Exchangeable cation location

Figure 3.8. Schematic of the smectite structure showing one gibbsite sheet between two silicate sheets. The basic unit is repeated many times in the horizontal directions to produce layers. The basic unit with the 9.6 A c-axis spacing expands to 14 A when water enters between layers. The exchangeable cations located between the layers produce the counter charge for the isomorphous substitution. It occurs in the layers marked with an asterisk (from Taylor and Ashcroft, 1972, with permission).

By isomorphous substitution, the Mg2+ ion can be replaced by other divalent ions such as Ni2+ and Co + or by monovalent cations such as Li. In this latter case, the excess of charge must be compensated by an additional exchangeable cation located between the layers. Talc and chrysotile of cobalt or nickel were prepared in our laboratory as precursors Of Ni and Co/SiOg catalysts... 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

Zeolites present a porous structure of channels and cages of varying dimension (106-108). The exchangeable cations are located in different positions. Since more sites are available than charges to be neutralized, the neutralization pattern may vary with the kind of exchangeable cation (109-110). 

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. 

Cation Siting in Linde X. Cation siting for the Na+ and K+exchanged forms of hydrated Linde X has been determined by x-ray analysis (9, 10) (Table IV). The cations located by x-ray analysis are distributed over three different sites (I, I, and II) on the threefold axes. These cations bind onto the framework such that the closest cation-oxygen distances are approximately equal to the sum of their ionic radii. The linear relationships in Figure 5 indicate that the Li+ Na+ Ag+ K+, and Tl+ exchanged forms have an approximately similar distribution of those cations which are strongly bound to the framework. This has been found by x-ray... 

The structure of and possible cation location in these materials is fairly well known (2, 8, 4, )> and their ion-exchange behavior toward a multitude of pairs of ions, mostly including sodium, has been measured and interpreted in terms of basic properties of ions, crystal structures, and pore dimensions. The major part of these studies is with alkali- and alkaline-earth cations, alkylammonium ions, rare-earth cations, and silver and thallium ions (1). In contrast, the ion adsorption of transition metals in faujasite has received little attention. 

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... 

Montmorillonites (smectite clays) have structures resembling that of pyrophyllite but the structure is not electrically neutral. Exchangeable cations are located in interlamellar regions of the clay and, furthermore, the clay can be flocculated such that the plate-like crystals compact with parallel c-axes to give coherent layers. The smectites are then attractive materials with which to modify electrodes. 

Dehydrated halloysites have C.E.C. in the range of 6—10 mequiv./lOO g (Van der Marel, 1958 Garrett and Walker, 1959). Garrett and Walker have shown that the exchangable cations are located on the external surfaces of the crystals and not in the interlayer position of halloysite. Until it is possible to obtain accurate chemical analyses of the kaolinite minerals, it will be difficult to determine their exchange capacity and the source of the charge. 

In the Liver Dialysis Unit (HemoCleanse Technologies, Lafayette, USA), the adsorbents (powdered activated charcoal and cation-exchangers) are located in the dialysate-moving phase [31]. Dialysate content is adjusted so as to prevent... 

Clay minerals are hydrous aluminum phyllosilicates made of sheets or layers composed of tetrahedra and octahedra. This mineral type includes the following groups kaolinite, smectite, illite, and chlorite. In the case of smectite, each layer comprises two sublayers of tetrahedra with an inserted octahedral layer, where, between layers, an interlayer space where the exchangeable cations are located is formed [131-133], In Figure 2.24... 

Structure VI is transformed into the more stable compound VII. This transformation is restricted to NaX zeolites. Although LiX and KX give rise to the formation of species VI at room temperature, carbonate structures were completely absent on CaX, SrX, and BaX at room temperature (280) but were formed at elevated temperatures (286). This behavior of various zeolites, which in detail depends on the degree of exchange, is explained by the location of the exchangeable cations on different sites in the zeolite framework (282). 

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