Chapter 185 The use of rare-earth-containing zeolite catalysts

Publisher Summary This chapter discusses the use of rare-earth-containing zeolite catalysts. Zeolites are tridimensional crystalline aluminosilicates with the following formula in the as-synthesized form: mM2/nO[xAl2O3-ySiO2]wH2O, where M is a cation which can belong to group IA or IIA or can be an organic cation, while n is the cation valence, and w represents water contained in the zeolite voids. The main characteristic of the zeolites is that the tetrahedral primary building blocks are linked in such a way that a three-dimensional network containing channels and cavities of molecular dimensions is produced. In the case of rare-earth-exchanged zeolite, structures A, X, and Y have been the most studied. The synthesis and structures of these zeolites are well known, and the structures are all based on the sodalite (β-cage) unit. The A zeolite is obtained by replacing each 4-ring of the sodalite cage at the corner of the unit cell by a cube. Each sodalite cage at the body center of the unit cell transforms into a truncated cuboctahedron with twelve 4-rings, eight 6-rings, and six 8-rings. The truncated octahedra form a system of pores linked in three directions by windows of 8-rings. The sodalite units are separated from each other, and access to them is via 6-rings. The topology of faujasite (and of the related X and Y zeolites) is obtained by linking sodalite units with hexagonal prisms. Each sodalite unit is linked to four sodalite units in a tetrahedral configuration by hexagonal prisms which are attached to four of the eight hexagonal faces; the other four hexagonal faces are unshared, as are the 4-rings.