7Li{19F} TEDOR NMR to observe the lithium migration in heated montmorillonite

The distribution of lithium ions in heated synthetic montmorillonites was studied by using 19F, 7Li MAS and 7Li{19F} TEDOR nuclear magnetic resonance technics. To get the best of NMR spectroscopy, Na-montmorillonites, free of paramagnetic centres (Fe2+, Fe3+), were synthesized in fluoride medium, in order to have substitution of some hydroxyl groups by fluorine in the octahedral layer (Na0.35[(Al1.69Mg0.36)VI(Si3.86Al0.14)IVO10(OH0.95F0.05)2]). After Li+ exchange, mild heat treatments (from 110 to 250 °C during 24 h) were applied to follow the lithium migration. Both 19F and 7Li MAS spectra indicate that the lithium migration operates in two steps: at 110 °C, 86% of the lithium ions have moved to the hexagonal site. The remaining interlayer ions move progressively to this site when the heating temperature is in the range [170-250 °C]. For the first time a direct fluorine-lithium distance of 2.23 Å was measured by 7Li{19F} TEDOR, which uses fluorine as a highly selective and NMR-sensitive witness of atomic-scale motions inside porous media, and which agrees with the preferred hexagonal site for Li+ migration.