Competitive Adsorption of Benzene and Ammonia ... - ACS Publications

61 rue de Bruxelles, 5000, Namur, Belgium. Received November 23, 2000. In Final Form: February 13, 2001. The adsorption of a single component such as ...
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Langmuir 2001, 17, 3341-3347

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Competitive Adsorption of Benzene and Ammonia on NaEMT Zeolite: A Quantitative Infrared Study Bao-Lian Su* and Fanny Docquir Laboratoire de Chimie des Mate´ riaux Inorganiques, The University of Namur (FUNDP), 61 rue de Bruxelles, 5000, Namur, Belgium Received November 23, 2000. In Final Form: February 13, 2001 The adsorption of a single component such as benzene or ammonia and the competitive adsorption of benzene with ammonia have been quantitatively studied on NaEMT zeolite at room temperature using Fourier transform infrared spectroscopy. The benzene and ammonia adsorption capacity and behavior of NaEMT have been compared with those of NaY zeolite. The benzene adsorption behavior in this zeolite with and without the presence of ammonia has been correlated with the structural and chemical properties of zeolite. The present study shows that benzene molecules adsorb more strongly than NH3 on NaEMT and in the presence of ammonia, both Na+ ions and 12R windows are adsorption sites for benzene whereas in the presence of benzene alone, only Na+ ions can interact with benzene. It reveals that with changing the ratio of benzene/NH3, benzene molecules can migrate from one type of site toward another. This evidences that the adsorption selectivity of benzene on one site or another can be modified by adding ammonia.

1. Introduction Zeolites are a class of crystalline microporous materials with well-defined pores. The use of zeolitic adsorbents as a means of separating and purifying gas mixtures is a growing unit operation in chemical engineering. Because zeolite exhibits a high affinity for certain gases, the separation of a gas mixture of a number of compounds on zeolites is, for example, in separation of aromatics, performed by selective adsorption which is less costly than fractionated distillation or crystallization. The adsorbed molecules on zeolites can be then selectively desorbed by raising the temperature, decreasing the partial pressure of an adsorbate, or introducing a compound that is able to compete with adsorbed molecules. The competitive adsorption is a very important step in a separation process. The effectiveness of desorbents relies on their ability to displace adsorbed phases, that is, to compete with the adsorbate. It is then of fundamental and industrial interest to better understand the way the gas molecules interact with and in zeolites. EMT zeolite, the hexagonal analogue of faujasite, has the same building units as the X and Y zeolites. Owing to the arrangement of sodalite cages and hexagonal prisms differing from that in faujasite, the EMT structure contains two nonidentical 12R windows (Figure 1A) and two different large cages (Figure 1B). The two large cages occurring in the EMT structure are referred to as cages I and II and are different from that existing in faujasite (cage III). For the sake of clarity and further use of the present work, Figure 1 illustrates a geometrical representation of EMT structure, showing two different 12R openings (type I, 0.71 × 0.71 nm, and type II, 0.74 × 0.65 nm in diameter), the location of possible cation positions (Figure 1A), and two different large cages (Figure 1B) in EMT. The presence of two nonequivalent hexagonal prisms, two different sodalite cages, and two different large cages in the EMT structure induces the presence of two different I (Ia and Ib), I′ (Ia′ and Ib′), and II (IIa and IIb) sites in this zeolite (Figure 1A). EMT zeolite, since its first * Corresponding author. Phone: 31 81 72 45 31. Fax: 31 81 72 45 30. E-mail: [email protected].

