Two-dimensional 23Na MQ MAS NMR study of layered silicates

The two-dimensional multiple-quantum MAS (2D MQ MAS) NMR method was applied to Na-23 in the hydrous layered silicates makatite, kanemite, octosilicate, magadiite and kenyaite in order to obtain precise information about the coordination structure of sodium ions in the inter-layer spaces. The makatite spectrum with high-power H-1 decoupling clearly showed the existence of three different sites for sodium ions, in agreement with the structure derived from single-crystal X-ray diffraction. Lineshape fitting was performed for the cross sections of the 2D spectrum for the three sites. The derived Na-23 NMR parameters indicate the value of the isotropic chemical shift to describe the coordination number of sodium ions in this type of compound. In the kanemite 2D spectra, well resolved signals from three different sites for sodium ions were also observed but only because of mixed sample composition. Two of the resonances were assigned to environments which we believe can best be described as NaHSi2O5 . 3H(2)O and NaHSi2O5 . 4H(2)O, respectively, from a comparison of the spectra of two separately synthesized samples and by contrasting the results with those of samples dried under vacuum at room temperature for 6 h. The sodium ions in the third site are coordinated by more water molecules than in the other two sites. In all three sites, sodium ions were expected from their chemical shifts to have six-coordinated octahedral coordination. Although the structure of the samples changed reversibly by drying under vacuum at room temperature and re-moisturizing, drying the sample under vacuum at 100 degrees C resulted in structural change irreversibly altering the 2D NMR spectra. Octosilicate, magadiite and kenyaite showed only one sharp peak in their 2D spectra, and octahedral coordination with relatively high symmetry was deduced for each of these silicates. Samples of octosilicate and magadiite dried under vacuum at room temperature showed reversible changes in their 2D spectra, and the observed broadened signals were deduced to be due to the formation of less symmetrical coordination sites of sodium ions by the removal of water molecules.