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Langmuir 2000, 16, 1132-1140
Infrared Spectroscopic Study on the Location of Benzene in KL Zeolite upon Coadsorption of Ammonia and Methylamine Bao-Lian Su,* Vale´rie Norberg, and Carine Hansenne Laboratoire de Chimie des Mate´ riaux Inorganiques, I.S.I.S., Universite´ de Namur, 61 Rue de Bruxelles, B-5000 Namur, Belgium Received May 17, 1999. In Final Form: September 13, 1999 The location of benzene in KL zeolite upon coadsorption of ammonia and CH3NH2 has been investigated and the interaction strength of ammonia, methylamine, and benzene with KL has been evaluated by means of infrared spectroscopy. The adsorption behavior of benzene in KL zeolite has been correlated with the average negative charge of oxygen atoms, with the counterions, with the zeolite structure, and with the interaction strength of ammonia, methylamine, and benzene with KL zeolite. The ammonia and methylamine molecules can interact with K+ ions via the lone pair on nitrogen atoms and with the residual silanols present in dehydrated KL zeolite. The desorption experiments demonstrate that the interaction strengths of ammonia, methylamine, and benzene with KL rank in the following order: methylamine/KL > benzene/KL > ammonia/KL. It has been found that adsorption of methylamine, but not ammonia, causes the appearance of some broad features in the skeleton vibration range, indicating a modification of the lattice parameter: precisely, a deformation of the zeolite framework. It has also been observed that besides benzene adsorption on counterions, the 12R windows occurring in KL, which are not the preferential adsorption sites for benzene in the presence of benzene alone or upon coadsorption of ammonia, become favorable adsorption sites for benzene upon coadsorption of methylamine. The present work evidences once again the structural effect in adsorption of benzene. We think that the fact that the 12R windows become indeed adsorption sites for benzene upon coadsorption of methylamine should be related to the deformation of the zeolite framework, precisely the 12R windows. It is possible that the 12R windows in KL zeolite are initially not compatible with benzene molecules from the point of view of the structure but that the deformation gives a beneficial effect and the 12R windows become favorable for adsorption of benzene.
1. Introduction LTL zeolite contains a system of non-interconnecting straight 12R channels (Figure 1). In KL zeolite, four positions for cations are found, that is, sites A, B, C, and D. In dehydrated KL zeolite, an additional position is also detected. However, only sites D are located in the 12R straight channels: precisely, on the walls of the main channels. L zeolite has been widely used as a catalyst and a catalyst support in various reactions.1-9 For instance, KL zeolite has been used as a support for highly dispersed Pt. The obtained catalyst was shown to be very selective for the aromatization of hexane to benzene.1-4 Cu2+exchanged L zeolites and other multivalent metal-modified L zeolites have been applied as highly active and selective catalysts for the substitution of benzene to aniline or phenol and chlorination of aromatics.7,9 KL zeolite loaded * Corresponding author. (1) Jentoff, R. E.; Tsaputsis, M.; Davis, M. E.; Gates, B. C. J. Catal. 1998, 179, 565. (2) Besoukhanova, C.; Guidot, J.; Barthomeuf, D.; Breysse, M.; Bernard, J. R. J. Chem. Soc., Faraday Trans. 1, 1981, 77, 1595. (3) Davis, R. J. Heterog. Chem. Rev. 1994, 1, 41. (4) Jacobs, G.; Patro, L.; Resasco, D. E. J. Catal. 1998, 179, 43. (5) Burgers, M. H.; Van Bekkum, H. Zeolites and Related Microporous Materials: State of the Art 1994; Elsevier Sciences: Amsterdam, 1994; Stud. Surf. Sci. Catal. 1994, 84c, 1981. (6) Jing, M.; Tatsumi, T. J. Phys. Chem. 1998, 102B, 10879. (7) Yoo, J. W.; Kim, D. S.; Chang, J. S.; Park, S. E. In Progress in Zeolites and Microporous Materials; Chon, H., Thu, S. K., Uh, Y. S., Eds.; Elsevier Sciences: Amsterdam, 1996; Stud. Surf. Sci. Catal. 1996, 105C, 2035. (8) Okamoto, Y.; Kikuta, H.; Otho, Y.; Nasu, S.; Terasaki, O. In Progress in Zeolites and Microporous Materials; Chon, H., Ihm, S. K., Uh, Y. S., Eds.; Stud. Surf. Sci. Catal. 1996, 105C, 2051. (9) Botta, A.; Buysch, H. J.; Puppe, L. Angew. Chem., Int. Ed. Engl. 1991, 30, 1689.