synthesis in 1990 using 18-crown-6-ether as a template,1-4 has been widely used in different catalytic and adsorption processes3-7 and has shown quite interesting properties in particular in the treatment of aromatics.8-13 However, the adsorption behavior of aromatics in this large pore zeolite is not yet clear and well studied. Furthermore, the results reported in the literature are quite discrepant. The present work deals with the adsorption behavior of benzene and ammonia and the competitive adsorption of these two components. This study has been made in order to shed some light on the effect of coadsorption of a basic molecule on the benzene adsorption behavior and to develop a new adsorbent with advanced performance and new processes for separation of gas molecules. 2. Experimental Section NaEMT was synthesized using 18-crown-6 as a template.1-4 To remove the organic template, a special procedure14 was used because it was shown that rapid heating in a pure oxygen atmosphere was too severe and part of the crystallinity can be lost. In a tubular oven, sample powders were placed. The temperature of the oven was increased at a rate of 50 K/h from room temperature to 423 K and maintained for 1 h at this value. (1) 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, Vol. 221; Plenum Press: New York, 1990; p 69. (2) Dougnier, F.; Patarin, J.; Guth, J. L.; Anglerot, D. Zeolites 1992, 12, 160. (3) Delprato, F.; Delmotte, L.; Guth, J. L.; Huve, L. Zeolites 1990, 10, 546. (4) Annen, M. J.; Young, D.; Arhancet, J. P.; Davis, M. E.; Schramm, S. Zeolites 1991, 11, 98. (5) Su, B. L.; Barthomeuf, D. Appl. Catal., A 1995, 124, 81. (6) Hari Prasad Rao, P. R.; Massiani, P.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1994, 84, 81. (7) Stoˆcker, M.; Mostad, H.; Roˆrrik, T. Catal. Lett. 1994, 28, 203. (8) Su, B. L.; Barthomeuf, D. Zeolites 1993, 13, 626. (9) Su, B. L.; Barthomeuf, D. Appl. Catal., A 1995, 124, 73. (10) Su, B. L.; Manoli, J. M.; Potvin, C.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1993, 89, 857. (11) Su, B. L. Ph.D. Thesis, University of Paris, 1992. (12) Su, B. L. Zeolites 1996, 16, 25. (13) Su, B. L. J. Chem. Soc., Faraday Trans. 1997, 93, 1449. (14) Feijen, E. J. P.; Lievens, J. L.; Martens, J. A.; Grobet, P. J.; Jacobs, P. A. J. Phys. Chem. 1996, 100, 4970.

10.1021/la001623t CCC: $20.00 © 2001 American Chemical Society Published on Web 05/03/2001

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Su and Docquir amount of a single component (benzene or ammonia) was first added onto the NaEMT wafer, and after 1 h of equilibration, the known and increasing amounts of another were introduced into the IR cell. The amount of adsorbed benzene or ammonia at each condition was evaluated as described in refs 15-25. The interaction strength of benzene/zeolite and NH3/zeolite was evaluated using desorption experiments which were conducted by 0.5 or 1 h of evacuation at different temperatures. The spectrum of zeolite and that of the gas phases have been subtracted in all the spectra presented in this study.

3. Results and Discussion 3.1. Adsorption of Single Components. Adsorption of Benzene on NaEMT: Selectivity, Capacity, and Strength. In the C-H out-of-plane (oop) vibration range, liquid benzene gives a pair of bands at 1960 and 1815 cm-1, relating to (ν5 + ν17) and (ν10 + ν17) bending vibrations of C-H, respectively. This pair of bands can be shifted toward higher wavenumbers or can be split into two pairs of bands after adsorption on zeolites compared to that of liquid benzene. These two pairs of bands were assigned to benzene adsorbed on the cations and on the 12R windows of zeolites.15-25 Such assignments and benzene locations based on the IR results have been verified by various techniques and theoretical calculations.26-40 The extent of the shift value can be thus used to distinguish the adsorption of benzene on one site or the other. Benzene, interacting with the compensating ions of zeolites, gives a relatively small shift of ca. 20-40 cm-1 because the interaction of the ions with the π electron cloud of the benzene rings affects indirectly and weakly the C-H oop bending vibration. This pair of bands will be referred to here as the low-frequency bands (LF). Adsorption of benzene on the oxygen atoms of the 12R windows induces a relative high shift of ca. 40-100 cm-1; because the hydrogen atoms of benzene interact with oxygen atoms of zeolites, the C-H vibration is directly affected. This pair

Figure 1. Schematic description of EMT structure, possible cation locations, 12R channels, and two different 12R windows (A) and two different cages of EMT structure (hypercage, cage I, and hypocage, cage II) (B). For comparison, the supercage of faujasite is also given (B, cage III). It was then increased at a rate of 5 K/h up to 443 K and kept constant for 1 h. The next step to 573 K was achieved by heating at 50 K/h. After 1 h at 573 K, the final temperature of 773 K was reached at the same heating rate and maintained for 4 h. The sample, free of the organic template, with formula [Na21(AlO2)21(SiO2)75] can be used for our study. Self-supported wafers of template-free NaEMT were loaded in an IR cell and calcined in a flow of dry oxygen for 8 h at 723 K and then in a vacuum for 4-6 h. The present study was carried out using Perkin-Elmer Fourier transform infrared spectrometers 1750 and 2000. After pretreatment, the IR cell containing the zeolite sample was slowly cooled to room temperature (RT) and the spectrum of the zeolite phase was recorded as a baseline. The known and increasing amounts of single components (benzene or ammonia) were then introduced. After 1 h of equilibration, the spectrum of the adsorbed phase was recorded. For the studies on the competitive adsorption of benzene and ammonia, a known

