The Mechanism Responsible for Extraordinary Cs Ion Selectivity in Crystalline Silicotitanate

Combining information from time-resolved X-ray and neutron scattering
with theoretical calculations has revealed the elegant mechanism whereby
hydrogen crystalline silicotitanate (H-CST; H2Ti2SiO7·1.5H2O) achieves its remarkable ion-exchange
selectivity for cesium. Rather than a simple ion-for-ion displacement
reaction into favorable sites, which has been suggested by static
structural studies of ion-exchanged variants of CST, Cs+ exchange proceeds via a two-step process mediated by conformational
changes in the framework. Similar to the case of ion channels in proteins,
occupancy of the most favorable site does not occur until the first
lever, cooperative repulsive interactions between water and the initial
Cs-exchange site, repels a hydrogen lever on the silicotitanate framework.
Here we show that these interactions induce a subtle conformational
rearrangement in CST that unlocks the preferred Cs site and increases
the overall capacity and selectivity for ion exchange.