Molecular Sieves - Science and Technology - Vol. 6 - Characterization II / 04-NMR of Physisorbed 129Xe Used as a Probe to Investigate Molecular Sieves

NMR of Physisorbed 120XE Used as a Probe to Investigate Molecular Sieves

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We have seen that the variation of δ with the water concentration can be used to locate water molecules outside the supercages when the concentration is low (2 H2O molecules per 1/8 unit cell), then, at higher concentrations, at the windows between the supercages, thus reducing the diffusion of xenon between the supercages [170]. Finally, the increase of δS and of the slope of the δ vs. [H2 O] plots indicate an increase in the frequency of molecular encounters of xenon atoms between themselves or with water molecules, i.e. a perceptible reduction of the intracrystalline free space and of course, again, a decrease in the diffusion.

With the help of a few examples it was also possible to show that there is a correlation between the 129 Xe shift of adsorbed xenon and 1H PFG NMR

diffusion of hydrocarbons used as probe molecules [179, 208].

The second parallel between 129Xe NMR and 1H PFG NMR concerns the effect of coking. We have seen from [172] that the variation of the δ = f (N) curve with the degree of coking makes it possible to locate the coke, to deduce its distribution outside and inside the zeolite crystallites and, in the latter case at the windows or the surface of the cages or channels, whether it is homogeneous or not. The PFG NMR technique in combination with the NMR tracer desorption technique gives the same conclusions ( [179] and [209] for 5A zeolite, [210] and [211] for ZSM-5 and HY zeolites).

2.6.3.2

Industrial Zeolite Catalysts

Zeolite + Binder

Although qualitative, such studies make it possible to demonstrate the effects of various factors such as the dilution of the zeolite by another solid, the compression, the experiment temperature, etc. on the δ = f (N) curves (or δ = f (PXe) and therefore on the diffusion of xenon from one crystallite to another. The information obtained is vitally important from the point of view of the application of the Xe technique to the study of industrial zeolite catalysts. Chen et al. [60] have studied the effects of these factors on Na – Y and ZSM-5 either pure or mixed or associated with Na – A, a zeolite which does not adsorb xenon under normal conditions and which therefore serves simply as a diluent, the samples being in the form of powder or compressed at pressures PC between 0 and 314 MPa.

For a pure zeolite, at a given xenon pressure, δ increases very little with the compression. For example in the case of Na – Y (diameter of windows

˚

7.5A), δN→0 increases from 57 to 60 ppm when PC goes from 0 to 314 MPa. The variation decreases further with the pore diameter of solids.

In the case of a mixture with Na – A, where W is the concentration of the Na – Y or ZSM-5 zeolite:

	W
δN→0 =	W + C δS,	(30)

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Fig. 32 δP→0 variations with NaY concentration W, in :NaY-NaA mixture: (a) 314 MPa compressed; (b) powder; (c) difference between line a and b. (Reprinted with permission from [55] Copyright (1992) American Chemical Society)

C is a constant characteristic of the mixture. At low dilution, δN→0 does not change much with dilution and compression (Fig. 32). Therefore, for routine application of the xenon NMR technique to a pure zeolite or samples containing less than 50% of nonporous impurity the result should be acceptable. At higher dilution, δN→0 decreases rapidly with dilution. This tells us that special attention must be paid to the application of the technique to industrial catalysts which sometimes contain more than 50% of binder. For a mixture, the increase of linewidth with xenon pressure can be also a good indication of the presence of a second phase in the sample.

Equation 30 can be written:

1/δ	= 1/δ	+ (C/δ )W–1.	(31)
N→0	S	S

Hence the 1/δN→0 = f (1/W) plot should be a straight line whose intercept gives the value of 1/δS. This allows us to check whether the zeolite has been perturbed by its environment, either during the formation of the pellet or during a catalytic reaction. It is, to the best of our knowledge, the only technique by which the zeolite component alone in a mixture can be studied. It should be however noted that, when the binder is also porous, it is better to record the spectra at low temperature. It is known that δS (corresponding to the pure zeolite), decreases with a temperature increase. δN→0 shows the same variation, but the influence of the temperature increases with the degree of dilution.

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Finally, the form of the xenon NMR signal depends markedly on the homogeneity of mixtures. This can be used as a tool for checking the homogeneity of industrial samples.

