Infrared Evidence of Three Types of Interaction between Methylamine

Infrared Evidence of Three Types of Interaction between ... Laboratoire de Chimie des Mate´riaux Inorganiques (CMI), The University of Namur (FUNDP),...
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Infrared Evidence of Three Types of Interaction between Methylamine and a Series of Alkali Cation Exchanged Faujasite Zeolites F. Docquir,†,§ H. Toufar,‡ and B. L. Su*,† Laboratoire de Chimie des Mate´ riaux Inorganiques (CMI), The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium, and Tricat Zeolites GmbH, Chemiepark, Tricat-Strasse, G-06749 Bitterfeld, Germany Received May 23, 2001. In Final Form: July 12, 2001 The acid-base properties of a series of cationic faujasite zeolites have been studied using methylamine as a probe molecule by means of in situ infrared spectroscopy. The results have been correlated with the nature of counterions and the Si/Al ratio of the studied zeolites. The present work reveals three kinds of interaction between methylamine and cationic zeolites. Methylamine can interact not only with alkali cations via the lone electron pair on nitrogen atoms but also with the negatively charged oxygen atoms of the framework via the hydrogen atoms of both NH2 and CH3 groups as well. The interaction between the methyl groups and these oxygen atoms is for the first time postulated. However, the Lewis acidity of counterions remains the dominating factor in the interaction between methylamine and cationic zeolites. A deformation of the zeolite framework due to the adsorption of methylamine has been observed. This work shows clearly that methylamine can be an efficient probe molecule for the characterization of acid-base properties of zeolites.

1. Introduction 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. A better understanding concerning the adsorption behavior of molecules in zeolites is therefore of considerable interest to describe the catalytic and adsorptive properties of zeolites and to design new catalysts with enhanced performance. Recently, experiments have shown that adsorption properties of zeolites can be accommodated by modification of their chemical or structural properties by introducing a coadsorbate.1-5 A successive migration of benzene previously adsorbed on cationic sites to the 12R window sites has been evidenced by the coadsorption of ammonia on HY and NaEMT zeolites. A more basic molecule such as methylamine has been used to increase the zeolite basicity and to favor migration of benzene molecules from cations to 12R windows. Moreover, it was reported that the adsorption of methylamine in zeolites led to the modification of the lattice parameter, indicating the deformation of the zeolite framework. However, only a few works dealing with the interaction of methylamine can be found in the literature, and the studies concerning the adsorption of methylamine in dehydrated faujasites exchanged with alkali metals,6 alkaline earth metal cations,7 HY, HMOR,8 and metallic catalysts such as Cu * Corresponding author. E-mail: [email protected]. † University of Namur. ‡ Tricat Zeolites GmbH. § F.R.I.A. Fellow. (1) Su, B. L.; Norberg, V.; Hansenne, C. Langmuir 2000, 16, 1132. (2) Su, B. L.; Norberg, V. Langmuir 2000, 16, 6020. (3) Su, B. L.; Norberg, V. Langmuir 1998, 14, 7410. (4) Su, B. L. Zeolites 1996, 16, 25. (5) Su, B. L.; Norberg, V.; Hansenne, C.; de Mallmann, A. Adsorption 2000, 6, 61. (6) Morishige, K.; Kittaka, S.; Ihara, S. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2525. (7) Morishige, K.; Kittaka, S.; Takao, S.; Morimoto, T. J. Chem. Soc., Faraday Trans. 1 1984, 80, 993.

