Water in Interaction with Acid Sites in H-ZSM-5 Zeolite Does Not Form

Dec 12, 1996 - Inelastic neutron scattering (INS) has been used to study the adsorption of water, at different concentrations, in H-ZSM-5. INS is the ...
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J. Phys. Chem. 1996, 100, 19545-19550

19545

Water in Interaction with Acid Sites in H-ZSM-5 Zeolite Does Not Form Hydroxonium Ions. A Comparison between Neutron Scattering Results and ab Initio Calculations Herve´ Jobic,*,† Alain Tuel,† Mariann Krossner,‡ and Joachim Sauer‡ Institut de Recherches sur la Catalyse, CNRS, 2 AVenue Albert Einstein, 69626 Villeurbanne, France, and Max Plank Society, Research Unit “Quantum Chemistry”, Humbolt UniVersity Berlin, Ja¨ gerstrasse 10/11, 10117 Berlin, Germany ReceiVed: July 8, 1996X

Inelastic neutron scattering (INS) has been used to study the adsorption of water, at different concentrations, in H-ZSM-5. INS is the only vibrational technique where the intensities can be calculated with reasonable accuracy from atomic displacements. This feature is used here to simulate the INS spectra of the two possible structures resulting from water interaction with the Bro¨nsted acid sites of the zeolite: hydrogen-bonded water or hydroxonium ion. The atomic displacements for the two structures are derived from recent ab initio MP2 calculations (Krossner, M.; Sauer, J. J. Phys. Chem. 1996, 100, 6199-6211). The comparison between experimental and calculated INS spectra confirms that the first water molecule is attached to the acid site via two hydrogen bonds, in agreement with the conclusion made by Krossner and Sauer. Hydroxonium ions are not found in H-ZSM-5; however, this protonated species might be present in zeolites with a different structure.

1. Introduction Zeolites in the proton form are extensively used in acid catalysis. The active sites are formed by the bridging hydroxyl groups (Bro¨nsted sites) as depicted in Figure 1. A question which is much debated at the moment is whether the Bro¨nsted acidity of these solids (aluminosilicates or aluminophosphates) is high enough to protonate simple molecules such as water, methanol, or acetonitrile. It is well admitted that water molecules interact with the bridging OH groups, but the nature of the resulting species is still a controversial subject. Two possible structures have been envisaged: an hydrogen-bonded water molecule or a protonated molecule, H3O+, the hydroxonium ion. Most of the experimental results have been obtained by infrared or NMR spectroscopies, but the interpretation of the results is complicated. In NMR, rapid exchange between molecules in different adsorption states can take place at room temperature.1 Thus, Hunger et al. analyzed quantitatively their 1H MAS NMR results by a fast proton exchange between water molecules, bridging OH groups and hydroxonium ions.2 To suppress thermal motions and exchange processes, Dore´mieux-Morin et al. measured the 1H NMR spectra at 4 K, decomposing the broad signals into several contributions: free OH groups, isolated water, hydrogen-bonded water, and hydroxonium ions.3 The early infrared results of Jentys et al. in H-ZSM-5 were interpreted as an indication of the presence of hydroxonium species, at low pressure.4 In another infrared study5 the presence of hydroxonium ions was not supported, but partial proton transfer was proposed. In H-SAPO-34, the first assignment of the infrared spectra by Marchese et al. was strongly in favor of H3O+6, but other data obtained in CoAPO-18 were interpreted in terms of a hydrogen-bonded and/or protonated water molecule.7 It therefore appears that, like in NMR, the interpretation of the infrared spectra is complicated. Pelmenschikov et al. have shown that there are resonant interactions in the stretching OH region with overtones of OH deformations.8,9 By using 18O * Author to whom correspondence should be addressed. † CNRS. ‡ Humbolt University Berlin. X Abstract published in AdVance ACS Abstracts, November 1, 1996.

