Cation distribution in zeolites

Cation distribution in zeolitic structures is one of the key aspects to the understanding of the adsorption mechanisms and selectivities. Many experimental and simulation methods have been used to try to localise the cations. The present work confronts the different analytical methods and gives general distribution trends in accordance with results from the literature. 

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

NH3 is similar to H2O in that they both possess large dipole moments and are both small molecules. The presence of NH3 in a zeolite is chemically similar to the presence of H2O in a zeolite. Therefore, the hydrated cation distribution in zeolites is probably more typical of NH3 adsorption in zeolites than the dehydrated cation distribution. According to Breck (18), for hydrated zeolite X, cations are found in sites SI, SI, SII, and SIV. Of these sites, SI, SII, and SIV would all be adsorption lattice solution sites. The cationic and anionic lattice solution sites (in the supercavity of NaX) are illustrated in Figure 8. For NH3, the subscript J1 will refer to SII sites, the subscript J2 will refer to SI sites, and J3 will refer to SIV sites. The anionic sites are two and are (l) in the center U-membered ring of the connecting frame and (2) near the center of the 0(2)—0(1)—0(l) triad of oxygen atoms. For NH3, the subscript il will refer to the first anionic site the subscript i2 will refer to the second anionic site. 

The cation distribution in zeolites is the result of an energy-minimization process. The site energy is determined by the interaction of the cations with the framework, with the adsorbed molecules and by the mutual repulsion between them. Provided that an equilibrium distribution is possible, we may expect that the cation distribution contains information about the enerav levels of the sites. 

Recently, a statistical mechanical model was proposed for explaining the cation distribution in zeolites as a function of... 

Erising, T. and Leflaive, P. (2008) Extraframework cation distributions in X and Y faujasite zeolites a review. Micropor. Mesopor. Mat., 114, 27-63. 

The cation distribution in the faujasite zeolites is much more complex than in zeolite A. Five different sites have been identified and it is apparent that the distribution is dependent on the nature of the cations and the presence of water. 

Sodium-23 MASNMR measurements have been used to examine the extent to which this method can be used to determine the cation distribution in hydrated and dehydrated Y-zeolites. Results have been obtained on Na-Y and series of partially exchanged (NH, Na)-Y, (Ca,Na)-Y and (La,Na)-Y zeolites which demonstrate that the sodium cations in the supercages can be distinguished from those in the smaller sodalite cages and hexagonal prisms. For the hydrated Y zeolites, spectral simulation with symmetric lines allows the cation distribution to be determined quantitatively. 

The sodium-23 MASNMR results are consistent with the selective removal of sodium cations from the Y zeolite supercages by the partial cation exchange. This demonstrates that this technique can be used to monitor how cation distributions in Y zeolites change with various sample treatments. 

Kinetics of Sorption Processes as a Basis for Estimating Cation Distribution in Unit Cells of Zeolites... 

Kinetic studies of ion exchange on partially ion-exchanged type A zeolites of Mg Ca and Mn " revealed that mini-mums and maximums characterize the differential coefficients of internal diffusion for every exchange of 2 Na " ions for one divalent cation per unit cell of the zeolite. On the basis of these observations, assuming definite interactions between the cations and the zeolite lattice, predictions can be made concerning the distribution and arrangement of cations in the unit cells of a type A zeolite. Research on liquid phase adsorption of n-alkanes on partially ion-exchanged type A zeolites indicated that the differential diffusion coefficients for alkane adsorption are influenced likewise by cation distribution in the unit cells of the zeolite. 

Cation distribution in type A zeolites can be calculated by using the equations above, taking these conditions into consideration. Thus, for example, with 4 Ca " and 4 Na" ions per zeolite cavity—i.e., Nm = Np = 4—condition (a) is met if the 4 Ca " and 4 Na" ions occupy all 8... 

From the electrochemical point of view, an important class of materials is that constituted by aluminosilicates incorporating cobalt, iron, etc., centers. In the case of Fe-based zeolites with Mobil Five structure (FeZSM-5) materials, different forms of iron can coexist. These include isolated ions either in framework positions (isomorphously substituting silicon centers), isolated ions in cationic positions in zeolite channels, binuclear and oligonuclear iron complexes in extra-framework positions, iron oxide nanoparticles (size <2 nm), and large iron oxide particles (FcjOj) in a wide distribution (up to 25 nm in size) located in the surface of the zeolite crystal (Perez-Ramirez et al., 2002). The electrochemistry of such materials will be reviewed in Chapter 8. 

