Silicate clays exchange cation position

The most significant class of inorganic supports, which is used for the direct ion exchange of positively charged transition-metal complexes, are smectite clays. Pin-navaia has introduced the use of these swelling, layered silicate clays for catalysis. Other clays include montmorillonite, bentonite, and laponite. As shown by Pinna-vaia, cationic transition-metal complexes can be readily exchanged (intercalated) into the solvated interlayers of these silicates (Eq. (1)) [117] ... 

Use of organic templates In the current preparation of a new series of microporous crystals, organic templates are often used. Organic chemicals are also used in the preparation of pillared clays to control the pore sizes and the porosity as follows The interlayer cations of silicate layer are exchanged with positively charged sol particles, and then a part of the sols are exchanged with organic template cations such as octadecyl trimethyl ammonium (OTMA). 

In addition to MC and MD simulations, several quantum-chemical studies of hydrated clay minerals with or without exchangeable cations in the interlayer space have been performed at the ab initio and semiempirical level of theory. The total energy of hydrated layered silicates (talc and pyrophyllite) without exchangeable counterions, the position and interactions of interlayer water... 

Unlike layer silicate clays, the oxides of Fe and A1 are not inclined to develop structural charge as a result of isomorphous substitution. Consequently they have very low cation exchange capacities despite sometimes possessing impressively large surface areas. The surfaces do, however, develop limited charge (negative or positive) in response to the pH of the surrounding solution, and this process will be discussed... 

This was shown, for example, by electrokinetic measurements of Kavanagh and Quirk177) in the system Fe203-illite at pH 2.5. Electrophoretic and cation-exchange data indicated that the net charge of clay surface became strongly positive at low pH values as a result of adsorption of polycations Fe3+. Further examples of metal ion hydrolysis in oxide and silicate flotation systems were published by Fuerstenau178) and Stumm et al.62). 

Montmorillonite, a layered clay mineral, shows interesting behavior in the adsorption of organic molecules due to the presence of interlayer space containing exchangeable metal cations which compensate the positive charge deficiency in the alumino-silicate sheet. Various kinds of aromatic molecules are adsorbed in the Interlayer of montmorillonite, replacing water molecules which surrounds exchangeable metal cations in the interlayer(1). 

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