Adsorption of m-Xylene on Prehydrated Zeolite BaX: Correlation

Langmuir 2000, 16, 1931-1936

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Adsorption of m-Xylene on Prehydrated Zeolite BaX: Correlation between Temperature-Programmed Desorption and Low-Temperature Neutron Powder Diffraction Studies Christophe Pichon,†,‡ Alain Me´thivier,*,† and Marie-He´le`ne Simonot-Grange‡ Institut Franc¸ ais du Pe´ trole, 1 et 4 Avenue de Bois-Pre´ au, 92852 Rueil-Malmaison Cedex, France, and Laboratoire de Recherches sur la Re´ activite´ des Solides, UMR 5613 Universite´ de Bourgogne-CNRS, 9 Avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France Received June 24, 1999. In Final Form: October 27, 1999 Adsorption of m-xylene on partially hydrated zeolite BaX is studied by thermogravimetry, mass spectrometry, temperature-programmed desorption and neutron diffraction for different m-xylene loadings of the zeolite. Macroscopic and microscopic measurements, for both molecules, were correlated and relationships were found between the crystallographic positions of the adsorbed molecules on the zeolite and macroscopic properties such as the number and the area of the peaks in the desorption spectra for m-xylene or water. The influence of water molecules on the adsorption phenomenon of m-xylene on BaX zeolite was characterized. In particular, it was shown that the increase of the filling of the prehydrated zeolite with m-xylene leads to a rearrangement of the adsorbed aromatic molecule on its adsorption site. This displacement produces a weakening of the m-xylene-adsorbent interactions. It was also shown that the filling of the zeolite BaX with m-xylene modifies the distribution (and the energy of activation for desorption phenomenon) for water molecules, but not the location of the water adsorption site.

1. Introduction At present, the adsorption of C8 aromatics on zeolites is the most common way to process the separation of p-xylene from C8 aromatic mixtures (mainly p-xylene/mxylene mixtures). Faujasite-type zeolites (i.e., X- or Y-type zeolites) are currently used for this separation, especially the prehydrated BaX because of its high selectivity for p-xylene.1 Despite its economic interest, very few studies were devoted to the adsorption of xylenes on the prehydrated zeolite BaX.2 In this work, the adsorption of m-xylene on prehydrated zeolite BaX is studied by two experimental techniques: temperature-programmed desorption and low-temperature neutron powder diffraction. The compared results from both macroscopic and microscopic studies are reported in order to improve the understanding of adsorption of aromatics on zeolites. 2. Literature Survey

Figure 1. Structure and cationic sites of the faujasite zeolite.

2.1. Structure of the Zeolite BaX. The zeolite BaX belongs to the faujasite family (type FAU3 structure). X zeolites are characterized by a Si/Al ratio in the range 1-1.5. The framework structure has cubic symmetry. It is built of sodalite cages (also called β-cages) linked together by hexagonal prisms creating a large cavity called a supercage (Figure 1). Each supercage is connected to four sodalite cages through a 6-ring window (free aperture

diameter of 0.22 nm) and to four other supercages through a 12-ring window (free aperture diameter of 0.75 nm). There are eight supercages and eight sodalite cages per unit cell. For zeolite BaX,4-6 barium cations are located in three crystallographic sites (Figure 1): Ba(1) is situated at the center of the hexagonal prism (type I), Ba(1′) is located in the sodalite cage in front of the 6-ring window connected to the hexagonal prism (type I′), and the last site, Ba(2), is in the supercage (type II) and lies in front of the 6-ring window connected to the sodalite cage. Whereas large aromatic molecules such as xylene molecules enter only in supercages, small molecules such

* To whom correspondence should be sent. Fax: (33) 1.47.52. 70.25. E-mail: [email protected]. † Institut Franc ¸ ais du Pe´trole. ‡ Universite ´ de Bourgogne (1) Neuzil, R. W. U.S. Patent 3,558,730, 1971. (2) Furlan, L. T.; Chaves, B. C.; Santana, C. C. Ind. Eng. Chem. Res. 1992, 31, 1780. (3) Meier, W. M.; Olson, D. H.; Baerlocher, Ch. Atlas of Zeolite structure types, 4th ed.; Elsevier: London, 1996; Zeolites, Vol. 17, pp 1-230.

