Hydrogen Bonding of Methanol with Bridged OH Groups of Zeolites

María Fernanda Zalazar , Esteban Nadal Paredes , Gonzalo D. Romero Ojeda , Néstor Damián Cabral , and Nélida María Peruchena. The Journal of Phys...
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J. Phys. Chem. B 1998, 102, 2454-2463

ARTICLES Hydrogen Bonding of Methanol with Bridged OH Groups of Zeolites: Ab Initio Calculation, 1H NMR and FTIR Studies J. Kotrla, D. Nachtigallova´ , and L. Kubelkova´ † The HeyroVsky Institute of Physical Chemistry and Electrochemistry, DolejskoVa 3, 18223 Prague 8, Czech Republic

L. Heeribout, C. Doremieux-Morin, and J. Fraissard* Laboratoire de Chimie des Surfaces, associe´ au CNRS URA 1428, UniVersite´ Pierre et Marie Curie, 75252 Paris Cedex 05, France ReceiVed: June 3, 1997; In Final Form: October 1, 1997

FTIR spectroscopy at ambient temperature, broad-line 1H NMR spectroscopy at 4 K, and magic angle spinning (MAS) spectroscopy at ambient temperature, together with ab initio calculations at the Hartree-Fock and the second-order perturbation theory levels on the skeleton fragment HO-SiH2-O-SiH2-OH-AlH2-O-SiH2OH are used to study interaction complexes of methanol-d3 with bridging hydroxyls of zeolites (represented by H-mordenite and HZSM-5). The two-site neutral hydrogen-bonded methanol complex with bridging hydroxyl Si-OH-Al and the zeolite skeletal oxygen, which is predicted by the theory, is confirmed by the experimental observations provided that the number of adsorbed molecules is less than the number of bridging OH groups (Θ(OH) < 100%). Simulation of 1H broad-line NMR indicates an average distance 193-200 pm between the zeolite and methanol hydrogens in magnetic interaction, which is in a reasonable agreement with ab initio calculations value 198.5-195 pm. The MAS NMR signal assigned to the exchange between methanol and bridging hydroxyls at very low OH coverage is found at 8.6 ppm, which is below the value reported for methoxonium ion. Using the Fermi resonance theory, the seven bands of OH groups observed in the infrared spectra allows us to determine the fundamental stretching and bending vibrations of bridging hydroxyls (1810-1790, 1375-1370, 880-866 cm-1) and OH groups of the methanol (3560-3555, 1375 cm-1) in the surface complex. The ab initio calculations on a large zeolite fragment show new possibilities of the localization of methanol on the zeolite surface. Nevertheless, compared to the theoretical calculations, the experiment still proves that the zeolitic hydrogen is much closer to the center between methanol and zeolite oxygens, and the methanol OH group is only little affected by the skeletal oxygen. The clusters of methanol adsorbed on bridging hydroxyls and affected by skeletal oxygens appear in methanol excess ((Θ(OH) ) 100-280%). The zeolite increases the charge of the cluster and the hydrogen bonding, which is higher than that for the liquid methanol.

Introduction Organic cations are generally believed to function as active intermediates in reactions catalyzed by hydroxyls of strong solid acids. Nevertheless, the question whether they appear as transition-state complexes on the top of the activation energy barrier or in the equilibrium

AH‚‚‚B S A-‚‚‚+HB

(1)

between the hydrogen-bonded neutral complex AH‚‚‚B and ionpair (protonized) complex A-‚‚‚+HB established directly on the surface is still a matter of living debate. Interaction complexes of simple alcohols (above all methanol), aldehydes, and ketones with strongly acidic zeolites (or other solids) and the activation †

Deceased on December 17, 1996.

of these molecules for catalytic transformations pertain to the most discussed problems nowadays. According to rules derived for solutions,1 these surface species should attain at ambient temperature predominantly ion-pair character. Therefore, the interpretation of Fourier-transform infrared spectra (FTIR) and 1H and 13C magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of methanol/ solid acid2-4 and acetone/H-zeolite5-7 as well as the first ab initio self-consistent field (SCF) calculations on the methanol/ H-zeolite fragment,8 which favored the formation of ion-pair complexes dominating on the surface4 or appearing together with neutral hydrogen-bonded species,3,5,6 is not surprising. However, more advanced ab initio studies9-13 of methanol on zeolite models revealed the minimum on the potential energy surface only for neutral species while the methoxonium cation corresponded to a transition structure for the proton transfer

S1089-5647(97)01805-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/19/1998

H-Bonding of Methanol with Bridged OH Groups between adjacent zeolite skeletal oxygens. The value estimated for the proton-transfer energy was very low (e12,11 11,12 or 6 kJ/mol14), which allowed some authors14 to propose the prevalence of methoxonium ion on the zeolite surface. A new application of Fermi resonance (FR) theory for the interpretation of FTIR spectra seemed to support the concept of the stable neutral hydrogen-bonded complex.11,15 In a very recent study Shah et al. calculated a methanol-zeolite complex using density functional theory with generalized gradient approximation and plane wave basis set. Different minima were found depending on the structure of the zeolite fragment used for calculation.16 However, a neutral hydrogen-bonded structure was found to be a minimum regardless of the structure of the zeolite by Haase et al.17 The differences between experimental and ab initio vibrational frequencies for surface complexes were so large that the unambiguous conclusion could not be reached.11,14 The addition of the methanol molecules to zeolite pores in amount exceeding one methanol molecule per bridging hydroxyl facilitates, according to ab initio10,13 and 1H MAS NMR18 studies, the formation of methoxonium ion. The problem again needs experimental attention. For acetone on H-zeolites, the latest low-temperature 13C MAS NMR measurements,19,20 ab initio calculations,21 and FTIR studies at ambient temperature,22 which employed the Fermi resonance theory for the evaluation of the bands of OH groups,23 seriously doubted the occurrence of the proton transfer. These results showed a predominant formation of the medium-strong hydrogen-bonded species acetone‚‚‚HO(zeol). This latter finding encouraged us to reinvestigate the problem of methanol using the combination of several spectroscopic experimental methods: 1H broad-line NMR, 1H MAS NMR, and FTIR supported by an ab initio study of the methanol complex with the zeolite fragment HO-SiH2-O-SiH2-OHAlH2-O-SiH2-OH at the HF/6-31G* and MP2/6-31G* levels.24,25 To distinguish clearly hydrogens in OH groups from those in methyls, the study was performed using CD3OH. It has been already shown that 1H broad-line NMR in “rigid lattice conditions” can serve as a valuable tool for the identification of specific groups of adjacent H atoms and their relative distances.26-28 Because the configuration of H atoms in the complex CH3OH‚‚‚HO(zeol) should be different from that in the species CH3OH2+‚‚‚-O(zeol), this method can appreciably contribute to the solution of the problem of methoxonium ions and neutral H-bonded species on the acidic zeolite surface.

