J. Phys. Chem. 1996, 100, 15917-15922
15917
Study of the Dispersion of Sodium Chloride in Zeolite NaY A. Seidel, U. Tracht,† and B. Boddenberg* Lehrstuhl fu¨ r Physikalische Chemie II, UniVersita¨ t Dortmund, Otto-Hahn-Strasse 6, D-44227 Dortmund, Germany ReceiVed: April 10, 1996; In Final Form: July 2, 1996X
Inclusion of sodium chloride in the large and small cavities of zeolite NaY was performed with the aid of the solid-state dispersion technique. Samples with different amounts of NaCl were subjected to various pretreatment conditions, including intensive washing with water, and were investigated with the methods of X-ray diffraction and nitrogen and xenon adsorption as well as with 129Xe and 23Na nuclear magnetic resonance spectroscopy. The results obtained permit determination of the extent of salt inclusion in both types of cavities, the stability of the salt clusters toward treatment with water, and the characterization of these clusters by their 23Na NMR behavior under static and magic angle spinning conditions.
Introduction The inclusion of salts and other solids in host materials with regularly arranged voids of dimensions in the nanometer range is a subject of current interest in the search for materials with new optical, electrical, and catalytic properties.1-10 The inclusion of salts in the mineral sodalite which exhibits a system of identical interconnected cavities (sodalite cages) has been investigated intensively in recent years.11-16 Zeolites of the Faujasite type, like the synthetic X and Y versions, have attracted much less attention for the purpose of occluding salt guests although the regular arrangement of cavities of different sizes, namely the supercages and β-cages, could offer the opportunity of constructing compound materials with interesting new properties. On the basis of the pioneering research of Barrer et al.17,18 and of Rabo,19 we have prepared sodium chloride inclusion compounds with NaY as the host by means of the solid-state dispersion technique.20 The properties of these materials are studied by various techniques with emphasis on 23Na NMR spectroscopy which, in recent years, has proven to be a versatile tool for investigations in the field of zeolite science.13,14,21-28 Experimental Section The salt inclusion compounds NaCl(y)NaY were prepared from hydrated zeolite NaY (Union Carbide; Linde LZ Y-52; Si/Al ) 2.4) and polycrystalline NaCl (Fluka) under conditions that are summarized in Figure 1. Samples with NaCl contents y ) 7.0, 12.5, and 24.8 wt %, corresponding to nominal salt concentrations Nnom(NaCl) ) 2.8, 5.3, and 12.2 NaCl per 1/8 unit cell (uc), were obtained by thoroughly mixing appropriate amounts of the starting materials at the normal atmosphere and heating the mixtures under high vacuum (p < 10-5 hPa) to 823 K. This temperature was maintained for 24 h. In short-hand notation, the so-obtained samples are denoted P(y). Subsequently, these materials were kept at ambient atmosphere for at least 3 days to obtain rehydrated samples designated R. The rehydrated samples were repeatedly washed with bidistilled water at ambient temperature, filtered, dried at 353 K, and saturated with water again (H). Finally, the materials were dehydrated under high vacuum at 673 K for 16 h (D). The same conditions were applied to obtain dehydrated zeolite NaY (NaYD). † Present address: Max-Planck-Institut fu ¨ r Polymerforschung, Mainz, Germany. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01073-8 CCC: $12.00
Figure 1. Route of preparation and designation of NaCl/NaY inclusion compounds.
