Structure and Characterization
Zeolite A exhibits the LTA (Linde Type A) structure. It has a 3-dimensional
pore structure with pores running perpendicular to each other in the x,
y, and z planes, and is made of secondary building units 4, 6, 8, and 4-4.
The pore diameter is defined by an eight member oxygen ring and is small
at 4.2Å. This leads into a larger cavity of minimum free diameter
11.4Å. The cavity is surrounded by eight sodalite cages (truncated
octahedra) connected by their square faces in a cubic structure. The unit
cell is cubic (a = 24.61Å) with Fm-3c symmetry. Zeolite A has a void
volume fraction of 0.47, with a Si/Al ratio of 1.0. It thermally decomposes
at 700ºC. [1,2,3]
Zeolite A, like other zeolites, is synthesized in a gelling process. Sources
of alumina (usually sodium aluminate) and silica (usually sodium silicate)
are mixed in basic aqueous solution to give a gel. The alkali agent can
be NaOH or solutions of quaternary ammonium salts, amines, or other polar
organics. The gel is then heated to 70-300ºC to crystallize the zeolite.
The zeolite is normally synthesized in the Na+ form. 
Zeolite A is of much interest because its supercage structure is useful
in spacio-specific catalysis. The inner cavity is large enough for structure
changing reactions to take place, but the small pore means only a specific
structure can get into the cavity for reaction, typically n-paraffins and
olefins. One use is in paraffin cracking. The small entry pore is selective
towards linear paraffins, and cracking can occur on sites within the supercage
(alpha-cage) to produce smaller chain alkanes. Zeolite A is also widely
used in ion exchange separation. 
1. Herreros, B., The
X-Ray Diffraction Zeolite Database on the web.
2. Meier, W. M., et. al., The
Atlas of Zeolite Structure Types on the web.
3. Subhash Bhatia, Zeolite Catalysis: Principles and Applications, CRC Press,
Inc., Boca Raton, Florida, 1990.
4. Ribeiro, F. R., et. al., ed., Zeolites: Science and Technology, Martinus
Nijhoff Publishers, The Hague, 1984.
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