LOCATION OF THE 18-CROWN-6 TEMPLATE IN EMC-2 (EMT) RIETVELD REFINEMENT OF THE CALCINED AND AS-SYNTHESIZED FORMS

The structure of the synthetic zeolite EMC-2, in both its calcined [a = 17.3864(4) angstrom and c = 28.3459(9) angstrom] and its as-synthesized, partially dehydrated [a = 17.3741(1) angstrom and c = 28.3646(2) angstrom] forms, has been investigated in the space group P6(3)/mmc using powder diffraction methods. The calcined material proved to have a pure EMT (hexagonal) framework topology with no significant FAU (cubic) intergrowths. In the partially dehydrated sample, one 18-crown-6 molecule with an associated Na+ ion was found in each of the medium and large cages of the framework. The atoms of the crown ether molecules in the medium cages are well defined and fit snugly into the curvature of one half of the cage. The 18-crown-6 molecule in the large cage is also located on one side of the cage, where it is anchored by three Na+ ions associated with 6-rings of the framework. A certain amount of conformational disorder is observed in this crown ether. The stacking of the disk-shaped crown ether molecules and their ideal fit into the depressions of the sodalite cage layer suggest an explanation for the strong specificity of the 18-crown-6 molecule for the hexagonal framework topology.