Ion and water mobility in hydrated Li-LSX zeolite studied by 1H, 6Li and 7Li NMR spectroscopy and diffusometry

Crystallites of zeolite LSX with a diameter of about 10 μm were synthesized. Crystals of this size are shown to allow the simultaneous investigation of intracrystalline mass transfer phenomena of water molecules and lithium ions in hydrated zeolite Li-LSX by NMR diffusometry. By MAS NMR spectroscopy with the 1H and 6Li nuclei, the water molecules and lithium ions are found to yield two signals, a major and a minor one, which may be attributed to locations in the sodalite cages and the supercages, respectively. By 1H and 6Li exchange spectroscopy the mean residence times in the sodalite cages at 373 K are found to be about 150 ms for the water molecules and about 40 ms for the lithium cations. PFG NMR self-diffusion measurements at 373 K yield a diffusivity of about 2 × 10−11 m2 s−1 for the lithium ions, which is about one order of magnitude below the water diffusivity.

Figure optionsDownload as PowerPoint slideHighlights
► Large LSX crystals were purposefully synthesized for PFG NMR diffusion studies.
► Exchange between sodalite and supercages is studied by 1H and 6Li MAS NMR.
► 1H MAS NMR is applied for high-resolution diffusion measurement of water.
► For the first time cation diffusion in zeolites was measured by PFG NMR.
► Lithium diffusion is exceeded by water diffusion by one order of magnitude.