The mechanical mixture (short-hand notation M) with 12.5 wt % NaCl as well as the compounds R were investigated with X-ray diffraction using Cu KR radiation. Nitrogen (77 K) and xenon (298 K) adsorption isotherms of NaYD and of the compound samples P and D(12.5) were measured using allsteel equipments. Ambient-temperature 129Xe NMR spectra were recorded with a solid-state NMR spectrometer type CXP 100 (Bruker) operating at the resonance frequency ω/2π ) 21.4 MHz. Single π/2-pulse excitation was applied. The 129Xe NMR chemical shifts reported are referenced to xenon gas at vanishing pressure. 23Na NMR experiments on hydrated and dehydrated zeolite NaY and on the differently pretreated salt inclusion compounds were performed with a Bruker spectrometer type MSL 400 (ω/2π )105.8 MHz) employing π/8-pulse excitation. The spectra were recorded in a magic angle spinning probe at frequencies of rotation νr ) 0 (static) and 10 kHz (MAS). Samples P and D, which are liable to rehydration, were transferred into the ZrO2 rotors (4 mm o.d.) under dry argon © 1996 American Chemical Society
15918 J. Phys. Chem., Vol. 100, No. 39, 1996
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Figure 3. Adsorption isotherms of nitrogen (T ) 77 K) in dehydrated NaY (×), in samples P with y ) 7.0 (9), 12.5 (b), and 24.8 ([) wt % NaCl, and in sample D(12.5)(O).
TABLE 2: Saturation Capacities of Nitrogen and Specific Pore Volumes of NaYD and Salt Inclusion Compounds P and D Figure 2. XRD patterns of NaCl/NaY materials: mechanical mixture M(12.5) (a) and rehydrated compounds R with 7.0 (b), 12.5 (c), and 24.8 (d) wt % NaCl. The asterisks designate reflexes of crystalline NaCl.
TABLE 1: X-ray Diffraction Data of NaYH, the Mechanical Zeolite/Salt Mixture M(12.5), and the Rehydrated Salt Inclusion Compounds R
a
sample
a (nm)
Irel(NaCl)
N(NaCl) (1/8 uc)-1
NaYH M(12.5) R(7.0) R(12.5) R(24.8)
2.468 2.468 2.463 2.467 2.467
0 10.0 0.5 1.3 2.0
0 0a 2.5 4.6 11.1
Assumption.
atmosphere and were investigated immediately afterward. The 23Na NMR chemical shifts are reported with reference to 1 mol dm-3 aqueous NaCl solution. Results Figure 2 shows the XRD patterns obtained for the mechanical mixture of hydrated NaY with 12.5 wt % NaCl (M) and for the rehydrated materials after heating at 823 K (R). Each sample exhibits sharp reflexes characteristic of the zeolite matrix and of crystalline NaCl. The first of these observations proves that the crystallinity of the zeolite is retained after the heating procedure, the second that some salt has remained outside the zeolite crystallites. In Table 1 the results of the quantitative analysis of the diffractograms are collected. Besides the lattice constants, it contains in column 3 the intensity ratio, Irel(NaCl), of the strongest reflex of NaCl (2θ ) 31.7°) relative to the most intense line of the host (2θ ) 23.7°), representing a quantity proportional to the concentration of nondispersed salt. Using these and the composition data of the mechanical mixtures, the concentrations of the dispersed salt N(NaCl), expressed in the unit [1/8 unit cell (uc)]-1 of the zeolite, were calculated (column 4, Table 1). Figure 3 shows the adsorption isotherms at 77 K of nitrogen in dehydrated NaY as well as in the compounds designated P
sample
nsat(N2) (mmol g-1)
Vp (cm3 g-1)
NaYD P(7.0) P(12.5) P(24.8) D(12.5)
10.0 8.1 6.3 0.06 9.8
0.35 0.28 0.22 0.002 0.34
and D(12.5). The sample P with the highest salt content exhibits negligible adsorption. In the other cases initial steep rises are followed by slightly increasing linear portions. The extrapolation of the latter (p f 0) yields saturation capacities, nsat(N2), which are considered to reflect the complete filling of the intracrystalline space accessible to nitrogen. The obtained values of nsat(N2) as well as of the corresponding pore volumes Vp calculated on the basis of Gurvitch’s rule with FN1 2(77 K) ) 0.808 g cm-3 29 (both quantities are referred to the mass of the dehydrated host) are collected in Table 2. In the case of samples P, the pore volume decreases with increasing salt content and becomes zero for the highest loading investigated. Interestingly, after washing (sample D(12.5)) the accessible pore volume of the parent zeolite NaY is completely restored. Figures 4 and 5 show the ambient-temperature adsorption isotherms and 129Xe NMR chemical shifts δ of xenon, respectively, in dehydrated NaY as well as in the samples P(12.5) and D(12.5). In each case, almost linear adsorption isotherms and linear δ vs xenon concentration (NXe) curves are observed. Remarkably, both the adsorption and NMR results are the same for the parent zeolite and the washed compound sample whereas the as-prepared sample P(12.5) exhibits lower adsorption and higher chemical shifts. The large body of experimental 23Na NMR data, namely static and MAS spectra of NaY and of each of the zeolite/salt compounds under the various pretreatment conditions applied, requires restriction of the presentation to selected examples. Preferentially, the samples containing 12.5 wt % NaCl will be shown. If not stated otherwise, the spectra obtained for the samples of different salt content under the same pretreatment conditions exhibit similar features. The presentations also contain computer-simulated MAS spectra demonstrating the
Dispersion of NaCl in Zeolite NaY
Figure 4. Adsorption isotherms of xenon (T ) 298 K) in samples P(12.5) (b), D(12.5) (O), and in NaYD (×).
