| Abstract
| - The distribution of fluoride ions has been studied in the pure silica IFR, ITH, IWR, STF and STT zeolitestructures using computational techniques. The interactions between the F- and SDA+ ions (where SDA isthe organic structure directing agent) are able to explain the F- cage occupation found experimentally. Whilestudying the short-range fluoride−framework interactions, a relationship was found between the Si atomsforming the pentacoordinated units and the lowest F- defect energies, which rationalizes the experimentalSi−F bonding in terms of energetic stability. It is proposed that the F- location is governed by a two stepprocess. In a first stage, the electrostatic long-range forces and, especially, the interactions between the F-and the SDA+ ions, decide which cage will be filled with F-; in a second stage, once the F- cage locationis decided, the F- forms a covalent bond with a Si site to form an energetically stable pentacoordinated unit[SiO4/2F]-.
|
