Dynamics of xylene molecules in zeolite NaY

261

Microporous Materials, 2 (1994) 261-267 Elsevier Science B.V., Amsterdam

Dynamics of xylene molecules in zeolite NaY C. Kirschhock and H. Fuess* Fachgebiet Strukturforschung,

Fachbereich

Materialwissenschaf,

Technische Hochschule Darmstadt. Petersenstrasse

20.64287

Darmstadt, Germany

(Received 19 March 1993; accepted 3 November 1993)

The dynamics of p and m-xylene in the cavities of zeolite NaY has been studied between 77 and 300 K. Three temperature regions with different types of motion are distinguished by analysis of the second moment of ‘H nuclear magnetic resonance (NMR) signals. At temperatures up to 125 K the molecules are adsorbed in front of the SW Na cation. Between 125 and 250 K experimental data are best described by a tetrahedral jump model. Above 250 K enhanced mobility of the arenes is explained as continuous diffusion. Keywords:

xylene; zeolite NaY, nuclear magnetic resonance; dynamics; guest-host interaction

Introduction One of the most outstanding features of zeolites is their ability to host molecules in their cavity systems. Besides the highly polar inner surface and the acid properties of these aluminosilicates their catalytic activity is conditioned by the definite space and geometry inside the cavities. In order to improve the microscopic understanding of the mutual effects during catalytic processes the motion of the adsorbed molecules inside the host lattice has to be considered. Zeolite Y offers supercages about 1.27 nm in diameter and a pore opening of 0.78 nm. These faujasites are widely used in the isomerisation and separation of alkylbenzenes [l J. Adsorption sites and possible movements of these substances are therefore of a high technical interest. Xylene molecules especially should show an interesting dynamic behaviour due to their comparable small size (-0.5 nm) and their tlat geometry. The motion of one p- and m-xylene molecule and two p-xylene molecules per supercage of zeolite Y has been * Corresponding author. 0927-6513/94/$7.00 0 1994 - Elsevier Science B.V. All rights reserved. SSDI 0927-65 13(93)E006 1-K

analysed by ‘H NMR techniques. The mobility of these guest molecules has been investigated by means of the second moment of their ‘H NMR signals. This method [2-61 permits an investigation of the motion of an arrangement of magnetic dipoles such as hydrogen nuclei in organic molecules. In comparison to other spectroscopic methods NMR does not perturb the investigated system, which remains in thermal equilibrium with respect to its state of motion. In this respect, NMR experiments correspond to quasi-elastic neutron scattering. However, both methods have complementary time scales. Quasi-elastic neutron spectroscopy is concerned with the analysis of the inelastic broadening of Bragg reflections [7]. The observed changes in energy are of the order of a few millielectron volts, corresponding to motions with characteristic times shorter than lo-’ s. Oscillations, librations and fast rotational motions are therefore readily detected. In an earlier investigation of the xylene isomers in NaY by means of quasi-elastic neutron scattering a very fast libration of the molecules was detected [S]. Accordingly, investigation of the second moments of these systems should complement the earlier results on a

262

longer time scale, observed with NMR As will be shown in of the results of both the integral dynamic in zeolite Y.

C. Kirschhock and H. Fuess / Microporous Mater. 2 (1994) 261-267

as typical correlation times are of the order 10-9-10-6 s. the discussion a combination independent methods renders behaviour of xylene molecules

Experimental Samples of zeolite Y with a Si/Al ratio of 2.5 were activated in a deep bed by heating slowly up to 520 K with subsequent cooling to room temperature in a vacuum-tight glass apparatus at pressures below lo-’ Pa. Heating rates were: 30 K/h up to 360 K, 15 K/h up to 390 K and 60 K/h up to 520 K. The liquid aromatic molecules were introduced into the host lattice by means of a vacuum tight stopcock system. The vapour pressure of the liquids was sufficient for a complete uptake of the guest molecules. Finally, the samples were enclosed in glass ampoules, fused to the activation equipment. Measurements were carried out on a wide-line spectrometer with a fixed high frequency of 8.2 MHz and variable magnetic field. A temperature range of 77-300 K was covered, and cooling was achieved by nitrogen evaporation. The data obtained were fitted by the simplex method [9] with an expression composed of Gauss and Lorentz components [lo]. The resulting function served to calculate the experimental second moment. Increasing mobility causes a narrowing of the resonance lines, which is mathematically expressed by the second moment:

with K = th2 for S = Z and K = +h2 for S #I. The left side of this equation represents the mathematical description of the second moment. The distribution function f(H) describes the measured intensities at a given magnetic field of value Z-Z.The centre of the resonance line is indicated by Ho. Correlation with dynamic processes is achieved by the calcula-

tion of theoretical values for the second moment regarding the possible motion of the protons and comparison with the experimental values. This is expressed by the right side of the above equation, where index S denotes the nucleus influencing with its spin .Zs the magnetic field at the observed nucleus I. As this work is concerned with ‘H NMR, index Z always denotes protons, whereas possible other nuclei that take part in the dipolar interaction are 23Na and 27Al belonging to the host. The local field at the nuclei is determined by the gyromagnetic ratios ys and yl, which furthermore influence the resonance condition for the interacting partners. In the case of non-equivalent interacting nuclei, e.g. 27Al interacting with ‘H, the constant K is considerably smaller, because for 27Al the resonance condition is not fulfilled. The summation takes into account the geometrical arrangement of the nuclei investigated. The parameter rk,l is the vector connecting nucleus k with nucleus 1and 0 is the angle included by the vector and the direction of the magnetic field. The motion of the molecules has to be taken into account by averaging this sum over the trajectory of the vectors between the NMR-active nuclei.

Results The second moment as function of temperature (Figs. 1 and 2) displays for each sample three temperature regions which correspond to different modes of movement and, therefore, require separate theoretical treatment. Furthermore, a comparison between the results of one and two p-xylene molecules reveals that intermolecular interaction of the magnetic dipoles may be neglected. Within experimental error the two experimental curves are congruent between 50 and 200 K. Below 125 K the second moment of both guest molecules can be associated with fixed molecules adsorbed near a Na+ cation in front of a six-ring of the supercage as reported by various authors [ 1l-l 33. Up to this temperature, all motions except the rotation of the methyl groups are suspended. This rotational motion has been assumed to be faster than the NMR time scale which is sensitive for dynamic processes slower than 10m9 s.

263

C. Kirschhock and H. Fuess 1 Microporous Mater. 2 (1994) 261-267

temperature

/

K

Fig. 1. Second moments of one (circles) and two (triangles) p-xylene molecules as a function of temperature. The curves differ only slightly. Two changes of dynamical behaviour can be detected. At 125 K a decrease of second moment takes place simultanuously for both samples. The second change of motional behaviour occurs at 210 K for the sample with two molecules per supercage and at 220 K for the sample with the lower coverage.

50

100

150

temperature

200

/

250

300

K

Fig. 2. Second moment of one m-xylene molecule per supercage exhibiting a behaviour similar to the experimental data of p-xylene. Again three temperature regions with differing molecular motions can be distinguished. The variation of line width of the.NMR signals is displayed on the upper right.

C. Kirschhock and H. Fuess / Microporous Mater. 2 (1994) 261-267

264

Therefore the average of the geometrical part in eqn. 1 was calculated for 3600 different orientations of each methyl group and used for the calculation of the second moment. Furthermore, the resting centre of gravity of the molecule indicates that a nuclear magnetic dipole interaction of the protons with the nearest Na and Al nuclei of the host lattice may not be neglected [14]. To evaluate these contributions to the second moment, the distances of sodium cations and tetrahedral sites of the lattice to the molecular protons were calculated, based on crystallographic data of xylenes in NaY, known from neutron diffraction [12]. The resulting good agreement between experiment and theory (Fig. 3) confirms the model of strongly adsorbed fixed molecules. In an intermediate temperature range (125-175 K) both xylene isomers exhibit a drastic decrease of their second moment (Figs. 1 and 2). For an explanation of the experimental values of 1.3. IO-* T2 (m-xyiene) and I.O*lO-* T2 (p-xylene) a setting in of rotation has been assumed. The second moments for xylene molecules rotating around different axes have consequently been computed. While doing so, a suitable motional average of the space-dependent part of eqn. 1 has to be obtained. The angle 0 is therefore split into an angle B which describes the orientation of the rotation axis with respect to the orientation of the external magnetic field and an angle r which is determined by the orientation of the vector rk,i to the rotation axis (Fig, 4) [4,5]. The statistical distribution of orientation of the crystallites in the sample leads to an average of the b-dependent part. By selecting the rotation axis the valid equation can be readily computed (eqn. 2). $(H-H0)2f(H)

para-xylene

meta-xylene

hH-lH

*

5.9 10-8T2

5 .9 10-8T2

*H-2”’ t!h

!!iq%$ i 0 . 1 10‘8T2

0.3 lo-*T2

.$g$Jxgr2?YJ?+z& T

.,

T

T 0.6lnfq T

&%q: o,4 i O.Slnm

1

‘, 0.65ill$l

,T T

T

0.3 lo-*T2

T

fhlmt 0.4

: b.s%n! I

’

