New insights into fluoride's role in MFI zeolites: unveiling the link between location and synthesis conditions

Abstract

This study examines the fluoride distribution in pure silica MFI zeolite synthesized in fluoride medium using various tetra-alkyl ammonium and phosphonium cations as organic structure directing agents (OSDAs). The ¹⁹F NMR spectra show a resonance at ∼−65 ppm, related with the presence of fluoride atoms in the t-mel cage bonded to the silicon at T9 site, and another signal at ∼−80 ppm. Through DFT calculations, the ∼−80 ppm ¹⁹F NMR resonance is attributed to fluoride within the t-mel cage bonded to silicon at the T12 site. Thus, the results reported here reveal that, contrary to previous reports, there is more than one position for fluoride in the zeolite structure. The location of fluoride at T12 is stabilized by using bulky OSDAs and by increasing the concentration of defects in the MFI framework. Moreover, it is shown that the OSDA used in the synthesis determines the content and distribution of siloxy–silanol defects and fluoride in pure silica MFI zeolite. These findings highlight the impact of synthesis parameters in the charge distribution in zeolites, which ultimately governs key structural properties and potential applications of zeolite materials in catalysis and adsorption processes.