S0022-3654(96)01995-8 CCC: $12.00

Figure 1. Structural bridged hydroxyl groups.

labeled water, Wakabayashi et al. were recently able to make unequivocal assignments of the infrared bands due to either the zeolitic OH or the adsorbed water molecule, and they concluded that neutral water molecules were present.10 The assignment of the infrared spectra obtained with H-SAPO-34 was recently reconsidered,11 and a coexistence of the protonated and hydrogenbonded water was proposed. The presence of both species was also found by powder neutron diffraction.11 Recent ab initio calculations performed on small clusters indicate that only the hydrogen-bonded structure is a minimum whereas the hydroxonium species is a transition structure for proton transfer.12 However, the energy difference between the two structures is small, a few kJ/mol. This difference in energy could become even smaller when the electrostatic potential of the infinite 3-D lattice is introduced,13 although recent embedded cluster calculations14 show that this effect is small. It has also been found that the structure of the zeolite, through the size of the cages and of the channel apertures, had an effect on the energy of proton transfer.15 Therefore the situation is still confused and more comparisons between experimental data and theoretical calculations are needed. We report here new experimental results obtained on water adsorbed in H-ZSM-5, using inelastic neutron scattering (INS). This technique is particularly sensitive to vibrational modes involving hydrogen motions because of the large incoherent cross section and the low mass of the proton.16 Furthermore, vibrational frequencies below 1600 cm-1 are easier to observe with INS compared with infrared. Another important feature is that the INS intensities are directly related to atomic displacements which can be computed from empirical force fields or from ab initio quantum chemical methods. Excellent © 1996 American Chemical Society

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agreement has been reported between experimental INS spectra and simulated spectra using theoretical frequencies and atomic displacements as imputs.17 Since atomic displacements have been recently calculated for the two possible water structures by Krossner and Sauer,12 INS spectra of water in interaction with an acid site can be simulated and compared with the experimental data. A previous INS study of water adsorbed at low loading in H-mordenite has been reported, and the observed vibrational features were assigned to hydroxonium ions, hydrogen-bonded water, and free hydroxyl groups.18 The presence of hydroxonium ions was derived from the presence of two peaks at 1385 and 1670 cm-1, which were assigned respectively to the symmetric and antisymmetric bending modes of this species. The calculation of the shift of a deformation mode of the bridging OH group to 1317 cm-1 upon formation of the hydrogen-bonded complex,12 and new data obtained at different water loadings in H-ZSM-5 prompted us to test the previous assignment. 2. Experimental Section Zeolite Synthesis and Characterization. The ZSM-5 sample was prepared by mixing a solution containing 18 g of silica (A200, Degussa), 7 g of NaOH, and 90 mL of distilled water to a second solution containing 8.2 g of Al2(SO4)3‚18H2O and 8 g of tetrapropylammonium bromide in 100 mL of distilled water. The mixture was vigorously stirred for 30 min, and a dilute solution of H2SO4 (0.1 M) was added dropwise until the formation of a thick gel (pH ≈ 10.6). This gel was transferred into a Teflon flask and heated in a autoclave under autogeneous pressure for 36 h at 443 K followed by 36 h at 463 K. The solid was removed by centrifugation, washed several times, and dried at 383 K. The zeolite was then calcined in air at 823 K. H-ZSM-5 was obtained by exchanging Na-ZSM-5 with a saturated solution of NH4Cl in water (three exchanges at 353 K for 1 h). Between consecutive exchanges, the solid was abundantly washed with distilled water. It was then calcined in air at 773 K. The Si/Al ratio determined from 29Si NMR, 12.5, is in perfect agreement with the value derived from chemical analysis. The chemical formula of the dehydrated zeolite was found to be H5.7Na1.4Al7.1Si88.9O192. The crystallinity of the material was checked by X-ray diffraction, and no octahedrally coordinated aluminum could be measured by 27Al NMR. Infrared spectroscopy showed a sharp peak at 3610 cm-1 associated with acidic OH groups, a small peak at 3740 cm-1 due to silanol groups, and no signal near 3650 cm-1, where extralattice Al may contribute. Neutron Spectrometer. The neutron experiments were performed on the spectrometer IN1BeF at the Institut LaueLangevin in Grenoble, France. The INS spectra were recorded from 70 to 2000 or 2350 cm-1 using Cu (200), (220), and (331) monochromator planes. A beryllium filter was placed between the sample and the detector.16 This setting gives a moderate energy resolution, the instrumental resolution varying from 25 cm-1, at low energy transfers, to 50 cm-1 at large energy transfers. A better resolution can be obtained on this instrument, using an additional graphite filter, but at the expense of the signal intensity.19 The frequency values given within the figures and in the text have been corrected from a systematic shift due to the beryllium filter. The estimated absolute accuracy is ca. (15 cm-1. After activation at 700 K, the zeolite (15 g) was transferred, inside a glovebox purged with Ar, into a cylindrical vacuumtight aluminum container. The INS spectra were recorded at 4 K, the neutron cell being placed in a helium cryostat. Such a

Figure 2. Inelastic neutron scattering spectra of dehydrated H-ZSM5: (a) experimental; (b) calculated.

