J. Phys. Chem. B 1999, 103, 4323-4329
Temperature Dependence of Nuclear Magnetic Resonance Chemical Shifts of r-Cages of NaY Zeolite
4323 129Xe
in the
Andrea Labouriau,*,† Tanja Pietrass,‡ William A. Weber,§ Bruce C. Gates,§ and William L. Earl† Chemical Science and Technology DiVision, Mail Stop J514, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, and Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: December 7, 1998; In Final Form: March 16, 1999
One- and two-dimensional NMR spectra of 129Xe sorbed in NaY zeolite at a loading of approximately 1 Xe atom/4 R-cages were obtained as a function of temperature. One-dimensional spectra were measured over the range of 60-300 K, by far the largest range yet investigated with this method. The data were fitted to the simple statistical mechanical cylindrical pore model of Cheung yielding a van der Waals adsorption energy of 3.3 kJ/mol. 2D-NOESY spectra were obtained at 300 and 355 K. The two-dimensional data show that the intercrystalline diffusion is slow compared to intracrystalline diffusion. The results indicate that the Xe atoms spend most of their time in the R-cages and at the lower temperatures, near the cage walls.
Introduction Even small distortions in the xenon spherical electronic cloud result in large changes in the resonance frequency in the NMR spectrum due to its large polarizability. Consequently, Xe gas is a sensitive probe of microenvironments where it is sorbed or trapped, and 129Xe NMR chemical shifts have been used to characterize polymers and porous solids, particularly zeolites.1-4 Zeolites are crystalline microporous structures consisting of SiO4 and AlO4 tetrahedra linked together in three-dimensional networks of pores and cages of varying sizes. They contain exchangeable cations that balance the framework negative charge resulting from Al in the structure. Zeolites not only sorb guest species but often transform them catalytically, and the regular pores with molecular dimensions allow molecular sieving and shape-selective catalysis. Zeolites are also used for ion exchange and selective sorption for gas purification.5,6 The NMR spectrum of 129Xe sorbed in a zeolite is normally a single peak, the resonance frequency of which changes with temperature, Xe loading, and the zeolite pore structure. In the limit of zero Xe content in a zeolite, the 129Xe chemical shift is roughly inversely proportional to the cavity size. These characteristics of Xe have led to extensive use of 129Xe NMR as a tool to characterize the pore space in zeolites and other microporous materials. Unfortunately, the 129Xe chemical shiftpore structure relationship has been oversimplified and data overinterpreted, which have led to some general distrust of this tool for pore structure characterization. The objectives of the research presented here were to extend the limits of characterization of the pores of NaY zeolite by applying 129Xe NMR spectroscopy over a much wider temperature range than has yet been reported. A temperature study of the 129Xe NMR chemical shifts is an important element in order to understand the energetics of xenon sorption in porous †
Los Alamos National Laboratory. New Mexico Institute of Mining and Technology. § Univeristy of California. ‡
materials. NaY zeolite was chosen because its structure is relatively simple and well-known.7,8 Dehydrated NaY zeolite (NaxSi192-xO384Alx), a faujasite, has a cubic structure with space group Fd3hm and a cell parameter a ) 24.85 Å (Figure 1), as determined by X-ray and powder neutron diffraction.7-9 The zeolite used in this study has a Si/ Al ratio of 2.6; thus each unit cell has 53 Al, 139 Si, 384 O, and 53 Na atoms. There are eight sodalite and eight R-cages per unit cell. The sodalite cages are cuboctahedral with 24 T (Si or Al) atoms and 36 O atoms each (Figure 1). The largest ring in a sodalite cage is an 8-ring, with a diameter of about 4 Å; the van der Waals diameter of Xe (4.4 Å) is too large for it to pass through the windows and enter the small cavities of the sodalite cages. The sodalite cages are linked through double 6-rings (pseudohexagonal prisms) to form the R-cages. Each R-cage is tetrahedrally linked to four others through 12-ring windows with 7.5-Å diameters; the approximate inside diameter of the R-cage is 11.8 Å. Sorbed Xe atoms readily enter the R-cages and probe their interiors while being excluded from the