Transferable Force Field for Water Adsorption in Cation-Exchanged Titanosilicates

Abstract

A new, transferable classical force field potential for titanosilicate ion-exchange materials is reported. Plane-wave pseudopotential density functional theory (DFT) simulations are used to compute partial charges, where the atomic boundaries are partitioned through the Wigner−Seitz approach. The repulsion−dispersion interactions are modeled with a 12-6 Lennard-Jones potential. The force field is parametrized with the acid-exchanged titanosilicate and transferred to the sodium-, cesium-, and strontium-exchanged titanosilicates and their niobium-substituted counterparts. Grand canonical Monte Carlo simulations are conducted to compute the sorptive properties of water in the acid-exchanged titanosilicate. Excellent agreement of the saturated water loading, positions, and occupancies with experimental neutron diffraction results is observed, while the adsorption energies at low water loading match DFT calculations. In transferring the force field to the sodium-, cesium-, and strontium-exchanged titanosilicates, canonical replica exchange Monte Carlo simulations are used to enhance cation sampling. The Wigner−Seitz force field parametrization effectively predicts the adsorption properties and provides a molecular-scale picture of the origins of titanosilicate selectivity in these materials. Overall, the methodology for translating electronic structural information derived from DFT calculations to classical force field based simulations can easily be extended to alternative crystalline materials such as zeolites and clays.