Figure 1. Geometrical description of the main straight channels, the large cages, and the 12R windows occurring in KL.
with Mo3S44+ clusters6 or iron oxide clusters8 has been also employed for production of alcohols by CO hydrogenation6 or for partial oxidation of hydrocarbons,8 respectively. As known, catalysis and separation using zeolites must always be preceded by adsorption. The adsorption of molecules within the cavities of the zeolite is a central step which affects strongly the catalytic activity and selectivity of the reaction and the separation efficiency. However, the adsorption properties of L zeolite are still little known, and only very few papers were devoted to this matter.10-16 Knowledge of the adsorption properties of KL zeolite should be of important interest in the context of designing new catalysts with advanced performance. (10) Su, B. L.; Barthomeuf, D. Appl. Catal., A 1995, 124, 81. (11) Silbernagel, B. G.; Garcia, A. R.; Hulme, R.; Newsam, J. M. In Zeolites: Facts, Figures, Future; Jacobs, P., Van Santen, R. A., Eds; Elsevier Sciences: Amesterdam, 1999; Stud. Surf. Sci. Catal. 1999, 49B, 615. (12) Su, B. L. J. Chem. Soc., Faraday Trans. 1997, 93, 1449. (13) Su, B. L.; Barthomeuf, D. Zeolites 1995, 15, 470. (14) Silbernagel, B. G.; Garcia, A. R.; Newsam, J. M.; Hulme, R. J. Phys. Chem. 1989, 93, 6506. (15) Newsam, J. M.; Silbernagel, B. G.; Garcia, A. R.; Hulme, R. J. Chem. Soc., Chem. Commun. 1987, 664. (16) Su, B. L. Zeolites 1996, 16, 75.
10.1021/la990593v CCC: $19.00 © 2000 American Chemical Society Published on Web 11/17/1999
Location of Benzene in KL Zeolite
It was reported that only the K+ cations located in the main channels of KL zeolite are adsorption sites for benzene and the 12R windows occurring in this zeolite are not preferential adsorption sites for benzene, at least in the presence of benzene alone in the system. Similar adsorption properties for benzene were also observed in NaEMT17 and NaBeta12,16,18 zeolites, which also contain the 12R window channel systems. On the contrary, the location of benzene on both cations and 12R windows was detected in a series of faujasite X and Y zeolites exchanged with alkali cations.19 Although some chemical and structural factors have been used to explain the adsorption properties of NaEMT, KL, and NaBeta zeolites for benzene, the reasons why the 12R windows in these three zeolites are not preferential adsorption sites for benzene compared to those in faujasite X and Y zeolites exchanged with alkali cations have been the object of a dispute. The situation is further complicated by the more recently published IR results that the benzene adsorption behavior in NaEMT could differ from that in KL and NaBeta,12,16,18,20 since, upon coadsorption of a basic molecule such as NH3, the 12R windows, at least part of them in NaEMT, become indeed the adsorption sites for benzene and, in the presence of a high quantity of ammonia in the system, the 12R windows are more preferential sites for benzene. This was observed neither in KL16 nor in NaBeta.18 It is evident that a general conclusion cannot be made to explain the adsorption properties of a large family of zeolites with different structures. Each zeolite should have its particularity in adsorption, which could be related to the location and type of the extraframework counterions, the chemical composition, and the zeolite structure. The present paper, being a continuation of our research in this field, reports a fundamental study of the interaction of cations with ammonia and methylamine and the changes in the location of benzene in KL upon coadsorption of methylamine. We try to elucidate, on the molecular level, how the interaction of ammonia and methylamine with zeolites can modify the acid-base character of the zeolites and consequently the adsorption behavior of zeolites. 2. Experimental Section 2.1. Material. KL, provided by Union Carbide, has the chemical formula K8.25Na0.15(AlO2)8.4(SiO2)27 for one unit cell. 2.2. Infrared Studies. The adsorption of molecules 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 Perkin-Elmer Spectrum 2000 spectrometer. The adsorption of increasing and known amounts of ammonia and methylamine was carried out at room temperature, as previously reported.12,13,17-20 After each adsorption, the sample was maintained at room temperature for at least 1 h for equilibration before recording the spectra. The influence of coadsorption of ammonia and methylamine on the benzene location was examined on a KL sample wafer having a benzene loading of 2.5 molecules per unit cell (molecules/u.c.). Benzene (2.5 molecules/ u.c.) was introduced onto the KL zeolite wafer. After 1 h of equilibration and recording the spectra of adsorbed benzene, (17) Su, B. L.; Manoli, J. M.; Potvin, C.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1993, 89, 857. (18) Su, B. L.; Norberg, V. Langmuir 1998, 14, 25. (19) de Mallmann, A.; Barthomeuf, D. Stud. Surf. Sci. Catal. 1986, 28, 609. (20) Su, B. L. Zeolites 1996, 16, 25.
Langmuir, Vol. 16, No. 3, 2000 1133 increasing and known amounts of ammonia and methylamine were introduced into the IR cell. After 1 h of equilibration at room temperature, the IR spectra were recorded. It should be noted that the quantities of ammonia (or methylamine) introduced into the IR cell were expressed in molecules/u.c., although, after saturation of the zeolite, this has no real physical meaning. However, it does facilitate the comparison of the results. The interaction strengths of benzene, ammonia, and methylamine with KL zeolites were evaluated by desorption at different temperatures for 30 min.
