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Langmuir 1998, 14, 7410-7419
Adsorption Behavior of Benzene in NaBeta Zeolite: An in-Situ Infrared Study of Molecular Recognition Bao-Lian Su* and Vale´rie Norberg Laboratoire de Chimie des Mate´ riaux Inorganiques, I.S.I.S., University of Namur, 61 Rue de Bruxelles, B-5000 Namur, Belgium Received July 30, 1998. In Final Form: September 25, 1998 The adsorption behavior of benzene in NaBeta zeolite at different temperatures has been investigated by means of in-situ infrared spectroscopy. The effect of coadsorption of ammonia on the benzene location has been studied at low and high benzene loading levels. The location of Na+ ions has been discussed, and the interaction strength of benzene with NaBeta zeolite has been evaluated by desorption experiments. The present work demonstrated that benzene molecules sit mainly on Na+ ions. The large number of silanols present in this zeolite can also weakly interact with benzene molecules. Two kinds of 12R windows occurring in the Beta zeolite structure are not the preferential adsorption sites for benzene at the studied temperature range even in the presence of ammonia. Quantitative analysis of the changes in the absorbance of the CH out-of-plane vibrations with benzene loadings has shown that all the Na+ ions are accessible to benzene molecules. This led us to suggest that a part of the Na+ ions located initially in the small cages of Beta zeolite is probably attracted by benzene molecules toward and is finally located in the 12R channels. The absence of benzene molecules on the two kinds of 12R windows of NaBeta zeolite has been correlated with the calculated average basicity of framework oxygen atoms and the geometry of the two kinds of 12R windows. This results very likely from the structural incompatibility of the benzene molecules and the 12R windows of Beta zeolite. This work confirmed that the location of benzene should very likely be governed by a molecular recognition effect where the adsorbate and absorbent should have the suitable chemical and structural properties as in substrate-enzyme systems.
1. Introduction Beta zeolite was first synthesized by Mobil in 1967 in the presence of alkaline cations.1 However, the structure of this zeolite was determined only in 19882-4 due to its complexity. It was reported that in the Beta zeolite structure, ordered and disordered frameworks coexist, and there are three mutually intersecting 12R channel systems and two kinds of 12R windows (Figure 1).5 Beta zeolite, like other large pore zeolites, has been widely used as a catalyst and catalyst support in various catalytic processes of aromatics such as isomerization,6,7 alkylation,8-12 and disproportionation13,14 and as a sorbent in separation of aromatics.15-17 These interesting properties are most likely related to the very special structure and the wide * Corresponding author. Phone: 32-81-72 45 31. Fax: 32-81-72 45 30. E-mail:
[email protected]. (1) Wadlinger, R. L.; Kerr, G. T.; Rosinski, E. J. U.S. Patent 3 308 069, 1967. (2) Treacy, M. M. J.; Newsam, J. M. Nature 1988, 332, 249. (3) Higgins, J. B.; LaPierre, R. B.; Schlenker, J. L.; Rohrman, A. C.; Wood, J. D.; Kerr, G. T.; Rohrbaugh, W. J. Zeolites 1988, 8, 446. (4) Meier, W. M.; Olson, D. H.; Baerlocher, Ch. Zeolites 1996, 17, 63. (5) McCusker, L. B. In Comprehensive Supramolecular Chemistry, Vol. 7: Solide-state Supramolecular Chemistry: Two- and Threedimensional Inorganic Networks; Alberti, G., Bein, Th. Eds.; Pergamon: New York, 1996; p 393. (6) Pe´rez-Pariente, J.; Sastre, E.; Forne´s, V.; Martens, J. A.; Jacobs, P. A. Appl. Catal. 1991, 69, 125. (7) Pardillos, J.; Brunel, D.; Coq, B.; Massiani, P.; De Me´norval, L. C.; Figueras, F. J. Am. Chem. Soc. 1990, 112, 1313. (8) Bellussi, G.; Pazzuconi, G.; Perego, C.; Girotti, G.; Terzoni, G. J. Catal. 1995, 157, 227. (9) Sirasanker, S.; Tangaraj, A.; Abdulla, R. A.; Ratnasamy, P. Stud. Surf. Sci. Catal. 1993, 75, 397. (10) Hari Prasad Rao, P. R.; Massiani, P.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1994, 84, 1449. (11) Reddy, K. S. N.; Rao, B. S.; Shiralkar, V. P. Appl. Catal., A 1993, 95, 53. (12) Parikh, P. A.; Subrahmanyam, N. Appl. Catal., A 1992, 90, 1. (13) Tsai, T. C.; Ay, C. L.; Wang, I. Appl. Catal. 1991, 77, 199. (14) Chen, W. H.; Pradhan, A.; Jong, S. J.; Lee, T. Y.; Wang, I.; Tsai, T. C.; Liu, S. B. J. Catal. 1996, 163, 436.
Figure 1. Geometrical description of the 12R channels and two kinds of 12R windows occurring in the Beta zeolite structure.
Si/Al ratio range of this zeolite. However, the acid-base sites in this zeolite are still little known, and few papers have been devoted to this kind of study.18-23 Because of (15) Dzwigaj, S.; de Mallmann, A.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1990, 86, 431. (16) Barthomeuf, D. U.S. Patent 4.584.424, 1986. (17) Shen, J. P.; Ma, J.; Sun, T.; Xu, Z.; Jiang, D. Z.; Min, E. Z. Stud. Surf. Sci. Catal. 1994, 90, 163. (18) Su, B. L.; Norberg, V. Zeolites 1997, 19, 65. (19) Jia, C. J.; Massiani, P.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1993, 89, 3659. (20) Hegde, S. G.; Kumar, R.; Bhat, R. N.; Ratnasamy, P. Zeolites 1989, 9, 231.
