(169d) Exploring the Role of Functional Groups and Nanoconfinement on the Structural and Dynamical Properties of Water and Ions inside Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are potential candidates for next-generation membranes with high ion selectivity due to the presence of specific ion binding sites created by incorporating functional groups into the organic linkers of MOFs. To design MOF membranes with high water flux and excellent ion-selectivity performance, a fundamental understanding of the structural and dynamical properties of water and ions inside MOFs is essential. Although a handful of previous studies have explored a few such structural and dynamical properties of water inside MOFs, no studies have been carried out to identify how functional groups affect these properties of water and ions. Here, we take a computational approach to determine the effect of different functional groups and confinement on the water/ion coordination environment and the transport inside Zr-based UiO-66 MOF. First, we perform classical molecular dynamics (MD) simulations of water and ions inside functionalized UiO-66 MOFs, which reveal that water coordination structure and ion hydration structures are significantly different under nanoconfinement than bulk water. The MD simulations also indicate that water undergoes both Fickian diffusion and sub-diffusion inside UiO-66, and the mode of diffusion depends on the functional group size and the number of functional groups on each linker. Next, we performed density functional theory (DFT) calculations to determine the binding affinity of water and ions with different functional groups. The DFT calculations show that water and ion diffusion coefficients strongly correlate with binding affinity. Finally, we extend our analysis to other MOFs to validate the generalizability of our results and provide design guidelines for MOF-based membranes with fast water transport and unprecedented ion-selectivity for applications in battery recycling, wastewater treatment, and rare earth element recovery.