Structural Stability and Brønsted Acidity of Thermally Treated

19470

J. Phys. Chem. C 2008, 112, 19470–19476

Structural Stability and Brønsted Acidity of Thermally Treated AlPW12O40 in Comparison with H3PW12O40 Urszula Filek,† Arne Bressel,‡ Bogdan Sulikowski,*,† and Michael Hunger*,‡ Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krako´w, Poland, and Institute of Chemical Technology, UniVersity of Stuttgart, Stuttgart, Germany ReceiVed: September 8, 2008; ReVised Manuscript ReceiVed: October 14, 2008

The thermal stability and hydroxyl coverage of aluminum salt of 12-tungstophosphoric acid, AlPW12O40, was studied by X-ray diffraction and solid-state NMR spectroscopy in comparison with the pure H3PW12O40 heteropoly acid. The most important differences of these materials consist in the higher thermal stability of the local structure and the stronger temperature dependence of the concentration of acidic protons of AlPW12O40. H3PW12O40 showed shrinkage of the unit cell in the X-ray patterns and changes of the local structure in the 31 P MAS NMR spectra already upon dehydration at 473 K, while no structural modifications were found for AlPW12O40 up to 673 K. On AlPW12O40, acidic protons and AlOH groups are formed by dissociation of water molecules at the multivalent aluminum cations upon dehydration at temperatures of 373-423 K. After dehydration at temperatures higher than 423 K, the hydroxyl groups of AlPW12O40 dehydroxylate and nonhydroxylated Al3+ cations are formed. The accessibility of acidic protons as studied by adsorption of pyridine was found to be similar for dehydrated H3PW12O40 and AlPW12O40, that is, a lower accessibility occurs with increasing dehydration temperature. The adsorption of probe molecules for studying the acid strength indicated that AlPW12O40 dehydrated at 373-523 K is as superacidic as dehydrated H3PW12O40. 1. Introduction In the past decades, heteropoly acids (HPA) and their salts gained raising importance as catalysts in a number of reactions.1-3 Often, these materials are applied in aqueous solution or as solid catalysts with a “pseudoliquid phase”.4 The catalytic activity of solid heteropoly acids at low temperatures is closely related to this property. Polar molecules, such as water, alcohols, and ethers, move into the three-dimensional bulk phase and expand the distance between the structural units, such as Keggin ions. In this case, selective catalytic reactions occur in spite there are no intrinsic micropores in the crystal structure. The crystal structure of the hexahydrate of H3PW12O40 · 6 H2O as determined by neutron diffraction,5 consists of PW12O403- polyanions, which are packed in a cubic structure, and the acidic protons are present in the form of H5O2+ cations. The water molecules of the H5O2+ species are hydrogen bonded together with an O-O distance of 0.24 nm.5 In the anhydrous H3PW12O40, without water molecules as spacers between the Keggin ions, the acidic protons exist in an isolated manner. These acidic protons cause a 1H MAS NMR signal at the chemical shift of about 9.1 ppm. They exhibit a low thermal mobility in the temperature range of 298-373 K because no effect of the temperature on the 1H MAS NMR line width was found.6 The thermal stability of heteropoly acids and their salts is of great importance for their application in heterogeneous catalysis at elevated temperatures.7-9 The aluminum salt of H3PW12O40 is an interesting catalyst for Friedel-Crafts acylation of aromatic compounds and regioselective ring opening of epoxides.9 Both processes were carried out at mild conditions, that is, at * To whom correspondence should be addressed. Fax: +49 711 68564081 (M.H.); +48 12 4251923 (B.S.). E-mail: [email protected] (M.H.); [email protected] (B.S.). † Polish Academy of Sciences. ‡ University of Stuttgart.

313-373 and 273 K to ambient temperature, respectively. For these catalytic applications of AlPW12O40, Lewis acid sites were discussed as active sites. Generally, Keggin-type heteropoly compounds are the most stable among various polyoxometalates. The temperature behavior of heteropoly acids was investigated by thermal gravimetric analysis (TGA) and differential thermal analysis (DTA), which indicated an onset of the thermal decomposition of H3PW12O40 at about 673 K.10 Heteropoly salts were found to be more stable than the parent acidic materials.11 The above-mentioned analytical methods, however, do not allow an insight into the structural evolution and the changes of Brønsted acid sites of polyoxometalates upon thermal treatment. In the present work, AlPW12O40 was studied upon dehydration at 373 to 673 K in comparison with H3PW12O40. Treatment of the sample materials under vacuum at elevated temperatures led to a loss of the water spacers. The accompanied structural changes were investigated by X-ray diffraction and 31P MAS NMR spectroscopy. Quantitative 1H MAS NMR spectroscopy was utilized for determining the hydroxylation and dehydroxylation behavior of anhydrous H3PW12O40 and AlPW12O40 materials. Finally, adsorption of deuterated pyridine and 13Cenriched acetone on the dehydrated samples was performed for clarifying the accessibility and acid strength of the acidic protons. 2. Experimental Section Parent dodecatungstophosphoric acid (H3PW12O40) was commercially obtained from POCH S.A., Poland. The others reagents used were analytical grade, commercially available and used without further purification. The salt aluminumdodecatungstophosphate (AlPW12O40) was obtained according to Baba et al. by the addition of a stoichiometric amount of aqueous solution of Al(NO3)3 · 9H2O to the aqueous solution of H3PW12O40 at room temperature for 1 h.12 Because this salt is water soluble, the mixture was heated to dryness at 323 K.

