Zeolite Type Frameworks: Connectivities, Configurations and Conformations

Abstract

Zeolite-type framework structures are normally based on fully cross-linked tetrahedra containing Si, Al, P and occasionally other atoms. These are generally called T-atoms. The bridges are invariably formed by oxygen atoms. In a few cases, a small fraction of the T-O-T bridges are missing, and in such instances the term ‘interrupted framework’ is used. The topology of a particular observed crystalline framework is called a structure type (ST) irrespective of composition, distribution of T-atoms, unit cell dimensions, or symmetry. General terms used when dealing with structure types are the topology of a 4-connected network or the connectivity of a framework, the latter implying a chart or an enumeration scheme of the T-atoms. It should be noted that the actual geometry including bond distances, valence angles etc. does not matter in describing structure types, topologies or connectivities. The same applies to the term configuration which has a similar meaning but refers to a local part of the framework and not to the entire 4-connected assemblage. Examples of frequently encountered configurations include cages, apertures, secondary building units (dealt with in Sect. 5), chains, and loop configurations (see Sect. 7 for details). When considering conformations, however, the actual geometry of configurations or of an entire framework structure must be taken into account. Unlike conformations of molecular structures, those of tetrahedral frameworks are completely fixed by T-O bond distances, O-T-O and T-O-T angles [1].