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

A molecular simulation study is reported for ion exchange in a rho zeolite-like metal organic framework (ZMOF). The nonframework Na+ ions in rho-ZMOF are observed to exchange with Pb2+ ions in PbCl2 solution. At equilibrium, all Pb2+ ions are exchanged and reside in rho-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 rho-ZMOF is estimated to be -10k(B)T, which is more favorable than -5k(B)T 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 rho-ZMOF without exchanging with other ions in solution due to strong interaction with rho-ZMOF; however, Na+ ions have a shorter residence time and a larger mobility than Pb2+ ions. The exchanged Pb2+ ions in rho-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 rho-ZMOF might be an intriguing candidate for water purification.