6 - Hydrothermal Synthesis and Growth of Zeolites

Abstract

Zeolites form an important group of hydrated aluminosilicate minerals of the alkalies and alkaline earths, with an infinitely three-dimensional anion network and with the atomic ratio O:(Al + Si) = 2. This chapter discusses the mineralogy, crystal chemistry, synthesis and applications of zeolites. A comparison has also been drawn between natural and synthetic zeolites. Aluminophosphate zeolites have also been introduced, which is a new class of microporous inorganic solids with the potential to be as useful and as scientifically challenging as the aluminosilicate zeolites and form a part of the history of framework of oxide molecular sieves. The synthesis of zeolites at temperatures around 100°C usually produces crystals in the size range 0.1–10 μm. In applications as catalysts, sorbents, and ion exchangers, this size range is fully suitable. The small size indicates that once nucleation begins there is rapid formation of numerous tiny particles all competing for the available chemical nutrients. In order to carry out the single crystal X-ray diffraction studies, zeolites should be 100 μm, or even more, in diameter. Similarly, for other examinations, like molecule and ion-selective membranes or other electrochemical studies, bigger crystals are required. Therefore, crystal growth of zeolites can be both challenging and interesting as well.