(4) Barrer, R. M. Zeolites and Clay Minerals as Sorbent and Molecular Sieves; Academic Press: New York, 1978. (5) Mellot, C. Thesis, Universite´ de Paris VI, France, 1993. (6) Descours, A. Thesis, Universite´ de Bourgogne, Dijon, France, 1997.

10.1021/la990819u CCC: $19.00 © 2000 American Chemical Society Published on Web 12/15/1999

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as water can be adsorbed in both supercages and sodalite cages. The adsorption sites are mainly located near the cations which neutralize the negative charges of the framework. 2.2. Adsorption of m-Xylene on Dehydrated Zeolite BaX. A large number of microscopic5-18 or macroscopic2,7,11,19-32 studies were previously performed on the adsorption of xylene on zeolite BaX. However, all those studies were done on fully dehydrated zeolite, whereas it seems that the water molecules influence the adsorption phenomenon.7 Moreover, no studies were devoted to the correlation between macroscopic and microscopic measurements about adsorption of m-xylene on dehydrated zeolite BaX. Mellot et al.16 investigated the adsorption of pure gaseous m-xylene on zeolite BaX by thermogravimetry, isothermal differential calorimetry, and neutron diffraction. The results of this study are reported and compared to ours in the results and discussion sections. In this paper we focuse on the adsorption of m-xylene on prehydrated zeolite BaX. A microscopic approach using low-temperature neutron powder diffraction (NPD) and a macroscopic approach using temperature-programmed desorption (TPD) are compared. The TPD spectra for both water and m-xylene molecules, obtained by coupling a microbalance with a mass spectrometer, are analyzed, and the results are correlated with crystallographic position obtained for those molecules by the refinement of the diffraction patterns with the Rietveld method.34 3. Experimental Section 3.1. Preparation of the Zeolite BaX. The zeolite BaX was obtained by ion exchange of a synthetic zeolite NaX supplied by Carlo Erba. The ion exchange was performed under the following (7) Czjzek, M.; Fuess, H.; Vogt, T. J. Phys. Chem. 1991, 95, 5255. (8) Czjzek, M.; Jobic, H.; Be´e, M. J. Chem. Soc., Faraday Trans. 1991, 87, 3455. (9) Germanus, A.; Ka¨rger, J.; Pfeifer, H.; Samulevic, N. N.; Zdanov, S. P. Zeolites 1985, 5, 91. (10) Ka¨rger, J.; Pfeifer, H. Zeolites 1987, 7, 90. (11) Ka¨rger, J.; Ruthven, D. M. Zeolites 1989, 9, 267. (12) Kitagawa, T.; Tsunekawa, T.; Iwayama, K. Microporous Mater. 1996, 7, 227. (13) Lachet, V. Thesis, Universite´ de Paris-Sud, France, 1998. (14) Lachet, V.; Boutin, A.; Tavitian, B.; Fuchs A. H. Langmuir, in press. (15) Mellot, C.; Espinat, D.; Rebours, B.; Baerlocher, Ch.; Fisher, P. Catal. Lett. 1994, 27, 159. (16) Mellot, C.; Simonot-Grange, M. H.; Pilverdier, E.; Bellat, J. P.; Espinat, D. Langmuir 1995, 11, 1726. (17) Schrimpf, G.; Tavitian, B.; Espinat, D. J. Phys. Chem. 1995, 99, 10932. (18) Sousa Gonc¸ alves, J. A.; Porthmouth, R. L.; Alexander, P.; Gladden, L. F. J. Phys. Chem. 1995, 99, 3317. (19) Bellat, J. P.; Cottier, V.; Simonot-Grange, M. H.; Allain, X.; Me´thivier, A. Re´ cents Progre` s en Ge´ nie des Proce´ de´ s 1995, 9, 159. (20) Bellat, J. P.; Simonot-Grange, M. H. Zeolites 1995, 15, 219. (21) Bellat, J. P.; Simonot-Grange, M. H.; Jullian, S. Zeolites 1995, 15, 124. (22) Cottier, V.; Bellat, J. P.; Simonot-Grange, M. H.; Me´thivier, A. J. Phys. Chem. B 1997, 101, 4798. (23) Goddard, M.; Ruthven, D. M. Stud. Surf. Sci. Catal., New Dev. Zeolite Sci. Technol. 1986, 28, 467. (24) Goddard, M.; Ruthven, D. M. Zeolites 1986, 6, 283. (25) Goddard, M.; Ruthven, D. M. Zeolites 1986, 6, 445. (26) Hsiao, H. C.; Yih, S. M.; Li, M. H. Adsorpt. Sci. Technol. 1989, 6, 64. (27) Li, M. H.; Hsiao, H. C. Adsorpt. Sci. Technol. 1990, 7, 9. (28) Li, M. H.; Hsiao, H. C.; Yih, S. M. J. Chem. Eng. Data 1991, 36, 244. (29) Ruthven, D. M.; Goddard, M. Zeolites 1986, 6, 275. (30) Santacesaria, E.; Gelosa, D.; Danise, P.; Carra`, S. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 78. (31) Santacesaria, E.; Morbidelli, M.; Danise, P.; Mercenari, M.; Carra`, S. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 440. (32) Simonot-Grange, M. H.; Bertrand, O.; Pilverdier, E.; Bellat, J. P.; Paulin, C. J. Therm. Anal. 1997, 48, 741. (33) IUPAC Pure Appl. Chem. 1985, 57-4, 603. (34) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 165.

