3412
J. Phys. Chem. B 1998, 102, 3412-3416
Accessibility of Cation Site in Zeolites by 6Li MAS NMR Spectroscopy Using Paramagnetic O2 as a Chemical Shift Agent. The Example of Zeolite Li-LSX J. Ple´ vert, L. C. de Me´ norval,* F. Di Renzo, and F. Fajula Laboratoire de Mate´ riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS, ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Ce´ dex 05, France ReceiVed: NoVember 21, 1997; In Final Form: February 18, 1998
A method based on the paramagnetic effect of physisorbed molecular oxygen is proposed for the discrimination of the different extraframework lithium cation sites in faujasite-type zeolites by solid-state NMR spectroscopy. The method allows a quantitative evaluation of the population of differently exposed cation sites.
Introduction Zeolites are used in a variety of catalysis, ion-exchange, and gas separation processes owing to their well-defined pore system, high thermal and chemical stability, and adjustable composition. Faujasite-type zeolites are among the most widely applied and studied systems because of the high accessibility of the cavities and the large range of Si/Al ratios attainable. The nature and location of the cations compensating the negative charge of the zeolite framework strongly influence the properties of the materials. It is generally recognized that the counterions are mainly located in five extraframework crystallographic sites1 (Figure 1). Sites I are located in the hexagonal prisms with a nearly octahedral oxygen coordination. Sites I′ and II′, in sodalite units, and sites II, in supercages, lie on sixring windows and are coordinated to three oxygen atoms. The remaining cations are located in the supercage sites III, of lower symmetry. Numerous investigations of the distribution of the cations in the sites of dehydrated faujasite zeolite X have been carried out by X-rays diffraction techniques.2-8 The results show different position and fractional occupancy of the sites. The cation distribution is very sensitive to chemical composition, preparation, and dehydration processes. In addition, the determination of lithium distribution in zeolite X with lithium cation as counterion is complicated by physical properties of the cation. X-rays are weakly scattered by lithium atoms, preventing laboratory-scale diffraction experiments. Furthermore, the different crystallographic sites are practically indistinguishable by 6Li and 7Li NMR spectroscopy because of the small range of chemical shifts even at high magnetic field under magic-angle spinning (MAS) conditions. The 7Li spin-3/2 nucleus is generally used because of its high natural abundance (92.6% and 7.4% of natural abundance for 7Li and 6Li, respectively) and short relaxation time. On the other hand, the 6Li spin-1 nucleus has a very poor receptivity, and quadrupolar contributions to the relaxation time T1 are usually absent because the 6Li isotope has one of the smallest quadrupolar moments (Q ) -8.0 × 10-4 barns). The MAS signals of 6Li nuclei do not suffer from extensive second-order broadening as in the case of other quadrupolar nuclei like 23Na. In this work, we propose a new method to obtain structural information from 6Li MAS NMR spectra of microporous * To whom correspondence should be adressed. FAX 33-(0)4-67-1443-49 or 43-53; E-mail:
[email protected].
Figure 1. 1. Framework of faujasite-type zeolite with possible cationic sites.
materials. This method is based on the paramagnetic effects of physisorbed molecules to discriminate selectively the lithium cations located in different crystallographic sites of zeolites. NMR signals of nuclei in the vicinity of paramagnetic species are subjected to large paramagnetic shifts, line broadenings, and short relaxation times T1.9,10 The effect arises primarily from hyperfine interactions9,10 of the unpaired electron spin with the nuclear spin and depends on the configurations of the system paramagnetic species-observed atoms. Paramagnetic shifts are used for instance in lanthanide complexes11 or transition-metal complexes,12 to obtain structural information on the conformation of ligands. In the presence of paramagnetic species the T1 of the 6Li isotope are strongly reduced.13,14 We take advantage of this effect to obtain spectra of higher signal-to-noise ratio. We carried out experiments as a function of O2 pressure and temperature to show the effect of physisorbed molecular oxygen as a site-discrimination agent in the cavities of faujasite-type zeolite Li-LSX (LSX for low silica zeolite X). Experimental Section Lithium-exchanged faujasite zeolites LSX (LSX: low silica X with ratio Si/Al ) 1) were obtained by ion exchange from as-synthesized zeolite with a composition of K19Na77Si96AL96O384, prepared as reported in ref 15. Three sodium exchanges were carried out with 1 M NaCl solution followed by lithium exchanges with 1 M LiCl solution at 368 K with stirring. The samples were washed with slightly alkaline solution and dried under vacuum at room temperature overnight after each exchange. X-ray diffraction measurements of
S1089-5647(98)00293-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/11/1998
Accessibility of Cation Site in Zeolites
Figure 2. 2. 6Li MAS NMR spectra of dehydrated zeolite LSX with Li82Na14Si96Al96O384 composition as a function of O2 pressure at room temperature: (a) under vacuum, (b) PO2 ) 200 Torr, (c) PO2 ) 600 Torr, and (d) PO2 ) 2000 Torr. Repetition delay d1 under vacuum is d1 ) 42 s, while d1 ) 0.5 s for spectra under O2 pressure. Rotor frequency is 3 kHz and number of scans is 1000.
