Combined Solid-State NMR and Theoretical Calculation Studies of

We shall proceed to discuss the case of two and three TMPO adsorbed on different acid ..... Gayraud , P. Y.; Stewart , I. H.; Derouane-Abd Hamid , S. ...
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Combined Solid-State NMR and Theoretical Calculation Studies of Brønsted Acid Properties in Anhydrous 12-Molybdophosphoric Acid Ningdong Feng,† Anmin Zheng,*,† Shing-Jong Huang,‡ Hailu Zhang,§ Ningya Yu,‡ Chih-Yi Yang,‡ Shang-Bin Liu,*,‡ and Feng Deng*,† State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, China, Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 10617, Taiwan, Department of Chemistry, National Taiwan Normal UniVersity, Taipei 11677, Taiwan, and Suzhou Institute of Nano-tech and Nano-bionics, the Chinese Academy of Sciences, Suzhou 215125, China ReceiVed: June 20, 2010; ReVised Manuscript ReceiVed: July 29, 2010

The strength and distribution of Brønsted acidic protons in anhydrous phosphomolybdic acid (H3PMo12O40, HPMo) have been studied by solid-state magic-angle-spinning (MAS) NMR, using trimethylphosphine oxide (TMPO) as the probe molecule in conjunction with density functional theory (DFT) calculations. Brønsted acid sties with strengths exceeding the threshold of superacidity (Zheng, A. et al. J. Phys. Chem. B 2008, 112, 4496) were observed for HPMo. In addition, the locations and adsorption structures of Brønsted protons on various oxygen sites in HPMo were also identified. The preferred location of the acidic proton was found to follow the trend: corner-sharing (Ob) > edge-sharing (Oc) . terminal (Od) sites. Moreover, a tendency of hybridization among Brønsted protons residing at Ob and Oc sites of HPMo was inferred by experimental as well as theoretical 31P chemical shifts of the adsorbed TMPO. 1. Introduction Heteropolyacids (HPAs), especially those of the Keggin series, have been extensively employed as solid acid catalysts1-5 and photocatalysts4,5 in various homogeneous solutions, liquid-solid, and gas-solid heterogeneous reactions due to their high thermal stability, acidity, and oxidizing ability compared to other conventional solid acids, such as zeolites, metal oxides, and mixed oxides. Owing to their strong Brønsted acidity, HPAs generally exhibit superior catalytic activities during catalytic reactions, for example, conversion of methanol to hydrocarbons, isomerization of aromatics, cracking of n-alkanes, etc.4,6-10 Among the HPAs family, the phosphomolybdic acid (H3PMo12O40, hereafter denoted as HPMo), which possesses a Keggin-type primary structure, has demonstrated itself for potential applications in homogeneous oxidation reactions, for example, ketone to aldehyde and cyclohexene to adipic acid.4 The primary structure of HPMo consists of a Keggin unit (KU; Figure 1), in which the central P atom with tetrahedral coordination (PO43-) is surrounded by 12 metal-oxygen octahedra (MoO66-). As such, each KU of HPMo contains three negative charges that are neutralized by three protons in the form of acidic hydroxyl groups that are situated at the exterior of the structure. Four types of oxygen atoms per KU can be envisaged in Keggin-type HPAs, including 4 central oxygen (Oa) sites, 12 corner-sharing oxygens (Ob sites) which bridge * To whom correspondence should be addressed. A.Z.: e-mail [email protected] and fax +86-27-87199291. S.-B.L.: e-mail sbliu@ sinica.edu.tw and fax +886-2-23620200. F.D.: e-mail [email protected] and fax +86-27-87199291. † State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences. ‡ Institute of Atomic and Molecular Sciences, Academia Sinica, and Department of Chemistry, National Taiwan Normal University. § Suzhou Institute of Nano-tech and Nano-bionics, the Chinese Academy of Sciences.

