PROBING INTRAZEOLITE SPACE

Molecular sieves are inorganic framework structures generally composed of crystalline aluminosilicate tetrahedra which are arranged to form channels of 2-10 Angstrom diameters and cages with dimensions from 6-1 5 Angstrom. Absorption of probe molecules of varying geometries and si zes is used to characterize the framework dimensions and topography in concert with X-ray diffraction identification of the specific structu re. From unit cell dimensions and assumptions about the size of the fr amework forming species, a pore volume can be calculated. The volumes of the absorbed probe molecules, using their liquid densities, are the n compared to the calculated pore volume. The constraint on the packin g of the absorbed molecules is quantified by comparing their packing d ensity to their density in the liquid state. Further, the packing of d ifferent probe molecules into the same pore volume is compared via a r atio technique called the 'packing ratio'. The effect of the lattice g eometry and framework dimensions on the 'packing ratios' is to provide a set of characteristic values for a given molecular sieve. The packi ng ratios for zeolites rho and ZSM-5 are presented as expectation valu es for other scientists to use as bases of comparison.