A Comparative Spectroscopic Study on the Location of Benzene and

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Langmuir 2001, 17, 1267-1276

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A Comparative Spectroscopic Study on the Location of Benzene and Cations in a Series of Si-Rich NaY Zeolites Bao-Lian Su,*,† Vale´rie Norberg,† and Johan A. Martens‡ Laboratoire de Chimie des Mate´ riaux Inorganiques, Universite´ de Namur, 61 Rue de Bruxelles, B-5000 Namur, Belgium, and Centrum voor Oppervlaktechemie en Katalyse (C.O.K.), Katholieke Universiteit Leuven, 92 Kardinaal Mercierlaan, B-3001, Heverlee, Belgium Received September 26, 2000. In Final Form: November 20, 2000 Benzene adsorption in two Si-rich NaY (NaYs and NaYd) zeolites (Si/Al ratio ≈ 3.5), prepared from different methods, has been investigated using FTIR spectroscopy and compared with that in NaY (Si/Al ratio ) 2.3) and NaEMT (Si/Al ratio ) 3.6) zeolites. The interaction strength of benzene with one site or another in these zeolites has been evaluated by a successive desorption of benzene at different temperatures. The location of cations in these two different Si-rich zeolites has been discussed. The quantitative analysis of the changes of the absorbance of the C-H out-of-plane vibrations with benzene loadings gives access to total benzene adsorption capacity and the amounts of benzene adsorbed on one site or another. The adsorption behavior of benzene in these zeolites has been then correlated with the Al distribution, the location of Na+ ions, the average negative charge of oxygen atoms, the zeolite structure, and the interaction strength of benzene with one site or another. As observed in NaY zeolite, two adsorption sites for benzene, Na+ ions and 12R windows, have been evidenced in two Si-rich NaY, whereas benzene molecules sit mainly on Na+ ions in NaEMT. No significant influence of the Al distribution on the benzene location and on the distribution of cations in the supercages of these two different Si-rich NaY zeolites is observed. A new pair of shoulders at high wavenumbers (2050-1913 cm-1) at high benzene loadings is detected for the first time. In all the NaY zeolites studied here, the interaction of benzene with the Na+ ions is stronger than that with the 12R windows. This explains the observation that the number of benzene molecules adsorbed on the 12R windows is lower than that on the Na+ ions. The low adsorption capacity for a supercage of NaYs and NaYd zeolites is most likely related to the low number of Na+ ions in a supercage and consequently to the high mobility of benzene molecules in the zeolite structures. It reveals that there are the diverse strengths of interaction of benzene with Na+ ions in NaEMT due to the migration of Na+ ions from the small cages to the large cages in the presence of benzene.

1. Introduction Industrial gas separation technology is a critical part of the petroleum, natural gas, and organic chemical industrial processes. Because of zeolite developments, rapid advances in new technology have been made in recent years. Alkali metal cation exchanged faujasite zeolites have been widely used as adsorbents in the separation of air and aromatics. It is then of fundamental and industrial interest to better understand the way the gas molecules interact with and in zeolites. Knowledge of the host-guest interaction and the location of cations in zeolites can suggest to us new materials with advanced performance. Zeolite Y is a synthetic FAU-type zeolite that has been extensively used as a catalyst in a series of reactions and as an adsorbent in the gas separation.1-9 This type of zeolite is usually synthesized under hydrothermal condi* Corresponding author. Fax: 32 81 72 54 14. E-mail: [email protected]. † Universite ´ de Namur. ‡ Katholieke Universiteit Leuven. (1) Venuto, P. B. Stud. Surf. Sci. Catal. 1996, 105, 811. (2) Lee, H. Molecular Sieves; Advances in Chemistry Series No. 121; American Chemical Society: Washington, DC, 1973; p 311. (3) Kiselev, A. V. Molecular Sieves; Advances in Chemistry Series No. 102; American Chemical Society: Washington, DC, 1971; p 37. (4) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: New York, 1982. (5) Corma, A. In Zeolite Microporous Solids: Synthesis, Structure, and Reactivity; Derouane, E. G., Lemos, F., Naccache, C., Riberio, F. R., Eds.; NATO ASI Series, Series C; Kluwer Academic Publishers: Dordrecht, 1992; Vol. 352, p 373. (6) Barthomeuf, D. Catal. Rev. 1996, 38, 521. (7) Su, B. L.; Barthomeuf, D. Appl. Catal., A 1995, 124, 81.

tions using a traditional sodium aluminosilicate hydrogel. This gives a framework with a Si/Al ratio ranging from 1.5 to 3.0.10,11 Silicon-enriched faujasites (Si/Al ratio higher than 3.0) can be prepared by using a dealumination process.12-14 Recently, the utilization of the crown ethers as organic templates has led the successful synthesis of cubic and hexagonal faujasites with Si/Al ratios higher than 3.5.15-19 However, it has been revealed20 that the Al distribution in FAU zeolite from a crown ether mediated synthesis is not random and the Si and Al ordering and the population of Na+ ions at the different cation sites were significantly different from those of Y zeolite crystallized in the absence of crown ether and then (8) Su, B. L.; Barthomeuf, D. Appl. Catal., A 1995, 124, 73. (9) Su, B. L.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1995, 94, 598. (10) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry and Use; John Wiley & Sons: London, 1974. (11) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (12) Beyer, H. K.; Beleny, I. M.; Hange, F. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2889. (13) Grobet, P. J.; Jacobs, P. A.; Beyer, H. K. Zeolites 1986, 6, 47. (14) Szostak, R. Stud. Surf. Sci. Catal. 1991, 58, 153. (15) Delprato, F. Ph.D. Thesis, Universite´ de Haute Alsace, Mulhouse, France, 1989. (16) Guth, J.-L.; Caullet, P.; Seive, A.; Patarin, J.; Delprato, F. In Guidelines for Mastering the Properties of Molecular Sieves; Barthomeuf, D., Derouane, E. G., Ho¨lderich, W., Eds.; NATO ASI Series B: Physics; Plenum Press: New York 1990; Vol. 221, p 69. (17) Dougnier, F.; Patarin, J.; Guth, J.-L.; Anglerot, D. Zeolites 1992, 12, 160. (18) Delprato, F.; Delmotte, L.; Guth, J.-L.; Huve, L. Zeolites 1990, 10, 546. (19) Annen, M. J.; Young, D.; Arhancet, J. P.; Davis, M. E.; Schramm, S. Zeolites 1991, 11, 98. (20) Dewer, J.; Karim, K.; Smith, W. J.; Thompson, N. E.; Harris, R. K.; Apperley, C. J. Phys. Chem. 1991, 95, 8826.

