Decoding the Water Harvesting Mechanism of MIL-100(Fe) Across Short- and Long-Range Length Scales

: Metal-organic frameworks (MOFs) hold promise as designer materials for atmospheric water harvesting, due to their unrivaled porosity, chemical tunability, and water affinity. Although an accurate understanding of the pore filling sequence is critical to developing improved MOF water harvesters, obtaining molecular-level details of the evolution of water clusters in MOFs has proven to be experimentally challenging. Here, a novel approach based on X-ray absorption spectroscopy (XAS), X-ray pair distribution function, powder X-ray diffraction, molecular dynamics (MD) simulations and in-depth theoretical XAS calculations is presented to gain quantitative insights into the structural and dynamical properties of water adsorbed within MIL-100(Fe), a prototypical MOF with giant pores. The complementary synchrotron X-ray techniques shed light on the behavior of water confined in MIL-100(Fe) with unprecedented structural sensitivity at the short-, intermediate- and long-range length scales, while the MD and theoretical XAS simulations revealed the order according to which water molecules populate the MOF mesopores and tracked the evolution of the hydrogen-bond network topology as a function of water content. The developed method can provide often elusive information on how the local structure affects the behavior and performance of MOF water harvesters, which is key to the development of rationally optimized MOF systems.