J. Phys. Chem. B, Vol. 102, No. 14, 1998 2455 the bridged zeolite hydroxyl and methanol hydroxyl group, respectively. For the measurements of the NMR spectra, the zeolite was “shallow bed” evacuated at 400 °C (“dehydrated sample”). The CD3OH was stepwise adsorbed at 30 °C to the final desired loading. The amount of adsorbed methanol was gravimetrically controlled. After the ampule was sealed off, the sample was shaken and allowed to homogenize for at least 48 h at room temperature. The same sealed sample was used for both MAS and broad-line 1H NMR experiments. 1H MAS NMR measurements were carried out using a Bruker 400 MHz spectrometer at room temperature. The homemade probe allowed the sample to rotate at 4 kHz. The delay between the successive pulses amounted to 20 s for the dehydrated samples to 5 s for the samples with adsorbed methanol. A special pulse sequence for 1H{27Al} dipolar dephasing was used to distinguish H atoms in the near neighborhood of Al atoms from those which are located in more remote positions.29-32 Broad-line 1H MAS NMR measurements were performed at 60 MHz on samples that were quenched at 4 K using a modified Varian DP 60 spectrometer. The spectra remained unchanged if the sample was kept under helium atmosphere to improve the thermal conductivity. The conditions of the measurements avoided the spectra modification by artifacts. The spectra were recorded as derivatives of the absorption NMR versus the magnetic field; only half of the centrosymmetric spectra are shown in the figures. The geometries of the stationary points corresponding to two possible orientations of methanol on the zeolite fragment HOSiH2-O-SiH2-OH-AlH2-O-SiH2-OH were optimized at the HF/6-31G* and MP2/6-31G* levels.24,25 The interaction energies calculated at the same levels of theory were corrected for the basis set superposition error (BSSE) by the counterpoise correction method.33 Because the optimization procedure of the zeolite fragment monomer resulted in the formation of unrealistic internal hydrogen bonds, the calculation of this monomer for obtaining interaction energies had to be performed with the geometry of the zeolite fragment obtained for the zeolitemethanol system. The same structure was also used for the calculations of the zero-point vibrational energy (ZPE) of the zeolite fragment. The ZPE corrections as well as the fundamental vibrational frequencies were obtained at the HF/6-31G* level. Vibrational frequencies were scaled by the factor 0.9.34 All the calculations were performed using the Gaussian94 program.35

Experimental and Computational Methods Mordenite (Si/Al ) 8.4) and ZSM-5 zeolite (Si/Al ) 13.5) in the Na and H forms were synthesized in the Institute for Oil and Hydrocarbon gases, Bratislava. Methanol-d3 (deuterium enrichment 99.6%) was supplied by Merck. Liquid adsorbate was degassed by repeated freezing and thawing and stored over dry KA zeolite. The infrared spectra of zeolite plates with a thickness of 7 mg‚cm-2 were measured in situ using a Nicolet MX-1E FTIR spectrometer (resolution 2 cm-1). Prior to the adsorption of methanol-d3 in successive doses at room temperature, the zeolite plate was activated in a spectroscopic cell in vacuum 10-4 Pa at 400 °C for about 4 h (“dehydrated sample”). The fraction of bridged OH groups interacting with the adsorbate (coverage Θ(OH)) was determined from intensities of free hydroxyls before and after the adsorption. In this case, the values exceeding 100% were calculated from the number of methanol molecules dosed and the number of bridged OH groups on the sample. For further discussion we call OZ-HZ and OM-HM

Results FTIR Spectra. The FTIR spectra of methanol-d3/HM and methanol-d3/HZSM-5 are displayed in Figures 1 and 2, respectively. Parts a and b of each figure show the spectra of zeolites before and after the adsorption of subsequent doses of methanol at ambient temperature in the spectral region 4000-2000 and 1000-800 cm-1, respectively. The spectral changes in the wavenumber interval 4000-1300 cm-1 that are caused by each subsequent dose are seen in part c. These difference spectra are obtained from the spectra of the zeolite with methanol by the subtraction of the spectrum of the zeolite with preceding dose of methanol. The character of the spectra of both HM and HZSM-5 after the activation in a vacuum at 400 °C is typical for acid forms of zeolites. The most intense bands at 3608 cm-1 for HM (Figure 1a) and 3612 cm-1 for HZSM-5 (Figure 2a) arise from the stretching vibrations ν(OZHZ). A sharp peak at 3745 cm-1 of relatively small intensity originates from the vibration

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Figure 1. FTIR spectra of methanol adsorption on HM at ambient temperature with the subsequent doses of methanol: a, original spectra in the region 2000-4000 cm-1; b, original spectra in the region 8201000 cm-1 (transmission window); c, differential spectra (see the text).

Figure 2. FTIR spectra of methanol adsorption on HZSM-5 at ambient temperature with the subsequent does of methanol: a, original spectra in the region 2000-4000 cm-1; b, original spectra in the region 8201000 cm-1 (transmission window); c, differential spectra (see the text).

νOH(SiOH) of free silanol groups on the zeolite. These groups are not affected by the interaction with surrounding surface oxygens or other hydroxyls. Other silanols present on both samples vibrate at lower wavenumbers which indicate the OH bond weakening due to the interaction with surrounding solid. The weak bands or shoulders are found between 3730 and 3700 cm-1, and a very broad band, recognizable on the background with difficulties, appears near 3300 cm-1 (Figures 1a and 2a). The spectrum of the zeolite with surface complexes of methanol depends on the concentration of the adsorbate. The following information was found. (1) If Θ(OH) is lower than 1, the surface complexes are predominantly formed by the direct interaction of adsorbate with Si-OH-Al. The intensity of the stretching vibration band of free Si-OZHZ-Al at 3612 cm-1 (HZSM-5) and 3608 cm-1 (HM) decreases, and seven new bands of OH groups appear at lower wavenumbers. (Figures 1 and 2, Table 5). The new band at 962 cm-1 (Figures 1b and 2b) can be attributed to the CO stretching vibration (νCO) of adsorbed methanol.36 The bands of stretching vibrations of CD3 groups appear at the spectral range 2268-2083 cm-1 (Figures 1a and 2c) as expected from the spectra of pure methanol.36 (2) A new set of bands starts to develop at coverages of bridging OZHZ groups near to 100% (Figures 1c and 2c, ∆Θ(OH) ) (100-75)%). The adsorbed methanol added in excess after the complete coverage of bridging hydroxyls is then reflected only by these new bands that appear at 3310-3155