dispersed on Al2O39 showed great difficulties in the vibrational assignment of CH3NH2. In recent works, it was observed that the adsorption of ammonia in a series of alkali-exchanged faujasite zeolites10 led to the interaction between not only the lone electron pair on the nitrogen atom and the included counterions but also hydrogen atoms of ammonia and negatively charged framework oxygen atoms. The latter can strongly weaken the N-H bond and is dependent on the negative charge of oxygen atoms. This observation showed that ammonia could be a potential and an efficient probe molecule in characterization of acid-base properties of zeolites. In fact, the weakening of N-H bonds which reflects in the wavenumber shift of the N-H bond and the interaction strength between NH3 and cations which can be measured by desorption temperature can give access to the Lewis acidity of cations and the basicity of framework oxygen atoms of zeolites. The decomposition of ammonia, even at room temperature, on zeolites with strong Lewis acidic cations and high negative charge on oxygen atoms was detected. This decomposition should be of important interest from an industrial point of view for developing new and efficient catalysts for methylation of ammonia to produce methylamines. If a hydrogen atom in the ammonia molecule is replaced by an alkyl group, which is more electron donating than hydrogen toward electronegative elements, the electron density on the nitrogen atom of the methylamine molecule is increased and this results in an enhanced basicity. Ion cyclotron resonance studies of gas-phase affinity have shown that the intrinsic basicity of methylamine is higher than that of ammonia11,12 and that Li+ has a higher affinity to ammonia than methylamine because of the repulsion (8) Ghosh, A. K.; Curthoys, G. J. Chem. Soc., Faraday Trans. 1 1984, 80, 99. (9) Jobson, E.; Baiker, A.; Wokaun, A. J. Chem. Soc., Faraday Trans. 1990, 86 (7), 1131. (10) Gilles, F.; Docquir, F.; Su, B. L. Adsorption Science and Technology; Do, D. D., Ed.; World Scientific: Singapore, 2000; p 578. (11) Brauman, J. I.; Riveros, J. M.; Blair, L. K. J. Am. Chem. Soc. 1971, 93, 3914.

10.1021/la010757j CCC: $20.00 © 2001 American Chemical Society Published on Web 09/08/2001

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between Li+ and the methyl group.13 It is well recognized that the basicity of the methylamine and ammonia molecules depends on both the bond structure and the environment of the acid-base pairs. The present work deals with the interaction of methylamine with alkali cation exchanged faujasite zeolites. The aim of this work is to correlate the type and the strength of methylamine adsorption with the type of counterions and the Si/Al ratio of alkali-exchanged faujasite zeolites and to compare with results obtained with those for ammonia in order to develop a new molecular probe in characterization of simultaneously acidity and basicity of zeolites. This work is also expected to shed some light on the strong modification in the adsorption behavior of benzene in 12R window zeolites induced by coadsorbed ammonia and methylamine. 2. Experimental Section 2.1. Materials. Starting materials NaX (Si/Al ) 1.2) and NaY (Si/Al ) 2.5) were provided by Union Carbide; Na-LSX (Si/Al ) 1.0), by Tricat Zeolites GmbH; and Si-rich NaY (Si/Al ) 3.6), by the KUL. LiX, KX, RbX, and CsX were obtained by ion-exchanging of NaX (Si/Al ) 1.2) with a chloride solution of the corresponding alkaline cation. 2.2. Sample Characterizations. The XRD patterns were obtained with a Philips PW 170 diffractometer, using Cu KR radiation. The crystal morphology was studied using a Philips XL-20 scanning electron microscope (SEM) with conventional sample preparation and imaging techniques. Specific surface areas were measured by nitrogen adsorption using a Micromeritics ASAP 2010 apparatus. The adsorption-desorption isotherms were obtained at 78 K. The samples were further degassed under vacuum overnight at 593 K before nitrogen adsorption measurements. The chemical compositions of the samples were obtained by elemental analysis using a Philips PU 9200 X spectrometer. 2.3. Infrared Study. The adsorption of molecules was performed on a self-supported zeolite wafer prepared with a pressure of 5 tons/cm2. The sample wafer placed in the IR cell was heated in a dry oxygen flow from room temperature to 723 K at a rate of 3 K/min. The temperature was maintained overnight in the same atmosphere. The sample was then treated under vacuum for 4 h at 723 K. 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 Perkin-Elmer Fourier Transform Spectrum 2000 spectrometer. The adsorption of welldefined and increasing amounts of methylamine was then performed on the wafer. After each adsorption, the sample was maintained at room temperature for 0.5 h for allowing it to reach an equilibrium before recording the spectra. Each spectrum reported was averaged from 20 scans. The resolution of the recorded spectra is 1 cm-1. The desorption experiments were carried out at different temperatures during 0.5 h in order to evaluate the adsorption strength of methylamine in the studied zeolites.