10.1021/la9809574 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/25/1998
Adsorption Behavior of Benzene in NaBeta Zeolite
the disordered structure of this zeolite, the location of counterions, such as protons and Na+ ions, has been a matter of debate. Recently, much research attention has been devoted to the study of the location of aromatics in zeolites, especially in 12R window zeolites, using benzene molecule as a probe,24-42 due to the importance of these zeolites in the transformation and separation processes of aromatics.6-17 Different techniques, such as 2H NMR,24,25 neutron diffraction,26,27 IR,28-39 quasi-elastic neutron scattering,40 and theoretical calculation,41,42 have been employed. On the basis of a series of studies on X and Y faujasite zeolites exchanged with alkali cations, it has been found that, in addition to the benzene location on cations, the 12R windows are also adsorption sites for benzene.24,25,27-31,42 The correlation between the average negative charge of the framework oxygen atoms of zeolites, δO, calculated using the Sanderson electronegativity equalization principle,38,43,44 and the location of benzene on 12R windows has been established,27-30,33,34,38 and showed that the adsorption of benzene on the 12R windows was strongly dependent on the negative charge of the framework oxygen atoms. The high negative charge of the framework oxygen atoms can reinforce the interaction of hydrogen atoms of benzene molecules with the oxygen atoms in the 12R windows, and the location of benzene on these sites can be stabilized.28-34 No significant adsorption of benzene on the 12R windows of HY (Si/Al ) 2.3),35,36 HEMT (Si/Al ) 4.0),36,37 HSAPO-37,35,36 HBeta (Si/Al ) 17.3),18 NaEMT (Si/Al ) 4.0),37 and dealuminated NaY (Si/Al ) 6.1)31 zeolites could successfully be explained by their low negative charge on the framework oxygen atoms (δO ) -0.240 for HY, δO ) -0.228 for HEMT, δO ) -0.318 for HSAPO-37, δO ) -0.225 for HBeta, δO ) -0.317 for NaEMT, and δO ) -0.275 for dealuminated NaY). It was shown that the adsorption of a basic molecule (21) Hunger, M., Ernst, S.; Steuernagel, S.; Weitkamp, J. Microporous Mater. 1996, 6, 349. (22) Maache, M.; Janin, A.; Lavalley, J. C.; Joly, J. F.; Benazzi, E. Zeolites 1993, 13, 419. (23) Beck, L. W.; Haw, J. F. J. Phys. Chem. 1995, 99, 1076. (24) Bull, L. M.; Henson, N. J.; Cheetham, A. K.; Newsam, J. M.; Heyes, S. J. J. Phys. Chem. 1993, 97, 11776. (25) Auerbach, S. M.; Bull, L. M.; Henson, N. J.; Metiu, H. I.; Cheetham, A. K. J. Phys. Chem. 1996, 100, 5923. (26) Vatale, G.; Bull, L. M.; Powell, B. M.; Cheetham, A. K. J. Chem. Soc., Chem. Commun. 1995, 2253. (27) Fitch, A. N.; Jobic, H.; Renouprey, A. J. Phys. Chem. 1986, 9, 1311. (28) de Mallmann, A.; Barthomeuf, D. J. Chem. Soc., Chem. Commun. 1986, 476. (29) de Mallmann, A.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1986, 28, 609. (30) de Mallmann, A.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1987, 37, 365. (31) de Mallmann, A. Ph.D. Thesis, University of Paris, Paris, France, 1989. (32) Su, B. L.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1989, 94, 598. (33) de Mallmann, A.; Barthomeuf, D. J. Chem. Soc., Chem. Commun. 1989, 129. (34) de Mallmann, A.; Barthomeuf, D. Zeolites 1988, 8, 292. (35) Su, B. L.; Barthomeuf, D. J. Catal. 1993, 139, 81. (36) Su, B. L.; Barthomeuf, D. Zeolites 1993, 13, 626. (37) Su, B. L.; Manoli, J. M.; Potvin, C.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1993, 89, 857. (38) Barthomeuf, D. Catal. Rev., Sci. Eng. 1996, 38, 521. (39) Barthomeuf, D.; de Mallmann, A. In Chemistry, Ecology and Health; Ione, K. G., Ed.; Nova Science Publishers: Commack, NY, 1996; p 279. (40) Jobic, H.; Renouprey, A.; Fitch, A. N.; Lauter, H. J. J. Chem. Soc., Faraday Trans. 1987, 83, 3199. (41) Kitagawa, T.; Tsunekawa, T.; Iwayama, K. Microporous Mater. 1996, 7, 227. (42) Auerbach, S. M.; Metiu, H. J. Chem. Phys. 1996, 105, 3573. (43) Mortier, W. J. J. Catal. 1978, 55, 138. (44) Sanderson, R. T. Chemical Bonding and Bond Energy, 2nd ed.; Academic Press: New York, 1976.
Langmuir, Vol. 14, No. 26, 1998 7411
such as NH3 on zeolites can modify the charge of framework atoms and that the negative charge of framework oxygen atoms could be redistributed and further increased through the interaction of NH3 with the framework atoms and compensating cations.33,45-48 This weakens and strengthens the interaction of benzene molecules with the cations and the 12R windows, respectively. The benzene adsorption on the compensating cations was finally displaced to the 12R windows.31,33,47 The experimental observation that the 12R windows of NaEMT, dealuminated NaY, and HY do indeed become the adsorption sites for benzene in the presence of ammonia31,33 proves that the location of benzene on 12R windows of zeolites is related to the negative charge of framework oxygen atoms. However, our recent works showed some opposite results. There is no location of benzene on 12R windows of KL zeolite (Si/Al ) 3.2) even in the presence of ammonia48,49 despite the negative charge of the oxygen atoms similar to that of NaY (Si/Al ) 2.5 and δO ) -0.352) and despite the theoretical calculations predicting that the 12R windows in KL zeolite should be a potential adsorption site for benzene.50,51 The above discussion suggests that the negative charge of the framework oxygen atoms should not be the only dominating factor in governing the benzene location on the 12R windows of zeolites. Beta zeolite also contains 12R windows, but it has not been clarified yet whether the benzene molecules can locate on its 12R windows.15,17 Knowledge concerning the hydrocarbon adsorption is important to better understand and to explain the catalytic and adsorptive properties of zeolites. This is also fundamental for the design of new catalysts and new absorbents with improved performances. The present paper deals with the adsorption behavior of benzene and the effect of coadsorption of ammonia and temperature on the benzene location within NaBeta zeolite. These studies have been conducted in order to shed some light on the acid-basic properties of Beta zeolites, on the location of Na+ ions, and on the dominating factors for benzene location on 12R windows. 