two molybdenum atoms, 12 edge-sharing oxygens (Oc sites) which not only bridge two molybdenum atoms but also share a central oxygen atom, and 12 terminal oxygens (Od sites) which bound to a single Mo atom.1,2,11 It is well-known that the Brønsted acid properties of the HPMo strongly depend on the electronic charge of the oxygen atoms to which the protons are attached. Thus, knowledge regarding the locations of the acidic protons in anhydrous HPMo is essential in understanding the nature of the catalytic active sites and hence its catalytic performance during acid-catalyzed reactions. Although the atom positions in crystalline solids or amorphous materials can normally be determinated by X-ray diffraction,12 this may not apply to hydrogen atom due to its small scattering cross section for X-ray. In this context, solid-state NMR is a powerful tool for characterizing the nature of acid sites on solid catalysts, such as HPAs and zeolites.13-19 For example, 1H magic-angle-spinning (MAS) NMR has been widely used to directly probe the structure of various hydroxyl groups, including the bridging OH groups (i.e., Brønsted acid sites). Alternatively, the acid properties of solid acid catalysts may also be probed through suitable probe molecules, especially those that contain a nucleus with desirable NMR characteristics (e.g., sensitive to the local environments) and those that tend to form adsorption complexes with protons at the Brønsted acid sites. For example, 13C MAS NMR of the adsorbed 2-13Cacetone13,14 and 31P NMR of adsorbed trimethylphosphine (TMP)15,16 or trialkylphosphine oxides (R3PO)17-19 have been used to determine acid strengths of various solid catalysts, whereas the locations of acid protons, interaction parameters between the probe molecule and the acid centers, as well as the reaction transition state structure and activation energies over various solid catalysts may further be explored by theoretical calculations.20-24 For example, on the basis of the relative energies among protons located at different oxygen sites predicted by density functional theory (DFT) calculations,

10.1021/jp105683y  2010 American Chemical Society Published on Web 08/20/2010

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Figure 1. Ball-and-stick and polyhedral representations of anhydrous HPMo.

possible distributions of acid protons on a 12-tungstophosphoric acid (H3PW12O40; HPW) system have been identified and compared with the experimental chemical shift (CS) parameters obtained from 1H and 31P MAS NMR of the adsorbed trimethylphosphine oxide (TMPO) probe molecule.25 Such combined DFT calculations and NMR experimental technique has also been used to determine the acidic structure and strength of other solid acid catalysts, such as BF3/Al2O3,26 WO3/ZrO2 and MoO2/ ZrO2,27 and MCM-22 zeolite.28 In continuation of our previous studies on Keggin-type HPAs,5,25 the acid locations and strengths of protonic sites in anhydrous HPMo have been studied by DFT calculations in conjunction with 1H and 31P MAS NMR experiments. In particular, the relative energies of acidic protons located at different oxygen sites (Ob, Oc, and Od) of the PMo12O403- (PMo) polyanions surfaces have been derived from DFT calculations by which the preferred oxygen sites for protons may be inferred and compared with the chemical shift results obtained from 31P MAS NMR of adsorbed TMPO.

and 4.4 µs was used for 1H and 31P resonance, respectively. For the single-pulse 31P MAS NMR experiment, an excitation pulse equivalent to ca. π/4 and a recycle delay of 10 s were used during spectrum acquisition. Typically, a contact time of 0.2 ms and a recycle delay of 2 s were used for the 31P{1H} Lee-Goldberg cross-polarization heteronuclear correlation (LGCP HECTOR) experiments.30 The chemical shifts (CSs) for the 1 H and 31P resonance were referred to tetramethylsilane (Si(CH3)4; TMS) and 85% H3(PO4) solution, respectively. All experiments were carried out with a MAS frequency of 12 kHz. 2.3. Computational Method. Geometry optimizations were carried out based on DFT employing the Dmol3 program in the Material studio 4.4 Modeling package31 at the generalized gradient approximation (GGA) level with the PW91 exchange and correlation functional.32 A double numerical basis set with polarization functions (DNP) was used in all calculations. This basis set is comparable to 6-31G**.33 The convergence threshold tolerance was set to the default medium level: total energy ) 2 × 10-4 Ha; maximum force ) 4 × 10-3 Ha/Å.