(15) Su, B. L.; Norberg, V. Langmuir 1998, 14, 2352. (16) Su, B. L.; Norberg, V.; Martens, J. A. Microporous Mesoporous Mater. 1998, 25, 151. (17) Su, B. L.; Norberg, V. Langmuir 1998, 14, 7410. (18) Su, B. L. Zeolites 1996, 16, 75. (19) Su, B. L.; Norberg, V. Langmuir 2000, 16, 6020. (20) Su, B. L.; Barthomeuf, D. Zeolites 1995, 15, 470. (21) Su, B. L.; Norberg, V.; Hansenne, C. Langmuir 2000, 16, 1132. (22) Su, B. L.; Barthomeuf, D. J. Catal. 1993, 139, 470. (23) Su, B. L.; Norberg, V. Zeolites 1997, 19, 65. (24) Su, B. L.; Norberg, V. Colloids Surf., A, accepted for publication. (25) Su, B. L.; Norberg, V.; Hansenne, C.; de Mallmann, A. Adsorption 2000, 6, 61. (26) Lechert, H.; Wittern, K. P. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1054. (27) Hasha, D.; Miner, V.; Garces, J.; Roche, S. In Proceedings of the Symposium of the new Surface, Science in Catalysis; American Chemical Society Meeting, Philadelphia, PA, Aug 1984; p 953. (28) Bull, L. M.; Henson, N. J.; Cheetham, A. K.; Newsam, J. M.; Heyes, S. J. J. Phys. Chem. 1993, 97, 11776. (29) Auerbach, S. M.; Bull, L. M.; Henson, N. J.; Metiu, H. I.; Cheetham, A. K. J. Phys. Chem. 1996, 100, 5923. (30) Fitch, A. N.; Jobic, H.; Renouprey, A. J. Phys. Chem. 1986, 90, 1311. (31) Fitch, A. N.; Jobic, H.; Renouprey, A. J. Chem. Soc., Chem. Commun. 1985, 284. (32) Jobic, H.; Renouprey, A.; Fitch, A. N.; Lanter, H. J. J. Chem. Soc., Faraday Trans. 1, 1987, 83, 3199. (33) Sauer, J.; Deininger, D. Zeolites 1982, 2, 114. (34) Auerbach, S. M.; Metiu, H. I. J. Chem. Phys. 1996, 105, 3753. (35) Auerbach, S. M. J. Chem. Phys. 1996, 106, 7810. (36) Auerbach, S. M.; Henson, N. J.; Cheetham, A. K.; Metiu, H. I. J. Phys. Chem. 1995, 995, 10600. (37) Kitagawa, T.; Tsuneekawa, T.; Iwayama, K. Microporous Mater. 1996, 7, 227. (38) Demontis, P.; Yashonath, S.; Klein, M. L. J. Phys. Chem. 1989, 93, 5016. (39) Henson, N. J.; Cheetham, A. K.; Redondo, A.; Levine, S. M.; Newsam, J. M. Stud. Surf. Sci. Catal. 1994, 84, 2059. (40) Cartledge, G. H. J. Am. Chem. Soc. 1930, 52, 3076.

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Figure 3. Changes in the relative absorbance (Ar) of adsorbed benzene on NaEMT as a function of the number of benzene introduced: (a) ν5 + ν17 and (b) ν10 + ν17.

