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H2O Interaction with Solid H3PW12O40: An IR Study C. Paze´, S. Bordiga, and A. Zecchina* Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita` di Torino, Via P. Giuria 7, I-10125, Torino, Italy Received March 31, 2000. In Final Form: June 22, 2000 The interactions of H2O with solid H3PW12O40 have been studied by IR spectroscopy, and the results are compared with those obtained for H2O interacting with other acidic or superacidic solid systems (acidic zeolites and the perfluorosulfonic membrane H-Nafion). In fully dehydrated HPW a heterogeneous family of hydroxyls is evidenced which, by interaction with water, leads to the formation of neutral 1:1 H-bond adducts. At higher water pressures, the formation of symmetric diaquahydrogen ions, and of clusters of protonated water is also observed. The H2O‚‚‚H+‚‚‚OH2 symmetrical adducts are characterized by a very broad absorption in the 1400-700 cm-1 range, which, by comparison with theoretical studies on gas-phase clusters and with experimental results obtained on H5O2+ salts, is assigned to the νO‚‚‚H+‚‚‚O mode. On the contrary, the IR spectra of fully dehydrated HPW do not show any absorption in this range.
Introduction H3PW12O40 (HPW) is one of the strongest Keggin-type heteropolyacids and is considered to have strong Brønsted acidity at the border between acids and superacids.1-6 Its primary structure is characterized by units (“Keggin”) where a central P atom in tetrahedral coordination is surrounded by 12 edge-sharing metal-oxygen octahedra (WO6).7 The negative charge of this structure is neutralized in the acidic form by protons, which in the hydrated forms are variably solvated by water. In particular, the most structurally defined hydrated form contains 6 H2O molecules per Keggin unit (KU); the KU are interconnected by ‚‚‚H2O‚‚‚H+‚‚‚OH2‚‚‚ groups.8 Upon dehydration at temperatures lower than 593 K, the H5O2+ species are destroyed by loss of H2O: however, the Keggin structure is still retained although with modification of the cell parameters (29 H2O: cubic Fd3m;7 21 H2O: orthorombic Pcca;9 14 H2O: triclinic P11; 6 H2O: cubic Pn3m8). Two kinds of Brønsted acid protons can be present in solid HPW: (i) hydrated protons in protonated water clusters H(H2O)n+ (hydrated forms), and (ii) non-hydrated protons localized at the peripheral oxygens of the Keggin units as hydroxyl groups (dehydrated forms). The first family of protons is characterized by high mobility, so that the proton conductivity of HPW is comparable to that of water.10,11 The presence of the first family of protons has been observed by neutron diffraction experiments performed at different hydration levels (219 and 68 H2O per KU). However, the precise localization by neutron diffraction of the protons bonded to the KU is still uncertain. This is probably due to the difficult experi* To whom correspondence should be addressed. Tel: +39011 6707537. Fax: +39011 6707855. E-mail:
[email protected]. (1) Corma, A. Chem. Rev. 1995, 95, 559. (2) Jansen, R. J. J.; Veldhuizen, H. M. v.; Schwegler, M. A.; Bekkum, H. v. Recl. Trav. Chem. Pays-Bas 1994, 113, 115. (3) Drago, R. S.; Dias, J. A.; Maier, T. O. J. Am. Chem. Soc. 1997, 119, 7702. (4) Kozhevnikov, I. V. Catal. Rev.sSci. Eng. 1995, 37, 311. (5) Misono, M.; Okuhara, T. Chemtech 1993, Nov, 23. (6) Misono, M. Catal. Rev.sSci. Eng. 1987, 29, 269. (7) Keggin, J. F. Proc. R. Soc. A 1934, 75, 144. (8) Brown, G. M.; Noe-Spirlet, M.-R.; Busing, W. R.; Levy, H. A. Acta Crystallogr. 1977, B33, 1038. (9) Spirlet, M.-R.; Busing, W. R. Acta Crystallogr. 1978, B34, 907. (10) Dias, J. A.; Osegovic, J. P.; Drago, R. S. J. Catal. 1999, 183, 83. (11) Kozhevnikov, I. V. Russ. Chem. Rev. 1987, 56, 811.
