The structures of zeolite sorption complexes. I. The structures of dehydrated zeolite 5 A and its iodine sorption complex

Abstract

The structure of dehydrated synthetic zeolite 5A (Ca4Na4AlazSix2048, cubic, Pm3m, a0 = 12-42 + 0.01/~) has been refined by least-squares analysis of X-ray powder diffraction data, and the structure of its sorption complex with nearly six iodine molecules per unit cell (Ca4Na4Al12048.5.6512, a0= 12.29+ 0.07/~) has been determined and similarly refined. In the 'empty' dehydrated zeolite the two kinds of cation appear to have somewhat different positions on the threefold axes, the Ca 2+ ion being close to the plane of the 6-ring oxygen window of the sodalite unit and the Na + ion being displaced about 0"4/~ inward from that position into the sodalite unit. Si, A1-O framework distances are 1-63-1.69/~,. In the iodine sorption complex, the I2 molecules lie in mirror planes of the framework structure; they are near the large windows and tipped at 32.5 ° to the window planes. The relative orientations of I2 molecules could not, however, be completely determined. The I2 molecules in one unit cell have a maximum possible aggregate point symmetry 3 and the overall structure is presumably disordered. Two different arrangements with point symmetry 3 are possible, and are regarded as more probable than some less symmetrical arrangements. The I2 interatomic distance uncorrected for thermal motion is 2.72/~, in fair agreement with the value 2"68 ,~ reported for solid iodine; the corrected value, 2.79/~,, may be in significant excess of reported values of about 2.67/~ for gaseous I2 molecules. Each I2 mol-ecule makes close approaches in its axial direction to a framework oxygen atom and to an iodine atom in an adjacent molecule, with an I. • • O distance of 3.29/~, and an I. • • I distance of 3-46/~. The latter, and a non-axial I. • .I contact distance of 4.01 A, compare roughly with analogous distances of 3-54 and 4-06 A in solid iodine.