2. Experimental Section 2.1. Sample Preparation. H3PMo12O40 · xH2O (Sinopharm Chemical Reagent Co., Ltd.) was purified by extraction with diethyl ether, then recrystallized at room temperature. The purity of the sample was checked by 31P MAS NMR (see Figure S1 in the Supporting Information). Prior to the adsorption of the probe molecule, each sample was subjected to dehydration treatment at 473 K under vacuum ( Oc . Od. In this context, Od sites, which have an energy difference as much as 123 kJ/mol relative to the Ob sites, are thermodynamically unstable for acidic protons in HPMo. In other words, the three Brønsted acid protons in bare HPMo are most likely to locate at the bridging oxygen (Ob and Oc) sites, provided that the system is thermodynamically controlled.

Now, since Ob and Oc are the preferable sites for acidic protons in bare HPMo, one could in principle exclude most possibilities of hybridization distributions (particularly those associated with the Od sites) but only to consider the following cases: (i) three H+ at Ob (i.e., O3b), (ii) three H+ at Oc (i.e., O3c), (iii) three H+ at Od (i.e., O3d), (iv) two H+ at Oc and one at Ob (i.e., O1b2c), (v) two H+ at Ob and one at Ob (i.e., O2b1c), and (vi) one H+ each at Ob, Oc, and Od (i.e., Obcd). Note that, for convenience for discussion, the first three cases have been categorized as case A, whereas the latter three as case B in Table 2. For the latter three cases, the trend for hybridization stability may also be evaluated based on the relative substitution energy in Table 2, which goes as follows: O2b1c > O1b2c > Obcd. Among them, the energy difference between O2b1c and O1b2c was only ca. 13.4 kJ/mol, representing the two most probably protonic sites. The aforediscussed results are in good agreement with those obtained from 1H{31P}/31P{1H} REDOR MAS NMR and DFT calculations.40 It is noted that, although HPMo has a similar Keggin structure with its HPW analogue, their proton distributions and acidic strengths (vide infra) are rather different. While the acid protons in HPW have been shown to locate preferably at the terminal oxygen (Od) sites,25 the results obtained from this study reveal that the protons are most likely located at hybridized Ob and Oc sites. Such a difference between protonic sites in HPW and HPMo indicates that the electronic configurations of W (5d46s2) and Mo (4d55s1) indeed have considerable effects on the distribution of electrostatic potentials, which in turn influence the acid features on the surfaces of the HPAs. The proton affinity (PA) value, which represents the energy required to remove an acidic proton (i.e., energy required for H3PMo12O40 f H2PMo12O40- + H+), may be used as a criterion to evaluate the intrinsic acidic strength of solid acids.19 The smaller the PA value, the easier the Brønsted acidic proton can be deprotonated, and thus the stronger the strength of the acid site. On the basis of results obtained from DFT calculations, the PA values estimated for deprotonating the first bridging proton from HPMo are 1100.3 and 1101.4 kJ/mol for Brønsted acidic protons located at O2b1c and O1b2c models, which is notably greater than that in HPW superacids (PA ≈ 1079 kJ/mol),41 suggesting that HPMo has a weaker acid strength than HPW. For comparison, calculated PA values in the range from 1171 to 1200 kJ/mol were found for zeolites Y, CHA, MOR, and ZSM-5 (in order of increasing acid strengths),42,43 indicating that HPMo indeed has a much stronger acid strength than typical zeolites. 3.3. Scenario I: Adsorption of One TMPO per HPMo Keggin Unit. Having addressed the preferred locations of Brønsted acid protons at various oxygen sites of HPMo, a new question arises regarding the adsorption of TMPO guest molecules on HPMo: Does it make a difference when TMPO molecules are introduced onto the solid acid sequentially or simultaneously? To address this question, let us first examine the first case (scenario I): adsorption of one TMPO molecules on one of the Brønsted acidic protons located either at Ob or Oc site of the HPMo. Again, since the preferable sites for acidic protons in bare HPMo follow the trend Ob > Oc . Od, discussions of TMPO associates with the most unfavorable Od sites are excluded herein. The PW91/DNP-optimized adsorption structures of all possible cases for scenario I are shown in Figure 5 and their corresponding geometric parameters and 31P CSs are listed in Table 3. Note that four possible scenarios may be