sodalite cages and the pseudohexagonal prisms. Only the extra framework cations in the R-cages are readily accessible to sorbed Xe atoms and probed by 129Xe NMR spectroscopy. Eulenberger et al.7 first characterized the location of the Na+ ions required for charge neutrality in Y zeolite. Later, Smith8 published an extensive study of cation locations in faujasite zeolites more accurately defining the cation sites and naming them. Smith’s nomenclature has become accepted, and the sites have been verified in several more recent papers.9-11 There are essentially five different cation sites: SI, SI′, SII, SII′, and SIII. The SI site is in the center of the pseudohexagonal prism, and SI′ is in the sodalite cage, symmetrically placed with respect to the 6-ring separating the prism and the sodalite cage. The SII site is in the R-cage just above the 6-ring that separates the R-cage and the sodalite cage. The SII′ site is in the sodalite cage, symmetrically placed with respect to the 6-ring. The SIII site is in the R-cage at the 4-ring joining the R-cage with the pseudohexagonal prism. In Na Y zeolites only the SI, SI′, and
10.1021/jp9846835 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/30/1999
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SII sites are occupied. Fitch9 studied a Y zeolite with a Si/Al ratio of 2.43 (very similar to the zeolite in this work: Si/Al ) 2.6) and found that the occupancy of the SII, SI′, and SI sites was 100%, 58%, and 44%, respectively, with a standard error of 2%. On the basis of these results, we expect all of the SII sites (in the R-cages) to be occupied by Na+ in our sample. Since only cations in the R-cages are readily accessible to sorbed Xe atoms, only those in SII sites are probed by 129Xe NMR spectroscopy.
transfer. The average bulk density of each zeolite sample was 0.4 g/cm3, which corresponds to a loose powder as defined by Ripmeester and Ratcliffe.12 NMR experiments were performed with a Varian Unity 400 spectrometer operating at 110.629 MHz. Low temperatures were achieved with an Oxford model CF 1200 cryostat using a homebuilt transmission line NMR probe.13 Temperature was measured with the Au-Fe/constantan thermocouple that is part of the temperature sensing and controlling circuit in the Oxford cryostat and with a calibrated carbon-glass resistor that is mounted in a copper block that is part of the probe structure. The sample was cooled in steps from room temperature to 60 K, with equilibration for 20-30 min between temperature steps. Chemical shifts were measured at each step. Accurate chemical shift referencing is difficult in these experiments. We measured the chemical shift, at room temperature, of an external standard sample of Xe gas at 2.0 atm prior to each set of measurements at variable temperatures. The shift was then corrected to the shift at zero pressure using equations given by Jameson.14,15 The typical π/2 pulse width was 10 µs, and a 3-s recycle delay was used for most experiments in the temperature range investigated. To ensure that we were obtaining most of the NMR signal, 129Xe spin-lattice relaxation times were measured as a function of temperature for several samples of Xe in NaY zeolite with a standard inversion-recovery pulse sequence. The longest relaxation time in the temperature range was 1.3 s at 60 K. The recycle delay used is too short to observe the resonance of xenon in the intercrystalline space (gas phase), so we obtained a 129Xe NMR spectrum at room temperature with a 600-s recycle delay to try to detect gas-phase Xe. The number of transients per spectrum varied from 44 to 1024, depending on the signalto-noise ratio, which depends on the sample temperature. We performed two-dimensional chemical exchange NMR experiments16,17 with different mixing times at different temperatures to provide qualitative information about the homogeneity and molecular dynamics of Xe in the sample. These involve the same pulse sequence as 2D nuclear Overhauser NMR experiments.18 These were hypercomplex, StatesHaeberkorn-Reuben19 experiments with a sweep width of 6 kHz, 64 free induction decays (fid’s) in the t1 direction, 256 scans per fid, and 256 points per acquisition (t2). The zeolite starting material was very well-characterized by a number of techniques: X-ray powder diffractograms were obtained on a Scintag xds2000; scanning electron micrographs were obtained on a JEOL 6300 FEG/SEM; elemental analysis was performed at the department of Geology at the University of New Mexico by using a combination of inductively coupled plasma-atomic adsorption and X-ray fluorescence techniques. We used 29Si NMR spectra obtained on the Varian 400 instrument mentioned above to determine Si/Al ratios.20
