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Location of Benzene in NaBeta Zeolite upon Coadsorption of Ammonia and Methylamine: A Further Confirmation of Molecular Recognition Effect in Benzene Adsorption in 12R Window Zeolites Bao-Lian Su* and Vale´rie Norberg Laboratoire de Chimie des Mate´ riaux Inorganiques, I.S.I.S., Universite´ de Namur, 61 Rue de Bruxelles, B-5000, Namur, Belgium Received January 20, 2000. In Final Form: March 31, 2000 The adsorption of a single component such as ammonia and methylamine in NaBeta zeolite has been first investigated by means of in-situ infrared spectroscopy, and the effect of coadsorption of ammonia and methylamine on the location of benzene has been then checked. The benzene adsorption behavior in NaBeta zeolite with or without the presence of coadsorbates has been correlated with structural and chemical properties of zeolite. The hypothesis of the molecular recognition effect in benzene adsorption has been further verified. The present work shows that ammonia and methylamine can interact not only with Na+ ions via the lone pair on nitrogen atoms but also with the large number of silanols present in NaBeta zeolite. The interaction strength of ammonia, methylamine, and benzene with NaBeta ranks in the order of methylamine/NaBeta > benzene/NaBeta > ammonia/NaBeta. It is found that the interaction of ammonia and methylamine with NaBeta causes an important modification of lattice parameters, indicating the deformation of zeolite framework. Despite this deformation of zeolite framework upon coadsorption of either ammonia or methylamine, only adsorption of benzene in Na+ ions and no change in the location of benzene are observed. The 12R windows in NaBeta remain less favorable sites for benzene adsorption, being contrary to what we observed in KL zeolite upon coadsorption of methylamine, in NaEMT and HY upon coadsorption of ammonia, and in NaY upon adsorption of benzene alone. The present work confirms further again that the adsorption of benzene in 12R window zeolites is governed by a molecular recognition effect. The location of benzene in 12R windows of zeolites, which is a phenomenon of multiple interaction, i.e., six hydrogen atoms of benzene with six oxygen atoms of 12R window, is possible only if the structural and chemical characters are compatible between the benzene molecule and 12R window. In regard to the benzene adsorption properties, 12R window zeolites can be divided into four different categories. This classification could be quite useful in the design of new catalysts and adsorbents for industrial treatments of aromatics.
1. Introduction The study of the location of aromatics in 12R window zeolites attracts growing research attention due to the importance of these zeolites in the catalytic transformation and separation processes of aromatics.1-10 Various techniques such as FTIR-Raman,11-20 neutron diffraction,21,22 * Corresponding author. (1) Pe´rez-Pariente, J.; Sastre, E.; Forne´s, V.; Martens, J. A.; Jacobs, P. A. Appl. Catal. 1991, 69, 125. (2) Pardillos, J.; Brunel, D.; Coq, B.; Massiani, P.; De Me´norval, L. C.; Figueras, F. J. Am. Chem. Soc. 1990, 112, 1313. (3) Bellussi, G.; Pazzuconi, G.; Perego, C.; Girotti, G.; Terzoni, G. J. Catal. 1995, 157, 227. (4) Sirasanker, S.; Tangaraj, A.; Abdulla, R. A.; Ratnasamy, P. Stud. Surf. Sci. Catal. 1993, 75, 397. (5) Hari Prasad Rao, P. R.; Massiani, P.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1994, 84, 1449. (6) Reddy, K. S. N.; Rao, B. S.; Shiralkar, V. P. Appl. Catal., A 1993, 95, 53. (7) Parikh, P. A.; Subrahmanyam, N. Appl. Catal., A 1992, 90, 1. (8) Tsai, T. C.; Ay, C. L.; Wang, I. Appl. Catal. 1991, 77, 199. (9) Chen, W. H.; Pradhan, A.; Jong, S. J.; Lee, T. Y.; Wang, I.; Tsai, T. C.; Liu, S. B. J. Catal. 1996, 163, 436. (10) Dzwigaj, S.; de Mallmann, A.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1990, 86, 431. (11) Shen, J. P.; Ma, J.; Sun, T.; Xu, Z.; Jiang, D. Z.; Min, E. Z. Stud. Surf. Sci. Catal. 1994, 90, 163. (12) Su, B. L.; Norberg, V. Zeolites 1997, 19, 65. (13) Jia, C. J.; Massiani, P.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1993, 89, 3659. (14) Hegde, S. G.; Kumar, R.; Bhat, R. N.; Ratnasamy, P. Zeolites 1989, 9, 231. (15) Hunger, M.; Ernst, S.; Steuernagel, S.; Weitkamp, J. Microporous Mater. 1996, 6, 349.
2 H NMR,23-25 inelastic neutron scattering,26 and theoretical calculation27-28 have been employed. It was found that benzene molecules can sit not only on compensating ions through an interaction between the π electrons of the ring and the cations but also on 12R windows through a weak interaction between six hydrogen atoms of benzene with six oxygen atoms of a 12R window. However, the adsorption of benzene on one site or another or on both sites depends on the Lewis acidity of counterions, the basicity of framework, and the shape of 12R windows. The adsorption of benzene in the counterions is a general trend
(16) Maache, M.; Janin, A.; Lavalley, J. C.; Joly, J. F.; Benazzi, E. Zeolites 1993, 13, 419. (17) Beck, L. W.; Haw, J. F. J. Phys. Chem. 1995, 99, 1076. (18) de Mallmann, A.; Barthomeuf, D. J. Chem. Soc., Chem. Commun. 1986, 476. (19) de Mallmann, A.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1986, 28, 609. (20) de Mallmann, A.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1987, 37, 365. (21) Vatale, G.; Bull, L. M.; Powell, B. M.; Cheetham, A. K. J. Chem. Soc., Chem. Commun. 1995, 2253. (22) Fitch, A. N.; Jobic, H.; Renouprey, A. J. Phys. Chem. 1986, 90, 1311. (23) Bull, L. M.; Henson, N. J.; Cheetham, A. K.; Newsam, J. M.; Heyes, S. J. J. Phys. Chem. 1993, 97, 11776. (24) Auerbach, S. M.; Bull, L. M.; Henson, N. J.; Metiu, H. I.; Cheetham, A. K. J. Phys. Chem. 1996, 100, 5923. (25) Norberg, V.; Su, B.-L. Stud. Surf. Sci. Catal. 1999, 125, 253. (26) Jobic, H.; Renouprey, A.; Fitch, A. N.; Lauter, H. J. J. Chem. Soc., Faraday Trans. 1987, 83, 3199. (27) Kitagawa, T.; Tsunekawa, T.; Iwayama, K. Microporous Mater. 1996, 7, 227. (28) Auerbach, S. M.; Metiu, H. J. Chem. Phys. 1996, 105, 3573.