10.1021/jp807947v CCC: $40.75  2008 American Chemical Society Published on Web 11/13/2008

Stability and Acidity of AlPW12O40 Powder X-ray diffraction patterns were acquired on a Siemens D5005 diffractometer using Cu KR radiation (30 kV, 20 mA). The samples were heated in a high-temperature chamber with a rate of 2 K/min from room temperature to 873 K, and diffractograms were acquired in temperature steps of 100 K for 1 h. For studying the thermal stability and dehydroxylation behavior of the heteropoly acids under study by solid-state NMR spectroscopy, about 250 mg of material were thermally treated with the following temperature program in a 5 mm glass tube connected to a vacuum line. The temperature was raised with 0.5 K/min from room temperature to the final temperatures of 373-673 K and evacuated at a pressure of p < 10-2 mbar for 6 h. Subsequently, the samples were sealed or loaded with probe molecules before sealing. Deuterated pyridine (C5D5N, 99.9% deuterated) and acetone-2-13C (CH313COCH3, 99.5% 13C-enrichment) were purchased from Acros and Sigma-Aldrich, respectively. About three probe molecules per Keggin ion were loaded on the dehydrated materials. To avoid rehydration of the dehydrated materials before the NMR studies, the samples were filled into gas-tight rotors in a glovebox under dry nitrogen gas. The solid-state NMR experiments were performed on a Bruker MSL 400 spectrometer at resonance frequencies of 400.3, 100.4, and 161.9 MHz for 1H, 13C, and 31P nuclei, respectively. Flip angles of π/2 for 1H, 13C, and 31P and repetition times of 10 s for 1H, 30 s for 13C, and 60 s for 31P MAS NMR spectroscopy were used. The 1H and 31P MAS NMR spectra were recorded with a sample spinning rate of about 10 kHz, while 13C MAS NMR spectra were obtained with a spinning frequency of about 4.0 kHz. The 1H{27Al}-TRAPDOR MAS NMR experiment was performed by irradiating a 27Al pulse with an rf field of 50 kHz in the first pulse delay τ of the π/2-τ-π-τ pulse sequence (spin-echo) applied to the 1H nuclei (νrot ) 5 kHz, τ ) 200 ms).13,14 The concentration of the hydroxyl groups of the calcined heteropoly acids was determined by comparing the 1H MAS NMR intensities with that of an external standard consisting of zeolite Na,H-Y, which was obtained by exchange of 35% of the sodium cations by ammonium ions and calcination at 673 K. To separate the different MAS NMR signals and for the quantitative evaluation of spectra, the data were processed with the Bruker software WINNMR and WINFIT. 3. Results and Discussion 3.1. Thermal Stability of H3PW12O40 and AlPW12O40. The X-ray diffraction patterns obtained for hydrated H3PW12O40 and AlPW12O40 and during the dehydration upon increasing the temperature with a heating rate of 2 K/min are illustrated in Figures 1a and b, respectively. While the diffractograms of the hydrated H3PW12O40 show the characteristic lines described in literature,15 a shrinkage of the heteropolyacid structure starts upon dehydration at 473 K, as indicated by shifts of the characteristic diffraction lines at 2Θ ) 25.2 ° and 34.5° to 2Θ ) 26.1° and 35.6°. The corresponding unit cell contraction is probably due to the removal of physically adsorbed water acting as spacers. In the temperature range of 573-773 K, the intensity of diffractograms is decreased, but the Keggin structure was stable. At 873 K, the formation of bronze can be noticed by lines at 2Θ ) 23.5° and 33.3°.16 In the case of AlPW12O40, the structure is stable up to 673 K, as indicated by characteristic lines, for example, at 2Θ ) 20.5° and 24.9°. An additional increase of the dehydration temperature leads to a change of the structure, and a new compound, such as bronze, is formed causing characteristic lines at 2Θ ) 23.7° and 33.6°.