dehydrated samples and N2 adsorption/desorption measurements at 77 K on the exchanged solids outgassed at 523 K indicated that no loss of crystallinity took place during cation exchange. 29Si MAS NMR and 27Al MAS NMR measurements of solids after ion exchanges showed no dealumination of zeolite framework. Atomic absorption spectroscopy indicated a Si/Al ratio 1.01 ( 0.1 and cation contents of 86.0 + 0.5% lithium, 14.0 + 0.5% sodium and 99.5 + 0.5% lithium, 0.5 + 0.5% sodium (molar) for samples obtained after one and three lithium exchanges, respectively. The anhydrous unit cell composition of the solids deduced from the elemental analyses were then Li82Na14Si96Al96O384 and Li95NaSi96Al96O384. Dehydrated materials were prepared on a vacuum line in 5 mm NMR tubes by slow heating up to 723 K under vacuum (10-6 Torr). Adsorption of O2 was carried out at room temperature, under various pressure levels between 200 and 2000 Torr. The NMR tubes were then sealed, preventing moisture adsorption and air contamination. O2 adsorption isotherms were measured using a conventional static apparatus between 50 and 1200 Torr. 6Li MAS NMR spectra were recorded on a Bruker ASX 400 spectrometer at 58.8 MHz using a specially designed homemade NMR probehead spinning the sealed tube up to 4 kHz.16 The instrument was equipped with a gas-flow system allowing heating and cooling of the sample in the temperature range from 200 to 360 K. The acquisition of 6Li MAS NMR spectra consisted of a single π/2 pulse of 9 µs pulse length, with a repetition delay varying from 0.5 to 42 s. Spinning frequencies were in the range of 2-3 kHz, and 1000-2000 scans were accumulated for each measurement. Chemical shift measurements are precise to within 0.2 ppm and are referenced to an aqueous solution of LiCl. Results and Discussion The 6Li MAS NMR spectrum of dehydrated faujasite LSX with a composition of Li82Na14Si96Al96O384, measured under vacuum at room temperature, exhibited no signal when a repetition delay d1 ) 0.5 s was applied between the scans. A signal developed only with a long repetition delay, d1 ) 42 s, as shown in Figure 2a. With a spinning frequency of 3 kHz, a
J. Phys. Chem. B, Vol. 102, No. 18, 1998 3413
Figure 3. 3. 6Li MAS NMR spectra of dehydrated zeolite LSX with Li82Na14Si96Al96O384 composition under PO2 ) 600 Torr as a function of the temperature. Repetition delay d1 ) 0.5 s, number of scans is 2000, and rotor frequency varies from 2.0 kHz (higher temperature levels) to 2.3 kHz (* spinning sidebands).
