Water Confined In Mil-101(Cr): Unique Sorption-Desorption Behaviors Revealed By Diffuse Reflectance Infrared Spectroscopy And Molecular Dynamics Simulation

MIL-101(Cr) is considered a candidate for use as adsorbent materials in sorption-based heat exchangers because of its superior water uptake and high hydrothermal stability. Understanding the sorption-desorption behavior of water in MIL-101(Cr) is required for its real industrial applications. However, the sorption-desorption mechanism of water in MIL-101( Cr) cannot be revealed from the employed standard characterizations involving sorption-desorption isotherms. Here, we report a combined investigation of infrared molecular adsorption and molecular dynamics simulation to analyze the phase transitions of water confined in MIL-101(Cr). The water molecules at a low pressure preferentially coordinate with the metal sites and form one-dimensional water chains from the unsaturated Cr3+. As the pressure increases, the water chains grow in length and connect, gradually forming a water monolayer on the inner surface of the MIL cages. This monolayer changes the cage surface property from hydrophobic to hydrophilic, which induces the beginning of water condensation in the 29 and 34 angstrom cages. The entire pores are filled with condensed water as the experimental pressure gradually reaches 1 atm. A reverse behavior of water is observed as the pressure decreases, and this systematic analysis of water in MIL-101(Cr) suggests the further development of superior materials of sorption-based heat exchangers.