Natural and synthetic crystalline zeolites are characterized
by frameworks originating from linkages of (Si, Al)-tetrahedra,
in which structural cages and channels are occupied
by water molecules and an appropriate amount of non·
framework cations for balancing the negative charge of the
available (AI02)- groups. Most common among these
non-framework cations are Na+, Ca+ and K+ (in both
natural and as-synthesized zeolites). Reversible and facile
exchange of these cations with others is one of the characteristic
properties of zeolites. The extents of such exchanges
are guided by factors like crystallographic sites
and structural openings involved, ionic hydration energies,
hydrated radii of the ions etc. Technologically important
ion exchanges in zeolites include Ni+, RE3+ (e.g. La3+,
CeH) and NH4 + for sodium. |