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Mobility of benzene molecules in NaEMT and KL zeolitic nanostructures studied by 2H NMR spin–lattice relaxation experiments
Chemical Physics Letters 387 (2004) 188–192 www.elsevier.com/locate/cplett
Mobility of benzene molecules in NaEMT and KL zeolitic nanostructures studied by 2H NMR spin–lattice relaxation experiments F. Docquir 1, V. Norberg, B.L. Su
*
Laboratoire de Chimie des Materiaux Inorganiques (CMI), The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium Received 14 November 2003; in final form 30 January 2004 Published online:
Abstract The motions, diffusion coefficients and activation energies of C6 D6 in NaEMT (Ea ¼ 18.0 kJ/mol) and KL (Ea ¼ 7.3 kJ/mol) zeolites have been examined by 2 H NMR technique. Two types of movement of C6 D6 exist. Below 183 K for KL and 263 K for NaEMT, benzene molecule adsorbed on cation rapidly rotates around its sixfold symmetry axis. Above these temperatures, aromatics are in isotropic motion and jump from one adsorption site to another. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Studies of benzene adsorption are of great importance because catalytic processes using zeolites involve often the production and transformation of aromatics. Two elementary steps occur in the catalytic and separation processes based on zeolites: the diffusion of molecules within the cavities of zeolites and their adsorption on active sites. The knowledge of the motions of reactants and products in the pores of zeolites is of interest to understand the activity and selectivity of these solids in catalytic applications and may lead to developing new materials with advanced performance. FTIR [1–7], neutron diffraction [8–10] and NMR [3,10–12] were used to study the dynamics and the location of benzene and other organic molecules in zeolites. In the case of faujasite zeolites, combination of neutron diffraction and NMR studies showed that at low temperatures benzene molecule interacts with cations via p-electron cloud of the aromatic and executes a rapid rotation around its sixfold axis. At higher tem-
*
Corresponding author. Fax: +3281725414. E-mail address: [email protected] (B.L. Su). 1 FRIA fellow. 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.02.019
peratures benzene diffusion from one SII site to another was observed. EMT zeolite is a hexagonal analogue of faujasite. LTL zeolite, contrary to faujasite type zeolites, contains a system of non-interconnecting 12R channels parallel to c-axis. This Letter deals with the mobility of benzene molecules in these two very important large pore zeolites at various temperatures by using 2 H NMR technique. We try to understand at a molecular level the effect of zeolite structure and benzene loading on the motion of benzene molecules by measuring the diffusion coefficients and activation energies. This study is the first relative to the motion of aromatics in EMT zeolitic nanostructure.
2. Experimental KL (K9 Al9 Si27 O72 ) and NaEMT (Na21 Al21 Si75 O192 ) zeolites were provided by Union Carbide and KUL, respectively. 50 mg of powdery samples were packed into NMR tubes (5.0 mm with constrictions) and calcined at 723 K, 8 h in a dry oxygen flow and 4 h in vacuum. The NMR tube containing calcined sample was then cooled to room temperature. The adsorption of known amount of C6 D6 (2.2 and 4.0 molecules per unit
F. Docquir et al. / Chemical Physics Letters 387 (2004) 188–192
cell (m/u.c.) for KL and NaEMT, respectively) was carried out. 2 H NMR spectra were collected on a Bruker MSL400 spectrometer at a frequency of 61.4 MHz. Spinlattice relaxation times (T1 ) were measured using the inversion–recovery method followed by quadrupole echo sequence for observation of the signal, i.e. 180°delay-90°x s 90°y s-echo. The peak separation, i.e. the splitting between the singularities of NMR spectra in kHz is calculated by using Eq. (1) Dm ¼ 3=4DQCC;
ð1Þ
where Deuterium Quadrupole Coupling Constant (DQCC) is the pseudo quadrupolar constant and dependent on the CQ (the static value of the quadrupole interaction) and the angle b between C–D bond direction and the axis of rotation of benzene molecules and can be expressed by Eq. (2) DQCC ¼ 1=2ð3 cos2 b 1ÞCQ :
ð2Þ
The rate of relaxation R1 for a spin I ¼ 1 nucleus, like H, undergoing isotropic motion can be described using the BPP theory [13]
2
1 T1 3 2 2 g2 Q ¼ p v 1þ ð J ðx0 ; sc Þ þ J ð2x0 ; sc ÞÞ; 2 3
R1 ¼
ð3Þ
where v is the quadrupolar coupling constant and J ðx0 ; sc Þ is the spectral density function, which is dependent on the Larmor frequency x0 and the correlation time of the motion sc . For motions with a single correlation time, J ðx0 ; sc Þ may be represented by J ðx0 ; sc Þ ¼ sc =1 þ ðx0 ; sc Þ2 .