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19471

Figure 1. X-ray diffraction patterns of H3PW12O40 (a) and AlPW12O40 (b) recorded upon heating with a rate of 2 K/min from room temperature to 873 K.

As shown in a number of previous studies, the chemical shift of the 31P MAS NMR signals of tungstophosphoric acids and their salts depend strongly on their water contents.4,17 The 31P MAS NMR spectra of both H3PW12O40 and CsxH3-xPW12O40 in the hydrated state are dominated by narrow signals at -15.0 to -15.6 ppm.4,17 The same observation was made for hydrated AlPW12O40 in the present work (not shown). Upon dehydration of H3PW12O40 and their salts, however, different 31P NMR spectrospcopic behaviors of the phosphorus atoms in these materials were found. Figure 2 shows the 31P MAS NMR spectra of H3PW12O40 (top) and AlPW12O40 (bottom) recorded after dehydration in vacuum at 373-673 K. According to the literature,4,17 the 31P MAS NMR spectrum of weakly hydrated (n ) 0.1-0.5 of H2O per Keggin unit) H3PW12O40 consists of a broad signal at about -11 ppm. A corresponding signal occurs in Figure 2a for H3PW12O40 dehydrated at 373 K. With increasing dehydration temperature, this signal becomes narrow and has a shift value of about -11.5 ppm (Figures 2b,c). This 31 P MAS NMR signal indicates that most of the H5O2+ cations of the H3PW12O40 polyanions were transformed into acidic protons, H+, bound to oxygen atoms of the Keggin units. Upon dehydration at 673 K (Figure 2d), the 31P MAS NMR signal of H3PW12O40 shifts to about -12.5 ppm. Essayem et al. explain this spectroscopic behavior by local structure changes.17 As indicated by X-ray diffraction, structural changes in the longrange order require a dehydration temperature of more than 773 K (see Figure 1a).

19472 J. Phys. Chem. C, Vol. 112, No. 49, 2008

Filek et al.

Figure 2. 31P MAS NMR spectra of H3PW12O40 (a-d) and AlPW12O40 (e-h) recorded upon dehydration at 373-673 K from top to bottom.

Figure 3. 1H MAS NMR spectra of H3PW12O40 (a-c) and AlPW12O40 (d-f) recorded upon dehydration at 373 (a, d), 473 (b, e), and 523 K (c, f).

In contrast to the above-mentioned observation, the 31P MAS NMR spectra of AlPW12O40 recorded upon dehydration at 373 to 673 K are dominated by signals at about -15 ppm. In agreement with studies on partially dehydrated CsXH3-XPW12O40,2 dehydration of AlPW12O40 at moderate temperatures of 373-573 K (Figures 2e-g) leads to a weak signal at -11 ppm, while at dehydration temperatures of 573 and 673 K (Figures 2g,h) a weak shoulder at -13.5 ppm occurs. In contrast to dehydrated H3PW12O40, however, no changes of the local structure are indicated by the 31P MAS NMR spectrum of AlPW12O40 dehydrated at 673 K (Figure 2h). Generally, the 31P MAS NMR signal at -15.0 to -15.6 ppm is characteristic for tungstophosphoric polyanions with counter cations like H5O2+ in the case of hydrated H3PW12O40 or Cs+ in the case of CsxH3-xPW12O40.4,17 In the case of dehydrated AlPW12O40, these counter cations are Al(OH)mn+ species. On the other hand, the weak 31P MAS NMR signal occurring at -11 ppm in the spectra of dehydrated AlPW12O40 (Figures 2e-g) indicates the presence of acidic protons, H+, as found for dehydrated H3PW12O40 (Figure 2b,c). 3.2. Nature of Hydroxyl Species in Dehydrated H3PW12O40 and AlPW12O40. The 1H MAS NMR spectra of H3PW12O40 and AlPW12O40 dehydrated at 373, 473, and 523 K are shown in Figure 3, top and bottom, respectively. In agreement with studies of Baba and Ono and Uchida et al.,18,4 the spectra of H3PW12O40 are dominated by a signal of acidic protons, H+, at about 9.1 ppm. According to Ganapathy et al.,19 these protons are located near oxygen atoms bound to a single tungsten atom (terminal Od). An increase in the calcination

