Micro- and mesoporous materials play an important role in modern techniques, the fields of applications ranging from classical ones like molecular sieving, washing and drying agents, via crude oil cracking, gas storage and scavenge of radioactive pollution, up to shape-selective catalysis under various conditions and operation as carrier of functional species, such as luminescent or magnetic ones. Nature has bestowed mankind a number of microporous minerals, e.g. the classical aluminosilicate zeolites and the complex heteropolyhedral titanosilicates of Kola peninsula, to mention just a few, but scientific curiosity and real economic needs have led to enormous efforts worldwide to find new examples of framework types and to improve the synthesis of already existing ones.
The crystallogenesis of micro- and mesoporous materials differs from that of most inorganic salts or organic molecules in several aspects. The isotropic ionic forces of the former, and the weak, short-range, intermolecular van-der-Waals interactions of the latter are favourable for crystal growth, such that in many cases relatively big crystals can be obtained in short times. Micro- and mesoporous materials often consist of ionic-covalent bonds which have strong directional dependencies and often lead to surface reconstruction. In addition, these materials are commonly metastable, and their nucleation and growth is kinetically rather than thermodynamically controlled. The need to control so many, often not well-defined, parameters renders crystal growth of these materials difficult. As a result the crystals grow relatively slowly and the obtained crystals are often quite small.
In my talk a review of the most salient features of the synthesis of micro- and mesoporous materials will be given. In addition, the results of our own studies on the transformation of layered precursors into three-dimensional microporous frameworks will be discussed.
Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged (grants DE 412/27-1,2; DE 412/31-1).