10.1021/la0000670 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/14/2000
Benzene in NaBeta
while the location of benzene in the 12R window of zeolite is generally governed by a molecular recognition effect29,30 where the adsorbate and 12R windows should have the suitable chemical and structural properties. More recently, we have reported the interesting experiments that adsorption properties of zeolites can be accommodated by modification of the chemical or structural properties of zeolites by introduction of a coadsorbate.29-32 For instance, in NaEMT, in the presence of benzene alone, Na+ ions are the main adsorption sites for benzene; 12R windows of this zeolite are not preferential sites for benzene adsorption due to the low negative charge of oxygen atoms in 12R windows. Upon coadsorption of ammonia, the negative charge of oxygen atoms is increased;33-35 the 12R windows become quite favorable sites for benzene adsorption.31,32 In KL, the negative charge of oxygen atoms calculated by using Sanderson electronegativity equalization is high enough, but benzene molecules do not adsorb on its 12R windows because of the structural incompatibility between benzene molecules and 12R windows.36 Upon coadsorption of ammonia, no change in the location of benzene was thus observed,37 while upon coadsorption of methylamine, the 12R windows of this zeolite become the most preferential adsorption sites for benzene due to the framework deformation induced by the strong interaction between methylamine and KL zeolite framework. This deformation gives a beneficial effect, and the 12R windows in KL zeolites which are initially not compatible with benzene molecules become mostly preferable adsorption sites for benzene in the presence of methylamine.30 Beta zeolite contains three mutually intersecting 12R channel systems and two kinds of 12R windows. Although this zeolite is still a poorly defined material and one can think that any interpretation of data obtained from this zeolite is dangerous, it is one of the most important 12R window zeolites and has been widely used as catalyst and catalyst support in various catalytic processes of aromatics.1-10 A recent study showed that Beta zeolite is a very active and selective catalyst for alkylation of benzene.38 The knowledge on benzene adsorption in Beta zeolite should be therefore of important interest to better understand the reaction mechanism and to develop new catalysts with advanced performances for alkylation of benzene. It has been reported that in NaBeta in the presence of benzene alone benzene molecules adsorb mainly on Na+ ions at any benzene loading, and only very few benzene molecules sit in 12R windows at low benzene loading.29 As known, the coadsorption of different molecules in zeolites deserves particular attention on practical as well as fundamentals grounds. The present paper deals with the locations of benzene upon coadsorption of ammonia and methylamine within NaBeta zeolite. We try to understand on the molecular level the effect of structure on the location of benzene and verify the hypothesis that the adsorption of benzene in 12R windows of zeolites is governed by a molecular recognition effect proposed previously.29,30 This study can (29) Su, B. L.; Norberg, V. Langmuir 1998, 14, 7410. (30) Su, B. L.; Norberg, V.; Hansenne, C. Langmuir 2000, 16, 1132. (31) Su, B. L.; Manoli, J. M.; Potvin, C.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1993, 89, 857. (32) Su, B. L. Zeolites 1996, 16, 25. (33) Mortier, W. J. J. Catal. 1978, 55, 138. (34) Sanderson, R. T. Chemical Bonding and Bond Energy, 2nd ed.; Academic Press: New York, 1976. (35) Mortier, W. J. In Proceedings of Sixth International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworths: Guildford, 1984; p 734. (36) Su, B. L.; Barthomeuf, D. Zeolites 1995, 15, 470. (37) Su, B. L. Zeolites 1996, 16, 75. (38) Siffert, S.; Gaillard, L.; Su, B.-L. J. Mol. Catal. 2000, 153, 267.
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answer also whether the 12R windows of zeolite Beta can be accommodated chemically or structurally by coadsorbing ammonia and methylamine and become finally the preferable adsorption sites for benzene. On the basis of the present and previous results, we try also to classify 12R window zeolites into different categories in regard to their adsorption behavior. This classification and verification of molecular effect should be very useful in the design of new catalysts and adsorbents used in the industrial treatments of aromatics. 2. Experimental Section 2.1. Materials. The zeolite precursor with chemical composition obtained from chemical analysis for a unit cell of 64 T atoms of (TEA)5.6Na1.4(AlO2)3.5(SiO2)60.5 was kindly provided by the Instituto tecnologia Quimica, Polytechnical University of Valencia, and synthesized in alkaline medium with the presence of tetraethylammonium hydroxide.39 Na+ ion exchanged Beta zeolite was prepared using the previous reported procedure in ref 29. The chemical composition of NaBeta from chemical analysis for a unit cell is Na3.5(AlO2)3.5(SiO2)60.5. 2.2. Infrared Studies. The adsorption of molecule (ammonia or methylamine) was performed on self-supported zeolite wafers (15 mg/cm2) prepared with a pressure of 5 tons/cm2. The sample wafer placed in the IR cell was heated in a dry oxygen flow from room temperature to 723 K at a rate of 3 K/min. The temperature was maintained overnight in the same atmosphere. The sample was treated in vacuo for 4 h at the same temperature. The IR cell was then cooled slowly to room temperature, and the spectrum of zeolite phase alone was recorded as a reference using a PerkinElmer Spectrum 2000 spectrometer. The adsorption of increasing and known amounts of molecule was carried out on the pretreated wafer. After each adsorption, the sample was maintained at room temperature for 1 h equilibration. The spectra were then recorded. The influence of coadsorption of ammonia and methylamine on the benzene location was performed on two samples having benzene loading of 1.0 and 6.0 molecules per unit cell (molecules/ uc), respectively. After recording the spectra of adsorbed benzene, an increasing and known amount of ammonia or methylamine was introduced into the IR cell. After 1 h equilibration at room temperature, the IR spectra were recorded. The quantities of benzene, ammonia, and methylamine introduced into the IR cell were expressed in molecules per unit cell (molecules/uc), although, after saturation of the zeolite, this has no real physical meaning. However, it does facilitate the comparison of the results. The interaction strength of benzene, ammonia, and methylamine with NaBeta zeolite was evaluated by desorption at different temperature for 0.5 h.