Zeolite-Supported Metals

The theoretical analysis of a spectrum containing several components due to the inhomogeneity of the sample can also provide information about the mobility of xenon. For example, we have seen in paragraph Sect. 2.5.2 that at the beginning of hydrogen chemisorption on a Pt/Na – Y catalyst, two Xe-NMR lines are observed; these are attributed to xenon atoms in contact either with pure platinum particles or with particles bearing hydrogen. This interpretation clearly implies that as long as not all particles are in contact with H2, the mean lifetime of the Xe atoms in these two regions must not be much less than the inverse of the frequency difference between the two lines, since otherwise the two lines should coalesce. By a rigorous determination of the NMR line shape of a two-region system with finite exchange times, the mean lifetime of the xenon atoms in the inner core of the Pt – H/Na – Y crystal-

lites proves to be τ = 1.25 ms, and the self-diffusion coefficient of xenon in the uncovered Pt – Na – Y part is found to be equal to D 10–11 m2 s–1 [211].

2.6.3.3 Na–A/Xe system

There have been many experimental and theoretical studies on the distribution, the calculation of the chemical shift and the dynamics of xenon adsorbed in the α-cages of Na – A, essentially because Xe exchange between α-cages is very slow and because one has also a good model of (Xe)n clusters with a well-defined number of atoms, n, in the cages. Indeed, 129 Xe NMR can be used to determine the distribution of xenon atoms among the α-cages of Na – A (Fig. 33a) [82–84].

The xenon is generally adsorbed at high temperature (523 K) and pressure (0.5 to 20 MPa) because the window openings of the α-cages are roughly

˚	+	. However, this
4.2 A and in addition are blocked by the presence of Na

does not entirely rule out intercage movement of the xenon, since pore sizes determined from adsorption studies have almost always turned out to be greater than their crystallographically computed counterparts, for two reasons [212, 213]:

1.Neither the guest molecule nor the host lattice is rigid, in that both the molecule and the oxygen framework are polarizable (i.e. capable of distortion);

2.Both guest and host are in a continuous state of vibration as the bonds holding them together bend under the influence of temperature.

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Fig. 33 a 129Xe 1D NMR spectrum and b 129Xe 2D exchange NMR spectra of xenon adsorbed on NaA zeolite at 523 K (PXe = 3 MPa). 2D spectra were recorded using mixing times of 0.2, 0.5, and 2.0 s. The diagonal peaks correspond to 129Xe resonances from xenon in α-cages containing different numbers of occluded atoms as indicated above by the respective peaks of the 1D spectrum. The cross peaks are a result of intercage motion of xenon during the mixing time of the 2D experiment. (Reprinted from [86], with permission from Elsevier Science)

Nivarthi and McCormick [214] were particularly interested in the movement of xenon within the cage. They have demonstrated how 129Xe NMR relaxation of adsorbed molecules can provide valuable information about sorbate location and dynamics. The decoalescence of the 129 Xe NMR peak observed with a loading of 1 Xe/α-cage indicates the presence of distinct adsorption sites in this cage (in front of the 4-oxygen ring windows) and a very rapid intracage motion of the Xe atoms at room temperature. The magnitude of the longitudinal relaxation time, T1, observed for the peak corresponding to the high loading case suggests the existence of polyhedral configurations of the xenon atoms inside the α-cage with enhanced stability.

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The frequency of the intracage exchange (hopping from one adsorption site to another) is, at a maximum, of the order of 3450 Hz and the corresponding activation energy, 32.0 kJ mol–1, of the same order as that observed for intracrystalline diffusion. Nivarthi et al. [214] conclude that the rate-limiting step for diffusion through a zeolite may not be configurational diffusion through small windows but might be the hop between adsorption sites within a single cage, especially at high loadings, since motion is in that case restricted by a “molecular traffic jam”. This conclusion should apply generally to all zeolite systems at high sorbate loadings.

Li et al. [215] extended the methods used for rare gas clusters in free space to investigate the relation between the thermodynamic and dynamic properties of occluded clusters in terms of the adsorption site of a single (Xe)n cluster inside the α-cavity. They reported how the sodium cations, primarily the type III Na+, and the Xe – Xe interactions influence aspects of Xe adsorption, particularly the site occupancy, the site-to-site exchange rate, and the way the number of Xe atoms inside the α-cavity influences the 129 Xe chemical shift. They have shown that there is a locally stable site for one adsorbed xenon atom in each eight-membered and four-membered ring; the effective number of potential energy minima is 11, the number of minima found by Jameson et al. [216].