Figure 2. IR absorbance of the C-H oop vibration of benzene adsorbed on NaEMT at different benzene loadings (molecules/ uc): (a) 1.4, (b) 6.3, (c) 10.5, (d) 14.7, (e) 29.7, and (f) saturation of zeolite (a pressure of 20 Torr in the IR cell).

will be referred in this study to as the high-frequency bands (HF). However, the exact shift value and the presence of two pairs of bands are strongly dependent on the zeolites used, the Lewis acidity of the cations, the basicity of the framework oxygen atoms, the number and the type of cations present in zeolite, and the benzene loading.15-25 Figure 2 reports the IR spectra of benzene adsorbed on NaEMT at different benzene loadings (spectra a-e) and saturation (spectrum f) of zeolite. One main pair of bands at 1988 and 1849 cm-1, corresponding to LF bands because of the shift value of 28-34 cm-1, appears when the first benzene molecules were introduced onto the zeolite wafer. One pair of small shoulders at 2010 and 1869 cm-1, assigned to HF bands, is observed with increasing amount of introduced benzene. At higher benzene loadings, another pair of shoulders at 1960 and 1815 cm-1, related to the condensation of benzene in the zeolites according to their wavenumbers, is detected. The above observation shows clearly that only the Na+ ions are principle adsorption sites for benzene and the 12R windows in NaEMT are not adsorption sites for benzene. The relative absorbance (Ar) of adsorbed benzene at each benzene loading is measured and drawn in Figure 3 as a function of benzene loading. The coverage at which the Ar reaches a constant reflects the saturation of zeolite with benzene.15-25 NaEMT has therefore an adsorption capacity of ca. 21 molecules per unit cell (molecules/uc). This value coincides with the number of Na+ ions present in one unit cell of NaEMT, indicating that all Na+ ions present in NaEMT interact with benzene molecules because one Na+ ion is supposed in the literature to interact with only one benzene molecule.15-25 It is known that 8.9 out of 21 Na+ ions per unit cell are present in the large cages and others are located in the hexagonal prisms and sodalite cages.41 The present results suggest that in the presence of benzene, Na+ ions located initially in the small cavities are attracted by benzene and migrate toward the large cages to interact with benzene molecules. This possible migration is in agreement with that observed on the change in location (41) Lievens, J. L.; Verduijin, J. P.; Bons, A. J.; Mortier, W. J. Zeolites 1992, 12, 698.

of alkali metal or alkaline earth metal cations upon adsorption of H2O, H2S, and C6H6.42 It was reported that NaY has a total benzene adsorption capacity of around 33 molecules/uc, consisting of 28 molecules/uc on Na+ ions and 5 molecules/uc on 12R windows. Note that the unit cells contain 192 tetrahedra for faujasite and 96 for EMT. The comparison of the adsorption capacity for these two zeolites should be made at the same number of tetrahedra. For a number of 192 tetrahedra, the adsorption capacity of NaEMT for benzene is around 42 molecules, being higher than that of NaY (a total of 33 molecules) although in NaEMT only Na+ ions, but not the 12R windows, are adsorption sites for benzene in the presence of benzene alone as shown by the present study. On the contrary, both Na+ ions and 12R windows are adsorption sites for benzene. It has been described in the Introduction that two different cages are larger (hypercage) and smaller (hypocage) than the supercage of faujasite (Figure 1B). The EMT and faujasite have similar framework densities, 12.9 T/1000 Å3 for EMT and 12.7 T/1000 Å3 for faujasite.43 The higher adsorption capacity observed for NaEMT is very likely due to the presence of large 12R straight channels (Figure 1A) and the lack of adsorption of benzene on 12R windows. The adsorption of benzene on 12R windows can, on one hand, block the access of benzene into the large cages and, on the other hand, force the benzene molecules to localize in a fixed orientation, hindering the possible compact package of benzene molecules in 12R straight channels, and reduce consequently the total adsorption capacity. The interaction strength of benzene with NaEMT was determined. Figure 4 depicts the IR spectra of the C-H oop vibration (Figure 4A) and the C-C stretching vibration (Figure 4B) of adsorbed benzene after desorption at different temperatures. A 1 h desorption at 383 K is needed to completely eliminate benzene adsorbed on NaEMT, indicating a strong interaction between benzene and Na+ ions of NaEMT. Adsorption of Ammonia on NaEMT: Capacity and Strength. The interaction of NH3 with protonated zeolites has intensively been studied and used to evaluate the number and strength of acid sites in zeolites.44 However, no detailed studies on the interaction of NH3 with cationic (42) Peuker, C. H.; Moeller, K.; Kunath, D. J. Mol. Struct. 1984, 114, 215. (43) Meier, W. M.; Olson, D. L. Atlas of Zeolites Structure Types, 4th ed.; Elsevier: London, 1996. (44) Kro¨nzinger, H. In Handbook of heterogeneous catalysis; Ertl, G., Kro¨nzinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997.

