Formation, Location, and Photocatalytic Reactivity of Methoxy Species

Publication Date (Web): September 12, 2008. Copyright © 2008 American Chemical ... Graduate School, Chinese Academy of Sciences. , §. Xiamen Univers...
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J. Phys. Chem. C 2008, 112, 15765–15770

15765

Formation, Location, and Photocatalytic Reactivity of Methoxy Species on Keggin 12-H3PW12O40: A Joint Solid-State NMR Spectroscopy and DFT Calculation Study Hailu Zhang,†,‡ Anmin Zheng,† Huaguang Yu,†,‡ Shenhui Li,†,‡ Xin Lu,§ 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, Chinese Academy of Sciences, Wuhan 430071, China, State Key Laboratory of Physical Chemistry of Solid Surface & Center for Theoretical Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China, and Graduate School, Chinese Academy of Sciences, Beijing 100049, China ReceiVed: December 3, 2007

Solid-state NMR experiments and quantum chemical Density Functional Theory (DFT) calculations were employed to investigate the formation, location, and photocatalytic reactivity of methoxy species on anhydrous 12-H3PW12O40. Two different types of methoxy species were identified on the methanol-adsorbed 12-H3PW12O40 catalyst. Rotational Echo DOuble Resonance (REDOR) NMR experiments combined with quantum chemical DFT calculations demonstrated that the two corresponding methyl groups reside on the Oc and Od atoms of the Keggin anion, forming the surface OcCH3 and OdCH3 species. Photocatalytic experiments further indicated that the two methoxy species are both photochemically reactive species with the OdCH3 species being much more reactive, and the methoxy species are preferentially mineralized to the final product CO2 directly. Introduction With carbenium ion-like properties, surface alkoxy species are considered important catalytic intermediates in a number of heterogeneous catalytic reactions, such as the conversion of methanol to hydrocarbons, the alkylation of aromatic compounds, the alkene transformations, the photocatalytic degradations of alcohols and alkane, and so on.1-4 Accordingly, the formation and the property of alkoxy groups on the surface of solid catalysts are attracting considerable attention. Recently, the existence and reactivity of surface alkoxy species have been demonstrated by both theoretical calculations and experiments, and some important information about the mechanism of the relevant heterogeneous catalytic reactions has been obtained.1-5 However, the experimental evidence about the formation and evolution of alkoxy species was mostly obtained on zeolites and metal oxide, such as H-Y, H-SAPO-34, and TiO2, 1-4 while on another type of important heterogeneous catalyst, polyoxometalates (POMs), the experimental investigation is still lacking.6 Due to their strong acidities, redox properties, interesting photochemical characteristics, and solid state structures, POMs have received much attention for many years.7-11 Presently, POMs have been used as catalysts in several industrial processes and many promising reactions, such as isomerizations,12 alkylations,13,14 and photocatalytic reactions.15-20 Although there are many structural types of POMs, the Keggin-type POMs, such as 12-H3PW12O40, are more extensively used in the numerous catalytic applications. Figure 1 shows the primary Keggin unit (KU) of the phosphotungstic anion, which is composed of a central PO4 tetrahedron surrounded by 12 WO6 octahedra.21 There are three kinds of oxygen atomsscentral (Oa), * To whom correspondence should be addressed. Fax: (+86) 278-7199291. E-mail: [email protected]. † Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences. ‡ Graduate School, Chinese Academy of Sciences. § Xiamen University.

bridging, and terminal (Od) onessin the Keggin structure, and the bridging oxygen atoms are further subdivided into cornersharing oxygen atoms (Ob) and edge-sharing oxygen atoms (Oc). Recently, by using solid-state NMR spectroscopy of 2-13Cacetone adsorption in combination with DFT calculation, we demonstrated that acidic protons were localized on both Oc and Od oxygen atoms of the Keggin unit, and their Brønsted acid strengths were also identified.22 It is generally assumed that alcohol molecules are catalyzed by Brønsted acid sites, forming alkoxy species by eliminating one water molecule, which are considered active intermediates in catalytic reactions. The knowledge of detailed acidic properties of 12-H3PW12O40 allows us to further study the formation as well as the reactivity of alkoxy species present on the catalyst. Herein, we present the first experimental observation of methoxy species on 12H3PW12O40, a representative Keggin-type POMs, and study the location as well as the photocatalytic reactivity of methoxy species. Experimental Section Sample Preparation for MAS NMR. 12-H3PW12O40 · nH2O was purified by extraction with diethyl ether and recrystallization from water at room temperature. The purity of the sample was checked by 31P MAS NMR and FTIR. A sample was first placed in a quartz tube (with a diameter of 8 mm) connected to a vacuum. The temperature was slowly (1.5 deg/min) increased to 523 K and held in a vacuum for 2 h to dehydrate the material. After the sample cooled to room temperature, a controlled amount of 13C-methanol (99% 13C enriched, Cambridge Isotope Laboratories, Inc.) was introduced from a vacuum line, frozen at liquid N2 temperature, and then flame sealed. The sealed sample was kept at room temperature for the NMR measurements. For the samples used for photocatalytic reactions, O2 was also introduced and the sealed samples were irradiated for different time by using a UV lamp (λ ) 254 nm, 18 W). The temperature of the sealed quartz tube was monitored and found to be nearly constant (295 ( 2 K) during photocatalytic

