Abstract
In zeolites, i.e., crystalline microporous aluminosilicates [1 2 3], the framework exhibits negative charges as a consequence of the incorporation of trivalent aluminum atoms instead of tetravalent silicon. The same situation is encountered in most cases of related crystalline microporous (cf. [4, 5]) and crystalline-like mesoporous materials, e. g., in isomorphously (by Fe3+, Ga3+, B3+, etc.) substituted zeolite structures and Al-containing M41S materials, respectively (cf., e.g., [6, 7]). These negative charges of the frameworks must be compensated by the positive charges of extra-framework cations or via the interaction of the framework oxygen atoms with protons under formation of acid hydroxyls, i.e., so-called Brønsted centers (cf., e.g., [8]). According to the synthetic procedures (see, e.g., [9, 10]), usually Na+, K+ or template cations are present in as-synthesized micro- or mesoporous materials and play the role of charge-compensating species; proton attack of the oxygen atoms of the framework occurs, e.g., on removal of the organic template molecules. However, the charge-compensating entities (alkali metal cations, protons, etc.) can be replaced by other cations, and this makes zeolites inorganic cation exchangers (cf., e.g., [ 11, 12]). The ion-exchange capacity of microporous materials, especially of zeolites such as LTA- and P-type zeolites (see [13, 14]), is the basis for their worldwide application as detergent builders [15]. However, ion exchange is also one of the most important processes for post-synthesis modification of microporous materials, for instance, in order to tailor adsorbents and catalysts.