3. Results and Discussion 3.1. Zeolite Characterization. The crystallinity of our materials was checked using X-ray diffraction. Figure 1 reports the diffractograms of starting material NaX (Figure 1a) as well as those of LiX (Figure 1b) and CsX (Figure 1c). The XRD pattern of LiX (Figure 1b) is quite similar to that of NaX (Figure 1a). The introduction of a larger cation such as Cs in the zeolite framework causes an important modification in the XRD pattern (Figure 1c). A decrease in intensity of the peaks situated at low angle values and an increase in the region of high angles were observed. Crystallite shapes of starting NaX particles (12) Aue, D. H.; Webb, H. M.; Bowers, M. T. J. Am. Chem. Soc. 1972, 94, 4726. (13) Woodin, R. L.; Beauchamp, J. L. J. Am. Chem. Soc. 1978, 100, 501.

Figure 1. XRD diffraction patterns of starting NaX zeolite (a), LiX (b), and CsX (c).

are octahedral (Figure 2a). Lithium and cesium ion exchange does not seem to alter the morphology (Figure 2b,c). Adsorption isotherms of N2 (not presented here) are typical of microporous materials. A decrease in specific surface areas with ion exchange of a larger counterion was observed. In the case of rubidium or cesium, the access to supercages by the adsorbed molecules is limited due to the size of cations. Specific surface areas, chemical compositions, and characteristics of all the studied samples are collected in Table 1. 3.2. Adsorption of Methylamine in Cationic Faujasite Zeolites. On the basis of experimental observations and theoretical calculations,14 the gaseous methylamine molecule is characterized by the following absorption bands in the mid-infrared spectrum (Table 2): the antisymmetric and symmetric stretching vibrations of the N-H bond at 3427 and 3361 cm-1, 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 2985, 2961, and 2820 cm-1, and a group of bands related to the bending vibration of H-C-H at 1485, 1473, and 1430 cm-1. 3.2.1. Adsorption of CH3NH2 in LiX. Figure 3 reports the IR absorbance spectra of LiX in the ranges 40002500 cm-1 (A), 2200-1800 cm-1 (B), and 1800-1300 cm-1 (C) after adsorption of increasing amounts of methylamine from one molecule per unit cell to saturation. The spectrum of the zeolite phase alone and that of the gas phase of the IR cell have been subtracted. With introduction of increasing amounts of methylamine, the intensity of all (14) Dellepiane, G.; Zerbi, G. J. Chem. Phys. 1968, 48, 3573.

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Docquir et al. Table 1. Chemical Composition and Characteristics of the Studied Zeolites

zeolite

chemical compositiona

Si/Al ratioa

Na-LSX LiX NaX KX RbX CsX NaY (UC) Si-rich NaY

Na96(AlO2)96(SiO2)96 Li86Na1(AlO2)87(SiO2)105 Na87(AlO2)87(SiO2)105 K53Na34(AlO2)87(SiO2)105 Rb42Na45(AlO2)87(SiO2)105 Cs20Na67(AlO2)87(SiO2)105 Na55(AlO2)55(SiO2)137 Na42(AlO2)42(SiO2)150

1.0 1.2 1.2 1.2 1.2 1.2 2.5 3.6

-δ0b

specific surface area (m2/g)c

0.429 0.408 0.412 0.440 0.441 0.431 0.348 0.318

629 659 527 518 246 275 621 465

a Elementary analysis. b Negative charge on oxygen atoms calculated using Sanderson electronegativity equalization principle (ref 13). c BET results.

Figure 2. Scanning electron micrographs of NaX zeolite (a), LiX (b), and CsX (c).

