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

185

The effect of the dehydration and rehydration of zeolites on the influence of Ca2+ cations can also be studied by 129 Xe NMR [99]. For example, the

chemical shift δ and the signal width ∆ω of xenon adsorbed on Mg70 – Y increase with increasing dehydration of the solid. The values of δ and ∆ω are greatest when dehydration is complete. Conversely, the spectra evolve with rehydration: for a given xenon pressure, the line due to xenon in the supercages containing only bare Mg2+ decreases in favor of the line corresponding to Mg2+ surrounded by OH ligands and water molecules yielding hydrated (MgOH)+. 129Xe NMR, therefore, makes it possible to follow the diffusion of an adsorbate in a zeolite crystallite.

The problem is more difficult in the case of paramagnetic cations (Ni2+, Co2+). One must consider the term δM which influences the chemical shift. This term may be very large and leads to δ values of several thousand of parts per million [125]. In this case, the location and the oxidation state of the cations can be studied, in particular during reduction, oxidation or dehydration of such samples. For example, Scharpf et al. [126] have succeeded, using this technique, in following the reduction and reoxidation of the Ni7.5 Na – Y zeolite dehydrated under vacuum at 623 K. When the sample was reduced at lower temperatures (about 373 K), two types of environments for the xenon atoms were evident: one corresponding to xenon in the nickel-exchanged material, and the other corresponding to xenon in contact with Na – Y or HY. Reduction at higher temperatures produced a low-frequency shift of the first resonance, indicating that this environment becomes more like the environment of xenon in the Na – Y zeolite. At the highest temperature (643 K) studied, only the line corresponding to xenon in Na – Y was detected. These results prove that nickel ions are removed from the supercages upon reduction. Using this method, Scharpf et al. showed that reoxidation under 80 kPa of oxygen at various temperatures does not reverse the process of reduction. The distribution of the Ni2+ cations in the different sites of the zeolites has been studied by Bansal et al. for zeolites containing a low extent of exchangecations [127] and by Gédéon et al. [128, 129] in the case of Ni – Na – Y zeolites with a high Ni2+ concentration. By following the variation of the chemical shift with the number of atoms adsorbed, the evolution of the environment of these cations can be studied as a function of the pretreatment temperature, for each level of exchange, and their migration from the supercages.

In the case of Co – Na – Y, with 15% of Na+ cations exchanged, Bonardet et al. [130] have studied the dependence of the δM term on the pretreatment temperature Tt (Fig. 13). For 300 ≤ Tt ≤ 423 K the chemical shift extrapolated to zero pressure, δN→0, and the slopes of the δ = f (N) plots decrease. This corresponds to the departure of water molecules freeing the pores. The small shift difference compared to Na – Y is due to the presence of Co(H2O)2+6 partially blocking the pores. When Tt is higher than 423 K, the dramatic increase in chemical shift and the change in the shape of the δ = f (N) plots arising from paramagnetic Xe – Co2+ interactions prove that, despite the migration

186	J.-L. Bonardet et al.

Fig. 13 δ = f (NS ) variation for CoNaY (15% of Na exchanged): (a) Tt (K) 300, • 323,373, 423; (b) Tt (K) 373, • 423, 523, 573, 623, 773. Full line: δ = f (NS) curves for pure NaY zeolites treated at the same Tt as CoNaY. (Reprinted from [130], with permission from Elsevier Science)

of Co2+ out of supercages as shown by X-ray diffraction [131], Xe – Co2+ interactions are still detectable when water molecules are progressively eliminated from the coordination sphere of Co2+ cations. It has been proposed that Co2+ cations are located in SII sites of the sodalite cages.