mental procedure which should be adopted to carefully control the hydration conditions of the sample for the long time needed for neutron diffraction studies (which is a necessary prerequisite to obtain significant results). A study of the acid strength and of the heterogeneity of hydroxyls of HPW therefore still represents an interesting subject of investigation. The problem of location of protons in the fully dehydrated form of HPW has been faced by several authors, and it is widely accepted that the hydrogen atoms are localized on the oxygens linked to W atoms as hydroxyl groups. However, the precise hydroxyl location is still the object of debate, since two contrasting hypothesis have been advanced: (1) protons are located on terminal oxygens, or (2) protons are located on the W-O-W bridges, as in the aqueous solution. The first hypothesis is supported by 17O NMR results,11,12 while the second finds support in IR spectra and extended Hu¨ckel MO calculations.13,14 On the bases of NMR results, Kozhevnikov11,12 proposed that the proton migrates between four equivalent WdO‚‚‚H+‚‚‚OdW positions, and is thus linking four Keggin unit together, as does the H5O2+ ion in the partially hydrated H3PW12O40‚6H2O form. Quite recently, calculations based on density functional theory (DFT) have been made to elucidate the problem of proton location,15 and it is concluded that the most favorable sites of the acidic protons are bridging oxygen atoms. Apart from the problem of proton location, the other great problem concerning the HPW is the heterogeneity (if any) of acid hydroxyls, and their acid strengths. A lot of techniques (microcalorimetry,3,10,15-20 catalytic probe reactions,4,11,15,19-22 NMR,10,12,18,23-27 UV-vis,28 XRD,10,16,17 (12) Kozhevnikov, I. V.; Sinnema, A.; Bekkum, H. v. Catal. Lett. 1995, 34, 213. (13) Lee, K. W.; Mizuno, N.; Okuhara, T.; Misono, M. Bull. Chem. Soc. Jpn. 1989, 62, 1731. (14) Moffat, J. B. J. Mol. Catal. 1984, 26, 385. (15) Bardin, B. B.; Bordawekar, S. V.; Neurock, M.; Davis, R. J. J. Phys. Chem. B 1998, 102, 10817. (16) Hodnett, B. K.; Moffat, J. B. J. Catal. 1984, 88, 253. (17) Lefebvre, F.; Liu-Cai, F. X.; Auroux, A. J. Mater. Chem. 1994, 4, 125. (18) Jozefowicz, L. C.; Karge, H. G.; Vasilyeva, E.; Moffat, J. B. Microp. Mater. 1993, 1, 313. (19) Hu, C.; Hashimoto, M.; Okuhara, T.; Misono, M. J. Catal. 1993, 143, 437. (20) Okuhara, T.; Kasai, A.; Hayakawa, N.; Yoneda, Y.; Misono, M. J. Catal. 1983, 83, 121. (21) Cavani, F. Catal. Today 1998, 41, 73.
