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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Fig. 7 Room-temperature spectrum of xenon adsorbed in Na – A zeolite at 523 K and xenon pressure of 4 MPa. Each peak corresponds to a definite number of xenon atoms (1, 2, 3, 4 and 5) in the α-cages. (Reprinted with permission from [82]. Copyright (1988) American Chemical Society)

gas particles among the cells [88]. To describe 129Xe NMR data for xenon trapped in the α-cages of the Na – A zeolite, it is necessary that, in addition to the attractive interactions between the particles, the repulsive interactions should also be included when the xenon atoms begin to fill the α-cages of the zeolite.

With more sophisticated NMR experiments (multisite magnetization transfer experiments or two-dimensional exchange NMR) and simulations, the dynamics of xenon movement between α-cages can be investigated [86, 89]. Larsen et al. give an activation energy of about 60 kJ/mole for cage-to- cage migration [86].

2.2.6

Porosity Studies

In order to obtain from δ(129 Xe) precise data about the void space of a zeolite of unknown structure or on the dimensions of structural defects, the term δS has been related to the size and shape of the internal volume by means

of the mean free path of adsorbed xenon imposed by the structure (Eq. 8 and Fig. 3). In the fast exchange model, for a sufficiently high temperature, a xenon atom moves freely inside the void space. This random movement is the basis of the determination of the mean free path, either calculated for a closed sphere or an infinite channel, or using a computer in the general case [56].

The computer is given the analytical expressions of the modeled structures of zeolites. A xenon atom is then randomly circulated within the space. The distance, , travelled between two successive collisions is calculated. The mean free path, , is obtained by averaging these distances (Table 3). The plot

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Table 3 Values of δS and mean free path for various molecular sieves. The case of infinite

cylinder and sphere can be rigourously solved by calculation: infinite cylinder: = DC – DXe ; sphere: = 0.5(DS – DXe ) where DC, DS and DXe are the diameters of the cylinder, the sphere and the xenon atom, respectively. The values of pore size given above are from [90]

Molecular	δS				Characteristics of the void spaces
sieves	(ppm)	(nm)			accessible to xenon atom
A, ZK4	87	0.37			sphere, diameter 1.14 nm with six 8-ring
					openings of 0.4–0.5 nm, depending on the cation
L	90	0.31			one dimensional barrel-shaped channels: 12-ring
					openings of 0.71 nm, max. diameter 0.74 nm
Ferrierite	227	0.01			c-channel, 10-ring: 0.54 × 0.42 nm
	157	0.04			b-channel, pseudo-sphere, diameter: ≈ 0.7 nm
					with 2 8-ring openings: 0.48 × 0.35 nm
Mordenite	115	0.245			one dimensional channel 12-ring: 0.67 × 0.7 nm
	250	0.005			side-pocket: 0.57 × 0.26 × 0.48 nm
Offretite	108	0.2			one dimensional channel 12-ring: 0.67 × 0.68 nm
ZSM-12	90	0.13			one dimensional channel, non planar
					12-ring 0.55 × 0.59 nm
ZSM-20	58	0.56			intergrowth of cubic and hexagonal faujasite: normal
					supercage with four 12-ring apertures of 0.74 nm
					diameter, “maxi” supercage with five 12-ring
					openings, 0.71 × 0.71 nm (for two) and
					0.74 × 0.65 nm (for three) and “mini” supercage with
					three 12-ring openings 0.74 × 0.65 nm
ZSM-23	114	0.045			one dimensional channel, 10-ring: 0.52 × 0.45 nm
ZSM-48	96	0.11			one dimensional channel, 10-ring: 0.53 × 0.56 nm
EU-1	94	0.17			one dimensional channel, 10-ring: 0.41 × 0.57 nm
					“side pockets”: 0.68 × 0.58 nm, 0.81 nm deep
Ω	75	0.30			one dimensional channel, 12-ring, 0.74 nm
Theta-1, Nu-10, 130		0.05			one dimensional channel, 10-ring: 4.4 × 5.5 nm
ZSM-22					cavity 0.63 × 0.63 × 1.51 nm with six openings:
Erionite	99	0.17
AlPO4-5,					0.36 × 0.52 nm
	56	0.29			one dimensional channel, 12-ring: 0.73 nm
SAPO-5					one dimensional channel, 14-ring: 0.79 × 0.87 nm
AlPO4-8	58	0.39
AlPO4-11,	120	0.056			one dimensional channel, 10-ring: 0.39 × 0.63 nm
SAPO11
AlPO4-17	72	0.17			erionite structure
SAPO-31	85	0.09			one dimensional channel, non planar 12-ring: 5.3 nm
SAPO-34	84	0.18			chabazite structure: cavity 0.67 × 0.67 × 1 nm with
					six 8-ring openings 0.38 nm
MAPO-36	74	0.26			one dimensional channel, 12-ring: 0.65 × 0.75 nm
Y, SAPO-37	58	0.56			faujasite structure
SAPO-41	108	0.11			one dimensional channel, 10-ring: 0.70 × 0.43 nm
VPI-5	49	0.77			one dimensional channel, 18-ring: 1.21 nm