peaks increases. When 20 molecules of CH3NH2 are adsorbed per unit cell, two intense groups of bands are observed in both the ranges 4000-2500 and 1800-1300 cm-1 (Figure 3A,C). A weak peak at 3194 cm-1 as well as two features and a broad shoulder at 3716, 1638, and 3470 cm-1, respectively, are also present. The peaks at 3360 and 3301 cm-1 can be assigned to the antisymmetric and symmetric stretching vibrations of N-H, respectively, and the peak at 1609 cm-1 may be correlated to the antisymmetric bending vibration of H-N-H. The group of bands observed in the regions 3000-2800 and 15001400 cm-1 corresponds to the antisymmetric and symmetric stretching vibrations and to the antisymmetric bending vibration, respectively, of the methyl group of methylamine. By comparison with infrared spectra of

gaseous methylamine, the frequencies of all the peaks of methylamine molecules adsorbed on the zeolite are shifted toward lower wavenumbers, indicating that the zeolite and the methylamine molecules are in interaction. As the wavenumber is related to the force constant by Hooke’s law, we can thus conclude that both N-H and C-H bonds are weakened by the interaction of methylamine molecules with the LiX zeolite. The feature at 3716 cm-1 can be assigned to the OH group, generated by the interaction of H+ protons produced by the possible decomposition of a small amount of CH3NH2 and interacting with the negatively charged framework oxygen atoms. The intensity of this peak is weak, indicating a slight decomposition of the methylamine molecules as observed previously with the adsorption of ammonia on cationic zeolites.10 The bands of protonated amine should be superimposed with the bands of adsorbed amine. The broad shoulder at 3470 cm-1 could stem from the N-H stretching vibration of the CH3NH- group which is produced by the decomposition of CH3NH2 in LiX. It was reported7 that the N-H stretching vibration of Me2NH molecules adsorbed on alkaline earth metal exchanged X zeolites gives a broad band centered at 3500 cm-1, while its free stretching frequency is 3384 cm-1. This has been explained by the general tendency observed by Chatt et al.15 that the N-H stretching frequency, for a given N-H bond, increases with increasing negative charge on the nitrogen atom. In Me2NH molecules, the nitrogen atom is strongly polarized by the strong electrostatic interaction with the alkaline-earth-metal zeolites and becomes more negative, which results in an increase in the N-H stretching frequency. In the present case, after decomposition of CH3NH2, this molecule (especially the N atom) bears a negative charge, and the frequency of N-H stretching increases also compared to the free N-H stretching vibration frequency. This broad shoulder is observed only when the oxygen atoms have a very enhanced negative charge and the counterions are hard Lewis acids. The weakness of the N-H bond or the decomposition of CH3NH2 is a cooperative effect of oxygen atoms and cations. The decomposition of methylamine is thus possible considering that CH3NH2 is a more basic molecule than NH3, so the negative charge carried by oxygen atoms is enhanced. The band at 1638 cm-1 can be attributed to a split of the symmetric stretching vibration of the N-H bond, due to a change of the CH3NH2 symmetry at the time of the interaction with zeolites. The peak at 3194 cm-1 results from the combination of the H-N-H bending antisymmetric vibration (2υ). A slight negative feature is also observed at 3650 cm-1. This band is present after pretreatment, meaning that it can be generated (15) Chatt, J.; Duncanson, L. A.; Venanzi, L. M. J. Chem. Soc. 1955, 4456.

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Table 2. Wavenumbers (cm-1) of Different Vibration Bands of Methylamine in the Gaseous (Experimental and Theoretical) and Adsorbed States CH3NH2 adsorbed on type of vibrationa N-H A.S. N-H S.S. comb. of H-N-H A.B. C-H Str.

H-N-H A.B. H-C-H bend. H-N-H S.B. a

gaseous CH3NH2 expt calcd Na-LSX ∆υ 3427 3361

3440 3377

3358 3292 3195 2970 2948 2893

2985 2961

2966 2965

2820

2810

2808

1623 1485 1473 1430 968

1630 1471 1466 1426 1060

1613 1480 1461 1424 b

LiX

∆υ NaX ∆υ

KX

69 3360 67 3363 64 3355 69 3301 60 3305 56 3296 3194 3196 3205 15 2985 0 2975 10 2957 13 2960 1 2952 9 2936 2927 2928 2887 2902 2896 12 2822 -2 2812 8 2800 2796 10 1609 14 1611 12 1616 5 1480 5 1478 7 1482 12 1466 7 1460 13 1460 6 1429 1 1426 4 1423 b b b