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Figure 5. Optimized geometries for one TMPO adsorbed on one of the Brønsted acid protons (scenario I: see Table 3) in HPMo at (a) Ob and (b) Oc sites in the O1b2c model, and (c) Ob and (d) Oc sites in the O2b1c model. Selected interatomic distances (in Å) are depicted.

TABLE 3: Geometric Parameters (Bond Lengths in Å) and 31 P Chemical Shifts (in ppm) for One TMPO Adsorbed on Acid Sites of HPMo (Scenario I) 1b2c PdOTMPO OTMPO · · · H H · · · O(PMo) σabsolute calcd 31P CS exptl 31P CS

2b1c

Ob

Oc

Ob

Oc

1.566 1.065 1.418 220.9 84.7 86.2

1.570 1.043 1.503 218.5 87.1 89.9

1.568 1.050 1.475 220.1 85.5 87.5

1.567 1.064 1.428 220.0 85.6 87.5

envisaged, namely the adsoption of one, two, or three TMPO molecules on Brønsted acidic protons located at Ob, Oc sites

in two hybridized models, O1b2c and O2b1c (i.e., excluding those associated with the less favorable Od sites). In the case of scenario I, the most obvious change after the adsorption of one TMPO per HPMo KU is the notable increase in the OH bond length (i.e., H · · · O(PMo); typically 1.418-1.503 Å) compared to that of the bare HPMo (ca. 0.975 Å), indicating the occurrence of proton transfer from the host (HPMo) to the guest (TMPO), forming TMPOH+ cationic complex as a consequence. Moreover, by comparing the PdO bond length of TMPO (denoted as PdOTMPO in Table 3) when adsorbed on respective protons at Ob (1.566 Å) and Oc (1.570 Å) sites of the HPMo represented by the O1b2c model, it is indicative that the acid proton at the Oc site has a slightly stronger acidic

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Feng et al. TABLE 4: Geometric Parameters (Bond Lengths in Å) and 31 P Chemical Shifts (in ppm) for Two TMPO Adsorbed on Acid Sites of HPMo (Scenario II) 1b2c PdOTMPO OTMPO · · · H H · · · O(PMo) σabsolute calcd 31P CS exptl 31P CS

Figure 6. Optimized geometries for two TMPO adsorbed on different Brønsted protons of HPMo associated with Ob and Oc sites (scenario II: see Table 4) in the (a) O1b2c and (b) O2b1c models. Selected interatomic distances (in Å) are depicted.

strength than at the Ob site, whereas the acidic strengths of the two proton sites in the O2b1c model are nearly the same due to their close resemblance in PdOTMPO bond lengths. On the basis of the optimized structures above, it is indicative that the calculated 31P CSs for the O1b2c model at 84.7 and 87.1 ppm should be due to one TMPO molecule adsorbed on acid protons at Ob and Oc sites, respectively, likewise, for the O2b1c model at 85.5 and 85.6 ppm. Comparing with the experimental 31 P CSs, it may be concluded that the 31P signals observed at 89.9, 87.5, and 86.2 ppm are responsible for one TMPO bounded to a Brønsted acid proton, as shown in the Figure 5. Earlier, we have concluded that the 31P CS of the adsorbed TMPO should increase linearly with the increasing acidic strengths.19 In scenario I, the adsorption of one TMPO on a Brønsted acid sites in HPMo should result in the formation of monoprotonated TMPOH+ complex, giving rise to 31P CSs beyond the threshold of superacidity (86 ppm).19 Indeed, these CS values are notably larger than typical microporous zeolites and mesoporous molecular sieves, such as HY (65 ppm),17 H-Beta (78 ppm),16 Al-SBA-15 (66 ppm),44 and Al-MCM-41 (69 ppm).19 3.4. Scenarios II-IV: Adsorption of Two and Three TMPO per HPMo Keggin Unit. Since HPMo is a tribasic acid, thus, upon increasing the average TMPO loading to g2 TMPO/ KU, it is anticipated that the possibility in finding more than one TMPO adsorbed on Brønsted acid protons at different oxygen sites should prevail. We shall proceed to discuss the case of two and three TMPO adsorbed on different acid sites in HPMo; they are categorized as scenario II and III, respectively. The PW91/DNP-optimized adsorption structures of two TMPO interacted with Brønsted acidic protons located at Ob and Oc sites of HPMo in the O1b2c and O2b1c models (scenario II) are shown in Figure 6 and the corresponding geometric parameters and 31P CSs are depicted in Table 4. By comparing