10.1021/jp806588q CCC: $40.75  2008 American Chemical Society Published on Web 09/12/2008

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Figure 1. The Keggin structure of the PW12O403- anion. There are three types of exterior oxygen atoms (Ob, Oc, and Od) in the Keggin unit. Oa is the central oxygen atom.

experiments. Before NMR measurement, the samples were transferred into ZrO2 NMR rotors with airtight cap under a dry nitrogen atmosphere in a glovebox. A CAVERN device23 was used for preparing sample used for in situ variable-temperature (VT) NMR experiments. The catalyst was loaded into the CAVERN device that was connected to a vacuum line, followed by the dehydration of catalyst and the adsorption of 13Cmethanol. The sealed rotor (tightly sealed by a Kel-F cap) was kept in liquid N2 and then transferred into a precooled NMR probe before the NMR measurements. Compared with the methanol adsorption in the small quartz tube, the low surface area of the 12-H3PW12O40 catalyst and the large volume of the CAVERN device lead to a relatively poor adsorption of methanol. Solid-State NMR Experiments. All NMR measurements except for the in situ VT NMR experiments were performed with a triple-resonance 5 mm probe on a Varian Infinityplus400 spectrometer. 13C MAS NMR spectra were acquired by using a single-pulse sequence with 1H proton decoupling and a 13C pulse width (π/2) of 4.0 µs and a repetition time of 10 s. 1H-13C CP MAS experiments were performed with a contact time of 2 ms and a repetition time of 2 s. T2 relaxation time was measured under MAS condition, using CPMG sequence. 13C{31P} REDOR NMR spectra were acquired with use of a standard REDOR pulse sequence. The 13C NMR signal intensities were measured separately for the REDOR spectra with (S) and without (S0) applying a series of rotor-synchronized π pulses to the 31P channel. The π-pulse widths of 13C and 31P were 8.0 and 7.6 µs, respectively. The in situ VT NMR experiments (223-293 K) were performed with a double-resonance 7.5 mm probe on a Varian Infinityplus-300 spectrometer. The sealed rotor was kept at a desired temperature for 30 min for temperature equilibrium before data acquisition. VT 13C MAS NMR spectra were acquired by using a single-pulse sequence with 1H proton decoupling and a 13C pulse width (π/2) of 5.3 µs and a repetition time of 10 s. Online Gas Chromatography (GC) Analysis. The gas-phase organic species formed during the photocatalytic reaction were detected by a homemade online GC analysis system, which was equipped with a continuous-flow fix-bed photoreactor and Flame Ionization Detector (FID). The product CO2 formed during the reaction was detected by using saturated limewater.15 Dehydrated catalyst (200 mg) was spread on the bottom of a quartz reactor. Then a stable anhydrous methanol/air mixture gas (Fmethanol ) 1.3 g m-3 and νgas ) 7 mL min-1) was passed through the catalyst. Until the concentration of the outlet gas

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Figure 2. Proton-decoupled 13C MAS NMR spectra of different amounts 13C-methanol adsorbed on anhydrous 12-H3PW12O40. The loading levels of 13C-methanol were varied from 0.3 to 0.9 molecules per Keggin unit. The MAS spinning speed was set to 6000 ( 1 Hz. The signals at ca. 110 ppm in the spectra are due to the background of the NMR rotor.

