Ion Exchange in Metal–Organic Framework for Water Purification: Insight from Molecular Simulation

Abstract

A molecular simulation study is reported for ion exchange in a <i>rho</i> zeolite-like metal–organic framework (ZMOF). The nonframework Na⁺ ions in <i>rho</i>-ZMOF are observed to exchange with Pb²⁺ ions in PbCl₂ solution. At equilibrium, all Pb²⁺ ions are exchanged and reside in <i>rho</i>-ZMOF, while Na⁺ ions are in a dynamic equilibrium with solution. By umbrella sampling, the potential of mean force for Pb²⁺ moving from solution into <i>rho</i>-ZMOF is estimated to be −10<i>k</i>_B<i>T</i>, which is more favorable than −5<i>k</i>_B<i>T</i> for Na⁺ and contributes to the observed ion exchange. The residence-time distributions and mean-squared displacements reveal that all the exchanged Pb²⁺ ions stay continuously in <i>rho</i>-ZMOF without exchanging with other ions in solution due to strong interaction with <i>rho</i>-ZMOF; however, Na⁺ ions have a shorter residence time and a larger mobility than Pb²⁺ ions. The exchanged Pb²⁺ ions in <i>rho</i>-ZMOF are located at eight-, six-, and four-membered rings. As attributed to the confinement effect, distinctly different dynamic properties are found for Pb²⁺ ions at the three locations. Pb²⁺ ions at 8MR have the highest mobility due to the largest ring size, while those at 4MR have a negligible mobility. This simulation study provides microscopic insight into the ion-exchange process in ionic MOF and suggests that <i>rho</i>-ZMOF might be an intriguing candidate for water purification