3. Results and Discussion To better understand the effect of coadsorption of ammonia and methylamine on the location of benzene in KL, the interaction of ammonia and methylamine alone with KL has first been studied. 3.1. Interaction of KL Zeolite with Basic Molecules. 3.1.1. With Ammonia. The interaction of NH3 with protonated zeolites21-24 and oxides such as MgO, CuO, Al2O3, SiO2, and SiO2-Al2O3 has been widely studied.25-29 However, the interaction with cationic zeolites is less reported in the literature.30-32 Ammonia molecules, having permanent electric dipole moments, are known to be able to interact with the “effective” anionic framework and mobile cations.33 The NH3 molecule is a symmetrical top and would have four fundamentals, two totally symmetric (A1) and two doubly degenerate (E). Gaseous NH3 molecules give four vibration peaks in the midinfrared spectrum at 3444,34 [341435], 3336, 1627.5, and 931.6 cm-1 which were already assigned to the asymmetric (ν3) and symmetric (ν1) stretching vibrations of N-H and the asymmetric (ν4) and symmetric (ν2) bending vibrations of H-N-H, respectively. Ammonia molecules adsorbed in KL give more complicated spectra than that of gaseous NH3. Figure 2 depicts the infrared spectra of ammonia adsorbed on KL at different ammonia loadings in the range 4000-1300 cm-1. The infrared spectrum of KL zeolite alone and that of the gas phase of the IR cell have been subtracted. The wavenumber range below 1300 cm-1 cannot be studied due to the strong absorption of the zeolite wafer. Three main peaks at 3387, 3307, and 3252 cm-1, two shoulders at 3500 and 3219 cm-1, and a negative feature at 3740 cm-1 are detected in the range 40002500 cm-1 (Figure 2A). Only one strong vibration feature at 1636 cm-1 is observed in the range 1800-1300 cm-1 (Figure 2C). These peaks are present at any ammonia loading. The peaks at 3387, 3307, and 3252 cm-1 can be (21) Zecchina, A.; Buzzoni, R.; Bordiga, S.; Geobaldo, F.; Scarano, D.; Riccjardi, G.; Spoto, G. Stud. Surf. Sci. Catal. 1995, 97, 213. (22) Bra¨ndle, M.; Sauer, J. J. Mol. Catal. 1997, 119, 19. (23) Liepold, A.; Roos, K.; Reschetilowski, W.; Schmit, R.; Sto¨cker, M.; Philippov, A.; Anderson, M. W.; Esculcas, A. P.; Rocha, J. Stud. Surf. Sci. Catal. 1997, 105, 423. (24) Ghosh, A.; Curthoys, G. J. Chem. Soc., Faraday Trans. 1984, 80, 90. (25) Coluccia, S.; Lavagnino, S.; Marchese, L. J. Chem. Soc., Faraday Trans. 1987, 83, 477. (26) Basila, M. R.; Kantner, T. R. J. Phys. Chem. 1967, 71, 467. (27) Kagami, S.; Onishi, T.; Tamaru, K. J. Chem. Soc., Faraday Trans. 1 1984, 80, 29. (28) Peri, J. B. J. Phys. Chem. 1965, 69, 21. (29) Peri, J. B. J. Phys. Chem. 1965, 69, 231. (30) Kiselev, A. V.; Lygin, V. I.; Titova, T. I. Zh. Fiz. Khim. 1964, 38, 2730. (31) Morishige, K.; Kittaka, S.; Ihara, S. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2525. (32) Morishige, K.; Kittaka, S.; Takao, S.; Morimoto, T. J. Chem. Soc., Faraday Trans. 1 1984, 80, 993. (33) Woltman, A. W.; Hartwig, W. H. In Adsorption and Ion Exchange with Synthetic Zeolites; Flank, W. H., Eds.; ACS Symposium Series 135; American Chemical Society: Washington, DC, 1980; p 1. (34) Herzberg, G. Molecular Spectra and Molecular Structure, II. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold Company: NewYork, 1945. (35) Jacox, M. E.; Milligan, D. E. Spectrochim. Acta. 1963, 19, 1173.
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Figure 2. Changes in the infrared absorbance spectra of the N-H stretching vibration range (A) and the H-N-H bending vibration range (C) of ammonia adsorbed in KL zeolite as a function of amount of ammonia introduced into the IR cell (molecules/ u.c.). The range 2200-1800 cm-1 (B) is also given to check the deformation of the framework. (a) 3.0; (b) 5.0; (c) 7.0; (d) 10.0; (e) 15.0; (f) 25; and (g) saturation of KL zeolite (a pressure of 20 Torr of ammonia present in the IR cell).
assigned to the asymmetric (ν3) and symmetric (ν1) stretching vibrations of N-H and the combination bands 2ν4. The first two bands of NH3 are shifted toward low wavenumbers after adsorption in KL compared with the bands of gaseous NH3. The IR band at 1636 cm-1 arises from the asymmetric bending vibration of H-N-H and is also shifted, but to high wavenumber. It is observed that, after thermal pretreatment in a vacuum, some hydroxyls such as silanols which give rise to an infrared band at 3740 cm-1 are present in our KL (not shown here). It is known that these silanols, acting as weak Bro¨nsted acid sites, can interact with some basic molecules such as benzene and ammonia and can be shifted toward low wavenumbers.13,18 The negative feature at 3740 cm-1 and the shoulder at 3219 cm-1 result therefore from the interaction of NH3 with silanols of KL. The consumption of silanols at 3740 cm-1 gives a negative feature at same wavenumber and a positive shoulder at low wavenumber (3219 cm-1) after subtracting the spectrum of the KL zeolite phase from the spectra of adsorbed NH3 in KL. The assignment of the shoulder at 3500 cm-1 is not clear yet. This band with medium intensity is very likely due to the adsorption of NH3 in KL, which can split the doubly degenerate asymmetric stretching vibrations of N-H, which have the same energy level in the gaseous state, into two components at upper (3500 cm-1) and lower (3387 cm-1) energy levels. The splitting of the doubly degenerate asymmetric bending vibrations, however, is not clearly observed. The assignment of all observed bands is given in Table 1. For comparison, the bands of gaseous and crystalline NH3 are also listed in this table. The splitting of the doubly degenerate asymmetric stretching vibrations and the displacement of the stretching vibration bands of N-H and the bending band of H-N-H toward lower and higher wavenumbers, respectively, after adsorption of NH3 in KL may be not only the consequence of the disturbing action of the zeolite cations on the unbounded electron of the nitrogen atoms but also the result of the formation of hydrogen bonds between the NH3 and the lattice oxygen atoms. Adsorption of NH3 in KL results thus from the interaction of the electron lone pair of the nitrogen atoms
Table 1. Wavenumbers (cm-1) of Different Vibration Bands of Ammonia in the Gaseous, Crystalline and Adsorbed States state of ammonia type of vibration N-H stretching asym (ν3)
gaseous
crystalline
3444a (3414)b
3378
N-H stretching sym (ν1) 3336 H-N-H bending asym (ν4) 1627 2ν4 3219 H-N-H bending sym (ν2) 968
3223 1646 3237 1060
adsorbed in KL 3387 3500c 3307 1636 3252 d
a From ref 35. b From ref 34. c Doubly degenerate band split into two bands at the upper (3500 cm-1) and lower (3387 cm-1) energy levels. d Not observable.