temperature from 373 to 523 K does not change the resonance position of this signal, however, its intensity decreases (Figures 3a-c). In contrast, the 1H MAS NMR spectra of AlPW12O40 dehydrated at 373-523 K consist of up to four signals due to strongly physisorbed water and different hydroxyl groups (Figures 3d-f). Upon dehydration at 373 K (Figure 3d), the 1H MAS NMR spectrum is dominated by a signal at 7.7 ppm, which is tentitatively assigned to water molecules physisorbed on acidic Brønsted sites.20 The strong low-field shoulder at 9.1 ppm indicates the formation of acidic protons, as already observed for H3PW12O40. In addition, there is a broad high-field shoulder at about 5.4 ppm. In the 1H MAS NMR spectrum of AlPW12O40 dehydrated at 473 K (Figure 3e), this high-field shoulder is split into two signals at 4.2 and 5.6, while the signal of physisorbed water at 7.7 ppm disappeared. Upon dehydration of AlPW12O40 at 523 K (Figure 3f), the spectrum exclusively consists of a small signal of acidic protons at 9.1 ppm. It is interesting to note that the signal at 9.1 ppm in the 1H MAS NMR spectra of AlPW12O40 is not accompanied by spinning sidebands, but the signals at 4.2 and 5.6 ppm have strong sideband patterns. The latter finding is an indication of a strong heteronuclear dipolar interaction of the hydroxyl protons at 4.2 and 5.6 ppm with heteronuclei, such as 27Al nuclei in AlOH groups. 1H{27Al}-TRAPDOR MAS NMR experiments are suitable to investigate the dipolar coupling between 1H nuclei with spin I ) 1/2 and quadrupole nuclei, such as 27Al nuclei characterized by a spin of S ) 5/2. If a long 27Al pulse is irradiated during the pulse delay of a spin-echo sequence applied to 1H nuclei, those 1H nuclei contribute to the echo

Stability and Acidity of AlPW12O40

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19473

Figure 4. 1H{27Al}-TRAPDOR MAS NMR study of AlPW12O40 dehydrated 473 K. Spectrum (c) was obtained by substracting the 1H spin-echo MAS NMR spectrum obtained without (a) and with (b) 27Alirradiation.

Figure 5. Temperature dependence of the number of acidic protons (δ1H ) 9 ppm) of H3PW12O40 and AlPW12O40 dehydrated at 373673 K.

intensity, which are not involved in a dipolar interaction with neighboring 27Al nuclei. Subtraction of the 1H spin-echo MAS NMR spectrum recorded with 27Al irradiation (Figure 4b) from the 1H spin-echo MAS NMR spectrum obtained without 27Al irradiation (Figure 4a) gives the spectrum of hydroxyl protons, which are involved in a dipolar coupling with 27Al nuclei. The difference spectrum in Figure 4c exclusively consists of signals at 4.2. and 5.6 ppm, which indicates that these signals are really caused by AlOH groups. The simultaneous occurrence of 1H MAS NMR signals of acidic protons at 9.1 ppm and AlOH groups at 4.2 and 5.6 ppm indicates that the dehydration of AlPW12O40 is accompanied by a dissociation of water molecules coordinated to Al3+ cations under formation of the above-mentioned hydroxyl groups.1 The observation of up to two signals of AlOH groups (4.2 and 5.6 ppm) could be an indication that AlOH groups are formed according to eqs 1 and 2

molecules as a prerequisite for the formation of acidic protons and AlOH groups. Upon dehydration at temperatures higher than 423 K, dehydroxylation of acidic protons and AlOH groups by recombination to water molecules occurs, which are desorbed as a result of the thermal treatment:

Al(H2O)n3+ f Al(OH)2+ + 2H+ - (n - 2)H2O

(1)

Al(H2O)n3+ f Al(OH)2+ + 1H+ - (n - 1)H2O

(2)

In the case of a larger number of water molecules coordinated to Al3+ cations during dehydration at low temperatures, a higher content of Al(OH)2+ species are formed via eq 1 and vice versa. The homonuclear dipol-dipol interaction between the two neighboring hydroxyl protons of Al(OH)2+ species may be the reason of the larger residual line width of the 1H MAS NMR signal at 5.6 ppm in comparison with the signal at 4.2 ppm. As indicated by the 1H MAS NMR spectra in Figure 3, an increase in the dehydration temperature leads to a characteristic change of the number of acidic protons in H3PW12O40 and AlPW12O40. Quantitative evaluation of the 1H MAS NMR spectra of these materials obtained upon dehydration in the temperature range of 373-673 K give the concentrations of acidic protons at 9.1 ppm summarized in Figure 5. While the number of acidic protons in H3PW12O40 decreases monotonically with increasing calcination temperature, the number of acidic protons in AlPW12O40 reaches a maximum upon calcination at 423 K before the number of these protons is rapidly decreased upon dehydration at higher temperatures. The latter finding is very similar to the recently studied behavior of hydroxyl groups in aluminum-exchanged zeolites X and Y.21 Heating of AlPW12O40 at temperatures up to 423 K is accompanied by a dehydration and dissociation of water