2b1c

Ob

Oc

Ob

Oc

1.558 1.110 1.345 226.3 79.3 81.2

1.566 1.056 1.462 221.3 84.3 83.7

1.562 1.098 1.373 223.2 82.4 81.2

1.564 1.076 1.438 224.2 81.4 81.2

with the structures described in scenario I, a notable decrease in the extent of protonated TMPOH+ was observed after the adsorption of the second TMPO molecule. Taking the Ob site in model O1b2c as an example, the PdOTMPO bond length decreased from 1.566 Å of one TMPO adsorption in scenario I (Table 3) to 1.558 Å, meanwhile, the H · · · O(PMo) distance was also decreased from 1.418 to 1.345 Å, indicating that the acidic protons are closer to the base oxygen atom of the PMo. Likewise, the same trends were also observed for the rest of the cases in scenario II. In this case, the calculated 31P CSs were found to be 79.3 (Ob) and 84.3 (Oc) ppm in the O1b2c model, and 82.4 (Ob) and 81.4 (Oc) ppm in the O2b1c model, which are in close proximity with the experimental data. Thus, we attribute the 31P NMR resonances observed experimentally with CSs of 83.7 and 81.2 ppm (region I) to two TMPO adsorbed on Brønsted acid protons associated with Ob (or Oc) in O1b2c (or O2b1c) models as shown in Figure 6. Next, let us consider scenario III, in which three TMPO are adsorbed on different Brønsted acid sites. Again, the corresponding PW91/DNP-optimized adsorption structures obtained from (Ob, Oc, Oc) sites in the O1b2c model and (Ob, Ob, Oc) sites in the O2b1c model are shown in Figure 7 and their corresponding geometric parameters and 31P CSs are summarized in Table 5. Taking the TMPO adsorbed on the O2b1c model as an example, in this case, two of the TMPO molecules were protonated by the Brønsted acidities located at Ob and Oc sites to form the TMPOH+ complex, while the remaining TMPO interacts with a Brønsted acid proton located at an additional Ob site. By analyzing the theoretical 31P CSs in scenario III, the resonances observed experimentally at CSs of 57 and 77 ppm may further be assigned, as shown in Table 5. Note that the resonance predicted at 49.3 ppm for the Ob site in the O2b1c model was not found in the experimental 31P NMR spectra of the TMPO/ HPMo, suggesting that such a three TMPO adsorbed per KU in HPMo case does not exist in perfectly thermally treated (Tb g 473 K) systems. However, such 3TMPO/KU adsorption structures are likely to present in systems with inadequate thermal pretreatment, as can be seen in the spectra treated at Tb ) 393 K in Figure 3. Finally, let us discuss another scenario, in which two TMPO molecules are simultaneously adsorbed on one Brønsted acid proton in HPMo (scenario IV). Since bulk TMPO is crystalline (mp at ca. 140 °C) at ambient temperature and the HPMo adsorbent is a nonporous solid typically with surface area less than 10 m2/g, thus, it is anticipated that the TMPO molecules will be difficult to diffuse into the bulk HPMo to reach a homogeneous adsorption in absence of a proper sample pretreatment at desirable temperatures (say, with Tb g 423 K). In this case, an adsorption complex with more than one TMPO on one acidic site, such as (TMPO)2H+/HPMo, is likely to be formed. Similar observations have also been made for an analogous system such as (CD3CN)2H+ adducts in anhydrous

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Figure 8. Optimized geometry for two TMPO adsorbed on the same Brønsted acid proton associated with the Ob site in the O1b2c model (scenario IV: see Table 6). Selected interatomic distances (in Å) are depicted.