Even though the Xe-wall interaction is much larger than Xe – Xe interaction, the energy differences between the adsorption sites are not much larger than the Xe – Xe repulsions. Atoms in smaller clusters, which have low-energy sites available for all the Xe atoms, can move around without encountering the hindrance of large short-range Xe – Xe repulsive forces; but in larger clusters either congestion forces Xe atoms to occupy higher-energy sites, which makes them mobile, or some of the Xe atoms occupy the low-energy sites and experience large Xe – Xe nearest neighbor interactions. Both cases bring the Xe – Xe repulsive forces into play in the dynamics. Nonetheless, even though the Xe – Xe interaction seems to play the dominant role in the dynamics of a cluster of xenon atoms inside the Na – A α-cavity, it is the balance of this interaction against the Xe-cavity interaction that governs the size and temperature dependence of the chemical shift.

The other studies are mainly concerned with xenon diffusion from one cage to another. The well-separated peaks of the one-dimensional spectra and their relatively small line-widths indicate that intercage exchange frequency is smaller than the frequency differences between the resolved peaks. However, intercage xenon exchange is inferred from the observation of changes in the one-dimensional 129 Xe spectra over the time as samples equilibrate [84]. Larsen et al. [86] demonstrated that this slow intercage motion can be monitored directly by 129 Xe 2D exchange NMR in which magnetization transport during a time interval, tmix, is measured. In Fig. 33b the 129Xe frequencies before and after the mixing time are correlated.

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Denoting the frequency of a n-cage (i.e. a cage containing n xenon atoms) as ω(n), the normalized spectral intensity M(ω(n) , ω(m)) is the joint probability of finding a xenon in a n-cage before the mixing time and in a m-cage afterwards. In the experimental spectra of Fig. 33b, for the shortest mixing time (0.2 s), most of the spectral intensity is confined to the diagonal, indicating negligible intercage motion during this mixing time. For tmix = 0.5 s, one-off diagonal cross peaks M(ω(n) , ω(n±1)), i.e. cross-peaks adjacent to the diagonal, at frequencies corresponding to cages differing in occupancy by one xenon, have significant intensity. This indicates that the dominant change in xenon frequencies with time corresponds to an increase or decrease of the cage occupancy by one xenon. When tmix = 2.0 s, exchange is observed between all but the least populated cages. This progression of spectral features with mixing time is characteristic of mass transport in this system.

The overall rate, Rn,m, of xenon atoms going from any n-cage to any m-cage is proportional to the probability P(n) × P(m) of finding a n-cage next to a m- cage, and to the rate coefficient kn of a given xenon leaving a n-cage

Rn,m = nP(n)P(m)kn ,	(32)
where kn exhibits an Arrhenius form
kn = An exp(– Ea(n)/kT).	(33)

From the simulation of 129 Xe 2D spectra, Larsen et al. [86] have determined kn as a function of cage occupancy n and the diffusivity D = 10–19 m2 s–1 at

ambient temperature, which is much smaller than the values accessible by PFG NMR [198].

Another possibility for studying the chemical exchange by monitoring the transfer of polarization is a one-dimensional experiment in which one selectively inverts the magnetization at site A and monitors the recovery both of this resonance and that at sites B, C, etc., with which it is exchanging [217]. The use of the DANTE (Delays Alternating with Nutations for Tailored Excitation) sequences for selective inversion, has the advantage that the total flip angle and selectivity may be controlled independently, without changing the pulse power.

By using these techniques, Jameson et al. [217] proposed and confirmed experimentally the relationships between the set of microscopic rate constants kmn and the phenomenological rate constant Kmn. These relationships are analogous to those proposed by Larsen et al. [86]. (It should be noted, however, that the notations are different). kmn is associated with the rate transfer of a single Xe atom from a cage containing the cluster (Xe)n into a neighboring cage containing the cluster (Xe)m–1 , thereby making the new cluster (Xe)m . These authors have also shown that the rate constant associated with a single Xe atom leaving a particular (Xe)n is relatively independent of the destination, except when the destination cage is already highly populated.

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2.6.4

129Xe NMR as a Tool

for Probing Intracrystalline Concentration Profiles and Transport Diffusion

In opposition to the self-diffusion measurements (PFG NMR, quasi-elastic neutron scattering) which allow the microscopic observation of molecular mobilities, the measurement of transport diffusion can only be carried out from a macroscopic point of view, i.e. by monitoring the time dependence of the molecular concentration in the gas phase or the bulk phase of the bed of zeolites. “Microscopic” observation of intracrystalline transport diffusion by 129 Xe NMR spectrum evolution allows us to go back to the time dependence of the intracrystalline concentration profile.

