Cation mobility, zeolites

The low frequency absorption II originates from the Maxwell-Wagner effect already observed in dehydrated X-type zeolites (8). In the presence of water the enhanced cationic mobility intensifies this effect. This interpretation disagrees with that of Matron et al. (10). They ascribed their low frequency a-process to cations on site I and site II. This is improbable in view of the correspondence with the Maxwell-Wagner effect in dehydrated X-type zeolites, observed by us (8). 

The sorptive, catalytic, and ion-exchange properties of zeolites depend strongly on the kind, position, and mobility of the charge-balancing cations. Since chemical shifts and multiplicities of lines are related to site occupancy and their widths to cationic mobility, NMR can in principle provide important information on the nature of the intracrystalline environment. 

Besides, the interaction of divalent cations with the zeolite framework and water is stronger on account of the higher charge and lower cationic radius exhibited by Ca2+ (Ca2+ 0.99 A) in contrast with Na+ and K+ (Na+ 0.95 A and K+ 1.33 A). This effect induces a lower mobility of divalent cations, on account of the fact that divalent cations are more intimately linked with the zeolite framework. Hence, the lower cationic mobility is an additional reason for the decrement in the permittivity of tested samples, and this effect is also detected by the thermodielectric analyzer as a decrease in V0 [110,119],... 

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 values in this table refer to zeolites with sodium cations. The cations are mobile in the lattice and can be exchanged. Upon exchanging sodium cations in zeolite A by potassium ions the pore diameter decreases to 0.3 nm. Cation exchange also affects other properties, such as their adsorption properties and, with appropriate cations, also their catalytic properties. 

In the last decade, xenon has proven to be an efficient sorbate for probing the pore structure and the internal surface of adsorbents by NMR spectroscopy [28-30]. The advantage of xenon in comparison with other adsorbates is brought about by the large chemical shifts of Xe NMR as a consequence of the large electron shell and by the fact that xenon as a noble gas leaves the adsorbent structure essentially unaffected. In particular, in zeolite research Xe NMR has been successfully applied to probe pore and channel dimensions [31], cation distributions [32], cation sites [33], and cation mobilities [34,35] as well as matter depositions and lattice defects [36-38]. 

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. 

P-14 - Cation mobility and the sorption of chloroform in zeolite NaY a molecular dynamics study... 

Zeolites can contain charge-compensating cations. These cations can be mobile and can be exchanged, usually with an aqueous solution. Molecular dynamics simulations of the cation mobility in anhydrous zeolite A have been performed by Shin et al.2S2 Xhe most complete studies to date of a real zeolite are molecular dynamics simulations of zeolite A containing both sodium cations and water.283-285... 

Faujasites are of particular importance for industrial catalysis. As do all zeolites, they exhibit ionic conductivity, by motion of the cations in the internal channel structure. The cationic mobility is considerable, as shown by Table IX. The conductivity of zeolites is greater than that of most other crystals. At a temperature of 25°C the diffusion coefficient of the sodium ion in hydrated zeolite is only three orders of magnitude smaller than that of simple ions in aqueous solution. The conductivity increases with temperature at 335°C the conductivity of the anhydrous zeolite is greater than that of the... 

M.L. Lind, B.H. Jeong, A. Subramani, X. Huang, E.M.V. Hoek, Effect of mobile cation on zeolite-polyamide thin film nanocomposite membranes. Journal of Materials Research 24 (2009) 1624-1631. 

The advent of the addition of a quaternary ammonium cation as template or structure directing agent (SDA) to the alkaline gel by Barter and coworkers, and Mobil Oil coworkers, led to the Si02 enriched zeolite A in the case of Barter and to the high silica zeolites. Beta and ZSM-5, by the Mobil group. The latter synthesis temperature typically is 100-200 °C, higher than Milton s original work. 

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