Zeolites cation migration

Klein et al. (1995) NMR NaLaY zeolite Cation migration/ localization of reactant + + + Xylene isomerzation... 

Solid State Ion Exchange and Intra-zeolite Cation Migration... 

Kucherov and Slinkin reported solid-state reactions of H-mordenite and HZSM-5 zeolites with metallic oxides such as CuO (13), Cr2O-, Mo03, and V205 (14-17). The resulting samples were studied by EPR (electron paramagnetic resonance) spectrometry. The authors have shown that the metal cations migrate to cationic sites, where they are coordinately unsaturated. 

The molecular sieve behaviour of zeolites can be controlled by a hydrolytic process at elevated temperatures. Water vapour in contact with zeolite crystals at elevated temperatures results in a variation of the zeolitic adsorption characteristics. The amount of water vapour, the pretreatment temperature and the pretreatment time, can control the effect pore size of zeolites. It appears, that a steam treatment causes a cation migration and a cation hydrolysis of the exchangeable cations. However, the effect of steam on the adsorption behaviour of zeolites is influenced by the nature of the initial exchangeable cations. 

As shown in Fig. 4, there are 192 possible cation sites in a unit cell of faujasite (or X zeolite). For LSX, there are only 96 cations (monovalent). Upon activation of the zeolite, i.e., heating at 350°C, the cations migrate to the sites with the lowest energies. Migration is an activated process, which depends on the temperature, time, and the size of the cation. Unfortunately, the most stable sites (that have the lowest energies) are the hidden sites, not exposed to the supercage cavity. So relatively few exposed sites are occupied. 

The effective pore diameter of Y zeolite is determined by the kind of cation that balances the negative charge on the structure. Table IV shows micro-calorimetric measurements of different probe molecules adsorbed on cation-exchanged Y zeolite. Adsorption microcalorimetry has also proved to be a useful technique to study cation migration in zeolites 152). Specifically, repeated adsorption-desorption calorimetric measurements increased the heat of CO adsorption on a Cu-exchanged Y zeolite, indicating that Cu " cations were migrating from inaccessible sites for CO to accessible sites. Previously it had been shown that addition of Cu to NaY increased the differential heat of CO adsorption on these materials. 

The Cu luminescence and IR spectra of Cu -NO yield discrete characteristic bands in a wide Cu concentration range. It is neccessary to determine a proper Cu zeolite treatment as well as to introduce a quantitative analysis of the spectra for evaluation of the Cu siting in zeolites. The limitations with this respect are following. The Cu ion siting and its distribution should reflect those of the Cu " ions. Adsorption of NO on Cu should proceed on all the individual Cu sites, with exclusion of the cation migration and inaccessibility of its coordination sphere. It follows that the spectral data for the Cu-zeolites should be carefully evaluated. 

On the other hand, if zeolites are treated at high temperatures, a complete ion exchange can be achieved. The radius of the hydrated La3+ ion is 3.96 A whereas the free diameter of the entrances to the network of the small cavities is 2.4 A (Herreros et al. 1992). However, when the zeolite is heated to about 320°C, the hydration sphere is lost (the Pauling radius of La3+ is 1.13 A) and some of the lanthanum cations migrate into the sodalite cages. 

Radiolytic spin labeling of molecules adsorbed in zeolites occurs by ionization to form radical cations and by formation of H-adduct radicals by H atom addition. Ionization of adsorbed molecules is a two-step process, equations (1) and (2). Because the adsorbate loading used in experiments is low (typically one percent or less by weight), energy is absorbed by the matrix and not directly by the adsorbate. Holes (Z" ) created in the zeolite lattice migrate to adsorbate (A) by charge transfer. Stabilization of radical cations is made possible at low temperature by sequestration in the zeolite pores and by trapping of electrons by the matrix. 

Multinuclear solid state nuclear magnetic resonance (NMR) has been applied to study the interaction of pyrrole with extra framework compensating cations in zeolites LiNaY and LiNaX. Upon adsorption over zeolite LiNaY, Na and Li cations migrate towards accessible positions in the supercage to interact with one molecule of pyrrole. The adsorption over zeolite LiNaX decreases the mobility of SIIT Na cations, while pyrrole molecules do not interact with Li" cations. At lower loading, pyrrole adsorbs over more basic sites, which are associated with Na cations in zeolite LiNaY. 

X-ray diffraction can readily be applied in situ to follow struetural changes associated with thermal transformations, cation migrations and crystallisation from solution. One simple example of the first application is to determine the temperature at which a material loses crystallinity, as illustrated for the scandium terephthalate Sc2(02CC6H4C02)3 in Figure 3.6. Another application is to measure the kinetics of cation migration throughout zeolites, as previously described for Na,Ni-Y. 

Figure 6.8 Cation migration in zeolite L. When K-L zeolite is stirred at 70 °C with aqueous solutions of lanthanide salts, only potassium ions with direct access to the large channels (left, above and below) are exchanged, giving ca. 1.2 Ln cations per unit cell. These lanthanide cations move readily into sites between the cancrinite cages (B sites) upon heating to 300 °C, but motion into the cancrinite cages is highly thermally activated. Motion of Ln cations into and cations out of the cancrinite cage occurs only above 400 °C, as shown by the results of Rietveld refinement of the X-ray powder diffraction data of heated samples (right, below).

As described in Sect. 5.1.10, Na MAS NMR is a suitable tool for detection and determination of cation migration upon solid-state reaction between M Cl or M +Clj and sodium forms of zeolites. Similar results obtained for the system LaCl3 7H2O /Na-Y are illustrated by Fig. 22 [35,74]. 

ESR and ESEM studies of Cu(II) in a series of alkali metal ion-exchanged Tl-X zeolites were able to demonstrate the influence of mixed co-cations on the coordination and location of Cu(II) (60). The presence of Tl(l) forces of Cu(II) into the -cage to form a hexaaqua species, whereas Na and K result in the formation of triaqua or monoaqua species. In NaTl-X zeolite, both species are present with the same intensity, indicating that both cations can influence the location and coordination geometry of Cu(II). The Cu(II) species observed after dehydration of Tl-rich NaTl-X and KT1-X zeolites was able to interact with ethanol and DMSO adsorbates but no such interaction was observed with CsTl-X zeolites. This interaction with polar adsorbates was interpreted in terms of migrations of the copper from the -cages. 

Su, B-L and Norberg, V. (1998) Migration of adsorbed benzene molecules from cations to 12R windows in the large cages of Cs(Na)-EMT zeolite upon coadsorption of NH3. Langmuir, 14, 2353-2360. 

Cation-radicals, stabilized in zeolites, are excellent one-electron oxidizers for alkenes. In this bimolecular reaction, only those oxidizable alkenes can give rise to cation-radicals, which are able to penetrate into the zeolite channels. From two dienes, 2,4-hexadiene and cyclooctadiene, only the linear one (with the cylindrical width of 0.44 nm) can reach the biphenyl cation-radical or encounter it in the channel (if the cation-radical migrates from its site toward the donor). The eight-membered ring is too large to penetrate into the Na-ZSM-5 channels. The cyclooctadiene can be confined if the cylindrical width is 0.61 nm, however the width of the channels in Na-ZSM-5 is only 0.55 nm. No cyclooctadiene reaction with the confined biphenyl cation-radical was detected despite the fact that, in solution, one-electron exchange between cyclooctadiene and (biphenyl) proceeds readily (Morkin et al. 2003). 

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