10.1021/la0013704 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/18/2001

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dealuminated with silicon hexafluoride to a comparable framework Al fraction. On the basis of the combination of a 29Si magic angle spinning (MAS) NMR study and a model generated by computer algorithm, the heterogeneous Al sitting throughout the zeolite lattice and the existence of Al-enriched and Al-depleted zones in the FAU and EMT zeolites synthesized in the presence of crown ether were reported.21 The 12R windows occurring in FAU and EMT zeolites from crown ether mediated synthesis were classified as Si-rich and Al-rich. The heterogeneity of basic sites has been evidenced by Murphy et al.22 using pyrrole adsorption, confirming heterogeneous Al distribution in EMT zeolite. This heterogeneous Al sitting is believed to be induced by the Na-crown ether complexes which are sitting at distinct surface holes during zeolite synthesis.21 Chao et al.23 even assumed that in the presence of crown ethers, the frameworks of FAU and EMT zeolites are constructed from β-cage units containing either five or four aluminum T-atoms. These differences in the location of Na+ ions and Al distribution in the FAU zeolites synthesized with and without the presence of crown ethers should invoke the different acid-base character and, consequently, the different catalytic and adsorptive properties. The higher catalytic activity in hexane cracking of materials from a crown ether mediated synthesis was observed and was most likely due to this different Al sitting.24 However, few papers deal with this matter. It was reported6,8,9,25-30 that benzene molecules can interact both with Na+ ions (acting as Lewis acids) and with the highly negative charged framework oxygen atoms (intrinsic basic sites) located in the 12R windows of NaY, which is synthesized from the sodium aluminosilicate gel and has a Si/Al ratio around 2.4. On the basis of the amount of benzene adsorbed in a supercage, it was concluded that about four Na+ ions were located in each supercage and others are present in the hexagonal and sodalite cages of this NaY zeolite. However, only the Na+ ions in NaEMT were found to interact with benzene molecules. The high number of benzene molecules found in the large cages of this zeolite led to the important finding that a migration of Na+ ions from the small cages toward the large cages occurs, and all the Na+ ions are present in the large cages in the presence of benzene.31-34 The questions can then arise as to (1) whether two Si-rich NaY, prepared from two different methods cited above and having similar Si/ Al ratios (≈3.5) but different Al distributions in the framework, should have the similar benzene adsorption behavior, (2) whether the distribution of cations in the supercages of these two zeolites is different, and (3) (21) Feijin, E. J. P.; Lievens, J. L.; Martens, J. A.; Grobet, Piet. J.; Jacobs, P. A. J. Phys. Chem. 1996, 100, 4970. (22) Murphy, D.; Massiani, P.; Franck, R.; Barthomeuf, D. J. Phys. Chem. 1996, 100, 6731. (23) Chao, J. J.; Shy, D. S.; Sheu, S. P.; Lin, C. F. Microporous Mater. 1994, 2, 91. (24) Feijin, E. J. P.; Martens, J. A.; Jacobs, P. A. Stud. Surf. Sci. Catal. 1996, 101, 721. (25) de Mallmann, A.; Barthomeuf, D. J. Phys. Chem. 1989, 93, 5636. (26) de Mallmann, A. Ph.D. Thesis, University of Paris, Paris, France, 1989. (27) de Mallmann, A.; Barthomeuf, D. Zeolites 1988, 8, 292. (28) Barthomeuf, D.; de Mallmann, A. In Chemistry, Ecology and Health; Ione, K. G., Ed.; Nova Science Publishers: Commack, NY, 1996; p 276. (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) Su, B. L.; Manoli, J. M.; Potvin, C.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1993, 89, 857. (32) Su, B. L. Ph.D. Thesis, University of Paris, Paris, France, 1992. (33) Su, B. L. J. Chem. Soc., Faraday Trans. 1997, 93, 1449. (34) Su, B. L. Zeolites 1996, 16, 25.

Su et al. Table 1. Chemical Composition and Preparation Methods of Zeolites Studied zeolites

chemical composition for one unit cell

Si/Al ratio

labeled as

Si-NaYa Si-NaYb NaYc NaEMTd

Na42(AlO2)42(SiO2)150 Na44(AlO2)44(SiO2)148 Na56(AlO2)56(SiO2)136 Na21(AlO2)21(SiO2)75

3.6 3.4 2.4 3.6

NaYs NaYd NaY NaEMT

a Prepared using 15-crown-5 as template.15-19,21 b Prepared from a dealumination process, see the description in the Experimental Section. c Synthesiszd using a hydrogel. d Prepared using 18crown-6 as template.15-19,21

whether the adsorption behavior of NaY synthesized in the presence of crown ether is similar to NaY or to NaEMT, i.e., whether the different Al distribution affects the adsorption properties of zeolites. The answers to these questions are fundamental in understanding their catalytic properties and in designing the new materials with advanced performances. The present work describes the cation location and the adsorption behavior of benzene in two Si-rich NaY zeolites (Si/Al ) 3.4-3.6) prepared from different methods. By comparison with NaEMT, and a commercial NaY (Si/Al ) 2.4), we try to shed some light on the effect of the structure, Al distribution, and Si/Al ratio on the cation location and the adsorption behavior of benzene in zeolites. 2. Experimental Section 2.1. Materials. The first Si-rich NaY (Si/Al ) 3.6) (labeled as NaYs) and NaEMT (Si/Al ) 3.6) zeolites were synthesized, as described in refs 15-19 and 21, using 15-crown-5 ether and 18crown-6 ether as templates, respectively. The chemical compositions of these two samples (organic templates free) are listed in Table 1. The second Si-rich NaY with Si/Al ratio around 3.4 (labeled as NaYd) was obtained from a dealumination process using a commercial NaY (provided by Union Carbide) as starting material. NaY, whose chemical composition is listed in Table 1, was first extensively exchanged in a NH4NO3 solution to obtain highly exchanged NH4Y. A weight of 2.5 g of NaY zeolite was added to 30 mL of solution of NH4NO3 (10 wt %) heated at 368 K. The suspension was mixed for 3 h at 368 K with stirring. The hot mixture was filtered and washed with abundant hot bidistilled water (368 K) in order to eliminate NO3- ions. The full exchange procedure was repeated seven times. The recovered solid was dried in an oven at 373 K overnight and has the formula (NH4)56.4Na0.7Y (labeled as NH4Y). The crystallinity of the obtained powder was checked using X-ray diffraction (XRD) and was shown to be good. The dealumination was then conducted using the method reported by Skeels and Breck35 and others.36 The NH4Y zeolite was introduced at 343 K in a 0.4 M ammonium hexafluorosilicate solution, the zeolite to (NH4)2SiF6 ratio being 3.3 g/g. After the zeolite was well-dispersed, the temperature of the bath was increased to 368 K and kept at this value for 3 h with stirring. The product was then filtered and thoroughly washed with boiling water. The recovered sample was dried at 373 K overnight and then exchanged three times with a 1 M solution of sodium chloride at 333 K. The obtained material was dried at 373 K overnight before any use. The chemical composition of this sample was made from chemical analysis, 27Al and 29Si MAS NMR, and elemental analysis by atomic adsorption and is also listed in Table 1. Its crystallinity was checked using XRD and was shown to be good and no extraframework Al was detected from 27Al MAS NMR. 2.2. Infrared Studies of Benzene Adsorption and Desorption. The preparation and pretreatment of the selfsupported zeolite wafers (15 mg/cm2) and the adsorption and (35) Skeels, G. W.; Breck, D. W. In Proceedings of the Sixth International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworths: Guildford, 1984; p 87. (36) Garralon, G.; Fornes, V.; Corma, A. Zeolites 1988, 8, 268.