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IR,13,29-32 Raman,29,31 inelastic neutron scattering,29,33 and photoacustic34 spectroscopies) have been used until now to tackle this problem on HPW samples at different hydration levels. As far as IR spectroscopy is concerned, it is most interesting to remark that while a great number of data showing the low-frequency region of the Keggin skeletal absorptions (below 1200 cm-1) have been published, surprisingly no data are available for ν(OH) vibrational modes, which are directly influenced by interaction with H2O and subsequent hydrogen bonding (except ref 29). This is quite unexpected, because it is well-known that IR spectroscopy is the most powerful tool to investigate systems where hydrogen bonds are present. It is a matter of fact that the acid strength of hydroxyls (OH) can be estimated by measuring the perturbation induced on the ν(OH) by complexation with proper bases B, because the shift of frequency of the νOH induced by the formation of the OH‚‚‚B adducts is roughly proportional to the enthalpy of the reaction OH + B ) OH‚‚‚ B.35,36 To this end, many different B probes have been used, either binuclear (N2, CO) and polynuclear (ethers, alcohols, nitriles), ranging in intervals of proton affinity (i.e., of basicity) comprised between 118 (N2) and 196 (tetrahydrofuran) kcal/mol.37 In the simplest cases, i.e., when OH interacts with B through a weak H-bond, the ν(OH) stretching undergoes a negative shift ∆νj, accompanied by a proportional increase of integrated intensity and of width. When the strength of the H-bond is medium strong, the ν(OH) becomes very broad and is modulated by Fermi resonance effects with low-frequency modes; the most immediate consequence of this phenomenon is that more than one band associated with a single ν(OH) mode is observable. Under these circumstances, the interpretation of IR spectra can become quite troublesome. Because of these reasons, the spectra of H2O (proton affinity ) 166.5 kcal/mol) interacting with acidic zeolites like H-ZSM-5 or H-Mordenite have been almost fully understood, after long debates, only in these last few years.38,39 In our opinion, these difficulties explain why almost no IR data about the OH modes of HPW (which is a strong acid) are reported and discussed in the chemical literature. The aim of this paper is to investigate by IR (22) Okuhara, T.; Mizuno, N.; Misono, M. Adv. Catal. 1996, 41, 113. (23) Kozhevnikov, I. V.; Sinnema, A.; Jansen, R. J. J.; Bekkum, H. v. Catal. Lett. 1994, 27, 187. (24) Chidichimo, G.; Golemme, A.; Imbardelli, D.; Iannibello, A. J. Chem. Soc., Faraday Trans. 1992, 88, 483. (25) Chidichimo, G.; Golemme, A.; Imbardelli, D.; Santoro, E. J. Phys. Chem. 1990, 94, 6826. (26) Lee, K. Y.; Arai, T.; Nakata, S.; Asaoka, S.; Okuhara, T.; Misono, M. J. Am. Chem. Soc. 1992, 114, 2836. (27) Ghanbari-Siahkali, A.; Philippou, A.; Dwyer, J.; Anderson, M. W. Appl. Catal. 2000, 192, 57. (28) Varga, G. M.; Papaconstantiou, E.; Pope, M. T. Inorg. Chem. 1970, 9, 667. (29) Kearley, G. J.; White, R. P.; Forano, C.; Slade, R. C. T. Spectrochim. Acta A 1990, 46, 419. (30) Fournier, M.; Thouvenot, R.; Rocchiccioli-Deltcheff, C. J. Chem. Soc., Faraday Trans 1991, 87, 349. (31) Rocchiccioli-Deltcheff, C.; Fournier, M.; Franck, R.; Thouvenot, R. Inorg. Chem. 1983, 22, 207. (32) Southward, B. W. L.; Vaughan, J. S.; O’Connor, C. T. J. Catal. 1995, 153, 293. (33) Kearley, G. J.; Pressman, H. A.; Slade, R. C. T. J. Chem. Soc., Chem. Commun. 1986, 1801. (34) Highfield, J. G.; Moffat, J. B. J. Catal. 1984, 88, 177. (35) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman and Co.: San Francisco, 1960. (36) Schuster, P.; Zundel, G.; Sandorfy, C. The Hydrogen Bond; NorthHolland Publishing Co.: Amsterdam, 1976. (37) Paze´, C.; Bordiga, S.; Lamberti, C.; Salvalaggio, M.; Zecchina, A.; Bellussi, G. J. Phys. Chem. B 1997, 101, 4740. (38) Wakabayashi, F.; Kondo, J. N.; Domen, K.; Hirose, C. J. Phys. Chem. 1996, 100, 1442. (39) Zecchina, A.; Geobaldo, F.; Spoto, G.; Bordiga, S.; Ricchiardi, G.; Buzzoni, R.; Petrini, G. J. Phys. Chem. 1996, 100, 16584.
