Five‐Coordinate Silicon in Zeolites: Probing SiO_4/2F⁻ Sites in Nonasil and ZSM‐5 with ²⁹Si Solid‐State NMR Spectroscopy

Abstract

With fluoride as a mineralizing agent, the high-silica zeolites nonasil and ZSM-5 were obtained by hydrothermal syntheses. 29Si NMR spectroscopy showed that five-coordinate silicon exists in the form of SiO4/2F− in the zeolite frameworks (see the 29Si{19F} CPMAS NMR spectrum of nonasil on the right). In ZSM-5 at room temperature the fluoride ion undergoes a dynamic exchange between different silicon atoms, which is frozen out at 140 K.