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of δS = f ( ) for a few classical zeolites (A, Y, L, MOR, Rho ...), with usual Si/Al ratios, has hyperbolic shape, curve I in Fig. 8, and Fraissard et al. first wrote:

	1
δs =	λ + µ δa,	(20)

which is consistent with Eq. 8, λ and µ are fitting parameters. This expression has to be modified in the light of more recent work: δa cannot be constant for all structures. It was said above that δa is related to the molecular interaction energy (W) of Derouane’s model, which depends on the surface curvature, hence on . Moreover, δV has been assumed to be equal to zero; this is certainly true for large void volumes (compared to the xenon diameter), such as in faujasite; but for zeolites whose channels are small compared to the xenon diameter, δV is not zero and depends on δa and , as we have seen in Sect. 2.2.

On the other hand, Chen and Fraissard have shown that the fast site exchange hypothesis is valid in the case of Y zeolite for T > ca. 303 K, which is not far from the experimental temperature [60]. This is certainly not true for zeolites with smaller void volumes.

Fig. 8 δS = f ( ). See Table 1 for the coordinates of all molecular sieves.

Curve I (classical zeolites): Rho (cavity and prism), Ferrierite: F (channels b and c), Mordenite: Z (main channel), L, A, Y.

Curve II (new molecular sieves): Theta-1: Θ-1, AlPO4-11: A-11, ZSM-5: Z-5, ZSM-48: Z-48, EU-1, ZSM-12: Z-12, SAPO-34: S-34, AlPO4-17: A-17, AlPO4-5: A-5, SAPO-37: S-37, VPI-5. (Reprinted from [2–4] with permission from Springer-Verlag)

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Another important point is the dependence of the chemical shift on the chemical composition of the zeolite surface, in particular upon the number of charges, i.e. on the Si/Al ratio. This dependence is more pronounced when the temperature is low and the pore diameter is small. In the case of faujasite, either in the Na+ form or decationized, the δS values decrease monotonically by 4 ppm when Si/Al increases from 1.28 to 54 [2–4]. Such variations have been also observed by Chao et al. [90]. These authors showed that the reduction of the δS value with decreasing Al content of faujasite type zeolites is not linear and depends on the pore size and extra-framework species formed by dealumination. For ZSM-5 and ZSM-11 (narrow channels) the dependence of δS on the aluminum concentration is greater than for faujasite (large cavities) [91–93]. For instance, δS increases linearly with the Al content of the framework on ZSM-5 and ZSM-11 zeolites [93]. In addition, this variation shows a break at about [Al] = 2 atoms per unit cell (Fig. 9) which demonstrates that the xenon-wall interactions change at this concentration. This can be explained by a change of the Al distribution in the framework. Finally, we mention the difference between the δS = f (N) relationships for ZSM-5 and ZSM-11 for [Al] ≥ 2 Al atoms per unit cell (Fig. 9). Despite the similarity of the structure of these zeolites, the sensitivity of the 129Xe NMR technique is sufficient to differentiate between them. The variation of the δS value with framework composition has been also reported for SAPO-37 [94], AlPO4-5 and SAPO-5 [95] molecular sieves.

These various deviations from the simple model are responsible for the fact that many newly studied structures, mainly structures with very low cation content, do not fit the first δS = f ( ) curve; they form a cloud of points

limited by a second curve for 0.05 < < 0.4 nm, curve II in Fig. 8 [48]. Although the models are essentially qualitative this technique has been

widely used to study microporous solids.