with the counterions, acting as Lewis acid sites, and that of the hydrogen atoms of NH3 with negatively charged lattice oxygen atoms. Both interactions can weaken the N-H bond and make the NH3 stretching and bending frequencies of NH3 adsorbed in KL lower and higher than those of the free NH3 molecules. The NH3 stretching frequencies may be used thus as a rough measure of the interaction strength of NH3 with cationic zeolites. The stronger the interaction of NH3 with cationic zeolites, the weaker is the N-H bond and the lower are the NH3 stretching frequencies if other conditions are equal. Since the weakening of the N-H bond is due to the interaction of NH3 simultaneously with counterions and lattice oxygen atoms, the NH3 stretching frequencies can also be used as an efficient molecular probe to characterize the acidbase pairs of cationic zeolites. We do not go further to discuss this matter here. The spectrum in the range 2200-1800 cm-1 (Figure 2B) is also given, since some IR features in this region were observed by Zecchina et al.36 upon adsorption of molecules such as H2O, NH3, CH3OH, C2H5OH, (CH3)O, (C2H5)2O, and C4H8O in a series of protonated zeolites such as HY, H-ZSM-5, and H-Mordenite, and they (36) Zecchina, A.; Bordiga, S.; Spoto, G.; Scarano, D.; Spano, G.; Geobaldo, F. J. Chem. Soc., Faraday Trans. 1996, 92, 4863.
Location of Benzene in KL Zeolite
Langmuir, Vol. 16, No. 3, 2000 1135
Figure 3. Changes in the infrared absorbance spectra of the N-H and C-H stretching vibration range (A) and the NH2 and CH3 bending vibration range (C) of methylamine adsorbed in KL zeolite as a function of amount of methylamine introduced into the IR cell (molecules/u.c.). The range 2200-1800 cm-1 (B) is also given to check the deformation of the framework. (a) 1.0; (b) 3.0; (c) 5.0; (d) 7.0; (e) saturation of KL zeolite (a pressure of 20 Torr of methylamine present in the IR cell).
attributed these features to framework overtone modification. The present results show clearly that no significant framework overtone modification occurs upon adsorption of NH3. This is probably due to the relatively weak interaction of NH3 with KL zeolite. 3.1.2. With Methylamine. Until now, only very few papers on the adsorption of methylamine can be found in the literature24,31,32,37 due to the great difficulties in the vibrational assignment of CH3NH2, and these works dealt with the interaction of methylamine with dehydrated faujasites exchanged with alkali metal and alkaline earth metal cations, HY, HMOR, and metallic catalysts such as Cu dispersed on Al2O3 using FTIR and microbalance techniques. No paper dealing with the adsorption of methylamine in KL zeolite has been reported. Replacement of a hydrogen atom in the ammonia molecule by alkyl groups, which are normally electron donating (more so than hydrogen) toward electronegative elements, results in increased electron density on the nitrogen atom and increased basicity. We might expect stronger interaction of KL zeolite with methylamine than with NH3. The gaseous methylamine molecule in the midinfrared spectrum, on the basis of experimental observation and theoretical calculation (Table 2),38 gives the asymmetric and symmetric stretching vibrations of the N-H bond at around 3427 and 3361 cm-1 and the asymmetric bending vibration of H-N-H at around 1623 cm-1, a group of bands corresponding to the stretching vibrations of C-H at around 2961, 2985, and 2820 cm-1, and a group of bands related to the bending vibration of H-C-H at around 1485, 1473, and 1430 cm-1. Figure 3 depicts the changes in the infrared absorbance spectra of methylamine adsorbed in KL zeolite at different amounts of methylamine introduced into the IR cell in the ranges 4000-2500 cm-1 (A), 2200-1800 cm-1 (B), and 1800-1300 cm-1 (C). The spectrum of the zeolite phase alone and that of the gas phase of the IR cell have been subtracted. Two intense groups of bands are observed in (37) Jobson, E.; Baiker, A.; Wokaun, A. J. Chem. Soc., Faraday Trans. 1990, 86, 1131. (38) Dellepiane, G.; Zerbi, G. J. Chem. Phys. 1968, 48, 3573.