2. Experimental Section 2.1. Materials. The starting material was kindly provided by the Instituto de Tecnologia Quimica, Polytechnical University of Valencia, Spain, and synthesized in alkaline medium with the presence of tetraethylammonium hydroxide, as reported in refs 52-54. The crystallinity of the sample was checked using XRD and IR and showed to be good. The chemical composition for 64 T atoms per unit cell, obtained by chemical analysis, was Na1.4(TEA)5.6(AlO2)3.5(SiO2)60.5. To remove organic templates, the starting sample was first calcined in a dry N2 flow with a heating rate of 100 K per hour. When the desired temperature (823 K) was reached, the gas was replaced by O2 and the temperature was maintained for 5 h. The calcined sample, that is, organic template-free, was labeled as H(Na)Beta. Five grams of calcined sample was added in 50 mL (45) Mortier, W. J. In Proceedings of Sixth International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworths: Guildford, 1984; p 734. (46) Gutmann, V.; Resch, G. Stud. Surf. Sci. Catal. 1987, 37, 239. (47) Su, B. L. Zeolites 1996, 16, 25. (48) Su, B. L. Zeolites 1996, 16, 75. (49) Su, B. L.; Barthomeuf, D. Zeolites 1995, 15, 470. (50) Newsam, J.; Silbernagel, B. G.; Garcia, A. R.; Hulme, R. J. Chem. Soc., Chem. Commun. 1987, 664. (51) Silbernagel, B. G.; Garcia, A. R.; Newsam, J. M.; Hulme, R. J. Phys. Chem. 1989, 93, 6506. (52) Loeffler, E.; Lohse, U.; Peuker, Ch.; Oehlmann, G.; Kustov, L. M.; Zholobenko, V. L.; Kazansky, V. B. Zeolites 1990, 10, 266. (53) Camblor, M. A.; Pe´rez-Pariente, J. Zeolites 1991, 11, 202. (54) Camblor, M. A.; Mifsud, A.; Pe´rez-Pariente, J. Zeolites 1991, 11, 792.
7412 Langmuir, Vol. 14, No. 26, 1998 of 1 M sodium chloride solution. The mixture was stirred and heated under reflux conditions for 5 h. After filtration, the obtained solid was washed with distilled water and filtered until all Cl- ions were removed. The recovered solid was then dried in an oven at 373 K overnight. The chemical composition of this sample from elementary analysis is Na3.5(AlO2)3.5(SiO2)60.5, labeled as NaBeta. 2.2. Infrared Studies. The adsorption of benzene 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 per min. The temperature was maintained overnight in the same atmosphere. The sample was treated under vacuum for 4 h at the same temperature. The IR cell was then cooled slowly to room temperature, and the spectrum of the zeolite phase alone was recorded as a reference using a Fourier Transform Bio-Rad FTS-60A or Perkin-Elmer Spectrum 2000 spectrometer. The adsorption of increasing and known amounts of benzene was carried out as described in refs 28-37 on the pretreated wafer. After each adsorption, the sample was maintained at room temperature for 1 h for equilibration. The spectra were then recorded. The amount of benzene adsorbed on cations has been evaluated as previously described in refs 28-37. The influence of coadsorption of ammonia on the benzene location was examined on two samples having benzene loadings of 1.0 and 6.0 molecules per unit cell (molecules/u.c.), respectively. After the spectra of adsorbed benzene were recorded, an increasing and known amount of ammonia was introduced into the IR cell. After 1 h of equilibration at room temperature, the IR spectra were recorded. The quantities of benzene and ammonia introduced into the IR cell were expressed in molecules per unit cell (molecules/u.c.), although, after saturation of the zeolite, this has no real physical meaning. However, it does facilitate comparison of the results. The interaction strength of benzene with NaBeta zeolite was evaluated by desorption at different temperatures for 30 min. The study on the effect of temperature on the location of benzene and the interaction of benzene with NaBeta zeolite was carried out as described below. After the pretreatment of the zeolite wafer, 6 molecules of benzene per unit cell was introduced onto the zeolite wafer at room temperature. After equilibration, the IR cell containing the zeolite wafer was cooled in liquid nitrogen for 15 min. The first spectrum of the adsorbed phase was recorded as soon as the IR cell was taken out of the liquid nitrogen. The temperature of the IR cell increased progressively to room temperature, and the spectra of the adsorbed phase were recorded at each desired temeprature. The spectrum of the adsorbed phase at 373 K was taken after heating the IR cell at 373 K for 30 min.
3. Results and Discussion 3.1. Exchange of Sodium Ions and the Hydroxyls in H(Na)Beta and NaBeta Zeolites. An ion-exchange reaction is carried out to convert H(Na)Beta, an almost completely protonated framework, to NaBeta. Figure 2 gives the comparison of the IR absorbance spectra of the starting zeolite material (a) and exchanged sample (b) in the region 3900-3300 cm-1. H(Na)Beta (Figure 2a) shows three OH groups at 3789, 3749, and 3612 cm-1, attributed previously to the Al-O-H species being near one or more SiOH groups, generated when Al atoms leave the framework, to the terminal silanols, and to the bridged framework OH groups, respectively.18 After Na+ ion exchange (Figure 2b), the peaks at 3789 and 3612 cm-1 are not detected. The complete disappearance of the bridged framework OH groups at 3612 cm-1 demonstrates the complete exchange of H+ by Na+. It was reported that the Al-O-H groups at 3789 cm-1 can be removed by acid leaching.52 A NaCl solution and a reflux condition have been used for the ion exchange. The present results show that this treatment is also able to remove this kind of OH groups. Moreover, two OH groups at 3749 and 3672 cm-1
Su and Norberg
Figure 2. Infrared absorbance spectra of hydroxyls of H(Na)Beta (a) and NaBeta (b) zeolites after pretreatment at 723 K.