of the reactor was stable, a UV lamp (λ ) 254 nm, 18 W) was turned on to start the reaction. Computational Methods. Geometry optimizations were carried out by using density functional theory (DFT) and employing the Dmol3 program in the MS Modeling package24 at the generalized gradient approximation (GGA) level with the PW91 exchange and correlation functional. Double numerical basis set with polarization functions (DNP) was used in all calculations.25 This basis set is comparable to 6-31G**. The convergence threshold tolerance was set as follows: total energy ) 4 × 10-7 Ha; maximum force ) 4 × 10-5 Ha/Å. The chemical shift predictions were carried out by using the Amsterdam Density Functional (ADF) package,26 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.27,28 The triple-ξ basis set, including the polarization functions (TZP) based on the Slater-type orbital (STO), was chosen. It was demonstrated that the relativistic effects are important for the W atom in defining the geometry of structure, therefore, the calculations were performed within the relativistic scheme with the zero order regular approximation (ZORA)29 including the scalar effects. The NMR calculations were performed by using the gaugeindependent atomic orbital (GIAO) method on the optimized structures. Results and Discussion 13C

MAS NMR Experiments. Figure 2 shows the 13C MAS NMR spectra (acquired with a spinning rate of 6 kHz) of different amounts of 13C-methanol adsorbed on anhydrous 12H3PW12O40. At a low methanol loading (0.3 molecule/KU), two well-resolved signals at ca. 55 and 59 ppm are observable. The presence of nonsymmetric spinning sidebands at a low spinning rate due to the characteristic anisotropy of chemical shift of strongly bound surface methoxy groups is an indication for the presence of these species.2 We also acquired the MAS and CP/ MAS NMR spectra of 13C-methanol adsorbed on anhydrous 12H3PW12O40 with a loading of 0.3 molecule/KU at a MAS spinning rate of 2.0 kHz (a commonly used spinning rate in the literature2c,d). The spinning sidebands of signals at 55 and 59 ppm can be clearly observed (see Figure S1 in the Supporting Information), indicating that the two signals should originate from two different types of strongly surface-bound methoxy groups. With increasing the adsorption loading from 0.3 to 0.6 13C-methanol molecules per KU, a new signal at ca. 63 ppm

Reactivity of Methoxy Species on Keggin 12-H3PW12O40

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Figure 4. 13C{31P} REDOR NMR results of 13C-methanol adsorbed on anhydrous 12-H3PW12O40 with a methanol loading of 0.3 molecule/ KU. “N” refers to the number of rotor cycles, 2 (59 ppm) and b (55 ppm) denote the experimental data points. The dotted lines were calculated by varying the internuclear distance (RC-P, Å) between the central P atom of the Keggin anion and the C atom of methoxy groups. The MAS spinning speed was 6000 ( 1 Hz.

Figure 3. In situ VT 13C MAS NMR spectra of 13C-methanol adsorbed on 12-H3PW12O40 at various temperatures. The MAS spinning speed was set to 4000 ( 2 Hz.

appears. The signal can be attributed to the dehydration products of methanol and is most likely due to adsorbed dimethyl ether (DME).2,30 Since the previous studies of methanol dehydration were mostly demonstrated on zeolites,2 the structural difference between POMs and zeolites may make the spectral assignments uncertain. Here, in situ VT NMR experiments were employed to assist in the spectral assignments. The 13C MAS NMR spectra of 13Cmethanol adsorbed on 12-H3PW12O40 at various temperatures are shown in Figure 3. At 223 K, a main peak at 53.7 ppm probably due to hydrogen-bonded methanol is visible with downfield shoulder peaks at ca. 59 and 63 ppm. The appearance of the downfield signals indicates the dehydration of methanol on 12-H3PW12O40 can occur at very low temperature. Upon increasing the temperature to 248 K, the signal at ca. 59 ppm grows at the expense of the upfield signal, which shifts to 54.6 ppm. As the temperature increases to 273 K, the signal at ca. 59 ppm further increases and two upfield signals at 55.3 and 54.3 ppm can be resolved. At 293 K, the hydrogen-bonded methanol signal disappears thoroughly and two signals at 59.1 and 55.3 ppm dominate the 13C NMR spectrum. We used multiple components at 63, 59.1, 55.3, and 53.7 ppm to deconvolute the VT NMR spectra. Obviously, with an increase of temperature, the two signals at 59.1 and 55.3 ppm gradually increase at the expense of the hydrogen-bonded methanol signal at 53.7 ppm, indicating that the two former signals result from the dehydration products of methanol. According to its chemical shift, the peak at 55.3 ppm is most likely due to the methoxy group. Since the formation of DME needs the adsorption of two methanol molecules on one acid site,30b the signal at ca.

