Zeolites From the synthesis to applications

Zeolites are perfectly crystallized minerals with a regular nanoporous system consisting of a network of channels with diameters of less than 10 Å (1 nm) and cages or intersections of channels; these nanodimensions, close to those of organic molecules, explain why zeolites are used in many industrial processes. Their history begins with the discovery in 1756 by the Swedish mineralogist Crönstedt of a mineral (stilbite) which, due to its high water content, swelled when heated in a flame. This property led him to attribute the name zeolite, derived from the Greek words zeo and lithos (zeolite: boiling stone) to this new family of minerals (aluminosilicates). For a very long time, natural zeolites were mainly used in jewelry for the beauty of their crystals. The development of hydrothermal synthesis and the discovery of large sedimentary basins have enabled zeolites to be used on a large scale and for a wide range of applications: separation by molecular sieving, purification by adsorption, cation exchange and catalysis. Both hydrothermal synthesis and post-synthesis processing lead to zeolites of well-defined structure and composition, which has greatly facilitated their industrial use.

This article has two main objectives:

• describe the key steps involved in zeolite synthesis and post-synthesis treatments: cation exchange, dealumination, desilication, etc. ;

• present and explain the effect of their physical and physicochemical characteristics: confinement of reagents in nanopores, nature, concentration, strength of active sites, etc. on their activity, selectivity and stability in the main industrial processes operating by acid, bifunctional redox-acid, basic or oxidation catalysis.