Figure 3. Infrared absorbance spectra of NaBeta zeolite pretreated at 723 K without (a) and with (b) adsorbed benzene.
are still present in the exchanged sample, and the peak at 3749 cm-1, observed already in H(Na)Beta, maintains its intensity after ion exchange. This indicates that the protons of the silanol groups are not exchangeable. Its high intensity stems from the small crystal size of Beta zeolites, since smaller particles require more hydroxyl groups to complete the coordination spheres of Si on the exterior surface.55 The peak at 3672 cm-1 is new and is not detected in H(Na)Beta. This peak has previously been observed in different zeolites.19,22,52 It often appears after treatment with water vapor or a severe heating treatment.19,22 Its attribution is not clear yet. Three origins have previously been envisaged: nonacidic extra framework cationic aluminic species, acidic protons compensating negative charges of AlO4 tetrahedra partially disconnected from the framework, and strong acidic cationic Al species bonded to framework oxygen atoms.19,22,52,56,57 The assignment of these Al-OH groups will be discussed in the following section. The presence of these Al-OH groups in our sample reveals that a dealumination, but very slight due to its very low IR intensity, occurred during the pretreatment of the zeolite wafer. 3.2. Benzene Adsorption. Figure 3 reports the comparison of IR absorbance spectra of NaBeta zeolite after pretreatment at 723 K (a) and after adsorption of (55) Kiricsi, I.; Flego, C.; Pazzuconi, G.; Parker, W. O.; Millini, R.; Pergo, C.; Bellussi, G. J. Phys. Chem. 1994, 98, 4627.
Adsorption Behavior of Benzene in NaBeta Zeolite
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Figure 5. Variation of the wavenumber of the new broad band assigned to the interaction of silanols with benzene molecules (A) and the shifts of this band compared to the wavenumbers of the initial peaks of silanols (B) as a function of the amount of benzene introduced.
Figure 4. Changes in the infrared absorbance spectra of silanol hydroxyls of NaBeta upon adsorption of increasing amounts of benzene (molecules/u.c.). Amounts of benzene introduced: (a) 0.0; (b) 0.5; (c) 2.0; (d) 3.0; (e) 4.0; (f) 6.0; (g) 8.0; saturation of the zeolite (a pressure of 20 Torr present in the IR cell).
benzene at room temperature (b). Changes are observed at four vibration ranges (I, 4000-3100 cm-1; II, 31002900 cm-1; III, 2200-1700 cm-1; IV, around 1479 cm-1) corresponding to the OH groups, the combination of stretching C-H and bending and stretching C-C vibrations, the C-H out-of-plane bending vibration domain, and the stretching C-C vibration mode, respectively. In the present study, only the first and the third domains will be described in detail. Interaction of OH Groups with Benzene Molecules. Figure 4 shows the changes in the IR absorbance of the silanol hydroxyls of NaBeta upon adsorption of benzene. As can be seen, the intensity of silanol groups at 3746 cm-1 decreases with introduction of benzene, and a broad band, superimposed on the peak at 3672 cm-1, appears simultaneously. This broad band, whose wavenumber decreases with benzene adsorption, corresponds to the silanol groups interacting with benzene molecules and shifted toward a lower wavenumber. It was recognized that the extent of the shift should be characteristic of the strength of the acid sites.20,35,36,49 Figure 5 illustrates the variation in wavenumber of the broad band (curve A) and of the shift value (curve B) as a function of benzene loading. At low benzene loading, a small shift value (less than 100 cm-1) is obtained, while a high benzene loading leads to a more important shift value. In the presence of a high pressure of benzene in the IR cell, a shift value of about 145 cm-1 is obtained. This value is higher than that of KL (113 cm-1)53 and dealuminated NaY (136 cm-1) but is similar to that of LZY-82 (148 cm-1).36 This means that the silanol groups in NaBeta have a relatively high acidity compared to the same OH groups in other zeolites.
However, they are less acid than the bridging hydroxyls of HBeta, which give a shift value larger than 300 cm-1 upon adsorption of benzene.18 The interaction of benzene with the Al-OH groups at 3672 cm-1 could not be evaluated very precisely due to the superimposition of the broad band, which arises from the interaction of benzene with silanol groups, on this peak. However, a broad band centered at 3520 cm-1 is still observed in addition to the first broad band after introduction of the first benzene molecules into the IR cell (Figure 4b). Since no other OH groups are present in the exchanged sample, this broad band should very probably result from the interaction of benzene molecules with the Al-OH groups at 3672 cm-1, giving a shift value of about 152 cm-1. These groups are more acid than silanol groups but less acid than the bridged framework OH groups present in the HBeta zeolite.18 The present work shows that the AlOH groups present in the studied NaBeta zeolite are neither the nonacidic nor the strong acidic OH groups, being in line with what was proposed in the previous studies.9,22,52,57 They are most likely related to the OH groups bonded to Al partially disconnected from the framework. Type and Amount of Adsorbed Benzene. Benzene adsorption has successfully been used in determining the number and the location of cations or protons and the types of adsorption sites in zeolites.28-37,47,48 In the C-H out-of-plane vibration range, liquid benzene gives a pair of bands at 1960 and 1815 cm-1, due to (ν5 + ν17) and (ν10 + ν17) bending vibrations of C-H, respectively. It was shown that this pair of bands can be shifted toward higher wavenumbers or can be split into two pairs of bands. These two pairs of bands were assigned to the benzene molecules adsorbed on the cations and on the oxygen atoms of 12R windows of zeolites. The extent of the associated shift values can be used to distinguish the adsorption of benzene on one site or the other. Benzene molecules, interacting with cations or protons of zeolites, give a relatively small shift in wavenumber of around 20-40 cm-1 compared to liquid benzene, since the interaction of the cations with the π electron cloud of benzene rings affects indirectly, (56) Sun, Y.; Chu, P.; Lunsford, J. Langmuir 1991, 7, 3027. (57) Borade, R. B.; Clearfield, A. J. Phys. Chem. 1992, 96, 6729.
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Su and Norberg
Figure 6. Changes in the infrared absorbance spectra of the C-H out-of-plane vibrations of benzene adsorbed on NaBeta zeolite as a function of the amount of benzene introduced (molecules/u.c.) in the IR cell: (a) 0.5; (b) 2.0; (c) 3.0; (d) 4.0; (e) 6.0; (f) 8.0; (g) 10; (h) saturation of the zeolite (a pressure of 20 Torr present in the IR cell).