Table 2. Wavenumbers (cm-1) of Different Vibration Bands of Methylamine in the Gaseous (Experimental and Theoretical) and Adsorbed States state of methylamine type of vibration N-H stretching asym N-H stretching sym C-H stretching vibrations H-N-H bending asym combination of H-N-H bending asym H-C-H bending vibrations H-N-H bending sym
gaseous (exp)(38)
gasous (calc)(38)
adsorbed in KL(a)
3427 3361 2985, 2961, 2820 1623
3440 3223 2966, 2965, 2810 1630
3373 3377 2952, 2917, 2894, 2816 1602 3252
1485, 1473, 1471, 1466, 1480, 1462, 1430 1426 1426 968 1060 b
(a) From present work. (b) Not observable.
both the ranges 4000-2500 cm-1 (Figure 3A) and 18001300 cm-1 (Figure 3C). A weak peak at 3189 cm-1 and a negative feature at 3740 cm-1 are also present. The twin bands at 3373 and 3318 cm-1 are immediately suggestive of the ν(NH) the antisymmetric and symmetric vibrations of N-H, respectively, for methylamine, and the band at 1602 cm-1 may then be correlated with the δ(NH2) of methylamine. The groups of bands observed in the regions 3100-2700 cm-1 and 1550-1400 cm-1 correspond to the asymmetric and symmetric stretching vibrations and to the asymmetric bending vibration, respectively, of the methyl group of methylamine molecules adsorbed in KL. It is observed that the frequencies of the stretching vibration bands of N-H and C-H and those of the bending vibrations of H-C-H of methylamine molecules adsorbed in zeolite are shifted toward lower and higher wavenumbers compared with the corresponding bands of gaseous molecules, indicating the weakness of the N-H and C-H bonds due to the interaction of the methylamine molecule with the KL zeolite. It should be noted that the frequency of the band corresponding to the asymmetric bending vibration of H-N-H is displaced exceptionally toward lower wavenumbers, being contrary to what was observed in the case of NH3. This should be very likely related to
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Figure 4. Changes in the infrared absorbance spectra of the N-H and C-H stretching vibration range (A) and the NH2 and CH3 bending vibration range (C) of methylamine adsorbed in KL zeolite as a function of desorption temperature (K). The range 22001800 cm-1 (B) is also given to check the deformation of the framework. (a) saturation of KL zeolite; (b) 298; (c) 323; (d) 348; (e) 363; (f) 383.
the replacement of a hydrogen atom by a methyl group, which affects the bending vibration mode of H-N-H. We will not go further to try to interpret the shift of the bending vibration band of H-N-H toward lower wavenumbers instead of higher wavenumbers. The band at 3189 cm-1 and the negative feature at 3740 cm-1 are relative to the interaction of silanols present in dehydrated KL zeolite with methylamine molecules, as observed in the case of NH3. Owing to this interaction, the silanol band is shifted toward low wavenumber, giving a peak at 3189 cm-1 and a negative feature at 3740 cm-1. However, the shift value in wavenumbers is quite higher in the case of methylamine (∆ν ) 551 cm-1) than that in the case of NH3 (∆ν ) 521 cm-1). It is well-known that the shift value ∆ν of the ν(OH) stretching frequency is roughly proportional to the interaction enthalpy with bases and in turn the proton affinity of the bases. The present results suggest that the interaction of silanols is stronger with methylamine than with ammonia. This is in line with the order of basicity (pKb value) of these two bases. In the range 2200-1800 cm-1, some features which are not present in the case of NH3 are observed and are not originated from the vibration of methylamine molecules. As indicated above, the spectra of methylamine adsorbed in KL are obtained after subtraction of the spectrum of the zeolite phase and that of the gas phase of the IR cell. If the zeolite skeletal vibration modes are not modified by adsorption of methylamine, no peaks should be observed, as in the case of adsorption of NH3 in KL. The presence of some features in this region indicates that zeolite skeletal vibrations are indeed affected by the adsorption of methylamine. Zecchina et al. reported21,36 that the presence of adsorbates in the channels of zeolites causes a small increase of the lattice parameters, indicating a slight deformation of the zeolite framework and consequently a small shift of the skeletal modes (especially the overtones and combination bands in the 2100-2550 cm-1 interval) toward lower wavenumbers. As a result, some features are present in this region after subtraction of the spectra. We will take care of this disturbing, confusing, and undesired consequence, since some real adsorbate bands can be altered. Zecchina et al. claimed also that
this effect is not dependent on the specific structure of the adsorbed molecule. Our results obtained with adsorption of NH3 and methylamine in KL zeolite show very clearly that the increase in the lattice parameters is observed with adsorption of methylamine but not with NH3. This suggests that this effect is related to the strength of interaction between the zeolite framework and the adsorbate and depends indeed on the structure and the chemical properties of the adsorbates. The interaction strength of methylamine with KL zeolite has been evaluated using a zeolite wafer in contact with a high pressure of methylamine. Figure 4 reports the changes in the infrared absorbance spectra of methylamine adsorbed in KL zeolite upon desorption at different temperatures. The two broad bands, indicating the deformation of the zeolite framework, are clearly present when a high pressure is in contact with KL zeolite. A desorption at room temperature for 30 min can remove a great part of the methylamine adsorbed in KL zeolite, since the intensity of all peaks stemming from adsorbed methylamine molecules is reduced by about 60%. The deformation of the zeolite framework disappears after a desorption at 348 K for 30 min. A small amount of methylamine is still present in the KL zeolite even though the desorption temperature is raised to 483 K, indicating the strong interaction between KL zeolite and methylamine. It was reported that ammonia adsorbed in KL can be removed easily even at a temperature less than 343 K.16 This indicates that methylamine will interact more strongly with KL zeolite than NH3. The desorption results are in accordance with the shift value of the N-H bond observed for methylamine and ammonia. Due to the strong interaction between KL zeolite and methylamine, the chemical and electronic properties of KL zeolite will be also strongly modified. 3.2. Location of Benzene in KL Zeolite upon Coadsorption of a Basic Molecule. 3.2.1. Upon Coadsorption of Ammonia. The adsorption capacity of KL zeolite for benzene has been previously evaluated by one of present authors and is around 2.0 molecules/u.c.13 It should be noted that the framework overtone modifications in the range 2200-1700 cm-1 observed after
Location of Benzene in KL Zeolite
Langmuir, Vol. 16, No. 3, 2000 1137
Figure 5. Infrared absorbance spectra of the C-H out-of-plane vibrations of benzene adsorbed (A) and the N-H bending vibration of ammonia adsorbed (B) on KL after evacuation for 0.5 h at (b) 298, (c) 323, (d) 348, (e) 363, and (f) 383; spectrum a represents the zeolite wafer in contact with 2.5 molecules/u.c. of benzene and a high pressure of ammonia.