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Figure 1. Temperature programmed desorption of HPW. Helium flow ) 20 mL/min. Pretreatment at RT, 1 h.
spectroscopy the water arrangement in the secondary structure of HPW at different hydration levels, and to compare the results with those obtained on other acid systems, like protonic zeolites and the superacid perfluorosulfonic Nafion membrane (in which the vibrational modes of undissociated SO3H groups are well distinguishable from those of dissociated SO3- anion).40 Experimental Section 12-Tungstophosphoric heteropolyacid, H3PW12O40‚nH2O, was a commercial sample supplied by Strem Chemicals Inc. H3PW12O40 at different hydration levels have been obtained by degassing in ultrahigh vacuum at room temperature (RT) and at increasing temperatures. The starting thin film of heteropolyacid was obtained by dissolving proper amounts of H3PW12O40‚nH2O in the powder form in distilled water, by depositing a portion of the solution on a IR transparent silicon plate and by drying the liquid film at RT. This procedure allows to obtain very thin films of HPW. The silicon wafer was then transferred in a IR cell where both degassing at increasing temperatures and in situ water vapor dosage could be made. The IR spectra were recorded, at 2 cm-1 resolution, on a Bruker IFS66 FTIR spectrometer equipped with a MCT cryogenic detector. TPD results were obtained with a Thermoquest TPD/ R/O 1100 (TCD detector) equipped with Baltzer QUADSTAR 422 quadrupole mass spectrometer. The sample was in powder form. Since it has already been shown16 that the TPD profile of HPW cannot be determined without pretreatment at RT in flowing He (owing to the too large amounts of physisorbed water which is desorbing at RT in the helium flow with subsequent saturation of the TCD detector) the measurement was made after fluxing with He at RT (for 1 h; the speed of the helium flow was 20 mL/min). Results and Discussion 1. TPD Results. The TPD profile is shown in Figure 1. Three main groups of bands can be observed: (i) an absorption at temperature lower than 373 K (centered at 343 K), which is due to physisorbed molecular water; (ii) a peak centered at about 438 K, with an unresolved shoulder on the high-temperature side (about 473 K); (iii) a very broad asymmetrical band between 573 and 773 K, (40) Buzzoni, R.; Bordiga, S.; Ricchiardi, G.; Spoto, G.; Zecchina, A. J. Phys. Chem. 1995, 99, 11937.
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and a peak at 818 K. The weak intensity of the first peak is due to the pretreatment procedure in flowing helium, which has removed most of the weakly adsorbed H2O. The second peak is due, as shown by Hodnett et al.,16 to the loss of 6-7 H2O molecules per Keggin unit. The remaining peaks in the 573-823 K interval are probably due to dehydration rearrangements of the Keggin unit, like for example16 Chart 1
H3PW12O40 h PW12O38.5 + 1.5H2O (mass spectrometric analysis indicate in fact that all the peaks were due entirely to water). The peak at 438 K (corresponding to the loss of 6-7 H2O molecules) is due to the breaking of the high symmetric structures H2O‚‚‚H+‚‚‚OH2 which bond four KU. The asymmetric shape of the peak gives information about the breaking reaction of this adduct, which is likely a two-step process (in fact at least two overlapped components are present, presumably associated with water dimers and monomers). Due to the overlapping, the area and the shape of the two components cannot be easily obtained; nevertheless, a higher width can be inferred for the high-temperature component (likely associated with the elimination of the last water molecule directly interacting with the KU). The higher width of the monomers component with respect to that of the dimers (which are all equivalent in geometry and location) likely means that the Brønsted sites of the dehydrated form are not all equivalent, while the strong overlapping reveals that diffusion and interconversion of the species are playing a role in the system during dehydration. In relation to this, it should be noticed that data deriving from thermal analysis performed with flow systems are much influenced by parameters like the He flow speed and ∆temperature/time, so