Figure 7. Changes in the infrared absorbance spectra of benzene in the range 4000-2800 cm-1 in the gas phase of the IR cell with the introduction of benzene (molecules/u.c.) into the IR cell: (a) 1.0; (b) 2.0; (c) 3.0; (d) 4.0; (e) 6.0; (f) 8.0; (g) 10.0; (h) saturation of the zeolite (a pressure of 20 Torr present in the cell).
and only weakly, the C-H out-of-plane bending vibration. This pair of bands was previously referred to as the lowfrequency (LF) bands.28-37,47,48 Adsorption of benzene on the oxygen atoms of the 12R windows induces a relatively high shift in wavenumber of around 40-100 cm-1 because the hydrogen atoms of benzene molecules interact with the oxygen atoms of zeolites; the C-H vibration is directly and strongly affected. This pair was, hence, referred to as the high-frequency (HF) bands.28-37,47,48 However, the exact shift value and the presence of two pairs of bands are strongly dependent on the zeolites used, the Lewis acidity of cations, the basicity of framework oxygen atoms, the number and the type of cations present in zeolite, and the benzene loading.28-37,47,48 Figure 6 depicts the changes in the IR absorbance spectra of C-H out-of-plane vibrations of benzene molecules adsorbed on NaBeta at different benzene loading levels. The spectrum of the gas phase and zeolite phase is subtracted. One main pair of bands at 1979 and 1842 cm-1 is observed at low benzene loading (Figure 6a). With regards to the extent of the shift value of around 23-27 cm-1, this pair of bands can be attributed to the interaction of Na+ ions with benzene molecules, that is, the LF bands. A pair of very weak shoulders at 2001 and 1867 cm-1 is also detected and is related to the benzene molecules adsorbed on the 12R windows of Beta zeolite, that is, HF bands according to the higher shift value of around 41-52 cm-1. With increasing the amount of benzene introduced, no significant modification is observed in the wavenumber of the pair of LF bands, which increases only in intensity, while the pair at 2001 and 1867 cm-1
is maintained as weak shoulders. A new pair of weak shoulders appears at 1962 and 1818 cm-1 at a benzene loading of around 2.0 molecules/u.c. (Figure 6b). The wavenumbers of this pair of bands are very close to those of liquid benzene, and at higher benzene loadings, this pair becomes more and more intense. As discussed above, the silanols can also, but weakly, interact with benzene molecules. This pair should arise from the benzene molecules interacting with silanols, labeled as BS bands (benzene-silanol interaction bands). In the presence of a high pressure of benzene (Figure 6h), the BS bands at 1962 and 1818 cm-1 are the most intense due to the large number of silanols present in NaBeta zeolite. Compared to the intensity of the LF and BS bands, that of the HF bands is very low at any benzene loading, indicating that the amount of benzene adsorbed on 12R windows of NaBeta zeolite is very low and that the adsorption of benzene on 12R windows is not favored. The presence of benzene molecule in the gas phase of the IR cell is checked after each addition of benzene (Figure 7). When around 4.0 molecules/u.c. of benzene are introduced into the cell (Figure 7d), very weak peaks appear in the region of the combination of stretching C-H and bending and stretching C-C vibrations: 3100-2900 cm-1. This reflects the presence of traces of benzene in the gas phase of the IR cell and implies that not all the benzene molecules introduced into the cell are adsorbed by the zeolite above this benzene loading. The amount of benzene molecules adsorbed on Na+ ions (Figure 8) is evaluated after deconvolution of the spectra
Adsorption Behavior of Benzene in NaBeta Zeolite
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Figure 8. Changes in the relative absorbance (Ar) of the (ν5 + ν17) (a) and (ν10 + ν17) (b) bands of benzene adsorbed on NaBeta zeolite as a function of the number of benzene molecules introduced into the IR cell.
using the method described in refs 18,28-39,47,48. The relative absorbance (Ar) of benzene molecules adsorbed on Na+ ions increases with the introduction of benzene and reaches a plateau at around 4.0 ( 0.3 molecules/u.c. of benzene introduced. Further introduction of benzene does not result in significant modification in the relative absorbance of LF bands, and only the amount of benzene in the gas phase and that interacting with silanols increase (Figure 6). The amount at which a band becomes constant reflects the saturation of the corresponding adsorbing center.31,34-36 Since at a benzene loading of 4.0 molecules per unit cell, only very few benzene molecules are detected in the gas phase of the cell; this suggests that almost all the benzene introduced into the cell adsorbs on the zeolite wafer. However, at each benzene loading, it is observed that (a) a very small amount of benzene is located on the 12R windows and, (b) from a benzene loading of around 2.0 molecules/u.c., a small portion of introduced benzene interacts with silanols, indicating that not all the benzene introduced into the cell adsorbs on Na+ ions. The amounts of benzene molecules adsorbed on the 12R windows and interacting with silanols can be determined by assuming that the extinction coefficients of the peaks associated with the (ν5 + ν17) and (ν10 + ν17) vibrations have similar values. After subtraction of these two quantities, although they are small at the benzene loading 4.0 molecules/u.c., from the amount of benzene introduced at which the Ar of LF bands become constant, the number of benzene molecules adsorbed on Na+ ions is deduced and is around 3.4 ( 0.3 molecules/u.c. This value is very close to the number of Na+ ions compensating the framework negative charges induced by Al atoms (around 3.5). The good agreement between these two values suggests that one benzene molecule interacts with one cation.37 This reveals also that, in the presence of benzene alone, all the Na+ ions in NaBeta zeolite are accessible to the aromatic ligand. 3.3. Effect of the Temperature on the Interaction of Silanols with Benzene Molecules and on the Benzene Location in NaBeta Zeolite. Experiments were carried out with a sample in contact with 6 molecules/ u.c. of benzene. The spectra of the adsorbed phase in the range 4000-3200 cm-1 at different temperatures are displayed in Figure 9. The zeolite phase alone is also given for comparison (Figure 9a). This range of the spectra can provide important information on the interaction of silanols with benzene molecules. Spectrum b was recorded at 235 K. All spectra recorded below this temperature
Figure 9. Infrared absorbance spectra of the silanol hydroxyls of NaBeta without benzene (a) and with a benzene loading of 6 molecules/u.c. at (b) 235, (c) 248, (d) 262, (e) 272, (f) 278, (g) 284, (h) 288, (i) 298, and (j) 373 K.
