Molecular insight into the adsorption and diffusion of water in the versatile hydrophilic/hydrophobic flexible MIL-53(Cr) MOF

Abstract

Prior to envisage any implication of metal-organic framework (MOF) materials in industrial applications such as gas storage/separation, it is of primary importance to examine beyond their stability under humidity, the interactions between water and MOF surfaces. Regarding the MOF type MIL-53(Cr), the situation becomes more complex due to the breathing of its structure upon water adsorption that induces a structural transition between a narrow pore (NP) and a large pore (LP) forms. The resulting shrinkage/reopening of the framework leads to crucial modifications of the hydrophobicity/hydrophilicity character of the material. A combination of molecular simulations (Grand Canonical Monte Carlo and molecular dynamics) and experimental techniques (quasi-elastic neutron scattering and gravimetry/volumetry) allows a complete elucidation of the adsorption and diffusion mechanisms in play in this structure. It also provides a microscopic explanation of the hydrophilic and mildly hydrophobic behaviors of the NP and LP forms respectively. The arrangements of water within the two porosities are then discussed and compared to those usually observed for the liquid state. Furthermore, the self-diffusivity for water in these two structural forms significantly differs both in magnitude and mechanism with a normal and single file one-dimensional diffusion mechanisms in the LP and NP forms, respectively. Indeed, this anomalous behavior of the MIL-53(Cr) upon water adsorption implies a strong modification of its hydrophilic/hydrophobic character. This unusual behavior opens a promising direction to improve the selectivity of MOFs in industrial applications such as purification of alcohols without the need for functionalization procedures. © 2011 American Chemical Society.