Experimental Methods
Results
The Davison Division of W. R. Grace and Co. provided the NaY zeolite used in these experiments. Samples were prepared by weighing some of the zeolite in an 8-mm glass tube, evacuating to 10-3 Torr, and heating to 150, 250, and 350 °C for 1 h each and then heating to 450 °C for 12 h to remove all the sorbed water. Each sample was cooled to room temperature under vacuum. A measured volume of Xe gas was condensed and frozen inside the glass tube, and the tube was then flamesealed. The sample reported here had a Xe loading of 0.25 atom/ R-cage along with 13 Torr of He, added to facilitate heat
The peak ratios from the 29Si NMR spectroscopy yield a Si/ Al atomic ratio of 2.6 for the starting zeolite and indicate that the sample is highly crystalline with few defects, as there was virtually no signal detected in the 103-ppm region characteristic of silanols.20 The sharpness of the X-ray powder diffraction pattern and the lack of a broad underlying feature indicative of amorphous material confirmed the high crystallinity. The scanning electron micrograph of Figure 2 shows that the crystallites are nearly uniform regular cubes with a mean size of approximately 0.5 µm. An elemental analysis showed that
Figure 1. Two views of the structure of zeolite Y. (a) Drawing of the overall structure of zeolite Y looking into the R-cage through the large window. Note the tetrahedral arrangement of the windows between R-cages. (b) Drawing of a portion of the structure of zeolite NaY showing the cation positions. This drawing contains two sodalite cages and some of the associated pseudohexagonal prisms. The positions of some of the cations are shown. All of the unique sites are shown. Note that sites SII and SIII are in the R-cage.
NMR of
129Xe
in the R-Cages of NaY Zeolite
J. Phys. Chem. B, Vol. 103, No. 21, 1999 4325
Figure 4. Two-dimensional chemical exchange NMR spectra of 129 Xe in zeolite Y taken at room temperature (300 K). The contour plot at the left was taken with a mixing time of 5 ms and the one at the right with 600 ms.
Figure 2. Scanning electron microscope image of the NaY zeolite particles used in this work.
Figure 5. Two-dimensional chemical exchange NMR spectra of 129Xe in zeolite Y taken at elevated temperature (355 K) to increase the diffusion rate. The contour plot at the left was taken with a mixing time of 5 ms and the one at the right with 500 ms.
TABLE 1: Xe-Zeolite Lennard-Jones Parameters Figure 3. NMR spectra of 129Xe sorbed in NaY zeolite at temperatures ranging from 300 to 60 K.
the zeolite contained impure Fe, equivalent to 200 ppm by weight of Fe2O3. The 129Xe spectra of Xe in NaY zeolite at temperatures from 60 to 300 K are shown in Figure 3. The spectrum of solid bulk Xe is well-known.21,22 Although Xe in the bulk phase freezes at 161 K at 1 atm, we observed no signal corresponding to solid Xe (317 ppm at 77 K)23 at any temperature. Furthermore, we detected no NMR resonance corresponding to bulk Xe gas at room temperature even though we made special measurements with pulse recycle delays of 600 s to accommodate the very long relaxation time, T1, of Xe in the gas phase. As shown in Figure 3, both the chemical shift and line width increased as the temperature decreased. At room temperature, the 129Xe NMR spectrum is a single, isotropic line, which indicates that on a time scale of milliseconds, Xe is not adsorbed on an anisotropic framework site. Ripmeester24 demonstrated that sorbed Xe in an anisotropic environment has a line shape that reflects the chemical shift tensor interaction. At 60 K, the NMR spectrum is a very broad line with no noticeable singularity. However, it is possible that this resonance contains an anisotropic component that is not apparent because it is small or hidden by broadening from other interactions. The results of two-dimensional chemical exchange experiments are shown as contour plots in Figures 4 and 5. Figure 4 is the plot for mixing times of 5 and 600 ms at ambient temperature (≈300 K), and Figure 5 is for mixing times of 5 and 500 ms at 355 K.
