The crystal structures of the synthetic zeolites (Cs, K)-ZK5 and (Cs, D)-ZK5 determined from neutron powder diffraction data

The structure of a dehydrated ZK5 zeolite with the calculated composition Cs9 7K13.0Si73.2Al22.8O192 was refined using data collected on a high resolution neutron powder diffractometer at 294 K (a= 18.671(1)Å) and at 493 K (a = 18.660(1)Å) using a combination of Rietveld and Fourier techniques. The structures of a deuterated ZK5 with the composition D17.5Cs3.9K0.8Si73.8Al22.2O192 (a = 18.673(1)Å), and Cs-exchanged ZK5 (D13.0Cs6.0K2.7Si74.3Al21.7O192, a = 18.688(1)Å) were also refined at 294K. There is no evidence for Si/Al ordering (space group Im 3m). Cs occupies the flat 8-ring site (〈Cs—O〉 = 3.3 Å), while K prefers the puckered 8-ring site (〈K —O〉 = 2.9Å). There are significant distortions of pore openings upon extraction of the cations. The flat 8-ring is distorted elliptically. Both the ion-exchange properties of ZK5 and the positions of Cs and K can be explained in terms of the refined structure. The Cs preference for the flat 8-ring site sets an upper limit of 12 atoms/unit cell to its exchange into ZK5. The proximity of the flat and puckered 8-ring sites (5.8 Å) causes the Cs atom either to move towards the α cage if K is present or to relax toward the γ cage if K is extracted.