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desorption of benzene onto zeolite wafers were previously detailed in refs 31-33. All the IR spectra were recorded using a PerkinElmer Fourier Transform Spectrum 2000 spectrometer. The determination of the total adsorption capacity of zeolites for benzene, the amounts of benzene adsorbed on one site or other, and the quantity of benzene molecules present in the gas phase of the IR cell at each benzene loading have been thoroughly described in our recent paper.37 For the sake of further use in the present paper, we give only a very brief description of the equations used. The total relative absorbance, Ar(total), of benzene IR bands for one supercage (s.c.) was obtained from the area of the peaks using the expression25-34,38-41

Ar/s.c. ) k

M 8m

∫

band

A(ν) dν

(1)

where M is the weight of one unit cell of the zeolites, m is the weight of the wafer in grams, ν is the wavenumber in cm-1, and k is a coefficient arbitrarily chosen equal to 2.95 × 10-7. The relative absorbance, Ar, of the infrared band characterizing adsorption of benzene on cations or on 12R windows at each benzene loading, n, was also obtained by using this equation after deconvolution of the bands. The number of benzene molecules adsorbed on the cations, nC, or on the 12R windows, nW, at each benzene loading, n, can be evaluated using eq 225-34,38-41

Ar/s.c. ) InC (or nW)/2491

(2)

where I is the integrated intensity of each peak and 2491 is a constant depending on the units chosen25-34,38-41 (24 tetrahedra, i.e., a supercage, is chosen in this study). For more details, please see the refs 25-34 and 38-41. na is the quantity of benzene molecules adsorbed on zeolite at a benzene loading n, hence

na ) nC + nW

Figure 1. Changes in the infrared absorbance spectra of C-H out-of-plane vibration (v5 + v17) and (v10 + v17) bands of benzene absorbed on NaYs zeolite pretreated at 723 K at different amount of benzene introduced into the IR cell (molecules/s.c.): (a) 0.5; (b) 1.0; (c) 1.5; (d) 2.0; (e) 2.5; (f) 3.0; (g) 3.5; (h) 4.5; (i) saturation (26 mbar of benzene in the cell). Chart 1

(3)

na is normally different from n, which is the sum of benzene molecules adsorbed on the wafer (na) and those contained in the gas phase of the cell. At very low benzene loadings, na and n should be the same or similar since all benzene molecules introduced are adsorbed by zeolite.

3. Results and Discussion 3.1. Adsorption of Benzene on NaYs. 3.1.1. Adsorption Sites for Benzene. Figure 1 reports the changes in the infrared absorbance spectra of C-H out-of-plane (o.o.p.) vibrations of benzene adsorbed on NaYs zeolite, which was synthesized in the presence of crown ether, with increasing amounts of benzene. The spectrum of NaYs zeolite and that of the gas phase of the IR cell have been subtracted. When the first molecule of benzene is introduced into the infrared cell, one main pair of bands at 1985 and 1845 cm-1 and a pair of shoulders at 2015 and 1877 cm-1 appear immediately (Figure 1a). It was known that liquid benzene gives a pair of bands at 1960 and 1815 cm-1 in the range of C-H o.o.p. vibration, assigned to (v5 + v17) and (v10 + v17) vibrations, respectively, and the adsorption of benzene on zeolites can shift this pair of bands toward higher wavenumbers. The shift value compared to liquid benzene can be used to distinguish the different types of sites interacting with benzene.6-9,25-34,37-41 Benzene adsorbed on the counterions of zeolites can normally give a pair of bands shifted around 20-40 cm-1. This shift results from an interaction of benzene through (37) Su, B. L.; Norberg, V.; Martens, J. A. Microporous Mesoporous Mater. 1998, 25, 151. (38) Su, B. L.; Barthomeuf, D. J. Catal. 1993, 139, 470. (39) Su, B. L.; Barthomeuf, D. Zeolites 1993, 13, 626. (40) Su, B. L.; Norberg, V. Zeolites 1997, 19, 65. (41) Su, B. L. Zeolites 1996, 16, 75.

a Key: •, T atoms (Si or Al atoms); b, Na+ cation; O, oxygen atoms; o, hydrogen atoms of benzene.

its π electron cloud with the counterions (Chart 1a). This interaction affects only indirectly and weakly the C-H out-of-plane bending vibration. Since the shift value is relatively low, this pair is therefore referred to as lowfrequency (LF) bands. A higher shift value of around 50100 cm-1, referred to as a high-frequency (HF) band, can be observed if benzene molecules sit on the 12R windows of zeolites.6-9,25-34,37-41 This is due to the interaction of hydrogen atoms of benzene molecules with negatively charged oxygen atoms located in the 12R windows. This interaction affects directly and strongly the C-H out-ofplane vibration; thus the shift value is higher (Scheme 1b). The above assignments and benzene locations based on the infrared results have been verified by NMR,42-45 (42) Lechert, H.; Wittern, K. P. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1054. (43) Hasha, D. Miner, V.; Garces, G.; Roche, S. Proceedings of Symposium of the new Surface, Science in Catalysis, ACS meeting, Philadelphia, August, 1984; p 953.