(broad), 2715-2690 (broad), 1590-1580, and 1525 cm-1 (Figures 1a,c and 2a,c; the latter two bands are overlapped). The previous bands characteristic for the methanol adsorbed on bridging OZHZ groups still remain in the spectra (Figures 1a,b and 2 a,b). Among them the bands at 866 (HM, Figure 1b) and 880 cm-1 (HZSM-5, Figure 2b) decrease in intensity. The spectra above 2000 cm-1 are rather complex (Figures 1a and 2a). Nevertheless, the negative band near 3560-3550 cm-1 in the difference spectra obtained at the conditions of the methanol excess (Figures 1c and 2c) might be caused by the intensity decrease of the corresponding band of CD3OH‚‚‚HO(zeol) species. NMR Spectra. 1H MAS NMR. After the activation at 400 °C, the HM sample exhibits the following signals (Figure 3, refs 37, 38): (i) 4.0 ( 0.2 ppm signal which is attributed to free acidic H of bridging hydroxyls OZHZ. The signal intensity corresponds to 5.0 ((5%) groups per unit cell (uc). (ii) 2.0 ( 0.2 ppm signal originates from the free silanol H atoms, denoted silanol1, which are not in a strong hydrogen interaction with the surface of the solid and whose concentration is very small. (iii) 6.6 ( 0.2 ppm signal appears in the region where H from both SiOZHZ-Al and silanol groups can resonate. Because the 1H{27Al} dipolar dephasing affects this signal only slightly while the signal at 4.0 ppm is completely suppressed (Figure 4), we can attribute the 6.6 ppm signal in accordance with Freude30 to silanols (denoted as silanol-2) in a strong electrostatic interaction

H-Bonding of Methanol with Bridged OH Groups

J. Phys. Chem. B, Vol. 102, No. 14, 1998 2457

Figure 3. 1H MAS NMR spectra at ambient temperature of activated HM.

Figure 5. 1H MAS NMR spectra of HM after the adsorption of methanol. The coverage of bridging hydroxyls Θ(OH) ) 1.

Figure 4. 1H MAS NMR spectra at ambient temperature of activated HM before the adsorption of methanol. The effect of 1H{27Al} dipolar dephasing spin-echo for various durations (D′) of 27Al irradiation. Asterisks denote the spinning side bands.

with the solid surface.29-31 This assignment is also supported by the fact that the signal is always narrow and that it cannot be simulated by Lorenzian curves.30 The concentration of these groups is 1.2/uc ((40%). The increasing adsorption of methanol-d3 results in an intensity decrease of the signal of free bridging hydroxyls at 4.0 ppm while the two signals of silanols at 2.0 and 6.6 ppm remain almost unchanged (Figure 5). A new signal, assigned to the exchange of H atoms between adsorbed methanol and bridging hydroxyls, is found at 8.6 ppm after the first dose of

methanol is adsorbed (Θ(OH) ) 32%). The chemical shift and the line widths increase with Θ(OH) until a maximum 9.9 ppm and 2.7 kHz, respectively, for Θ(OH) about 100%, and then decrease with increasing Θ(OH), for example, until 9.5 ppm and 1 kHz for Θ(OH) about 280%. 1H Broad-Line NMR. The spectrum of mordenite before the adsorption of methanol (not shown) is typical for the absence of hydrogen-containing adsorbed species. It is of a Lorentzian shape (parameter 1.35 × 10-4 T) and contains no contribution for h > 6 × 10-4 T, where h is the difference between the applied magnetic field and the Zeeman resonance field. When methanol-d3 is adsorbed, the experimental spectra (Figure 6) can be simulated by only two distinct contributions. They correspond to r-distant pairs of H atoms described using a two-spin magnetic configuration according to Pake,39 and “free” H atoms without any particular near neighbor H, for which the absorption is either of the Lorentzian (L) or Gaussian (G) character (Table 1). The numbers of H-pairs and remaining “free” H atoms related to the total number of bridging hydroxyls together with calculated r-distance and typical configuration parameters are given in Table 1 for various coverages of bridging OH groups (Θ(OH)). These data show the following: (i) For Θ(OH) < 100%, almost all methanol-d3 interacts with bridging hydroxyls and the r-distance slightly decreases from 200 pm for Θ(OH) ) 32% to 192 pm for Θ(OH) ) 100%. The interpair distance X is almost constant (285-273 pm). (ii) If an excess of methanol-d3 is added (Θ(OH) > 100%), the number of remaining free hydroxyls agrees with the number of silanol groups. All other H atoms are associated as pairs. The r-value increases with the concentration of methanol molecules from 193 pm for Θ(OH) ) 100% to 205 pm for Θ(OH) ) 280%. Compared to the coverages near or below 100%, the shortest interpair distance X decreases with increasing Θ(OH); at Θ(OH) g 200% it is equal to the r-distance. It should be noted that whatever the experimental technique used, IR or NMR, we have not been able to find any H/D isotope exchange at ambient temperature between the CD3 of the adsorbed molecule and the acidic H of the zeolite, in agreement with refs 40 and 41. Ab Initio Calculations. The fragment was chosen to imitate both a four-member ring and part of the channel of mordenite and HZSM-5. Two local minima of the hydrogen-bonded neutral complex of methanol with the zeolite fragment HOSiH2-O-SiH2-OH-AlH2-O-SiH2-OH are found on the

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a a

b b

c

Figure 7. The methanol-zeolite complex calculated at the MP2/631G* level: a, structure 1, b, structure 2. Figure 6. 1H broad-line NMR spectra of H-mordenite after the adsorption of CD3OH: experimental spectrum (dotted line), simulated spectrum (full line), two-spin contribution (---), Lorenzian or Gaussian absorption (-‚-). A, B, C refer to Θ(OH) ) 32, 100, and 176%, respectively.