adsorption of methylamine were not observed in KL zeolite after adsorption of benzene, indicating once again the effect of the structure of the adsorbed molecules on the modification of the skeleton vibration modes of the zeolite. The effect of coadsorption of ammonia on the location of benzene was studied on a KL zeolite wafer in contact with 2.5 molecules/u.c. of benzene. Under this condition, the KL zeolite was completely saturated by benzene molecules. The results showed that KL zeolite does not adsorb benzene molecules in its 12R windows even in the presence of NH3.16 The adsorption strength of benzene and ammonia in KL zeolite was evaluated on a sample wafer in contact simultaneously with 2.5 molecules of benzene per unit cell and a high pressure of ammonia in the IR cell. Figure 5 reports the changes in the absorbance of the C-H outof-plane vibration bands of adsorbed benzene (Figure 5A) and the C-C deformation vibration of benzene and the N-H asymmetric deformation vibration of adsorbed NH3 (Figure 5B) in KL zeolite as a function of desorption temperature. In the presence of 2.5 molecules of benzene per unit cell and a high pressure of ammonia in the cell, only one main pair of bands at 1994 and 1885 cm-1, corresponding to benzene molecules adsorbed on K+ ions, is present in the range 2200-1800 cm-1. Coadsorption of ammonia does not give any modification of the benzene location in KL zeolite. The peaks at 1635 and 1479 cm-1 observed in the range 1800-1400 cm-1 (Figure 5B) arise from the deformation vibration bands of the H-N-H of ammonia and of the C-C of benzene, respectively. With increasing desorption temperature, the intensities of all these peaks decrease. At the desorption temperature of around 336 K, the peak stemming from the H-N-H bending vibration disappears completely, and the vibration bands of adsorbed benzene disappear only after a desorption at 393 K. The above results indicate clearly that benzene molecules adsorb more strongly in KL zeolite than ammonia molecules.
3.2.2. Upon Coadsorption of Methylamine. This was also studied on a KL zeolite wafer in contact with 2.5 molecules/u.c. of benzene, that is, the saturation of zeolite by benzene. Figure 6 shows the changes in the absorbance of the CH out-of-plane vibration bands of adsorbed benzene upon coadsorption of methylamine. The spectra, which present only the adsorbed benzene, were obtained by subtraction of the spectrum of methylamine adsorbed in KL zeolite at a defined methylamine loading from the spectrum of benzene and methylamine adsorbed in KL zeolite with the same methylamine loading. The spectrum of benzene adsorbed alone in KL zeolite is given in curve a of Figure 6 for comparison. When benzene molecules alone are present in KL zeolite, one pair of bands at 1985 and 1844 cm-1 and a pair of small shoulders at 2012 and 1883 cm-1 are observed and correspond respectively to the adsorption of benzene on the K+ cation, referred to as LF bands, and on the 12R windows, referred to as HF bands, according to our previous results.12,13,16 These assignments have been verified by a series of techniques such as NMR,11,39 neutron diffraction,40 and theoretical calculation.41,42 The very low intensity of the HF bands compared with that of the LF bands implies the low amount of benzene molecules adsorbed in the 12R windows and that the 12R windows are not the preferential adsorption sites for benzene. This is in accordance with what we previously observed. No significant change is observed after introduction of 1 molecule/u.c. of methylamine into the IR cell (Figure 6b). However, when 3.5 molecules/u.c. are introduced, the intensity of the LF bands decreases sharply and that of the HF bands increases (Figure 6c). In the presence of a high pressure of (39) Auerbach, S. M.; Bull, L. M.; Henson, N. J.; Metiu, H. I.; Cheetham, A. K. J. Phys. Chem. 1996, 100, 5923. (40) Fitch, A. N.; Jobic, H.; Renouprey, A. J. Phys. Chem. 1986, 9, 1311. (41) Sauer, J.; Deininger, D. Zeolites 1982, 2, 114. (42) Auerbach, S. M.; Metiu, H. J. Chem. Phys. 1996, 105, 3573.