are very similar to this spectrum and are not shown here. As can be seen, below 235 K, only a small part of silanols can interact with benzene molecules (Figure 9b). Two broad bands are observed at 3625 and 3520 cm-1 and are attributed to the silanols and the Al-OH groups interacting with benzene molecules, as discussed in section 3.2. No significant change is observed below 278 K (Figure 9f). The intensity of the silanol peak at 3747 cm-1 decreases, and that of the broad band at 3625 cm-1 increases. At 284 K, the silanol band at 3747 cm-1 is the less intense while the broad band at 3625 cm-1 is the most intense (Figure 9g). This indicates that, in the range 166284 K, with increasing temperature, the amount of silanols interacting with benzene molecules increases and, at 284 K, most of the silanol groups are interacting with benzene molecules. This indicates also that a kinetic energy is needed for the adsorption of benzene on external silanols. This should be due to the intercrystalline diffusion of benzene. As described in the Experimental Section, the crystal size of the studied NaBeta zeolite is quite small and the zeolite wafers are prepared by the zeolite crystallites which form the pores between them. The intercrystalline diffusion at low-temperature becomes evident. As the temperature further rises, the intensity of the silanol band increases while the broad band, corresponding to the silanols interacting with benzene molecules, decreases in intensity. This indicates that the amount of silanols interacting with benzene molecules decreases, and a part of benzene molecules is present in the gas phase of the cell due to the high thermal agitation of benzene molecules at higher temperatures. This suggests also that the interaction of the silanols with
7416 Langmuir, Vol. 14, No. 26, 1998
Su and Norberg
Figure 10. Infrared absorbance spectra of benzene adsorbed on NaBeta with a benzene loading of 6 molecules/u.c. in the C-H out-of-plane vibration range at (a) 235, (b) 248, (c) 262, (d) 272, (e) 278, (f) 284, (g) 288, (h) 298, and (i) 373 K.
Figure 11. Infrared absorbance spectra in the C-H out-ofplane vibrations after evacuation for 0.5 h at (b) 298, (c) 323, (d) 343, (e) 373, (f) 393, and (g) 423 K; spectrum a represents the saturation of zeolite with benzene.
benzene molecules is weak. This will be confirmed by desorption experiments which will be presented in the following section. Figure 10 reports the spectra of the adsorbed phase in the range 2200-1700 cm-1 at different temperatures. Spectrum a was recorded at 235 K. Spectra recorded below this temperature are very similar to this spectrum and are therefore not shown here. At 235 K, a pair of bands at 1981 and 1845 cm-1, corresponding to the LF bands, is observed (Figure 10a). The pair of weak shoulders at 1962 and 1818 cm-1, as discussed in section 3.2 and assigned to the benzene molecules interacting with silanols, that is, BS bands, is also present. As the temperature increases, no change is observed for the LF bands; however, the intensity of the BS bands increases and then decreases. The maximum in intensity for BS bands is found at 284 K (Figure 10f). The changes observed in this range are in agreement with those in the range 4000-3000 cm-1. This part of the results demonstrates that the interaction of benzene with Na+ ions is strong and that, even at very low temeprature, this interaction is already observed. However, the interaction of benzene with silanols is relatively weak, and at low (below 235 K) or high temperatures (above room temperature), this interaction is not favored due to the low and high kinetic energy of benzene molecules, respectively. The silanol groups are therefore not preferential adsorption sites for benzene, but Na+ ions are. 3.4. Interaction Strength of Benzene with NaBeta Zeolite. The desorption of benzene molecules adsorbed on NaBeta zeolite at different temperatures has been
performed on a sample saturated with benzene to evaluate the interaction strength of benzene with NaBeta zeolite. Figure 11 shows the spectra of benzene adsorbed on NaBeta after desorption at different temperatures. At saturation of zeolite with benzene, one main pair of bands at 1983 and 1844 cm-1, corresponding to the interaction of benzene with Na+ ions (LF bands), is present. A pair of shoulders at 1962 and 1820 cm-1, assigned to the interaction of benzene with silanols (BS bands), is also observed. After desorption at room temperature for 30 min, the pair of shoulders at 1962 and 1820 cm-1 disappears, indicating again the weak interaction of benzene with silanols. The intensity of LF bands decreases also after desorption at room temperature and decreases continuously with the desorption temperature, and the pair of weak shoulders at 2001 and 1867 cm-1, attributed to the HF bands and hidden by the large LF bands, becomes more evident. The intensity of the HF bands is only very slightly affected when the desorption temperature rises from 343 to 423 K while that of LF bands is strongly reduced. The above results illustrate that only a desorption at 423 K during 30 min can eliminate most of the benzene adsorbed, that a very small fraction of benzene molecules interacts very strongly with the 12R windows, and that most of the 12R windows are not the preferential adsorption sites for benzene. 3.5. Effect of the Coadsorption of Ammonia on the Benzene Location. This analysis was carried out on two samples having benzene loading levels of 1.0 and 6.0 molecules/u.c., which correspond to around 30 and 100% Na+ ions occupied by benzene molecules, respectively. Figures 12 and 13 report the changes in the
Adsorption Behavior of Benzene in NaBeta Zeolite
Figure 12. 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 molecumes/u.c. with increasing amount of introduced ammonia (molecules/u.c.) over NaBeta zeolite, (a) 0; (b) 1.0; (c) 3.0; (d) 5.0; (e) 8.0; (f) 10.0; (g) 12.0; (h) 15.0; (i) 17.0; (j) 20.0; (k) a pressure of 20 Torr of ammonia present in the IR cell.
absorbance of the C-H out-of-plane vibrations in the range 2200-1700 cm-1 of adsorbed benzene on NaBeta at low (Figure 12) and high (Figure 13) benzene loadings upon coadsorption of ammonia. At Low Benzene Loading. One main pair of bands at 1983 and 1845 cm-1 and a pair of small shoulders at 1997 and 1864 cm-1 are observed (Figure 12a)58 when around 1.0 molecules/u.c. of benzene is introduced onto a NaBeta zeolite wafer. These two pairs correspond to the LF and HF bands, respectively. With introduction of an increasing and known amount of ammonia into the cell, the LF bands are broadened and increased in intensity. However, no growth of HF bands upon coadsorption of ammonia, as observed in NaEMT,47 dealuminated NaY, and HY,31 is detected, indicating that no displacement of benzene molecules adsorbed on cations toward 12R windows occurs. The broadening and increasing in intensity of LF bands should arise from the changes in the extinction coefficient of LF bands of benzene upon coadsorption of ammonia. This was previously observed in NaX and NaY zeolites.31,33 A new broad band centered at 1938 cm-1 appears as the amount of ammonia introduced into the cell increases. This broad band should not arise from benzene adsorbed (58) The wavenumbers of the HF and LF bands are slightly different from those observed in Figure 6a at the same benzene loading, this is because the different spectrometers are used for these two experiments. The spectra of Figure 6 were recorded using a Bio-Rad FTS 60A Spectrometer while those in this section were recorded using a PerkinElmer Spectrum 2000 Spectrometer.