J. Phys. Chem., Vol. 100, No. 39, 1996 15919 missing in the spectra of the hydrated and dehydrated host. For samples R(12.5) and H(12.5) this line exhibits the same line width (0.6 kHz). The spectrum of R(12.5) contains a further line at -2 ppm (0.3 kHz) and a doublet of very narrow lines at +7 ppm which are assigned to NaCl at the external surface of the zeolite crystals. Sample H(12.5) shows a further line at -2 ppm (0.4 kHz) and a very small shoulder at about +7 ppm. The hydrated parent zeolite NaYH exhibits a narrow line at -1 ppm (0.4 kHz) and a broad line at -5 ppm (1.0 kHz).21,22,27 Figure 7a,b shows the 23Na NMR spectra of the samples P(12.5) and D(12.5) and of their hydrated counterparts R(12.5) and H(12.5), respectively, measured under both static and magic angle spinning conditions. It is noted that the spectra shown in a and b cover different frequency ranges. Considering the response of the width of the component lines to the recording technique applied, it is observed that the MAS line at +2 ppm, common to all compound samples, broadens by a factor of about 3 when the static mode is applied. This is best seen in the case of sample D(12.5) (Figure 7a), where the least overlap with other components occurs. Similarly, strong broadening is observed for the lines at +7 ppm (P, R) and -14 ppm (P) and for the quadrupole pattern (D). On the other hand, only small broadening is obtained for the lines centered at -2 ppm (H, R). For each dehydrated washed compound sample D, the maximum of the MAS line at +2 ppm is displaced to lower field by about 4 ppm when the rotor frequency is set to zero. Simultaneously, the line becomes asymmetric. Figure 8 shows the 23Na MAS NMR spectra of the dehydrated washed samples D as function of the nominal salt concentration. It is seen that in each case the spectrum consists of the line at +2 ppm and of the second-order quadrupole pattern, as was shown in Figure 6a for sample D(12.5). Interestingly, the intensity ratio of these two components is practically independent of the nominal salt concentration. Discussion
Figure 5. 129Xe NMR chemical shifts of xenon in samples P(12.5) (b), D(12.5) (O), and in NaYD (×).