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neutron diffraction46-48 and theoretical calculation49-55 in a series of faujasite X and Y zeolites exchanged with alkali metal cations. The exact shift values and the presence of two types of adsorption sites depend on the cation types, the zeolite structures, and benzene loadings. As described above, the first pair of bands gives a shift value of around 25-30 cm-1 and corresponds therefore to the benzene molecules interacting with the cations of NaYs zeolite (LF bands). A high shift value of around 55-62 cm-1 has been found for the pair of shoulders which is thus attributed to the location of benzene on the 12R windows (HF bands). As the amount of benzene introduced into the IR cell increases, these two pairs of bands become more and more intense. The wavenumbers of these two pairs of bands vary, however, only very slightly. The intensities of both HF and LF bands remain practically unchanged from a benzene loading of 2.50 molecules/s.c. (Figure 1e). At this benzene loading, another pair of shoulders at 1958 and 1816 cm-1 is detected. This pair of shoulders whose wavenumbers are the same as that of liquid benzene should be assigned to benzene molecules in the pseudo-liquid phase, referred to as PL bands. These shoulders remain rather small even in the presence of a high pressure of benzene in the cell. The above observation indicates that in NaYs zeolite, besides the adsorption of benzene on Na+ ions, benzene molecules can also interact with the oxygen atoms of the 12R windows. This is not observed in NaEMT which was also synthesized in the presence of crown ether. The simultaneous presence of HF and LF bands at any benzene loading suggests that when a defined amount of benzene is introduced, a part of the benzene will interact with the cations and another part will locate on the 12R windows. The adsorption on the cations and on the 12R windows occurs simultaneously. This is different from the previous observation that the adsorption of benzene on the 12R windows appears only when all the Na+ ions accessible to benzene are occupied by benzene molecules in NaY zeolite.25-30 It should be indicated that, for the first time, a pair of very small shoulders is detected at 2052 and 1906 cm-1 at any benzene loading (Figure 1). The assignment of this pair of shoulders is not clear. Whether this pair of shoulders is generated by the interaction of benzene with the sites other than Na+ ions in SII sites and 12R windows will be further ascertained. 3.1.2. Determination of the Amounts of Benzene Adsorbed. Total Amount of Benzene Adsorbed on NaYs Zeolite. The total amount of benzene adsorbed on NaYs zeolite, i.e., the sum of benzene adsorbed on the cations and the 12R windows, is first evaluated by plotting the total relative absorbances, Ar (total), of (v10 + v17) and of (44) Bull, L. M.; Henson, N. J.; Cheetham, A. K.; Newsam, J. M.; Heyes, S. J. J. Phys. Chem. 1993, 97, 11776. (45) Auerbach, S. M.; Bull, L. M.; Henson, N. J.; Metiu, H. I.; Cheetham, A. K. J. Phys. Chem. 1996, 100, 5923. (46) Fitch, A. N.; Jobic, H.; Renouprey, A. J. Phys. Chem. 1986, 90, 1311. (47) Fitch, A. N.; Jobic, H.; Renouprey, A. J. Chem. Soc., Chem. Commun. 1985, 284. (48) Jobic, H.; Renouprey, A.; Fitch, A. N.; Lanter, H. J. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3199. (49) Sauer, J.; Deininger, D. Zeolites 1982, 2, 114. (50) Auerbach, S. M.; Metiu, H. I. J. Chem. Phys. 1996, 105, 3753. (51) Auerbach, S. M. J. Chem. Phys. 1996, 106, 7810. (52) Auerbach, S. M.; Henson, N. J.; Cheetham, A. K.; Metiu, H. I. J. Phys. Chem. 1995, 99, 10600. (53) Kitagawa, T.; Tsuneekawa, T.; Iwayama, K. Mircoporous Mater. 1996, 7, 227. (54) Demontis, P.; Yashonath, S.; Klein, M. L. J. Phys. Chem. 1989, 93, 5016. (55) Henson, N. J.; Cheetham, A. K.;Redondo, A.; Levine, S. M.; Newsam, J. M. Stud. Surf. Sci. Catal. 1994, 84, 2059.

Su et al.

Figure 2. Changes in the relative absorbance of benzene adsorbed on NaYs with increasing amounts of benzene introduced into the cell (molecules/s.c.): (a) relative absorbance, Ar, of benzene adsorbed on the Na+ ions; (b) relative absorbance, Ar, of benzene adsorbed on the 12R windows; (c) total relative adsorbance, Ar(total), of benzene adsorbed on NaYs zeolite; (A) (v10 + v17) vibration and (B) (v5 + v17) vibration.

(v5 + v17) vibrations, as a function of the amount of benzene introduced into the IR cell. The Ar (total) values for each vibration can be calculated using eq 1 and are displayed in Figure 2 (curves c). It can be seen that the total relative absorbances for (v10 + v17) vibration (curve c of Figure 2A) and that for (v5 + v17) vibration (curve c of Figure 2B) increase with the introduction of benzene molecules into the IR cell and reach a constant value at an abscissa P of around 2.50 ( 0.30 molecules/s.c., marked by the dotand-dash line. The further addition of benzene into the cell does not result in further changes in the total relative absorbance. This indicates that the saturation of the zeolite is reached at this benzene loading. The changes in the gas phase of the infrared cell have been checked after each benzene addition. Some very small peaks in the range of 3100-2950 cm-1 are detected only after the introduction of around 2.50 molecules/s.c. of benzene into the infrared cell (Figure 3e). The intensities of these peaks increase continuously when benzene molecules are further introduced into the cell. This indicates that when benzene loadings are below 2.50 molecules/s.c., all the benzene molecules introduced into the infrared cell can be fully adsorbed by NaYs zeolite. Above this benzene loading, benzene molecules are in

Location of Cations in Zeolites

Figure 3. Absorbance infrared spectra of benzene in the gas phase of an IR cell after addition of benzene onto NaYs zeolite: (a) 0.5; (b) 1.0; (c) 1.5; (d) 2.0; (e) 2.5; (f) 3.0; (g) 3.5; (h) 4.5; (i) saturation (26 mbar of benzene in the cell).

excess, and a part of benzene molecules introduced is present in the gas phase of the infrared cell. The quantity of benzene present in the gas phase of the cell at each benzene loading is determined using an external reference as previously reported.32,40 At a benzene loading of 2.50 molecules/s.c., around 0.30 ( 0.20 molecules/s.c. is present in the gas phase. The total amount of benzene adsorbed on NaYs zeolite is obtained by deducting the amount of benzene present in the gas phase from the value P from which the total relative absorbances become unchanged. This gives a value of 2.20 ( 0.50 molecules/s.c., indicating that the total adsorption capacity of NaYs zeolite for benzene is 2.20 ( 0.50 molecules/s.c. Selectivity of Benzene Adsorbed on Na+ Ions and on 12R Windows. The changes in Ar (LF) and Ar (HF) of (v5 + v17) vibration of benzene (curves a and b) and those of (v10 + v17) vibration of benzene (curves a and b) at each amount of benzene introduced are obtained using eq 1 after the decomposition of bands, and reported in Figure 2. As we can see that the Ar (LF) and Ar (HF) values increase simultaneously with the introduction of increasing amount of benzene. The values of Ar (LF) reach a plateau at an abscissa value P′ ) 2.00 ( 0.30 m/s.c.. However, the values of Ar (HF) attain a constant value only at an abscissa value P ) 2.50 ( 0.30 molecules/s.c. The latter is the same as that observed for the values of Ar (total). The above observation suggests that the cations able to interact with benzene molecules are saturated by benzene molecules before the 12R windows will be. This implies that the interaction of benzene with the cations is stronger than that with the 12R windows in NaYs zeolite. When all the 12R windows able to interact with benzene molecules are occupied, no additional benzene molecules can chemically adsorb on this zeolite. Benzene adsorption follows the Beer-Lambert law before zeolite is saturated by benzene. Furthermore, if benzene is adsorbed under one form, I in eq 2 is a constant for a given material and Ar is proportional to the amount