Al(OH)n+ + nH+ f Al3+ + nH2O

(3)

Hence, hydroxylation of the Al3+ cations of AlPW12O40 during dehydration at 373-423 K (eqs 1 and 2) is a reversible process leading to the formation of nonhydrated and nonhydroxylated Al3+ cations upon thermal treatments at temperatures higher than 423 K (eq 3). Comparison of the X-ray patterns and 31P MAS NMR spectra, discussed in Section 3.1, with the dehydroxylation behavior of H3PW12O40 shown in Figure 5 indicates that the loss of acidic protons is directly accompanied by local structure changes of this material. In contrast, the dehydration and dehydroxylation of AlPW12O40 at temperatures up to 673 K leads to a transformation of Al(OH)2+ and Al(OH)2+ species into nonhydroxylated Al3+ cations (see eq 3), which compensate the negative charges of the polyanions and stabilize the Keggin ions so that no significant structural changes occur in the abovementioned temperature range. 3.3. Accessibility and Acid Strength of Brønsted Acid Sites in H3PW12O40 and AlPW12O40. As demonstrated by Baba et al.,22 adsorption of deuterated pyridine (C5D5N) on partially dehydrated H3PW12O40 leads to a protonation of the probe molecules by the acidic protons. The corresponding 1H MAS NMR signal of pyridinium ions (C5D5NH+) occurs at about 13 ppm.22 For comparison, the 1H MAS NMR signals of C5D5NH+ on dehydrated H-form zeolites are observed at 15 to 19 ppm.23,24 In the present work, the deuterated pyridine was utilized for investigating the accessibility and acid strength of acidic protons of H3PW12O40 and AlPW12O40 dehydrated at 373 and 523 K (Figure 6, top and bottom, respectively). Upon adsorption of three C5D5N molecules per Keggin unit on H3PW12O40 dehydrated at 373 K (Figure 6a), the signal of acidic protons at 9.1 ppm is decreased by about 80% and lowfield signals at 12 to 13 ppm and 19 ppm can be observed. Upon adsorption of C5D5N on H3PW12O40 dehydrated at 523 K (Figure 6b), however, less than about 30% of the acidic protons at 9.1 ppm are affected by the probe molecules. This observation indicates a strong decrease of the accessibility with increasing dehydration temperature.

19474 J. Phys. Chem. C, Vol. 112, No. 49, 2008

Figure 6. 1H MAS NMR spectra of H3PW12O40 (a, b) and AlPW12O40 (c, d) recorded upon dehydration at 373 K (a, c) and 523 K (b, d) and loading with three C5D5N molecules per Keggin unit.

In agreement with Baba et al.,22 the signal at 12-13 ppm was assigned to pyridinium ions formed by adsorption of C5D5N on acidic protons of H3PW12O40. According to recent DFT calculations of Zheng et al.,24 the shift values of C5D5NH+ ions of 12-13 ppm correspond to Brønsted acid sites on solid acids with a proton affinity of about 1108 kJ/mol. This proton affinity is about 209 kJ/mol lower than the proton affinity of acid sites in H-Form zeolites.24 The DFT calculations of Zheng et al. indicate that a smaller 1H NMR shift of C5D5NH+ ions formed on solid acids corresponds to a higher acid strength and vice versa.24 It must be stressed that in this case a complete proton transfer from the solid acid to the probe molecule occurs. As a consequence, the second low-field signal at 19 ppm is due to the C5D5NH+ ions located on acid sites with lower acid strength than those responsible for the C5D5NH+ ions at 13 ppm. These acid sites could be remaining H5O2+ cations of the partially dehydrated H3PW12O40. In this case, the low-field signal at 19 ppm should be strongly decreased with increasing dehydration temperature, which agrees with the experimental finding in Figure 6b. The protonation of the probe molecule C5D5N by the acid sites of H3PW12O40 is supported by 31P MAS NMR spectroscopy of the loaded materials (Figures 7a,b). The 31P MAS NMR spectrum of H3PW12O40 dehydrated at 373 K is characterized by a signal at -15.6 ppm and weak signals at -11 to -12 ppm (Figure 7a). The signal at -15.6 ppm is due to tungstophosphoric polyanions compensated in their negative charges by countercations, in this case by C5D5NH+ ions. In contrast, the 31 P MAS NMR spectrum of H3PW12O40 dehydrated at 523 K

Filek et al.