atom pair
(kJ/mol)
σ (Å)
Xe-O Xe-Na
1.539 0.269
3.32 3.73
Discussion Xe-Zeolite Interactions. In interpreting the NMR data, it is important to understand the energetics of the interactions of Xe atoms in the zeolite. The interactions are usually modeled as two pairwise, short-range interactions: Xe-O and Xe-Na. It is common to neglect interactions between Xe and the T atoms because the latter are well-shielded from Xe by the O atoms of the zeolite lattice. This atom-atom approximation has been successfully applied in investigations of Xe sorption in zeolites. In general, a Lennard-Jones (6-12) potential energy function is used to calculate the potential energy UaXe(r) of the intermolecular interaction of a Xe atom with atoms i of the zeolite at different points on the internal surface (r is the distance between the centers of the Xe atom and the O or Na atom):
[( ) ( ) ]
UiXe(r) ) 4iXe
σiXe riXe
12
-
σiXe riXe
6
(1)
The index i represents the atom O or Na. The depth of the potential well is given by , and σ is the sum of the radii of the Xe atom and the O or Na atom, i.e., the van der Waals separation between the atomic centers. Kiselev and Du25 derived a set of interaction parameters for eq 1, which are listed in Table 1. The total potential experienced by a Xe atom is the sum of all pairwise Lennard-Jones potentials over the framework O and Na ions.
4326 J. Phys. Chem. B, Vol. 103, No. 21, 1999
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In addition to the standard Lennard-Jones potential, some molecular statistical calculations include a long-range interaction term in eq 1 that accounts for the electrostatic interaction between a Xe atom and the electrostatic field associated with the cation. In our case this is the partial charge on Na+. This is an induced dipole-charge interaction given by:
( ) RXeFi
δ)
2
U′iXe(r) ) -
4 2riXe
[ ]
∫Vδzeo(r) exp
(2)
where RXe is the Xe polarizability, Fi is the charge of the ion i, and riXe is the distance between the centers of a Xe atom and the cation. There are two approaches reported to account for the electrostatic term. One is to assume that the zeolite is purely ionic,26 and the second is to assign a fraction of a negative charge to each O atom and a full positive charge to each Na atom to retain electroneutrality of the lattice.25,27 A structural assumption, included in most molecular dynamics simulations, is that the zeolite is a rigid lattice; i.e., there is no energy exchange between sorbed molecules and the framework. There are a few simulations based on the proposal that the force field includes a harmonic and an anharmonic potential that represent the framework as a heat reservoir (for details, see Schrimpf28). NMR Chemical Shift of 129Xe in Zeolites. At room temperature xenon exchanges rapidly between zeolite cages and crystallites, so that the chemical shift is an average over all these environments. Ito and Fraissard1 realized that the chemical shift of Xe in a porous material is a function of several parameters, including temperature, the possible presence of paramagnets, the Xe-Xe collisional interaction, and collisions with the zeolite pore walls. They suggested a very simple equation to approximate the chemical shift:
δ ) δ0 + δS + δE + δXe
Recently, Fraissard’s concepts have been supplanted by less empirical models of the Xe shift. Cheung35 used a statistical mechanical model that expresses the chemical shift relative to a canonical ensemble expression for the location of Xe in the pores. This results in the equation:
(3)
where δ0 is the shift reference, normally taken to be the chemical shift of Xe gas at zero pressure, δS accounts for Xe-wall collisions, δE is the shift caused by electric fields created by the cations, and δXe is a pressure-dependent term accounting for Xe-Xe collisions. The δS term is associated with the frequency of collision of a Xe atom with the pore wall and thus is related to the Xe mean free path.29 On the basis of this assumption, equations were derived for δS as a function of pore size for different pore structures (cylinders, spheres, etc.).30 To eliminate Xe-Xe interactions, the 129Xe shift is plotted as a function of Xe pressure. The extrapolation to zero pressure yields δS, which is related to the pore dimensions. An alternative to Fraissard and Ito’s simple additive approach to chemical shifts is to use molecular dynamics simulations. Thus, some researchers have attempted to combine statistical mechanical or molecular mechanical models with NMR chemical shifts, usually 129Xe or 13C shifts. Jameson et al.31-34 successfully simulated the equilibrium distribution of Xe atoms sorbed in zeolite A using Monte Carlo methods. Zeolite A is a good starting point for the work because (1) it has a Si/Al ratio of 1 and consequently lacks structural disorder and (2) it has small pore windows which lead to constraints on the xenon exchange between R-cages. Although molecular dynamics and ab initio theoretical models may provide detailed information at the atomic level, they cannot predict zeolite structures accurately. Thus, the zeolite structure must be well-known from X-ray or neutron diffraction for an accurate prediction.