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Figure 4. Changes in the infrared absorbance spectra of C-H out-of-plane vibration (v5 + v17) and (v10 + v17) bands of benzene absorbed on NaYd zeolite pretreated at 723 K with different amounts of benzene introduced into the IR cell (molecules/s.c.): (a) 0.5; (b) 1.0; (c) 1.5; (d) 2.0; (e) 2.5; (f) 3.0; (g) 3.5; (h) 4.0; (i) saturation (26 mbar of benzene in the cell).

of benzene adsorbed up to the total adsorption capacity of a given zeolite or to the saturation of the sites.6-9,25-34,37-41 However, it should be noted that the integrated intensity should be different for one site or another and can vary with the structure of zeolite.26,27 Previous work has determined that the integrated intensity I was around 2.5 times higher for the adsorption of benzene onto the 12R windows than onto the cations for faujasite X and Y zeolites exchanged with the alkali metal cations.26,27 By use of eqs 2 and 3 and IC/IW ) 2.5, the amounts of benzene adsorbed on the cations and on the 12R windows at each benzene loading can separately be drawn. The calculation of nC and nW for NaYs zeolite gives 1.86 ( 0.30 and 0.34 ( 0.20 molecules/s.c. of benzene adsorbed on the cations and on the 12R windows, respectively, at the saturation of this solid with benzene. It was reported that one Na+ ion can interact only with one benzene molecule. The value of 1.86 ( 0.30 molecules/ s.c. implies that there are 1.86 ( 0.30 Na+ ions located in one supercage of NaYs zeolite. 3.2. Adsorption of Benzene on NaYd. 3.2.1. Adsorption Sites for Benzene. Figure 4 gives the infrared absorbance spectra of C-H o.o.p. vibration of benzene adsorbed on NaYd zeolite at different benzene loadings. The spectra of NaYd zeolite and that of the gas phase of the IR cell are subtracted. It can be seen that the adsorption behavior of benzene on NaYd zeolite is very similar to that on NaYs zeolite. One main pair of bands at 1987 and 1847 cm-1 and one pair of shoulders at 2019 and 1881 cm-1 are observed as soon as the first molecule of benzene is introduced onto this zeolite. These two pairs of bands correspond to the interaction of benzene molecules with the Na+ ions (LF bands) and the oxygen atoms of the 12R windows (HF bands) of NaYd zeolites due to their shift values of 24-29 and 58-66 cm-1, respectively. With the introduction of increasing amounts of benzene, the

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Figure 6. Absorbance infrared spectra of benzene in the gas phase in an IR cell after addition of benzene (molecules/s.c.) onto NaYd: (a) 0.5; (b) 1.0; (c) 1.5; (d) 2.0; (e) 2.5; (f) 3.0; (g) 43.5; (h) 4.0; (i) saturation (26 mbar of benzene in the cell).

Figure 5. Changes in the relative absorbance of benzene adsorbed on NaYd with increasing amounts of benzene introduced into the cell (molecules/s.c.): (A) (v10 + v17) vibration and (B) (v5 + v17) vibration; (a) relative absorbance, Ar, of benzene adsorbed on the Na+ ions; (b) relative absorbance, Ar, of benzene adsorbed on the 12R windows; (c) total relative absorbance, Ar(total), of benzene adsorbed on NaYd zeolite.

intensity of these two pairs of bands increases. In the presence of a high pressure of benzene in the IR cell (Figure 4i), a new pair of shoulders at 1962 and 1816 cm-1 appears. This pair of shoulders is attributed to the benzene molecules in the pseuso-liquid phase, i.e., PL bands. As observed in NaYs zeolite, both the Na+ ions and the 12R windows of NaYd zeolite are adsorption sites for benzene. The simultaneous presence of the HF and LF bands is observed on this zeolite. The pair of small shoulders at 2053 and 1912 cm-1 is also detected as in NaYs zeolite. 3.2.2. Determination of Amounts of Benzene Adsorbed. Total Amount of Benzene Adsorbed on NaYd Zeolite. The total amount of benzene adsorbed on this zeolite is first evaluated using the same method as made for NaYs zeolite. Figure 5 reports the changes in Ar (total) for (v10 + v17) (curve c in Figure 5A) and (v5 + v17) (curve c in Figure 5B) vibrations as a function of the amount of benzene introduced into the cell. The values of Ar (total) for (v10 + v17) and (v5 + v17) vibrations reach a plateau at an abscissa P of around 2.50 ( 0.30 molecules/s.c., indicated by the dot-and-dash line. As observed in the case of NaYs zeolite, only a trace of benzene is detected in the gas phase of the cell when around 2.50 molecules/ s.c. of benzene are introduced into the cell (Figure 6e).

This indicates that all the benzene introduced into the cell is adsorbed onto a NaYd zeolite wafer when benzene loadings are below 2.50 molecules/s.c. At a benzene loading of 2.50 molecules/s.c., most of benzene introduced is adsorbed by zeolite and only a small part of benzene is present in the gas phase of the cell. The amount of benzene present in the gas phase at each benzene loading is determined using an external reference as described above. The total amount of benzene adsorbed on NaYd zeolite is obtained by subtracting the amount of benzene present in the gas phase from the value P. This gives a value of 2.30 ( 0.50 molecules/s.c., being very similar to that for NaYs zeolite. This suggests that these two Si-rich NaY zeolites with the similar Si/Al ratio have a similar adsorption capacity for benzene. Selectivity of Benzene Adsorbed on the Cations and on the 12R Windows. These are made using the same method as in the case of NaYs zeolite. The values of Ar for LF bands (curves a) and those for HF bands (curves b) at each benzene loading are obtained using the eq 1 after the decomposition of the HF and LF bands and are plotted as a function of the amounts of benzene introduced into the cell in Figure 5. The values of Ar of LF bands reach a plateau at P′ ) 2.15 ( 0.30 molecules/s.c., while those for HF bands become constant only at P ) 2.50 ( 0.20 molecules/s.c. The amounts of benzene adsorbed on the cations and on the 12R windows are therefore 2.00 ( 0.30 and 0.30 ( 0.20 molecules/s.c., respectively. The value of 2.00 ( 0.30 molecules/s.c. indicates that there are 2.00 ( 0.30 Na+ ions located in one supercage of NaYd zeolite. 3.3. Benzene Adsorption on NaYs, NaYd, NaEMT, and NaY Zeolites. Benzene adsorption behavior in NaYs and NaYd zeolites is compared with that in NaEMT and NaY zeolites. Figure 7 depicts the infrared spectra of benzene adsorbed on NaY(spectrum a), NaYs(b), NaYd(c), and NaEMT(d) zeolites in the C-H o.o.p. vibration range at room temperature at the saturation of the zeolites by benzene. It is clearly observed that the benzene adsorption

