Zeolites cation interactions

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. 

A notable exception are chemisorbed complexes in zeolites, which have been characterized both structurally and spectroscopically, and for which the interpretation of electronic spectra has met with a considerable success. The reason for the former is the well-defined, although complex, structure of the zeolite framework in which the cations are distributed among a few types of available sites the fortunate circumstance of the latter is that the interaction between the cations, which act as selective chemisorption centers, and the zeolite framework is primarily only electrostatic. The theory that applies for this case is the ligand field theory of the ion-molecule complexes usually placed in trigonal fields of the zeolite cation sites (29). Quantum mechanical exchange interactions with the zeolite framework are justifiably neglected except for very small effects in resonance energy transfer (J30). ... 

Zeolites can be hydrophilic or hydrophobic due to the different Si/Al ratios within the zeolite framework. Organic molecules rely on H-bonding, electrostatic, and 77-cation interactions for effective zeolite absorption, and these interactions will clearly be influenced by the number of cation sites present. As expected, the more Si present, the more hydrophobic the zeolite and, therefore, the greater the ability of these materials to interact with hydrophobic organic molecules or to exclude hydrophilic molecules, such as water. Zeolites X/Y have a Si/Al content at or close to 1 and are highly hydrophilic absorbants. Pentasil zeolite ZSM-5, which... 

The cluster calculations for Li+, Na+, and K+ ions in six-membered windows (S,. and Sn sites) were performed by Beran (104). It was concluded that in this series the properties of a zeolite framework (charge distribution, bond orders, Lewis acidity or basicity as characterized by LUMO and HOMO energies) only slightly depend on the type of cation. The decrease of water adsorption heats in this sequence was explained by the assumption that the strength of the water-cation interaction correlates with the strength of the interaction between a cation and lattice oxygen atoms. 

Similar computations were carried out by Beran (106) for Mg2+- and MgOH+-containing zeolites. The results were compared with those obtained for Ca-containing zeolites. The distinctions were mainly ascribed to the stronger electron-acceptor properties of magnesium as well as to stronger electrostatic fields in the Mg-containing zeolites. In both cases the water-cation interaction was predicted to be rather weak. 

Zeolitic materials have been widely used in the last decades in the chemical and petrochemical industries. This increasing interest on these materials is based in their unique properties a uniform intra-crystalline microporosity that provides aceess to a large and well-defined surface, the molecular sieve effect, and the electrostatic field centered at zeolite cations. Furthermore, some properties of zeolites can be tailored by changing the nature of the compensating cation located in the inner part of the cavities by means of their ion-exchange capability. In this way, the pore accessibility of some zeolites used in gas separation processes, as well as the adsorbent-adsorbate interactions, can be tailored by the introduction of cations with different size and chemical nature. Similarly, different cations can be used to introduce new chemical properties (acid-base, redox, etc.), which are needed for a given application in catalytic processes. 

Up to now, infrared spectroscopy has been used mainly to determine the types of hydroxyl groups and the acidity of zeolites (39). The frequencies of the vertical and horizontal vibrations (with respect to the cavity wall) of H2O molecules adsorbed in zeolite A were determined by measurements in the far infrared ( 220 and —75 cm" ) (37). These values are in agreement with a simple theoretical model. A number of ultraviolet and ESR studies are reviewed (33). The difference has been established between the specific molecular interaction of aromatic molecules on zeolites cationized with alkali cations and the more complex interactions involving charge transfer in CaX and deca-tionized X and Y zeolites. These more complex interactions with CaX zeolites containing protonized vacancies and with decationized zeolites are similar. These phenomena are related to the interactions of molecules with acidic centers in zeolites which are stronger, as compared with the molecular adsorption. 

For the samples studied, no pyridine adsorbed on Lewis acid sites was detected, indicating that the zeolite had not been partially dehy-droxylated to form such sites. The band reflecting pyridine-cation interaction was detected only after about 16 alkaline earth ions had been introduced into the unit cell. It grew steadily in intensity as more divalent ions were introduced into the structure. In Figure 1, the intensity of the pyridinium ion band, expressed as absorbance/sample mass, is plotted as a function of the per cent exchange by divalent ion. The concentration of acid sites is a function of the degree of exchange. 

The infrared spectrum of adsorbed nitrogen can also be used to probe cation sites in zeolites. Zecchina et al [34] compared vibrational frequencies of CO and N2 adsorbed at low temperatures in mordenite containing different alkali metal cations. In both cases the vibrational frequencies could be correlated with (Rx + Rm) > where Rx is the cation radius and Rm the radius of the adsorbed molecule, suggesting a simple electrostatic field explanation for the frequency shifts between different cations. The appearance of a band due to N2 interacting with a particular zeolite cation will also mean that that particular cation is located in sites accessible to the N2 molecule. 

This procedure and, importantly, the force fields used to describe the zeolite-template interactions in the atom-atom approximation, have been well validated against experiments. Firstly, the locations of a number of templates within zeolites have been determined by single crystal or powder X-ray diffraction. Thus, for example, the templates 1-aminoadamantane and N-methylquinuclidinium cation have been used to crystallize the LEV... 

Infrared Spectroscopic Detection of Interactions between Organic Substrates and Zeolite Cations... 

C. O. Kowenje, Spectroscopic characterization of the metal cation siting and the adsorbate-cation interactions in copper(II) and cobalt(II) exchanged Faujasite-X Zeolite, PhD thesis SUNY Binghamton USA, 2006, available at www.il.proreauest.com (UMI number 3220349). 

The vibration frequencies were calculated ab initio for a model structure comprising a bare metal cation interacting with the fliran ring. For both the experimental and calculated spectra the interaction of furan with the zeolite caused the out-of-plane vibrational modes to move to higher wave numbers, see Fig. 7.28. This result can be explained if one assumes an interaction, oriented perpendicular to the molecular plane of furan, between the zeolite cation and the 7t electron system of furan, which lead to an increase of the CH out-of-plane bending force constant. However, for the experimental spectra the wavenumbers of the out-of-plane modes then decrease in the series Li > Na > > Cs whereas the computed... 

In our opinion, the unusual Xe NMR chemical shifts of the silver -exchanged X zeolite are due to specific xenon / silver - cation interactions [ 7 ]. 

Infrared spectroscopy has been a valuable technique for exploring zeolite structures. It is useful for studying the nature of hydroxyl groups in zeolites, the interaction of cations with adsorbed molecules, and the fundamental framework structures of zeolites. 

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