-U(r) dr kBT
(4)
where V is the volume of the cavity, kB is Boltzmann’s constant, δzeo(r) is the 129Xe NMR chemical shift at a distance r from the surface, and U(r) is the Xe-zeolite wall potential energy. Knowing δzeo(r), we can in principle calculate the chemical shift from eq 4. However, eq 4 cannot be solved analytically, and a numerical solution is required. For simplicity, Cheung replaced the Lennard-Jones potential for the Xe-zeolite interaction with a square-well potential, and Xe-cation interactions are not explicitly considered. Although this model is oversimplified, it provides useful information about the strength and range of the interaction between a Xe atom and the wall of the pore as well as qualitative information about pore size. Following Cheung’s model, the chemical shift is given by:
δ(T) )
(
c
( ))
1 + F exp
- kBT
(5)
where is the depth of a square-well potential, c is a phenomenological constant, and F is a geometric term given by
F)
L - 2aXe 2lm - 1
(6)
In this equation aXe is the van der Waals radius of the Xe atom and m is 1, 2, or 3 for one-, two-, or three-dimensional pores, respectively. The potential well has a width of l and a value of - inside the well and zero outside, becoming infinite when the distance between Xe and O atoms is less than the sum of their van der Waals radii. The quantity L expresses the free pore size. At low temperatures (when kBT , ) the chemical shift as given by eq 5 becomes independent of the geometric parameter F. Cheung used this model to simulate the 129Xe shifts measured at two temperatures, 144 and 296 K, for five molecular sieves: silicalite, CaA zeolite, NaA zeolite, offretite, and LTL zeolite. In addition, he included the temperature dependence of the 129Xe shift in NaY zeolite at temperatures from 160 to 300 K. The chemical shifts were fit using this model, from which he obtained values for the parameters described above. In the open and unconstrained structure of zeolite Y, the fast exchange of xenon between R-cages and crystallites can be slowed by reducing the temperature, which decreases the thermal energy of xenon. In the following section, we apply Cheung’s simplified square-well model to the 129Xe chemical shifts observed for very low loadings of Xe in zeolite NaY over a wide range of temperature as a first step toward obtaining values for the potential energy functions. One-Dimensional NMR. The sample used in this work had a very low Xe loading, less than 1 Xe atom/supercage. Woods and Rowlinson36 suggested that the distribution of occupancies of Xe in the unit cell of NaY zeolite is broad and quite symmetric around the most probable occupancy. Furthermore, molecular modeling calculations performed by Santikari et al.37 show that most Xe atoms exist as monomers when Xe is present
NMR of
129Xe
in the R-Cages of NaY Zeolite
Figure 6. Plot of the 129Xe chemical shift of xenon in zeolite Y as a function of temperature. The solid line is a fit to eq 5 as described in the text.
in NaY zeolite at low loadings. Consequently, Xe-Xe interactions are inferred to be negligible for the xenon concentration considered here, although they might become more significant at low temperatures. Figure 6, a plot showing the experimental chemical shifts as a function of temperature, includes a solid line representing a fit to Cheung’s model, using eq 5. The van der Waals interaction energy is of the order of 3.3 kJ/mol, which is in qualitative agreement with the value of 1.5 kJ/mol reported by Kiselev and Du.25 Cheung rationalized the discrepancy between the NMR-derived values and Kiselev and Du’s value. Kiselev and Du’s potential represents a simple two-body interaction. However, in the NMR measurement, a single Xe atom no doubt interacts with more than one O atom in the zeolite framework, which would give a correspondingly larger value for the potential. Also, the value of the free pore size determined in the fitting, L ) 15 Å, is close to the internal size of the R-cage. The value for the potential well width (l) is 0.5 Å, which is similar to that one reported by Cheung. One can develop a qualitative picture for Xe atoms in zeolite Y that is consistent with these results. In the zeolite Y structure, Xe atoms can occupy the R-cages or the double 8-rings that connect them. The potential energy function is pictured as a hump at the center of the R-cage with a low-energy trough along the inner surface of the cage wall. At high temperatures, Xe atoms have sufficient thermal energy to sample spaces near the center of the cavity, which are associated with small chemical shifts (gaseous xenon), but as the temperature decreases, the Xe atoms spend more and more time close to the wall, giving rise to larger chemical shifts. Figure 6 shows that at temperatures less than 100 K the chemical shift reaches a plateau. Experimentally, we also find that the NMR resonance line becomes very broad at these low temperatures (Figure 3). These results are consistent with the interpretation that at low temperatures the Xe atom stays mainly on the internal surface of the pore. Thus, as the temperature becomes low, the Xe atoms spend almost all of their time along the walls, so the shift does not change and reaches an asymptotic value near 98 ppm. The NMR resonance lines in this work are much broader than those reported by Ratcliffe and Ripmeester,38 for xenon sorbed in NaY. In their work, the xenon loading was varied from 1 to 12 xenon atoms/supercage and the samples contained no helium gas to aid in rapid temperature equilibration. Their samples were