that when these parameters change, the maximum of the associated peaks is also influenced. Finally, it is worth underlining that the desorption temperatures obtained with flow systems are not directly comparable with those obtained in vacuum (although from a qualitative point of view, the results are absolutely parallel). This difference must be kept in mind when comparing TPD results and IR results obtained in vacuum (next paragraph). In general, it can be stated that for the same temperature the vacuum treatment of thin films has a stronger dehydration effect than the flow treatment on powders, and consequently that the desorption temperatures in vacuum experiments are remarkably lower than the corresponding ones in flowing He. In conclusion of this paragraph, it is suggested that five different levels of hydration are possible for HPW: (i) full hydration in the presence of H2O vapor (presence of H2n+1On+, H2O/Brønsted sites (n) > 2); (ii) region of H5O2+ (n ) 2) which is obtained by removal of physisorbed molecular water by prolonged flowing in He at RT; (iii) region of 0 e n e 2, corresponding to the removal of the water molecules which determine the secondary structure (this is likely a two-step process); (iv) region of progressive structural dehydration of the KU (following the reaction path shown in Chart 1). 2. Evolution of the IR Spectra of HPW during Dehydration at Increasing Temperatures. In Figure 2, the evolution of the IR spectra of HPW upon heating in vacuum at increasing temperature (between RT and 723 K) is shown. For the sake of clarity, two main regions of frequencies (1150-650 and 3800-1200 cm-1) are discussed separately:
Figure 2. IR spectra of H3PW12O40 at different hydration levels. Lines: (1) full hydrated sample; (2) after short degassing at RT (3 min); (3) after long degassing at RT (150 min); (4) after heating in vacuum at 433 K; (5) after heating in vacuum at 523 K; (6) after heating in vacuum at 623 K; (7) after heating in vacuum at 723 K.
1150-650 cm-1. In this region the absorptions associated with metal-oxygen skeletal modes of the Keggin unit are mainly present. Several distinct components are present in this region at 1090-1060 cm-1 (νas(P-O)), 1010-930 cm-1 (ν(WdO)), 900-870 and 850-700 cm-1 (ν(W-O-W)) (the most common assignments reported in the literature are reported in parentheses).13,32 Of course it must be kept in mind that the assignment of each absorption to a vibration localized only on a specific bond or a specific family of bonds is an abstraction, each mode being due to collective vibrations of the complex Keggin structure. 3800-1200 cm-1. In this region the absorptions associated with OH modes (stretching ν and bending δ) of acidic hydroxyls and of water molecules are present. As the temperature increases, the spectra are progressively modified. For the sake of clarity we will discuss the spectra corresponding to increasing dehydration states in succession. (i) Fully Hydrated HPW (Virgin Sample: n . 2). The spectrum of the fully hydrated HPW sample in equilibrium with H2O vapor present in the atmosphere (Figure 2, line 1) is characterized by a broad asymmetric absorption centered at 3510 cm-1, associated with both neutral and protonated water species (νOH modes), and by two bands at 1720 and 1620 cm-1 (δ(OH) modes). More specifically, the bands at 3510 and 1620 cm-1 are mainly due to neutral H2O in the outer solvation sphere (see Chart 2), while the band at 1720 cm-1 is the bending mode of H2O in the H5O2+ groups (which could be either solvated H3O+ groups or symmetric H-bond adducts). This spectrum strongly resembles those of concentrated aqueous solutions of strong acids (i.e., triflic acid, H2SO4, HCl) and of films of hydrated H-Nafion.40 In the region of metal-oxygen skeletal modes of the Keggin unit, bands are present at 1074, 974, 903, and 770 cm-1. (ii) H5O2+ in HPW As Obtained after Short Degassing at Room Temperature (n = 2). After short degassing at RT
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Chart 2
Figure 3. IR spectra of interaction of increasing doses of H2O on HPW dehydrated at 433 K: (1) maximum pressure ) 20 Pa. Spectra in the inset correspond to lines 2-5 from which line 1 has been subtracted.
