Tracing Water and Cation Diffusion in Hydrated Zeolites of Type Li-LSX by Pulsed Field Gradient NMR

The pulsed field gradient (PFG) technique of NMR is exploited for recording the time-dependent mean diffusion path lengths of both the water molecules (via NMR) and the cations (via Li-7 NMR) in hydrated zeolite Li-LSX. The observed propagation patterns reveal, for both the water molecules and the cations, two types of transport resistances, acting in addition to the diffusion resistance of the genuine pore network. They are attributed to the interfaces at the boundary between the purely crystalline regions (crystallites) within the Li-LSX particles (intergrowths) under study and to the external surface of either the particles themselves or crystallite aggregates within these particles. The cation diffusivity is retarded by about 1 order of magnitude in comparison with the water diffusivity. This notably exceeds the retardation of cation diffusion in comparison with water in free solution, reflecting the particular influence of the zeolite lattice on the guest mobility.