Molecular dynamics study of water confined in MIL-101 metal–organic frameworks

Abstract

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 Confined in MIL-101(Cr): Unique Sorption–Desorption Behaviors Revealed by Diffuse Reflectance Infrared Spectroscopy and Molecular Dynamics Simulation

“…When a large amount of water is loaded, all of the supertetrahedrons in the MIL-101­(Cr) will also be filled. Although the number of the inside water molecules in one 8 Å structure is no more than five, they still may result in a small increase in the isotherm . In addition, no interaction of water molecules with the carboxylate O is found in the MIL-101­(Cr).…”

“…Although the number of the inside water molecules in one 8 Å structure is no more than five, they still may result in a small increase in the isotherm. 47 In addition, no interaction of water molecules with the carboxylate O 48 is found in the MIL-101(Cr). This may be caused by the synthesis process: when the terephthalic acid is connected to the Cr trimer, one of the Cr atoms could replace the H atom of the carboxylic group and form a coordinate covalent bond with the O atom, which makes it difficult for this O atom to further form the hydrogen bond (H-bond) with the water molecules.…”

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

“…The highly hydrophilic metal sites of MIL-101­(Cr) facilitate the adsorption of water, whereas hydrophobic organic ligands support the diffusion of water within the material. As a result, MIL-101­(Cr) represents a promising material for water adsorption applications, which has stimulated various multidisciplinary studies of water adsorption in MIL-101­(Cr). ,,,− …”

High cubicity of D2O ice inside spherical nanopores of MIL-101(Cr) framework: a neutron diffraction study

“…38 In MOF networks with crystalline and flexible pores, the H-bonding network of water molecules extends to the neighboring pores. [56][57][58] The extended H-bonding network may form large crystalline ice. Although the formation of a single crystal extending into the neighboring pore may not be guaranteed, the slow H-bonding dynamics [59][60][61] may make it possible to form a crystal with extended correlation length.…”