3. Results and Discussion The adsorption of ammonia and methylamine alone in NaBeta has been first studied, and the effect of coadsorption of these bases on zeolite framework and further on the location of benzene in this zeolite has been then analyzed. 3.1. Adsorption of Ammonia Alone in NaBeta. The adsorption of NH3 in protonated zeolites40-43 and oxides such as MgO, CuO, Al2O3, SiO2, and SiO2-Al2O3 has been widely studied.44-48 However, the adsorption of ammonia (39) Camblor, M. A.; Mifsud, A.; Pe´rez-Pariente, J. Zeolites 1991, 11, 792. (40) Zecchina, A.; Buzzoni, R.; Bordiga, S.; Geobaldo, F.; Scarano, D.; Riccjardi, G.; Spoto, G. Stud. Surf. Sci. Catal. 1995, 97, 213. (41) Bra¨ndle, M.; Sauer, J. J. Mol. Catal. 1997, 119, 19. (42) Liepold, A.; Roos, K.; Reschetilowski, W.; Schmit, R.; Sto¨cker, M.; Philippov, A.; Anderson, M. W.; Esculcas, A. P.; Rocha, J. Stud. Surf. Sci. Catal. 1997, 105, 423. (43) Ghosh, A.; Curthoys, G. J. Chem. Soc., Faraday Trans. 1984, 80, 90. (44) Coluccia, S.; Lavagnino, S.; Marchese, L. J. Chem. Soc., Faraday Trans. 1987, 83, 477. (45) Basila, M. R.; Kantner, T. R. J. Phys. Chem. 1967, 71, 467. (46) Kagami, S.; Onishi, T.; Tamaru, K. J. Chem. Soc., Faraday Trans. 1 1984, 80, 29.
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Figure 1. Changes in the infrared absorbance spectra of N-H stretching vibration range (A) and H-N-H bending vibration range (C) of ammonia adsorbed in NaBeta zeolite as a function of amounts of ammonia introduced into the IR cell (molecules/ uc). The range of 2200-1700 cm-1 (B) is also given to check the deformation of framework. (a) 1.9, (b) 5.8, (c) 9.6, (d) 15.4, (e) 19.3, (f) 28.9, (g) 33.0, (h) 38.6, and (i) saturation of NaBeta zeolite (a pressure of 20 Torr of ammonia present in the IR cell).
in cationic zeolites is less reported in the literature.49-51 The NH3 molecule is a symmetrical top and would have four fundamentals: two totally symmetric (A1) and two doubly degenerate (E). Gaseous NH3 molecules give four vibration peaks in the mid-infrared spectrum at 344452 [341453], 3336, 1627.5, and 931.6 cm-1 which were already assigned to the asymmetric (ν3) and symmetric (ν1) stretching vibration of N-H and the asymmetric (ν4) and symmetric (ν2) bending vibration of H-N-H, respectively. Ammonia molecules adsorbed in NaBeta zeolite give more complicated spectra. The infrared spectra of increasing doses of ammonia adsorbed on NaBeta in the range 40001300 cm-1 are depicted in Figure 1. The infrared spectrum of NaBeta zeolite alone and that of the gas phase of the IR cell have been subtracted. The wavenumber range below 1300 cm-1 cannot be studied due to the strong absorption of zeolite wafer. Three main peaks at 3400, 3315, 3253 cm-1, a shoulder at 3475 cm-1, two overlapped broad bands centered at 2985 and 2789 cm-1, and a negative feature at 3740 cm-1 are detected in the range 4000-2400 cm-1 (Figure 1A). One strong vibration feature at 1637 cm-1 and a triplet consisting of two overlapped bands at 1507 , 1473 cm-1 and a weak peak at 1554 cm-1 are observed in the range 1800-1300 cm-1 (Figure 1C). These peaks are present at any ammonia loading. The peaks at 3400, 3315, and 3253 cm-1 can be assigned to the asymmetric (ν3) and symmetric (ν1) stretching vibration of N-H and the combination bands 2ν4. The first two bands of NH3 are shifted toward low wavenumbers after adsorption in NaBeta compared with the corresponding bands of gaseous NH3. It is well-known that the asymmetric bending vibration of ammonia coordinately bonded to Lewis acid sites falls in the range 1600-1660 cm-1 and the deformation band (47) Peri, J. B. J. Phys. Chem. 1965, 69, 21. (48) Peri, J. B. J. Phys. Chem. 1965, 69, 231. (49) Kiselev, A. V.; Lygin, V. I.; Titova, T. I. Zh. Fiz. Khim. 1964, 38, 2730. (50) Morishige, K.; Kittaka, S.; Ihara, S. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2525. (51) Morishige, K.; Kittaka, S.; Takao, S.; Morimoto, T. J. Chem. Soc., Faraday Trans. 1 1984, 80, 993. (52) Herzberg, G. Molecular Spectra and Molecular Structure, II. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold Company: NewYork, 1945. (53) Jacox, M. E.; Milligan, D. E. Spectrochim. Acta 1963, 19, 1173.