Fig. 9 δS variation versus Al content of the framework: ◦ NaZSM-5; NaZSM-11. (Reprinted with permission from [93]. Copyright (1992) American Chemical Society)

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2.2.6.1

Zeolite with a Single Type of Void Volume

When the zeolite structure contains only one type of pore (more or less cylindrical channels or spherical cavities) accessible to xenon, the xenon spectrum consists of a single line characteristic of the adsorption zone.

The main information is generally obtained by analyzing the dependence of the chemical shift of this signal on the xenon concentration. The amount of xenon adsorbed, e.g., at 300 K is expressed as the number, N, of atoms per gram of anhydrous zeolite or the number, Ns, per cage or part of the channel. At 300 K, the δ = f (N) variation is a straight line when the distribution of Xe – Xe collisions is isotropic, i.e. in the case of large cavities (Y, ZK4 ...). The slope, dδ/ dN, is proportional to the local Xe density and therefore inversely proportional to the internal free volume. If the distribution of Xe – Xe colli-

sions is anisotropic (narrow channels, diameters between 4 and 7.5 ˚), the

A

slope of this curve increases with N [48, 96] (Fig. 10).

The chemical shift, δS, at zero concentration is clearly related to the structure: the smaller the channels or the cavities, or the more restricted the diffusion, the greater δS. For example, δS (ZK4) is greater than δS (Y) not only because the α-cage diameter (1.14 nm) is slightly smaller than the supercage one (1.3 nm) but, also and especially, because the diffusion of xenon in ZK4 zeolite is more difficult than in Y. The dimensions of the openings allowing passage from one α-cage to another are similar to those of xenon atoms, whereas those of the supercages are twice as big as Xe.

Fig. 10 δ = f (N) variations for different molecular sieves: ZSM-23, + SAPO-41, ♦ silicalite-1, ◦ EU-1, ZK-4, MAPO-36, ♦ Beta, LZY-52, (•) VPI-5. (Reprinted from [2–4], with permission from Springer-Verlag)

180 J.-L. Bonardet et al.

Many papers deal with studies of Y zeolites [97–100], where it is often a question if structural changes arise after dealumination. Conclusions are frequently based on variations of the internal volumes [90, 101, 102]. Some other less common zeolites or related materials have been investigated, such as ZSM [92, 93], L [103], Theta [104], VPI-5 [105, 106], SAPO-34 [107], SAPO37 [95, 108], SAPO and AlPO4 – 5 [95], SAPO and AlPO4 – 11 [109], etc.

The chemical shifts, δS, of Na – Y and SAPO-37, which is isostructural with Na – Y, are very similar, showing that to a first approximation the fast exchange hypothesis is valid for such an open structure [94, 95]; the chemical variation of the surface does neither modify the residence time of the adsorbed xenon on the surface nor the δa value. Note that Davis et al. found chemical shift differences for these two structures, but this was due to a partial destruction of the SAPO-37 structure, which is fragile in the presence of humidity [110].

For very small pore diameters, the Si/Al ratio is relatively more important and for a given mean free path, , the chemical shift varies from curve I (say Si/Al < 6 or 7) to curve II (higher Si/Al ratios), Fig. 8. The influence of the chemical composition of the surface is highlighted by the existence of these two curves. The main reason for this chemical shift variation is that the residence time of a xenon atom on the surface, τa, is not negligible at room temperature.

Nevertheless, the empirical and experimental relationship δS = f ( )— Eq. 20—was used with success for the approximate determination of the internal void volume space of beta zeolite when the structure was still unknown [111].

2.2.6.2

Zeolites with Several Types of Void Volume

Except in the case of Na – A, already discussed, 129 Xe-NMR spectra have as many components as there are different types of void volume in the zeolites, at least if exchange between xenon adsorbed in the different zones is slow on the NMR time scale.

This is the case of ferrierite whose spectra have two lines corresponding to the two types of channels [112]. This example is particularly interesting: the diffusion from one channel to another is slow and into one type of channels only one Xe atom can enter. Therefore, the corresponding signal is independent of the xenon pressure. Only the line intensity increases with the filling degree.