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Figure 4. IR absorbance of the C-H oop (A) and C-C stretching (B) vibrations of adsorbed benzene after 1 h of evacuation at (a) 298, (b) 333, (c) 353, (d) 368, and (e) 383 K.

Figure 6. IR absorbance spectra of the N-H stretching (A) and bending (B) vibrations of NH3 adsorbed on NaEMT after 0.5 h of evacuation at (a) 298, (b) 323, (c) 343, and (d) 373 K.

Figure 5. IR absorbance of the N-H stretching (A) and bending (B) vibrations of NH3 adsorbed on NaEMT at different NH3 loadings (molecules/uc): (a) 1.0, (b) 5.0, (c) 10.0, (d) 145.0, (e) 20.0, and (f) saturation of zeolite (a pressure of 20 Torr in the IR cell).

zeolites can be found in the literature because of the complexity of the interactions. Figure 5 reports the IR spectra of NH3 adsorbed on NaEMT at different ammonia loadings (spectra a-e) and saturation (spectrum f) of zeolite. After introduction of different amounts of NH3, a slight negative feature is observed at 3745 cm-1 and corresponds to the interaction of NH3 with the small quantity of silanols present in NaEMT. Four peaks at 3474 (i), 3385 (ii), 3312 (iii), and 3260 (iv) cm-1 in the N-H stretching vibration range (Figure 5A) and only a broad band at 1639 (v) cm-1 in the H-N-H bending vibration range (Figure 5B) are observed, and their intensities increase with increasing amount of introduced NH3. Gasous ammonia gives three peaks at 3414 (ν3), 3338 (ν1), and 1628 (ν4) cm-1 in the present studied ranges.45 The peaks at 3385 (ii), 3312 (iii), and 1639 (v) cm-1 can be assigned to N-H asymmetric and symmetric stretching (ii and iii) and H-N-H asymmetric bending (v) vibrations of NH3 coordinately bonded to Na+ ions (Lewis acids) via the N atom of NH3. Because of this interaction, the N-H (45) Herzberg, G. Molecular spectra and molecular structure, II Infrared and Raman spectra of polyatomic molecules; Van Nostrand Reinhold Co.: New York, 1945.

bond is weakened and the N-H IR bands are shifted toward lower wavenumbers for the N-H stretching vibration and higher wavenumbers for the N-H bending vibration. The vibration band at 3260 cm-1 can be attributed to the combination band (2ν4 ) 2 × 1639 ) 3278 cm-1) of the H-N-H bending vibration. A very weak band is also observed at 1502 cm-1. Because our NaEMT zeolite contains a small amount of silanols (3745 cm-1) after pretreatment in oxygen and then in a vacuum, the band at 1502 cm-1 is, therefore, very likely due to the bending vibration of NH4+ formed between silanols and NH3 adsorbed. The interaction between NH3 and silanols is confirmed by production of a negative feature at 3745 cm-1 upon adsorption of ammonia. On the basis of previous studies on the adsorption of NH3 on MgO,46 the large peak at 3474 cm-1 could be generated from the interaction of NH3 with the negatively charged oxygen atoms via H atoms of NH3. As the interaction of NH3 with negatively charged oxygen atoms should shift the frequency of the N-H vibration toward lower wavenumbers and what we observed here is a shift toward higher wavenumbers, this vibration band should not be from the N-H vibration. This band is very likely due to the bond formed (H- - -O) between H atoms of NH3 and oxygen atoms of the framework, giving a vibration in the region of hydroxyls as observed in HY and HEMT. The adsorption capacity of NaEMT for NH3 is also determined and shows that NaEMT has an adsorption capacity of 70 molecules/uc. As described above, NaEMT contains 96 tetrahedra per unit cell instead of 192 for NaY. The comparison should be made at the same tetrahedra number. For 192 tetrahedra, NaEMT has an ammonia adsorption capacity of around 140 molecules, slightly higher than that of NaY, that is, around 130 molecules at room temperature. Figure 6 reports the IR spectra of adsorbed ammonia after desorption at different temperatures for 30 min in the range of 4000-2800 cm-1 (Figure 6A) and 1800-1400 cm-1 (Figure 6B). It is observed that a desorption at 373 K can remove all adsorbed ammonia molecules. Compared with benzene (383 K and 1 h), the interaction of ammonia with Na+ ions is weaker. (46) Coluccia, S.; Lavagnino, S.; Marchese, L. J. Chem. Soc., Faraday Trans. 1 1987, 83, 477.