NaY NaY ∆υ (UC) ∆υ (KUL) ∆υ

∆υ RbX ∆υ

CsX

72 3356 71 65 3293 68 3197 28 25 2935 26 2887

3358 69 3367 60 3298 63 3310 51 3200 3197 2984 1 2938 23 2958 3 2884 2925 2902 2803 17 2824 -4 2793 1618 5 1607 16 1478 7 1481 4 1460 13 1466 7 1422 8 1430 0 b b

20 2802 18 7 3 13 7

1617 6 1480 5 1460 13 1422 8 b

3371 3315 3191 2975 2951 2918 2896 2815 2787 1607 1478 1462 1426 b

56 46 10 10 5 16 7 11 4

A.S. ) antisymmetric stretching, S.S. ) symmetric stretching, A.B. ) antisymmetric bending, S.B. ) symmetric bending. b Not observable.

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 LiX zeolite as a function of amount of methylamine introduced into the IR cell (molecules/unit cell). The range 2200-1800 cm-1 (B) is also given to check the deformation of the framework. (a) 1.0, (b) 5.0, (c) 10.0, (d) 20.0, and (e) saturation of LiX zeolite.

during the ion exchange since no such band can be observed in the spectrum of NaX zeolite pretreated in oxygen and then in a vacuum. The wavenumber of this band corresponds to the OH groups of HY. This suggests that during the ion exchange procedure, very small amounts of OH groups were generated. Due to the very low quantity, we will not go further to discuss the behavior of these OH groups upon adsorption of methylamine. An additional broad band appears in the range 22001800 cm-1 (Figure 3B), which cannot be due to the vibration of methylamine molecules. The intensity of this broad band is weak for low quantities of methylamine but increases when a high amount of CH3NH2 is adsorbed. Normally, a straight line should be observed in this region. With regard to the works of Zecchina et al.,16 this band can be attributed, as in the case of NaBeta and KL,1-3 to the vibrations of the zeolite lattice. They reported that the presence of absorbates 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 lattice modes (especially the overtones and combination bands in the 2100-1550 cm-1 interval) toward lower wavenumbers. As a result, some features are present in this region after subtraction (16) Zecchina, A.; Geobaldo, F.; Spoto, G.; Bordiga, S.; Ricchiardi, G.; Buzzoni, R.; Petrini, G. J. Phys. Chem. 1996, 100, 16584.

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 five molecules/unit cell of methylamine adsorbed in a series of cationic zeolites. The range 2200-1800 cm-1 (B) is also given to check the deformation of the framework. (a) Na-LSX, (b) LiX, (c) NaX, (d) KX, (e) RbX, (f) CsX, (g) NaY (UC), and (h) NaY (KUL); str. ) stretching vibrations, bend. ) bending vibrations.

of the spectrum corresponding to the zeolite without any adsorbate. The presence of a relatively large band thus suggests a deformation or at least a modification of the zeolite lattice. 3.2.2. Adsorption of CH3NH2 in a Series of Cationic Zeolites. Figure 4 depicts the changes in infrared absorbance spectra when five molecules of methylamine per unit cell are adsorbed on a series of cationic faujasite zeolites. The studied ranges are 4000-2500 cm-1 (A), 2200-1800 cm-1 (B), and 1800-1300 cm-1 (C). All the spectra are similar to those obtained with LiX. However, the vibration bands of methylamine are situated at different wavenumbers when the counterions or the Si/Al ratio is changed (Table 2). Effect of the Compensating Ions. For the X zeolites (Si/ Al ) 1.2), the stretching vibrational bands of N-H and C-H bonds tend to a higher wavenumber displacement when the Lewis acidity of counterions is reduced. The Lewis acidity of an ion corresponds to its ionic potential which is conversely proportional to the ionic radius. The Lewis acidity of alkali cations thus increases from large to small cations, that is, from Cs+ to Li+. It is observed that the shift values of both antisymmetric and symmetric vibrations of the N-H bond and stretching vibrations of the C-H bond for the zeolites exchanged with larger radius cations are greater than for the LiX one. This is contrary

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Figure 5. Displacement in wavenumber of stretching vibrational bands of N-H and C-H bonds according to the negative charge on oxygen atoms. A.S. ) antisymmetric stretching vibrations, S.S. ) symmetric stretching vibrations, Str. ) stretching vibrations.