Figure 3. Inelastic neutron scattering spectrum of water + H-ZSM-5 (loading θ1: 3.5 molecules per unit cell, on average).

low temperature is necessary to reduce the high dynamical disorder which has been observed for small molecules adsorbed in zeolites.20 Water adsorption was performed at room temperature, out of the cryostat. 3. Results The INS spectrum of the dehydrated zeolite was first recorded. The spectrum is shown in Figure 2a between 70 and 2000 cm-1. The signal from the empty can in the cryostat has been subtracted. The assignment of the main features is in line with previous work.18,21-23 The peaks at 320 and 405 cm-1 are assigned to out-of-plane (γ) deformations of bridged OH groups and the peak at 1080 cm-1 to the in-plane (δ) deformations. Other peaks are found at 155, 575, and 785 cm-1; they correspond to framework modes coupled with proton motions. A quantity of water corresponding to 3.5 molecules per unit cell, on average, was then adsorbed at room temperature (this loading will be called θ1). After sealing the cell, the sample was warmed at 350 K and was left 24 h to equilibrate. The neutron can was put back into the cryostat, and another spectrum was recorded at 4 K. It is shown in Figure 3, after background subtraction (empty can + cryostat). The spectrum obtained for θ1 is quite different from the spectrum of the dehydrated zeolite. The main band is now measured at 427 cm-1, the peak at 1080

Interaction of Water with H-ZSM-5 Acid Sites

J. Phys. Chem., Vol. 100, No. 50, 1996 19547

Figure 4. Inelastic neutron scattering spectra of water adsorbed at low loading in H-ZSM-5 (θ1): (a) Difference spectrum between Figures 3 and 2a; (b) Spectrum simulated for a water molecule hydrogen-bonded to a bridging hydroxyl group; (c) Spectrum simulated for a hydroxonium ion (multiphonon are taken into account in parts b and c).

Figure 5. Inelastic neutron scattering spectra of water adsorbed at increasing loading in H-ZSM-5: (a) θ2, 6.2 molecules per unit cell; (b) θ3, 10.2 molecules per unit cell; (c) θ4, 35 molecules per unit cell, on average.

cm-1 is relatively much smaller, and new bands can be observed at 1380 and 1650 cm-1. All the bridging OH groups are not in interaction with water molecules: the ratio H2O/H+ is 0.61, and the δ(OH) peak is still visible. A difference spectrum (between Figures 3 and 2a) was therefore calculated; it is shown in Figure 4a. This was not possible in the previous study on Hmordenite18 because the experimental conditions had changed between the two recordings, whereas the spectrometer settings and the zeolite quantity are the same in this work. The δ(OH) deformations appear as a negative peak at 1080 cm-1 in Figure 4a, indicating interaction of water with the acid sites. It is more difficult to observe the negative contribution from the γ(OH) deformations because they occur on the low-frequency side of the most intense band (a small negative peak can be found at 320 cm-1). The subtraction of the γ(OH) deformations has however the effect of shifting the main band from 427 to 455 cm-1. Increasing concentrations of water were studied for the same H-ZSM-5 sample. The loadings θ2 , θ3, and θ4 correspond respectively to 6.2, 10.2, and 35 water molecules per unit cell, on average (the high loadings were obtained by condensing the adsorbate onto the zeolite, the cell being left 24 h at 350 K to equilibrate). The spectra are shown in Figure 5 after subtraction of the dehydrated zeolite and of the background. For θ2 and θ3, the spectra were recorded up to 2350 cm-1. The most intense peak is found near 450 cm-1 for intermediate loadings, and it is shifted to 480 cm-1 for θ4. The band at 800 cm-1 is

well separated from the peak near 450 cm-1 for θ2, but it becomes progressively a shoulder as the loading increases; this is due to a broadening of the two bands. The band at 1650 cm-1 does not shift in frequency when going from θ1 to θ4; it appears to be split for θ1, but this is due to poorer statistics in that range. The full width at half-maximum (fwhm) of this band does not change, it is close to 150 cm-1. The peak measured at 1380 cm-1 for θ1 is not observed at higher loadings, and no extra peaks appear in that range. 4. Computation of the INS Spectra For the system under study, the incoherent scattering from hydrogen atoms dominates because of the large incoherent cross section, σH, of hydrogen. The intensity which is measured in the neutron scattering experiment corresponds to the incoherent double-differential cross section, which is given by

