Crystallographically different site exchange

Highly selective exchange as related to the presence of crystallographically different site groups... 

To explain the appearance of a magnetic moment by the segregation of differently charged Fe ions to each antiferromagnetic sublattice, coexistence of two kinds of Fe ions is necessary in a crystallographically equivalent site. If the 2+ ions are other than Fe, exchange of ions cannot be expected below room... 

Zeolite catalysts may also be regarded as mixed oxides, but the crystallographic structures differ from the solids discussed above in that active sites for catalysis occur within the open lattice framework. In consequence, rate data are not directly comparable with similar observations for other heterogeneous reactions since the preexponential factors are calculated and reported on a different basis. For completeness, however, it is appropriate to mention here that instances of compensation behavior on zeolite catalysts are known. Taylor and Walker (282) described such an effect for the decomposition reactions of formic acid and of methyl forma te on cation-exchanged 13X molecular sieves, and comparable trends may be found in data reported for reactions of propene on similar catalysts (283). 

Natural zeolites are aluminosilicates with rigid, three-dimensional crystal lattices containing cavities connected by windows. The size and shape of these varies greatly from one material to another. In a few instances, e.g., analcite-leucite, the crystal structure may change discontinuously with conversion to another ionic form (Barrer and Hinds, 1953). Moreover, most zeolites contain several physically and crystallographically distinct exchange sites with different accessibility and selectivity (Barrer and Falconer, 1956). 

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