Pichon et al. Table 1. Characterization of the Starting Zeolite NaX and the Final Zeolite BaX composition molar mass/g mol-1 Si/Al ratio crystallite sizea/µm Dubinin’s volumeb/ cm3 g-1 crystallinityc

NaX

BaX

Na83Si108Al85O384d 13346 1.27 0.5 to 1 0.324

Na3Ba43Si109Al84O384d 17470 1.29 0.5 to 3 0.266

95% ((3%)

100% ((3%)

a

Measured by scanning electron microscopy. b Performed by nitrogen. c Appraised by comparison between the theoretical microporous volume and the Dubinin volume. d Including error bars on the elemental compositions, framework and cation charges are balanced.

conditions: The zeolite NaX was suspended and refluxed in a 1 mol L-1 aqueous solution of Ba(NO3)2. The zeolite was then calcined at 573 K under nitrogen flux (5 L g-1 h-1) for 6 h. This procedure was repeated in order to obtain a cation exchange ratio of 96%. The characterization of the starting material and of the final zeolite BaX (Table 1) was performed by the measurement of the crystallite size by scanning electron microscopy, the measurement of the Dubinin volume by nitrogen adsorption at 77 K, and chemical analysis by atomic absorption and XR fluorescence. 3.2. TPD Measurements. TPD measurements were performed on a Setaram Tag 24-10 thermogravimetric balance, composed of two furnaces in which a reference crucible and a sample carrier crucible, both in quartz, are suspended. The gas vector was helium with a delivery of 2 L h-1. The balance was coupled with a mass spectrometer (Balzers Thermostar mass spectrometer). The sampling was performed by a capillary (situated below the furnace on the sample carrier crucible side) which was heated to improve the yield of this operation. After saturation with distilled water at 323 K, 20 mg ((1 mg) of zeolite BaX was introduced in the balance, dehydrated at 573 K, and then loaded with water and m-xylene (supplied by Fluka, which warrants a purity in excess of 98%). Two loadings were studied. In the first experiment the zeolite BaX was loaded only with water (3.6% of the dried weight of the zeolite sample, i.e., around 35 water molecules per unit cell), in the second one the zeolite BaX was loaded with 3.6% of water and then filled with m-xylene (14% of the dried weight of the zeolite, i.e., around 23 m-xylene molecules per unit cell). The desorption spectra were recorded for four different temperature rates: 0.5, 1, 3, and 5 K/min. 3.3. NPD Experiments. For diffraction experiments the zeolite BaX powder was placed into glass cells. The cells were connected to vacuum and adsorption branches to allow dehydration and adsorption of heavy water or pure deuterated m-xylene (supplied by Eurisotop, which warrants an isotopic enrichment in excess of 98%). The dehydration of the sample was performed under vacuum (10-4 Pa) at 623 K for 24 h. The glass cells were then isolated from the pumping system to begin the adsorption step. The adsorption of heavy water was done first at 473 K. It is emphasized that the water content was controlled very accurately. The adsorption step was then completed, at 423 K, by admitting pure deuterated m-xylene vapor at a controlled pressure. In this way three different samples were prepared: BaX/D2O, which has only 5% water adsorbed; BaX/D2O/1mX, which has similar water content and in addition is loaded with 4% m-xylene; BaX/D2O/3mX, which has again similar water content but contains 14% m-xylene. The amount of adsorbed water and m-xylene was determined by weighing the samples before and after the adsorption, and the result for the three samples is reported in Table 2. The cells were then transferred to a nitrogen-conditioned glovebox where the zeolite samples (about 6 g of powder) were placed in airtight vanadium cylindrical sample holders (1.5 cm diameter and 6 cm height). The sample holders were made airtight with an indium seal. The powder patterns of the three samples were recorded at low temperature on two different diffractometers: the G4.2 diffractometer at the Laboratoire Le´on Brillouin CEA-CNRS (Saclay, France) and the D2B diffractometer at the Institut Lau¨eLangevin (Grenoble, France). Both instruments gave sufficiently

