K+ ion distribution in zeolite ZK-4's with various Si/Al ratios and the contribution of K+ ions to K cluster formation

The detailed distribution of K+ ions on dehydrated K-type zeolite ZK-4's having various Si-to-Al atomic (Si/Al) ratios was determined using a high-resolution X-ray powder diffraction technique. The space group changes from Fm (3) over barc (a similar to 24.6 Angstrom ) to Pm (3) over barm (a similar to 12.1 Angstrom) at an Si/Al ratio of between 1.1 and 1.5 as the increases. The site occupancy of the K+ ion at the six-membered ring (6-MR) decreases as the Si/Al ratio increases, whereas that at the eight-membered ring (8-MR) is close to unity, independent of the Si/Al ratio. The K- ion site at the 6-MR was split into two independent positions, which were displaced in alpha- and beta-cage sides. The K clusters can be stabilized in the alpha-cages of ZK-4 having a high Si/Al ratio by K atom adsorption onto the ZK-4 powder. The mechanism of stabilization of the K clusters in the alpha-cage is strongly correlated with the K+ ion distribution. i.e., the high occupancy of the K+ ions at the 8-MR site. (C) 2002 Elsevier Science Inc. All rights reserved.