TABLE 1: 1H Broad-Line NMR Data: Concentration (c) of the H-H Pairs and Free H Atoms per Bridging OH Group, Intrapair r(H-H) and Interpair X Distance in pm, Absorption Shape, and Parameters free H atoms (OH groups) H-H pairs Θ(OH)a abs (%) c × 100b r ( 5 X ( 5 c × 100b shapec par. Ld par. Xe 32 74 100 106 122 150 176 200 240 280

31 74 100 102 111 125 138 149 170 191

200 196 193 196 198 198 200 200 205 205

285 265 273 252 251 240 210 210 205 205

101 50 24 24 24 24 24 24 24 24

L L G L L L L G G G

1.40 1.70 315 1.20 0.98 0.85 0.99 280 280 291

a,b ( 10% of the value reported. c L ) Lorentzian, G ) Gaussian. Parameter for the absorption by free OH groups in 10-4 T for the Lorentzian shape. e Distance in pm corresponding to the magnetic field parameter for the Gaussian absorption.

d

potential-energy surface (PES) both at the HF and MP2 computational levels. In both structures (Figure 7a,b), the methanol is hydrogen bonded to two surface sites: the hydrogen of acidic bridging hydroxyl OZHZ via the methanol oxygen OM and the basic skeletal oxygen OF (O15 or O16 for structures 1

TABLE 2: Selected Optimized Bond Lengths, r (pm), and Angles, a (deg), for Structures 1 and 2 and Shell-2a of the Neutral Hydrogen-Bonded Complex of Methanol with Zeolite Fragment structure 1 r(OZ-OM) r(OM-HZ) r(OF-HM) r(OZ-HZ) r(OM-HM) r(Al-O1) r(Al-O2) r(C-OM) r(Si3-O1) r(Si8-O4) r(HM-HZ) a(O1-Al-O2) a(Si3-O1-Al) a(Si8-O4-Al) a

structure 2

HF

MP2

HF

MP2

266.6 172.1 207.2 97.7 95.2 172.1 196.3 141.3 168.8 160.1 218.5 98.98 130.08 179.11

255.7 154.1 190.0 103.7 98.0 175.6 195.3 144.4 170.7 163.6 198.5 99.13 128.79 142.97

264.1 165.9 188.8 98.4 95.7 173.2 194.3 141.2 169.0 158.8 214.2 100.42 131.34 176.19

253.2 147.2 178.4 106.1 98.8 171.6 194.2 145.2 169.6 161.9 194.8 98.0 131.84 147.43

Shell-2a MP2 145.3 176.2 104.9 99.3 177.0 188.4 143.1

98.0

Reference 11, zeolite fragment (H3Si-O)3-Al-OH-SiH3.

and 2, respectively) via the hydrogen of methanol OMHM group. The former interaction is considerably stronger than the latter one. This is reflected in the shorter bond length of HZ-OM compared to HM-OF by about 30 pm (Table 2). The structures previously suggested by all other authors9-13 consisted of two surface sites in a close neighbor which are bound to the same central Al atom. In contrast, the adsorption sites of both new structures are separated by one skeletal oxygen. The orientations of methanol in structure 1 (Figure 7a) and structure 2 (Figure 7b) are different because of the different locations of the OF site (Figure 7a,b). The intermolecular OM-HZ and OF-HM

H-Bonding of Methanol with Bridged OH Groups

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TABLE 3: Interaction (Adsorption) Energies ∆E (kJ/mol) and Interaction (Adsorption) Enthalpies ∆H0ad (kJ/mol) of Structures 1 and 2 of the Neutral Hydrogen-Bonded Complex of Methanol with the Zeolite Fragment ∆Ead

a

∆H0adb

HF/6-31G*//HF/6-31G* +BSSE MP2/6-31G*//MP2/6-31G* +BSEE HF/6-31G*//HF/6-31*+BSSE+ZPE(HF) MP2/6-31G*//MP2/6-31G*+BSSE+ZPE(HF)

structure 1

structure 2

-62.3 -86.5 -53.6 (8.7) -77.8 (8.7)

-74.9 -103.8 -66.3 (8.6) -95.2 (8.6)

a Interaction energy calculated as a difference between the total energy of the compact and the sum of the energies of its monomers corrected for BSSE: ∆Ead ) Etot - Emet - Ezeol - BSSE. b The BSSE-corrected interaction enthalpy at 0 K: ∆Had ) ∆Ead - ∆ZPE where zero-point energies (given in the parentheses) were calculated at the HF/6-31G* level.

distances are always shorter and the intramolecular OM-HM and OZ-HZ distances longer in structure 2 than those in structure 1. This complies with a deeper minimum on PES for the structure 2. Selected geometrical parameters of two stationary points calculated at the HF and MP2 levels are compared in Table 2. In agreement with calculations of Haase and Sauer,11 the inclusion of electron correlation effects shortens intermolecular hydrogen bonds by about 10-20 pm and elongates OH bonds in both zeolite bridging and methanol OH groups by less than 10 pm. In addition, large Si8-O4-Al angles (adjacent to the Si3-O1H-Al angle) predicted at the HF level (179.1 and 176.2° for the structures 1 and 2, respectively) are reduced to the values closer to real zeolite parameters at the MP2 level (143.0 and 147.4°, respectively). The calculated distances between HM and HZ atoms (r(HH)) that are important for the interpretation of 1H broad-line NMR spectra, are 218.5 and 214.2 pm for structures 1 and 2, respectively, at the HF level and 198.5 and 194.8 pm, respectively, at the MP2 level. The difference in the distance between the pair of H in the neutral hydrogen-bonded methanol complex CD3OH‚‚‚HO(zeol) and the corresponding ion-pair (IP) complex CD3OH2+‚‚‚-O(zeol) derived from the structure 1 was determined at the HF level. Because no local minimum on the PES corresponds to the IP complex, the optimization was performed after fixing the distance HZ-OM at 111 pm. With this distance, the optimization of the IP complex on the smaller zeolite fragment H3Si-OH-AlH2-OH gave the energetically most favorable structure. The resulting r(HM-HZ) in the methoxonium ion is 169 pm which is significantly shorter (by about 30 pm) than r(HM-HZ) in the neutral methanol surface complex. Note that the same difference in the HM-HZ distances is predicted also for the model H3Si-OH-AlH2-OH: the HMHZ is 192 and 142 pm for the neutral complex and the ion-pair complex, respectively.42 The adsorption (interaction) energies ∆Ead given in Table 3 were calculated at the HF and MP2 levels as a difference between the total energy of the complex and the sum of total energies of its isolated constituents, corrected for BSSE. In accordance with the trends in geometrical parameters (Table 3), structure 2 of the neutral hydrogen-bonded complex of methanol-d3 with zeolite bridging hydroxyls is more stable than structure 1 (by 12.6 kJ/mol at the HF level and 17.3 kJ/mol at the MP2 level, respectively). The electron correlation effects stabilize structure 1 and structure 2 by 24.1 and 29.2 kJ/mol, respectively. The adsorption (interaction) enthalpies ∆H0 at 0 K that were obtained from ∆Ead by the subtraction of zeropoint energy (∆ZPE) (8.7-8.6 kJ/mol, Table 3) exhibit the same trend as the interaction energies. The size of the zeolite model does not allow calculations of fundamental vibrational frequencies of the neutral hydrogenbonded complex of methanol at the MP2 level to be performed; Table 4 displays the values obtained at the HF level. As

