Natural zeolites and X-ray crystallography: Opportunities and limitations (Natuurlijke zeolieten en rontgenkristallografie: Mogelijkheden en beperkingen).

摘要

Zeolites are porous Si,Al-silicates which find important applications in industry. They are used as ion exchangers, as molecular sieves and as catalysts. These inorganic compounds are found in natural deposits and are produced synthetically. Due to this open structure (with cages and channels), cations and (polar) molecules (e.g. water molecules) can freely move through the structure. The interaction between the framework and the cation/water molecules is weak.; The aim of our experiments was to shed light on this peculiar framework cation/water molecule relationship in the case of natural zeolites. This requires a better understanding of the zeolite structure and the position of the cations/water molecules therein. X-ray diffraction is the most important technique used to study the structure of solids. This technique has been developed into a strong tool which enables us to get accurate structural data. In most cases, satisfactory results are obtained. The standard approach fails however to encompass all structural peculiarities one encounters in the case of zeolites. Si/Al-order/disorder phenomena, space group problems and cations/water molecules positioned on many different, low occupancy sites in the case and channels hamper our efforts to obtain an accurate structure model. Most refinements found in the literature strand upon these problems. We have tried out some fresh ideas with changing success.