J. Phys. Chem. B, Vol. 103, No. 21, 1999 4327 cooled by quenching the sample from 295 to 77 K. These samples took many months to reach equilibrium as determined by the 129Xe NMR spectrum having different peaks, with different chemical shifts, during the equilibration period. Nonetheless, after a long time, all spectral components converged to a single, sharp, and isotropic line at 77 K. The authors concluded that the various components were associated with different xenon densities across the sample, giving rise to a distribution of chemical shifts. Although the samples reached equilibrium in a time scale ranging from hours to years, their experiment demonstrates that xenon is mobile at 77 K. Ratcliffe and Ripmeester suggested that at this temperature, xenon is probably sorbed on the inner surface of the zeolite; however, they were unable to determine if the isotropic chemical shift observed is the result of a rapid exchange between sorption sites or if the xenon clusters behave as a true fluid. At this point we cannot explain why our resonance lines are much broader than the ones reported in Ratcliffe and Ripmeester’s work. A possible explanation is that paramagnetic centers in the form of iron impurities in the zeolite in our sample are responsible for the broadening despite their very low concentration. They do not report the concentration of paramagnetic centers in their samples. A number of molecular dynamics calculations are reported in the literature for small molecules in zeolites. Keffer, McCormick, and co-workers mapped the potential energy surface for xenon in Y zeolite (and about 20 other zeolites and molecular sieves) using pairwise potentials.39 These maps reveal the accessible pore volume, low-energy sorption sites, and the activation barrier for xenon motion from site to site. For a purely siliceous Y zeolite (no Na+ cations), they computed 10 favorable sorption sites for xenon: 6 are equivalent sites located in front of the 4-rings in the supercage, and the other 4 sites are equivalent sites in front of the 6-rings in the R-cage (Figure 1). These are sites where the xenon-oxygen contact is maximal because in the first case the Xe atom is in close contact with four O atoms and in the second case it is in slightly more distant contact with six O atoms. In other work, Santikary et al. found xenon to be closely associated with the sodium cations in NaY zeolite at low temperatures.37 Yashonath et al. made several molecular dynamics studies of xenon and hydrocarbon motion in zeolites, including zeolite Y.40-43 In summary, these reports find that at reduced temperatures the sorbed atoms travel along the wall and a low point in the potential exists where they pass through the windows between R-cages. Although the experimental xenon NMR data presented here do not allow us to identify exact xenon locations, they are consistent with the observation of xenon increasingly adhering to the wall as the temperature is lowered. There are only few experimental results that locate Xe atoms or other sorbates in faujasites or other zeolites. Heink et al. used synchrotron X-ray diffraction at 304 K. Their data suggest that Xe atoms are located at the Na 6-ring sites, SII, in the R-cages of NaX zeolite.44 However, there is still no NMR evidence that supports specific xenon sorption sites. The 129Xe NMR spectrum shows only one spectral line, even at very low temperatures. The spectra show no clear chemical shift powder pattern line shape. If xenon occupies an asymmetric site for times on the order of the inverse of the expected chemical shift anisotropy (CSA), i.e., ≈10-3 s, then we would expect to see residual CSA powder patterns.45 In fact, 129Xe NMR spectra cover a large range of chemical shifts with small changes in the local environment. Two-Dimensional NMR. The 2D exchange NMR experiment can be used as a qualitative indicator of homogeneous
