Computational Study of Location and Role of Fluoride in Zeolite Structures

The distribution of fluoride ions has been studied in the pure silica IFR, ITH, IWR, STF and STT zeolite structures using computational techniques. The interactions between the F- and SDA+ ions (where SDA is the organic structure directing agent) are able to explain the F- cage occupation found experimentally. While studying the short-range fluoride-framework interactions, a relationship was found between the Si atoms forming the pentacoordinated units and the lowest F- defect energies, which rationalizes the experimental Si-F bonding in terms of energetic stability. It is proposed that the F- location is governed by a two step process. 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 location is decided, the F- forms a covalent bond with a Si site to form an energetically stable pentacoordinated unit [SiO4/2F]-.