to the expectation that the interaction of LiX with N atoms of methylamine is stronger, N-H and C-H bonds will be more weakened, and the shift values of N-H and C-H bands will be more pronounced. We will discuss this in following sections. Effect of the Si/Al Ratio. Methylamine adsorption has been realized in a series of Na+-exchanged faujasite zeolites with different Si/Al ratios. The adsorption of methylamine on Na-LSX (Si/Al ) 1.0) gives a displacement value of 69 cm-1 (antisymmetric and symmetric stretching vibrations of the N-H bond) and 12 cm-1 (stretching vibration of the C-H bond), which decreases up to 56, 46, and 5 cm-1, respectively, in the case of Si-rich NaY (Si/Al ) 3.6). As the exchange degree is relatively low in the case of RbX and CsX, to better correlate the adsorption properties with zeolite characters, the negative charges on oxygen atoms have been used. Figure 5 represents the displacement value in wavenumber for all the studied zeolites as a function of the negative charge carried by the oxygen atoms of zeolites which can be evaluated using the Sanderson electronegativity equalization principle.17 The shift value is enhanced with the increasing of the zeolite basicity. This point will be discussed in the following section together with the results obtained from desorption measurements. 3.3. Evaluation of Interaction Strength. The interaction strength of methylamine with zeolites has been evaluated using a zeolite wafer in contact with a high pressure of methylamine. A desorption is then realized at different temperatures during 0.5 h to remove the methylamine adsorbed. Figure 6 reports the IR absorbance spectra of LiX in the ranges 4000-2500 cm-1 (A), 2200-1800 cm-1 (B), and 1800-1300 cm-1 (C) upon desorption of methylamine. After a desorption at room temperature, the peaks corresponding to methylamine molecules are still present. When the desorption temperature is raised, the intensity of all the peaks is reduced. The deformation of the zeolite framework disappears after a desorption at 473 K, and finally, after evacuation at 533 K, no bands due to methylamine molecules are observed. It can be concluded that this temperature is necessary to realize a complete desorption and that there is a quite strong interaction between methylamine molecules and LiX zeolite. From Figure 7, which depicts the desorption temperature of methylamine according to the negative charge of (17) Mortier, W. J. J. Catal. 1995, 55, 138.

Docquir et al.

Figure 6. 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 LiX zeolite as a function of desorption temperature (K). The range 2200-1800 cm-1 (B) is also given to check the deformation of the framework. (a) 293, (b) 323, (c) 343, (d) 373, (e) 393, (f) 423, (g) 473, and (h) 533 K.

Figure 7. Desorption temperature of methylamine as a function of negative charge on oxygen atoms of studied zeolites.

the oxygen atoms in the case of NaX zeolite (Si/Al ) 1.2) exchanged with different alkali cations, we can see that the temperature at which methylamine can be completely removed is reduced when the charge on the oxygen atoms increases, passing from 533 K for LiX to 323 K for CsX. The results agree with the Lewis acidity of counterions, since it increases from large to small alkali cations. From the above-cited results, it appears thus that the adsorption strength of methylamine in Li-faujasite is the strongest among all of the alkali-exchanged faujasite X zeolites studied here. 4. General Discussion The shift value of stretching and bending vibration bands of methylamine could be expected to be related to the interaction of counterions with the lone electron pair of the nitrogen atom of the methylamine molecule. This interaction can weaken the N-H bond and results in a shift of the stretching and bending vibration bands of methylamine. As explained before, the Lewis acidity of alkali counterions increases from large to small cations and the zeolite acidity is enhanced when the Si/Al ratio increases. The results of desorption give the strongest interaction strength between methylamine and LiX zeolite, for which the Lewis acidity of the counterion is the highest, proving the interaction between compensating ions and the lone electron pair on the nitrogen atom. We can thus expect to observe a higher shift value in wavenumber by adsorption results when the counterion acidity is en-

Interaction between Methylamine and Zeolites

Figure 8. Representation of three types of interactions between methylamine and zeolites.