It is also the case of Rho zeolite, but here the spectrum depends on the nature of the cation and on the temperature. For H – Rho, there is a rapid exchange between cavities and prisms. The two characteristic lines are then obtained only at low temperature [113]. On the contrary, for Cs – Rho there is only one line, since the Cs cations are located in the prisms and prevent Xe atoms from being there. The 129 Xe NMR study of this structure was later

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extended more thoroughly to a set of H – Cs Rho samples with variable Cs content [114] and for Cd Rho [115]. It must be noted that in these two papers the attributions of the lines have been changed: the high-frequency signal previously attributed to xenon adsorbed in the prisms (or at least xenon exchanging between cavity and prism, depending on the temperature) is assigned to the cavities, and the low-frequency signal previously attributed to xenon adsorbed in the cavities is assigned to xenon adsorbed at the external surface. The flexibility of this structure as a function of the temperature has also been studied [116].

Mordenite, whose porous structure consists of one-dimensional channels connected to side pockets in the perpendicular direction, has also been studied by 129 Xe NMR [61, 117]. The spectra have two signals corresponding to channels and side-pockets, respectively, at least if the temperature is low enough to prevent exchange between these two sites. The temperature of coalescence depends on the nature of the cations. It is about 273 K for H+ and about 370 K for Na+. If the cation is Cs+, the side-pockets are no longer accessible to xenon, and there is only one line. Moudrakovski et al., using 2D NMR could determine rate constants for xenon exchange between the main channels and side pockets [118].

Cloverite is a zeolite with two types of void spaces for xenon adsorption. But in the only published paper, to our knowledge, its 129Xe-NMR gives a single-line spectrum corresponding to the large supercages [119]. The authors conclude that the apertures of the second type are blocked by residual carbonaceous products arising from template decomposition.

When there are several signals corresponding to various types of void volumes, the line intensities can be used to study the distribution of adsorbed xenon in these different spaces for a given total xenon concentration as a function of the temperature and the location of cations. In this way one can obtain interesting information about the structures of zeolites, e.g. zeolite intergrowths and crystallinity.

2.2.6.3

Zeolite Intergrowths and Crystallinity

In view of what has been said about intercrystallite diffusion, if there is a mixture of zeolites or a structure intergrowth, each zeolite component will give rise to its own NMR lines in the spectra, providing that the diffusion of Xe between monocrystalline domains is not too fast and prevents the averaging of Xe-zeolite interactions. If the latter condition is satisfied and since NMR spectroscopy is quantitative, the measurement of peak areas allows the determination of the zeolite composition. Of course, the adsorption isotherms of standard components must be known. This has been checked with a synthetic mixture of Ca-A and Na – Y. The composition was easily and precisely (to ± 1%) found [120].

182 J.-L. Bonardet et al.

In the same way, the composition of a ferrierite-mordenite intergrowth, which was very poorly detected by X-ray diffraction, was determined [112]. It should be pointed out that the zeolite structures which give intergrowths are usually very similar, which means that the void volumes and the corresponding lines are very close. The case of ferrierite-mordenite is particularly favorable, because mordenite gives a high-frequency line (about 250 ppm, due to side-pockets) well separated from the others. Moreover, after line decomposition, a signal coming from ferrierite can be used to obtain additional information about the composition of the mixture. On the contrary, it has so far been impossible to resolve offretite-erionite intergrowths [121].

Chen et al. obtained a line corresponding to the AlPO4 – 8 structure in a VPI-5 sample; AlPO4 – 8 is the degradation product of VPI-5 [105]. A ZSM- 5-ZSM-11 intergrowth in a ZSM-8 sample could also be seen [121, 122].

The problem of crystallinity is of the same type; there are several approaches:

•At a given pressure, comparing the intensity of the “well crystallized” lines with that of the standard zeolite gives a measure of crystallinity [120].

•It has been said that the slope of the δ = f (N) curve, expressed in Xe atoms/g, depends on N. If the amount of xenon adsorbed by the amorphous phase is negligible at room temperature, then the ratio of the slopes of the two δ = f (N) curves for one sample and the standard is also a measure of the crystallinity.

•Finally, in the most favorable case, treatment of the zeolite can create secondary porosity. This situation reduces to that of zeolites with several zones and has been observed for Y zeolite (Fig. 11), mordenite, or with the dealumination of ZSM5 during the methanol to gasoline conversion [121].