Competitive Adsorption of Benzene and Ammonia

Figure 7. IR absorbance of the C-H oop vibration of adsorbed benzene at the benzene loading levels of 4.5 (A) and 20.7 (B) molecules/uc with increasing amounts of introduced NH3 (molecules/uc). A: (a) 0.0, (b) 16.6, (c) 24.8, (d) 35.6, (e) 72.3, and (f) 99.6. B: (a) 0.0, (b) 99.6, (c) 153.2, and (d) 215.3.

3.2. Dual Adsorption of Benzene and Ammonia on NaEMT. Change in Location of Benzene upon Coadsorption of Ammonia. Low (4.5 molecules/uc) and high (20.7 molecules/uc) quantities of benzene have been introduced onto two NaEMT wafers, and the different amounts of NH3 are subsequently introduced. Figure 7A,B demonstrates the changes in absorbance of the C-H oop vibration of adsorbed benzene at the benzene loading levels of 4.5 molecules/uc (Figure 7A) and 20.7 molecules/uc (Figure 7B) with introduction of known and increasing amounts of NH3. Only LF bands at 1988 and 1848 cm-1, corresponding to the adsorption of benzene on Na+ ions, are observed after introduction of benzene alone (spectra a in Figure 7A,B). With introduction of increasing amounts of NH3, a new pair of bands at 2014 and 1874 cm-1, related to the interaction of benzene with 12R windows of NaEMT, that is, HF bands, according to the shift value of 54-59 cm-1, appears and their intensities increase. At high ammonia loading, HF bands become the most intense at both low and high benzene loading levels. The above results indicate that upon coadsorption of NH3, the 12R windows also become the adsorption sites for benzene. Adsorption Selectivity of Benzene upon Coadsorption of NH3. The amounts of benzene adsorbed on Na+ ions and 12R windows upon coadsorption of NH3 at benzene loading levels of 4.5 and 20.7 molecules/uc have been evaluated according to the variation in the absorbance of each type of band characteristic of benzene adsorbed on Na+ ions and on 12R windows. The plot of relative absorbance of each type of adsorption band as a function of amount of

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introduced ammonia is given in Figure 8A for a benzene loading of 4.5 molecules/uc and Figure 8B for 20.7 molecules/uc. At both benzene loadings, the amount of benzene adsorbed on Na+ ions (curve a) decreases whereas that on 12R windows (curve b) increases with introduction of ammonia. This implies that benzene molecules adsorbed on 12R windows upon coadsorption of NH3 come from those initially adsorbed on Na+ ions. This also suggests that in the presence of ammonia, a migration of benzene molecules from Na+ ions toward 12R windows occurs. At low benzene loading (Figure 7A), the total amount of benzene molecules adsorbed on NaEMT (curve c), that is, the sum of amounts of benzene molecules adsorbed on Na+ ions and on 12R windows, remains constant and equal to the number of introduced benzene (dashed line), indicating that all of the introduced benzene are retained in NaEMT even though a high pressure of NH3 is present in the IR cell. At high benzene loading (Figure 7B), the total amount of benzene adsorbed on NaEMT (curve c) decreases with increasing amount of NH3 and is lower than that of benzene adsorbed initially on NaEMT (dashed line), indicating the removal of part of the adsorbed benzene molecules from the NaEMT wafer because of the presence of high pressure of NH3 in the IR cell. The effect of the benzene/NH3 ratio on the location of benzene was previously studied using a zeolite wafer in contact simultaneously with 20.7 molecules/uc of benzene and 215.3 molecules/uc of ammonia.13 The additional amounts of benzene were introduced back into the IR cell. The quantitative analysis concerning the variation of the absorbance of the bands corresponding to adsorption of benzene on Na+ ions (curve a) and on the 12R windows as a function of benzene/ammonia molecular ratio has been made and is shown in Figure 9. The absorbance of HF bands remains constant, whereas that of the LF bands progressively increases and the ratio of amounts of benzene adsorbed on Na+ ions and on 12R windows changes with the ratio of benzene/ammonia. This indicates that the 12R windows are saturated with benzene molecules and no more benzene molecules can adsorb on 12R windows even in the presence of a high pressure of benzene and ammonia in the IR cell. Adsorption Capacity of NaEMT for Ammonia in the Presence of Preadsorbed Benzene. Figure 10 shows the IR spectra of the N-H band in the range of 1750-1575 cm-1 in the presence of different quantities (4.5 molecules/uc, Figure 9A, and 20.7 molecules/uc, Figure 9B) of benzene preadsorbed on NaEMT. Only a broad band at 1639 cm-1 is seen as in the case of ammonia alone. On the basis of the plot of relative absorbance of the N-H band in the

