Ionic Clusters in Zeolites

Abstract

Many established molecular sieve applications depend either directly or indirectly on the existence of a net anionic charge on the zeolite framework. In order to preserve charge neutrality, the framework charge is normally balanced by the familiar exchangeable cations residing within the zeolite pores. A large and rapidly growing body of literature, however, attests to the possibility of more complex and unusual chemical species fulfilling this charge-balancing role. My task in the following pages is to provide an overview of the preparation and properties of the many ionic cluster species, almost all of them cationic, that can be formed within zeolites. Methods of synthesis range from straightforward ion exchange, dehydration and calcination procedures to direct elemental vapourzeolite reactions under high vacuum conditions. The resulting clusters, diverse both in chemical composition and in physical and chemical properties, may conveniently be divided into metal and non-metal species. Although it is the burgeoning corpus of published work on the synthesis and characterization of cationic metal clusters in zeolites that primarily shapes this review, research into non-metal clusters, and in particular into clusters of ionic and semiconducting compounds in zeolites, is a major growth area producing many new ionic clusters. A brief outline of this work is also included.