- © The Mineralogical Society Of America
A few years ago, the IUPAC recognized a need in the area of ordered microporous and mesoporous materials for a system of terms, whose definitions are generally accepted. A subcommission was formed to address this problem, and eventually a set of recommendations was published (McCusker et al. 2001). These recommendations, which include both basic nomenclature and a standardized crystal chemical formula, are based on common usage and on a systematic classification scheme recently published by Liebau (2003).
Zeolites and zeolite-like materials, with their 3-dimensional, 4-connected inorganic frameworks, served as the basis for this terminology, because they constitute the largest group of ordered microporous materials and because a nomenclature already existed for them (Barrer 1979). However, the new nomenclature was developed with the idea of encompassing all ordered, microporous and mesoporous materials with inorganic hosts, including those with non-zeolitic chemical compositions, with non-tetrahedral building units and/or with host structures that do not extend in three dimensions. The only restrictions imposed were that the pores must be ordered, accessible, and have free diameters of less than 50 nm (generally accepted range for microporous and mesoporous materials).
In most cases, the atoms of the host (and therefore the voids) are arranged periodically with long-range order and produce sharp maxima in a diffraction experiment. That is, the materials are crystalline. However, there are some materials for which the host displays only short-range order (i.e., is amorphous with respect to diffraction experiments), but the pores are of uniform size with long-range order and produce diffraction maxima at d-values reflecting the pore-to-pore distance. That is, the pore structure is “crystalline.” The following recommendations apply to any material in which the arrangement of the pores within the inorganic host is highly ordered. They do not apply to materials such as pillared clays, where the pores …