Figure 7. 31P MAS NMR spectra of H3PW12O40 (a, b) and AlPW12O40 (c, d) recorded upon dehydration at 373 K (a, c) and 523 K (b, d) and loading with three C5D5N molecules per Keggin unit.

consists of a strong signal at -11.1 ppm due to the large number of nonaccessible acidic protons and a significantly weaker signal at -15.6 ppm due to the acidic protons transferred to the probe molecules (Figure 7b). The decrease of the signal at -15.6 ppm in comparison with the spectrum in Figure 7a is due to the lower accessibility of the acidic protons in the material dehydrated at 523 K in comparison with the material dehydrated at 373 K. In the case of AlPW12O40, the number of acidic protons formed after dehydration at 373 K is small and, therefore, only a weak 1H MAS NMR signal of pyridinium ions occurs at 13 ppm (Figure 6c). The absence of the 1H MAS NMR signal at 19 ppm in the spectra of dehydrated and pyridine-loaded AlPW12O40 indicates that no H5O2+ cations exist. As found for H3PW12O40 dehydrated at 523 K, also most (ca. 70%) of the acidic protons of AlPW12O40 dehydrated at 523 K are not accessible for the probe molecules (Figure 6d). As indicated by the shift value of 13 ppm for the pyridinium ions protonated by the accessible acidic protons, the acid strength of dehydrated AlPW12O40 agrees with that of dehydrated H3PW12O40. The 31P MAS NMR spectra of dehydrated AlPW12O40 in Figures 7c,d are both dominated by the signal at -15 ppm. This is due to the fact that the tungstophosphoric polyanions are compensated in their negative charges by countercations, such as cationic aluminum species and C5D5NH+ ions. In the case of AlPW12O40 dehydrated at 523 K, however, the acidic protons, which are not accessible for pyridine, are responsible for the weak signal at -11 ppm in Figure 7d. An additional method for characterizing the strength of Brønsted acid sites, such as the acidic protons in dehydrated tungstophosphoric polyanions, is the adsorption of acetone interacting by hydrogen bonding with these surface sites.25 The

Stability and Acidity of AlPW12O40

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19475 TABLE 1: Resonance Positions, δ13C, and Low-Field Shifts, ∆δ13C, of Acetone-2-13C Adsorbed at Brønsted Acid Sites of Various Liquids and Solids liquid and solid acids

δ13C/ppm

∆δ13C/ppm

ref.

CDCl3 zeolite H-X zeolite H-Y zeolite H-ZSM-5 AlPW12O40 H3PW12O40 H2SO4, 100% AlCl3/MCM-41 (SG)nAlCl2 FSO3H/SbF5 (4:1) SbF5

205 215 220 223 235 235 244 245 245 248 250

0 10 15 18 30 30 39 40 40 43 45

26 26 26 24, 26 p.w.a 27 26 28 29 26 26

a

Figure 8. 13C MAS NMR spectra of H3PW12O40 (a, b) and AlPW12O40 (c, d) recorded upon dehydration at 373 K (a, c) and 523 K (b, d) and loading with three acetone-2-13C molecules per Keggin unit.

adsorbate-induced low-field shift ∆δ13C of acetone-2-13C upon interaction with solid acids is a well-accepted scale of acid strength.26 In Figures 8a,c, the 13C MAS NMR spectra of acetone-2-13C adsorbed on H3PW12O40 and AlPW12O40 dehydrated at 373 K are depicted. While the 13C MAS NMR spectrum of acetone-loaded H3PW12O40 is dominated by a single signal at 235 ppm (Figure 8a), the spectrum of acetone-loaded AlPW12O40 consists of two signals at 235 and 217 ppm (Figure 8c). According to Yang et al.,27 the signal at 235 ppm is due to acetone molecules adsorbed at acidic protons located near oxygen atoms bound to a single tungsten atom (terminal Od). This assignment was obtained by comparing results of 13 C{31P}REDOR NMR experiments and DFT calculations.27 In contrast to the present study, Yang et al. observed an additional signal at 246 ppm,27 which was assigned to acetone molecules interacting with acidic protons located at oxygen atoms that bridge two tungsten atoms (edge shearing Oc). The data summarized in Table 1 allow a comparison of the chemical shift values δ13C and low-field shifts ∆δ13C of acetone2-13C adsorbed on acidic protons of dehydrated H3PW12O40 and AlPW12O40 with those of acetone-2-13C interacting with acid sites of reference materials. Generally, the shift values δ13C range from 205 ppm for acetone-2-13C in CDCl3 to 250 ppm for acetone-2-13C interacting with SbF5 corresponding to low-field shifts ∆δ13C of 0 and 45 ppm, respectively. The shift value δ13C of 235 ppm and the low-field shift ∆δ13C of 30 ppm obtained for acetone-loaded H3PW12O40 and AlPW12O40 lie in the middle between the values of H-ZSM-5, which is the most acidic zeolite, and 100% H2SO4. This finding supports the results obtained for pyridine-loaded H3PW12O40 and AlPW12O40 upon dehydration at 373 and 523 K and indicates that both these materials have superacidic behavior. The strong 13C MAS NMR signal at 217 ppm occurring in the spectrum of acetone-loaded AlPW12O40 dehydrated at 373

p.w. ) present work.