Location of Cations in Zeolites

Figure 7. Infrared spectra of benzene adsorption on NaY (a), NaYs (b), NaYd (c), and NaEMT (d) zeolites at room temperature at saturation of the zeolites.

behavior in NaYs and NaYd zeolites is very similar to that in NaY zeolite (without considering the total adsorption capacity for benzene) but quite different from that in NaEMT zeolite. The adsorption of benzene on NaEMT zeolite at the saturation condition gives only a main pair of bands at 1986 and 1845 cm-1, and this pair of bands is assigned to the interaction of benzene with Na+ ions of NaEMT. Two pairs of weak shoulders are also observed at 2014-1882 and 1961-1819 cm-1 and were attributed to the benzene interacting with the oxygen atoms of 12R windows and to the pseudo-liquid phase of benzene, respectively. It is evident that in NaEMT, benzene molecules sit mainly on Na+ ions. The present work indicates that despite the difference in the Si/Al ratio between NaYd and NaY zeolites and in the Al distribution between NaYs and NaYd zeolites, both the Na+ ions and the 12R windows in these three NaY zeolites are adsorption sites for benzene. However, despite the similarity of the Al distribution, the same Si/Al ratio ()3.6), and the existence of the Si-rich and Al-rich 12R windows in NaYs and NaEMT zeolites, these two zeolites give very different adsorption properties for benzene. It should be noticed that almost no PL bands can be detected on NaY. This is very important for the further discussion. The amounts of benzene adsorbed on one site or another in NaEMT and in NaY zeolites were previously determined.29-31 These values together with those of NaYs and NaYd zeolites are listed in the Table 2. Since EMT structure contains only 96 tetrahedra for one unit cell instead of the 192 in the FAU structure, the comparison is made at the same number of 192 tetrahedra. It is found that the number of benzenes interacting with the Na+ ions is the highest in NaEMT. In three NaY zeolites, NaY zeolite gives the highest benzene adsorption capacity and number of benzene adsorbed on the cations and on the 12R windows. There is only a slight difference between NaYs and NaYd zeolites in the total adsorption capacity for benzene and in the selectivity of benzene adsorbed on

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cations and on 12R windows. A general trend in three NaY zeolites is that the selectivity of benzene adsorbed on the Na+ ions is much higher than that on the 12R windows. 3.4. Interaction Strength of Benzene Molecules with the Na+ Ions and the 12R Windows. As described above, NaYs and NaYd give very similar benzene adsorption behavior, the interaction strength of benzene molecules with the Na+ ions and with the 12R windows is performed only on NaYs zeolite. This has been evaluated on a NaYs sample contacting with a benzene pressure of around 26 mbar. In this condition, the zeolite is completely saturated by benzene molecules. Figure 8 illustrates the infrared spectra in absorbance of benzene adsorbed on NaYs zeolite in the range of C-H o.o.p. vibrations at different desorption temperatures. At the saturation condition, the LF bands at 1985 and 1844 cm-1 and the HF bands at 2020 and 1882 cm-1 are observed. The weak PL shoulders at 1961 and 1813 cm-1 are also present (Figure 8a). After an evacuation at room temperature (Figure 8b), the PL bands disappear completely and no benzene can be detected in the gas phase of the IR cell. The absorbance of LF and HF bands also decreases. When the desorption temperature is raised to 323 K (Figure 8c), a great part of benzene adsorbed on the 12R windows (HF bands) is evacuated and the absorbance of LF bands decreases to half. This indicates that only around half of the total amount of benzene adsorbed on the cations is still retained by the Na+ ions. A desorption at 353 K (Figure 8d) removes all the benzene adsorbed on the 12R windows and a few of the benzene molecules also present on the Na+ ions. The above observation suggests again that the interaction of benzene molecules with the Na+ ions is stronger than that with the 12R windows. After evacuation at 363 K (Figure 8e), no trace of benzene is detected, indicating that all the adsorbed benzene molecules are desorbed. It was reported that only a desorption at 383 K during 1 h can eliminate completely the benzene molecules adsorbed in NaY zeolite.26 This means that the interaction of benzene with NaY is stronger than that with NaYs zeolite. For comparison, the same experiments have been conducted on a NaEMT zeolite wafer contacting also with a benzene pressure of about 26 mbar. The changes in absorbance of C-H oop vibration bands as a function of desorption temperature are given in Figure 9. A main pair of bands at 1986 and 1845 cm-1 and two pairs of small shoulders at 2014 and 1882 cm-1 and at 1961 and 1819 cm-1 are present. These three pairs of bands correspond to LF, HF, and PL bands, respectively. The evacuation at room temperature can remove completely the benzene molecules in the pseudo-liquid phase, mostly the benzene molecules located on 12R windows and more than half of benzene interacting with the Na+ ions (Figure 9b). This indicates that in NaEMT, a great part of Na+ ions interacts only weakly with benzene molecules. A similar spectrum as in the case of NaYs has been obtained after evacuation at 343 K (Figure 9c). This means that a part of the Na+ ions in NaEMT retains benzene molecules as strong as some Na+ ions in NaYs. All the benzene molecules previously adsorbed on NaEMT zeolite can be eliminated by an evacuation at 393 K (Figure 9d). 4. General Discussion 4.1. Effect of the Zeolite Structure, Al Distribution, and Si/Al Ratio on the Benzene Adsorption Behavior. Both NaYs and NaEMT are synthesized using crown ethers as templates and have a similar Si/Al ratio. NaY

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Table 2. Amounts and Selectivity of Benzene Adsorbed on Four Zeolites Studied amounts of benzene adsorbed (molecules)/192 TO4 on cations

b

on 12R windows

zeolites

on zeolite

amount

selectivity (%)

NaYs NaYd NaY NaEMTb

17.6 18.4 32.8 41

14.9 16.0 28.0 41

84.7 87.0 85.4 100

amount 2.7 2.4 4.8 trace

selectivity (%)

-δOa

15.3 13.0 14.6 ∼0

0.318 0.320 0.350 0.318

a Average negative charge of the framework oxygen atoms, calculated using Sanderson electronegativity equalization principle.6,56-58 A unit cell of EMT contains 92 tetrahedra while Fau contains 192 tetrahedra.