decomposition into separate spectral contributions. The calculations were performed with a home-written computer program based on procedures of theoretical analysis described in the literature.30 Figure 6a,b shows the 23Na MAS NMR spectra of the waterfree samples P(12.5), D(12.5), and NaYD and of their hydrated counterparts R(12.5), H (12.5), and NaYH, respectively. The spectrum of P(12.5) exhibits two well-resolved lines at +2 ppm (half-width δν ) 0.9 kHz) and -14 ppm (1.4 kHz) as well as a shoulder at +7 ppm (0.1 kHz) which can be attributed to nonoccluded NaCl. The dehydrated washed sample D(12.5) shows the line at +2 ppm (0.8 kHz) as well but is accompanied at lower frequencies by a second-order quadrupole pattern with maxima at about -24 and -50 ppm. Similar patterns are observed for the sample P(7.0) with strongly reduced relative intensity (spectrum not shown) and for the dehydrated parent zeolite NaYD which exhibits a further singlet line at -4 ppm (1.0 kHz).24,25,27 Interestingly, the line at +2 ppm common to both dehydrated compound materials is also found in the spectra of the hydrated compound samples R(12.5) and H(12.5) but is
The discussion pursues the aim to elucidate three main aspects of the presently studied zeolite/salt systems, namely the extent of sodium chloride inclusion in the voids of the zeolite matrix, the distribution of the salt between the large supercages (L 1.25 nm) and the small β(sodalite)-cages (L 0.66 nm) of the faujasite framework, and the physicochemical state of the guest within the cavities under the different pretreatment procedures applied. Rabo19 has shown that impregnation of dehydrated zeolite NaY with concentrated aqueous NaCl solution and subsequent evaporation of the solvent leads to reversible and irreversible occlusion of salt in the supercages and β-cages, respectively. It was concluded from chemical analysis that the irreversibly occluded salt amounts to just one NaCl per β-cage if sufficiently high temperature is applied. In the mineral sodalite, the exclusively available sodalite cages have been shown to contain sodium chloride in the form of [Na4Cl]3+ complexes.31 So, an interesting point of investigation is whether the presently applied salt dispersion technique leads to a similar degree of β-cage filling as impregnation and whether the salt is incorporated in these cages in a way comparable to that of sodalite. The analysis of the XRD data performed in the previous section leaves open the question whether the nondetectable salt is contained in the intracrystalline voids of the zeolite matrix or, to an appreciable extent, is dispersed on the outer surface of the zeolite crystals. The circumstance that for the investigated as-prepared samples P the pore volume detected by nitrogen adsorption is reduced in comparison to NaYD and decreases with increasing salt content proves that NaCl is incorporated at least in the supercages. It is expected that, approximately, a linear correlation between Vp (Table 2) and N(NaCl) from X-ray
15920 J. Phys. Chem., Vol. 100, No. 39, 1996
Seidel et al.
Figure 6. Experimental (top) and simulated 23Na MAS NMR spectra of dehydrated (a) and hydrated (b) zeolites. Dehydrated samples: NaYD, D(12.5), and P(12.5); hydrated samples: NaYH, H(12.5), and P(12.5).
analysis (Table 1) should exist if the vast majority of the noncrystalline salt is dispersed into the large zeolite cavities. Figure 9 shows that such a linear correlation, in fact, exists. These findings suggest that an amount of 11 - x NaCl units is required to fill completely a supercage where x is the unknown average number of NaCl incorporated in a β-cage. Certainly, a β-cage cannot accommodate more than one NaCl (x e 1) so that the maximum number of NaCl per supercage is in the range 10-11, which is in good agreement with literature data.20 The presence of NaCl in the supercages of the samples P is corroborated by the decrease of xenon adsorption and the increase of the 129Xe NMR chemical shifts in comparison to NaYD because the reduction of the width of the pores accessible to xenon atoms is known to lead, in general, to downfield displacement of the resonance position.32-34 We hesitate to plunge into a quantitative analysis of these findings because other effects such as the change of the chemical environment within the cavities may contribute to the observed displacements.32,35-39 Most importantly, however, the coincidence of the adsorption and NMR data of NaYD and sample D(12.5) tells us that, by washing the materials, the salt is practically completely removed from the supercages, thus confirming the findings of Rabo.19 A further confirmation comes from the nitrogen adsorption isotherms where the pore volume of sample D(12.5) has the same value as is found for dehydrated NaY (Table 2). From the results of the preceding discussions, systematic changes of the 23Na NMR features of both the hydrated and
dehydrated materials are expected to occur in the sequence parent/washed/nonwashed samples. In comparing the washed inclusion compounds with the parent zeolites, the spectral parts due to Na+ ions in the supercages are expected to remain essentially unchanged whereas the components due to β-cage sodium species should be strongly disturbed if, actually, NaCl is irreversibly occluded in these voids. Proceeding from the washed to the nonwashed materials, NMR spectral changes are expected to occur mainly for the Na+ cations contained in the supercages. The observation that the 23Na MAS NMR line centered at +2 ppm occurs for each of the investigated samples treated with NaCl but is missing in the spectra of the parent NaY suggests the assignment of this line to Na+ cations in the β-cages containing irreversibly occluded NaCl. Since this line broadens considerably upon switching the rotor spinning frequency to zero, these sodium species must be subjected to strong anisotropic couplings and, therefore, are concluded to be contained in immobile assemblies, most probably in [Na4Cl]3+ clusters (see below). The NMR parameters of these clusters, obviously, are not influenced by the physicochemical situation prevailing in the supercages except for a small difference in line width (0.6 kHz for hydrated and about 0.8 kHz for dehydrated samples). If the foregoing assignment holds true, then the further features of the 23Na NMR spectra are explainable on a chemically reasonable basis. We first compare the spectra of the dehydrated (D) and hydrated (H) washed compound samples
Dispersion of NaCl in Zeolite NaY
J. Phys. Chem., Vol. 100, No. 39, 1996 15921
Figure 8. 23Na MAS NMR spectra of samples D with y ) 7.0 (a), 12.5 (b), and 24.8 (c) wt % NaCl.