TABLE 4: Vibrational Frequenciesa (cm-1) Calculated at the HF Level for the Complex of Methanol with Zeolite Fragment: Structures 1 and 2 and Shell-2b methanol

methanol on zeolite

mode

free

structure 1

structure 2

Shell-2a

νOMHM

3706

3628

3550

3216

3066

3653 3260c 3188 2300c

2238 2199 2081 1334 1252 1148 815

2262

νOZHZ νCD δOMHM δOZHZ νCO γOZHZ a

2206 2159 2059 1289 1167

2077 1389 1292 1149 841

1350 1250 784

b

All frequencies scaled with 0.9. Reference 11, zeolite fragment (H3Si-O)3-Al-OH-SiH3. c Estimated from Badger’s correlation.11,43

expected from the energetic calculations, the OH stretching frequencies of methanol, νOMHM, and bridging hydroxyls of the zeolite, νOZHZ, are lower for structure 2 than for structure 1. The reverse is true for the in-plane bending vibrations of the methanol, δOMHM, and the zeolite, δOZHZ, and the out-of plane bending vibrations of the methanol, γOMHM, and the zeolite, γOZHZ. As a consequence of the influence of electron correlation effects on the bond distances, a substantial decrease in frequencies of both the methanol νOMHM and the zeolite νOZHZ stretching vibrations can be observed. Haase and Sauer11 revealed that this amounts to 360 and 750 cm-1 for the interaction complex of methanol with the relatively small fragment H3Si-OH-Al(OH)2-O-SiH3 (denoted as Shell-1.5). In contrast, δOMHM and δOZHZ were shifted upward only by 70 and 140 cm-1, respectively.11 For larger fragments, the same authors proposed11 to estimate “MP2” stretching frequencies from OH bond distances calculated on the MP2 level using Badger’s equation.43 This resulted in the downward shift of the HF frequencies νOMHM and νOZHZ for the methanol complex with the zeolite model (H3Si-O)3-Al-OH-SiH3 (denoted as Shell-2) by 393 and 888 cm-1, respectively (Table 4). Using the Badger’s equation, we found the redshifts of 270 and 290 cm-1 for the νOMHM of structures 1 and 2, respectively, and 860 and 835 cm-1 for νOZHZ of the structures 1 and 2, respectively (Table 4). Discussion In this section, we discuss the experimental and/or theoretical results of free OH groups of zeolites and the methanol-zeolite complex at the low and high coverage of methanol. OH Groups on Zeolites. Comparison of FTIR and 1H MAS NMR data of HM before the adsorption of methanol clearly shows the complementary character of these data. Both NMR and infrared spectroscopy unambiguously indicate free bridging OZHZ groups (4.0 ppm, 3608 cm-1), but these methods differ

2460 J. Phys. Chem. B, Vol. 102, No. 14, 1998 in the strength of the evidence of various types of silanols. The SiOH groups which are not affected by the interaction with adjacent surface atoms are clearly proved by the FTIR band at 3745 cm-1 (Figures 1 and 2) while the corresponding signal in NMR at 2.0 ppm is generally narrow and weak but sometimes obscured by the tail of the strong signal at 4.0 ppm (Figure 4). In contrast, silanols which are hydrogen-bonded to other surface OH groups or O atoms are clearly evidenced by the strong and narrow NMR signal at 6.6 ppm (Figure 3) whereas FTIR bands between 3730 and 3700 cm-1 are weak and narrow and peaks at 3300 cm-1 are diffuse and often badly recognizable on the sample background (Figures 1 and 2). In addition, the 1H MAS NMR gives quantitative information about different OH groups. Both the 1H MAS NMR and FTIR spectra affirm that first doses of methanol-d3 are adsorbed on the bridging OZHZ groups of both HM and HZSM-5 (Θ(OH) < 100%). All these groups are located on such positions inside the HZSM-5 that they can be easily reached by methanol molecules. However, as it is known44 a small fraction of these bridging OZHZ groups of HM are in small side pockets. H atoms of these groups have to be transferred into accessible positions where they are able to interact with the adsorbate when a slight excess of methanol is added (Figure 1c, ∆Θ(OH) ) 150-100%, negative peak near 3650 cm-1). Interaction of methanol with silanol groups is negligible even under the large excess of methanol at Θ(OH) ) 280%. This is indicated by FTIR, 1H MAS NMR spectra (Figures 1, 2, and 5), and 1H broad-line NMR (Figure 6). These latter data show the total number of OH groups that do not interact with methanol to be the same as the number of silanol hydroxyls on the sample before the adsorption (Table 1). CD3OH‚‚‚HO(zeol) Complex at Θ(OH) < 100%. The latest ab initio calculations11-14 (including this paper) on various zeolite fragments composed from at least two Si and one Al atoms always show a minimum on the potential energy surface for the neutral hydrogen-bonded complex of methanol irrespective of the various computational methods employed. However, the energy barrier corresponding to the formation of methoxonium ion was predicted so low (e12,11 11,12 or 6 kJ/mol14) that it seemed to justify the occurrence of this process even at ambient temperature. The suggestion of the methoxonium ion (CD3OH2+) was mainly substantiated2-4 by the FTIR band at 1700-1600 cm-1 that was assigned to the deformation vibration of OH2 group. Accordingly, Kubelkova´ et al.3 suggested the establishment of the equilibrium between the hydrogen-bonded neutral and ion-pair forms on HY and HZSM-5 zeolites using the following assignment of FTIR bands observed: (i) 3576, 1420-1360, 1320 cm-1 vibrations of methanol OMHM, 2900-2800 cm-1 vibration of zeolite bridging OZHZ in the “asymmetric” hydrogen-bonded neutral complex CH3OH‚‚‚HO(zeol); (ii) 3576, 2550-2400, 1720-1610 cm-1 vibrations of OH2 group in the “asymmetric” methoxonium ion CH3OH2+‚‚‚-O(zeol) with one OH bond longer than the other one. The ab initio calculations11,14 considered “symmetric” methoxonium ion, so that the highest experimental frequency (ii) was not predicted. However, the other two experimental frequencies at lower wavenumbers fitted reasonably with the frequency regions of the stretching and bending vibrations calculated at the HF and MP2 levels between 2500-2000 and 1700-1600 cm-1. The OMHM vibrational frequencies of the neutral complex of methanol were evaluated at the HF levels11,14 in the frequency regions 3628-3550 cm-1 for the νOMHM of adsorbed methanol, 3298-3066 cm-1 for the νOZHZ of interact-

