Silicate clays layer charge

Boyd SA, Jaynes WF (1994) Role of layer charge in organic contaminant sorption by organo-clays. In Mermut AR (ed) Layer charge characteristics of 2 1 silicate clay minerals. CMS Workshop Lecture Series, vol 6, The Clay Minerals Society, Boulder, CO, pp 48-77... 

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] ... 

Subsequent work showed that a modification of the synthesis procedure produced a 10A hydrate which> if dried carefully, would maintain the interlayer water in the absence of excess water (27). This material is optimal for adsorbed water studies for a number of reasons the parent clay is a well-crystallized kaolinite with a negligible layer charge, there are few if any interlayer cations, there is no interference from pore water since the amount is minimal, and the interlayer water molecules lie between uniform layers of known structure. Thus, the hydrate provides a useful model for studying the effects of a silicate surface on interlayer water. 

Electron spin resonance (ESR) is a useful technique for investigating the mobility and orientation of exchange cations at the surface of layer silicate clays in various states of hydration. Using Cu2+ and the charged nitroxide spin probe, TEMPAMINE+... 

Clay minerals are characterized by a high surface charge and a very small particle size. A detailed presentation of two types of layered silicate clay (kaoUnite and smectite) is given in Chapter 1. 

Low-temperature sheet structure silicates with a high Fe2+ content seem to be restricted to the 1 1 minerals. The Fe2+ content of the octahedral sheet of the 2 1 clays is seldom larger than 0.6 per 3.0 positions (less than 0.3 for most samples). Few low-temperature 2 1 clays have enough A1 substitution in the tetrahedral sheet to adjust its size to that of the large octahedral sheet. Substitutions of this magnitude, 1 A1 per four tetrahedral positions, at low temperatures, are favored more by the 1 1 than the 2 1 arrangement. Presumably, the lack of a layer charge and, therefore, the need for the tetrahedra to rotate to accommodate an interlayer K, and the fact that the tetrahedral sheet is sandwiched between two octahedral sheets allow interlayer size adjustments to be made more easily in the 1 1 than in the 2 1 clays. 

Clays, natural or synthetic, are the most widely investigated and understood nanoadditives used to enhance the flame retardancy of polymers through nanocomposite technology, because of their unique properties, particularly the ease of surface treatment and application in polymer matrices. Clay can be cationic and anionic materials, in accordance with the charge on the clay layers. In this chapter, the focus is on two kinds of clays montmorillonite (MMT), a naturally occurring cationic clay that belongs to the smectite group of silicates, and LDH, an anionic clay that does occur naturally but for which the synthetic form is more common. Other clays will also be mentioned as appropriate. 

Nickel silicate and ferrous silicate are the preferred catalysts in the Smuda process. The Smuda catalyst is a layered silicate clay framework with ordered nickel (or iron) atoms inside. The catalyst is charged at 10 wt% ratio of the plastic feedstock. The catalysts are based on layered silicates with Lewis acid activity [24]. Catalytic cracking results in very little noncondensable gas (<1%) and minimal carbonaceous char. The hfe of the Smuda catalyst is approximately 1 month [24]. 

Ion exchange or molecule exchange reactions, where the charge on the framework layer remains unchanged, e.g. sheet silicates, clays. 

Figure 2,14, Dependence of 2 1 layer silicate clay expansion in water on structural layer charge.

Figure 2.16. Common groups of layer silicate clay structures found in soils, pictured terms of their tetrahedral (iHk) and octahedral ( ) sheets. The usual locations of - /uctural charge and exchange cations are indicated by — and + signs.

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... 

---