TABLE 6: Geometric Parameters (Bond Lengths in Å) and 31 P Chemical Shifts (in ppm) for Two TMPO Adsorbed on the Same the Same Brønsted Acid Proton Associated with the Ob Site of HPMo (O1b2c Case; Scenario IV) Figure 7. Optimized geometries for three TMPO adsorbed on different Brønsted acid protons (scenario III: see Table 5) in HPMo at (a) Ob, Oc, and Oc sites in the O1b2c model and (b) Ob, Ob, and Oc sites in the O2b1c model. Selected interatomic distances (in Å) are depicted.

TABLE 5: Geometric Parameters (Bond Lengths in Å) and 31 P Chemical Shifts (in ppm) for Three TMPO Adsorbed on Acid Sites of HPMo (Scenario III) 1b2c PdOTMPO OTMPO · · · H H · · · O(PMo) σabsolute calcd 31P CS exptl 31P CS

2b1c

Ob

Oc

Oc

Ob

Ob

Oc

1.543 1.203 1.204 226.8 78.8 77

1.560 1.079 1.402 225.7 79.9 77

1.522 1.422 1.063 252.2 53.4 57

1.559 1.087 1.385 227.1 78.5 77

1.521 1.432 1.060 256.3 49.3 57

1.558 1.10 1.330 227.3 78.3 77

HPW based on the FTIR and ab initio calculations45 and 1H NMR of pyridinium salt of HPW.46 As an illustration, the optimized structure for two TMPO molecules adsorbed on a proton associated with Ob sites in the O1b2c model is shown in Figure 8 and the corresponding geometric parameters and 31P CSs are listed in Table 6. In this case (scenario IV), a Brønsted proton will be situated in-between two oxygen atoms for each respective TMPO, which are anticipated to be partially protonated. In view of the close resemblance in the PdOTMPO bond lengths and 31P CSs observed for the two TMPO molecules (Table 6), it is indicative that the

PdOTMPO OTMPO · · · H σabsolute calcd 31P CS avg 31P CS exptl 31P CS

TMPO1

TMPO2

1.550 1.181 234.9 70.7

1.545 1.227 237.8 67.8 69.3 63-67

two adsorbate molecules should be indistinguishable and most likely swap between each other in the form of (TMPO)2H+ adsorption complexes. As such, the observed CS should be a weighted average of the two resonances (70.7 and 67.8 ppm), leading to a broad peak at ca. 69.3 ppm. Therefore, such (TMPO)2H+ species should be responsible for the 31P resonances observed for the NMR spectra of the TMPO/HPMo system in the CS range of 63-70 ppm (region II). Thus, the assignments of the 31P CSs can readily be made by combining experimental results obtained from the adsorbed TMPO and theoretical DFT calculations to reveal the distribution of Brønsted (protonic) acid sites in HPMo. Clearly, strong dependences in 31P CSs with the loading of the adsorbed TMPO as well as the corresponding adsorption sites in HPAs may be inferred. For the system of TMPO/HPMo studied herein, resonance peaks with CSs greater than 84 ppm are assigned due to the adsorption of one TMPO per KU (i.e., formation of (TMPOH+)/KU complexes) and those with CSs in the range of 80-84 ppm are attributed to (TMPOH+)2/KU, whereas 31P resonances with CSs smaller than 80 ppm are likely associated