2.3.4

Case of Trivalent Cations

The 129 Xe NMR technique has also been used to study the dehydration processes of La3+ and Ce3+ exchanged Y zeolites [132, 133]. In these studies, the authors have confirmed that all the cations La3+ and Ce3+ migrate to the hexagonal prisms and the sodalite cages when the zeolites are completely dehydrated. Later, Fraissard and coworkers [134] have extended the 129 Xe NMR investigation to other faujasite-type zeolites with trivalent cations, i.e., Y, Na – Y, La, Na – Y, Ce, Na – X and Y, Na – X. The chemical shifts of adsorbed

xenon follow a concave plot against the adsorbed xenon concentration, very similar to the previous 129 Xe NMR study of Mg2+, Ca2+, Zn2+, Cd2+ , Ni2+ ... in

X and Y zeolites. This result suggests strong adsorption of xenon on the Y3+ ions located inside the zeolite supercage. In contrast, the 129 Xe NMR chemical shift for Ce – X, La – X, Ce – Y and La – Y zeolites [132, 134] indicates that Ce3+ and La3+ cations are located in hexagonal prisms or sodalite cages inaccessible to xenon atoms. The example of Y3+ cations proves that not all trivalent cations migrate out of the supercages during thermal treatment of the solid.

The location of Ru3+ ions in Y zeolites and their redox behavior have been studied by Shoemaker et al. [69]. The increase in shifts in the down-field

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

187

direction was attributed to the interaction of xenon atoms with the paramagnetic Ru3+ centers. From chemical shift and line-width variations, these authors have shown that after reduction at room temperature, the ruthenium particles are highly dispersed and located in the zeolite cages. However, migration of the metal occurs upon evacuation.

2.3.5

Effect of the Electronic Structure (nd10): Ag+, Cu+, Zn2+, and Cd2+ Cations

A particularly interesting case is that of faujasite-type zeolites containing Ag+ or Cu+ cations, the external electron structure of which is nd10. In experiments on silver-exchanged Na – Y and Na – X zeolites, Fraissard and coworkers found an unusual and unexpected up-field (i.e. low-frequency) 129 Xe chemical shift of about – 40 to – 50 ppm (relative to xenon gas at zero pres-

sure) for fully silver-exchanged zeolites in the limit N → 0 [135–139]. This is in contrast to all other 129 Xe NMR studies on cation-exchanged X and Y

zeolites which all show high-frequency shifts. Figure 14 shows the 129Xe NMR isotropic chemical shifts of xenon in Na – X and in the fully silver-exchanged zeolite Ag – X. The shifts in dehydrated and oxidized Ag – X are distinctly lower than that for Na – X over the range of concentration studied. Most remarkably, the shifts decrease with Xe concentration, exhibiting negative values in the range – 40 to – 50 ppm at low xenon concentration. In contrast to these results, the samples reduced at 373 K, and 673 K show high-frequency shifts with respect to Na – X. After reduction at 373 K, δ increases steadily with the number of xenon atoms per supercage (Ns ) from + 100 ppm to about

Fig. 14 δ = f (NS) variation for the zeolites: Na – X, • Ag – X (dehydrated at 673 K),Ag – X (oxidized at 723 K), Ag – X (reduced at 373 K); Ag – X (reduced at 673 K). (Reprinted from [135], with permission from Elsevier Science)

188 J.-L. Bonardet et al.

+ 170 ppm for about 0.3 < Ns < 4, whereas after reduction at 673 K the shift values are between + 140 and + 160 ppm for 0.1 < Ns < 1, with a shallow minimum at about Ns = 0.7. In the latter case, the plot of δ = f (N) has the classical shape of zeolite-supported metals. This unusual low-frequency shift in the dehydrated and oxidized samples has been attributed to specific Ag+ – Xe interactions which are caused by the existence of a fully occupied d-shell in this cation. In order to explain the up-field shift, the authors postulate a shortlived Ag+ – Xe complex and a 4d10-5d0 donation from the transition metal cation to xenon [135]. Similar phenomena have also been observed for the 77 Se resonance in the presence of Ag+ cations [140].

Later experiments showed that fully exchanged CuY [141, 142] and CuX [143] zeolites exhibit shifts which are much lower than expected. The parabolic form of the δ = f (N) curves, which was expected because of the presence of paramagnetic Cu2+ is not observed (Fig. 15). In order to explain this behavior, Gédéon et al. [143] attributed this down-field shift to a 3d10 -5d0 donation mechanism between Cu+ ion and Xe. Boddenberg et al. have attributed this to the presence of sites bereft of cations in the supercages [144]. Indeed, during dehydration, there is at the same time migration of Cu2+ from the supercages and partial autoreduction of Cu2+ to Cu+inside the supercages. These easily accessible Cu+ cations are able to behave like Ag+ and participate in 3dπ -5dπ electron donation. This behavior is not observed for Zn2+ and Cd2+ in X-type zeolite and seems not general among cations of nd10 configuration. It has been proposed that, after dehydration, Zn2+ and Cd2+ interact strongly with the matrix and are situated in SII sites, a position which

Fig. 15 δ = f (N) variation for the zeolites: ( ) NaX, (♦) AgX, (•) CdX, ( ) ZnX, (o) CuX. (Reprinted from [143], with permission from Elsevier Science)

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

189

prevents such dπ donation. The higher charge of the cation, responsible for a greater polarization of the electronic cloud leading to a high chemical shift, can also be put forward and may compete with the electron donation.