Fig. 11 Room temperature spectrum of xenon adsorbed in dealuminated Y zeolite at a pressure of 0.2 MPa. The smaller signal corresponds to secondary porosity. (Reprinted from [2–4], with permission from Springer-Verlag)

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2.3

Influence of Cations

2.3.1 Introduction

It has been explained in Sect. 2.1 that, when xenon adsorbs more strongly on some adsorption sites (SAS), a term δSAS, characteristic of this interaction, must be considered. Indeed, these sites can be saturated while the adsorption on the other sites continues to increase monotonically with [Xe]. At low Xe pressure, the term δSAS predominates. Therefore, when the pressure increases, adsorption occurs on the weaker sites. The result is a decrease of the observed chemical shift which is a weighted average of δXe and δSAS. Afterwards the chemical shift again increases due to important Xe – Xe interactions at higher pressures. These characteristic δ = f (N) plots have been observed for X and Y zeolites containing cations like Mg2+, Ca2+, Zn2+, Cd2+, rare earth cations Y3+, La3+, Ce3+ and even paramagnetic cations Ni2+, Co2+, Ru3+ . Most of these studies have been carried out as a function of the cation exchange degree or the temperature of thermal treatment (i.e. the extent of dehydration of the zeolite). Since Xe atoms can only interact with cations located in the supercages and not with those in the sodalite cages or prisms, the location of cations can be deduced. The migration of cations inside the crystals, between different sites, have been studied in terms of their hydration state.

2.3.2

Case of H and Alkali-Metal Ions

It has been shown that for X and Y zeolites the influence of the H+ and Na+ cations on the 129 Xe NMR is negligible at room temperature [1–4, 97, 99,

123, 124]. The chemical shift is then given by the terms δS and δXe of Eq. 6 and is roughly independent of the value of the Si/Al ratio, and therefore, of the number of H+ or Na+ ions (Fig. 12). These results prove that in X and Y supercages, the time-averaged field δE due to these cations is negligible at 300 K. At very low N, the motion of each atom is disturbed only by cage walls. Consequently, the chemical shift, δS (58 ± 2 ppm) obtained by extrapolation of the line δ = f (N) to N = 0 can be considered as characteristic of the zeolite structure. The increase of δ with N results from mutual interactions between Xe atoms. However, for analogous zeolites containing other monovalent cations, several 129 Xe NMR studies have shown that the chemical shifts vary linearly with xenon loading [99, 123, 124]. The plots δ = f (N) depend on the type of cation; the heavier the cation, the greater the shift. For example, Ito et al. [99] have shown that δN→0 is 78 and 99 ppm for KY and RbY, respectively.

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Fig. 12 δ = f (N) variation for HY, NaY with different Si/Al ratios: 1.35, ♦ 2.42, () 54.2, ◦ 1.28 and for MgY zeolite for various magnesium contents: 47%, 53%,

• 62%, 71%

2.3.3

Influence of Divalent Cations: Diamagnetic and Paramagnetic Ions

Fraissard et al. [97–99] have studied xenon adsorption on Mgλ – Y zeolites, under vacuum at 773 K, where λ denotes the degree of cation exchange with Na+ (Fig. 12). When Mg2+ cations are in the sodalite cages or the hexagonal prisms without any contact with xenon (λ < 53%), δ is a linear function of the xenon concentration (as for Na – Y). When some Mg2+ cations are situated within the supercages (λ > 53%), one observes values of δ (compared with Na – Y) that are greatest when λ is high, especially at low xenon concentrations. For each value of λ the curve of δ = f (N) passes through a minimum; the higher λ is, the more pronounced and further shifted to higher concentration is the minimum. According to Ito et al. [99], the experimental value of δN→0 for N = 0 is roughly proportional to the square of the electric field at the nuclei of xenon atoms adsorbed on Mg2+ cations.

The large positive shift and the parabolic behavior of the δ = f (N) curves in the case of divalent cations was attributed first by Fraissard et al. [2–4] to the high polarizability of xenon and the distortion of the xenon electron cloud by the strong electric fields created by the 2+ cations. According to Cheung et al. [50], the high value of δ is not solely due to that. They suggest the formation of a partial bond between these two species formed by the donation of a xenon 5p electron to the empty s orbital of the divalent cation. A similar model concerning electron transfer from xenon to platinum was proposed by Ito et al. to explain the large shift in δ for platinum supported on Na – Y (see Sect. 2.5 [182]).