K hints to the presence of additional acid sites with lower acid strength (Figure 8c). In the case of AlPW12O40, these acid sites are clearly formed by hydroxylated aluminum species. On the other hand, the 13C MAS NMR signal occurring at 222 ppm in the spectra of acetone-loaded H3PW12O40 (Figure 8a) indicates an interaction of acetone with H5O2+ ions having a lower acid strength in comparison with acidic protons. Therefore, both these signals are strongly decreased in the spectra of H3PW12O40 and AlPW12O40 loaded with acetone upon dehydration at 523 K (Figures 8b and 8d). Interestingly, no 13C MAS NMR signal occurred at 242-245 ppm, which would be caused by the interaction of acetone-2-13C molecules with strong Lewis acid sites, such as prepared by reaction of aluminum chloride with conditioned silica gel and subsequent dehydration at 473 K.29 4. Conclusions The thermal stability of the aluminumdodecatungstophosphate AlPW12O40 in comparison with the dodecatungstophosphoric acid H3PW12O40 and the formation and dehydroxylation of Brønsted acid sites on these materials were investigated by X-ray diffraction and solid state NMR spectroscopy. While dehydration of H3PW12O40 at 473 K already induced shrinkage of the unit cell, no significant structural changes could be observed in the X-ray patterns of AlPW12O40 as a result of dehydration up to 673 K. Similarly, the 31P MAS NMR spectrum of H3PW12O40, which was dehydrated at 673 K, indicated a significant modification of the local structure, while no changes in the local structure were found for AlPW12O40. This is a clear demonstration of a higher thermal stability of AlPW12O40 in comparison with H3PW12O40 at temperatures up to 673 K. Already upon dehydration of H3PW12O40 at the low temperature of 373 K, a high number of acidic protons exist causing a strong 1H MAS NMR signal at 9.1 ppm. In the case of AlPW12O40, acidic protons are formed due to the dehydration at moderate temperatures (373-423 K) by dissociation of water molecules at the multivalent aluminum cations. In addition to the acidic protons occurring in the 1H MAS NMR spectrum at 9.1 ppm, AlOH groups causing 1H MAS NMR signals at 4.2 and 5.6 ppm are formed on dehydrated AlPW12O40. Upon dehydration at temperatures higher than 423 K, the hydroxyl groups (acidic protons and AlOH groups) of AlPW12O40 dehydroxylate again and nonhydroxylated Al3+ cations are formed. These nonhydroxylated Al3+ cations compensate the negative charges of the dehydrated Keggin units and stabilize the structure of AlPW12O40 at higher thermal treatments. Adsorption of deuterated pyridine as probe molecule indicates that most of the acidic protons of H3PW12O40 and AlPW12O40

19476 J. Phys. Chem. C, Vol. 112, No. 49, 2008 dehydrated at 373 K are accessible for molecules with sizes comparable to the molecular diameter of pyridine. Upon dehydration at 523 K, however, less than 30% of the acidic protons are accessible for these molecules. The accessibility of acidic protons was found to be similar for dehydrated H3PW12O40 and AlPW12O40. Interaction of pyridine with the acidic protons of dehydrated H3PW12O40 and AlPW12O40 results in the protonation of the probe molecule and an 1H MAS NMR signal of pyridinium ions at 13 ppm. This chemical shift of C5D5NH+ ions corresponds to a proton affinity of the solids acids of ca. 1108 kJ/mol, which is about 209 kJ/mol lower than the proton affinity of Brønsted acid sites in H-form zeolites. Upon adsorption of acetone-213 C, a 13C MAS NMR signal at 235 ppm occurs in the spectra of dehydrated and acetone-loaded H3PW12O40 as well as AlPW12O40. This resonance shift corresponds to an acid strength of the acidic protons of dehydrated H3PW12O40 as well as AlPW12O40 between those of H-ZSM-5 and 100% H2SO4. The adsorption of both pyridine and acetone as probe molecules indicates that dehydrated AlPW12O40 is as superacidic as dehydrated H3PW12O40. No hint was found for the presence of Lewis acid sites on AlPW12O40 dehydrated at 373 and 523 K. Acknowledgment. Financial support by Deutsche Forschungsgemeinschaft, Volkswagenstiftung Hannover, and Fonds der Chemischen Industrie is gratefully acknowledged. The work was supported (in part) by the European project TOK-CATA (Brussels). References and Notes (1) Kozhevnikov, I. V. Chem. ReV. 1998, 98, 171–198. (2) Marosi, L.; Otero Arean, C. J. Catal. 2003, 213, 235–240. (3) Sultan, M.; Paul, S.; Fournier, M.; Vanhove, D. Appl. Catal., A 2004, 259, 141–152. (4) Uchida, S.; Inumaru, K.; Misono, M. J. Phys. Chem. B 2000, 104, 8108–8115. (5) Brown, G. M.; Noe-Spirlet, M.-R.; Busing, W. R.; Levy, H. A. Acta Crystallogr. 1977, B33, 1038–1046. (6) Baba, T.; Ono, Y. J. Phys. Chem. 1996, 100, 9064–9067.