Gédéon et al. [145] have also studied the oxidation state as well as the location of copper in CuY zeolites at various levels of exchange and hydration, and at different stages of redox treatment. Information about the nature of

Xe – Cu+ or Xe – Cu2+ interactions has been obtained by a combination of ESR and 129Xe NMR. Similar studies have been done by Liu et al. [146]. The

combined ESR and 129 Xe NMR studies enabled these authors to investigate the reduction/oxidation processes of the copper ions.

More recently, quantum chemical ab-initio calculations have been per-

formed by Freitag et al. [147] for the interaction between xenon atoms and the cations Li+, Na+, K+, Cu+ and Ag+. The 129 Xe NMR chemical shifts have

been calculated. For the alkali ions a small down-field shift is obtained which is attributed to the polarization of the Xe wave function by the charge of the cation. For the Cu+ and Ag+ an additional up-field shift is found which is caused by a mixing of the 5p, 4p, 3p orbitals at Xe with the 3d or 4d orbitals at Cu+ or Ag+. This leads to an increased magnetic shielding and consequently to low chemical shifts.

2.3.6

Location and Number of Cations: Quantitative 129Xe NMR

The concept of quantitative 129 Xe NMR spectroscopy concerns the analysis of experimentally detected isotropic chemical shifts in terms of site specific shifts and of the concentrations of various cation sites as well as the exploration of the information content of the widths of the resonance lines.

A systematic investigation of the effect of the cation on the chemical shift of 129 Xe adsorbed on zeolites has been carried out by Fraissard and coworkers. We have seen that these authors suggested an explanation of the 129 Xe NMR shift in terms of different contributions from Xe – Xe interactions and Xe-zeolite interactions. The magnetic and electric contribution has been evaluated in the case of nickel-exchanged Y zeolite [127, 128].

Cheung et al. has offered a quantitative interpretation by the use of a model in which the xenon atoms adsorbed on the wall of the supercages are in rapid exchange with those in the gaseous phase [50]. Later, Liu and coworkers [148] have carried out a detailed investigation of the effect of ion exchange in Na – Y zeolite on the adsorption strength toward xenon atoms and on the

change in the 129 Xe NMR chemical shifts. They found that the dependence of the 129 Xe NMR chemical shift on the amount of adsorbed xenon can be fit-

ted with a second order polynomial in all cases. This is interpreted by treating the adsorbed xenon as a two dimensional gas with a virial expansion model similar to the case of xenon in the normal gaseous state. According to this model, the major force for the adsorption and various kinds of interaction of

190	J.-L. Bonardet et al.

xenon is the van der Waals force, but the coulombic force of the cations also plays a role.

In a series of papers [137, 149–155], a model of xenon adsorption was introduced that is capable of explaining quantitatively the dependence of the chemical shift on the concentration of sorbed xenon in Y-zeolites exchanged with transition metal ions such as silver, copper, zinc and cadmium. According to this model, the adsorption isotherms of Xe in transition metal ion-exchanged zeolites can be quantitatively described by the isotherm equation:

NS = n1	k1P	+ n2	k2P	+ k3P,	(21)
	1 + k1P		1 + k2P

where NS is the concentration of Xe sorbed in the supercages at the pressure P; n1 and n2 are the concentrations of the transition metal cation sites which were identified with ions residing on the crystallographic SIII (index 1) and SII (index 2) positions; k1 and k2 are the Langmuir adsorption constants on the respective sites; k3 is a Henry adsorption constant which describes the interaction of Xe with the residual supercage surface not occupied by the transition metal cations. Figure 16 shows, as an example, the xenon adsorption isotherm for the oxidized AgY zeolite as well as its components. The concentrations of the cations, ni, and their adsorption constants ki are obtained from the curve fit.