Figure 7. Static (top) and MAS (bottom) 23Na NMR spectra of dehydrated (a) and hydrated (b) compound materials NaCl/NaY: Dehydrated samples: D(12.5) and P(12.5); hydrated samples: H(12.5) and R(12.5).
with the corresponding spectra of the parent zeolites. It has been shown that in zeolite NaYD the quadrupole pattern with quadrupole coupling constant QCC ) 4.3 MHz, asymmetry parameter η ) 0.3, and isotropic chemical shift δiso ) +3 ppm originates from sodium cations at the hexagonal window sites SII and SI′ in the supercages and β-cages, respectively.27 The narrow line at δ ) -4 ppm (QCC ) 0.1 MHz) was assigned to cations at the crystallographic position SI in the centers of the hexagonal prisms.27 In NaYH, the narrow line at -1 ppm and the broad line at -5 ppm were attributed to hydrated Na+ ions in the supercages and β-cages, respectively.27 The comparison with the spectra of the compound samples D and H leads us to the following conclusions. In samples D, the missing of the line at -4 ppm is due the displacement of the corresponding Na+ ions from the SI positions into the β-cages under the influence of the negative charge of the Cl- anions residing therein. The quadrupole pattern which is simulated with the parameters QCC ) 4.0 MHz, η ) 0.3, and δiso ) -1 ppm essentially originates from Na+ cations residing at the supercage positions SII, with possible minor contributions from cations at sites SI′ in residual β-cages not occupied by NaCl (if present). In samples H, the line at -2 ppm is due to hydrated
Figure 9. Pore volume of samples P(y) as function of the concentration of dispersed NaCl.
Na+ ions in the supercages since the frequency position and the width of this line are close to the values found for the corresponding sodium species in NaYH. Actually, the high mobiliy of these supercage cations is proven by the only slightly enlarged line width observed under static measuring conditions. The missing of the line at -5 ppm signifies the absence, or at least the very low concentration, of hydrated Na+ ions in β-cages. Thus, this is strong indication for the almost complete occupancy of these cages with NaCl. This conclusion is also supported by the independence of the nominal salt concentration observed for the relative intensities of the two spectral MAS components of samples D (Figure 8) which are assigned to the Na+ species within NaCl-containing β-cages and to Na+ cations at the hexagonal window sites within the supercages and the not salt-occupied sodalite-cages. This finding implies that a saturation of the β-cages with NaCl is already attained with the lowest applied guest/host ratio of the starting mixture.