Figure 8. Changes in the Ar (left) and number of benzene molecules (right) adsorbed on Na+ ions (a), on the 12R windows (b), and on NaEMT (c) at the benzene loading levels of 4.5 (A) and 20.7 (B) molecules/uc as a function of amount of introduced ammonia. The initial amount of benzene introduced is represented by the dashed line.

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Figure 9. Changes in the Ar of benzene adsorbed on Na+ ions (a) and on 12R windows (b) as a function of benzene/ammonia molecular ratio.

Figure 11. IR absorbance spectra of the C-H oop vibration of benzene adsorbed on NaEMT before (b) and after (c) 1 h of evacuation at room temperature. Spectrum b describes the adsorbed benzene phase on the NaEMT wafer in contact with 90.8 molecules/uc of benzene and 215.3 molecules/uc of ammonia. For comparison, the spectrum of adsorbed benzene on the NaEMT wafer under saturation conditions is also given (a).

Figure 10. IR absorbance spectra of the N-H bending vibration of adsorbed ammonia in the presence of amounts of 4.5 (A) and 20.7 (B) molecules/uc of preadsorbed benzene at different ammonia loadings (molecules/uc). A: (a) 9.1, (b) 16.3, (c) 25.0, (d) 38.6, (e) 56.8, and (f) 75.0. B: (a) 99.6, (b) 163.6, and (c) 227.3.

presence of 4.5 molecules/uc (curve b) and 20.7 molecules/ uc (curve c) of preadsorbed benzene as a function of amount of introduced ammonia,13 it is clear that the adsorption capacity for ammonia is highly reduced; NaEMT can adsorb ca. 55 and 42 molecules/uc of ammonia when 4.5 and 20.7 molecules/uc of benzene preadsorbed on this zeolite, respectively. This indicates also that ammonia molecules interact less strongly with NaEMT compared to benzene and adsorb only on the remaining sites after benzene adsorption. Change in Location of Benzene upon Desorption. Figure 11 reports the change of IR spectra of the C-H out-ofplane vibration range before (b) and after (c) 1 h of evacuation at room temperature. This was made on a sample contacted simultaneously with 90.8 molecules/uc of benzene and 215.8 molecules/uc of NH3. The IR spectrum of NaEMT saturated with benzene alone is also given for comparison. In the presence of benzene alone (spectrum a), only LF bands are detected. In the presence of benzene and ammonia (spectrum b), both LF and HF bands are observed, indicating that two kinds of adsorption of benzene, that is, adsorption on Na+ ions and 12R windows, coexist. After 1 h of evacuation (spectrum c), only LF bands are still present. No N-H band has been detected either in the gas phase of the IR cell or in the wafer. This means that all ammonia molecules have been removed from wafer and the IR cell. The intensity of LF bands increases