( )

σinc d ) ∑ Sinc d (Q,ω) dΩ dE inc k0 d 4π d2σ

k

(1)

The sum on d is over the hydrogen atoms. The neutron momentum transfer, pQ, is defined by Q ) k - k0, where k and k0 are respectively the final and incident wave vectors. The incoherent scattering function, Sinc d (Q,ω), can be calculated for fundamental transitions, for overtones, and for combinations. In general, the fundamentals will be the most intense features

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of the spectra. The incoherent scattering function for a fundamental λ at frequency ωλ can be written in our experimental conditions (pω . kBT):

Sinc d (Q,ω)

) exp(-Q

〈u2d〉)

2

p|Q‚Cλd|2 δ(ω - ωλ) 2ωλ

(2)

The intensity of the δ function is governed by the product p|Q‚ Cλd|2, where the vector Cλd describes the displacement of the dth atom during the λth vibrational mode. The effect of the DebyeWaller factor, exp(-Q2〈u2d〉), is to decrease the intensity as the momentum transfer Q increases (on the spectrometer IN1BeF, Q2 is proportional to the energy transfer). The Debye-Waller factor has a pronounced effect when the mean-square amplitude 〈u2d〉 is large. Small Debye-Waller factors imply not only a reduced intensity for the fundamentals but also a larger contribution from multiphonon. This happens to be the case for the hydrogen atoms of adsorbed water, hence the low temperature for recording the INS spectra: 4 K (one cannot avoid zero-point motions however). In a recent paper,12 the vibrational frequencies of free zeolite models and their 1:1 complexes with water were calculated at the MP2 level. We will use here these ab initio frequencies and atomic displacements to simulate the INS spectra. The frequencies have been scaled by a factor 0.96 to account for both systematic errors of the calculated harmonic force constants and neglected anharmonicity effects. Only the displacements from the hydrogen atoms of the acid site and of the water molecule will be considered in the computation of the INS spectra because of the much smaller scattering cross section of silicon, aluminum, and oxygen. The displacement vectors, in mass-weighted Cartesian coordinates, have the following normalization property for each mode:

∑d (Cλd)2 ) 1

(3)

5. Discussion Dehydrated Zeolite. The out-of-plane and in-plane deformations of dehydrated H-ZSM-5 are found at similar frequencies as for other acidic zeolites. The γ(OH) and δ(OH) deformations were found respectively at 420 and 1080 cm-1 in H-Y21-23 and at 320 and 1060 cm-1 in H-mordenite.18 The γ(OH) deformations are clearly split into two components in H-ZSM-5: 320 and 405 cm-1. At such low energy transfers, the Debye-Waller factor has a small influence so that the two peaks can only correspond to fundamentals. This indicates therefore that there are at least two different bridging OH groups. Only one species is observed with 1H NMR as well as in infrared since there is only one OH stretching mode at 3610 cm-1. Heterogeneity of the OH groups in NaH-ZSM-5 has however been derived from infrared experiments with probe molecules.24 Possible reasons for this heterogeneity were considered: (i) a nonhomogeneous Al distribution and (ii) various T-O bond lengths and T-O-T angles. These two explanations cannot be tested with INS, but neutron diffraction could be used to locate the different acidic OH species. The splitting of the δ(OH) deformations is less pronounced, although a peak can be measured at 1210 cm-1, on the highfrequency side of the large band at 1080 cm-1. This peak is more resolved than in other zeolites,18,21 but its assignment in terms of OH groups situated in different crystallographic positions is not certain. At such energy transfers, larger Q values are involved, so that multiphonon features have a higher contribution. The peak at 1210 cm-1 could result from a

