Proton dynamics in CsHSO(4) confined in mesoporous silica FSM-16 as studied by (1)H solid-state NMR

Cesium hydrogen sulfate CsHSO(4) was introduced in a nanospace of mesoporous silica FSM-16, and we have investigated proton dynamics of CsHSO(4) confined in the mesopores by means of (1)H solid-state NMR. X-ray powder diffraction patterns demonstrate accommodation of CsHSO(4) in the mesopores and suggest that confined CsHSO(4) is amorphous. Thermal analyses indicate that the confinement suppresses the transition from phases II to I and the melting. The motional narrowing in the (1)H static NMR spectra takes place in the temperature range lower than that for bulk CsHSO(4), and the spectral line shape indicates that the proton motional rate has a distribution. The proton mean residence times are estimated from the spectrum simulation assuming the distribution of the motional rate. The obtained central values are smaller by one order of magnitude than those for bulk CsHSO(4) although the central values of the activation energies are comparable.