Using this model with zeolites containing different levels of silver cations, it is possible to monitor the number of the silver cations (species 1 and 2) in the supercages as a function of the overall silver content (Fig. 17).

Fig. 16 Overall and composite adsorption isotherms of xenon in the zeolite AgY(ox) according to the model discussed in the text. The composite isotherms belong to — type 1,

- - - type 2, and -·- type 3 sites in the supercages. (Reprinted from [137], with permission from Elsevier Science)

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

191

Fig. 17 Concentrations of supercage silver cations in dehydrated (673 K) and oxidized (673 K) AgNaY zeolites as a function of the overall silver content. Type 1: Ag+ in (+) dehydrated and oxidized zeolites. Type 2: Ag+ in × dehydrated and ◦ oxidized zeolites. (Reprinted from [137], with permission from Elsevier Science)

The experimentally observed 129 Xe NMR chemical shifts, δ = f (P), can be quantitatively reproduced with the aid of the equation:

δ = Ns		1 + k1P δ1 +	1 + k2P δ2 + k3Pδ3		+ FNS .	(22)
1		n1k1P	n2k2P

The individual chemical shifts δi of xenon interacting with the metal cation sites (i = 1, 2) and the residual surface (i = 3) can be determined. The term FNS takes into account the mutual Xe – Xe interactions to a first-order approximation [38].

Boddenberg and coworkers [155] have demonstrated that the observed values of the linewidths as well as their quite different dependence on pressure can be explained on the basis of a theoretical approach which is capable of explaining semi-quantitatively the 129 Xe NMR line widths versus xenon concentration curves for a series of zincand cadmium-exchanged Y zeolites. This theoretical model traces the origin of line broadening back to inhomogeneous spatial distributions of transition metal cation sites interacting strongly with the encaged xenon atoms. Using Eqs. 21 and 22, these authors determined the distribution of Zn2+ [156].

2.3.7

Cation Exchange Between Different Zeolites

129 Xe NMR of adsorbed xenon can be used to follow the exchange of cations in the zones accessible to xenon in a given crystallite, and to visualize the

192 J.-L. Bonardet et al.

exchange of cations between different zeolites. Chen et al. have studied the cation exchange process between RbX and Na – Y zeolites. The two peaks detected at 144 and 89 ppm correspond to xenon adsorbed in dehydrated Rb – Na – X and Na – Y zeolites, respectively [73]. Upon mechanical mixing at 300 K, the intensities of the two signals decrease and a third broad signal at about 141 ppm appears. After further treatment of the mixture at 673 K and 0.01 Pa, there is a further larger decrease in the intensities of the Rb – Na – X and Na – Y signals, the peak at 141 ppm disappears and two additional broad signals are seen at 128 and 109 ppm.

The two xenon resonance signals corresponding to the two pure zeolites can be restored upon cooling the sample to 200 K, which proves that the third signal at 141 ppm is a coalescence of two signals. The mixture obtained directly in the tube by shaking is very inhomogeneous with regard to the distribution of the crystallites of the two zeolites. There are three types of zones: pure Rb,Na – X, pure Na – Y, and a Rb,Na – X,Na – Y mixture. Because of slow diffusion, the three corresponding lines can be distinguished at 300 K. Consequently, the information obtained by 129 Xe NMR at 300 K is characteristic of macroscopic zones, i.e. zones containing several crystallites. The extent of these zones depends on the mean lifetime of the xenon atom in the crystallites compared with the NMR timescale. These zones can be reduced by lowering the temperature of the NMR experiment in order to obtain more information about localization. The rate of exchange of xenon atoms between one crystallite and another must also depend on the crystallite size and, above all, on the barrier for diffusion to the external surface; all other things being equal, this barrier is related to the size of the windows of the cavities and

channels.

The low-temperature 129 Xe NMR spectra of xenon adsorbed on the zeolite mixture show that, down to 200 K, there are still four well-resolved peaks, two of them corresponding to the original Rb,Na – X and Na – Y zeolites. The other two signals located between those of RbNa – X and Na – Y should cor-

respond to the Rb58–x Na23+x X and RbxNa56–xY zeolites, that result from ion exchange between the two original solids.

