Clay properties tetrahedral sheet

Acid activation has long been known as a preparation method for porous materials. In general, the acid solution dissolves the octahedral sheets of the clay and produces a significant modification over the tetrahedral sheets [11]. The solids obtained have higher surface area and better adsorptive and catalytic properties than the original clays, depending on the starting material and the conditions of the treatment [12-15]. The activation of metakaolin has been reported only in a few articles, and the application of the solids obtained to different catalytic reactions has been described [3,7,16-18]. 

Swelling mica-type clay minerals composed of octahedral and tetrahedral sheets are widely utilized in preparing two-dimensional heterostructural nanohybrids, most likely because of their natural ubiquity, high stability, swelling property, and wide applications. In fact, smectite and vermiculite groups have been the preferred layer components to be hybridized with a variety of inorganic and organic components. 

Conceptually, the next simplest clay mineral is pyrophyllite, which is produced by attaching tetrahedral sheets above and below an octahedral layer (Fig. 6), compared with just one octahedral sheet for kaolin [15]. The resulting chemical composition of pyrophyllite is AI2Si4Ol0(OH)2, which is equivalent to the mineral formula Al20 4Si02-II20. The structure and properties of pyrophyllite are summarized in Table 2. 

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. 

The above results are related to the structural properties of the clay minerals. In the case of kaolinite, the tetrahedral layers of adjacent clay sheets are held tightly by hydrogen bonds. Therefore, only readily available planar external surface sites exist for exchange. With smectite, the inner peripheral space is not held together by hydrogen bonds, but instead it is able to swell with adequate hydration and thus allow for rapid passage of ions into the interlayer. 

Clay minerals are hydrous layered aluminosilicates. The aluminosilicate layer is composed of two types of sheets octahedral (0) and tetrahedral (T). Each sheet is composed of parallel planes of atoms, either oxygen or aluminum (silicon) in an alternating sequence. The large variability in the properties of clay minerals is related to the stacking sequence of the two basic types of sheets, T and 0, and to the uncompensated charge that develops by isomorphous substitution of unevenly charged cations. 

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