Variation of sodium coordination during the hydration processes of layered sodium disilicates as studied by 23Na MQMAS and 1H↔23Na CP/MAS NMR spectroscopy

Abstract

²³Na MQMAS and ¹H↔²³Na CP/MAS NMR techniques have been used to study the variation of the sodium coordination during the hydration processes of the layered sodium disilicates SKS-5 and SKS-6. Three new types of sodium ions, with different local coordination environments, were identified and correspond to sodium ions coordinated by oxygen atoms of the silicate framework, water molecules and hydrogen-bonded silanol I (with the hydroxy proton bonded to an oxygen atom in the adjacent layer), sodium ions coordinated by six water molecules and sodium ions coordinated by oxygen atoms of the silicate framework and hydrogen-bonded silanol II (with the hydroxy proton bonded to a non-bridging oxygen at the same silicon atom). The ²³Na NMR results suggest that the hydration processes of SKS-5 and SKS-6 are very similar, i.e. both layered sodium disilicates are gradually converted into new phases by hydration, which is consistent with previous ¹H and ²⁹Si NMR experiments. The unchanged isotropic chemical shift (δ_iso) of the first type of sodium ions in the hydrated SKS-5 samples suggests that the corresponding average Na–O bond length and the interlayer distance of the new phase in the hydrated SKS-5 samples remain constant during the hydration process. However, a decrease in δ_iso (ca. 2–3 ppm) for the first type of sodium ions is observed during the hydration process of SKS-6, implying that the corresponding interlayer distance of the new phase and the average Na–O bond length increase by ca. 0.03–0.04 Å.