Incoherent inelastic neutron scattering studies of nanoconfined water in clinoptilolite and heulandite zeolites

In a continued effort studying the role of water in ion exchange processes of zeolites, a synthetic series of alkali and alkaline earth metal cation variants (Na⁺, K⁺, Rb⁺, Mg²⁺, and Ca²⁺) of the hydrated clinoptilolite (Si/Al ≈ 5) and heulandite (Si/Al ≈ 3.5) aluminosilicate zeolites is examined by incoherent inelastic neutron scattering (TINS). The low-frequency librational modes of water reveal the impact of nanoconfinement and framework charge within these isostructural aluminosilicate structures. The experimental IINS spectra are correlated with power spectra derived through molecular dynamics simulations. The impact of ion-zeolite, ion-water, and water-zeolite interactions on the hindered rotations (librations) of water molecules is explored as a function of Si/Al substitution, cation identity, and subnanometer confinement. The results indicate that electrostatic charge on the overall framework has a stronger influence than charge density of a given ion in the channel and that these effects become more pronounced as the charge difference between ion and zeolite is increased.