narrow MAS band is observed at -0.4 ppm. We note a slight shift to high fields compared with the resonance at 0 ppm of the hydrated sample. This shift has already been observed in the case of the zeolite A,17 another zeolite with framework based on sodalite building blocks. No more information is available from this spectrum, and the single peak observed suggests the inability of 6Li MAS NMR to separate contributions of cations distributed among different crystallographic sites, in the case of measurements performed under vacuum. The spectra of the same sample recorded under various O2 pressure levels reveal important differences from Figure 2a, as shown in Figures 2b-d. The spectra were recorded using a very short repetition delay, d1 ) 0.5 s, (using such conditions, no signal appeared in the spectrum recorded under vacuum). The main characteristic of the spectra is the appearance of a second peak at lower field in addition to the original one at ca. 1 ppm. The downfield shift is more pronounced as the O2 pressure increases from 200 to 2000 Torr. Concomitant with the shift in the position, a slight broadening of the signal is also noticed. Furthermore, an increase of the intensity of the peak located around 1 ppm with increasing O2 pressure is observed relative to the shifted peak. Figure 3 shows the effect of temperature between 220 and 360 K on the 6Li MAS NMR spectra of the dehydrated zeolite LSX with composition Li82Na14Si96Al96O384, loaded at room temperature with an O2 pressure PO2 ) 600 Torr. At high temperature, 360 K, the two lines are very close. As the temperature decreases, the second peak is shifted toward low fields, and it broadens until complete disappearance into the background at 220 K. The modifications of the 6Li MAS NMR spectra are explained by the paramagnetic effect of molecular oxygen. The paramagnetic effects generally observed in silicates lead to a broadening of the NMR lines and a sharp decrease of the relaxation time T1.13,14 However, an additional effect of peak shift appeared in this case. The access of O2 molecules in faujasite-type zeolites is restricted to supercage cavities, the hexagonal prisms and the sodalite cages being inaccessible to these molecules under our measurement conditions. The signal at low field, which is the most affected in terms of chemical shift and shape, is assigned to lithium cations in close contact
3414 J. Phys. Chem. B, Vol. 102, No. 18, 1998
Figure 4. 4. Adsorption isotherms of O2 adsorbed on zeolite LSX with a Li82Na14Si96Al96O384 composition at five temperature levels: (a) 300 K, (b) 280 K, (c) 260 K, (d) 240 K, and (e) 220 K.
Ple´vert et al.
Figure 6. Results from Figure 3: (a) shift δ of the paramagneticshifted line as a function of temperature T and (b) variation of ln δ as a function of 1/T at constant pressure, PO2 ) 600 Torr (shifts δ are corrected from pressure reduction in the sealed tube as temperature decreases; see text).
TABLE 1: Integrated Relative Intensities of 6Li MAS NMR Spectra of Partially and Fully Exchanged Zeolite Li-LSX (Spectra from Figure 7), and Calculated Number of Cations Contributing to Each Signal Per Unit Cell Li82Na14Si96Al96O384 peak at 8 ppm peak at 1 ppm
Li95NaSi96Al96O384
int (%)
no. of Li+
int (%)
no of Li+
22 ( 1 78 ( 1
18 ( 1 64 ( 1
29 ( 1 71 + 1
28 ( 1 67 ( 1
to data reported in Figure 4, is proportional to the oxygen concentration in the supercages. From least-squares fitting of the experimental data, the paramagnetic shift at room temperature may thus be expressed as a linear form δ (ppm) ) aNO2 + b, with NO2 the number of O2 molecules per supercage:
δ (ppm) ) 39.0NO2 - 0.2
Figure 5. 5. Room-temperature variation of peak shift as a function of O2 pressure and O2 content in dehydrated zeolite LSX with Li82Na14Si96Al96O384 composition (results from Figure 2, abscissas correlated as from Figure 4).
with O2 molecules, i.e., in sites located in the supercages. The peak around 1 ppm corresponds to the remaining lithium cations in the other sites that are not in close contact with the oxygen molecules and are not subjected to paramagnetic shifts. Nevertheless, the paramagnetic effect affects the relaxation time T1 of all lithium cations. The relaxation time T1 mainly varies as a function of r-6, where r is the distance between the paramagnetic species and the nucleus under investigation.9 Thus, a decrease of T1 is also observed for lithium cations contributing to the signal at 1 ppm as the oxygen pressure increases. At room temperature, the observed paramagnetic shift changes with oxygen pressure in the sealed tubes. Figure 4a shows the isotherm for O2 adsorption on the dehydrated zeolite LSX with composition Li82Na14Si96Al96O384 measured at 300 K. At this temperature, the adsorbed O2 volume varies linearly in the pressure range of measurement according to Henry’s law. Figure 5 shows the shift of the downfield peak from the spectra of Figure 2 as a function of the oxygen pressure. The peak shift increases linearly with the oxygen pressure and, according
(1)