Filek et al. (7) Kamiya, Y.; Okuhara, T.; Misono, M.; Miyaji, A.; Tsuji, K.; Nakajo, T. Catal. SurV. Asia 2008, 12, 101–113. (8) (a) Sulikowski, B.; Haber, J.; Kubacka, A.; Pamin, K.; Olejniczak, Z.; Ptaszynski, J. Catal. Lett. 1996, 39, 27–31. (b) Olejniczak, Z.; Sulikowski, B.; Kubacka, A.; Gasior, M. Top. Catal. 2000, 11/12, 391– 400. (9) (a) Firouzabadi, H.; Iranpoor, N.; Nowrouzi, F. Tetrahedron 2004, 60, 10843–10850. (b) Firouzabadi, H.; Iranpoor, N.; Ali Jafari, A.; Makarem, S. J. Mol. Catal. A: Chem. 2006, 250, 237–242. (10) Tsigdinos, G. A. Top. Curr. Chem. 1978, 75, 1–64. (11) Eguchi, K.; Yamazoe, N.; Seiyama, T. Nippon Kagaku Kaishi 1981, 336, 342. (12) Baba, T.; Sakai, J.; Ono, Y. Bull. Chem. Soc. Jpn. 1982, 55, 2657– 2658. (13) Freude, D. Chem. Phys. Lett. 1995, 235, 69–75. (14) Wolf, I.; Freude, D. Microporous Mater. 1995, 5, 69–75. (15) Miocˇ, U. B.; Todorovic´, M. R.; Davidovic´, M.; Coloban, Ph.; Holclajtner-Antunovic´, I. Solid State Ionics 2005, 176, 3005–3017. (16) Miocˇ, U. B.; Dimitrijevic´, R. Z´; Davidovic´, M.; Nevic´, Z. P.; Coloban, Ph. J. Mater. Sci. 1994, 29, 3705–3717. (17) Essayem, N.; Tong, Y. Y.; Jobic, H.; Vedrine, J. C. Appl. Catal., A 2000, 194-195, 109–122. (18) Baba, T.; Ono, Y. Appl. Catal., A 1999, 181, 227–238. (19) Ganapathy, S.; Fournier, M.; Paul, J. P.; Delevyoe, L.; Guelton, M.; Amoureux, J. P. J. Am. Chem. Soc. 2002, 124, 7821–7828. (20) Hunger, M.; Freude, D.; Pfeifer, H. J. Chem. Soc., Faraday Trans. 1 1991, 87, 657–662. (21) Huang, J.; Jiang, Y.; Reddy Marthala, V. R.; Thomas, B.; Romanova, E.; Hunger, M. J. Phys. Chem. C 2008, 112, 3811–3818. (22) Baba, T.; Nomura, M.; Ono, Y.; Ohno, Y.-I. J. Phys. Chem. 1993, 97, 12888–12893. (23) Hunger, M. Catal. ReV.sSci. Eng. 1997, 39, 345–393. (24) Zheng, A.; Zhang, H.; Chen, L.; Yue, Y.; Ye, C.; Deng, F. J. Phys. Chem. B 2007, 111, 3085–3089. (25) Hunger, M. NMR Spectroscopy for the Characterization of Surface Acidity and Basicity. In Handbook of Heterogeneous Catalysis, 2nd ed.; Ertl, G., Knoezinger, K., Schueth, F., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 2008; Vol. 2, Chapter 3.2.4.4, pp 1163-1178. (26) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. W.; Ferguson, D. B. Acc. Chem. Res. 1996, 29, 259–267. (27) 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– 18280. (28) Xu, M.; Wang, W.; Seiler, M.; Buchholz, A.; Hunger, M. J. Phys. Chem. B 2002, 106, 3202–3208. (29) Xu, T.; Kob, N.; Drago, R. S.; Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 1997, 119, 12231–12239.

JP807947V