Investigation of proton dynamics within the hydrogen-bond network of the layer silicate Na-RUB-18

Solid-state H-1 and Si-29 MAS NMR spectroscopies were applied to the hydrous layer silicate Na-RUB-18 to investigate the hydrogen-bond networks formed within the crystals as a function of temperature. Two different proton environments can be differentiated. At temperatures below 230 K, the weak hydrogen bonds formed by the intercalate water and the strong hydrogen bonds formed by the silanol groups do not interact. At temperatures up to 360 K an increasing number of hydrogen bonds localized at silanol groups breaks up. At this temperature, 1D infinite hydrogen-bond networks are formed in the material, resulting in proton conductivity. Molecular dynamics (MD) simulations yielded increasing vibrations of these hydrogen bonds as an explanation for the breaking. From H-1 MAS NMR experiments with modified samples, it becomes obvious that only one-half of all water molecules participate in the network. Further, it is shown that a minimum coherence of the 1D network is necessary to stabilize the high-temperature state.