15922 J. Phys. Chem., Vol. 100, No. 39, 1996 The interpretation of the 23Na MAS NMR spectra of the nonwashed samples P and R is facilitated by a comparison with the spectra of the corresponding samples after washing (D and H). The line at -14 ppm of samples P has to be assigned to rigid cationic salt complexes of the type [Nax(NaCl)y]x+ (x e 4; y e 10) in the supercages which are generated from the zeolite cations of SII sites and the introduced salt. The rigid character of these species follows from the line broadening observed when the rotor frequency is set to zero. The incorporation of the SII cations in the formation of these clusters is deduced from the missing of the characteristic quadrupole pattern in the MAS spectra of samples P(y) with y g 12.5 wt % NaCl. In sample P(7.0) where, as was mentioned previously, the quadrupole pattern appears with strongly reduced intensity, there is such a low salt concentration within the supercages that part of the SII cations remains uninfluenced. In samples R, the line at -2 ppm, which is only slightly influenced by the rotor spinning speed, is due to hydrated Na+ cations in a fluid-like phase within the supercages. This fluid is best characterized as a cationic salt solution. The remaining question concerns the physicochemical state of the guest species within the β-cages of the faujasite structure. In this context, it is interesting to compare the resonance frequency position, as well as the width and the shape of the presently detected 23Na MAS NMR line at +2 ppm (δν ≈ 0.8 kHz) with the results found for NaCl incorporated in the cages of the mineral sodalite.13,14 Here, a single-symmetric MAS line centered at +6 ppm (δν ) 0.17 kHz) is observed which was interpreted to be due to [Na4Cl]3+ complexes within the sodalite cages.14 From the analysis of the manifold of the magic angle spinning sidebands due to the satellite transitions ((1/2 T (3/ 2), the underlying quadrupole coupling constant was determined to be QCC ≈ 0.3 MHz.13 The central transition MAS NMR results of the presently investigated samples preclude obtaining a reliable value of QCC of the +2 ppm line. The downfield shift of this line which is observed upon switching to the static measuring mode and the simulteneous appearance of line shape asymmetry might be due to either the presence of strong quadrupole couplings with high asymmetry parameter, to anisotropic chemical shift couplings, or to both effects simultaneously. Field-dependent MAS NMR measurements that could allow differentiation between these possibilities are in preparation. At present, the similarity of the shape and position of the line observed in the MAS NMR spectrum of NaCl sodalite with the presently detected line at +2 ppm leads us to assume that [Na4Cl]3+ clusters are present within the β-cages of the obtained faujasite structure as well. Actually, in dehydrated NaY, an overall amount of 3 Na+ per 1/8 uc is known from diffraction studies to occupy the crystallographic positions SI and SI′.40,41 So, from a stoichiometric point of view, the tetrahedral [Na4Cl]3+ complexes can readily be generated if one NaCl is occluded in each β-cage and the Na+ cations residing on sites SI in the dehydrated host are completely displaced to the SI′ positions under the influence of the negative charge of the Cl- anions. Conclusions By applying XRD, adsorption, and NMR techniques, sodium chloride has been shown to be occluded in the voids of zeolite NaY upon heating mechanical mixtures of the salt and hydrated NaY at a temperature of 823 K. By this solid-state dispersion technique, a maximum amount of about 11 NaCl can be occluded per 1/8 unit cell of the host. Whereas the salt deposits in the supercages can be removed by washing with water, NaCl is irreversibly incorporated in the β-cages, where the salt is most probably involved in the formation of tetrahedral [Na4Cl]3+
