Analysis of the Water Adsorption Mechanism in Metal–Organic Framework MIL-101(Cr) by Molecular Simulations

Water adsorption in MIL-101(Cr) (MIL = Materials Institute Lavoisier) was investigated by molecular simulations. Grand canonical Monte Carlo simulations were performed to understand the pore-filling processes of water in the large, medium, and small cavities of MIL-101(Cr) at different pressures. With increasing pressure, the water molecules first adsorb around unsaturated Cr atoms, forming multimolecular chains and clusters. The simulations show that capillary condensation in the medium and large cavities is complete at the relative pressures of 0.44 and 0.54, respectively. The pore-filling process in the small cavities of MIL-101(Cr) continues even at relative pressures larger than 0.56. To elucidate the confinement effect on the structural properties of water, the radial distribution functions, average hydrogen bond numbers, and order parameters are analyzed. The time evolution and diffusion properties of water molecules from the molecular dynamics simulation reveal that the stability of various sized water clusters in the small cavities plays an essential role in the pore-filling processes. The connection between multimolecular water chains and the growth of water clusters are observed on the inner surfaces of the large and medium cavities before condensation. Finally, a six-stage water adsorption mechanism in MIL-101(Cr) has been proposed.