Cation location, zeolites

The zeolite frameworks are built up of SiO tetraliedra, which are neutral, and AlO tetraliedra, which have a charge of-1. The charge of tire AlO tetraliedra is balanced by tire charges of cations located at various crystallographically defined positions in the zeolite, many of tliem exposed at tire internal surface. The cations are... 

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

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. 

In Mordenite. Smit and den Ouden (60, 144) reported a Monte Carlo investigation of methane adsorbed in mordenite of varying Si/Al ratios. In their calculations, both the zeolite and sorbate were held rigid, infinite dilution was assumed, and sorbate-zeolite interaction parameters were taken from Kiselev et al. (79). Electronic neutrality of the zeolite framework was preserved by compensating the trivalent aluminum exactly with sodium cations, located in experimentally determined crystallographic locations. 

Palladium ions were reduced by hydrogen at room temperature. The zeolite thus formed has hydroxyl groups identical to those found in de-cationated Y zeolites and probably has a Bronsted acid character. Furthermore, hydrogen reduction produces metallic palladium almost atomically, dispersed within the zeolite framework as demonstrated by our IR, volumetric, and x-ray (23) results. Palladium atoms are located near Lewis acid sites which have a strong electron affinity. Electron transfer between palladium atoms and Lewis acid sites occurs, leaving some palladium atoms as Pd(I). Reduction by hydrogen at higher temperatures leads to a solid in which metal palladium particles are present. The behavior of these particles for CO adsorption seems to be identical to that of palladium on other supports. 

Figure 12. Supercage structure, cation location (I, II, III or 1, 2, 3) within X- and Y-type zeolites. Bottom portion shows the reduction in available space (relative) within the supercage as the cation size increases.

Zeolites are somewhat like silica in their surface characteristics. Ketones and hydroxy-1,4-biradicals have very polar groups which can interact favorably with metal cations located along zeolite walls. The potential effect of the metal ions on the position of the reacting ketones is twofold. First, the cations may force a ketone molecule into a conformation or a site which it would normally not occupy based solely upon free-volume considerations. Second, the diffusion coefficient of a ketone or a hydroxy-1,4-biradical is probably much more than an order of magnitude smaller than that of benzene [289] so that the residence time of a ketone and its Norrish II intermediates in a zeolite site with at least one metal ion is expected to be closer to 100 ns than to 1 ns. 

The Al-rich (cationic) zeolites have highly polar internal surfaces. The polarity increases with increasing cation charge and decreasing cation size. However, the relationship between the nature of the cation and the surface properties is complex because the differences in cation location (sites) must also be considered. 

The reductive/oxidative properties of transitional metal elements in these zeolite catalysts were also examined by TPR and TPO, and it is shown that metallic species in certain cation locations may migrate under calcination, reduction, and reaction conditions [7], The different treatment, e g, coking or even the oxidative regeneration, will produce metallic species of varied oxidation states with different distributions in the molecular sieve structures as exemplified by the above XPS data. The redox properties of these metallic cations exhibit the influence of hydrogen and/or coke molecules, and it is further postulated that the electron transfer with oxygen species are considered responsible for their catalyzed performance in the TPO regeneration processes, as shown in Figure 2. 

Isomorphous substitution of Si4+ by a trivalent ion (this is often an Al3+ ion) results in a negative lattice charge. This negative charge can be compensated by a cation located in the zeolite cage or micropore or on the clay layer. When a... 

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. 

Zeolites containing 3d transition-metal ions were considered in Beran et al. (109-112). The peculiarities of the donor-acceptor interactions of these cations located within six-membered rings with a zeolite lattice were discussed in terms of atomic charges, bond orders, and orbital energies. The redox properties of the cations, the acid-base properties of zeolites, and the dependence of these characteristics on the Si/Al ratio were discussed as well. The authors noted that the forms containing univalent copper and nickel ions should possess the highest electron-donor ability and consequently the... 

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. 

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

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. 

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. 

Magnesia has strong basic sites but no acid sites (Table XVII) (e.g., 147, 179,180,189,203). However, acidity is generated when magnesia is added to silica (Table XVIII) (74,104). This acidity is exclusively of the Lewis type (59). Its acid sites are more widely distributed as compared with silica-alumina. The acid strength distribution of amorphous silica-alumina and silica-magnesia is more heterogeneous than that observed for any of the pure zeolites (H Y, ZSM-5, mordenite, etc.). This may in part be due to the presence of surface Al and Mg cations located in different environments. 

In present work the influence of cobalt additive on catalytic properties of CuH-ZSM-5 zeolite, in both ethane oxidation and N2O decomposition, was studied. ESR spectroscopy was used for monitoring the change in either the valence or coordinative state of Cu isolated cations, located inside zeolitic channels, upon different treatments of the samples. 

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