0.2 lo-’ T 2

sum 6.5 10” T 2

6.2 lo-‘T2 I

measured 7.0 lo-’ T 2

6.5 1O-8T 2

d

Fig. 3. Contributions to the second moment of the observed molecules at low temperatures. Distances of the protons (empty circles) to the framework nuclei are indicated.

dH Best agreement between experimental and theoretical [4-61 (Fig. 5) data has been achieved with rotation axes in the aromatic plane which divides the molecule between the two methyl groups. The steric situation inside the supercage requires the molecules to depart from their adsorption sites during the corresponding rotation. This necessary translational movement cannot be detected by the method used, because only the angles between the vectors connecting each pair of protons and the

Nk.t(1-3;:s2r)

=&y;h2z&IH + l)'C

Magnetic interaction between protons and nuclei of the host lattice was not taken into account. Due to the relative motion of the molecule and the lattice and the weak dipolar interaction between non-identical nuclei, these contributions proved to be cu. 5% smaller than the experimental error.

C. Kirschhock and H. Fuess / Microporous Mater. 2 (1994) 261-267

axis

Fig. 4. Illustration of vectors and angles used to calculate the second moment. Note that a parallel displacement of the axis does not change the values needed.

para-xylene AXiS

2%-t I104

meta-xylene Axis

2-’ Moment IOT

1.12

2.42

2nd Moment as measured I 1OT

1.0

1.3

Fig. 5. Comparison between the second moments calculated for different rotation axes and the values measured.

265

rotational axis are relevant ‘for calculating the second moment. Therefore, rotation around any axis in the same plane as the molecule and parallel to the previously described axis has to be considered. But any rotation about an axis outside the centre of gravity of the molecule results in additional translation of the molecule. As a consequence the rotational motion is in agreement with the steric requirements in the supercage. With further increasing temperature the second moment continues to decrease. The signals are getting more and more characteristics of liquidlike systems [ 151, indicating a diffusion throughout the zeolitic cavity system.

Discussion

The results reported here complement earlier measurements [S] of the dynamic properties of xylenes in Nay. Quasi-elastic neutron spectroscopy revealed modes of motion on a time scale faster than lo-’ s. The xylene molecules are adsorbed in front of a sodium cation in the supercage. At temperatures higher than 150 K they oscillate around their pseudo six-fold axes with a rate higher than lo- l1 s [8]. NMR investigations allow access to slower processes. As already mentioned a combination of both methods yields a complete description of molecular movement: the molecules perform a fast change of orientation, as shown by neutron spectroscopy. This motion is the prerequisite for the rotation, determined by NMR. As this rotation is accompanied by a separation from the adsorption site though, on the average, the molecules are still located in front of a six ring in the tetrahedral supercage, the only possible conclusion is that the starting and the end point of the rotation are crystallographically equivalent sites. To meet this requirement only few orientations of the molecule allow its rotation at all. The mechanism providing the molecules in the necessary steric situation is the libration, which takes place on a much higher time scale than the rotation. A strong hint that both modes of motion are interdependent is that the change of adsorption sites ceases as soon’ as the oscillation stops at 150 K. In brief, the xylene molecules perform tetra-

266

hedral jumps [14] (Fig. 6). Each jump is accompanied by a rotation through 70”32 around a rotational axis which is the intersection of the molecular planes before and after the change of adsorption site. Without knowledge of the fast libration of the molecules, the NMR results cannot account for this model because after the first jump the orientation of the molecule would only allow the return to the previous adsorption site. But in the time between two jumps the molecule quickly