hanced. However, the obtained results are contrary to expectation. Indeed, we observed a more important displacement in wavenumber of the stretching vibrational bands of N-H bonds in the case of CsX (∆υ ) 69 cm-1) than for LiX (∆υ ) 67 cm-1). We can also notice the same effect when the zeolite possesses the same cation but a different Si/Al ratio (∆υ ) 69 cm-1 for Na-LSX and ∆υ ) 56 cm-1 for NaY-KUL), resulting in different intrinsic framework acidities. This suggests that besides the interaction of counterions and the lone electron pair of the nitrogen atom of methylamine, other interactions should be present and play a role in weakening the N-H bonds. The shift value of stretching modes varies with the Si/Al ratio and size of the cation, which have a direct influence on the negative charge of framework oxygen atoms. This indicates that hydrogen atoms of amine groups can interact with negatively charged framework oxygen atoms as observed for NH3.10 This direct interaction can strongly affect the N-H bonds and induce a large shift of the different vibration modes of methylamine. This means that the interaction between cations and N atoms will displace the N-H bands (the displacement in the case of LiX should be higher than that in the case of CsX) and the direct interaction between H atoms of NH2 and framework oxygen atoms will further displace the N-H bands (this time, the displacement in the case of CsX will be higher than that in the case of LiX). The observed shift values are from two different contributions. This explains the slight difference between the shift values of the N-H band observed in CsX and LiX. Besides these two interactions, we can postulate a third one. In fact, according to the adsorption results, we can see that the stretching vibrational bands of C-H bonds are situated at different wavenumbers when the basicity of the zeolites is changed. We observe a more important displacement in the case of CsX (∆υ ) 17 cm-1) than for LiX (∆υ ) 2 cm-1) and for Na-LSX (∆υ ) 12 cm-1) compared to NaY-KUL (∆υ ) 5 cm-1), which is contrary to our expectation. This suggests that hydrogen atoms of methyl groups can also directly interact with negatively charged framework oxygen atoms. This interaction weakens the C-H bond and provokes a shift of vibrational bands of methyl groups in methylamine. These three types of interactions between cationic zeolites and methylamine molecules are represented in Figure 8. Moreover, concerning the stretching vibrations of the methyl group, we can detect more peaks when the zeolite basicity is reduced. Indeed, we observe six vibrational bands in the case of LiX or NaY (KUL), whereas only three can be evidenced for CsX and four in the case of Na-LSX. This can be explained as follows: in zeolites where the framework oxygen atoms have a weaker negative charge, hydrogen atoms of methyl groups are less tightly held giving methylamine molecules an enhanced mobility, and this implies that the peaks become more numerous. It was already admitted that the negatively charged framework oxygen atoms (intrinsic basic sites of zeolites) and the metal counterions (Lewis acid sites), forming acidbase pairs, are the active centers and jointly play a role

