Résumé
This chapter introduces molecular sieves. It discusses the preparation of zeolites. An important property of zeolites is microporosity. Zeolites—like any other tectosilicate—are formed by a continuous network of oxygen-sharing SiO4 or AlO4-tetrahedra. Alkali or alkaline earth cations are trapped inside the network and compensate the negative charge of trivalent aluminum atoms sharing four oxygen atoms with their neighbors. Zeolites are formed by the hydrothermal synthesis at temperature levels much lower than feldspar, and thus the cations of the zeolites are hydrated. After crystallization, the hydration water of the cations are evaporated by heating. The aluminosilicate framework is rigid enough to avoid collapsing and the micropore volume previously occupied by water is made available to adsorb other molecules. The first commercial application of zeolites was the adsorption of water molecules to dry gas flows. The definition of the zeolites as “molecular sieves” describes the screening of molecules of different size by their microporosity. The molecular-size microporosity of the zeolites is important for catalytic applications. The shape-selectivity effect in catalysis corresponds to the selection among possible reaction paths based on the match between shape and size of the reacting molecules and the micropores.