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Figure 6. Changes in the infrared absorbance spectra of the C-H out-of-plane (ν5 + ν17) and (ν10 + ν17) vibration bands of absorbed benzene at a loading level of 2.5 molecules/u.c. with increasing the amount of introduced methylamine (molecules/ u.c.) over KL zeolite. (a) 0; (b) 1.0; (c) 3.5; (d) 5.0; (e) 7.0; (f) saturation (a pressure of 20 Torr of methylamine present in the IR cell).
methylamine in the IR cell, the HF bands are most intense and the LF bands become small shoulders. It is important to note that the intensity of the HF bands under this condition is lower than that of the LF bands when benzene is present alone in the system (Figure 6a). Nevertheless, the present results show clearly, in the presence of methylamine, the adsorption of benzene on K+ cations is disfavored and that on the 12R windows is enhanced. The presence of benzene in the gas phase of the IR cell at each methylamine loading is also checked, and the spectra of the gas phase at each methylamine loading are reported in Figure 7. A series of bands in the region 32003000 cm-1, which corresponds to the combination of the stretching C-H and bending and stretching C-C vibrations of benzene, is observed when 2.5 molecules/u.c. of benzene is introduced into the IR cell, indicating that not all the benzene molecules introduced are adsorbed in KL zeolite. This is in agreement with the adsorption capacity of KL for benzene equal to around 2 molecules/u.c.13 With introduction of an increasing amount of methylamine into the IR cell, the intensity of the benzene bands in the range 3200-3000 cm-1 increases, indicating that some benzene molecules adsorbed in KL are removed to the gas phase of the IR cell. A broad band in the region 3000-2700 cm-1, which corresponds to the vibration of the C-H and the N-H of methylamine, is also observed. The above results indicate that, upon coadsorption of methylamine, a part of the benzene molecules adsorbed on the K+ ions migrates toward the 12R windows, a second part is removed from the KL zeolite wafer to the gas phase of the IR cell, and only a very small part is still retained by the K+ ions. Desorption experiments have been carried out on a sample wafer contacting simultaneously 2.5 molecules/ u.c. of benzene and a high pressure of methylamine to
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Figure 7. Changes in the infrared spectra for absorbance in the range 3200-2700 cm-1 in the gas phase of the infrared cell with the introduction of an increasing amount of methylamine (molecules/u.c.) into the cell at a benzene loading level of 2.5 molecule/u.c. (a) 0; (b) 1.0; (c) 3.5; (d) 5.0; (e) 7.0; (f) a pressure of 20 Torr of methylamine present in the IR cell.
compare the adsorption strengths of benzene and methylamine in KL zeolite. Figure 8 depicts the IR absorbance spectra of the CH out-of-plane vibration range of benzene (Figure 8A) and the N-H asymmetric bending vibration region of methylamine (Figure 8B). It is observed from Figure 8A that besides the pair of intense bands at 2012 and 1868 cm-1, assigned to the adsorption of benzene in the 12R windows (HF bands), a pair of weak shoulders at 1991 and 1847 cm-1, corresponding to benzene molecules adsorbed on K+ cations (LF bands), is present. As all spectra in these experiments were obtained by the subtraction of only the KL zeolite phase from those with adsorption of benzene and methylamine, the complication of the spectra due to the skeleton overtone modification is clearly observed by the appearance of two broad bands centered at 2044 and 1936 cm-1. Fortunately, their presence does not affect seriously our analysis. In the presence of a high pressure of methylamine in the cell, the HF bands are most intense (spectrum a of Figure 8A). An intense band at 1600 cm-1, stemming from the bending vibration of the N-H of methylamine, is also present in the range of the N-H bending vibration zone. After evacuation at room temperature for 30 min, the HF bands decrease sharply in intensity and become a pair of shoulders whereas the LF bands increase in intensity markedly and become the most intense bands. It is observed that the intensity of the N-H bending vibration band at 1600 cm-1 decreases significantly and that of the skeleton overtone vibrations becomes also less important. The present results indicate very clearly that when methylamine molecules adsorbed in KL are removed, even partly, from the zeolite wafer, the lattice parameter is less modified, the shift of the skeleton overtone modes disappears, and the benzene molecules emigrate from the 12R windows toward the K+ ions. With increasing further the desorption temperature, all the bands decrease
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Figure 8. Infrared absorbance spectra of the C-H out-of-plane vibrations of benzene adsorbed (A) and the N-H bending vibration of methylamine adsorbed (B) on KL after evacuation for 0.5 h at (b) 298, (c) 323; (d) 348, (e) 363, and (f) 383; spectrum a represents the zeolite wafer in contact with 2.5 molecules/u.c. of benzene and a high pressure of methylamine.