189
Activation energies and correlation times were obtained by fitting the temperature-dependent T1 data to the BPP theory assuming isotropic motion. Diffusion coefficients were calculated from sc using the Einstein relationship D ¼ hl2 i=6sc ;
ð4Þ
2
where hl i is the mean square displacement per site exchange jump.
3. Results and discussion The solid-state NMR spectra of nuclei with I > 1/2 are usually dominated by nuclear quadrupole interactions which are very sensitive to the molecular motion which is dependent on the temperature. In the NMR experiment, the three spin states of the 2 H nucleus are split by the static magnetic field [14] and the quadrupole interaction causes an additional shift of these spin energy states. For a powdery sample in which individual grains have an arbitrary orientation with respect to the applied magnetic field, the weighted average of the orientations yields a characteristic ÔhornedÕ spectrum named Pake pattern. In 2 H NMR experiments, molecular motions that are faster than the time scale of the NMR measurements (106 s) lead to reductions in the effective quadrupolar coupling constants [15]. Analysis of these effects provides information about the nature of the molecular motion. The temperature dependence of the 2 H NMR spectra of 2.2 m/u.c. of C6 D6 in KL (A) and of 4 m/u.c. in NaEMT (B) is depicted in Fig. 1. The shape of 2 H NMR spectra is very sensitive to molecular motion and shows
Fig. 1. 2 H NMR spectra of C6 D6 adsorbed in KL zeolite (A) obtained at (a) 153, (b) 163, (c) 183, (d) 193, (e) 213, (f) 223, (g) 243, (h) 263, (i) 283, (j) 303 and (k) 323 K and in NaEMT zeolite (B) at (a) 153, (b) 173, (c) 203, (d) 213, (e) 233, (f) 243, (g) 253, (h) 263, (i) 273, (j) 283, (k) 293 and (l) 323 K.
190
F. Docquir et al. / Chemical Physics Letters 387 (2004) 188–192
more than one type of movement of C6 D6 in the temperature range of 153 to 363 K. For KL (Fig. 1A), below 183 K, an axially symmetric anisotropic powder pattern, typical Pake pattern shape, with two horns predominates and a narrow central peak appears at 183 K and becomes dominating from 243 K. The same evolution was observed for NaEMT (Fig. 1B), only the singlet appears at higher temperature (263 K). The static value of the quadrupole interaction for C6 D6 is 183 10 kHz [16]. The peak separation is measured from Fig. 1 and is around 67 kHz for KL and 68 kHz for NaEMT. The effective quadrupolar constant is therefore obtained from Eq. (1) to be around 89 kHz both for KL and NaEMT. This value is half the magnitude of that in the static molecule. From Eq. (2), the angle between the gradient direction and the reorientation axis is b ¼ 90°. This means that the axis of spinning is perpendicular to the principal direction of the electric field gradients at the 2 H nuclei and that C6 D6 molecules in KL and NaEMT at the temperatures lower than 183 and 263 K, respectively, are mainly spinning in their molecular plane, i.e. about the sixfold axis. This mode of motion is consistent with the behaviour predicted on a structural study of KL-benzene system by neutron diffraction [17]. It was reported that benzene molecules exclusively adsorb on Kþ ions in KL and Naþ ions in NaEMT and 12R windows in both zeolites are not preferential sites for benzene [18,19]. For the lowest temperatures the dominant interaction is apparently cations-p electron cloud of benzene. Benzene molecules are then in rotation about the sixfold axis. The angle b between C–D bond direction and the axis of rotation of benzene molecules is thus equal to 90°. From 183 K for KL and 273 K for NaEMT to higher temperatures, there is a narrow central component which corresponds to highly mobile species of C6 D6 which jump from one adsorption site to another. With respect to the NMR time scale, benzene molecules are in isotropic motion which consists of the place exchanges among the cation sites. This means that at higher temperatures, the thermal agitation enables to benzene molecule to jump between adsorption sites at a rate that is sufficiently fast to average the quadrupolar tensor completely on the >106 s time scale of the NMR line shape experiment. In the temperature range of 183–243 K for KL while of 263–283 K, the rapid rotation of benzene molecule around its sixfold axis and its jump from one site to another coexist. The evolution of T1 is presented in Fig. 2. The curve obtained with benzene/KL (Fig. 2A) clearly shows a minimum which allows us to use the BPP theory to determine the activation energy and correlation times from Eq. (3). Diffusion coefficients can also be calculated from the correlation time sc using the Einstein relationship (4) which is the with a mean square displacement of 7 A, distance between two Kþ ions in 12R channels. How-
Fig. 2. Variation of spin–lattice relaxation time (T1 ) with temperature for C6 D6 in KL (A) and NaEMT (B) zeolites.