63 ppm (assigned to DME) is relatively weak in all the VT NMR spectra due to its low amount. 13C{31P} REDOR Experiments. The Rotational Echo DOuble Resonance (REDOR) technique, which was originally proposed by Gullion and Schaefer, is an efficient method to measure internuclear distances by recoupling the heteronuclear dipolar coupling which is removed by MAS.31 Here, 13C{31P} REDOR was used to determine the internuclear distance (RC-P) between the central P atom of the Keggin anion and the C atom of methoxy groups. The corresponding REDOR (S0 - S)/S0 curves are shown in Figure 4, where S0 and S represent the intensities of control signal and the dipolar dephasing signal, respectively. An obvious REDOR effect was observed for the 59 ppm signal. The best fit of the experimental data for the isolated spin pair gives rise to a RC-P of 5.65 ( 0.10 Å, while for the 55 ppm signal, the REDOR effect was relatively weaker than that of the 59 ppm signal, indicating that the corresponding 13C-31P distance is relatively longer. In addition, the shorter spin-spin relaxation time (T2 ) 0.94 ( 0.08 ms) of the 55 ppm signal compared with that (T2 ) 3.78 ( 0.32 ms) of the 59 ppm signal prevents us from measuring its whole REDOR curve, and thus determining the corresponding RC-P. The much shorter T2 value of the 55 ppm signal indicates a much more rigid nature of the corresponding methoxy species. DFT Calculations. Solid-state NMR technique combined with quantum chemical calculation is a powerful method to obtain the detailed structure of surface complexes. For example, we recently determined the location of acidic protons on 12H3PW12O40 by using 13C{31P} REDOR experiments in combination with DFT calculations;22 Mueller et al. reported the structure determination of the complex formed upon adsorption of mononucleotide 2′-deoxyadenosine 5′-monophosphate to the surface of a mesoporous alumina by solid-state NMR spectroscopy and ab initio computational chemistry.32 Here, quantum chemical calculations were also employed to determine the adsorbed species on 12-H3PW12O40. The internuclear distances between the central P atom of the Keggin anion and the C atom of methyl groups located at the different exterior oxygen sites (Ob, Oc, and Od) of the Keggin unit were determined. The three optimized methoxy complex structures are illustrated in Figure 5 (also see Table 1). For methyl groups binding to the Ob (Figure 5a), Oc (Figure 5b), and Od (Figure 5c) sites, the calculated C-P distances are 4.96, 5.51, and 6.45 Å, respectively. For the different DME adsorbed complexes on the OcH and OdH sites,

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Figure 6. The optimized structures of methanol physisorbed on the protons residing on the Oc (a) and Od (b) atoms of the Keggin unit.

Figure 5. The optimized structures of methyl groups binding to the Ob (a), Oc (b), and Od (c) sites of the Keggin unit. The C-O bond length in the surface methoxy species and the RC-P distance are indicated.

TABLE 1: Geometries of Methoxy Species and DME on 12-H3PW12O40 Calculated with DFT Method position

species

ads mode

RC-P (Å)

Ob Oc Od OcH

-CH3 -CH3 -CH3 DME DME DME DME

bonding bonding bonding side-ona end-on side-on end-on

4.96 5.51 6.45

OdH

6.99, 7.38 7.38, 8.49 8.22, 8.49

dipolar coupling constant (Hz) 100.32 73.18 45.62 33.00b 24.50b 20.58b

a The geometry optimization of the initial side-on structure results in an end-on complex. b Average 13C-31P dipolar coupling constant.

the calculated C-P distances are much larger than that (5.65 Å) measured for the signal at 59 ppm (see Table 1). Comparing these calculated data with those obtained from the REDOR NMR experiment, the two observed 13C NMR signals at 55 and 59 ppm can be definitely assigned to OdCH3 and OcCH3, respectively. Such an assignment is further supported by the 13C chemical shift calculations. The computed difference in the 13C chemical shifts of O CH and O CH species (+3.9 ppm) c 3 d 3 is in line with the experimental observation (ca. +4 ppm). Additionally, the computed difference in the 13C chemical shifts of OcCH3 and ObCH3 species is about +2.4 ppm. Therefore, our 13C chemical shift calculations also indicate that the signal at ca. 63 ppm is not due to methoxy species but can be assigned to DME. It is interesting that the ratio of the integral intensities of the 55 and 59 ppm peaks is ca. 1:1.7 for the methanol loadings of