compared with those before evacuation, which indicates that in the absence of ammonia, benzene molecules adsorbed on 12R windows remigrate to Na+ ions and the 12R windows are unable to retain benzene molecules in the absence of ammonia. The results just presented suggest that in the presence of benzene, the interaction of ammonia with NaEMT is highly weakened; whereas in the presence of ammonia alone a temperature of 373 K is necessary to remove entirely the ammonia molecules adsorbed, here room temperature is enough to eliminate all ammonia molecules in the presence of preadsorbed benzene. 3.3. Change in Adsorption Selectivity of Benzene on One Site or Another upon Coadsorption of Ammonia. The present work shows clearly that only Na+ ions in NaEMT, but not 12R windows, are preferential adsorption sites for benzene in the presence of benzene alone. However, upon coadsorption of ammonia, adsorption of benzene on 12R windows dominates and the adsorption of benzene on Na+ ions is highly reduced. A migration of benzene molecules from Na+ ions toward 12R windows upon coadsorption is observed. One may expect that the introduction of ammonia molecules which will interact with Na+ ions via the electron pair on nitrogen atoms can hunt the adsorbed benzene molecules out of Na+ ions. Because a remigration of adsorbed benzene molecules from 12R windows toward Na+ ions is really noted on removal of ammonia from the system, the desorption experiments show clearly that benzene molecules interact with Na+ ions more strongly than ammonia molecules. From the point of view of structure, in the EMT framework two kinds of 12R windows coexist. One is circular (0.71 × 0.71 nm) and another is elliptical (0.74 × 0.65 nm) (Figure 1A). Both are very similar to that occurring in faujasite (0.74 × 0.74 nm). The size of a benzene molecule is 0.34 × 0.62 × 0.69 nm. It is evident

Competitive Adsorption of Benzene and Ammonia

that the two kinds of 12R windows in EMT and that in faujasite are large enough to receive a benzene molecule. It seems that the migration of benzene from one site toward another upon coadsorption of ammonia is not simply due to the replacement of benzene molecules adsorbed on Na+ ions by ammonia, because at low benzene loading, a large number of Na+ ions remain unoccupied which can receive ammonia molecules introduced. However, the displacement of benzene from Na+ ions toward 12R windows occurs even at a very low amount of ammonia introduced. The chemical effect should be taken into account. Mortier47 reported that the interaction of ammonia with the zeolite framework can provoke a modification of a redistribution of the charge on framework atoms. As explained, the adsorption of benzene with 12R windows is made via the interaction of six hydrogen atoms of benzene with six oxygen atoms of 12R windows. A high negative charge on the oxygen atoms will reinforce this interaction. The negative charge on oxygen atoms (δ0) can be calculated using Sanderson’s electronegativity equalization principle which is currently employed in the field of zeolites in the literature.48,49 The δ0 of NaEMT is around -0.318; however, the δ0 of NaY (Si/Al ) 2.4) is around -0.350. The δ0 of NaEMT is not important enough to make the interaction of benzene and oxygen atoms of NaEMT possible. The interaction of ammonia with NaEMT can very probably modify the δ0 value, and therefore the adsorption of benzene on 12R windows occurs. (47) Mortier, W. J. In Proceedings of the 6th International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworth: Guildford, 1984; p 734. (48) Mortier, W. J. J. Catal. 1978, 55, 138. (49) Sanderson, R. T. Chemical Bonds and Bond Energy; Academic Press: New York, 1976.

Langmuir, Vol. 17, No. 11, 2001 3347

4. Conclusions The results obtained from the adsorption of single components and dual adsorption of benzene and ammonia demonstrate that benzene molecules adsorb more strongly than those of NH3 on NaEMT. The adsorption capacity of NaEMT for ammonia in the presence of benzene preadsorbed on NaEMT is highly reduced. In the presence of benzene alone, only Na+ ions are adsorption sites for benzene, and upon coadsorption of ammonia, benzene molecules adsorb also on 12R windows of NaEMT zeolite. The migration-remigration of benzene from one type of site toward another upon coadsorption of ammonia and on removal of ammonia from the system has been evidenced. The study on the adsorption strength of the single components indicates that although the interaction of NH3/zeolite is lower than that of benzene/zeolite, the interactions of ammonia with Na+ ions via N atoms of NH3 and with the negatively charged framework oxygen atoms via H atoms of NH3 very likely induce an increase in the negative charge of framework oxygen atoms and consequently the oxygen atoms of 12R windows of NaEMT, being initially not the adsorption sites for benzene because of their lower negative charge, become indeed adsorption sites for benzene. The present work reveals that the adsorption selectivity of benzene on one type of site or another can be modified by adding ammonia. This should be very interesting for separation processes. Acknowledgment. F.D. thanks the FRNS (Fonds national pour la Recherche Scientifique, Belgium) for a research scholarship (FRIA). Financial support from PAIIUAP 4/10, a Belgian federal research framework, is also gratefully acknowledged. LA001623T