The peak shift extrapolated to PO2 ) 0 Torr leads to δ0Torr ) -0.2 ( 0.2 ppm, in agreement with the expected value. Figure 6a shows the variation of the paramagnetic shift with temperature from the spectra of Figure 3. To find a correlation between the shift of the paramagnetic line and the adsorbed quantity NO2 at low temperatures, O2 adsorption measurements have been carried out at different temperatures (Figure 4). However, under our working conditions the NMR tubes are sealed at room temperature; the pressure is expected to decrease as the amount adsorbed in the solid grows when the temperature is lowered, and from the ideal gas law, the pressure will also decrease with temperature. The equilibrium pressure in a closed system, such as sealed tubes, and the adsorbed quantity in the zeolite may be estimated from adsorption isotherms, as the mass of product and the residual volume in the tube are known. Calculations are carried out in the Appendix on the dehydrated zeolite LSX with composition of Li82Na14Si96Al96O384 and an initial pressure PO2 ) 600 Torr at room temperature. Calculated pressures and loadings are reported in Table 2. It appears that pressure decreases nearly linearly in the sealed tube as the temperature is lowered and reaches, at 240 K, an equilibrium pressure PO2 nearly equal to half the initial one. In the meantime, the adsorbed quantity NO2 in the zeolite increases with the decreasing temperature. The O2 loading of the micropores remains relatively low as the supply of free O2
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J. Phys. Chem. B, Vol. 102, No. 18, 1998 3415
TABLE 2: Estimation of O2 Pressure, PO2, and Amount of O2 Adsorbed, NO2, in Sealed Tubes from Adsorption Measurements at Different Temperatures T, K
K ) NO2/PO2/ (103 Torr supercage)
PO2, Torr
NO2 (O2/supercage)
300 280 260 240 220
0.35 0.50 0.76 1.35 2.51
600 496 390 270 170
0.21 0.25 0.30 0.36 0.43
molecules is limited by the small volume of the NMR tube. The linear increase of O2 concentration in the supercages at low temperature cannot account for the exponential variation of the paramagnetic shift observed in Figure 6a. A second parameter is therefore required to describe the shift of the paramagnetic line induced by O2 molecules at different temperatures. The variation of the slope K of the isotherms has a behavior similar to the evolution of the paramagnetic shift as the temperature decreases. This may suggest a similar origin for the two effects. In the pressure and temperature ranges of the NMR experiments, the isotherms of O2 adsorption are nearly linear as shown in Figure 4. The amounts of O2 adsorbed, NO2, in the Langmuir model are expressed as NO2 ) NmbPO2, in the low-pressure region, where Nmb ) K; K is Henry’s constant and Nm the amount adsorbed at the monolayer point. The increase of the slope K at low temperatures is due to a temperature-dependent term τ, included in the Langmuir constant b, which follows a van’t Hoff form τ ) τ0eQ/RT, with Q the adsorption energy.18 The adsorption energy remains roughly constant in the temperature range of the experiments. Isosteric heats calculated from the data of Figure 4 indicate that the adsorption energy Q is close to 3.0 ( 0.2 kcal mol-1. The increase of the isotherm slope K at low temperatures depends mainly on the adsorption time τ, which corresponds to the average time of residence of the O2 molecule near the adsorption site.19 As the temperature decreases, it is expected that the mobility of O2 molecules decreases and the lifetime of the molecule in the vicinity of the lithium cation increases. Figure 6b shows the variation of ln δ of the shifted peak as a function of 1/T under constant pressure, PO2 ) 600 Torr. The paramagnetic shifts observed at low temperatures have been corrected by pressure reduction considering that at each temperature the paramagnetic shift varies linearly with the O2 pressure as observed at room temperature (Figure 5). The variation of ln δ with 1/T is linear, and the exponential variation of the paramagnetic shift with the temperature at constant PO2 suggests that the time scale of contact between the O2 molecule and the lithium cation is a critical factor. Additional factors, such as the 1/T dependence of the shift generally observed in the presence of paramagnetic species,20 are included in the curve of Figure 6b as ln T forms which do not change the linearity of the curve. The effect of the residence time of the molecules in the vicinity of the lithium cation is also revealed by the pronounced broadening of the downfield signal observed at low temperatures. In Figure 2d, corresponding to a sample recorded at room temperature under PO2 ) 2000 Torr, the downfield peak is shifted to about 30 ppm and features a relatively narrow line compared to the corresponding peak of Figure 3, shifted approximatively to the same position at 240-260 K under PO2 ) 600 Torr (PO2 measured at room temperature). This difference is due to a lower mobility of O2 molecules in the zeolite as the temperature decreases with a corresponding increase of
Figure 7. 7. 6Li MAS NMR spectra of dehydrated zeolite LSX with different exchange rates recorded under PO2 ) 600 Torr at room temperature with repetition delay d1 ) 42 s and number of scans ) 1000: (a) Li95NaSi96Al96O384, (b) Li82Na14Si96Al96O384.