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Table 2. Experimental and Crystallographic Data for Neutron Powder Diffraction Study BaX/D2O water/molecules per unit cell m-xylene/molecules per unit cell diffractometer temperature/K wavelength/nm total time/h range/deg 2θ step size/deg 2θ space group lattice parameter/nm no. of observations/steps, N no. of contributing reflections no. of structural parameters (P1) no. of profile parameters (P2) Rwp ) {∑w[y(o) - y(c)]2/∑wy(o)2}1/2 (%) Rexp ) [(N - P1 - P2)/∑wy(o)2]1/2 (%)

BaX/D2O/1mX

Composition 51.3

BaX/D2O/3mX

36.7 5.3

48.3 21.6

Data Collection G4.2 1.5 0.23433 24 0-150 0.1

D2B 5.0 0.15938 12 0-150 0.05

D2B 5.0 0.15938 12 0-150 0.05

Rietveld Refinement Fd3m 2.508(4) 1307 492 36 9 E4.9 E2.6

Fd3m 2.513(6) 2844 1452 42 9 E3.8 E2.0

Fd3m 2.512(6) 2888 1483 42 9 E3.1 E2.1

good counting statistics. The experimental conditions are specified in Table 2. The diffraction patterns were analyzed by the Rietveld refinement method34 using the GSAS software package.35 The space group used was Fd3m, since there was no indication in the powder pattern of a lower symmetry (e.g., additional lines) that may have been induced by the adsorption of the m-xylene molecules. First, the profile parameters, the background parameters, the unit cell constant, and the zeropoint were optimized. After a preliminary refinement of the framework geometry, the adsorbed m-xylene and water molecules were located with a series of difference Fourier maps. The m-xylene molecules were refined as rigid bodies. All restraints on the framework atoms and on population parameters of the host molecules were progressively released during the refinement. The Rietveld plot for the three refinements is shown in Figure 2.

4. Results 4.1. TPD Study. 4.1.1. TPD Spectra and Distribution of Water Molecules. The TPD spectra representing the rate of water desorption from zeolite BaX powder, for different heating rates, as a function of the temperature of the sample, exhibit a water single wide peak, centered on 501 K ((34 K), when there are no m-xylene molecules adsorbed on the zeolite (Figure 3). On the contrary, two distinct water desorption peaks (centered on 412 ( 31 K and 486 ( 26 K, respectively) are obtained when the prehydrated zeolite BaX is highly filled with m-xylene (Figure 4). These observations seem to indicate the presence, on the prehydrated zeolite BaX, of at least two adsorption sites for water. The width of the single peak may indicate that the sites have quite the same activation energy when there are no m-xylene molecules adsorbed on the zeolite. In the presence of m-xylene and whatever the heating rate, the ratio between the area representing the water low-temperature peak (SP1w) and the area representing the water high-temperature peak (SP2w) is constant (SP1w/ SP2w is qualitatively estimated by deconvolution and equal 0.7). 4.1.2. TPD Spectra and Distribution of m-Xylene Molecules. The TPD spectra representing the rate of m-xylene desorption from prehydrated zeolite BaX (Figure 5) for different heating rates as a function of sample temperature exhibit two distinct peaks: one very intense at low temperature (centered on 347 ( 23 K), and another, (35) Larson, A. C.; Von Dreele, R. B. GSAS General Structure Analysis System; LAUR 86-748, Los Alamos National Laboratory: Los Alamos, NM, 1994.