ever, the evolution of T1 of C6 D6 in NaEMT (Fig. 2B) shows two minima corresponding to two different movements. For higher temperatures, the minimum can be attributed to isotropic motion of C6 D6 and that for lower temperatures to the fixation of benzene on cations and its rotation around the sixfold axis. To calculate the activation energy of the isotropic motion, only the left part of the curve, corresponding to this motion, was used i.e. the mean with a mean square displacement of 5 A, distance between two Naþ ions, in a large cage. The data were fitted to the BPP theory (Eq. 3). All the extracted parameters are summarized in Table 1. Table 1 Results obtained using the BPP theory on relaxation time T1 for 2.2 and 4.0 molecules of C6 D6 per unit cell adsorbed in KL and NaEMT zeolites, respectively
Ea (J/mol) E0 (m2 /s) D298 (m2 /s) s0 (s) s298 (s)
KL
NaEMT
7 261 3.9 109 2.1 1010 4.3 1011 8.0 1010
18 003 5.84 109 4.08 1012 7.14 1012 1.02 108
F. Docquir et al. / Chemical Physics Letters 387 (2004) 188–192
Fig. 3. Evolution of diffusion coefficient of C6 D6 in KL and NaEMT zeolites versus 1/T.
The variation of the diffusion coefficient with temperatures for KL and NaEMT zeolites is shown in Fig. 3. We note that benzene molecules diffuse more easily in KL zeolite than in NaEMT, being in agreement with the appearance of the narrow central component at higher temperature (263 K) for NaEMT compared to 183 K for KL. This can be explained by a more important mean distance between two adsorption sites in KL. Comparing with the results obtained with a loading of 1.0 m/u.c. of C6 D6 in KL that there was no narrow central component in the temperature range of 150 to 350 K [20], while our experiments show that with a loading of 2.2 m/u.c., the narrow central component of an isotropic motion appears indeed from 183 K. A loading of 2.2 m/u.c. of C6 D6 corresponds to the saturation of KL zeolite by benzene [1]. This means that at higher loading, benzene molecules jump easier from one site to another. As a consequence, the activation energy of diffusion must be lower for 2.2 m/u.c. (Ea ¼ 7:3 kJ/ mol) than for 1.0 m/u.c. (Ea ¼ 20 kJ/mol). However, Bull et al. [11] reported that high benzene loading reduces the mobility of benzene molecules within the cavity of NaX and NaY zeolites. The presence of straight channels in KL zeolite should result in this discrepancy in faujasites and KL zeolites. It was found that the central component appeared at 165 K for NaX (Si/Al ¼ 1.2) and 265 K for NaY (Si/ Al ¼ 2.4) for a benzene loading of 1.0 m/u.c. [11]. Since one unit cell of EMT contains 96 T atoms while 192 in FAU structures. In a unit cell of EMT structure, there are 4 large cages. The loading of 4.0 m/u.c. in NaEMT can be considered as 1.0 molecule of benzene per large cage, similar to the loading in NaX and NaY. By comparison of results observed on NaEMT, NaX and NaY, i.e. the temperature of the appearance of the central peak, the diffusion of benzene molecules in NaEMT should be more difficult than in NaX but
191