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Figure 13. Changes in the infrared absorbance spectra of C-H out-of-plane vibration (ν5 + ν17) and (ν10 + ν17) bands of absorbed benzene at a loading level of 6.0 molecules/u.c. with increasing amount of introduced ammonia (molecules/u.c.) over NaBeta zeolite: (a) 0; (b) 1.0; (c) 3.0; (d) 5.0; (e) 8.0; (f) 10.0; (g) 15.0; (h) a pressure of 20 Torr of ammonia present in the cell.
and was already detected by Zecchina et al.59 in a study on H-Mordenite zeolite upon adsorption of ammonia. This broad band is consequently assigned to the framework overtone modification not dependent on the structure of the adsorbed molecules. The IR spectra of the gas phase are also recorded after each introduction of ammonia (Figure 14) and show that no benzene molecules are present in the gas phase of the cell at any benzene loading (Figure 14), and all the benzene molecules are present in NaBeta even in the presence of a high pressure of ammonia (Figure 14j). This also indicates that all the benzene molecules adsorb strongly on Na+ ions and that the coadsorption of ammonia cannot remove benzene molecules from the zeolite wafer. At High Benzene Loading. Adsorption of 6.0 molecules/ u.c. of benzene (Figure 13a) gives one main pair of LF bands and one pair of BS shoulders. The HF bands are not detectable due to their weak intensity. With introduction of increasing and known amounts of ammonia, the LF bands are broadened and increased in intensity, and the broad band centered at 1936 cm-1 appears, as observed in the case of low benzene loading, whereas the pair of BS bands disappears. No HF bands are present at any ammonia loading. The gas phase of the cell is studied to verify the presence of benzene molecules. After introduction of 6.0 molecules/u.c. of benzene (Figure 15), a group of peaks in the range 3200-3000 cm-1, corresponding to the combination of stretching C-H and (59) Zecchina, A.; Buzzoni, R.; Bordiga, S.; Geobaldo, F.; Scarano, D.; Riccjardi, G.; Spoto, G. Stud. Surf. Sci. Catal. 1995, 97, 213.
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Figure 14. Changes in the infrared absorbance spectra of benzene in the range 4000-2800 cm-1 in the gas phase of the infrared cell with the introduction of an increasing amount of ammonia (molecules/u.c.) into the cell at a benzene loading level of 1.0 molecule/u.c.: (a) 0; (b) 1.0; (c) 3.0; (d) 5.0; (e) 8.0; (f) 10.0; (g) 12.0; (h) 15.0; (i) 17.0; (j) a pressure of 20 Torr of ammonia present in the IR cell.
bending and stretching C-C of benzene, are observed. This indicates that only a part of the 6.0 molecules of benzene introduced adsorb on the NaBeta zeolite wafer. As the amount of ammonia introduced increases, the intensity of this group of peaks increases slightly. This suggests the removal of benzene molecules from the zeolite wafer to the gas phase of the IR cell. It is observed that the pair of BS bands disappears upon coadsorption of ammonia (Figure 13); the increase in intensity of the peaks of benzene in the gas phase is hence due to the removal of benzene molecules interacting with silanols from the zeolite wafer to the gas phase, and the amount of benzene in the gas phase increases. Both at low and high benzene loadings, the displacement of benzene molecules adsorbed on cations toward 12R windows and the appearance of HF bands are not observed. This indicates that the 12R windows are not the preferential adsorption sites for benzene. 4. General Discussion 4.1. Location of Na+ Ions in the Presence of Benzene. For Beta zeolite, because of its disordered structure and its wide Si/Al ratio range, little is known about the location of compensating cations.2,60 In the present study, a Beta zeolite with an Al content of around 3.5 per unit cell has been used. Since one Al atom can create one negative charge in the framework, which needs to be balanced by the compensating cations, there are
Su and Norberg
Figure 15. Changes in the infrared absorbance spectra of benzene in the range 4000-2800 cm-1 in the gas phase of the infrared cell with the introduction of an increasing amount of ammonia (molecules/u.c.) into the IR cell at a benzene loading level of 6.0 molecules/u.c.: (a) 0; (b) 1.0; (c) 3.0; (d) 5.0; (e) 8.0; (f) 10.0; (g) 15.0; and (h) a pressure of ammonia present in the IR cell.
around 3.5 Na+ ions per unit cell in this Beta zeolite. The results from the adsorption of benzene show that around 3.4 ( 0.3 molecules of benzene are strongly adsorbed on Na+ ions. This value is very close to the Na content in NaBeta zeolite. As one Na+ ion interacts only with one molecule of benzene, this means that all the Na+ ions present in NaBeta zeolite are accessible for benzene. Considering the large size of the benzene molecule (3.4 × 6.2 × 6.9 Å3), only the 12R channels in Beta zeolite are accessible for benzene. It is possible that all the Na+ ions are initially located in the 12R channels. Another hypothesis is that a part of the Na+ ions is initially located in the small cages and inaccessible,2,59 is attracted, by benzene molecules, migrates finally into the 12R channels, and becomes accessible for benzene molecules. The migration of Na+ ions from the different hexagonal prisms and sodalite cages into the large cages of NaEMT in the presence of benzene has already been observed,37 and it has also been reported that the protons in the sodalite cages of HSAPO-37 can be attracted into supercages and become accessible for benzene.35,36 Our recent study on the Bro¨nsted acidity of HBeta zeolite using benzene adsorption also showed that all the bridging protons can interact with benzene molecules, indicating the accessibility of all the protons to benzene.18 This redistribution of counterions in the presence of benzene was confirmed by neutron diffraction study on HSAPO-37.26 However, (60) Pe´rez-Pariente, J.; Sanz, J.; Forne´s, V.; Corma, A. J. Catal. 1990, 124, 217.
