Cation exchange, clay chemistry

An interesting aspect of thiophene chemistry is the differences in reactivity between thiophene and its more aromatic isostere, benzene, and its less aromatic isosteres, furan and pyrrole. One interesting facet of this contrast is that metal cation-exchanged clay catalyzed Diels-Alder reactions work for fiiran and pyrrole to produce reaction with a,p-unsaturated carbonyl compounds the thiophene examples do not react <94JCS(P1)761>. 

Clay minerals or phyllosilicates are lamellar natural and synthetic materials with high surface area, cation exchange and swelling properties, exfoliation ability, variable surface charge density and hydrophobic/hydrophilic character [85], They are good host structures for intercalation or adsorption of organic molecules and macromolecules, particularly proteins. On the basis of the natural adsorption of proteins by clay minerals and various clay complexes that occurs in soils, many authors have investigated the use of clay and clay-derived materials as matrices for the immobilization of enzymes, either for environmental chemistry purpose or in the chemical and material industries. 

Simple Models. The surface chemical properties of clay minerals may often be interpreted in terms of the surface chemistry of the structural components, that is, sheets of tetrahedral silica, octahedral aluminum oxide (gibbsite) or magnesium hydroxide (brucite). In the discrete site model, the cation exchange framework, held together by lattice or interlayer attraction forces, exposes fixed charges as anionic sites. 

Zeolite surface chemistry resembles that of smectite clays. In contrast to clays, however, natural zeolites can occur as millimeter- or greater-sized particles and are free of shrink-swell behavior. As a result, zeolites exhibit superior hydraulic characteristics and are suitable for use in filtration systems (Breck 1974) and as permeable barriers to dissolved chemical migration. Internal and external surface areas up to 800 m2 g have been measured. Total cation exchange capacities in natural zeolites vary from 250 to 3000 meq kg 1 (Ming and Mumpton 1989). External cation exchange capacities have been determined for a few natural zeolites and typically range from 10 to 50 percent of the total cation exchange capacity (Bowman et al. 1995). 

A wide variety of soils and sludge have now been treated. Soil characteristics that can impact the SET chemistry include the general soil type, which is treated (loam, sand, silt, and clay), the presence of humic material, the pH value, the soil s cation exchange capacity, its particle size, the amount of water present, and the iron content. Processes have been engineered to accommodate this wide range of variables [7,8,34]. Some soils can be treated... 

I was lucky to receive some jars of sodium-substituted Eucatex vermiculite and n-butylammonium-substituted Eucatex vermiculite. These synthetic systems had been obtained by cation exchange on the raw minerals. Such cation exchange plays a major role in clay chemistry, and the process is described in detail in standard books on clay colloids, like that of van Olphen [2], Some years later, I was able to obtain the following chemical formula for the dry sodium Eucatex vermiculite [3]... 

Adams, J. M. 1987. Synthetic organic chemistry using pillared, cation-exchanged and acid-treated montmorillonite catalysts a review. Appl. Clay Sci, 2 309-342. 

In this section several examples will be presented that are representative of ongoing research projects. Even though the results are still preliminary, the projects selected further illustrate the utility of MD simulations in soil chemistry. Topics include the calculation of free energy changes associated with cation exchange and the adsorption of proteins on clay mineral surfaces. 

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