similar to that in NaY in spite of the presence of straight channels in NaEMT which could favour the diffusion of benzene molecules. The adsorption strength of benzene on zeolites decreases with the Si/Al ratio for zeolites with the same structure, NaX (Si/Al ¼ 1.2) > NaY (Si/Al ¼ 2.4) > NaY (Si/Al ¼ 2.7) and the mobility of benzene was found to increase with increasing Si/Al ratio of zeolite [5]. The mobility of benzene in NaX can be expected to be lower than that in NaY. However, the results are contrary to expectation. The high benzene mobility found in NaX can be attributed to the non-localisable sodium cations in SIII sites which increase the number of adsorption sites and reduce the height of the potential barrier between them. As the Si/Al ratio of NaEMT (3.6) is higher than that of NaY (Si/Al ¼ 2.4), the mobility of benzene can also be expected to be higher than in NaY. However, similar mobility of benzene molecules is observed in these two zeolites. The lower mobility of benzene in NaEMT compared to expectation should be very likely due to the non-localisable sodium cations which migrate from small cages towards the large cages and are finally located near to the 12R windows upon benzene adsorption [21]. The location of cations near 12R windows can highly reduce the ease of benzene diffusion from one cage to another. Furthermore, the 12R windows in NaEMT are not adsorption sites for benzene. This reduces the number of adsorption sites compared to that in NaY and increases the height of the potential barrier between adsorption sites and consequently reduces the benzene mobility. In conclusion, in KL and NaEMT zeolites, at low temperatures, benzene rapidly rotates around its sixfold symmetry axis. At higher temperatures, an intracrystalline diffusion of benzene molecules, i.e. a rapid jump from one adsorption site to other is observed.
Acknowledgements F. Docquir thanks FNRS (Fonds National de la Recherche Scientifique, Belgium) for a FRIA scholarship. This Letter was performed within the framework of PAI-IUAP 5/1.
References [1] B.L. Su, D. Barthomeuf, Zeolites 15 (1995) 470. [2] A. de Mallmann, D. Barthomeuf, Zeolites 8 (1988) 292. [3] F. Gilles, J.L. Blin, H. Toufar, B.L. Su, Stud. Surf. Sci. Catal. 142 (2002) 1687. [4] B.L. Su, V. Norberg, J.A. Martens, Langmuir 17 (2001) 1267. [5] B.L. Su, F. Docquir, Langmuir 17 (2001) 3341. [6] B.L. Su, V. Norberg, Colloids Surf. A 187–188 (2001) 273. [7] B.L. Su, V. Norberg, Colloid. Surf. A 187–188 (2001) 297.
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[8] A.N. Fitch, H. Jobic, A. Renouprez, J. Phys. Chem. 90 (1986) 1311. [9] H. Jobic, A. Renouprez, A.N. Fitch, H.J. Hauter, J. Chem. Soc., Faraday Trans. 1 83 (1987) 3199. [10] A.G. Stepanov, T.O. Shegai, M.V. Luzgin, H. Jobic, Eur. Phys. J. E 12 (2003) 57. [11] L.M. Bull, N.J. Henson, A.K. Cheetham, J.M. Newsam, S.J. Heyes, J. Phys. Chem. 97 (1993) 11776. [12] H. Lechert, K.P. Wittern, Ber. Bunsen-Ges. Phys. Chem. 82 (1978) 1054. [13] N. Bloembergen, E.M. Purcell, R.V. Pound, Phys. Rev. 73 (1948) 679.
[14] A. Abragam, The Principles of Nuclear Magnetism, Oxford University Press, Oxford, UK, 1961 (Chapter 6). [15] C.H. Townes, B.P. Dailey, J. Chem. Phys. 20 (1952) 35. [16] F.S. Millet, B.P. Dailey, J. Chem. Phys. 56 (1972) 3249. [17] J.M. Newsam, B.G. Silbernagel, A.R. Garcia, R. Hulme, Chem. Commun. (1987) 664. [18] B.L. Su, J.M. Manoli, C. Potvin, D. Barthomeuf, J. Chem. Soc., Faraday Trans. 89 (1993) 857. [19] B.L. Su, D. Barthomeuf, Appl. Catal. A 124 (1995) 81. [20] B.G. Silbernagel, A.R. Garcia, J.M. Newsam, R. Hulme, J. Phys. Chem. 93 (1989) 6506. [21] B.L. Su, J. Chem. Soc., Faraday Trans. 93 (1997) 1449.