Adsorption Behavior of Benzene in NaBeta Zeolite
the migration of counterions was not observed in HY and NaY zeolites.36-38 The reason the counterions will be redistributed in the presence of benzene molecules is not clear yet. However, the present work suggests strongly that, in the presence of benzene, all the Na+ ions of NaBeta zeolite should be located in the 12R channels. 4.2. Location of Benzene in the 12R Windows of Zeolites with and without Coadsorption of Ammonia: Molecular Recognition Control. As just described, besides benzene molecules interacting weakly with the external silanols, only Na+ ions can strongly interact with benzene molecules, and they are the only kind of preferential adsorption sites for benzene. Very few benzene molecules are found to sit on the 12R windows of this zeolite. This indicates that the 12R windows of NaBeta zeolite are not the preferential adsorption sites for benzene even in the presence of ammonia. It was previously demonstrated that the location of benzene was strongly dependent on the negative charge of the framework oxygen atoms. The average negative charge of the present NaBeta zeolite lies around -0.240, is similar to those of HY and dealuminated NaY (Si/Al ) 6.0), and is lower than that of NaEMT. It seems that the absence of benzene location on 12R windows might result from its low basicity. It was observed that the coadsorption of ammonia, a basic molecule, can modify the benzene adsorption behavior on zeolite due to the interaction of ammonia with the zeolite framework, which induces the redistribution of the charge on the framework atoms. Because of the increase in negative charge of oxygen atoms upon coadsorption of ammonia, the 12R windows of HY, dealuminated NaY, and NaEMT, being not preferential adsorption sites for benzene in the presence of benzene alone, become indeed the adsorption sites for benzene. A displacement of benzene molecules adsorbed on Na+ ions toward the 12R windows was observed. Similar experiments have been performed on NaBeta zeolite. Upon coadsorption of ammonia, the negative charge of oxygen atoms of NaBeta zeolite can be supposed to be increased as in HY, dealuminated NaY, and NaEMT, and the 12R windows can be expected to become adsorption sites for benzene. However, no displacement of benzene from cations to 12R windows and no benzene molecules adsorbed on 12R windows are detected both at low and high benzene loadings upon coadsorption of ammonia. A similar situation was previously observed in KL zeolite.48,61 The above discussion suggests that the basicity of the framework oxygen atoms should not be the only dominating factor for the location of benzene on the 12R windows and that other factors should also be taken into account. It was known that the adsorption of benzene on the 12R windows is a multiple interaction; that is, six hydrogen atoms of benzene molecule interact simultaneously with six oxygen atoms of the 12R windows. The adsorption or fixation of benzene on the 12R windows is possible only when the oxygen atoms of the 12R windows have adapted chemical properties and the 12R window has a suitable geometry for the benzene molecule. The benzene molecule has a dimension around 3.4 × 6.2 × 6.9 Å3, and the six carbon atoms form a planar ring. It has been recognized that all synthetic zeolite Beta materials reported up to now show extreme disorder.5 It contains two topologically identical straight channel systems formed by the nonplanar orthogonal rings and a nonlinear tortuous channel system with saddle-shaped 12R windows (see Figure 1) which is parallel to the c (61) Su, B. L. J. Chem. Soc., Faraday Trans. 1997, 93, 1449.
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crystallographic axis and perpendicular to the other two linear channels.2-4 Hence, there are two kinds of 12R windows in Beta zeolite (types I and II). The 12R window (type II) in the tortuous channels, having the opening around 5.5 × 5.5 Å2 with a saddle shape (Figure 1), should be too small or too deformed to be an adsorption site for benzene molecules. This kind of 12R window is not, therefore, structurally suitable for benzene location. The 12R windows (type I) in the straight channels are large enough (6.4 × 7.6 Å2) (Figure 1) as host to receive benzene molecules as guest, and the negative charge on their 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. However, no benzene is found on these 12R windows. The lack of adsorption of benzene on this kind of 12R windows should therefore be, as observed in KL zeolite,47,48,61 related to a structural effect; that is, there is no suitable geometric compatibility between this kind of 12R windows and benzene molecules. The present study suggests strongly that the factors, other than chemical, should intervene in governing the location of benzene on a 12R window. Since there is no migration or redistribution of the counterions occurring in NaY zeolite in the presence of benzene and since the 12R windows of this zeolite are indeed the adsorption sites for benzene, hence, it is also possible that the lack of the adsorption of benzene on 12R windows of NaBeta zeolite results from this migration. However, the 12R windows in KL zeolite do not adsorb the benzene molecules, and no migration of counterions has been evidenced. The hypothesis that the lack of the adsorption of benzene on 12R windows of zeolites should be linked to the migration of counterions does not necessarily hold, and the question remains open. Our present work demonstrates that the adsorption of benzene on 12R windows is not a general trend. The location of benzene should be very likely controlled by a molecular recognition effect where the adsorbate and absorbent should have the suitable chemical and structural properties as in substrate-enzyme systems. 5. Conclusion The quantitative analysis of the benzene adsorption on NaBeta zeolite with a Si/Al ratio around 17.3 shows that all the Na+ ions present in this zeolite can strongly interact with benzene molecules and are located in the 12R channels and that a redistribution of the counterions occurs in the presence of benzene. The large number of silanols present in this zeolite can also weakly interact with benzene molecules at relatively high temperatures. The two kinds of 12R windows occurring in Beta zeolite are not the preferential adsorption sites for benzene even in the presence of ammonia. The present study reveals that the adsorption of benzene on 12R windows should very probably be dominated by a molecular recognition effect where the adsorbate and absorbent interaction involves the adapted chemical and structural properties as in substrate-enzyme systems. Acknowledgment. The authors thank Dr. Camblor at the Instituto de Tecnologia Quimica, Polytechnical University of Valencia, Spain, for his gift of starting zeolite sample Na(TEA)Beta. We would also like to thank Drs. David Mosley and Laurence Leherte at University of Namur for very helpful discussion and Mrs. Su-Virlet for her very helpful assistance. The FNRS (Fonds National de la Recherche Scientifique, Belgium) is also acknowledged for a scholarchip (FRIA) to V.N. LA9809574
