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 Pb 2+ ions in PbCl 2 solution. At equilibrium, all Pb 2+ 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 Pb 2+ moving from solution into rho -ZMOF is estimated to be −10 k B T, which is more favorable than −5 k B T for Na + and contributes to the observed ion exchange. The residence-time distributions and mean-squared displacements reveal that all the exchanged Pb 2+ 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 Pb 2+ ions. The exchanged Pb 2+ 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 Pb 2+ ions at the three locations. Pb 2+ 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.