0.3 and 0.6 molecule/KU. This ratio is in line with the ratio (ca. 1:1.7) of two kinds of accessible acidic protons localized on the Oc and Od oxygen atoms of anhydrous 12-H3PW12O40 that was previously disclosed.22,33 Generally, methanol molecules are physisorbed (hydrogen-bonded) on 12-H3PW12O40 before the formation of methoxy species. Our theoretical calculations indicate that the adsorption energy difference (0.5 kJ mol-1) for the two physisorbed complexes at the OdH and OcH sites is too small to cause a heterogeneous physisorption. Therefore, it can be expected that the populations of the two methoxy species follow the ratio of acid site density. On the basis of our experimental and computational results, the mechanism of formation of methoxy species is proposed as follows: the methanol molecule is first physisorbed on the Oc and Od sites through hydrogen-bonding interactions (see Figure 6); following the physisorption, it is protonated by the acidic proton (such as that residing on the Oc atom) and then the resulting carbocation interacts with the neighboring basic oxygen atom (such as the Od atom), forming a surface methoxy species (such as OdCH3) by eliminating one water molecule. The proposed mechanism is similar to that predicted by theoretical calculations for the formation of methoxy species on zeolites.30 In addition, the mechanism suggests that the OcCH3 species (corresponding to the 59 ppm signal) originates from the methanol molecule reacting with the acidic proton on the Od site, while the OdCH3 species (corresponding to the 55 ppm signal) is from the methanol molecule reacting with the acidic proton on the Oc site. Photocatalytic Degradation. As a kind of heterogeneous photocatalyst, POMs are attracting considerable attention for effectively removing low concentration organic pollutants in both the gas and the liquid phase, which is analogous to TiO2.15-20 Similar to other catalytic reactions, the investigations of activity sites on the catalysts and the reaction mechanisms are of great significance. The adsorption and photocatalytic degradation mechanisms of aliphatic alcohols on TiO2, the most widely used photocatalyst, were extensively studied by a variety of techniques.4,34 Two adsorption species, physisorbed (or hydrogen-bonded) alcohol molecules and chemisorbed alkoxy species, are usually found on the TiO2 surface.4,34c,35 It is generally accepted that chemisorbed alkoxy species are preferentially oxidized directly to final product CO2 during the photocatalytic degradation of aliphatic alcohols on TiO2, while physisorbed (or hydrogen-bonded) alcohol molecules are preferentially oxidized through intermediates (such as ketone or aldehyde).4c,d,34c,35 Since 12-H3PW12O40 is a strong solid acid with the acid strength of the isolated proton on the Oc site being

Reactivity of Methoxy Species on Keggin 12-H3PW12O40

J. Phys. Chem. C, Vol. 112, No. 40, 2008 15769 gas by using saturated limewater. For comparison, the GC spectra of the photocatalytic oxidation of methanol on 12K3PW12O40, an efficient photocatalyst17c on which methanol exists only in the form of physisorbed species (giving rise to a 13C signal at 50 ppm, see Figure S2 in the Supporting Information), were also recorded. In this case, the formation of formaldehyde was evident (Figure 8c,d). All these findings demonstrate that the chemisorbed alkoxy species are preferentially photooxidized to the final product CO2 directly, while the physisorbed methanol molecules are mineralized via intermediates on the Keggin-type POMs, similar to the photocatalysis of alcohols on TiO2.4c,d,34c,35

Figure 7. Proton-decoupled 13C MAS NMR spectra of 13C-methanol adsorbed on anhydrous 12-H3PW12O40 after different UV irradiation times. The MAS spinning speed was set to 6000 ( 1 Hz. The signals at ca. 110 ppm in the spectra are due to the background of the NMR rotor.

Figure 8. GC spectra recorded before (a, c) and during (b, d) the photocatalytic oxidation of methanol on the anhydrous 12-H3PW12O40 (a, b) and 12-K3PW12O40 (c, d) powdered catalysts.

