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 Cr 3+ . 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 Å 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.

Molecular dynamics simulations of water adsorbed in Material Institute Lavoisier MIL-101(Cr) metal–organic frameworks are performed to analyze the kinetic properties of water molecules confined in the framework at 298.15 K and under different vapor pressures and clarify the water adsorption mechanism in MIL-101(Cr). The terahertz frequency-domain spectra (THz-FDS) of water are calculated by applying fast Fourier transform to the configurational data of water molecules. According to the characteristic frequencies in the THz-FDS, the dominant motions of water molecules in MIL-101(Cr) can be categorized into three types: (1) low-frequency translational motion (0–0.5 THz), (2) medium-frequency vibrational motion (2–2.5 THz), and (3) high-frequency vibrational motion (>6 THz). Each type of water motion is confirmed by visualizing the water configuration in MIL-101(Cr). The ratio of the number of water molecules with low-frequency translational motion to the total number of water molecules increases with the increase in vapor pressure. In contrast, that with medium-frequency vibrational motion is found to decrease with vapor pressure, exhibiting a pronounced decrease after water condensation has started in the cavities. That with the high-frequency vibrational motion is almost independent of the vapor pressure. The interactions between different types of water molecules affect the THz-FDS. Furthermore, the self-diffusion coefficient and the velocity auto-correlation function are calculated to clarify the adsorption state of the water confined in MIL-101(Cr). To confirm that the general trend of the THz-FDS does not depend on the water model, the simulations are performed using three water models, namely, rigid SPC/E, flexible SPC/E, and rigid TIP5PEw.

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 porefilling 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.

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Mesoporous metal−organic frameworks (MOFs) are potential materials for efficient and reversible water adsorption and desorption processes. However, pore-filling/pore-emptying processes in mesoporous MOFs are complicated due to the complex pore geometry, nonuniform pore surface properties, and polarity of water molecules. In addition, the temperature effects on these processes are still unknown. Here, the temperature effect on the adsorption and desorption processes of water was investigated in a promising mesoporous MOF, namely, MIL-101(Cr). Experimentally measured water adsorption/desorption isotherms show a transition from a one-step to a two-step pore-filling/pore-emptying process with increasing temperature. Theoretical studies revealed that the corresponding mechanism of this transition is related to the relative pressures for the transition between two different stable adsorption states in two different-sized mesopores, the relative pressure at which the inner surface is completely covered with water, and the relative pressure at which condensed water overcomes the energy barrier from the metastable liquid state to the gas state.