Zr4+ sites (Lewis acid sites). In the optimized structure of a weak acidic proton in ZrO2 (Figure 6a), the acidic H-O(Zr) bond length is 0.965 Å. For MoOx/ZrO2 material, the corresponding bond length is elongated to 1.025 and 0.986 Å in the optimized structures of models II and III (Figure 6b and c), respectively, indicating an intrinsic acid strength order of model II > model III > ZrO2. It should be noted that there exists a hydrogen bond between the acidic proton and the OH group of the Lewis acid site. Obviously, the hydrogen-bonding interaction (1.566 Å) in model II is much stronger than that (2.350 Å) in model III. For WOx/ZrO2 material, the optimized models II (Figure 6d) and III (Figure 6e) have nearly the same acidic H-O bond length (0.992 Å) and slightly different hydrogen bond lengths, suggesting a similar intrinsic acid strength that is stronger than ZrO2. The calculated 1H chemical shifts of the five optimized structures are listed in Table 2. The differences between the results of our calculations and the experimental values range from 0.4 to 1.9 ppm for the five optimized structures. Because 1H NMR spectra usually have a relatively narrow range of chemical shift, we believe that the calculated results are acceptable and well in agreement with experimental data. Generally, the larger the 1H chemical shift, the stronger the acid strength. Our theoretical predictions also


Xu et al.

Figure 6. TZ2P-optimized structures of models I, II, and III for Brønsted acid sites present on the mesoporous materials: (a) zirconia, (b) and (c) MoOx/ZrO2, and (d) and (e) WOx/ZrO2. Selected bond lengths in angstroms are indicated.

TABLE 2: Experimental and Calculated 1H Chemical Shifts of Pyridine-d5 and 13C Chemical Shifts of 2-13C-acetone Adsorbed on Different Acid Site Models of Various Mesoporous Materials (Corresponding 1H Chemical Shifts of the Original Models Are Also Included) 1

original model model zirconia MoOx/ZrO2 WOx/ZrO2

I II III II III

13

H chemical shift

C chemical shift

pyridine adsorptioncomplex

acetone adsorption complex

calculated (in ppm)

experimental (in ppm)

calculated (in ppm)

experimental (in ppm)

calculated (in ppm)

2.5 8.6 6.0 8.5 7.6

4.4 7.2 5.6 7.5 5.8

11.8 20.7 18.8 20.5 18.6

8.7 19.3 16.6 19.5 16.3

215.1 222.4 228.8 221.9 229.1

suggest an intrinsic acid strength order of model II > model III > ZrO2 for both MoOx/ZrO2 and WOx/ZrO2 materials. We optimized the geometries of pyridine adsorbed on models I, II, and III, and the results are shown in Figure 7. In the complex structure of model I (Figure 7a), a hydrogen bond with a H-N bond length of 1.573 Å is formed between the acidic proton and the nitrogen atom of adsorbed pyridine and the H-O bond length increases from 0.965 Å (bare) to 1.019 Å. In the complex structure of pyridine adsorbed on the model II of MoOx/ ZrO2 (Figure 7b), the H-O bond length increases to 1.353 Å

experimental (in ppm) 226 228

and the H-N bond length reduces to 1.139 Å, probably indicative of the formation of a protonated pyridine (pyridine ion). In the complex structure of pyridine adsorbed on the model III of MoOx/ZrO2 (Figure 7c), the H-O bond length is further elongated to 1.503 Å and the H-N bond length is further reduced to 1.088 Å. A similar variation trend of the H-O and H-N bond length is observable after the adsorption of pyridine onto models II and III of WOx/ZrO2 material. The variation trend of both the H-O and H-N bond lengths indicates an acid strength order (the extent of proton transfer) of model III >



Figure 7. TZ2P-optimized structures of pyridine adsorbed on models I, II, and III proposed for Brønsted acid sites present on the mesoporous materials: (a) zirconia, (b) and (c) MoOx/ZrO2, and (d) and (e) WOx/ZrO2. Selected bond lengths in angstroms are indicated.

