Types of Inorganic Ion Exchange Material

An important distinction between ion exchange materials is whether they exhibit capacity for cations, anions, or both. Cation exchangers, and in particular zeolites, clays and zirconium phosphates, are the most common and best understood of the ion exchangers. Anion exchangers are also important but

Figure 1 The two majortypes of ion exchange site. (A) Type 1, structure-terminating and defect groups; (B) Type 2, charge-compensating groups. M is an oxide-forming metal with oxidation state 4; T is an oxide-forming metal with oxidation state 3. The regions enclosed in dotted lines are those giving rise to ion exchange where Z# (or Z-O~) is exchangeable. Shaded areas represent a continuation of the oxidic network.

Figure 1 The two majortypes of ion exchange site. (A) Type 1, structure-terminating and defect groups; (B) Type 2, charge-compensating groups. M is an oxide-forming metal with oxidation state 4; T is an oxide-forming metal with oxidation state 3. The regions enclosed in dotted lines are those giving rise to ion exchange where Z# (or Z-O~) is exchangeable. Shaded areas represent a continuation of the oxidic network.

the exchange of anions is often not fully reversible, thus the exchangers cannot be easily regenerated and the reactions are more difficult to treat thermo-dynamically. Multiply charged anions, in particular, may be held tenaciously by the exchanger. Examples of anion exchangers are certain clays such as hydroxy double salts (e.g. [CuNi(OH)3]Cl) and layered double hydroxides (e.g. hydrotalcite, Mg6Al2(OH)16 (CO3) • 4H2O). Amphoteric ion exchangers possess predominantly Type 1 exchange sites, e.g. hydrous oxides.

While ion exchange properties may be exhibited by both amorphous and crystalline solids, studies of the ion exchange properties of amorphous solids are often hampered by difficulties in preparing materials reproducibly and the difficulties in characterizing them fully. With crystalline materials, however, reproducible preparations can be easily verified and well-defined structural data aids in the interpretation of the results of ion exchange experiments.

Most crystalline inorganic ion exchangers are porous. This porosity may arise through the presence of void space between the layers in clay materials and layered double hydroxides, or through the intrinsic microporosity present in zeolitic materials. Many of the layered materials have the versatility to (revers-ibly) change their interlayer spacing and hence the size of the voids, which allows the ion exchange properties to be adjusted. The more rigid zeolite structures give rise to exchange reactions which may show extremely high selectivity to certain cations, or perform ion sieving.

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