Locations of cations in zeolites

At this time, the locations of cations in zeolites have been determined primarily by X-ray diffraction (XRD) techniques. Unfortunately, this method has the drawback of being able to locate only the most stationary cations in zeolites. In some studies of hydrated zeolites, less than 50% of the total cation population can be accounted for. A higher percentage of the cations can be located in dehydrated samples, but the effect of the dehydration step on the location of the cations is generally not well known. NMR measurements, on the other hand, are most sensitive to mobile cations and cations in high symmetry sites. 

The location of cations in zeolites is of considerable practical importance since their locations can affect, among others,... 

Computational modeling was successfully used to identify the locations of cations in zeolites and to clarify their interaction with zeolite hosts Cu" and Cu in ZSM-5 (MFI), ferrierite (PER), and faujasite (FAU) zeolites [134-136] alkali and alkaline-earth cations in MFI [133] Zn " in FAU and MFI [137,138] also some divalent cations in MFI [139]. Yet, these investigations represent only first steps in the exploration of the large variety of both metal cations (especially of transition metals) and zeolite structures. Moreover, the interaction of cations in zeolites with probe molecules or reagents [137,140,141] is much less investigated computationally, despite the fact that these interactions are crucial for interpreting results of spectroscopic methods used to characterize the cations as well as for rationalizing specific catalytic, adsorption, sensor, or other properties of the materials. 

It is known that the 29Si NMR chemical shift in zeolites is sensitive to the type of the exchangeable cation (56), which indicates the presence of interactions between cations and the framework. In particular, the substitution of Na+ by Li+ in zeolite A and in synthetic faujasite moves the 29Si resonances ca. 4 ppm downfield in both cases. Melchior et al. (235) have used this effect to study the location of cations in a series of partially exchanged zeolites (Li,Na)-A. They found that the average 29Si chemical shift is not a... 

Figure 6 Left Structure of the supercage and location of cations in X and Y zeolites. Right A cartoon representation of the proposed interaction of cation with the radical intermediates.

The locations of cations in particular zeolites have been determined in other zeolites, extensive studies have been completed on cations and the interaction of cations with other molecules in the structure. The extensive work carried out on faujasite and the topologically related... 

NMR ( C, Xe, metal) metal carbonyl clusters metd clusters structure, size, and location of duster in zeolite cages Chemical shifts of Xe NMR are sensitive to xenon pressure, temperature, type of cations, and size of zeolite crystals. 

The distribution of cations in a hydrated zeolite is mainly controlled by their sizes and can be described by a statistical model. In the dehydrated state, most of the cations are located on the intraframework sites their occupancies are governed by mutual repulsions and cation—framework interactions [1]. By which, the environments of the framework silicon atoms and their corresponding ssi NMR spectra are affected [2,3]. The chemical shift and lineshape of Si NMR have been found to depend on the nature and the distribution of cations in the small sodalite and double hexagonal prism (D6R) cavities of the dehydrated Y zeolites [3] The irreversible migration of La3 ions from the supercages to the small sodalite and/or D6R cavities by... 

The i29Xe chemical shift and the adsorption isotherm of xenon adsorbed on Y zeolites are dependent on the size, location and nature of cations in the zeolite intraframvork space. The variation of cation location in a partially cation-exchanged Na—Y can also be monitored by Na NMR. 

Correlations between structure and catalytic activity have been described for carbonium-ion type reactions (1). Much effort was also spent to establish a correlation between structural and compositional factors and the activity for redox type reactions (1, 9-12). Transition metal ions in zeolites were shown to be active in the oxidation and hydrogenation of hydrocarbons. In this connection various techniques were used to locate the cations in the framework of the faujasite-type zeolites (13-20). These ions migrate upon thermal treatment or by the adsorption of various substances. Thus, methods are needed to determine the location of the cations under reaction conditions. 

Recent work by Rabo et al. (57) opens new possibilities for controlling the activity and selectivity of zeolite catalysts. Occlusion of various guest molecules into the sodalite cavities of Y zeolites can significantly change the catalytic properties of the zeolites for carbonium-type reactions. Anions of occluded salts are located close to the center of the sodalite cavity and strongly influence the arrangement of cations in the faujasite lattice and hence the catalytic activity. 

The complementarity and interplay of the results of EXAFS and Mossbauer spectroscopies provide means by which crystal chemistry of certain cations in zeolites may be successfully studied. While neither technique is completely adequate as a "stand-alone tool for such studies, the combination of the two can be used to map the chemical nature, environment and location of cations where this information would otherwise be inaccessible. 

Quantum Chemical Modeling the Location of Extraframework Metal Cations in Zeolites... 

Removal of coordinating ligands by careful calcination prior to reduction, is therefore extremely important for metal/zeolite catalysts, because it controls the cation locations and thus the metal particle growth mechanism during subsequent reduction. It has been demonstrated that the ultimate metal dispersion depends on the temperature of the calcination (50,69,71,79,107). An optimum calcination temperature can be defined for obtaining maximum dispersion of metals in zeolites. 

Since the first " Xe NMR study of xenon adsorbed on a zeolite, this technique has been shown to be of interest for the investigation of the distribution and the size of supported metal particles, the quantitative distribution of phases chemisorbed on these particles, the dimensions of the void spaces of zeolites, the detection of structure defects, the location of cations and the effect of electric fields they create [i,2 ]. We report here some typical applications related to the study of intra-and inter-crystallite diffusion of cations in faujasite zeolites. 

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