7 Li{ 19 F} TEDOR NMR to observe the lithium migration in heated montmorillonite
Résumé
The distribution of lithium ions in heated synthetic montmorillonites was studied by using F-19, Li-7 MAS and Li-7{F-19} 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 (Na-0.35[(Al1.69Mg0.36)vi(Si3.86Al0.14)(IV)O-10(OH0.95F0.05)(2)]). After Li+ exchange, mild heat treatments (from 110 to 250 degrees C during 24 h) were applied to follow the lithium migration. Both F-19 and Li-7 MAS spectra indicate that the lithium migration operates in two steps: at 110 degrees 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 degrees C]. For the first time a direct fluorine-lithium distance of 2.23 angstrom was measured by Li-7{F-19} 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.