combination between the δ(OH) deformations at 1080 cm-1 and the lattice mode at 155 cm-1. A simulation of the INS spectrum based on the infinite framework and taking into account the different proton sites occupancies would be required to find out the correct explanation. The ab initio calculations were performed on the model H3SiO(H)Al(OH)2OSiH3 , i.e., the model presented in Figure 1, saturated by hydrogen atoms. The spectrum calculated for the dehydrated zeolite is shown in Figure 2b; the peaks corresponding to fundamentals and multiphonon have been broadened so that the width of the bands is comparable to the experimental. Even if the size of the model is limited, the agreement between the calculated and experimental spectra is satisfactory. Since there is only one bridging OH group in the model, there is only one peak at 295 cm-1 corresponding to γ(OH) and one at 1045 cm-1 due to δ(OH). In a given zeolite sample, the local structure and composition will influence the vibrational frequencies of the bridging OH groups, yielding a broad frequency distribution with eventually several maxima, like here for the γ(OH) deformations. This would also broaden the δ(OH) region so that the combination band at 1340 cm-1 would become only a shoulder. The modes of the aluminosilicate framework yield INS peaks at 100, 447, 600, and 690 cm-1 because they involve proton motions. An agreement between the experimental INS spectrum of H-Y and the spectrum calculated from an empirical force field has also been reported.23 Water Adsorption. After the first water loading (θ1), the resulting INS spectrum shown in Figure 3 is similar to the one previously reported in H-mordenite for a ratio H2O/H+ close to 1. In particular, the two bands which were observed at 1385 and 1670 cm-1 18 are measured at 1380 and 1650 cm-1 in H-ZSM-5, indicating a close similarity between the two systems. These two bands were tentatively assigned in H-mordenite to the symmetric (ν2) and antisymmetric (ν4) bending modes of the hydroxonium ion.18 The usual spectral regions for the bending modes of H3O+, collected from various spectroscopic studies,25 are 1050-1150 cm-1 for ν2 and 1670-1705 cm-1 for ν4. In H-mordenite, ν4 fit in that range but ν2 was out of the admitted spectral region. However, in the proton conductor HUO2PO4‚4H2O, ν2 and ν4 were placed at 1268 and 1650 cm-1, respectively.26 Moreover, the torsional vibration of H3O+ in HUP was situated at 497 cm-1, a frequency close to the one measured for an intense peak after water adsorption in Hmordenite: 460 cm-1. These results were the basis for the assignment of some bands observed in H-mordenite to hydroxonium ions.18 If one makes the hypothesis that hydroxonium ions are formed in H-ZSM-5 at low water concentration (θ1), one expects that for a much larger H2O/H+ ratio, 6.1 (θ4), only hydrogen-bonded water is present. However the position and the width of the band situated at 1650 cm-1 does not vary in the INS spectra. Furthermore, the calculated frequency of the deformation mode of the bridging OH groups, δ(OzHz) has been found to shift from 1045 to 1317 cm-1 upon formation of the hydrogenbonded complex.12 Since an INS band was measured at 1380 cm-1 in H-mordenite, Krossner and Sauer proposed to assign it to this perturbed δ(OzHz) mode.12 This leads us to revise our previous assignment using the calculated INS spectra as a support. One could simulate the INS spectra of the two possible water adsorption structures from empirical force fields, but this would involve several approximations. It is more profitable to use the vibrational frequencies and atomic displacements calculated by Krossner and Sauer. For the hydrogen-bonded water molecule, the INS spectrum calculated for the fundamentals,

Interaction of Water with H-ZSM-5 Acid Sites

Figure 6. Inelastic neutron scattering spectrum simulated under high resolution for a water molecule hydrogen-bonded to a bridging hydroxyl group (fundamentals only).

Figure 7. Inelastic neutron scattering spectrum simulated under high resolution for a hydroxonium ion (fundamentals only).

eq 2 but without considering the effect of the Debye-Waller factor, is shown in Figure 6. The peaks were broadened by convolution with the best available instrumental resolution.19,27 This spectrum quality is routinely obtained with molecular crystals (e.g., ref 28) but for zeolitic samples different sites and local composition give broader bands. A high resolution for the simulation of the INS spectra is useful to resolve the large number of vibrational modes resulting from water adsorption. Such a high resolution was not necessary in the case of the dehydrated zeolite (Figure 1b) because of the reduced number of modes. After introducing the effect of the Debye-Waller factor, adding multiphonon contributions, and using a bandwidth comparable to the experimental one, the spectrum shown in Figure 4b is obtained. It can be noted that a better instrumental resolution would not improve the quality of the experimental spectra because the intrinsic width of the bands is larger than the broadening due to the spectrometer. With the other hypothesis, the hydroxonium ion, the INS spectrum simulated under high resolution (Figure 7) is quite different (compare with Figure 6). After broadening fundamentals and multiphonon features (Figure 4c), some structure is lost but there are still major differences with the other simulated spectrum (Figure 4b), in particular in the number of bands between 1300 and 2000 cm-1. It appears that the hydrogen-bonded water model (Figure 4b) is in better agreement with the experimental spectrum shown in Figure 3 than the hydroxonium model (Figure 4c). Since there is a contribution from free hydroxyl groups in Figure 3, another comparison is made between the calculated spectra and

