Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework

Abstract

In silicogermanate zeolites containing double four-ring (d4r) building units, the germanium atoms preferentially occupy the corners of these cube-like units, but the absence of long-range order precludes the determination of the preferred arrangements of Si and Ge atoms by means of crystallographic methods. If fluoride anions are present during the synthesis, they are incorporated into these cages. Because of the sensitivity of the 19F chemical shift to the local environment, NMR experiments can provide indirect insights into the predominant (Si,Ge) arrangements. However, conflicting interpretations have been reported, both with regard to the preference for, or avoidance of, Ge–O–Ge linkages, and concerning the equilibrium position of fluoride inside the cage, where fluoride might either occupy the cage center or participate in a partly covalent Ge–F bond. In order to shed light on the energetically preferred local arrangements, periodic dispersion-corrected density functional theory (DFT) calculations we...