Mobility of benzene molecules in NaEMT and KL zeolitic nanostructures studied by 2H NMR spin–lattice relaxation experiments F. Docquir 1, V. Norberg, B.L. Su
*
Laboratoire de Chimie des Materiaux Inorganiques (CMI), The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium Received 14 November 2003; in final form 30 January 2004 Published online:
Abstract The motions, diffusion coefficients and activation energies of C6 D6 in NaEMT (Ea ¼ 18.0 kJ/mol) and KL (Ea ¼ 7.3 kJ/mol) zeolites have been examined by 2 H NMR technique. Two types of movement of C6 D6 exist. Below 183 K for KL and 263 K for NaEMT, benzene molecule adsorbed on cation rapidly rotates around its sixfold symmetry axis. Above these temperatures, aromatics are in isotropic motion and jump from one adsorption site to another. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Studies of benzene adsorption are of great importance because catalytic processes using zeolites involve often the production and transformation of aromatics. Two elementary steps occur in the catalytic and separation processes based on zeolites: the diffusion of molecules within the cavities of zeolites and their adsorption on active sites. The knowledge of the motions of reactants and products in the pores of zeolites is of interest to understand the activity and selectivity of these solids in catalytic applications and may lead to developing new materials with advanced performance. FTIR [1–7], neutron diffraction [8–10] and NMR [3,10–12] were used to study the dynamics and the location of benzene and other organic molecules in zeolites. In the case of faujasite zeolites, combination of neutron diffraction and NMR studies showed that at low temperatures benzene molecule interacts with cations via p-electron cloud of the aromatic and executes a rapid rotation around its sixfold axis. At higher tem-
*
Corresponding author. Fax: +3281725414. E-mail address: [email protected] (B.L. Su). 1 FRIA fellow. 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.02.019
peratures benzene diffusion from one SII site to another was observed. EMT zeolite is a hexagonal analogue of faujasite. LTL zeolite, contrary to faujasite type zeolites, contains a system of non-interconnecting 12R channels parallel to c-axis. This Letter deals with the mobility of benzene molecules in these two very important large pore zeolites at various temperatures by using 2 H NMR technique. We try to understand at a molecular level the effect of zeolite structure and benzene loading on the motion of benzene molecules by measuring the diffusion coefficients and activation energies. This study is the first relative to the motion of aromatics in EMT zeolitic nanostructure.
2. Experimental KL (K9 Al9 Si27 O72 ) and NaEMT (Na21 Al21 Si75 O192 ) zeolites were provided by Union Carbide and KUL, respectively. 50 mg of powdery samples were packed into NMR tubes (5.0 mm with constrictions) and calcined at 723 K, 8 h in a dry oxygen flow and 4 h in vacuum. The NMR tube containing calcined sample was then cooled to room temperature. The adsorption of known amount of C6 D6 (2.2 and 4.0 molecules per unit
F. Docquir et al. / Chemical Physics Letters 387 (2004) 188–192
cell (m/u.c.) for KL and NaEMT, respectively) was carried out. 2 H NMR spectra were collected on a Bruker MSL400 spectrometer at a frequency of 61.4 MHz. Spinlattice relaxation times (T1 ) were measured using the inversion–recovery method followed by quadrupole echo sequence for observation of the signal, i.e. 180°delay-90°x s 90°y s-echo. The peak separation, i.e. the splitting between the singularities of NMR spectra in kHz is calculated by using Eq. (1) Dm ¼ 3=4DQCC;
ð1Þ
where Deuterium Quadrupole Coupling Constant (DQCC) is the pseudo quadrupolar constant and dependent on the CQ (the static value of the quadrupole interaction) and the angle b between C–D bond direction and the axis of rotation of benzene molecules and can be expressed by Eq. (2) DQCC ¼ 1=2ð3 cos2 b 1ÞCQ :
ð2Þ
The rate of relaxation R1 for a spin I ¼ 1 nucleus, like H, undergoing isotropic motion can be described using the BPP theory [13]
2
1 T1 3 2 2 g2 Q ¼ p v 1þ ð J ðx0 ; sc Þ þ J ð2x0 ; sc ÞÞ; 2 3
R1 ¼
ð3Þ
where v is the quadrupolar coupling constant and J ðx0 ; sc Þ is the spectral density function, which is dependent on the Larmor frequency x0 and the correlation time of the motion sc . For motions with a single correlation time, J ðx0 ; sc Þ may be represented by J ðx0 ; sc Þ ¼ sc =1 þ ðx0 ; sc Þ2 .