The principle depends on the influence of an adsorbate on the spectrum of xenon coadsorbed as a probe. Bansal and Dybowski [218] have studied the diffusivity of H2O at 373 K between two layers of NiNa – Y zeolite (3.4 wt % nickel), one well dehydrated and the other very little (simply pumped at 298 K). According to these authors, if δ1 and δ2 correspond to the chemical shift of xenon in the two regions of H2O concentrations, C1 and C2, to a first approximation it holds:

δ1 – δ2 C1 – C2

(34)

The diffusion of H2O from one region to another changes the concentrations and the δ values. The evolution of shifts with time can be used to measure the diffusivity D. In particular, when the time is long, the following equation:

δ1(t) – δ2(t) K exp(– π2Dt/L2),

(35)

where 2L is the height of the sample, makes it possible to estimate the diffusivity of water: D(373 K) = (2 ± 1) × 10–10 m2 s–1.

Kärger [219] expressed some doubts about the comparison of this coeffi-

cient with the constant for the self-diffusion of water in 13 X zeolite: (4 ± 1) × 10–10 m2 s–1 [220]. He and his co-workers have looked at this type of study in

more detail. For this purpose, the adsorption/desorption process of benzene in ZSM-5 zeolite has been followed under xenon atmosphere by intimately mixing activated and loaded zeolite crystallites [221]. Figure 34 a (full line) shows the evolution of the spectrum of xenon with time, for an initial concentration of 6 and 0 molecules of benzene per u.c. and an average xenon concentration of 16 atoms/u.c. The signal of xenon situated in the supercages containing benzene (high δ) is broader and weaker than that of xenon in the benzene-free cages (small δ), because of the reduction of the adsorption capacity of xenon in the presence of benzene.

Corresponding to the two cases of intracrystalline limitation (i.e. limitation by intracrystalline diffusion) and extracrystalline limitation (i.e. limitation by external resistance, e.g. surface barriers), two different time dependences of the intracrystalline concentration profiles may be expected,

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Fig. 34 129Xe NMR spectra (full line) during the sorption of benzene on crystallites of ZSM-5-type zeolite at 293 K with an initial concentration of 6 and 0 benzene molecules per unit cell and a mean xenon concentration of 16 atoms per unit cell. Comparison with the simulated spectra (dashed lines) for the limiting cases of diffusion (a) and barrier-controlled sorption (b). (Reprinted with permission from [221]. Copyright (1992) American Chemical Society)

corresponding to two distinctly different 129 Xe spectra. Due to the fact that for barrier-controlled adsorption/desorption the intracrystalline concentration may assume only two values, the distinction between the two lines is preserved over nearly the whole process (Fig. 34b), while for diffusioncontrolled adsorption/desorption the wide range of intracrystalline concentrations leads to rapid coalescence of the two lines (Fig. 34a). These simulations indicate that the adsorption/desorption process of benzene in ZSM-5 zeolite is controlled by intracrystalline diffusion. The intracrystalline transport diffusivity deduced from these simulations is found to be (1.3 ± 0.3) × 10–14 m2 s–1. This value is in satisfactory agreement with the results of previous uptake experiments [222–224]. However, by the present analysis it has been shown unambiguously for the first time that the adsorption/desorption process under such experimental conditions (intimately mixed activated and loaded crystallites) is controlled by intracrystalline diffusion, so that the values obtained are in fact real intracrystalline transport diffusivities.

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The proposed method is limited to slow adsorption/desorption processes, since it is only applicable if the time constant of these processes is large in comparison with the time necessary for the measurement of the 129 Xe NMR spectra. Moreover, the use of sufficiently large zeolite crystallites is inevitable so that the displacement of the xenon atoms during the reciprocal of the difference of the chemical shifts is still much smaller than the crystallite diameter.

Springuel-Huet et al. have also studied the diffusion of benzene in H- ZSM-5 [225], varying the experimental conditions so as to approach those of applications (fluidized or fixed bed reactors, gas phase chromatography, etc.):

1.Sorbate equilibrium between loaded and unloaded well-mixed crystallites, as in the previous procedure. These authors verified that during this experiment the adsorption/desorption process is controlled by intracrys-

talline diffusion. In this case 1.5 hour after the onset of the experiment, the 129Xe NMR spectrum has already attained its final shape.