Su and Norberg
of H-N-H of NH4+ ions in the range 1350-1550 cm-1. The NH3 IR band observed at 1637 cm-1 arises very likely from the asymmetric bending vibration of H-N-H of molecularly adsorbed on Na+ ions and is also shifted but to high wavenumber compared with this band of gaseous NH3. The bending triplet at 1473, 1507, and 1554 cm-1 can be therefore attributed to NH4+ ions bidentately or tridentately bonded to framework according to Zecchina and co-workers.54 This is also in agreement with the observation by Uytterhoeven et al. and Vomsheid et al. on the basis of the study on adsorption of NH3 in protonated X and Y zeolites55 and in HSAPO-34 molecular sieve,56 respectively. However, NaBeta zeolite used does not contain bridging framework hydroxyls after ion exchange.29 One may ask, where are these NH4+ ions in our NaBeta zeolite after introduction of NH3? It has been observed that after thermal pretreatment in a vacuum a large number of silanols which give rise to an infrared band at 3740 cm-1 are present in our NaBeta zeolite.29 It is known that these silanols, acting as weak Bro¨nsted acid sites, can interact with some basic molecules such as benzene and ammonia and can be shifted toward low wavenumbers.29 The negative feature at 3740 cm-1 and the broad band centered at 2985 and 2789 cm-1 result therefore from the interaction of NH3 with silanols of NaBeta zeolite. The consumption of silanols at 3740 cm-1 gives a negative feature at same wavenumber after subtracting the spectrum of NaBeta zeolite phase from the spectra of adsorbed NH3 in NaBeta. The two broad bands centered at 2985 and 2789 cm-1 were already observed in HSAPO-34, HY, HMOR, HBeta, and HZSM-5 by different research groups and attributed to N-H stretching bands of triply (Zecchina et al.)54 or doubly bridged (Teunissen et al.)57 NH4+ ions. It is not the present work to distinguish that these two bands come from bidentately or tridentately coordinated NH4+ ions. Our study shows clearly and indeed that the silanol groups in NaBeta are acidic enough to interact with NH3 molecules to give NH4+ ions, and all the silanols present in NaBeta zeolite can interact with NH3 to form NH4+ ions. The formation of NH4+ due to the interaction of silanols and NH3 was previously observed in amorphous silicaalumina.45 The surface hydroxyls in γ-AlO3 was also found to be able to interact with NH3 to give NH4+ ions.47,48 The assignment of the shoulder at 3475 cm-1 is not clear yet. This shoulder with medium intensity is very likely due to the adsorption of NH3 in NaBeta which can split the doubly degenerate asymmetric stretching vibrations of N-H, which have the same energy level in gaseous state, into two components at upper (3475 cm-1) and lower (3400 cm-1) energy levels. The splitting of the doubly degenerate asymmetric bending vibrations, however, is not clearly observed. This part of the results shows that, upon adsorption of NH3 in NaBeta, most of the ammonia is adsorbed molecularly on Na+ ions via the lone electron pair on nitrogen atoms. NH3 molecules can also interact with weak Bro¨nsted acidic silanols to form NH4+ ions. These ions can be bidentately or tridentately bounded on the framework. Degenerate asymmetric stretching vibrations and the displacement of the stretching vibration bands of N-H (54) Zecchina, A.; Marchese, L.; Bordiga, S.; Paze, C.; Gianotti, E. J. Phys. Chem. B 1997, 101, 10128. (55) Uytterhoeven, J.-B.; Christner, L. G.; Keith Hall, W. J. Phys. Chem. 1965, 69, 2117. (56) Vomscheid, R.; Briend, M.; Peltre, M.-J.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1995, 91, 3281. (57) Teunissen, E. H.; Van Santen, R. A.; Jansen, A. P.; Van Duijneveldt, F. B. J. Phys. Chem. 1993, 97, 203.
Benzene in NaBeta
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and bending band of H-N-H toward lower and higher wavenumber, respectively, after adsorption of NH3 in NaBeta may be the consequence of the disturbing action of the zeolite cations on the unbounded electron of the nitrogen atoms, but also the results of the formation of hydrogen bonds between the NH3 and the negatively charged lattice oxygen atoms. Both interactions can weaken the N-H bond and make the NH3 stretching and bending frequencies of NH3 adsorbed in NaBeta lower and higher, respectively, than those of the free NH3 molecules. The spectrum in the range 2200-1700 cm-1 (Figure 1B) is also given since some IR features in this region were observed by Zecchina et al.58 upon adsorption of molecules such as H2O, NH3, CH3OH, C2H5OH, (CH3)O, (C2H5)2O, and C4H8O in a series of protonated zeolites such as HY, H-ZSM-5, and H-Mordenite, and they attributed these features to framework overtones modification. In this range, two broad bands at 1943 and 1834 cm-1 which are not present in the case of adsorption of NH3 in KL zeolite are observed and are not originated from the vibration of ammonia molecules. As indicated above, the spectra of ammonia adsorbed in NaBeta are obtained after subtraction of the spectrum of zeolite phase and that of gas phase of IR cell. If the zeolite skeletal vibration modes are not modified by adsorption of ammonia, no peaks should be observed in this range as in the case of adsorption of NH3 in KL. The presence of some features in this region indicates that zeolite skeletal vibrations are indeed affected by the adsorption of ammonia. Zecchina et al. reported40,58 that the presence of adsorbates in the channels of zeolites causes increase of the lattice parameters, indicating the deformation of zeolite framework and consequently a shift of the skeletal modes (especially the overtones and combination bands in the 2100-2550 cm-1 interval) toward lower wavenumbers. As a result, some additional structural vibration features due to deformation of framework are present in this region after subtraction of the spectra. We will take care of this disturbing, confusing, and undesired consequence since some real adsorbate bands can be altered. The interaction strength of ammonia with NaBeta zeolite has been evaluated using a zeolite wafer in contact with a high pressure of ammonia. Figure 2 reports the changes in infrared absorbance spectra of ammonia adsorbed in NaBeta zeolite upon desorption at different temperatures. The two broad bands, indicating the deformation of zeolite framework, are clearly present when a high pressure of ammonia is in contact with NaBeta zeolite. A desorption at room temperature for 0.5 h can remove a great part of ammonia adsorbed in NaBeta zeolite since the intensity of all peaks stemming from adsorbed ammonia molecules is reduced about 70%. As the evacuation temperature is raised, all the peaks decrease in intensity. After 0.5 h desorption at 423 K, the peaks resulting from the ammonia molecularly adsorbed on Na+ ions, NH4+ ions, and the two broad bands indicating the deformation of framework are still present despite their low intensity, and the silanols are not totally restored. It was reported that ammonia adsorbed in KL can be easily removed even at a desorption temperatures less than 343 K.30 Ammonia interacts therefore stronger with NaBeta than with KL. This may explain, at least in part, why the deformation of framework is observed upon adsorption of ammonia in NaBeta but not in KL zeolite although it is
well-known that some zeolite frameworks are more flexible than others. Nevertheless, the above discussion shows that the deformation of Beta zeolite framework is relative to the interaction of ammonia with this zeolite. The chemical, electronic, and structural properties of NaBeta zeolite are therefore modified strongly due to strong interaction between NaBeta and NH3, and adsorption properties of this zeolite could be consequently changed. 3.2. Adsorption of Methylamine Alone in NaBeta Zeolite. Because of the great difficulties in the vibrational assignment of CH3NH2, only very few papers on the adsorption of methylamine can be found in the literature43,50,51,59 that deal with the adsorption of methylamine on dehydrated faujasites exchanged with alkali metal and alkaline earth metal cations, HY, HMOR, and metallic catalysts such as Cu dispersed on Al2O3 using FTIR and microbalance techniques. Replacement of a hydrogen atom in the ammonia molecule by an alkyl groups, which are normally electron donating (more so than hydrogen) toward electronegative elements, results in increased electron density on the nitrogen atom and increased basicity. We might expect the stronger interaction of NaBeta zeolite with methylamine than with NH3. Gaseous methylamine molecule in the mid-infrared spectrum on the basis of experimental observation and theoretical calculation gives the asymmetric and symmetric stretching vibrations of N-H bond at 3427 and 3361 cm-1 and the asymmetric bending vibration of H-N-H at 1623 cm-1, a group of bands corresponding to the stretching vibrations of C-H at 2961, 2985, and 2820 cm-1, and a group of bands relative to the bending vibration of H-C-H at 1485, 1473, and 1430 cm-1. Figure 3 depicts the changes in infrared absorbance spectra of methylamine adsorbed in NaBeta zeolite at different amounts of methylamine introduced into the IR cell in the range 4000-2500 (A), 2200-1700 (B), and 1800-1300 cm-1 (C). The spectrum of zeolite phase alone and that of gas phase of the IR cell have been subtracted. Two intense groups of bands are observed both in the range of 4000-2500 cm-1 (Figure 3A) and 1800-1300 cm-1 (Figure 3C). A weak peak at 3184 cm-1 and a negative feature at 3740 cm-1 are also present. The twin bands at 3375 and 3314 cm-1 are immediately suggestive of ν(NH)
(58) Zecchina, A.; Bordiga, S.; Spoto, G.; Scarano, D.; Spano, G.; Geobaldo, F. J. Chem. Soc., Faraday Trans. 1996, 92, 4863.