J. Phys. Chem., Vol. 100, No. 50, 1996 19549 the difference spectrum in Figure 4. The peaks due to free OH groups appear as negative contributions; however, it is more clearly visible that the hydrogen-bonded model reproduces better the experimental profile. The splitting (445 cm-1) and the relative intensities of the two major peaks at 455 and 900 cm-1 in the observed spectrum are better reproduced by the hypothesis of hydrogen-bonded water (splitting 476 cm-1) than by the assumption of a hydroxonium ion (splitting 351 cm-1). The ab initio MP2 calculated frequencies are ≈80 cm-1 too low in energy when compared with the experiment, but the assignment of the main INS peaks is straightforward. The band situated at 1650 cm-1 is assigned to the water bending. The peak at 1380 cm-1 corresponds to the perturbed δ(OzHz) mode. The perturbed γ(OzHz) mode is more difficult to localize; it is calculated at 1020 cm-1, and it would then overlap with the δ(OzHz) mode of the free OH groups. However, since all the calculated modes are shifted down in energy, the peak measured at 1170 cm-1 may correspond to this mode. The band peaking at 900 cm-1 corresponds to the out-of-plane deformation of the bound proton γ(OwHb). The largest peak situated at 455 cm-1 is the sum of several contributions (see Figure 6): the out-ofplane deformation of the free water proton, γ(OwHf), at 325 cm-1, and intermolecular modes at 252, 277, 342, and 411 cm-1, which can be described as twisting and rocking modes of water. Translational modes of water are calculated at 66 and 176 cm-1 (Figure 6). In the final calculated spectrum (Figure 4b), only one band is discernible at 62 cm-1, in reasonable agreement with the peak measured at 90 cm-1. For the ratio H2O/H+ of 1.1 (θ2), it was expected that all the bridging OH groups would be in interaction with water so that the vibrational features of the hydrogen-bonded water molecule would be maximized. However, the δ(OzHz) mode at 1380 cm-1 is not visible anymore in the INS spectrum corresponding to this loading (Figure 5a) and the γ(OwHb) band has shifted from 900 to 800 cm-1, a frequency which is measured at high loading for weakly bound water. This shows that a large proportion of bridging OH groups (≈40%) is not accessible to water. They must be directed inside the pentasil chains and not in the 10-membered rings channels. A similar conclusion was reached from infrared studies.5 This inaccessibility of bridging OH groups in H-ZSM-5 implies that θ2 corresponds in fact to a ratio H2O/H+ close to 2. For this loading, the energy range was extended up to 2350 cm-1 in order to be able to observe other peaks which might result from protonated species. The ab initio calculations have shown that the hydroxonium ion is more stable if a second water molecule is added to the acid site.12 To test if new species are formed at high loadings, a simulation of the INS spectrum of a H5O2+ species was performed, based on a DFT calculation. The largest INS peaks were however found to fall in the range 800-100 cm-1, in a region where intense bands are already measured for a neutral complex. At higher energy transfers, the largest peak was found to overlap with the water bending mode. Hence it is not possible to decide whether protonated species are present or not at high loading because the experimental spectra show only broad features. This could be due to the presence of different species but also to the heterogeneity of the OH groups in the sample, which broadens the modes more than the instrumental resolution. The spectrum obtained for the highest loading, θ4, is similar to the one previously reported for ice.18 However, the fwhm of the HOH bending mode is only one-half the value in ice but the librational modes are 30% larger with a band maximum peaked at 480 cm-1 instead of 590 cm-1 for ice. This reflects differences in structure and intermolecular forces between the two systems.