189
Activation energies and correlation times were obtained by fitting the temperature-dependent T1 data to the BPP theory assuming isotropic motion. Diffusion coefficients were calculated from sc using the Einstein relationship D ¼ hl2 i=6sc ;
ð4Þ
2
where hl i is the mean square displacement per site exchange jump.
3. Results and discussion The solid-state NMR spectra of nuclei with I > 1/2 are usually dominated by nuclear quadrupole interactions which are very sensitive to the molecular motion which is dependent on the temperature. In the NMR experiment, the three spin states of the 2 H nucleus are split by the static magnetic field [14] and the quadrupole interaction causes an additional shift of these spin energy states. For a powdery sample in which individual grains have an arbitrary orientation with respect to the applied magnetic field, the weighted average of the orientations yields a characteristic ÔhornedÕ spectrum named Pake pattern. In 2 H NMR experiments, molecular motions that are faster than the time scale of the NMR measurements (106 s) lead to reductions in the effective quadrupolar coupling constants [15]. Analysis of these effects provides information about the nature of the molecular motion. The temperature dependence of the 2 H NMR spectra of 2.2 m/u.c. of C6 D6 in KL (A) and of 4 m/u.c. in NaEMT (B) is depicted in Fig. 1. The shape of 2 H NMR spectra is very sensitive to molecular motion and shows
Fig. 1. 2 H NMR spectra of C6 D6 adsorbed in KL zeolite (A) obtained at (a) 153, (b) 163, (c) 183, (d) 193, (e) 213, (f) 223, (g) 243, (h) 263, (i) 283, (j) 303 and (k) 323 K and in NaEMT zeolite (B) at (a) 153, (b) 173, (c) 203, (d) 213, (e) 233, (f) 243, (g) 253, (h) 263, (i) 273, (j) 283, (k) 293 and (l) 323 K.
190
F. Docquir et al. / Chemical Physics Letters 387 (2004) 188–192
more than one type of movement of C6 D6 in the temperature range of 153 to 363 K. For KL (Fig. 1A), below 183 K, an axially symmetric anisotropic powder pattern, typical Pake pattern shape, with two horns predominates and a narrow central peak appears at 183 K and becomes dominating from 243 K. The same evolution was observed for NaEMT (Fig. 1B), only the singlet appears at higher temperature (263 K). The static value of the quadrupole interaction for C6 D6 is 183 10 kHz [16]. The peak separation is measured from Fig. 1 and is around 67 kHz for KL and 68 kHz for NaEMT. The effective quadrupolar constant is therefore obtained from Eq. (1) to be around 89 kHz both for KL and NaEMT. This value is half the magnitude of that in the static molecule. From Eq. (2), the angle between the gradient direction and the reorientation axis is b ¼ 90°. This means that the axis of spinning is perpendicular to the principal direction of the electric field gradients at the 2 H nuclei and that C6 D6 molecules in KL and NaEMT at the temperatures lower than 183 and 263 K, respectively, are mainly spinning in their molecular plane, i.e. about the sixfold axis. This mode of motion is consistent with the behaviour predicted on a structural study of KL-benzene system by neutron diffraction [17]. It was reported that benzene molecules exclusively adsorb on Kþ ions in KL and Naþ ions in NaEMT and 12R windows in both zeolites are not preferential sites for benzene [18,19]. For the lowest temperatures the dominant interaction is apparently cations-p electron cloud of benzene. Benzene molecules are then in rotation about the sixfold axis. The angle b between C–D bond direction and the axis of rotation of benzene molecules is thus equal to 90°. From 183 K for KL and 273 K for NaEMT to higher temperatures, there is a narrow central component which corresponds to highly mobile species of C6 D6 which jump from one adsorption site to another. With respect to the NMR time scale, benzene molecules are in isotropic motion which consists of the place exchanges among the cation sites. This means that at higher temperatures, the thermal agitation enables to benzene molecule to jump between adsorption sites at a rate that is sufficiently fast to average the quadrupolar tensor completely on the >106 s time scale of the NMR line shape experiment. In the temperature range of 183–243 K for KL while of 263–283 K, the rapid rotation of benzene molecule around its sixfold axis and its jump from one site to another coexist. The evolution of T1 is presented in Fig. 2. The curve obtained with benzene/KL (Fig. 2A) clearly shows a minimum which allows us to use the BPP theory to determine the activation energy and correlation times from Eq. (3). Diffusion coefficients can also be calculated from the correlation time sc using the Einstein relationship (4) which is the with a mean square displacement of 7 A, distance between two Kþ ions in 12R channels. How-
Fig. 2. Variation of spin–lattice relaxation time (T1 ) with temperature for C6 D6 in KL (A) and NaEMT (B) zeolites.