(59) Jobson, E.; Baiker, A.; Wokaun, A. J. Chem. Soc., Faraday Trans. 1990, 86, 1131.
Figure 2. Changes in the infrared absorbance spectra of N-H stretching vibration range (A) and H-N-H bending vibration range (C) of ammonia adsorbed in NaBeta zeolite as a function of desorption temperature (K) at (b) 298, (c) 323, (d) 348, (e) 373, and (f) 423. The spectrum a represents the NaBeta saturated with NH3.
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Figure 3. Changes in the infrared absorbance spectra of N-H and C-H stretching vibration range (A) and NH2 and CH3 bending vibration range (C) of methylamine adsorbed in NaBeta zeolite as a function of amounts of methylamine introduced into the IR cell (molecules/uc). The range of 2200-1700 cm-1 (B) is also given to check the deformation of framework. (a) 1.9, (b) 3.9, (c) 5.8, (d) 7.6, (e) 9.6, (f) 11.6, (g) 13.5, (h) 15.4, and (i) 19.3.
Figure 4. Changes in the infrared absorbance spectra of N-H and C-H stretching vibration range (A) and NH2 and CH3 bending vibration range (C) of methylamine adsorbed in NaBeta zeolite as a function of desorption temperatures (K) at (b) 298, (c) 323, (d) 338, (e) 373, and (f) 423. Spectrum a represents 19.4 molecules of methylamine adsorbed in NaBeta, and the range of 2200-1700 cm-1 (B) is also given to check the deformation of framework.
antisymmetric and symmetric vibrations of N-H, respectively, for methylamine adsorbed molecularly via the lone pair on nitrogen atoms on Na+ ions of NaBeta zeolite, and the band at 1603 cm-1 may then be correlated with δ(NH2) of these methylamine molecules. The groups of bands observed in the region of 3100-2700 and 15501400 cm-1 correspond to the asymmetric and symmetric stretching vibration and to the asymmetric bending vibration, respectively, of methyl group of methylamine molecules adsorbed in NaBeta. It is observed that the frequencies of stretching vibration bands of N-H and C-H and those of bending vibrations of H-C-H of methylamine molecules adsorbed in zeolite are shifted toward lower and higher wavenumbers, respectively, compared with the corresponding bands of gaseous molecules, indicating the weakness of the N-H and C-H bond due to the interaction of methylamine molecule with NaBeta zeolite. It should be noted that the frequency of the band corresponding to asymmetric bending vibration of H-N-H is displaced exceptionally toward lower wavenumber, being contrary to what was observed in the case of NH3. This should be very likely related to the replacement of a hydrogen atom by a methyl group, which affects the bending vibration mode of H-NH. We will not go further to try to interpret the shift of bending vibration band of H-N-H toward lower wavenumber instead of higher wavenumber. The band at 3184 cm-1 very likely resulted from the combination of the H-N-H bending asymmetric vibration. The negative feature at 3740 cm-1, two broad bands at 2727 and 2521 cm-1, and the peak at 1662 cm-1 are relative to the interaction of silanols present in dehydrated NaBeta zeolite with methylamine molecules as observed in the case of NH3. The broad bands at 2627 and 2521 cm-1 and the weak band at 1662 cm-1 should correspond to the asymmetric and symmetric N-H stretching vibration and asymmetric H-N-H bending vibration of CH3NH3+ ions formed between methylamine and silanols of NaBeta zeolite. Owing to the interaction with methylamine, the silanols are totally consumed, giving a negative feature at 3740 cm-1 in the spectra after subtraction of the spectrum of zeolite. In the range 2200-1700 cm-1, some features are present and do not originate from the vibration of methylamine molecules. As indicated above, the spectra of methylamine
adsorbed in NaBeta are obtained after subtraction of the spectrum of zeolite phase and that of gas phase of IR cell. The presence of some features in this region indicates that zeolite skeletal vibrations are indeed affected by the adsorption of methylamine, and the adsorption of methylamine causes a deformation of zeolite framework as explained above in the case of NH3. The interaction strength of methylamine with NaBeta zeolite has been evaluated using a zeolite wafer in contact with a high pressure of methylamine. Figure 4 reports the changes in infrared absorbance spectra of methylamine adsorbed in NaBeta zeolite upon desorption at different temperatures. The two broad bands, indicating the deformation of zeolite framework, are clearly present when a high pressure is in contact with NaBeta zeolite. A desorption at room temperature for 0.5 h can remove around half the methylamine molecules adsorbed in NaBeta zeolite since the intensity of all peaks stemming from adsorbed methylamine molecules is reduced about 50%. The free silanols reappear, as demonstrated by decreased intensity of the negative band at 3740 cm-1, in the difference spectra in Figure 4A. But they are not completely restored since a small negative feature at 3740 cm-1 persists even at a desorption temperature of 423 K. We can see clearly also that the intensity of the peak at 1662 cm-1 increases as desorption temperature increases. This suggests that the formation of CH3NH3+ ions is favored by raising the temperature. A significant amount of methylamine molecules adsorbed is still present in NaBeta zeolite and part of them in interaction with silanols to protonate methylamine, giving the negative feature at 3740 cm-1 and the peak at 1662 cm-1. The deformation of framework is obviously observed even though the desorption temperature is raised to 423 K, indicating the strong interaction between NaBeta zeolite and methylamine. This part of the results demonstrates that methylamine can not only adsorb molecularly on Na+ ions but also interact with the large number of silanols present in NaBeta zeolite to give CH3NH3+ ions. The strong interaction between methylamine causes a deformation of framework of NaBeta zeolite. The chemical and structural properties of this zeolite are significantly modified. This will affect strongly the adsorption properties of this zeolite.