Kotrla et al. ing zeolite bridging hydroxyls, and 1350 cm-1 for δOMHM of adsorbed methanol. These data can be considered to support the above assignment of experimental values (i) to the neutral methanol complex. Nevertheless, the inclusion of the electron correlation effects on the MP2 level11 decreased the calculated frequencies of νOMHM and νOZHZ so much that it advocated an effort to think over the new interpretation of the FTIR spectra. A new approach to FTIR data has been stimulated by Pelmenschikov et al.15 who brought attention to the Fermi resonance (FR) effect that explains the bands at 2900-2800, 2550-2400, and 1700-1600 cm-1 as pseudobands ABC resulting from the resonant interaction of νOZHZ, 2δOZHZ, and 2γOZHZ vibrations of the CH3OH‚‚‚HO(zeol) neutral complex. The proof23 of the occurrence of this effect for medium-strong neutral hydrogen-bonded species on acidic zeolites has been done more lately by the study of the more simple molecule acetonitrile-d3 on zeolites of various acid strength. It was found23 that the fundamental δOZHZ and γOZHZ frequencies assessed from the AB and ABC spectral pattern using Overend’s equation45 are in a good agreement with the positions of the bands δOZHZ and γOZHZ observed in the spectrum. In addition, the AB contour was changed to the ABC when the acid strength of OZHZ groups increased. This was caused by the increasing downward shift of the νOZHZ and upward shift of the δOZHZ and γOZHZ accompanying the increasing hydrogen bonding in the CD3CN‚‚‚HO(zeol) complex. It was also pointed out that, in contrast to the neutral complex Adsorbate‚‚‚HO(zeol) (A‚‚‚zeol), the hydrogen bonding between AH+ and zeolite oxygen O(zeol) in the ion-pair surface complex AH+‚‚‚-O(zeol) decreases with the HO(zeol) acid strength, so that the νAH vibrational frequency decreases. Following this approach, the Overend’s FR theory45 was adopted for the estimation of frequencies of overtones of bending vibrations of bridged hydroxyls in the CD3OH‚‚‚HO(zeol) neutral complex in a way similar to that described in ref 23. The maxima and integral intensities of A and B components of the ABC Fermi resonance spectral pattern were used for the calculation of the overtone frequency of the in-plane bending vibration 2δOZHZ in the complex with methanol. Then the position of the gravity centrum of AB components νcg(AB) and the total intensity of AB together with the position and integral intensity of the component C allowed the overtone frequency of the out-of-plane bending vibration 2γOZHZ in the complex with methanol to be assessed. The gravity centrum νcg(ABC) of the ABC spectral pattern was considered as a frequency of the stretching vibration of bridged hydroxyls νOZHZ in the complex with methanol. On the basis of assumption described above, the FTIR spectra of methanol-d3 on HM and HZSM-5 in Figures 1 and 2 can be interpreted in terms of the neutral methanol complex strongly hydrogen bonded to bridging hydroxyls of both zeolites. This is implied namely from the following data and findings: (i) the contour of the ABC band and the shift of νOZHZ. Because methanol is a stronger base than acetonitrile, the intensity sequence A ) B , C observed for methanol complex complies well with the intensity sequence A < B . C reported23 for the acetonitrile-d3 neutral complex with bridging hydroxyls of HM and HZSM-5. Consequently, the shift ∆νOZHZ derived from the gravity centrum of the ABC band is much higher for the methanol complex (1820-1800 cm-1, Table 5) than that for the acetonitrile complex (1080-1040 cm-1, ref 23). (ii) The frequencies of the δOZHZ and γOZHZ calculated from the ABC components using Overend’s FR equation also aid

H-Bonding of Methanol with Bridged OH Groups

J. Phys. Chem. B, Vol. 102, No. 14, 1998 2461

TABLE 5: Experimentala and Calculated Using Fermi Resonance Theory (FR) Frequencies (cm-1) of Fundamental Stretching (ν), In-Plane (δ), and Out-of-Plane (γ) Bending Vibrations of Methanol OH (OMHM) and Zeolite OH (OZHZ) Hydroxyls of Free Constituents and CD3OH‚‚‚OH(zeol), CD3OH‚‚‚Na+(zeol) Complexes methanol freeb

HM

mode

gas

liquid

-180 °C

νOMHM

3690

3310

3195

free

NaM +Metc

3560

+Metc

HZSM-5 free

3600 3450

+Metc 3555

NaZSM-5 +Metc 3610 3450sh

νOZHZ δOMHM

3250sh 3608 1297

1391

1445

δOZHZ γOMHM γOZHZ

3612

1430 1050e,f

665

1790d 1325 1375sh

1375 1375g (FR)

1810d 1430

1042f

1320

1370 1345g (FR)

708

750 715 304f (∼420)h

Ai Bi Ci

866 925g (FR) 2800 2405 1690

305f (∼420)h

880 940g (FR) 2760 2360 1680

For Θ(OH) ) 25% and 50%. bReference 36. c Met ) methanol. dGravity centrum23 of ABC band component, (30 cm-1. e Reference 50. 51. gCalculated from FR theory, error for δOZHZ and γOZHZ is (10 and 30 cm-1, respectively. h Reference 52, measured for HY. i A, B, C ) trio of pseudobands caused by Fermi resonance effect, sh ) shoulder. a

fReference

the interpretation. They agree reasonably with positions of the bands of these fundamental vibrations found in the spectra at 1375 and 870-880 cm-1, respectively (Table 5). As expected, δOZHZ is higher for the complex with methanol (∼1370 cm-1) than with the less basic acetonitrile23 (∼1320 cm-1). (iii) The interpretation is also implied by the similarity in the band positions at 3560-3550 and 1430 cm-1 for methanol on HM and HZSM-5 with those of methanol on NaM and NaZSM-5 (3600-3610 and 1320-1325 cm-1, Table 5, refs 3, 4). This allows us to assign these bands to the stretching (high frequency) and bending (low frequency) vibrations of methanol OMHM. The frequency changes caused by the substitution of bridging hydroxyls for Na+ show that the OMHM bond of methanol is more perturbed (weakened) by the interaction with bridging hydroxyls and skeletal oxygens (Figures 6, and 7) than that with the Na+ ion and skeletal oxygen. The ab initio calculations of vibrational frequencies of the neutral complex of methanol with the zeolite fragments composed of two Si, one Al, one OH, and one oxygen along the main chain directed around the pore did not give a good agreement with experimental data.9,11,14 Namely, compared to the experiment (Table 5, νOMHM-νOZHZ ) 1770 or 1745 cm-1), the difference between the νOMHM and νOZHZ is too low on both the HF and the MP2 levels11,14 (465-320 and 960730 cm-1, respectively). To imitate the curvature of the pores, which might affect substantially the orientation of methanol on bridging hydroxyls, a longer fragment is employed in this paper. It comprises 3 Si, 1 Al, 1OH, and 4 O. The interaction with methanol bends a part of the fragment that mimics the 4-oxygenunit (Figure 7a, OZ, O5, O15) while the rest follows the zeolite pore or window (Figure 7b, OZ, O4, O16). The methanol crosses either the 4-ring or the window in structures 1 and 2, respectively. Even though structure 1 is less stable than structure 2, the adsorption enthalpy ∆H0ad calculated for both structures (Table 3) falls into the interval of measured adsorption heats11,46,47 that were found for HZSM-5 and HM between 75 and 120 kJ/mol for θ(OH) < 100%. Compared to other structures published in the literature,9,11,14 the ∆H0ad of structure 2 (95 kJ/mol, Table 3) approaches best the maximum value of calorimetric heats11,46,47 (115-120 kJ/mol). The formation of two stable arrangements of the methanol with the larger zeolite model thus demonstrates that due to the small energy