ever, the evolution of T1 of C6 D6 in NaEMT (Fig. 2B) shows two minima corresponding to two different movements. For higher temperatures, the minimum can be attributed to isotropic motion of C6 D6 and that for lower temperatures to the fixation of benzene on cations and its rotation around the sixfold axis. To calculate the activation energy of the isotropic motion, only the left part of the curve, corresponding to this motion, was used i.e. the mean with a mean square displacement of 5 A, distance between two Naþ ions, in a large cage. The data were fitted to the BPP theory (Eq. 3). All the extracted parameters are summarized in Table 1. Table 1 Results obtained using the BPP theory on relaxation time T1 for 2.2 and 4.0 molecules of C6 D6 per unit cell adsorbed in KL and NaEMT zeolites, respectively
Ea (J/mol) E0 (m2 /s) D298 (m2 /s) s0 (s) s298 (s)
KL
NaEMT
7 261 3.9 109 2.1 1010 4.3 1011 8.0 1010
18 003 5.84 109 4.08 1012 7.14 1012 1.02 108
F. Docquir et al. / Chemical Physics Letters 387 (2004) 188–192
Fig. 3. Evolution of diffusion coefficient of C6 D6 in KL and NaEMT zeolites versus 1/T.
The variation of the diffusion coefficient with temperatures for KL and NaEMT zeolites is shown in Fig. 3. We note that benzene molecules diffuse more easily in KL zeolite than in NaEMT, being in agreement with the appearance of the narrow central component at higher temperature (263 K) for NaEMT compared to 183 K for KL. This can be explained by a more important mean distance between two adsorption sites in KL. Comparing with the results obtained with a loading of 1.0 m/u.c. of C6 D6 in KL that there was no narrow central component in the temperature range of 150 to 350 K [20], while our experiments show that with a loading of 2.2 m/u.c., the narrow central component of an isotropic motion appears indeed from 183 K. A loading of 2.2 m/u.c. of C6 D6 corresponds to the saturation of KL zeolite by benzene [1]. This means that at higher loading, benzene molecules jump easier from one site to another. As a consequence, the activation energy of diffusion must be lower for 2.2 m/u.c. (Ea ¼ 7:3 kJ/ mol) than for 1.0 m/u.c. (Ea ¼ 20 kJ/mol). However, Bull et al. [11] reported that high benzene loading reduces the mobility of benzene molecules within the cavity of NaX and NaY zeolites. The presence of straight channels in KL zeolite should result in this discrepancy in faujasites and KL zeolites. It was found that the central component appeared at 165 K for NaX (Si/Al ¼ 1.2) and 265 K for NaY (Si/ Al ¼ 2.4) for a benzene loading of 1.0 m/u.c. [11]. Since one unit cell of EMT contains 96 T atoms while 192 in FAU structures. In a unit cell of EMT structure, there are 4 large cages. The loading of 4.0 m/u.c. in NaEMT can be considered as 1.0 molecule of benzene per large cage, similar to the loading in NaX and NaY. By comparison of results observed on NaEMT, NaX and NaY, i.e. the temperature of the appearance of the central peak, the diffusion of benzene molecules in NaEMT should be more difficult than in NaX but