Por the hydrated state, we can reasonably expect that the site I and site II cations are completely solvated with (three) water molecules. The interaction enerqy w can therefore he incorporated into e-p ejj and ejjj, such that the above equations formally reduce to those of the dehydrated state. Van Dun et. al. (2) found that as a function of the Al content, the cation distributions in dehydrated Na-exchanged FAU-type zeolites could he accurately predicted if it was assumed that the enerqy level differences relative to site I varied linearly with the framework negative charae. At around 48 Al/unit cell, the site preference chanqes from II > I > I (low Al content) to I > II > I (high Al content). 

To correlate the adsorption and diffusion properties to the cation distribution in the structure of CaNaA zeolites, temperature programmed desorption, anomalous X-ray powder diffraction and quasi elastic neutron scattering experiments were performed. It is shown that water adsorption and diffusion behaviour differs with the calcium content. Fourier maps obtained from anomalous diffraction experiments near the Ca K-edge allow to identify the calcium crystallographic sites unambiguously. 

IR framework spectra were used as a diagnostic tool by Occelli et al. [260] in detecting the presence of offretite (via a band at 600-610 cm ) and erionite (bands at 410-425,550-610,655-685 cm ) in mixtures of these two structures. Roessner et al. [261 ] considered, in their IR spectroscopic work on the cation distribution in dehydrated calcium-exchanged erionite, also the framework vibrations of Ca-erionite besides OD vibrations, CO adsorption and DRIFT spectroscopy in the NIR region. They were able to show that the Ca + cations were selectively located in the supercages in front of the six-membered rings. Similar to the features encountered with Y-type zeolites and mordenite (vide supra), also with offretite a sufficiently linear relationship was found between the wave-numbers of the asymmetric and symmetric T-0 vibrations and the number of framework Al atoms per unit cell [262]. 

The predicted location of extra-framework cation sites in zeolites can also be studied by MC methods, although the problem is further complicated by the disorder in both framework aluminium location (and associated charge) and partial occupancy of cation sites. In this case a large number of possible extraframework cation site distributions has to be considered, and a model assumed for the location of framework charge. 

Structures of cationic positions in zeolites are determined by location of aluminum atoms in the lattice. There are two factors which can disturb the stochastic distribution over the lattice. The first one is the availability of preferential lattice points for the location of Al atoms in zeolite. It can be important especially in the stabilization due to exchange of monovalent metal cations. The second one is mutual interaction of lattice Al atoms, which, for example, is the reason of direct adjacency impossibility for the placing of Al in the lattice (Lowenstein s rule). 

Fig. 15. Ferrierite. (a, b) Cation distribution in Co-exchanged (a) and Ni-exchanged (b) zeolites as determined from XRD data [05D1]. (cont.)...

As a conclusion of this section, it can be said that the method used has to be carefully chosen according to the sample studied and/or the expected results. Conventional XRD may be sufficient to localise a single cation species in a dehydrated zeolite whereas for bicationic zeolites more elaborate techniques like anomalous XRD or MAS and MQMAS NMR may be necessary. If the focus of the study is more on the influence of adsorbed molecules on the distribution of the cations, neutron scattering may be needed to complete the work. Finally, highly dealuminated zeolites may be difficult to study with diffraction techniques, in this case NMR techniques may be the best available option. 

To check this possibility we performed experiments with different concentrations of NaBr in the NaY zeolite. Table 2 presents the results. It can be seen that upon increasing the amount of NaBr impregnated on NaY, there is preference to formation of the cyclobutyl bromide over allylcarbinyl bromide, indicating that the relative position between the bromide ions and bicyclobutonium governs the product distribution. Hence, zeolites may act as solid solvent, favoring ionization of alkyl halides and nucleophilic substitution reactions. In contrast to liquid solvents, where solvation is mostly uniform, the zeolite surface seems to provide unsymmetrical solvation of the cations, leading to product distribution that is different from solution. 

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