model II> ZrO2 for the MoOx/ZrO2 and WOx/ZrO2 materials, which is different from the predicted intrinsic acid strength order of these materials (see above). We also calculated the 1H isotropic chemical shift of the acidic proton in the five adsorption complexes, and the corresponding results are listed in Table 2 as well. The calculated 1H isotropic chemical shift (11.8 ppm) of pyridine adsorbed on ZrO2 material is relatively larger than the experiment value (8.7 ppm), while the differences between the results of our calculations and the experimental values range from 1 to 2.3 ppm for pyridine adsorbed on the models II and III of the MoOx/ZrO2 and WOx/ZrO2 materials. Obviously, the predicted chemical shifts of pyridine adsorption complex can resolve the two kinds of Brønsted acid sites present on the mesoporous materials with different acid strengths. It is noteworthy that, as probed by adsorbed pyridine, the model III displays a much stronger acid strength (much longer O-H bond and shorter H-N bond) compared with the model II; however, the adsorption complex on model III has a relatively smaller (a more high-field shift) 1H chemical shift. This probably results from the much pronounced electron-donor effect from the pyridine ring due to the closer distance between the acidic proton and the N atom

of pyridine for the adsorption complex on model III. As a result, a relatively larger shielding effect and thus a more high-field 1H chemical shift can be expected. This is quite different from the previous assumption that a larger downfield 1H chemical shift of the pyridine adsorption complex usually corresponds to a much stronger acid strength.12c,d The optimized geometries of 2-13C-acetone adsorbed on the acidic proton of the three models are shown in Figure 8. Generally, the stronger the Brønsted acidity, the stronger the hydrogen bond between the carbonyl oxygen and acidic proton, resulting in a longer CdO bond length. In the complex structure of model I (Figure 8a), the acidic O-H bond distance increases from 0.965 to 0.985 Å, the distance between the acidic proton and the carbonyl oxygen is 1.676 Å, and the CdO bond length increases from 1.211 to 1.216 Å after the adsorption of acetone. For acetone adsorbed on the model II of MoOx/ZrO2 material (Figure 8b), obvious change occurs in the complex structure. For example, the O-H bond length increases further to 1.032 Å, the distance between the carbonyl oxygen and the acidic proton decreases to 1.524 Å, and the CdO bond length increases further to 1.224 Å. For acetone adsorbed on model III of the MoOx/ZrO2 material (Figure 8c), the O-H bond length increases


Xu et al.

Figure 8. TZ2P-optimized structures of acetone adsorbed on models I, II, and III proposed for Brønsted acid sites present on the mesoporous materials: (a) zirconia, (b) and (c) MoOx/ZrO2, and (d) and (e) WOx/ZrO2. Selected bond lengths in angstroms are indicated.

further to 1.111 Å, the distance between the carbonyl oxygen and the acidic proton decreases further to 1.333 Å, and the Cd O bond length increases further to 1.233 Å. A similar trend can be found for acetone adsorbed on models II and III of the WOx/ZrO2 material (Figure 8d and e). From the elongation extent of the CdO bond length of adsorbed acetone as well as the O-H bond, acid strength order of the three models can be predicted as follows: model I < model II < model III. The 13C isotropic chemical shifts of the carbonyl carbon for acetone adsorbed on the three proposed models were calculated for both mesoporous MoOx/ZrO2 and WOx/ZrO2 materials, and the corresponding results are listed in Table 2. For 2-13C-acetone adsorbed on model I, the calculated isotropic chemical shift is 215.1 ppm, indicative of a much weaker acid strength of mesoporous ZrO2 compared with zeolite HZSM-5 (having a calculated isotropic chemical shift of 223.9 ppm).15c Larger isotropic chemical shifts (∼229 ppm) of the carbonyl carbon were always predicted for acetone adsorbed on model III compared with acetone adsorbed on model II (∼222 ppm), indicating that the acid strength of model II is close to that of zeolite HZSM-5, while the acid strength of model III is similar

to that of sulfated zirconia (230 ppm). Because acetone is insensitive to different acid sites with close acid strengths, an average chemical shift was observed for acetone adsorbed on models II and III. Therefore, the calculated 13C chemical shift data suggest an acid strength order: model I < model II ≈ HZSM-5 < model III ≈ sulfated zirconia. Conclusions The mesoporous ZrO2, MoOx/ZrO2, and WOx/ZrO2 materials were prepared and their solid acidity was thoroughly studied by solid-state NMR techniques and DFT calculations. Two distinct types of Brønsted acid sites with acid strength stronger than zeolite HZSM-5, comparable to sulfated zirconia but still weaker than 100% H2SO4, were found to be present on the mesporous MoOx/ZrO2 and WOx/ZrO2 materials. With the help of theoretical calculations, the detailed structures of Brønsted sites formed on the surface of the mesoporous catalyst were revealed, and the predicted acid strengths of these sites were in good agreement with experimental observations. Besides the weak acidic Zr-OH groups, Lewis acid sites (coordinatively