the mean lifetime of the molecule in the vicinity of the cation. The effect of O2 as a paramagnetic agent is also revealed in the large decrease of the relaxation time T1, which is observed in the increase with O2 pressure and decrease in temperature of the integrated area of the peak at 1 ppm relative to the shifted peak in Figures 2 and 3, respectively. The influence of the repetition delay between scans was systematically investigated for the fully exchanged Li95NaSi96Al96O384 and the partially exchanged Li82Na14Si96Al96O384 zeolites LSX recorded under an oxygen pressure PO2 of 600 Torr. The repetition delay, d1 ) 42 s, is long enough to get full relaxation of all spins. Experiments at different repetition delay allow one to evaluate the relaxation time of the downfield signal (E0.5 s) while the relaxation time of the high-field line is about 8 s, both values depending on O2 concentration. Figure 7 shows the 6Li MAS NMR spectra of the two samples. In both cases the peak at low field is shifted to ca. 8 ppm, the main peak remaining nearly unchanged at the position of 1 ppm. The paramagnetic-shifted peak of the lithium fully exchanged zeolite is slightly more shifted than the peak of the partially exchanged zeolite. This effect is expected as the fully exchanged zeolite is a better O2 adsorbent (isotherm not shown) than the partially exchanged zeolite, leading for a given pressure to higher O2 concentration in the supercages of the former solid. Nevertheless, the main difference between the two spectra lies in the intensity of the paramagnetic-shifted peaks. Relative intensities are reported in Table 1 together with the corresponding number of lithium cations per unit cell contributing to the signal, assuming that all lithium cations are accounted for in the spectra (95 lithium cations for the fully exchanged zeolite and 82 lithium cations for the partially exchanged one, calculated from elemental analysis). The number of cations not affected by the paramagnetic shift (main signal at 1 ppm) is estimated in both cases as nearly 64 lithium cations per cell. This number corresponds to the cations that occupy sites I (I′) and II (II′) in dehydrated faujasite zeolite X.2-8 This value is also confirmed from neutron powder diffraction experiments on these two samples.21 In both cases the sites I′ and II are fully occupied by lithium cations corresponding to a population of 64 cations. It is quite logical that cations in site I′, at the center of double six-membered rings, are inaccessible to O2 and hence are unaffected by the paramagnetic shift. The absence of the paramagnetic shift for the cations in site II, in the windows between sodalite cages
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Ple´vert et al.
and supercages, deserves some discussion. Due to its small size and its short bond length with the coordination oxygen atoms, the lithium cation in site II is embedded in the six-ring window,21 contrary to larger cations such as the sodium cation which protrudes in the supercage of zeolite NaX.7 The location of the site II in zeolite Li-LSX may prevent a close contact between the oxygen molecule and the cation, explaining why this site is not submitted to the paramagnetic shift. The remaining cations are located in sites III in the supercage, with a degree of occupancy which is a function of the degree of lithium exchange. The integrated area of the paramagneticshifted peak is proportional to the number of lithium cations in site III. Conclusion
(4)
where T0 and P0 are the reference temperature and pressure. The total amount of oxygen is kept unchanged in a close system as the temperature is decreased from T1 to T2; hence
K1P1 + Kr(P1/T1) ) K2P2 + Kr(P2/T2)
(5)
The O2 pressure P2 may be estimated at different temperatures T2 from eq 5 as T1 and P1 are known. The amount of O2 adsorbed in the zeolite, Na, is obtained from eq 2. Deviations from Henry’s law are significant at the lowest temperature, 220 K, and account for errors up to 2% in the estimation of the adsorbed amount. The results are reported in Table 2. References and Notes
