Effects of Hydration during Strontium Exchange into Nanoporous Hydrogen Niobium Titanium Silicate

A Nb-substituted titanium silicate with the sitinakite (NbTS) topology was exchanged with Sr2+ to determine the mechanisms and pathways of ion diffusion through this mixed polyhedral nanoporous framework. The refined structural models yield unit cell parameters and atomic positions of Sr2+ and suggest that there was a two-step process during cation diffusion. The starting material of the exchange experiment was the H+-exchanged material, H1.4Nb0.6Ti1.4SiO7·1.9H2O, with space group P42/mcm. In the beginning of the exchange process, Sr filled the 8-membered-ring channel near the 42 axis in the center. Once the Sr2+ fractional occupancy reached approximate 0.11, Sr positions and extra-framework H2O molecules shifted away from the central 8-membered-ring toward the framework, and an increase in Sr hydration and framework bonding was observed. The new H2O positions resulted in a lowering of symmetry to the P4̅2m space group, and it is thought that the Sr migration served to enhance Sr2+ ion diffusion capacity into the channels of NbTS since the exchange rate briefly accelerated after the 0.11 fractional occupancy level was passed. Exchange of Sr2+ into the nanoporous material reached maximum fractional site occupancy of approximately 0.20 using a 10.0 mM SrCl2 solution.