The main feature of zeolite frameworks is the presence of pores, as can be seen in the example. 4-12 Angstrom in size and, due to the crystalline nature of the material, can be hundreds of Angstroms in length. This makes zeolites very interesting as catalytic agents, since the bulk of the crystal is accessible to chemical species that fit into the pores. Metal substitutions into the framework, Al for Si for example, will create areas of excess negative charge, where cations or non-framework species can bind. The difficulty stems primarily from the polycrystalline nature of most zeolite samples, with crystallite sizes typically below 5 um.
Zeolites continue to be synthesized at a furious pace.
Structure is sought after not only to understand the performance of newly synthesized catalysts but also to propose rational syntheses of homologous materials with tailored performance. Roughly 125 framework structures have been reported, yet several dozen distinct synthetic zeolites remain unsolved in the patent literature. The techniques of diversity synthesis have recently been introduced to the field, and this may soon lead to a tremendous explosion in the number of new, unsolved synthetic zeolites. The method then locates the positions of the T-atoms in the unit cell. The oxygens can subsequently be located by a Rietveld refinement.