smaller, at high temperature (centered on 448 ( 21 K). It indicates that there are two distinct desorption energies for m-xylene molecules adsorbed on the prehydrated zeolite BaX. The ratio between the area of the m-xylene low-temperature peak (SP1x) and the area of the m-xylene high-temperature peak (SP2x) is not constant: SP1x/SP2x decreases when the heating rate increases. 4.2. Diffraction Study. Remark on the Recording Temperature. To improve the quality of the diagrams, the powder patterns were recorded at low temperature whereas the adsorption process takes place between 423 K (for m-xylene) and 473 K (for water). However, it seems that the recording temperature has not a lot of influence on the crystallographic structure. As a matter of fact, a record, at 300 K, of the sample BaX/D2O/3mX does not show sensitive shifts in the refined structure (i.e., in the location and in the distribution of the atoms of the structure) compared to the low-temperature one (at 5 K). 4.2.1. Location and Distribution of Water Molecules on Prehydrated Zeolite BaX Filled or Not with m-Xylene. Whatever the composition of the adsorbed phase, there are always two different adsorption sites for water whose location is very similar for all samples. The site w1 is coordinated to the Ba2+ cation in site I′. It forms hydrogen bridges to the framework oxygens, indicated by OwaterOframework distances between 0.274(3) and 0.277(3) nm. The other adsorption site, w2, is in the supercage close to the Ba2+ cation in site II. In this position, a strong interaction is also observed between the water molecule and the framework. Here we notice a Owater-Oframework distance between 0.274(3) and 0.286(3) nm which may be associated with a hydrogen bridge. Although the adsorption sites are very similar in all three samples, the number of water molecules at these sites varies with the composition of the adsorbed phase (Table 3). The ratio between the number of water molecules located in sodalite cage the and the number of water molecules located in the supercage equals 2.4 for the prehydrated zeolite BaX which does not contain m-xylene. It decrease to 0.8 and 0.9 for the prehydrated zeolite BaX filled with 0.7 m-xylene molecules per supercage and highly filled with m-xylene (i.e., around three m-xylene molecules per supercage), respectively. 4.2.2. Location of m-Xylene Molecules on Prehydrated Zeolite BaX. The m-xylene molecules are located in the supercage, close to a Ba(2) cation. At low filling (0.7 m-xylene molecules per supercage), the aromatic molecule is adsorbed on a high symmetry position (the center of the

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Figure 3. TPD profile for water in prehydrated BaX zeolite (heating rate, 3 K/min).

Figure 4. TPD profile for water in prehydrated BaX zeolite filled with m-xylene (heating rate, 3 K/min).

Figure 2. Neutron powder patterns: (upper part) +, experimental data; s, calculated profile; (lower part) s, difference between experimental and calculated profiles. (a) Rietveld plot for the refinement of BaX/D2O. (b) Rietveld plot for the refinement of BaX/D2O/1mX. (c) Rietveld plot for the refinement of BaX/D2O/3mX.

aromatic ring is located on the 3-fold axis) and the plane of the aromatic ring is nearly parallel to the plane of the six-ring window (Figure 6). The m-xylene is stabilized on one hand by a very strong π-type interaction between the aromatic ring and the Ba2+ cation of type II (indicated by a short distance of 0.231(2) nm between the two) and, on the other hand, by two electrostatic interactions between the framework oxygens and the deuteriums of the mxylene (characterized by short Dmethyl-Oframework distances of 0.228(3) nm). At high filling (2.7 m-xylene molecules per supercage), the center of the aromatic ring moves away from the Ba(2) and leaves the 3-fold axis but remains in a high-symmetry position (on a mirror plane); the plane of the aromatic ring is no longer parallel with the six-ring window (Figure 7). The m-xylene is stabilized on one hand by a strong π-type interaction between the aromatic ring and the Ba2+ cation (indicated by a distance of 0.274(2)

Figure 5. TPD profile for xylene in prehydrated BaX zeolite filled with m-xylene (heating rate, 3 K/min). Table 3. Number of Adsorbed Molecules Per Cavity (or per 1/8 of Unit Cell) in the Prehydrated Zeolite BaX BaX/D2O water

supercage (site w2) sodalite cage (site w1) total m-xylene supercage

1.9 4.5 6.4

BaX/D2O/ BaX/D2O/ 1mX 3mX 2.5 2.1 4.6 0.7

3.1 2.9 6.0 2.7

nm between the two) and, on the other hand, by an electrostatic interaction between a framework oxygen and a deuterium of the m-xylene (characterized by a short Dmethyl-Oframework distance of 0.245(3) nm). In their work, Mellot et al.16 characterized, by a crystallographic study, the adsorption of gaseous m-xylene on the fully dehydrated zeolite BaX. They showed that, when the filling of the zeolite BaX increases, the m-xylene has to reorient its methyl groups while the center of the aromatic ring moves away from the Ba(2) cation (0.03 nm). In this work, a comparable displacement of the aromatic ring has been characterized (0.04 nm) but the reorientation of the aromatic molecules when the filling