Figure 9. Changes in the absorbance of C-H out-of-plane vibration bands of benzene adsorbed on NaEMT zeolite at saturation condition (a) and after evacuation 0.5h at (b) 298, (c) 343, and (d) 393 K. Figure 8. Changes in the absorbance of C-H out-of-plane vibration bands of benzene adsorbed on NaYs zeolite at saturation condition (a) and after evacuation 0.5 h at (b) 298, (c) 323, (d) 353, and (e) 363 K.

is an as-synthesized sample from a traditional method without using any organic templates, and NaYd is obtained by dealumination of the commercial NaY zeolite in silicon hexafluoride solution to get the comparable Al fraction to NaYs and NaEMT zeolites. There is the existence of the Si-rich and Al-rich 12R windows in NaYs and NaEMT zeolites, and the Al distribution is heterogeneous throughout these two zeolite frameworks.21 This was not observed in NaY or in NaYd zeolites.20,21 Although the Al distribution in NaYs and NaYd is different and although the Si/Al ratio in NaYd and NaY is different, these three NaY zeolites have similar adsorption properties for benzene (without considering the amounts of benzene adsorbed). Both the Na+ ions and the 12R windows are the adsorption sites for benzene. This suggests that the heterogeneous Al distribution and the variation of Si/Al ratio from 2.4 to 3.6 does not affect significantly the benzene location on the Na+ ions and on the 12R windows. However, this does not imply that the Si/Al ratio cannot influence the benzene location. If NaYd is further dealuminated, the location of benzene on the 12R windows disappears.26 In fact, we have reported that the location of benzene on the cations and on the 12R windows depends strongly on the Lewis acidity of the counterions and the negative charge of the oxygen atoms in the 12R windows. A strong Lewis acidity

of the cations stabilizes the adsorption of benzene on the counterions and the high negatively charged oxygen atoms reinforces their interaction with the hydrogen atoms of benzene. The average negative charge of the oxygen atoms, calculated using the Sanderson electronegativity equalization principle,6,56-58 which takes only the chemical composition of the zeolites into account, for four zeolites studied here is listed in Table 2. The framework oxygen atoms of NaYs and NaYd have sufficiently high negative charge to interact with benzene molecules. However, NaYs and NaEMT have quite different adsorption properties for benzene, despite their very similar negative charge value, their similarity in the Al distribution in the framework, and the existence of the Si-rich and Al-rich 12R windows in these two zeolites. EMT is an hexagonal analogue of faujasite, this difference is related to their different structures. The lack of benzene location on the 12R windows of NaEMT in the chemical and structural point of view has previously been discussed in detail.32-34 4.2. Selectivity of Benzene Adsorbed on One Site or Another and Cation Location. It is found from Table 2 that the total amount of benzene adsorbed on NaEMT and the number of cations interacting with benzene in this zeolite are the highest. This has been previously (56) Mortier, W. J. J. Catal. 1978, 55, 138. (57) Mortier, W. J. In Proceedings of the Sixth International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworth: Guildford, 1984; p 734. (58) Sanderson, R. T. Chemical bonds and bond energy; Academic Press: New York, 1976.

Location of Cations in Zeolites

explained by the migration of the Na+ ions from the sodalite cages and the hexagonal prisms toward the large cages in the presence of benzene. All the Na+ ions are present in the large cages and are accessible for benzene. The adsorption capacity and the number of the Na+ ions accessible to three NaY zeolites for benzene molecules are low compared to that of NaEMT. However, NaY zeolite gives a higher total adsorption capacity and number of benzene molecules interacting with the cations and the 12R windows compared to two Si-rich NaY zeolites. These result probably from the high negative charge on oxygen atoms and the high number of cations located in the supercages of NaY. As discussed above, a heterogeneous Al distribution which will be reflected in the cation site occupancy was observed in NaYs. The number of the Na+ ions on SII sites in NaYs and NaYd should be therefore different. However, only a slight difference in the number of benzenes is found in a supercage of these two zeolites, 1.86 ( 0.30 for NaYs and 2.00 ( 0.0 for NaYd. This means that there are around two Na+ ions located in a supercage of NaYs and NaYd. The number of the Na+ ions occupying SII sites in NaYs is quite similar to that in NaYd. This suggests strongly that the heterogeneous Al distribution does not significantly affect the SII site occupancy. However, the SI and SI′ site occupancy should be affected and the number of the Na+ ions in SI and SI′ sites in NaYs should be different from that in NaYd. The enhancement of the sodium cation population on SI′ sites in NaEMT and NaYs has been reported.59 This enhancement and the existence of the Si-rich and Al-rich 12R windows give probably a change in the local negative charge of oxygen atoms. That is why a small difference in the location of benzene on 12R windows is observed. The low amount of benzene observed in a supercage of NaYs and NaYd results therefore from the low number of the Na+ ions present in the supercages. However, as just discussed, a supercage of Y zeolite can contain four molecules of benzene. The amount of benzene found in a supercage of NaYs and NaYd zeolites lower than four suggests also that no migration of Na+ ions from small cages toward supercages occurs. In these three NaY zeolites, it is also observed that the number of benzene adsorbed on the cations is higher than that on the 12R windows. The desorption experiments show that the interaction of benzene with the Na+ ions is stronger than that with the 12R windows. The sites whose interaction with benzene is stronger will attract more benzene molecules. 4.3. Diverse Interaction Strengths of Benzene with Na+ Ions of NaEMT. The temperature to eliminate completely adsorbed benzene from NaYs and NaEMT zeolites is similar. However, some differences are still observed (Figures 8 and 9). At room temperature, 0.5 h of evacuation does not result in significant modification in intensity of LF bands of benzene adsorbed on NaYs (Figure 8a,b) while those on NaEMT decrease highly to less than half. This indicates that a great part of the benzene adsorbed on cations of NaEMT is easy to remove, implying that a great number of benzene molecules interact only weakly with Na+ ions in the large cages of NaEMT. Since both Na+ ions located in the SII sites and those migrated from the small cavities to the large cages interact with benzene molecules, Na+ ions migrating from the small cavities should interact weakly with benzene molecules, and benzene molecules interacting with these cations can thus be easily removed. There are around 11.5 (59) Lievens, J. L.; Verduijn, J. P.; Bons, A. J.; Mortier, W. J. Zeolites 1992, 12, 698.

