Role of water in the ion selectivity of niobate-based octahedral molecular sieves

The role of occluded water in ion exchange selectivity is examined in a class of molecular sieves named Sandia Octahedral Molecular Sieves (SOMS: Na₂Nb_2-xM_xO_6-x(OH) _x· H₂ O (i.e., M = Ti; 0 < x < 0.4)). SOMS exhibit a high selectivity for divalent cations only when the framework Nb(5+) are substituted by M(4+) atoms. Vibrational dynamics of the water molecules with varying charge balancing cations and M atoms are studied by inelastic neutron scattering (INS) measurements and ¹H MAS NMR and correlated to density functional theory and molecular dynamics data. The experimental INS spectra were compared with those of ice Ih to characterize the changes induced by confinement on the occluded H₂O and the resulting hydrogen-bonding network. Data indicates that, with increasing M(4+) content and divalent ion exchange, the trend of occluded water molecules is to change from extended rigid ice-like networks to restored bulklike arrangements with increased solvation effects on the channel charge-balancing cations.