difference some other types of neutral methanol complexes with bridging hydroxyls (not considered in these calculations) might be formed in the pores and that the localization of individual atoms in the zeolite skeleton plays a very important role here. The stretching frequencies of CD3 groups (2081-2238 cm-1Å and 2077-2262 cm-1 for structures 1 and structures 2, respectively), which are shifted toward higher values compared to liquid methanol-d3 (2059-2206 cm-1), are also in a good accord with the experiment (2083-2268 cm-1). It points to stronger CD bond in adsorbed molecules than in the liquid. However, an agreement worse than that for the above parameters is found for the vibrations of both the methanol and the zeolite hydroxyls. While the HF frequencies of the out-of-plane bending vibration γOZHZ and the in-plane bending vibrations δOZHZ and δOMHM of structure 2 (841, 1292, and 1389 cm-1, respectively) are in a reasonable agreement with the experimental values (925, 1375, and 1430 cm-1), the νOZHZ are far from the experimental values (3066 cm-1 compared to 1790 cm-1). Compared to the Shell-2 model,11 the “MP2” frequency of the νOMHM and νOZHZ are slightly improved (Tables 4, 5). According to the experimental frequencies of the fundamental OH vibrations, the OZHZ bond should be longer (weaker) than the calculated one, so that the zeolitic hydrogen can be expected to move near the center of the OZ-OM distance. In contrast, compared to the calculation, the OMHM bond is less perturbed and shorter. To reach a better agreement of the theory with the experiment, the calculation on a larger fragment which would better describe better the interaction of CD3 groups with skeletal oxygens and, possibly, the inclusion of electrostatic field effects might be advantageous. To get a more realistic description of the zeolite structure, we have also performed the calculations on the 4-oxygen ring model with the orientation of methanol crossing the ring. The optimized geometrical parameters, calculated adsorption enthalpies, and vibrational frequencies are in very good agreement with those obtained for structure 1 with the same orientation of methanol.48 Before the completion of coverage of bridging OZHZ groups, the clusters of methanol are formed inside the zeolite pores. These new cluster species are unambiguously proved by several new FTIR bands. However, on the time scale of the 1H MAS NMR experiment, the only one new H signal attributed to the methanol surface complexes is seen irrespective of the Θ(OH).

2462 J. Phys. Chem. B, Vol. 102, No. 14, 1998 The formation of clusters is manifested by the change of the position, shape, and width of this peak (Figure 5). As both zeolitic and methanol hydroxyls resonate in surface complexes at the same chemical shift, structures indicated by FTIR must be always in a fast exchange leading to the NMR signal averaging. This fast exchange was also suggested in refs 4 and 18. Similarly, the 1H broad-line NMR spectra can be simulated always by two contributions for all concentrations of methanol measured. The change in the character of methanol surface complexes is reflected in the r(H-H) pair distance, the interpair distance X, and the absorption shape (Table 1). Using MAS NMR the 1H chemical shift was found at 8.6 and 9.9 ppm at the coverage 32% and nearly 100%, respectively. Therefore, these values must be compared to the results obtained from calculations to predict the character of the complex. The ab initio 1H chemical shift for OH groups of the neutral methanol complex was obtained10,11 between 5.7 and 11.1 ppm while that of the methoxonium ions interacting with the zeolite fragment was predicted between 15.3 and 19.2 ppm. These values depend on both the computational method (HF, MP2) and the zeolite model;10,11 nevertheless, they seem to support the assignment of the signal at 8.6 or 9.9 ppm to the neutral complex rather than to the methoxonium ion. The prevalence of the neutral complex of methanol is confirmed by the 1H broad-line NMR study of methanol-d3/ HM (Figure 6, Table 1). It comes out that the high intraatomic distance r(HM-HZ) in the pairs of H atoms, which is derived from the spectral data for Θ(OH) < 100%, is close to the ab initio values obtained for models of neutral complexes. Note that the difference in the r(HM-HZ) distance in the neutral and ion-pair methanol species is high (50 pm). In addition, a previous 1H broad-line NMR investigation26 of water on zeolites demonstrated that ion pairs can be clearly distinguished from the neutral complexes which are present on the surface simultaneously. Therefore, we can suggest that only a very low, if any, number of methoxonium ions should appear on HM and, probably, also on HZSM-5. It may be surprising that CH3OH2+ is not detected, whereas on similar samples one finds H3O+ after the adsorption of water in amounts that depend on the nature of the zeolite. In aqueous solution CH3OH is slightly more basic than H2O. However, the media are very different. The effect of the environment for the definition of the pK has nothing to do with the “solvation” by the solid of a single molecule adsorbed on an acidic OH. The interactions between the adsorbed complex and the solid in each case should be sufficiently different to compensate (or not) the very small energy difference between SOH‚‚‚OH2 and SO- H3O+, SOH‚‚‚(HO-CH3) and SO-, (CH3OH2)+. Therefore, as experiment shows, depending on the medium, one can obtain different reactivities for two molecules with slightly different basicities. Methanol Clusters at Θ(OH) g 100%. The creation of methanol clusters in the zeolite pore begins below the complete coverage of bridging hydroxyls with CD3OH‚‚‚HO(zeol) species depending on experimental conditions. At present, assumption of several types of surface methanol clusters can be found in the literature: (i) The chain of methanol molecules grows into the pore free space.4,10 The hydroxyl of the methanol adsorbed on bridging hydroxyl is attached to the oxygen of the second molecule whose hydroxyl is bound to the oxygen of the next molecule. No special interaction with skeletal oxygens is expected. (ii) The skeletal oxygens affect the methanol cluster through the hydrogen bonding with the OH group of the second (and