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similar to that in NaY in spite of the presence of straight channels in NaEMT which could favour the diffusion of benzene molecules. The adsorption strength of benzene on zeolites decreases with the Si/Al ratio for zeolites with the same structure, NaX (Si/Al ¼ 1.2) > NaY (Si/Al ¼ 2.4) > NaY (Si/Al ¼ 2.7) and the mobility of benzene was found to increase with increasing Si/Al ratio of zeolite [5]. The mobility of benzene in NaX can be expected to be lower than that in NaY. However, the results are contrary to expectation. The high benzene mobility found in NaX can be attributed to the non-localisable sodium cations in SIII sites which increase the number of adsorption sites and reduce the height of the potential barrier between them. As the Si/Al ratio of NaEMT (3.6) is higher than that of NaY (Si/Al ¼ 2.4), the mobility of benzene can also be expected to be higher than in NaY. However, similar mobility of benzene molecules is observed in these two zeolites. The lower mobility of benzene in NaEMT compared to expectation should be very likely due to the non-localisable sodium cations which migrate from small cages towards the large cages and are finally located near to the 12R windows upon benzene adsorption [21]. The location of cations near 12R windows can highly reduce the ease of benzene diffusion from one cage to another. Furthermore, the 12R windows in NaEMT are not adsorption sites for benzene. This reduces the number of adsorption sites compared to that in NaY and increases the height of the potential barrier between adsorption sites and consequently reduces the benzene mobility. In conclusion, in KL and NaEMT zeolites, at low temperatures, benzene rapidly rotates around its sixfold symmetry axis. At higher temperatures, an intracrystalline diffusion of benzene molecules, i.e. a rapid jump from one adsorption site to other is observed.
Acknowledgements F. Docquir thanks FNRS (Fonds National de la Recherche Scientifique, Belgium) for a FRIA scholarship. This Letter was performed within the framework of PAI-IUAP 5/1.
References [1] B.L. Su, D. Barthomeuf, Zeolites 15 (1995) 470. [2] A. de Mallmann, D. Barthomeuf, Zeolites 8 (1988) 292. [3] F. Gilles, J.L. Blin, H. Toufar, B.L. Su, Stud. Surf. Sci. Catal. 142 (2002) 1687. [4] B.L. Su, V. Norberg, J.A. Martens, Langmuir 17 (2001) 1267. [5] B.L. Su, F. Docquir, Langmuir 17 (2001) 3341. [6] B.L. Su, V. Norberg, Colloids Surf. A 187–188 (2001) 273. [7] B.L. Su, V. Norberg, Colloid. Surf. A 187–188 (2001) 297.
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[8] A.N. Fitch, H. Jobic, A. Renouprez, J. Phys. Chem. 90 (1986) 1311. [9] H. Jobic, A. Renouprez, A.N. Fitch, H.J. Hauter, J. Chem. Soc., Faraday Trans. 1 83 (1987) 3199. [10] A.G. Stepanov, T.O. Shegai, M.V. Luzgin, H. Jobic, Eur. Phys. J. E 12 (2003) 57. [11] L.M. Bull, N.J. Henson, A.K. Cheetham, J.M. Newsam, S.J. Heyes, J. Phys. Chem. 97 (1993) 11776. [12] H. Lechert, K.P. Wittern, Ber. Bunsen-Ges. Phys. Chem. 82 (1978) 1054. [13] N. Bloembergen, E.M. Purcell, R.V. Pound, Phys. Rev. 73 (1948) 679.
[14] A. Abragam, The Principles of Nuclear Magnetism, Oxford University Press, Oxford, UK, 1961 (Chapter 6). [15] C.H. Townes, B.P. Dailey, J. Chem. Phys. 20 (1952) 35. [16] F.S. Millet, B.P. Dailey, J. Chem. Phys. 56 (1972) 3249. [17] J.M. Newsam, B.G. Silbernagel, A.R. Garcia, R. Hulme, Chem. Commun. (1987) 664. [18] B.L. Su, J.M. Manoli, C. Potvin, D. Barthomeuf, J. Chem. Soc., Faraday Trans. 89 (1993) 857. [19] B.L. Su, D. Barthomeuf, Appl. Catal. A 124 (1995) 81. [20] B.G. Silbernagel, A.R. Garcia, J.M. Newsam, R. Hulme, J. Phys. Chem. 93 (1989) 6506. [21] B.L. Su, J. Chem. Soc., Faraday Trans. 93 (1997) 1449.