2.Sorbate equilibrium between loaded and unloaded beds (each 5 mm deep)

of zeolite crystallites. The adsorption/desorption process is found to be significantly slowed down in this procedure. In this case the rate of attaining macroscopic equilibrium is controlled by the rate of molecular propagation through the bed; the equilibrium is achieved in about 7.5 hours. The intercrystallite diffusivity, Dinter , and the diffusion coefficient through the bed, D, are found to be 2 × 10–5 m2 s–1 and 1.3 × 10–9 m2 s–1, respectively.

3.Adsorption from the gas phase, corresponding to 6 C6 H6 molecules/u.c. at adsorption equilibrium. In this case, the retardation of uptake in comparison with the previous procedure (about 30 minutes) is a consequence of the spatial expansion of the gas volume over the bed of zeolite crystallites.

In conclusion, 129 Xe NMR demonstrates the dramatic effect of the arrangement of crystallites on the rate of intercrytalline exchange of guest molecules in zeolite beds. The exchange rate of benzene molecules in a bed of wellmixed crystallites of 129 H-ZSM-5 is at least one order of magnitude larger than the corresponding molecular exchange between two separate layers of crystallites, even for bed depths as small as 10 mm.

We can mention a more recent application of 129Xe NMR spectroscopy to study gaseous hydrocarbon diffusion in a fixed bed of ZSM5 zeolite [226].

3

Other Microporous Solids

3.1

Pillared Clays

Pillaring of clays by large cationic complexes (PILC) increases the thermal stability of materials which may have a potential catalytic importance, and

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generates microporosity between the layers, allowing reactive molecules to penetrate. The first study of PILC using 129 Xe spectroscopy was undertaken by Fetter et al. [227] in 1990. These authors observed practically no dependence of the chemical shift of xenon adsorbed at room temperature on the xenon pressure (7 ppm as PXe increases from 0 to 1.33 × 105 Pa). This suggests there is a rapid exchange with the gas phase and no Xe – Xe interactions or electric field effects. The value of the chemical shift extrapolated to zero pressure is 85 ± 2 ppm. This value is intermediate between those of Na – A (80 ppm) and L (90 ppm) zeolites. Assuming that xenon-lattice interactions are the same in zeolites and PILC and using the Demarquay–Fraissard relationship (Table 3) they obtained an average interlayer space of 1 nm, in good agreement with XRD and porosimetry measurements.

A more complete study was performed at variable temperature (110 < T < 298 K) by Barrie et al. [228]. The starting material was a gelwhite L containing Ca2+ and Na+ cations. Pillaring by treatment with aluminum chlorohydrate at 353 K, followed by calcination in air at 773 K, leads to the insertion of polymeric [Al13O4(OH)24]7+ cations which decompose to form alumina pillars after calcination. The interlayer spacing is 0.81 nm. Like Fetter, the authors did not find, whatever the adsorption temperature, an initial decrease in the chemical shift with xenon coverage, indicating that pillars do not act as strong adsorption sites; the value of the chemical shift at zero pressure is 96 ± 5 ppm. The Demarquay–Fraissard equation leads to values of the average interlayer space calculated for cylindrical or spherical pores (0.77 and 1.1 nm, respectively) close to the d-spacing obtained by XRD measurements.

From a detailed study of the effect of xenon concentration and lowering of the temperature, these authors concluded that in pillared clays xenon does not condense onto other xenon atoms but spreads out over the surface. These results show that the xenon NMR technique can be a useful tool to detect phases both in the interlayed spaces generated by pillaring and on the outer surface.

Yamanaka et al. [229] studied two kinds of SiO2 – TiO2 sol pillared clays; one prepared by air-drying (AD samples), the other by drying with CO2 under supercritical conditions (SCD samples). At room temperature, as observed by previous authors, the chemical shift does not depend on the xenon pressure, suggesting that the pores in pillared clays are isolated with no exchange of xenon atoms between the pores, at least at the NMR timescale. By extrapolation to zero xenon pressure, values of 98 and 103 ppm are obtained for AD samples calcined at 473 K and 773 K, respectively. The average pore diameters (for spherical cavities) extracted from (Eq. 20 and Table 3) were estimated to be 1.04 and 1 nm, respectively. In the case of SCD samples, which have a dual mode of porosity (micro as well as mesopores), the authors expected two distinct signals, but only one broad, slightly shifted (30 ppm) resonance line was observed. The Xe atoms must exchange rapidly between the two types of pores, because SCD samples have an open porous structure.