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with hybridized (TMPOH+)3/KU and (TMPO)2H+ adsorption complexes in TMPO/HPMo system. 4. Conclusions We have demonstrated that detailed information on distribution and strength of acidic protons in heteropolyacids (HPAs) may be obtained by combining DFT theoretical calculations with solid-state 31P MAS NMR of adsorbed trialkylphosphine oxides (R3PO) probe molecules. Accordingly, the preferred location of the Brønsted protons on various oxygen sites may also be readily inferred. For the system of HPMo investigated herein with trimethylphosphine oxide (TMPO) as the probe, the TMPOH+ adsorption complexes were found to preferentially reside at corner-sharing (Ob) and edge-sharing (Oc) oxygen sites rather than the terminal (Od) site, most likely forming hybridized adsorption complexes, such as O1b2c or O2b1c. On the basis of the calculated proton affinities and experimental and theoretical 31 P chemical shifts of adsorbed TMPO, it is conclusive that the acid strengths of Brønsted protons in HPMo, although slightly weaker than those in HPW, are much stronger than those in typical zeolites, thus, representing solid acid catalysts with superacidic characteristics. Such comprehensive studies, which focus on the structures and acid properties of HPAs, should provide new insights into the mechanism of the acid-catalyzed reaction occurring over the surfaces of the HPAs and should also be helpful for future designs and modifications of related catalysts for specified applications. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20703058, 20773159, and 20933009) and by the National Science Council (NSC98-2113M-001-017-MY3), Taiwan. The authors are grateful to the National Center for High-performance Computing (NCHC, Taiwan) and Shanghai Supercomputer Center (SSC, China) for their support in computing facilities. Supporting Information Available: Solid-state 31P MAS NMR spectrum of HPMo and assorted 1H MAS NMR spectra of bare and TMPO loaded HPMo. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Kozhevnikov, I. V. Chem. ReV. 1998, 98, 171. (b) Kozhevnikov, I. V. In Catalysis for Fine Chemical Synthesis; Vol. 2, Catalysis by Polyoxometalates; John Wiley & Sons: Chichester, UK, 2002; pp 216. (2) (a) Mizuno, N.; Misono, M. Chem. ReV. 1998, 98, 199. (b) Okuhara, T.; Misono, M. In Oxide Catalysts in Solid State Chemistry, Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; John Wiley and Sons: Chichester, UK, 1994. (c) Misono, M. Catal. Today 2009, 144, 285. (3) (a) Okuhara, T.; Watanabe, H.; Nishimura, T.; Inumaru, K.; Misono, M. Chem. Mater. 2000, 12, 2230. (b) Filek, U.; Bressel, A.; Sulikowski, B.; Hunger, M. J. Phys. Chem. C 2008, 112, 19470. (4) (a) Corma, A. Chem. ReV. 1995, 95, 559. (b) Katsoulis, D. E. Chem. ReV. 1998, 98, 359. (5) (a) Yang, J.; Janik, M. J.; Ma, D.; Zheng, A.; Zhang, M.; Neurock, M.; Davis, R. J.; Ye, C.; Deng, F. J. Am. Chem. Soc. 2005, 127, 18274. (b) Zhang, H.; Zheng, A.; Yu, H.; Li, S.; Lu, X.; Deng, F. J. Phys. Chem. C 2008, 112, 15765. (6) Blasco, T.; Corma, A.; Martı´nez, A.; Martı´nez-Escolano, P. J. Catal. 1998, 177, 306. (7) Gayraud, P. Y.; Stewart, I. H.; Derouane-Abd Hamid, S. B.; Essayem, N.; Derouane, E. G.; Ve´drine, J. C. Catal. Today 2000, 63, 223. (8) Corma, A.; Martı´nez, A.; Martı´nez, C. J. Catal. 1996, 164, 422. (9) Verhoef, M. J.; Kooyman, P. J.; Peters, J. A.; von Bekkum, H. Microporous Mesoporous Mater. 1999, 27, 365. (10) Na, K.; Okuhara, T.; Misono, M. J. Catal. 1997, 170, 96. (11) Marosi, L.; Platero, E. E.; Cifre, J.; Area´n, C O. J. Mater. Chem. 2000, 10, 1949. (12) Squires, G. L. In Introduction to the Theory of Thermal Neutron Scattering, 2nd ed.; Dover Publications Inc.: New York, 1996.

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