superacidic and that on the Od site being much stronger than that of zeolites, the protonation of methanol can occur more easily, leading to the sole formation of two methoxy species (without any physisorbed methanol). In this case, CO2 may be expected as the only final product after the photocatalytic degradation of methanol on the 12-H3PW12O40 catalyst. The difference in the photocatalytic reactivity of the two methoxy species was investigated as well. To avoid the influence of water and DME molecules, a small amount (0.3 molecule/ KU) of methanol coverage was chosen. Figure 7 shows the 13C MAS spectra of two methoxy species before and after photocatalytic oxidation. The intensities of the 59 and 55 ppm signals are gradually reduced after different times of UV irradiation with the latter being diminished more quickly; for example, the intensity of the 59 ppm signal is decreased by 39%, while that of the 55 ppm signal is decreased by 74% after 90 min of UV irradiation. This phenomenon indicates the two methoxy species are both photochemically reactive species and the OdCH3 species is more photocatalytically reactive than the OcCH3 species. The different reactivity of the two species is probably due to the different local electronic structures of the Oc and Od sites. In the 13C NMR spectra (Figure 7), the final product CO2 was undetectable, probably due to the low surface area (2.1 m3/ g) of the catalyst as well as the weak adsorption of the adsorbed species. We therefore employed GC spectroscopy to detect the gas-phase species qualitatively during the photocatalytic oxidation process. As shown in parts a and b of Figure 8, no reaction intermediates, such as formaldehyde (retention time: 4.1 min) and formic acid (retention time: 11.0 min), were identified in the gas phase. The formation of CO2 was detectable in the tail

Conclusions In summary, the formation, location, and photocatalytic reactivity of methoxy species on the Keggin 12-H3PW12O40 were investigated by solid-state NMR techniques as well as DFT calculations. Two surface methoxy species were identified on the methanol-adsorbed 12-H3PW12O40 catalyst. REDOR NMR experiments combined with DFT calculations demonstrated that the two corresponding methyl groups reside on the Oc and Od atoms of the Keggin anion, forming the surface OcCH3 and OdCH3 species, respectively. NMR experiments indicated that the two methoxy species are both photochemically reactive species with the OdCH3 species being much more reactive. The methoxy species are preferentially photooxidized to the final product CO2 directly on the anhydrous 12-H3PW12O40 catalyst, while the physisorbed methanol molecules are mineralized via intermediates on the Keggin-type 12-K3PW12O40 catalyst. The present results provide us additional insights into the photochemical properties of 12-H3PW12O40 as well as the photocatalytic reaction mechanism of methanol on the solid acid. This work opens up a new window for investigating the role of alkoxy species in numerous POMs-mediated hydrocarbon reactions. Acknowledgment. Financial support from the National Natural Science Foundation of China (Nos. 20425311 and 20673139) and the National Basic Research Program of China (No. 2009CB918603) are gratefully acknowledged. We also thank the Shanghai Supercomputer Center (SSC, China) for DFT calculations. Supporting Information Available: Proton-decoupled13C MAS and CP/MAS NMR spectra acquired with a MAS spinning speed of 2.0 kHz of 13C-methanol adsorbed on anhydrous 12H3PW12O40, proton-decoupled 13C MAS, and CP/MAS NMR spectra of 13C-methanol adsorbed on the 12-K3PW12O40 catalyst. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Murray, D. K.; Chang, J. W.; Haw, J. F. J. Am. Chem. Soc. 1993, 115, 4732. (b) Murray, D. K.; Howard, T.; Goguen, P. W.; Krawietz, T. R.; Haw, J. F. J. Am. Chem. Soc. 1994, 116, 6354. (2) (a) Jiang, Y. J.; Hunger, M.; Wang, W. J. Am. Chem. Soc. 2006, 128, 11679. (b) Ivanova, I. I.; Pomakhina, E. B.; Rebrov, A. I.; Hunger, M.; Kolyagin, Y. G.; Weitkamp, J. J. Catal. 2001, 203, 375. (c) Wang, W.; Buchholz, A.; Seiler, M.; Hunger, M. J. Am. Chem. Soc. 2003, 125, 15260. (d) Wang, W.; Seiler, M.; Hunger, M. J. Phys. Chem. B 2001, 105, 12553. (3) Li, X. B.; Nagaoka, K.; Simon, L. J.; Olindo, R.; Lercher, J. A.; Hofmann, A.; Sauer, J. J. Am. Chem. Soc. 2005, 127, 16159. (4) (a) Hwang, S. J.; Raftery, D. Catal. Today 1999, 49, 353. (b) Pilkenton, S.; Hwang, S. J.; Raftery, D. J. Phys. Chem. B 1999, 103, 11152. (c) Xu, W. Z.; Raftery, D. J. Phys. Chem. B 2001, 105, 4343. (d) Xu, W. Z.; Raftery, D.; Francisco, J. S. J. Phys. Chem. B 2003, 107, 4537.

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