(Figure 2, line 2) the two broad absorptions corresponding to H2O in the outer sphere of H5O2+ disappear, leaving two main bands at 3180 and 1710 cm-1, which are due to ν and δ modes of external OH groups of H5O2+. An absorption increment in the 1200-1100 cm-1 is also observed, which was less evident in the virgin sample. We anticipate that the observed spectrum is due to ν(O‚‚‚H‚‚‚O) of H5O2+. Notice that this spectrum is different from that of the hydrated sample and also from that of pure water, and that a certain resemblance is only found with that of spectra of strong acids in very concentrated aqueous solutions (i.e., H2SO4 (96%)).40 This undoubtedly means that the fraction of water molecules directly interacting with acidic protons is very high. As far as the region of metal-oxygen skeletal modes of the Keggin unit is concerned, bands are present at 1079, 982, 886, and 793 cm-1. (iii) HPW after Prolonged Degassing at Room Temperature (More Than 2 h). After this treatment, the absorptions in the region of metal-oxygen skeletal modes of the Keggin unit are not greatly modified (vide supra). On the contrary, the region of hydroxyls modes is completely changed (Figure 2, line 3): in fact the typical modes of water in acid media have disappeared, while a very broad flat band is now observable between 3400 and 1300 cm-1. At about 3400 and 1638 cm-1 also weak bands are superimposed. (iv) HPW after Heating in Vacuum at 433 and 523 K. Another huge modification is observable in the ν(OH) region after heating in vacuum at 433 K (Figure 2, line 4), i.e., at the temperature which Drago et al.3 consider as the optimal one to produce the anhydrous acid. A distinct modification of the bands due to structural Keggin vibrations is also evident: in fact the bands at 1080 cm-1 (νas(P-O)) and at 982 cm-1 (ν(WdO)) are now splitted in two components at 1085 and 1064 cm-1, and at 1006 and 975 cm-1 respectively. Meanwhile the band due to ν(WO-W) mode is shifted from 886 to 897 cm-1. As for the region of ν(OH) modes, while a residual weak and broad band is still observable between 3400 and 1300 cm-1, a new absorption is now clearly showing up between
3500 and 2750 cm-1. As the presence of water molecules can be excluded after this treatment (since almost no band due to water δ(OH) bending mode at about 1640-1700 cm-1 is present), the two broad absorptions are assigned mainly to two families of hydrogen bond OH groups bridging the KU. Essentially similar features characterize the spectrum of the sample heated at 523 K (Figure 2, line 5). (v) HPW after Heating in Vacuum at 623 and 723 K. Evacuation at 623 and at 723 K (Figure 2, lines 6, 7), leads to complete disappearance of the bands associated with OH groups: this testifies that the total removal of the hydroxyls (either belonging to water, or to acidic protons of the KU) has occurred. The elimination of the OH groups to form water molecules and modified neutral Keggin units (i.e., Chart 1) at 673 K is confirmed by the slight broadening and loss of intensity of powder XRD reflections.3 The apparent and broad absorption in the interval 3500-1300 cm-1 in spectrum 7 with apparent maximum at about 2000 cm-1, remaining after full dehydration, is presumably due to electronic absorptions in a non stoichiometric oxide (as observed for example on SnO2).41 3. Rehydration Process: IR Results. To fully prove the hypothesis made in the previous paragraph, the evolution of the IR spectra upon H2O readsorption have been studied (Figures 3-5). For the sake of clarity, the IR spectra have been divided in two groups corresponding to low and intermediate equilibrium pressures. 3.1. Low H2O Dosages (0 < p