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in the catalytic reaction and separation processes.18,19 The interactions between alkali cations and the lone electron pairs of nitrogen atoms, on one hand, and of the hydrogen atoms of N-H bonds with negatively charged framework oxygens, on the other hand, have also been described in the literature.10 However, the interaction concerning the framework oxygen atoms and the hydrogen atoms of methyl groups is for the first time postulated. It was reported in a theoretical study that the adsorption of methane on acidic zeolites could occur through an interaction between the framework oxygen atoms and the hydrogen of methyl groups.20 This interaction has however never been observed experimentally before. The existence of these three interactions is of important interest in order to better understand the zeolite behavior toward the reactant adsorption. The displacement of the N-H bond should be a cooperative effect of the oxygen atoms and cations. Both interactions weaken the N-H bond. That is why the highest displacement of the N-H bond is observed in KX, instead of in LiX as expected theoretically due to the high Lewis acidity of Li cations. According to results of Woodin et al.,13 the strong repulsion between Li+ ions and the methyl group of methylamine reduces the interaction strength between LiX and methylamine. However, the present desorption experiments showed clearly that in all studied zeolites, the interaction strength between LiX and methylamine is the highest. In the CsNaX sample, only 20 of 87 compensating cations are Cs. One may ask whether, if the interaction of amine is “basically” CH3-H2N-M+, we should expect to find an intense band corresponding to the interaction with Na+ and another less intense band corresponding to the interaction with Cs+. However, only one sharp and narrow band was observed. The same question can arise for the RbNaX sample. In our faujasite X zeolites, there are around 87 counterions. A maximum of 32 cations can be located in the possible sites (SI and SI′) in the small cages (hexagonal prisms and sodalite cages), and 64 can be located in the supercages (SII and SIII + SIII′). Since the cation positions in small cages are more energically favorable, all the 32 positions in the small cages will be first occupied by small cations such as Li, Na, or K. As Cs ions are larger, they (22 Cs in CsNaX) will occupy preferentially the positions in supercages. The remaining cations (87 - 32 - 22 ) 33) will also be located in supercages. The lone electron pairs on N atoms will interact with both Cs+ and Na+ ions in supercages at high loadings of methylamine since methylamine molecules cannot penetrate into the small cages. Two different displacements of N-H bands can really be expected. However, methylamine molecules are, at any moment, in motion, with migration and diffusion (jumping) from one site to another. They will not be fixed without movement. Our previous studies by NMR21 and FTIR22-24 have clearly shown the motion and jumping of benzene molecules from one site to another. The N-H band observed is only an average of displacements due to the interaction with Cs+ and Na+ ions. That is why we cannot observe two different (18) Yashima, T.; Suzuki, H.; Hara, N. J. Catal. 1974, 33, 486. (19) Su, B.-L.; Jaumain, D.; Ngalula, K.; Briend, M. In Proceedings of the 12th International Zeolite Conference, Baltimore, MD, July 5-10, 1998; Treacy, M. M. J. et al., Eds.; Materials Research Society: Warrendale, PA, 1999; p 2681. (20) Kramer, G. J.; van Santen, R. A. J. Am. Chem. Soc. 1995, 117, 1766. (21) Norberg, V.; Docquir, F.; Su, B. L. Stud. Surf. Sci. Catal. 1999, 125, 253. (22) Su, B. L.; Norberg, V. Langmuir 1998, 14, 2352. (23) Su, B. L.; Norberg, V.; Martens, J. A. Microporous Mesoporous Mater. 1998, 25, 151. (24) Su, B. L.; Norberg, V.; Martens, J. A. Langmuir 2001, 17, 1267.

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displacements. These two displacements can certainly be detected and distinguished with femtosecond techniques. Further study on methylamine molecule motion in zeolites by H2 NMR will be carried out. The above discussion demonstrates that adsorption of methylamine in cationic zeolites is a quite complex system; multiparameters can intervene in their interaction. However, due to the multifunction of methylamine, that is, the lone electron pair on the nitrogen atom and hydrogen atoms of NH2 and CH3 groups, multiple interactions can be monitored. 5. Conclusion The interaction of methylamine with a series of alkaliexchanged faujasite zeolites has been studied. The adsorption of methylamine in zeolites is the result of three interactions. The first one is the electrostatic interaction between the lone electron pair on the nitrogen of methylamine and the counterions of the zeolite framework. This interaction indirectly affects the N-H bond and increases with the Lewis acidity of cations. The two others are a hydrogen-bonding type between the oxygen atoms of zeolite and the hydrogen atoms of the amine and methyl groups of methylamine molecules. The latter interaction

Docquir et al.

becomes more important with increasing the zeolite basicity. However, the overall interaction strength is dominated by interaction between the cations and the lone electron pairs on nitrogen atoms. An additional band has appeared upon the adsorption of methylamine in the range of 2200-1800 cm-1. On the basis of previous studies, this band has been assigned to a modification or a deformation of the zeolite lattice, caused by the presence of adsorbates in the channels of zeolites. Although very small, this deformation has some disturbing and confusing consequences on the adsorptive properties of zeolites and must be taken into consideration when designing the catalysts for concrete applications. Acknowledgment. F. Docquir thanks F.N.R.S. (Fonds National de la Recherche Scientifique) for a F.R.I.A. scholarship. The authors thank Dr. D. Barthomeuf, Professor A. Zecchina, and Dr. C. Lamberti for the fruitful discussion and Dr. J. L. Blin and Mr. A. Le´onard for interesting suggestions. The gift of the Si-rich NaY sample from KUL is also greatly acknowledged. The present work has been performed within the framework of PAI-IUAP 4/10. LA010757J