Clays and Other Layered Materials

Clays are one of the most abundant materials present on the earth's surface. They constitute a large component of soil, while many ceramic and building materials as well as industrial adsorbents and catalysts contain clay. Soils owe their ability to sustain plant life largely to clays which have the ability to exchange ions with their surroundings. Clays are typically composed of sheets of linked SiO4 tetrahedra, which are connected to Al(OH)6 octahedra. If one sheet of silica interacts with a plane of Al(OH)6, then a two-tier sheet (Al2Si2O5(OH)4) typical of kaolinite is obtained. If the octahedral plane is sandwiched between two silica sheets, then a three-tier sheet is obtained (Al2Si4O10(OH)2), as found in the smectite and mica clays. The sheets are bonded to one another via covalent bonds between the silica and alumina sheets to yield a layer. It is how these layers stack together (via electrostatic and van der Waals forces only) which give clays many of their interesting properties, and gives a large degree of flexibility to the structures. Clay-like materials may be composed of oxides of elements other than silicon and aluminium.

The three principal types of clay - single-layer, nonexpandable double-layer and expandable double-layer - have been introduced by Dyer. Clays may be either cationic (exhibiting cation exchange properties) or anionic (anion exchangers). The former type is more common, accounting for the majority of naturally occurring clays; typical examples are montmorillonite and bentonite. Anionic clays, such as hydrotalcite, occur rarely in nature, but may be synthesized in the laboratory. Layered materials composed of neutral layers also exist, although they possess little or no intrinsic ion exchange capability. Table 3 lists some common types of layered material possessing cationic, anionic and neutral layers.

Pillared clays Expandable cationic clays may be converted into pillared clays by exchanging some or all of their charge-balancing cations with bulky inorganic species such as [Al13O4(OH)24(H2O)12]7+ or [Zr4(OH)14(H2O)10]2 + and then calcining the composites to dehydrate and dehydroxylate the pillaring species, leaving hydroxy/oxide pillars. An interesting pillaring process is that involving ion exchange with a cationic 'templating' agent (cetyltrimethylam-monium), followed by the synthesis of a mesoporous silica phase around the template cations. The resultant materials, in which the clay layers are propped apart by the mesoporous silica, possess surface areas up to 800 m2 g- and interlayer spacings of 3.3-3.9 nm.

For layered materials with anion exchange properties, like layered double hydroxides, species such as [V10O28]6~ and [H2W12O40]6~ may be exchanged with anions residing between the layers to increase the interlayer spacing.

Table 3 Examples of layered materials

Layer charge

Example

Neutral (no intrinsic ion exchange capability)3 Positive (anion exchange properties)

Negative (cation exchange properties)

TaS2 MoO3

Layered double hydroxides: [MlLxMx'(OH)2]x+[x;;/n]x- • ZH2O Hydroxy double salts:

[M(1 -x)M"1 +x)(OH)3d-y)](1 +3y) + [Xni + 3y)/n](1 +3y)- • ZH2O

Smectite clays (low charge density) Micas

MIVH-phosphates (high charge density, e.g. a-ZrP, -y-ZrP) Layered titanates Silicic acids aNeutral layered materials may undergo a type of ion exchange reaction via redox intercalation, whereby a neutral species is intercalated, followed by a transfer of electrons between the layer and the guest species. Thus both the layer and the intercalated species become charged.

While pillared clays usually offer advantages over normal clays in terms of their higher surface areas, higher sorptive capacities and greater ion exchange capacities, these properties begin to be diminished when the density of pillars becomes too great and the interlayer space becomes filled with pillars. Pillared clays are seldom employed as ion exchangers; their main applications lie in the fields of catalysis and adsorption.

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