4328 J. Phys. Chem. B, Vol. 103, No. 21, 1999 line broadening or molecular motion in solids. If the nucleus of interest moves through sites with slightly different chemical shifts in a time that is shorter than the mixing time or if the nucleus is completely static, then the two-dimensional contour plot will be nearly circular. If the molecular dynamics are slow (corresponding to an inhomogeneously broadened line), then the contour plot will be elongated along the diagonal. Solid samples usually have bulk magnetic susceptibility anisotropies associated with nonspherical crystallites, etc., which produce small shifts that can be exploited for these measurements. Thus, two-dimensional chemical exchange experiments with different mixing times at different temperatures provide qualitative information about sample homogeneity and the molecular dynamics of xenon that is relatively dilute in samples such as ours. Figure 4 contains two 129Xe contour plots taken at ambient temperature (≈300 K) with mixing times of 5 and 600 ms. Even at the relatively long mixing time of 600 ms, it is clear that Xe atoms are not able to sample all of the different chemical shifts in this sample. This result led us to repeat the two-dimensional experiment at a higher temperature (355 K) with mixing times of 5 and 500 ms. The results of these experiments (Figure 5) show that the contour plot for a mixing time of 500 ms at 355 K is more nearly circular than the comparable plot measured at 300 K, but the spectrum is still not entirely averaged even under these conditions. (It is difficult to extend this experiment to much longer mixing times because the relaxation time, T1, starts to attenuate the signal.) The molecular dynamics of xenon in zeolites has been measured and computed in several experiments. However, we could not find any reports on diffusion coefficients of xenon in zeolite NaY; thus we propose to compare our results with those obtained for other zeolites. The most relevant measurements were reported by Karger and co-workers,46,47 who used pulsed field gradient NMR to measure the self-diffusion of xenon in several zeolites. For instance, they measured intracrystalline selfdiffusion in NaX zeolite in two large samples with mean crystallite diameters of approximately 50 and 20 µm. Zeolite X has the same topology as zeolite Y (FAU) with a Si/Al ratio between 1 and 2 for X zeolite. This difference corresponds to only a small percentage change in the unit cell parameter and requires more extra framework cations. Karger et al.46,47 reported a room-temperature intracrystalline self-diffusion coefficient on the order of 5 × 10-9 m2/s for both samples. This result is in agreement with the self-diffusion coefficient estimated by Santikary et al.37,43 from molecular dynamics calculations, i.e., 6.1 × 10-9 m2/s at 285 K and 1 Xe/supercage. Also, Moudrakovski et al.48 used 2D-EXSY NMR methods to investigate xenon diffusion between a mixture of NaX and NaY zeolite particles. For example, the NMR spectra obtained for large particles show distinct resonance lines, denoting a slow interparticle exchange. As the size of the particles was reduced to less than 20 µm, the two peaks collapsed to a single line indicating a faster interparticle exchange. Consequently, their results indicate that xenon will diffuse through several thousand supercages in a few milliseconds for small particles. The mean particle size of the zeolite in our sample was 0.5 µm, corresponding to about 200 unit cells in length per crystallite. Then, according to the results mentioned above, one assumes that during the 500- and 600-ms mixing times a Xe atom should have diffused through the entire crystallite and even between numerous adjacent crystallites. The faujasite structure is cubic; thus, to a first-order approximation, this implies that the bulk susceptibility inside the
Labouriau et al. crystallites should be isotropic and should not depend on crystallite orientation. However, the crystallites are nearly cubic in aspect (not spherical), and so small anisotropies in the bulk susceptibility are expected. Furthermore, the zeolite contained about 200 ppm of iron impurity (expressed as Fe2O3). Xe atoms in close proximity to paramagnetic Fe3+ ions may be chemically shifted from those distant from these ions. These effects are sufficient to explain the observed line widths; i.e., some Xe positions in a crystallite may have resonances at 56 ppm whereas others have resonances at 60 ppm (Figure 4). Some crystallites may be different from others in orientation, shape, and/or Fe3+ content, giving rise to some of the observed inhomogeneity. However, there was no ordering of the crystallites in the NMR sample, and so “high-field crystallites” have an equal probability of being adjacent to “low-field crystallites” as to other “highfield crystallites”. The fact that the two-dimensional contour plots for 600 and 500 ms in Figures 4 and 5 are not perfectly circular implies that intercrystalline diffusion is relatively slow compared to intracrystalline diffusion. This result is corroborated by one-dimensional data, since we do not observe a peak associated with xenon in