Benzene in NaBeta
Figure 5. Infrared absorbance spectra in the C-C stretching and H-N-H bending vibration region (1800-1300 cm-1) of benzene and ammonia adsorbed in NaBeta after evacuation for 0.5 h at (b) 298, (c) 323, (d) 348, (e) 373, and (f) 423 K. The spectrum a represents the NaBeta in contact simultaneously with 6.0 molecules/uc of benzene and a pressure of 20 Torr of ammonia.
3.3. Location of Benzene in NaBeta Zeolite upon Coadsorption of Ammonia. Adsorption behavior of benzene and the location of benzene in NaBeta zeolite upon coadsorption of ammonia have been recently studied by the present authors29 and illustrated that benzene molecules sit mainly on Na+ ions, and only very few benzene molecules are found in 12R windows of NaBeta zeolite even in the presence of high pressure of ammonia. This means that 12R windows of NaBeta are not preferential adsorption sites for benzene, and coadsorption of ammonia does not induce a change in location of benzene although the deformation of framework is observed in present work. However, the reasons why the 12R windows are not preferential adsorption sites for benzene even in the presence of ammonia is not clear. To better understand the adsorption behavior of benzene in NaBeta zeolite, we have carried out some supplementary desorption experiments to compare the interaction strength of benzene and ammonia with zeolite. Figure 5 illustrates the infrared absorbance spectra of C-C stretching vibration of benzene and H-N-H bending asymmetric vibration of NH3 at different desorption temperatures. This study was made on a NaBeta sample in contact simultaneously with six molecules of benzene per unit cell and a high pressure of NH3, giving spectrum a. Two peaks at 1481 and 1636 cm-1, corresponding respectively to C-C stretching vibration of benzene and H-N-H bending vibration of NH3, are observed in spectrum a. A desorption of 0.5 h at room temperature can remove almost all of the NH3 molecules adsorbed while only half benzene molecules adsorbed initially in NaBeta are eliminated since the intensity of the peak from C-C vibration of benzene is reduced to half. Even after desorption at 423 K, a small amount of benzene molecules adsorbed is still present in NaBeta. This suggests that benzene molecules adsorb more strongly in NaBeta than ammonia. Comparing with the results obtained when ammonia is present alone in NaBeta that, after evacuation at 423 K, ammonia molecules adsorbed in NaBeta are
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Figure 6. Changes in the infrared absorbance spectra of C-H out-of-plane (ν5 + ν17) and (ν10 + ν17) vibration bands of absorbed benzene at a loading level of 1.0 molecule/uc (A) and in the range 4000-2700 cm-1 in the gas phase of the infrared cell (B) with increasing the amount of introduced methylamine (molecules/uc) over NaBeta zeolite: (a) 0, (b) 1.0, (c) 2.0, (d) 4.0, (e) 5.0, (f) 8.0, (g) 10.0, and (h) saturation (a pressure of 20 Torr of methylamine present in the IR cell).
Figure 7. Changes in the infrared absorbance spectra of C-H out-of-plane (ν5 + ν17) and (ν10 + ν17) vibration bands of absorbed benzene at a loading level of 6.0 molecules/uc (A) and in the range 4000-2700 cm-1 in the gas phase of the infrared cell (B) with increasing the amount of introduced methylamine (molecules/uc) over NaBeta zeolite: (a) 0, (b) 1.0, (c) 2.0, (d) 3.0, (e) 4.0, (f) 5.0, (g) 6.0, (h) 7.0, and (i) saturation of NaBeta (a pressure of 20 Torr of methylamine present in the IR cell).