Kotrla et al. next) methanol molecules.13,18 In this case, the second molecule of methanol is bound either to the hydroxyl of the first adsorbed methanol13,18 or to the same bridging hydroxyl as the first molecule.18 The spectral data of this paper show that the formation of clusters at Θ(OH) < 200% affects both the bridging hydroxyl and the OH group of the first methanol molecule (FTIR). Because the νOMHM of new species is much lower and the δOMHM is much higher than for the liquid methanol (Figures 1, 2, and Table 5), the hydrogen bonding in this cluster can be also considered much stronger. In addition, the high frequency of the δOMHM resembles the value assigned to the methoxonium ion on the H-Nafion.49 However, the interpair r(HM-HZ) distance still corresponds to the distances in hydrogen-bonded neutral species. We thus believe that small clusters bring higher charge than the original complex CD3OH‚‚‚HO(zeol) and that the zeolite strengthens the hydrogen bonding between adsorbed molecules. The fact that the r(HM-HZ) and X become similar (Table 1) as Θ(OH) increases from 100% to 200% seems to be consistent with the statement that the distances between hydrogens in OH groups of individual methanol molecules do not differ much from those in the methanol and bridging hydroxyl. The model considering the interaction of the second methanol molecule with the OH group of the first adsorbed methanol on bridging hydroxyl and with the skeletal oxygen seems to be most consistent with our results. If Θ(OH) is higher than 200%, the chemical shift begins to decrease (a similar effect was reported in ref 18) and the r(HM-HZ) and X distances inside and outside the pairs become similar. In accord with ref 18 it can be considered that large clusters become more liquidlike. Conclusions The combination of the study of methanol interaction complexes with HM and HZSM-5 using 1H broad-line NMR, 1H MAS NMR, and FTIR spectroscopies with ab initio calculation of methanol interaction with a large zeolite fragment containing bridging hydroxyl reveals the following: (i) The first doses of methanol are adsorbed on bridging hydroxyls of the zeolite. The “asymmetric” neutral complex of methanol CD3OH‚‚‚HO(zeol) is strongly hydrogen bonded to bridging hydroxyl which results in an appreciable shift of zeolitic hydrogen toward the oxygen of the methanol. The OH bond of methanol is weakened only a little by the interaction with skeletal oxygen. (ii) Using the Fermi resonance theory, the FTIR spectra can be interpreted in terms of the formation of the neutral hydrogenbonded complex of methanol with bridging hydroxyls of zeolites. This interpretation is in agreement with the results of the MAS NMR spectra. The intraatomic distance r(HM-HZ) also supports the statement of the formation of neutral complex. The HF and MP2 calculation of methanol on a large zeolite fragment HO-SiH2-O-SiH2-OH-AlH2-O-SiH2-OH revealed two stable structures where two active sites (the bridging hydroxyl and the skeletal oxygen) are separated further by skeletal oxygen. It shows new possibilities of the localization of methanol inside the zeolite pores and the importance of the arrangement of active sites on the zeolite structure. The use of this model gives better agreement of adsorption heats and vibrational OH frequencies with experimental data than other (shorter) models reported in the literature. However, further improvement is necessary (larger model, electrostatic field), especially for the computation of vibrational spectra. (iii) Methanol clusters bound to bridging hydroxyls and affected by skeletal oxygens appear below the complete

H-Bonding of Methanol with Bridged OH Groups coverage of bridging hydroxyls with one methanol molecule. The hydroxyls of adsorbed methanol molecules are more strongly affected by hydrogen bonding than in the liquid and solid methanol. The zeolite also increases the charge of the cluster. When more than two molecules per bridging hydroxyls are adsorbed, the clusters start to more resemble the liquid phase. Acknowledgment. This study has been carried out in the framework of the COST project D5/0002/94 partly under the PECO grant 926042. J.K. and L.K. acknowledge the support by the Ministry of Education of the Czech Republic (OCD5.10). D.N. acknowledges the support of the Grant Agency of the Academy of Sciences of the Czech Republic (C4040704). L.H. acknowledges the support by the Ministry of Research of France. References and Notes (1) The Hydrogen Bond, Recent DeVelopments in Theory and Experiment; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland Publishing Co.: Amsterdam, 1976. (2) Highfield, J. G.; Moffat, J. B. J. Catal. 1985, 95, 108. (3) Kubelkova´, L.; Nova´kova´, J.; Nedomova´, K. J. Catal. 1990, 124, 441. (4) Mirth, G.; Lercher, J. A.; Anderson, M. W.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1990, 86, 3039. (5) Kubelkova´, L.; Cejka, J.; Nova´kova´, J. Zeolites 1991, 11, 48. (6) Bosa´cek, V.; Kubelkova´, L. Zeolites 1990, 10, 64. (7) Diaz, C. D.; Locatelli, S.; Gonzo, E. E Zeolites 1992, 12, 85. (8) Sauer, J.; Kolmel, C.; Haase, F.; Ahlrichs, R. In Proceedings of the 7th International Zeolite Conference, Montreal, 1992, von Ballamos, R., Higgins, J. B., Treacy, M. M. J., Eds.; Butterworth: London, 1993; p 679. (9) Gale, J. D.; Catlow, C. R. A.; Carruthers, J. R. Chem. Phys. Lett. 1993, 216, 155. (10) Haase, F.; Sauer, J. J. Chem. Phys. 1994, 98, 3083. (11) Haase, F.; Sauer, J. J. Am. Chem. Soc. 1995, 117, 3780. (12) Blazskowski, S. R.; van Santen, R. A. J. Phys. Chem. 1995, 99, 11728. (13) Limtrakul, J. Chem. Phys. 1995, 193, 79. (14) Bates, S.; Dwyer, J. J. Mol. Struct. (THEOCHEM) 1994, 306, 57. (15) Pelmenschikov, A. G.; van Santen, R. A. J. Phys. Chem. 1993, 97, 1067 (16) Shah, R.; Gale, J. G.; Payne, M. C. J. Phys. Chem. 1996, 100, 11688. (17) Haase, F.; Sauer, J.; Hutter, J. Chem. Phys. Lett. 1997, 226, 397. (18) Thursfield, A.; Anderson, M. W. J. Phys. Chem. 1996, 100, 6698. (19) Biaglow, A. J.; Gorte, R. J.; Kokotailo, G. T., White, D. J. Catal. 1994, 148, 779. (20) Xuy, T.; Munson, E. J.; Haw, J. F. J. Am. Chem. Soc. 1994, 116, 1962. (21) Floria´n, J.; Kubelkova´, L. J. Phys. Chem. 1994, 98, 8734. (22) Kotrla, J., Kubelkova´, L. Stud. Surf. Sci. Catal. 1995, 94, 509. (23) Kubelkova´, L.; Kotrla, J.; Floria´n, J. J. Phys. Chem. 1995, 99, 10285.

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