ABSTRACT
For decades, zeolites have been described as crystalline aluminosilicate molecular sieves that have open porous structures and ion exchange capacities. The catalytic sites in aluminosilicate zeolites are associated with tetrahe-dral aluminum atoms in substitutional positions in the framework of silica. Zeolites of interest to shape selective catalysis may be divided into five major groups according to their pore/channel systems. The shape of the 8-membered oxygen rings varies from circular to puckered and elliptical. The dimension of the pore opening also varies accordingly. hus the actual pore size can be significantly larger than that calculated. Except for partheite and laumontite, which have puckered 10-membered oxygen rings in their structures, almost all medium pore zeolites of interest to shape selective catalysis are synthetic in origin. Medium pore zeolites may be synthesized with only a trace concentration of aluminum. As acid catalysts, the medium pore zeolites have exceptional shape selectivity and low coking tendencies.