still present. We can conclude that adsorption strength of ammonia on NaBeta is reduced due to the presence of benzene. 3.4. Location of Benzene in NaBeta Zeolite upon Coadsorption of Methylamine. This study was carried out on two samples having benzene loading levels of 1.0 and 6.0 molecules/uc. The first loading corresponds to around 30% Na+ ions occupied by benzene molecules.29 In the second situation, all the Na+ ions (3.5) are occupied by benzene molecules, and there are 2.5 molecules of benzene more which condense in the pores of zeolite or in interaction with silanols as described previously in refs 12 and 29. Figures 6A and 7A report the changes in the absorbance of the C-H out-of-plane vibrations in the range 2200-1700 cm-1 of adsorbed benzene on NaBeta at low (Figure 6A) and high (Figure 7A) benzene loadings upon coadsorption of methylamine. The spectra shown in Figures 6A and 7A present only the adsorbed benzene phases and were obtained by subtraction of the spectrum of methylamine adsorbed in NaBeta zeolite at a defined
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methylamine loading from the spectrum of benzene and methylamine adsorbed simultaneously in NaBeta zeolite with the same methylamine loading. At Low Benzene Loading. One main pair of bands at 1981 and 1845 cm-1 and a pair of small shoulders at 1997 and 1864 cm-1 are observed (Figure 6A, spectrum a) when 1 molecule/uc of benzene alone is introduced onto a NaBeta zeolite wafer. These two pairs of bands, which are referred to as HF and LF bands according to their wavenumbers, stem from the adsorption of benzene on Na+ ions (LF bands) and on 12R windows (HF bands), respectively. It is evident that at this benzene loading most of the benzene introduced adsorb on Na+ ions; only a very small part of benzene introduced is interacting with 12R windows of NaBeta, and the 12R windows of NaBeta are not preferential adsorption sites for benzene. With introduction of an increasing and known amount of methylamine into the IR cell, only the LF bands are broadened, and no growth of HF bands is detected. The broad band centered at 1949 cm-1, which is already observed in the case of NaBeta upon adsorption of methylamine, is also present when first molecules of methylamine are introduced (Figure 6A, spectrum b). This means that upon coadsorption of methylamine the framework of NaBeta is deformed. In the presence of a high pressure of methylamine in the IR cell, the peak at 1981 cm-1 is shifted toward the low wavenumber at 1974 cm-1, and that at 1845 cm-1 is completely vanished by another broad band at 1837 cm-1 which is also already observed when methylamine is present alone in NaBeta zeolite. Anyway, no HF bands corresponding to adsorption of benzene in 12R windows appear upon coadsorption of methylamine despite the strong deformation of framework induced by the strong interaction of methylamine with NaBeta zeolite. The IR spectra of the gas phase are equally recorded after each introduction of methylamine (Figure 6B) and show that when 4 molecules/uc of methylamine are introduced into the IR cell (Figure 6B, spectrum d), the peaks in the region 3200-3000 cm-1, coming from benzene molecules, appear, indicating the presence of benzene molecules in the gas phase of the IR cell. The above results suggest that, at low benzene loading, not only no displacement of benzene molecules adsorbed on Na+ ions toward the 12R windows upon coadsorption of methylamine is observed but also some of benzene molecules are removed from NaBeta zeolite wafer. At High Benzene Loading. Adsorption of 6.0 molecules/ uc of benzene (Figure 7A, spectrum a) gives one main pair of LF bands at 1982 and 1845 cm-1 and one pair of very small shoulders at 1963 and 1821 cm-1. The pair of shoulders has been previously attributed to the benzene molecules interacting with silanols of NaBeta zeolite (benzene-silanols interaction: BS bands).29 The HF bands are not detected. With introduction of an increasing and known amount of methylamine into the IR cell, the LF bands are broadened, decrease in intensity, and are shifted toward lower wavenumbers, and the peak at 1845 cm-1 vanished. The BS shoulders disappear and no HF bands appear. The broad bands centered at 1949 and 1834 cm-1, indicating the deformation of framework, are present at any methylamine loading and increase in intensity with increasing methylamine loading. The decrease in intensity of LF bands and the complete disappearance of BS shoulders suggest the removal of benzene molecules from zeolite wafer upon coadsorption of methylamine. This is proved by the increase in intensity of the peaks of benzene in the gas phase of the IR cell (Figure 7B) and the decrease in intensity of CH out-of-plane vibration of benzene adsorbed on NaBeta (Figure 7A).
Su and Norberg
Figure 8. Infrared absorbance spectra in the C-H out-ofplane vibrations of benzene adsorbed (A) and N-H bending vibration of methylamine adsorbed (B) on NaBeta after evacuation for 0.5 h at (b) 298, (c) 323, (d) 348, (e) 373, and (f) 423 K. Spectrum a represents the zeolite wafer in contact with 6.0 molecules/uc of benzene and a high pressure of methylamine.
The study made on NaBeta at low and high benzene loadings upon coadsorption of methylamine demonstrates clearly that no migration of benzene from cation toward the 12R windows occurs and 12R windows are not preferential adsorption sites for benzene although a deformation of framework is observed. 3.5. Interaction Strength of Benzene and Methylamine with NaBeta Zeolites. This was performed on a zeolite wafer contacted simultaneously with 6.0 molecules/uc of benzene and a pressure of 20 Torr of methylamine. Figure 8 illustrates the changes in infrared absorbance spectra of CH out-of-plane vibration of adsorbed benzene in the range 2200-1700 cm-1 (A) and of the N-H bending vibration at 1602 cm-1 of adsorbed methylamine (B) after 0.5 h evacuation at different temperatures. It can be seen that the LF bands decrease in intensity, and a pair of small shoulders, corresponding to HF bands, appears with evacuation temperature. At the desorption temperature of 423 K, only few benzene molecules adsorbed on NaBeta are detected. The methylamine molecules adsorbed on NaBeta are also progressively eliminated by increasing evacuation temperature. However, at 423 K, an appreciatable amount of methylamine molecules adsorbed is still retained by NaBeta zeolite. This can explain the presence of the broad band centered at 1953 cm-1 even at high evacuation temperature (spectrum f of Figure 8A). This signifies that methylamine molecules adsorb more strongly in NaBeta zeolite than benzene molecules. The appearance of HF shoulders after evacuation at high temperature was also observed when benzene molecules adsorb on NaBeta alone. This should be due to the presence of a very small amount of benzene adsorbed strongly on 12R windows. 4. General Discussion 4.1. Location of Benzene in NaBeta upon Coadsorption of Ammonia and Methylamine: A Molecular Recognition Control. It has been recognized that the adsorption of benzene on the 12R windows of zeolites is governed by the chemical and structural properties of zeolites.29,30,32,60 The high negative charges on oxygen atoms of the 12R windows can polarize the hydrogen atoms of benzene and enhance the interaction between the (60) Su, B. L. J. Chem. Soc., Faraday Trans. 1997, 93, 1449.