19550 J. Phys. Chem., Vol. 100, No. 50, 1996 6. Conclusion The adsorption of water in H-ZSM-5 has been studied at different loadings using inelastic neutron scattering. A large vibrational range can be covered with this technique, extending down to 70 cm-1 in this study or even to lower frequencies.18 Fundamentals can be observed in this spectral domain, here deformation and torsional motions of the hydroxyl groups and water molecules. A further advantage of this technique is that the INS spectrum can be calculated from the atomic displacements, providing a useful test of the vibrational assignment. Simulations of INS spectra based on ab initio MP2 calculations show that a water molecule is attached to an acid site via two hydrogen bonds. Protonated species are not observed. This conclusion is made for H-ZSM-5 with a small Si/Al ratio: 12.5. A similar conclusion was reached for a higher ratio: 50.10 However hydroxonium ions may be found in zeolites with a different structure, such as H-SAPO-34.11 It would be interesting to follow water adsorption in H-SAPO-34 with INS, to check whether vibrational features characteristic of the hydroxonium ion can be found. Acknowledgment. We thank Dr. B. Roessli for his help in performing the neutron experiments at the Institut LaueLangevin, Grenoble, France. References and Notes (1) Brunner, E. J. Mol. Struct. 1995, 355, 61. (2) Hunger, M.; Freude, D.; Pfeifer, H. J. Chem. Soc., Faraday Trans. 1991, 87, 657. (3) Batamack, P.; Dore´mieux-Morin, C.; Vincent, R.; Fraissard, J. J. Phys. Chem. 1993, 97, 9779. (4) Jentys, A.; Warecka, G.; Derewinski, M.; Lercher, J. A. J. Phys. Chem. 1989, 93, 4837. (5) Parker, L. M.; Bibby, D. M.; Burns, G. R. Zeolites 1993, 13, 107. (6) Marchese, L.; Chen, J.; Wright, P. A.; Thomas, J. M. J. Phys. Chem. 1993, 97, 8109.

Jobic et al. (7) Marchese, L.; Chen, J.; Thomas, J. M.; Coluccia, S.; Zecchina, A. J. Phys. Chem. 1994, 98, 13350. (8) Pelmenschikov, A.; van Santen, R. A. J. Phys. Chem. 1993, 97, 10678. (9) Pelmenschikov, A.; G.; van Wolput J. H. M. C.; Ja¨nchen, J.; van Santen, R. A. J. Phys. Chem. 1995, 99, 3612. (10) Wakabayashi, F.; Kondo, J. N.; Domen, K.; Hirose, C. J. Phys. Chem. 1996, 100, 1442. (11) Smith, L.; Cheetham, A. K.; Morris, R. E.; Marchese, L.; Thomas, J. M.; Wright, P. A.; Chen, J. Science 1996, 271, 799. (12) Krossner, M.; Sauer, J. J. Phys. Chem. 1996, 100, 6199. (13) Greatbanks, S. P.; Sherwood, P.; Hillier, I.; Hall, R. J.; Burton, N. A.; Gould, I. R. Chem. Phys. Lett. 1995, 234, 367. (14) Bra¨ndle, M.; Sauer, J. J. Mol. Catal. (submitted). (15) Shah, R.; Payne, M. C.; Lee, M. H.; Gale, J. D. Science 1996, 271, 1395. (16) Jobic, H. In Catalyst Characterization; Imelik, B., Ve´drine J. C., Eds.; Plenum: New York; 1994; p 347. (17) Cadioli, B.; Gallinella, E.; Coulombeau, C.; Jobic, H.; Berthier, G. J. Phys. Chem. 1993, 97, 7844. (18) Jobic, H.; Czjzek, M.; van Santen, R. A. J. Phys. Chem. 1992, 96, 1540. (19) Jobic, H.; Lauter, H. J. J. Chem. Phys. 1988, 88, 5450. (20) Jobic, H.; Renouprez, A.; Fitch A. N.; Lauter, H. J. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3199. (21) Jobic, H. J. Catal. 1991, 131, 289. (22) Jacobs, W. P. J. H.; Jobic, H.; van Wolput, J. H. M. C.; van Santen, R. A. Zeolites 1992, 12, 315. (23) Jacobs, W. P. J. H.; van Wolput, J. H. M. C.; van Santen, R. A.; Jobic, H. Zeolites 1994, 14, 117. (24) Datka, J.; Boczar, M.; Rymarowicz, P. J. Catal. 1988, 114, 368. (25) Williams, J. M. In The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland: Amsterdam, 1976; p 655. (26) Kearley, G. J.; Fitch, A. N.; Fender, B. E. F. J. Mol. Struct. 1984, 125, 229. (27) Penfold, J.; Tomkinson, J. Rutherford Appleton Laboratory report, RAL-86-019, 1994. (28) Coulombeau, C.; Jobic, H.; Bernier, P.; Fabre, C.; Schu¨tz, D.; Rassat, A. J. Phys. Chem. 1992, 96, 22.

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