NUCLEAR-MAGNETIC-RESONANCE AND X-RAY PHOTOELECTRON SPECTROSCOPIC INVESTIGATION OF LITHIUM MIGRATION IN MONTMORILLONITE

When Li-saturated montmorillonite is heated to 200-300 degrees C, the Li ions migrate from interlayer positions to sites in the layer struct ure. However, the identity of these sites has not been clearly establi shed. Here we have investigated the migration of Li ions in montmorill onite, after heat treatment at 250 degrees C, using chemical and instr umental analyses. The latter include X-ray diffractometry (XRD), Li-7- nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Heating causes a large reduction in cation exchang e capacity (CEC) and an almost complete loss of interlayer expansion w ith glycerol as shown by XRD. Static and magic angle spinning (MAS) Li -7-NMR spectroscopy shows that the quadrupole coupling constant of Li increases markedly over the corresponding value for unheated Li-montmo rillonite (where Li occupies exchange sites in the interlayer space) a nd for hectorite (where Li is located in the octahedral sheet). This w ould indicate that, in heated montmorillonite, Li occupies structural sites of low symmetry which, however, cannot be identified with octahe dral vacancies in the layer structure as is commonly assumed. XPS show s that the binding energy (BE) for Li in unheated montmorillonite is c omparable to that for other exchangeable cations. Heating broadens the Li Is band and decreases the BE. The BE for Li in heated montmorillon ite is significantly higher than that in either spodumene or lepidolit e, where Li is known to occupy octahedral sites. The combined data sug gest that heating induces Li to migrate from interlayer sites to ditri gonal cavities in the tetrahedral sheet, rather than into vacancies in the octahedral sheet, of montmorillonite.