C. Kirschhock and H. Fuess / Microporous Mater. 2 (1994) 261-267

reorientates and has therefore access to all four adsorption sites in the tetrahedral supercage. A rise in temperature results in very narrow NMR signals which are explained by diffusing guest molecules undergoing isotropic reorientation. This increased mobility can already be observed for two p-xylene molecules per supercage at 220 K, whereas for one p-xylene and m-xylene molecule per supercage the diffusion only sets in at 230 and 240 K, respectively. The slightly earlier start of increased mobility of the arenes in the higherloaded sample is due to the mutual hindrance of the two p-xylenes in the supercage during the tetrahedral jump mechanism. The additional arene present in the cavity blocks the access to the SII cations, so that diffusion sets in earlier. A similar effect on dynamic behaviour was encountered during the investigation of the second moment [16] and relaxation times [17] of different amounts of benzene in zeolite NaX. A rotation around the six-fold axis was always observed but only for the lowest coverages an additional rotation around the two-fold axis proved necessary to explain the experimental data. On the other hand, for starting diffusion m-xylene requires a temperature which is higher by 10 K since, on account of its geometry, its adsorption in front of a six-ring window of the zeolite is somewhat more favourable than that of p-xylene [S]. These results confirm the assumption that the strength of adsorption and the access to adsorption sites govern static and dynamic features of non-polar arenes in faujasites [18-20).

Conclusions

Fig. 6. Illustration of the tetrahedral jumps of pxylene (a) and m-xylene (b) between 125 and 250 K. On the average the molecules are located in front of the cations on position II (dashed circles). There they librate around the axis perpendicular to the aromatic plane. Out of a favourable position they rotate around the axis found by means of NMR through an angle of 72”32’. During this rotational motion the interaction with the cation on position II’ decreases. It is replaced by the Coulomb interaction of a symmetrically equivalent cation.

At low temperatures, the energy of the molecules is insufficient to allow departure from the adsorption site, whereas at high temperatures the potential field of the tetrahedrally arranged sodium cations is not able to confine the molecules inside the supercage. The molecules diffuse slowly through the host lattice, too slowly to be detected by neutron spectroscopy. Between these extremes of strict localisation and free diffusion, intermediate temperatures cause a movement, fitting the electrostatic requirements in the supercage. As various authors had already shown for other aromatic

C. Kirschhock and H. Fuess / Microporous Mater. 2 11994) 241-267

hydrocarbons such as benzene and toluene [16-201, the motion of xylene molecules is strongly coupled to the occupied cation sites. These complex dynamic processes point out anew that zeolitic guest-host systems cannot be visualised as an isolated host lattice with guest molecules. The symmetry of the zeolite and its cations determine the possible movement of the intercalated species. Furthermore, this work has shown that an adroit combination of spectroscopic methods is required to get full insight into the dynamics of guest molecules because partial movements on a wide time scale give rise to the molecular movement as a whole.

261 3 E.R. Andrew, J. Chem. Phys., 18 (1950) 607. 4 H.S. Gutowsky, G.B. Kristiakowsky, G.E. Pake and E.M. Purcell, J. Chem. Phys., 17 (1949) 972. 5 H.S. Gutowsky and G.E. Pake, J. Chem. Phys., 18 (1950) 162. 6 N. Bloembergen and E.M. Purcell, Phys. Reo., 73 (1948) 679. 7 M. Bee, Quasi-elastic Neutron Scattering, Adam Hilger, New York, NY, 1988. 8 M. Czjzek, H. Jobic and M. Bee, J. Chem. Sot. Faraday Trans., 87 (1991) 3455. 9 J.A. Nelder and R. Mead, Comput. J., 7 (1965) 308. 10 G.E. Pake, Solid State Physics Adu. Res. Appt., 2 (1956) I.

11 A.N. Fitch, H. Jobic and A. Renouprez, 1. Phys. Chem., 90 (1968) 1311. 12 M. Czjzek, T. Vogt and H. Fuess, J. Phys. Chem., 95 (1991) 5255. 13 M. Czjzek, T. Vogt and H. Fuess, Zeohtes, 12 (1992) 1535. 14 H. Lechert and W.D. Basler, J. Phys. Chem. Solids, 50 (1989) 492.

Acknowledgements

15 H. Pfeifer, NMR

We thank the Fonds der Chemischen Industrie for financial support. Furthermore we are grateful to Prof. A. Weiss and Dr. N. Weiden (Darmstadt, Germany) for permission to use equipment and for skilful guidance.

16 17 18 19

Basic Principles and Progress. Vol. 7, Springer-Verlag, Berlin, 1972. _ H. Lechert and K.-P. Wittern, Ber. Bunsenges. Phys. Chem., 82 (1978) 1054. H. Lechert and K.-P. Wittern, Ber. Bunsenges. Phys. Chem., 83 (1979) 596. W.D. Basler, H. Lechert and H. Weyda, Catal. Today, 3 (1988) 31. D. Geschke and H. Pfeifer, Z. Phys. Chem. Leipzig, 257 (1976) 365.

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