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Na+ ions per unit cell located initially in the small cages and 8.9 Na+ ions in the large cages.59 This means that 11.5 benzene molecules interact weakly with 11.5 Na+ ions migrating from small cavities, being more than half of the total number of benzene molecules adsorbed on NaEMT (around 20 molecules/u.c.). That is why the elimination of this part of benzene results in a reduction of the intensity of LF bands to less than half of the initial value. After this removal, only benzene molecules interacting strongly with Na+ ions in SII sites remain in the large cages. A high desorption temperature is needed to evacuate this part of adsorbed benzene molecules. The diverse interaction strengths of benzene molecules with Na+ ions are evidenced. The presence of the straight channels in NaEMT favors also the elimination of adsorbed benzene from zeolites. Since no migration of cations from small cavities is observed in either NaY, NaYd, or NaYs zeolites, the interaction of benzene molecules with Na+ ions in SII sites is relatively homogeneous compared to that in NaEMT. Benzene molecules are successively removed. 4.4. Adsorption Capacity of One Supercage and Mobility of Benzene. As just discussed, one supercage of Y zeolite can contain at least four molecules of benzene (the amount of benzene located on 12R windows is not included). One may notice that the number of benzene molecules found in one supercage of NaYs and NaYd is much lower than the capacity of a supercage. Furthermore the remaining free void of the supercage is not completely occupied by the condensed benzene since only a small amount of benzene in the pseudo-liquid phase is detected. This reduction of adsorption capacity for a supercage does not necessarily result from the steric effect since only around 2 Na+ ions are found to be present in a supercage. These results can be explained by the high mobility of the benzene molecules within the supercages which was already evidenced by Lechert et al.42 through NMR experiments. In the case of methane adsorption in NaA zeolite, neutron scattering expriments by Cohen de Lara and Kahn60 have shown that at room temperature one molecule of methane can move into the entire volume of one β-cage. As indicated by the results from the present work that there are only two Na+ ions located in a supercage, being lower than that in NaY (around four). The reduction in number of Na+ ions in a supercage leads to the elongation of the distance between the two nearest Na+ ions. It has been previously reported61,62 that the mobility of benzene in a series of NaY zeolites increases with increasing Si/Al ratio, and high benzene loading in these zeolites reduces the mobility of benzene. The mean residence time of benzene molecules on Na+ ions and that for a jump between two nearest Na+ ions, i.e., the mean residence time of benzene molecules in “gas phase” of the large cages, should be therefore respectively longer and shorter in NaY than those in NaYs and NaYd zeolites. This is confirmed by our desorption experiments which show that the interaction of benzene with Na+ ions is stronger in NaY than in NaYs. So, in NaYs and NaYd zeolites, benzene molecules should be more mobile than those in NaY. This is in agreement with that revealed by Cheetham et al.44 based on a comparative study by 2H NMR and molecular dynamics (MD) that the absence of SII sites in Y zeolites leads to facile diffusion and a lower activation energy, and consequently a high diffusion (60) Cohen de Lara; Kahn, R. J. Phys. (Paris) 1981, 42, 1029. (61) Liu, Sh.-B.; Ma, L.-L.; Lin, M.-W.; Wu, J.-F.; Chen, T.-L. J. Phys. Chem. 1992, 96, 8120. (62) Zibrowius, B.; Caro, J.; Pfeifer, H. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2347.

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coefficient but a low activation energy of benzene in siliceous Y and a low diffusion coefficient but a high activation energy of benzene in NaY were observed. The infrared study reported the same observation63 that in a series of dealuminated NaY zeolites, as the number of Na+ ions decreases, the intensity of the LF bands decreases but the intensity of PL bands increases. This high benzene mobility involves a high delocalization of these molecules in the supercages of NaYs and NaYd zeolites. This would limit the amount of benzene found in a supercage. That the high mobility of molecules reduces the adsorption capacity of zeolites was already reported by Barthomeuf et al.27 The infrared spectroscopy is a technique with time scales of about 10-13 s.64 The mean residence time of benzene during the jump between the two nearest Na+ ions, i.e., the residence time of benzene in “gas phase” of supercages, in NaY should be shorter than the time scale of the infrared technique, which is why no benzene in the pseudo-liquid phase is detected and only benzene molecules adsorbed on Na+ ions can be observed. These molecules as well as those observed on 12R windows are, in fact, “immobile” on the time scale of experiment as stated by Pfeifer et al..62 On the contrary, the mean residence time of benzene for a jump, i.e., mean residence time of benzene molecules in “gas phase” of supercages in NaYs and NaYd zeolites, should be longer and the pseudo-liquid phase is thus observed by infrared spectroscopy. It should be indicated that the benzene molecules adsorbed on 12R windows diffuse equally. However, due to the low amount of these benzene molecules, we consider only the mobility of benzene adsorbed on Na+ ions. Furthermore, the discussion on the mobility of benzene molecules is not the main aim of this paper. We would like to use this high mobility to explain the present observations on the low amount of benzene molecules on Na+ ions and the appearance of PL bands in two Si-rich NaY zeolites. The relatively stronger shoulders which are related to the pseudo-liquid phase of benzene observed in NaEMT (Figure 7d) suggest the high mobility of benzene in this zeolite. This can be attributed to the nonlocalizable Na+ ions, migrating from the small cavities, which increase the number of sorption sites and reduce the height of the (63) Su, B. L.; Norberg, V.; Hansenne, C.; de Mallmann, A. Adsorption 2000, 6, 61. (64) Nakamoto, K. Infrared and Raman Spectra, 4th ed.; John Wiley & Sons: New York, 1986.

Su et al.

potential barrier between them. This was already demonstrated in NaX.62 The relatively weak interaction of benzene with Na+ ions migrating from the small cages confirms further the high mobility of benzene in this zeolite. The presence of the 12R straight channels in this zeolite favors equally the diffusion of benzene molecules. A series of 2H NMR studies is being conducted to verify all the above explanations. 5. Conclusion The effects of the zeolite structure and Si/Al ratio on the benzene adsorption properties have been presented. Although NaYs and NaYd are prepared from the different methods and the Si and Al ordering is found to be different in these two zeolites, they have similar benzene adsorption behavior and adsorption capacity for benzene. Both the Na+ ions and the 12R windows are adsorption sites for benzene as observed in NaY. A new pair of shoulders at high wavenumbers (2050 and 1913 cm-1) is also observed on these two zeolites, and we will search for the origin of this pair of shoulders. The numbers of benzene molecules adsorbed on the Na+ ions and on the 12R windows of these two zeolites are determined and are 1.86 ( 0.30 and 0.35 ( 0.20 for NaYs and 2.00 ( 0.30 and 0.30 ( 0.20 molecules/ s.c. for NaYd, respectively. A similar number of benzene molecules found on Na+ ions in a supercage of NaYs and NaYd zeolites implies that the number of Na+ ions in a supercage of these two zeolites should be similar. This suggests further that the different Al distribution in these two zeolites does not significantly affect the location of Na+ ions in the supercages. The low adsorption capacity for a supercage of NaYs and NaYd zeolites compared to that of NaY is most likely related to the low amount of Na+ ions in a supercage and consequently to the high mobility of benzene molcules in the zeolite structures. The adsorption properties for benzene on NaEMT are quite different from those on two Si-rich NaY zeolites. The difference in zeolite structure results probably from the different adsorption behavior of benzene on these zeolites. Acknowledgment. V.N. thanks the FNRS (Fonds national de la Recherche Scientifique, Belgium) for a scholarship (FRIA). This work was partially sponsored by the Flemish Government in the frame of a G.O.A. grant to the C.O.K. and realized in the framework of PAI-IUAP 4/10. LA0013704