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Table 1. Classification of Zeolites with Regard to the Location of Benzene on 12R Windows category
chemical property
structural property
location on 12R windows
examples
1 2 3 4
favorable unfavorable favorable unfavorable
favorable favorable unfavorable unfavorable
yes noa nob noc
LiY, NaY, KY, RbY, CsX, NaYDI, CsEMT18-20,61,63,64 HY, NaEMT, NaYDII, NaYDIII31,32,61,63-65 KL25,30,37,63 NaBeta12,29,63
a Upon coadsorption of a basic molecule, the 12R windows can become adsorption sites for benzene.31,32,61,63-65 b The coadsorption of CH3NH2 induces the deformation of the framework, and the 12R windows become suitable for the location of benzene.25,30,37,63 c In any case, the 12R windows are not preferential adsorption sites for benzene.12,29,63
hydrogen atoms of benzene and the oxygen atoms of the 12R windows. The adsorption of benzene on the 12R windows becomes possible. However, the structural compatibility between benzene molecule and 12R windows is the key factor. The average negative charge of the oxygen atoms in KL, calculated using Sanderson electronegativity equalization principle,33,35 is around -0.350, even higher than that of NaY (Si/Al ) 3.6). In NaY zeolite, the adsorption of benzene on 12R windows was indeed observed;61 however, not in KL. It was known that the adsorption of a basic molecule like ammonia can modify and increase the negative charge of the oxygen atoms.35,62 No adsorption of benzene on the 12R windows in KL was observed even in the presence of ammonia. These results led us to suggest that the absence of the adsorption of benzene on 12R windows of KL both with and without the presence of ammonia should be linked to the lack of the structural compatibility between the 12R windows of KL and benzene molecules, although the 12R windows in KL zeolite are believed to be rather circular and large (7.1 × 7.1 Å) as those in NaY zeolite. Our more recent study has shown clearly a migration of benzene from cations to 12R windows upon coadsorption of methylamine and a remigration of benzene from 12R windows to cations on removal of methylamine in KL. It is true that the adsorption of methylamine can increase further the negative charge of the oxygen atoms of 12R windows compared with ammonia. Someone can also think that methylamine (but not ammonia) interacts more strongly with K+ ions than benzene; all the accessible K+ ions will be occupied by methylamine, and only 12R windows remain unoccupied and can be adsorption sites for benzene. If this is the case, at high loading of benzene, i.e., a high pressure of benzene vapor present in IR cell, and after the occupation of all the K+ ions by benzene molecule, the supplementary benzene molecules present in the system can also locate in 12R windows. It is known that the location of benzene in 12R windows should not be hindered by benzene molecules adsorbed on K+ ions. However, this not observed. As discussed above, we think that the fact that the 12R windows of KL zeolite become the adsorption sites upon coadsorption of methylamine is related to the deformation of framework. This modification of framework due to the interaction of methylamine with KL zeolite renders the 12R windows, which are initially incompatible with benzene molecules, favorable for adsorption of benzene. The present work shows that, in NaBeta zeolite, benzene molecules adsorb mainly on the Na+ ions even upon coadsorption of ammonia and methylamine and despite (61) Su, B. L.; Norberg, V.; Martens, J. A. Submitted to Colloids Surf. A. (62) Gutmann, V.; Resch, G. Stud. Surf. Sci. Catal. 1987, 37, 239. (63) Su, B.-L.; Norberg, V.; Hansenne, C.; de Mallmann, A. Adsorption 2000, 6, 61. (64) Su, B.-L.; Norberg, V.; Martens, J. A. Microporous Mesoporous Mater. 1998, 25, 151. (65) Su, B.-L.; Norberg, V. Langmuir 1998, 14, 2352.
the strong deformation of framework. This indicates clearly that the 12R windows occurring in NaBeta are not preferential adsorption sites for benzene. One may propose that the lack of adsorption of benzene in 12R windows in NaBeta even upon coadsorption of ammonia is due to the strong interaction of benzene with Na+ ions and ammonia cannot displace benzene molecules adsorbed on Na+ ions toward 12R windows. If this is the case, upon coadsorption of methylamine, benzene adsorption on 12R window should be observed since the interaction of methylamine with Na+ ions of NaBeta is stronger than that of benzene. So, the structural compatibility should be the key factor. There are two kinds of 12R windows in Beta zeolite. The 12R windows in the tortuous channels, having the opening around 5.5 × 5.5 Å with a saddle shape, should be too small or too deformed to be an adsorption site for benzene molecules (3.4 × 6.2 × 6.9 Å). This kind of 12R window is not, therefore, structurally suitable for benzene location. The 12R windows in the straight channels are large enough (6.4 × 7.6 Å) as host to receive benzene molecules as guest, and the negative charge on its oxygen atoms is supposed to be high enough to interact with benzene molecules due to the redistribution of the charge on the framework atoms and to the increase in the negative charge on oxygen atoms upon coadsorption of ammonia and methylamine. However, no benzene is found on these 12R windows. The lack of adsorption of benzene on this kind of 12R window should therefore be related to a structural effect; i.e., there is no suitable geometric compatibility between this kind of 12R windows and benzene molecules. It is also possible that the presence of a large number of silanols which can interact with benzene may hinder the location of benzene on 12R windows. It is true that we have observed a very small amount of benzene located in 12R windows at high loading of benzene and at increased temperature. This means that very few 12R windows exist chemically and structurally favorable for benzene location. They belong probably to a different polymorph than those normally used to describe the intergrowth structure of zeolite Beta, however, as the amount of benzene adsorbed on these sites is so small and does not increase by coadsorbing of ammonia or methylamine but decreases due to the deformation of framework. The general conclusion is that almost all of the 12R windows in Beta zeolite are neither chemically nor structurally favorable sites for benzene adsorption. The present work confirms once again that adsorption of benzene in 12R windows of zeolite is governed by a molecular recognition effect where benzene molecules (substrate) and 12R windows (adsorbent) should have adapted chemical and structural properties as in the system of enzyme-substrate. 4.2. Classification of Zeolites on the Basis of Molecular Recognition Effect with Regard to the Location of Benzene on 12R Windows. In regard to the benzene adsorption behavior, zeolites containing 12R windows can be classified into four different categories
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which are listed in Table 1. This classification should be very useful in the design of new catalysts and adsorbents used in the industrial treatments of aromatics. 5. Conclusion No change in benzene location has been observed upon coadsorption of ammonia and methylamine despite the deformation framework induced by adsorption of ammonia and methylamine. Benzene molecules adsorb mainly on Na+ ions. The 12R windows of NaBeta are not preferential adsorption sites for benzene due to their incompatibility in structure with benzene molecules. The present work confirms once again the existence of a molecular recognition effect in the location of benzene molecules in 12R windows of zeolites. The zeolite containing 12R windows
Su and Norberg
can be divided into four different categories in regard to the adsorption properties of benzene. This classification will guide us in design of new catalysts and adsorbents for industrial treatments of aromatics. Acknowledgment. This work has been performed within the framework of PAI-IUAP 4-10. The authors thank Dr. Camblor at the ITQ, Polytechnical University of Valancia, Spain, for his gift of starting zeolite sample Na(TEA) Beta and the FNRS (Fonds National de la Recherche Scientifique, Belgium) for a scholarship FRIA to V.N. Mrs. Su-Virlet is also acknowledged for her assistance. LA0000670
