Large-scale Investigations of Water Diffusion in Metal-Organic Frameworks with One-Dimensional Channels

Metal-organic frameworks (MOFs), known for their highly versatile nature, show considerable promise as adsorbents and membranes for water-related applications such as water harvesting and water filtration. One of the key factors that may influence their efficiency is the diffusion of water within MOFs. However, the behaviors and mechanisms of water diffusion in MOFs remain relatively underexplored. To this end, this study utilizes molecular dynamics (MD) simulations to comprehensively analyze water diffusion in hundreds of distinct MOFs. Herien, MOFs that exhibit rapid water diffusion under saturated conditions are identified. Moreover, structure-property relationships and underlying atomistic mechanisms are explored, showing that water mobility depends on a subtle interplay between pore topology and host-guest interaction. For instance, MOFs with similarly small pores can exhibit markedly different water diffusivities. In hydrophobic MOFs, weak framework-water interactions allow ultrafast single-file diffusion, but mobility plunges when the pores narrow further. In hydrophilic MOFs, strong framework-water interactions immobilize a layer of water on the pore walls; these fixed water molecules can interestingly shield strong adsorption sites, enhancing transport in wider pores yet impeding diffusion in narrower ones. Finally, to address the high computational cost associated with computing diffusivity, this work further evaluates the feasibility of using surrogate descriptors for rapid diffusivity estimation. Overall, this work offers molecular-level key insights into water transport phenomena.