However, in an investigation of a mixture of dehydrated Na – Y and Ca – Y, Ryoo et al. [70] pointed out that the single 129 Xe NMR peak observed after thorough mixing of the samples could result from fast xenon exchange between the crystallites, rather than from ion exchange. This is supported by the observation of two 129 Xe NMR peaks on cooling the sample, when the exchange rate of xenon between particles is reduced [71]. If solid-state ion exchange had occurred, the two signals would be seen if the temperature was reduced sufficiently to retard the exchange of xenon between cages, which would only be expected at much lower temperatures, while in fact two peaks are observed at temperatures as high as 263 K.

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

193

2.3.8

Effect of Nonframework Aluminum

It is well known that nonframework aluminum, AlNF, plays an important role in the catalytic properties of zeolites. The presence of these species is usually checked by 27 Al NMR spectroscopy but the nature of these species is

still unclear. Chen et al. [157, 158] have studied the nature of the AlNF in MFI-type zeolite by 129Xe NMR and 27Al NMR. 27Al NMR showed that the

reference sample (R) did not contain any nonframework aluminum (Table 4). Conversely, the samples prepared in fluoride medium, A, B and C always contained some AlNF. The quantity of AlNF in the samples depends on the synthesis conditions.

The four samples studied by 129 Xe NMR have different chemical shift variations with N (Fig. 18). For sample R, δ increases monotonically with N. For the samples synthesized in the fluoride medium, δ first decreases and then increases with N. This indicates the presence of some strong adsorption sites inside the channels of the samples. These sites can only be more or less

Table 4 Aluminum concentration per unit cell

Sample	Total aluminum	Framework Al	Nonframework Al
		(AlF)	(AlNF)
R	4.0	4.0	0
A	8.0	4.0	4.0
B	3.4	2.9	0.5
C	2.2	1.6	0.6

Fig. 18 δ = f (N) variations: • A; B; C; R. See Table 4. (Reprinted from [157], with permission from Elsevier Science)

194 J.-L. Bonardet et al.

charged AlNF species. Comparison of the number of AlNF and the δN→0 value shows that the charge on AlNF increases in the order A < B < C. The average charge on each AlNF atom depends on the amount of AlNF and on the AlNF/AlF ratio inside the zeolite.

Similar upward curvatures in xenon chemical shift plots have been reported by Bonardet et al. [159] for xenon adsorbed in steam-treated ultrastable NH4Y zeolites, and attributed by these authors to specific interactions of Xe with extra-framework aluminum species. Likewise, Bradley et al. [160] observed highly curved chemical shift versus coverage plots for xenon in aluminosilicate and gallosilicate MFI zeolites and proposed the existence

of charged extra-framework aluminum species. Theses authors showed that 129 Xe NMR can be used to indicate changes in the nature of the extra-

framework Al during the steam treatment.

To detect the AlNF species in Y zeolites, it is necessary to decrease the experiment temperature, i.e., the xenon mobility. This is justified by the fact that in Y zeolites, where the pores are bigger than in ZSM-5, the influence of the pore surface on the chemical shift is smaller at a given temperature.

2.4

Bulk and Distribution of Adsorbed Phases

2.4.1 Introduction

The use of NMR techniques such as relaxation time measurements, pulsefield gradient or line-shape analyses to study the molecular dynamics and consequently the location of adsorbed molecules (water, light hydrocarbons

...) in the micropores of zeolites does not allow us to easily obtain pertinent information about the adsorbate distribution. The simple 129Xe NMR method, sometimes coupled with multiquantum NMR measurements, has been successfully used to follow the encumbering or the blocking of the internal volume of molecular sieves during adsorption or coking.

2.4.2

Study of Adsorbed Organic Molecules

The first study of the bulk and distribution of organic molecules trapped in a zeolite was performed by Pines and coworkers [161, 162]. They examined the distribution of hexamethylbenzene (HMB) in an Na – Y zeolite dehydrated at 673 K under vacuum (10–4 Pa). They observed two signals, the relative intensities of which depended on the number of HMB molecules in the supercages. When the loading corresponded to an average of 0.5 molecule per supercage, the two signals had the same intensity (Fig. 19). When the sample was heated to 573 K, the spectra displayed a single line, suggesting