the gas phase. Recent two-dimensional chemical exchange experiments in ZSM-1249 seem to confirm this conclusion, although NaY and ZSM-12 have very different pore structures. At room temperature, Moudrakovski et al.49 obtained oblate two-dimensional contour plots for 129Xe spectra in siliceous ZSM-12 with a mixing time of 10 ms. They attributed the 129Xe line width to different orientations of the zeolite crystallites and concluded that intercrystalline diffusion was slow. The fact that their sample was purely siliceous means that it contained no Na+ ions or other exchange cations. The absence of such cations would be expected to increase the rate of diffusion because the electrostatic attraction between Na+ or other cations and Xe is likely to decrease xenon mobility. Furthermore, Moudrakovski et al. used larger crystallites than ours, which would require longer times for xenon to equilibrate between crystallites. At this point, we suggest that the discrepancy between our results and those mentioned above arises because of the very low Xe coverage in our sample and the relatively large attraction between the Xe atom and the zeolite pore. Simply stated, the potential difference between a Xe atom inside the zeolite pore and in the intercrystalline space (bulk xenon) is so high that intercrystalline exchange is very slow. However, different locations inside the crystallite have very small differences in potential energy so that intracrystalline diffusion is fast. When the xenon loading gets much higher than ours, van der Waals repulsions start to push Xe out of the pores into the intercrystalline space. Conclusions We applied a model proposed by Cheung to our variabletemperature chemical shift measurements, which provided reasonable values for the van der Waals potential energy (3.3 kJ/mol) and the free pore size of the R-cage (15 Å). However, the approximations in this model require cautious interpretation and underline the need for better models. One of the inferences of this work is that of better models are essential for further understanding of 129Xe NMR chemical shifts. The present state of understanding of 129Xe chemical shifts is insufficient to use those shifts to accurately predict energetics or locations of xenon atoms in pores, much less to make a quantitative measure of pore volumes or structures. Hopefully, continued work in this field will lead to models that are useful for obtaining pore structures.
NMR of
129Xe
in the R-Cages of NaY Zeolite
We also found that the intercrystalline diffusion in our samples is significantly slower than intracrystalline diffusion measured in pulsed field gradient experiments. Additional experiments are planned to test the generality of this result. Our data show that xenon at very low temperatures must spend most of its time at or very near the zeolite pore wall. At 60 K no chemical shift powder pattern line shapes were observed, which indicates the presence of atomic mobility to average the shift anisotropy. However, the large line widths may obscure typical CSA line shapes. Despite the results for MD calculations, no evidence for specific sorption sites on the zeolite pore wall could be observed. Acknowledgment. This work was supported by Los Alamos National Laboratory, U.S. Department of Energy Contract W-7405-ENG-36, as part of the Los Alamos Catalysis Initiative. References and Notes (1) Ito, T.; Fraissard, J. J. Chem. Phys. 1982, 76, 5225. (2) Ripmeester, J. A.; Davidson, D. W. J. Mol. Struct. 1981, 75, 67. (3) Dybowski, C.; Bansal, N.; Duncan, T. M. Annu. ReV. Phys. Chem. 1991, 42, 433. (4) Barrie, P. J.; Klinowski, J. Prog. NMR Spectrosc. 1992, 24, 91. (5) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular SieVes; Academic Press: New York, 1978. (6) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry, and Use; Wiley: New York, 1974. (7) Eulenberger, G. R.; Shoemaker, D. P.; Keil, J. G. J. Phys. Chem. 1967, 71, 1812. (8) Smith, J. V. AdV. Chem. Ser. 1971, 101, 171. (9) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311. (10) Mortier, W. J.; Van den Bossche, E.; Uytterhoeven, J. B. Zeolites 1984, 4, 41. (11) Koller, H.; Burger, B.; Schneider, A. M.; Engelhardt, G.; Weitkamp, J. Microporous Mater. 1995, 5, 219. (12) Ripmeester, J. A.; Ratcliffe, C. I. Anal. Chim. Acta 1993, 283, 1103. (13) Kim, Y.-W.; Earl, W. L.; Norberg, R. E. J. Magn. Reson. A 1995, 116, 139. (14) Jameson, A. K.; Jameson, C. J.; Gutowsky, H. S. J. Chem. Phys. 1970, 53, 2310. (15) Jameson, C. J.; Jameson, A. K.; Cohen, S. M. J. Chem. Phys. 1975, 62, 4424. (16) Jeneer, J. Proceedings of the Ampere International Summer School, Basko, Poland, 1971. (17) Jeneer, J.; Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546. (18) Bax, A. Two-Dimensional Nuclear Magnetic Resonance in Liquids; Delft University Press: Delft, 1984.
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