continuously in intensity. The skeleton overtone vibration bands disappear completely at 348 K, and the HF and LF bands disappear at 403 K, while the N-H bending vibration band is still present at this desorption temperature, indicating that methylamine molecules adsorb more strongly than benzene molecules in KL and that modification of lattice parameter, that is, the deformation of the zeolite framework, is only evident when a certain amount of methylamine is adsorbed in KL zeolite. 4. General Discussion Effect of Framework Deformation on the Adsorption of Benzene on the 12R Windows. It has been recognized that the adsorption of benzene on the 12R windows of zeolites is governed by the chemical and structural properties of the zeolites.12 The highly negative charged oxygen atoms of the 12R windows polarize the hydrogen atoms of benzene and enhance the interaction between the hydrogen atoms of benzene and the oxygen atoms of the 12R windows. The average negative charge of the oxygen atoms in KL, calculated using the Sanderson electronegativity egalization principle,43,44 is around -0.350, being similar to that of NaY (Si/Al ) 2.4). In NaY zeolite, the adsorption of benzene on 12R windows was indeed observed; however, it was not observed in KL. It was known that the adsorption of a basic molecule like ammonia could modify and increase the negative charge of the oxygen atoms.45,46 No adsorption of benzene on the 12R windows of KL was observed even in the presence of ammonia. These results (43) Mortier, W. J. J. Catal. 1978, 55, 138. (44) Sanderson, R. T. Chemical Bonding and Bond Energy, 2nd ed.; Academic Press: New York, p 1. (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.
led us to suggest that the absence of the adsorption of benzene on the 12R windows of KL both with and without the presence of ammonia should be linked to the lack of structural compatibility between the 12R windows of KL and benzene molecules. However, the present study (Figure 8) shows clearly a migration of benzene from cations to 12R windows upon coadsorption of methylamine and a remigration of benzene from 12R windows to cations on removal of methylamine. It is true that the adsorption of methylamine can increase further the negative charge of the oxygen atoms of the 12R windows compared with that of ammonia. However, we think that the fact that the 12R windows of KL zeolite become the adsorption sites upon coadsorption of methylamine is not because of the increase in the negative charge of the oxygen atoms but is related to the deformation of the framework. This modification of the framework due to the strong interaction of methylamine with KL zeolite renders the 12R windows, which are initially incompatible with benzene molecules, favorable for adsorption of benzene. The deformation of the framework was not observed in KL upon coadsorption of ammonia.12 That is why no adsorption of benzene on the 12R windows can be detected even though the negative charge of the oxygen atoms is increased due to the interaction of ammonia with this zeolite. The reason for which the adsorption of methylamine on KL can induce a deformation of the framework, but not that of ammonia, is the stronger interaction between methylamine and KL zeolite. The desorption experiments show that, to remove completely the methylamine from KL zeolite, a temperature higher than 403 K for 0.5 h is necessary. However for NH3, an evacuation at 330 K is enough. How this strong interaction can induce a framework deformation will be discussed elsewhere. On the basis of desorption experiments, which indicate that methylamine molecules adsorb more strongly on K+ ions than benzene molecules, one
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may propose also that although 2.0 out of 3.6 K+ ions per unit cell located in 12R straight channels at sites D are occupied by benzene molecules, the methylamine molecules introduced will interact first with the remaining K+ ions (3.6-2.0 ) 1.6) and more methylamine molecules introduced will attack the K+ ions interacting with benzene molecules and then bind on the K+ ions due to the stronger interaction and poison these sites. Benzene molecules will in consequence migrate from K+ ions toward 12R windows. We observe indeed that the migration of benzene molecules from K+ ions toward 12R windows occurs only when more than one molecule of methylamine per unit cell is introduced. Ammonia has no effect, since ammonia molecules interact less strongly with K+ ions than benzene molecules. This should indicate also that the 12R windows in KL zeolite can be adsorption sites for benzene, but due to the reason that benzene molecules interact more strongly with K+ ions than with 12R windows, in the presence of benzene alone, only adsorption of benzene on K+ ions is thus observed. There is a competition between K+ ions and 12R windows for benzene. We think that the explanation discussed above is less probable because the location of benzene molecules on K+ ions does not hinder sterically the adsorption of benzene in 12R windows and, theoretically, the available void in a large cage of KL zeolite is large enough to have simultaneous adsorption of benzene in K+ ions and 12R windows, as observed by the present work upon coadsorption of methylamine. If the appearance of adsorption of benzene on 12R windows is due to the poisoning of K+ ions by methylamine, in the presence of benzene alone and in the presence of a high amount of benzene, the two kinds of adsorption should be observed. However, this does not happen. The adsorption of
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benzene in 12R windows is observed only upon coadsorption of methylamine and in the presence of an observable deformation of the framework induced by adsorption of methylamine. 5. Conclusion The interaction of ammonia and methylamine with KL zeolite has been studied, and it has been shown that ammonia and methylamine can interact not only with K+ ions via the lone pair on the nitrogen atoms but also with residual silanols present in KL zeolite after pretreatment at 723 K. The interaction of KL zeolite with methylamine is found to be much stronger than that with ammonia and is so strong that the lattice parameter of the zeolite is modified, indicating the deformation of the zeolite framework. This deformation of the framework induced by adsorption of methylamine gives a beneficial effect for the location of benzene on 12R windows of KL. The deformation renders the 12R windows of KL zeolite, which are initially not adsorption sites for benzene, favorable for the location of benzene on these sites. This indicates that structural compatibility is the most important key factor in the location of benzene on 12R windows. It shows also that, in some zeolites, both the structural and chemical properties of 12R windows can be accommodated by introduction of a coadsorbate to the location of benzene. Acknowledgment. The authors thank the FNRS (Fonds National de la Recherche Scientifique, Belgium) for a scholarship FRIA to V.N. Mrs. Su-Virlet is also acknowledged for her assistance. LA990593V