Adsorption on Prehydrated Zeolite

Figure 6. Adsorption site for m-xylene in the prehydrated zeolite BaX at low filling: (a) front view of the adsorption site; (b) side view of the adsorption site.

increases is not seen. Thus, it seems that the water molecules prevent the adsorbed m-xylene molecules from reorienting its methyl groups, i.e., from rotating around the 3-fold axis. 5. Discussion 5.1. Water Molecules on Prehydrated Zeolite BaX. Both microscopic and macroscopic measurement show the existence of two adsorption sites for water. As the m-xylene can enter the supercage, it is assumed that the adsorption site for water located in the sodalite cage is the least modified, on the energy standpoint, during the adsorption of m-xylene. The adsorption site for water characterized by the low-temperature desorption peak (P1w) is then associated with the site w2, located in the supercage, and the adsorption site characterized by the high-temperature peak (P2w) of desorption is then associated with the site w1, located in the sodalite cage. These hypothesis are in good agreement with the ratios between the number of water molecules in each site, calculated for each measurement. 5.2. m-Xylene Molecules on Prehydrated Zeolite BaX. The TPD study shows the existence of, at least, two states for m-xylene molecules adsorbed on the prehydrated zeolite BaX (characterized by two desorption peaks, one very intense at low temperature, P1x, and an other, smaller, at high temperature, P2x). The results extracted from the diffraction experiments suggest that the lowtemperature peak, which represents the essential part of the adsorbed m-xylene, can be associated with the adsorption site for m-xylene found when the prehydrated zeolite BaX is highly filled with aromatic, whereas the

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Figure 7. Adsorption site for m-xylene in the prehydrated zeolite BaX at high filling: (a) front view of the adsorption site; (b) side view of the adsorption site.

high-temperature peak, which represents very few adsorbed m-xylene, can be associated with the adsorption site for m-xylene found at low filling on the prehydrated zeolite BaX. The decrease of the temperature of desorption as the filling of the prehydrated zeolite BaX increases can be correlated with a weakening of the strength of the adsorbate-adsorbent interactions, which is characterized by the decrease of the interaction between the aromatic ring and the Ba2+ cation (shown by the displacement of the aromatic ring to the center of the supercage) and the loss of one hydrogen bridge per m-xylene molecule. This decrease seems not to be compensated by the increase of the adsorbate-adsorbate interactions according to the observed distances between the methyl groups of the m-xylene molecules. These hypotheses allow an explaination of the decrease, observed in TPD, of the ratio between the area of the m-xylene low-temperature peak and the area of the m-xylene high-temperature peak. The more the heating rate increases, the less the m-xylene molecule can move to the strongest adsorption site. Furthermore, these assumptions are in good agreement with the thermodynamic part of their study, in which Mellot et al.16 measured, for a similar displacement of the aromatic ring of the m-xylene molecule produced by the filling of the dehydrated zeolite BaX, a slow decrease in the adsorption derivative enthalpies. 6. Conclusion This work has shown the great interest in the correlation between macroscopic and microscopic studies. It allows further insight in the adsorption process of pure m-xylene on the prehydrated zeolite BaX. The correlation has shown that the adsorption of aromatic molecules dramatically

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modifies the distribution of water molecules and the energy of the adsorption sites for water (especially for the site located in the supercage) but does not change the location of the water adsorption sites on the prehydrated zeolite BaX. Furthermore, the crystallographic position of adsorbed m-xylene on the prehydrated zeolite BaX has been proposed at low and high filling: During the filling of the zeolite the aromatic molecule moves away from the 3-fold

Pichon et al.

axis and goes to the center of the supercage. At least, it has been shown that the displacement of the m-xylene molecule leads to a weakening of the adsorbate-adsorbent interaction (cation-molecule and framework-molecule interactions), which is perceptible in the thermodynamic study. LA990819U