Seidel et al. clusters. The combination of XRD, adsorption, and 129Xe NMR techniques with 23Na NMR spectroscopy has been demonstrated to be a useful procedure to detect and characterize NaCl inclusions in microporous materials with cages of different sizes. Acknowledgment. Financial support of this work by “Fonds der Chemischen Industrie” is gratefully acknowledged. References and Notes (1) Stucky, G. D.; Mac Dougall, J. E. Science 1990, 247, 669. (2) Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 257. (3) Cox, S. D.; Gier, T. E.; Stucky, G. D. Chem. Mater. 1990, 2, 609. (4) Wark, M; Schulz-Ekloff, G.; Jaeger, N. I., Lutz, W. Mater. Res. Soc. Symp. Proc. 1991, 233, 133. (5) Schulz-Ekloff, G. Stud. Surf. Sci. Catal. 1991, 69, 65. (6) Ozin, G. A. AdV. Mater. 1992, 4, 612. (7) Stucky, G. D. Prog. Inorg. Chem. 1992, 40, 99. (8) Caro, J.; Marlow, F.; Wu¨bbenhorst, M. AdV. Mater. 1994, 6, 413. (9) Schu¨th, F. Chem. Unserer Zeit 1995, 29, 42. (10) Wark, M.; Schwenn, H.-J.; Warnken, M.; Jaeger, N. I.; Boddenberg, B. Stud. Surf. Sci. Catal. 1995, 97, 205. (11) Godber, J.; Geoffrey, A. O. J. Phys. Chem. 1988, 92, 4980. (12) Jacobsen, H. S.; Norby, P.; Bildsøe, H.; Jakobsen, H. J. Zeolites 1989, 9, 491. (13) Nielsen, N. Chr.; Bildsøe, H.; Jakobsen, H. J.; Norby, P. Zeolites 1991, 11, 622. (14) Engelhardt, G.; Sieger, P.; Felsche, J. Anal. Chim. Acta 1993, 283, 967. (15) Brenchley, M. E.; Weller, M. T. Zeolites 1994, 14, 682. (16) Mead, P. J.; Weller, M. T. Zeolites 1995, 15, 561. (17) Barrer, R. M. In Inclusion Compounds 1; Atwood, J. L., et al., Eds.; Academic Press: London, 1984; p 191. (18) Barrer, R. M.; Walker, A. J. Trans. Faraday Soc. 1968, 60, 171. (19) Rabo, J. A. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 332. (20) Xie, Y.-Ch.; Tang, Y.-Q. AdV. Catal. 1990, 37, 1. (21) Welsh, L. B.; Lambert, S. L. In PerspectiVes in Molecular SieVe Science; Flank, W. H., White, T. E., Eds.; ACS Symposium Series 368; American Chemical Society: Washington, DC, 1988; p 33. (22) Beyer, H. K.; Pal-Borbely, G.; Karge, H. G. Microporous Mater. 1993, 1, 67. (23) Jelinek, R.; O ¨ zkar, S.; Pastore, H. O.; Malek, A.; Ozin, G. A. J. Am. Chem. Soc. 1993, 115, 563. (24) Hunger, M.; Engelhardt, G.; Koller, K.; Weitkamp, J. Solid-State NMR 1993, 2, 111. (25) Engelhardt, G.; Hunger, M.; Koller, H.; Weitkamp, J. Stud. Surf. Sci. Catal. 1994, 84, 421. (26) Verhulst, H. A. M.; Welters, W. J. J.; Vorbeck, G.; van de Ven, L. J. M.; De Beer, V. H. J.; van Santen, R. A.; de Haan, J. W. J. Phys. Chem. 1994, 98, 7056. (27) Seidel, A.; Boddenberg, B. Z. Naturforsch. A 1995, 50, 199. (28) Hunger, M.; Engelhardt, G.; Weitkamp J. Microporous Mater. 1995, 3, 497. (29) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, FL, 1995. (30) Mu¨ller, D. Ann. Phys. 1982, 39, 451. (31) Pauling, L. Z. Kristallogr. 1930, 74, 213. (32) Fraissard, J.; Ito, T. Zeolites 1988, 8, 350. (33) Derouane, E. G.; Nagy, J. B. Chem. Phys. Lett. 1987, 137, 341. (34) Derouane, E. G.; Andre, J.-M.; Lucas, A. A. J. Catal. 1988, 110, 58. (35) Dybowski, C.; Bansal, N. Annu. ReV. Phys. Chem. 1991, 42, 433. (36) Barrie, P. J.; Klinowski, J. Prog. Nucl. Magn. Reson. Spectrosc. 1992, 24, 91. (37) Raftery, D.; Chmelka, B. F. NMR 1994, 30, 111. (38) Barrie, P. J. Annu. ReV. NMR Spectrosc. 1995, 30, 37. (39) Seidel, A.; Rittner, F.; Boddenberg., B. J. Chem. Soc., Faraday Trans. 1996, 92, 493. (40) Mortier, W. J. Compilation of Extra Framework Sites in Zeolites; Butterworth; London, 1982 and references therein. (41) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311.
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