Acidity of Mesoporous MoOx/ZrO2 and WOx/ZrO2 unsaturated Zr4+ sites) are present on the surface of mesoporous ZrO2. After the introduction of Mo or W species, the coordination of Mo-OH or W-OH to the unsaturated Zr4+ sites leads to the appearance of bridging Mo-OH-Zr (or W-OH-Zr) hydroxyl groups that act as Brønsted acid sites and a remarkable decrease in the concentration of Lewis acid sites present on the surface of ZrO2. The bridging Mo-OH-Zr or W-OH-Zr hydroxyl groups in the form of monomer and oligomer states are responsible for the strong Brønsted acidity of the MoOx/ ZrO2 and WOx/ZrO2 materials. Acknowledgment. We are very grateful for the support of the National Natural Science Foundation of China (20425311, 20573133, and 10234070) and State Key Fundamental Research Program (2002CB713806) of China. References and Notes (1) Kijenski, J.; Baiker, A. Catal. Today 1989, 5, 1. (2) Hino, M.; Arata, K. J. Chem. Soc., Chem. Commun. 1979, 101, 6439. (3) Corma, A.; Martıńez, A.; Martıńez, C. Appl. Catal., A 1996, 144, 249. (4) Arata, K. AdV. Catal. 1990, 37, 165. (5) Tanabe, K. Mater. Chem. Phys. 1985, 13, 347. (6) (a) Barton, D. G.; Soled, S. L.; Meitzner, G.. D.; Fuentes, G.. A.; Iglesia, E. J. Catal. 1999, 181, 57. (b) Iglesia, E.; Barton, D. G.; Soled, S. L.; Miseo, S.; Baumgartner, J. E.; Gates, W. E.; Fuentes, G. A.; Meitzner, G. D. Stud. Surf. Sci. Catal. 1996, 101, 533. (c) Khodakov, A.; Olthof, B.; Bell, A. T.; Iglesia, E. J. Catal. 1999, 181, 205. (7) Weitkamp, J.; Traa, Y. Catal. Today 1999, 49, 193. (8) Pacheco, G.; Zhao, E.; Gracia, A.; Sklyarov, A.; Fripiat, J. J. Chem. Commun. 1997, 491. (9) Knowles, J. A.; Hudosn, M. J. J. Chem. Soc., Chem. Commun. 1995, 20, 2083. (10) Melezhyk, O. V.; Prudius, S. V.; Brei, V. V. Microporous Mesoporous Mater. 2001, 49, 39. (11) Brei, V. V.; Prudius, S. V.; Melezhyk, O. V. Appl. Catal., A 2003, 239, 11. (12) (a) Klinowski, J. Chem. ReV. 1991, 91, 1459. (b) Farneth, W. E.; Gorte, R. J. Chem. ReV. 1995, 95, 615. (c) Hunger, M. Catal. ReV.sSci. Eng. 1997, 39, 345. (d) Hunger, M. Solid State Nucl. Magn. Reson. 1996, 6, 1. (13) Xu, M.; Arnold, A.; Buchholz, A.; Wang, W.; Hunger, M. J. Phys. Chem. B 2002, 106, 12140. (14) Fripiat, J. G.; Galet, P.; Delhalle, J.; Andre, J. M.; Nagy, J. B.; Derouane, E. G. J. Phys. Chem. 1985, 89, 1932. (15) (a) Ehresmann, J. O.; Wang, W.; Herreros, B.; Luigi, D. P.; Venkatraman, T. N.; Song, W.; Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 2002, 124, 10868. (b) Haw, J. F.; Xu, T.; Nicholas, J. B.; Gorguen, P. W. Nature 1997, 389, 832. (c) Xu, T.; Kob, N.; Drago, R. S.; Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 1997, 119, 12231. (16) (a) Zheng, A. M.; Chen, L.; Yang, J.; Yue, Y.; Ye, C. H.; Lu, X.; Deng, F. Chem. Commun. 2005, 2474. (b) Zheng, A. M.; Yang, M. Y.; Yue, Y.; Ye, C. H.; Deng, F. Chem. Phys. Lett. 2004, 399, 172. (17) (a) Li, N.; Jie, Q.; Zhu, S. M.; Wang, R. D. Ceram. Int. 2005, 31, 641. (b) Zhang, J.; Palaniappan, A.; Su, X. D.; Tay, F. E. H. Appl. Surf. Sci. 2005, 245, 304. (18) Cory, D. G.; Ritchey, W. M. J. Magn. Reson. 1988, 80, 128. (19) ADF 2004, SCM, Theoretical Chemistry; Vrije Universiteit: Amsterdam, 2004; (swww.scm.com). (20) (a) Vosko, S. H.; Wilk, L.; Nusair, M.; Can. J. Phys. 1980, 58, 1200. (b) Vosko, S. H.; Wilk, L. J. Phys. C: Solid State Phys. 1982, 15, 2139. (21) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671.

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ZrO2

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