In addition to the induced relaxation effect of the paramagnetic adsorbed O2 molecules, molecular oxygen acts as a paramagnetic shift agent which discriminates between the different lithium cation sites of faujasite-type zeolites depending on their accessibility to O2 molecules. The paramagnetic effect of molecular oxygen depends on both its concentration and mobility in the cavities of the zeolite which can be modulated respectively by pressure and temperature changes. Nevertheless, both parameters seem to stem from a single factor corresponding to the lifetime of molecules in the vicinity of cations under investigation. This phenomenon provides a quantitative method for the discrimination of cationic sites with different accessibility to the molecular probes. Further systematic studies of zeolites with different exchanged cations are in progress in order to validate this method for the direct identification of the cationic sites accessible to physisorbed molecules in porous solids with unknown structure. Acknowledgment. The authors wish to acknowledge Dr. F. Rachdi for fruitful discussions and R. Dutartre for assistance in adsorption measurements. Appendix The amount of O2 adsorbed in the zeolite, Na, and the residual O2 pressure in the sealed tube, P, are calculated assuming that the adsorption isotherms obey Henry’s law; hence
Na ) KP
(2)
The amount of O2 molecules, Nr, in the residual volume Vr of the NMR tube is estimated from
Nr ) Kr(P/T) with
Kr ) Vr(T0/P0)
(3)
(1) Mortier, W. J. Compilation of Extra Framework Sites in Zeolites; Butterworth: Guiford, 1982. (2) Smolin, Yu. I.; Shepelev, Yu. F.; Butikova, I. K.; Petranovskii, I. K. Kristallografiya 1983, 28, 72. (3) Al-Ajdah, G. N. D.; Al-Rished, A. A.; Beagley, B.; Dwyer, J.; Fitch, F. R.; Ibrahim, K. J. Inclusion Phenom. 1985, 3, 135. (4) Forano, C.; Slade, R. C. T.; Krogh Andersen, E.; Krogh Andersen, I. G.; Prince, E. J. Solid State Chem. 1989, 82, 95. (5) Shepelev, Yu. F.; Anderson, A. A.; Smolin, Yu. I. Zeolites 1990, 10, 61. (6) Shepelev, Yu. F.; Butikova, I. K.; Smolin, Yu. I. Zeolites 1991, 11, 287. (7) Olson, D. H. Zeolites 1995, 15, 439. (8) Koller, H.; Burger, B.; Schneider, A. M.; Engelhardt, G.; Weitkamp, J. Microporous Mater. 1995, 5, 219. (9) (a) Jesson, J. P. In NMR of Paramagnetic Molecules; La Mar, G. N., Horrocks, W. DeW., Holm, R. H., Eds.; Academic Press: New York, 1973; Chapter 1. (b) Swift, T. J. NMR of Paramagnetic Molecules; La Mar, G. N., Horrocks, W. DeW., Holm, R. H., Eds.; Academic Press: New York, 1973; Chapter 2. (10) Jameson, C. J.; Mason, J. In Multinuclear NMR; Mason, J., Ed.; Plenum Press: New York, 1987; Chapter 2 and references therein. (11) Horrocks, W. DeW.; Sipe, J. P.; Sudnick, D. In Nuclear Magnetic Resonance Shift Reagents; Sievers, R. E., Eds.; Academic Press: New York, 1973; p 53. (12) Acrete, R.; Casan-Pastor, N.; Bas-Serra, J.; Baker, L. C. W. J. Am. Chem. Soc. 1989, 111, 6049. (13) Cookson, D. J.; Smith, B. E. J. Magn. Reson. 1985, 63, 217. (14) Klinowski, J.; Carpenter, A.; Thomas, J. M. J. Chem. Soc., Chem. Commun.. 1986, 956. (15) Ku¨hl, G. H. Zeolites 1987, 7, 451. (16) Rachdi, F.; Reichenbach, J.; Firlej, L.; Bernier, P.; Ribet, M.; Aznan, R.; Zimmer, G.; Helmle, M.; Mehring, M. Solid State Commun. 1993, 87, 547. (17) Schimiczek, B.; Greth, R.; Boddenberg, B. Zeolites and Related Microporous Materials: State of Art 1994; Stud. Surf. Sci. Catal. 1994, 84. (18) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley-Interscience: New York, 1984; Chapter 2. (19) Adamson, A. W. Physical Chemistry of Surfaces; Wiley-Interscience: New York, 1990; Chapter 16. (20) Jameson, C. J.; Jameson, A. K.; Cohen, S. M. Mol. Phys. 1975, 29, 1919. (21) Ple´vert, J.; Di Renzo, F.; Fajula, F.; Chiari, G. J. Phys. Chem. B 1997, 101, 10340.
