Structure and thermodynamics of water adsorption in NU-1500-Cr

Abstract

Metal-organic frameworks (MOFs) are a class of materials with diverse chemical and structural properties, and have been shown to effectively adsorb various types of guest molecules. The mechanism of water adsorption in NU-1500-Cr, a high-performance atmospheric water harvesting MOF, is investigated using a combination of molecular dynamics simulations and infrared spectroscopy. Calculations of thermodynamic and dynamical properties of water as a function of relative humidity allow for following the adsorption process from the initial hydration stage to complete filling of the MOF pores. Initial hydration begins at the water molecules that saturate the open Cr3+ sites of the framework, which is then followed by the formation of water chains that extend along the channels connecting the hexagonal pores of the framework. Water present in these channels gradually coalesces and fills the hexagonal pores sequentially after the channels are completely hydrated. The development of hydrogen-bond networks inside the MOF pores as a function of relative humidity is characterized at the molecular level using experimental and computational infrared spectroscopy. A detailed analysis of the OH-stretch vibrational band indicates that the low-frequency tail stems from strongly polarized hydrogen-bonded water molecules, suggesting the presence of some structural disorder in the experimental samples. Strategies for designing efficient water harvesting MOFs are also proposed based on the mechanism of water adsorption in NU-1500-Cr.

Elucidating the Competitive Adsorption of H2O and CO2 in CALF-20: New Insights for Enhanced Carbon Capture Metal–Organic Frameworks

“…A similar variation in the entropy of water was observed for water adsorbed in ZIF-90 and NU-1500-Cr. 21,52 As mentioned above, the presence of CO 2 molecules within the framework leads to a decrease in the void space and the formation of a more connected hydrogen-bond network (Figure 2A). Consequently, the water entropy is consistently lower in the presence of CO 2 molecules within the framework, reflecting the more restricted motion of the H 2 O The effects of framework confinement and volume exclusion on the dynamics of H 2 O and CO 2 molecules within the framework can be better understood by examining the corresponding orientational correlation functions, C 2 (t) in eq 4, shown in Figure 4A,B, respectively.…”

“…A similar trend was also observed for water adsorbed in NU-1500-Cr. 21 Across the range between 27.5 and 59.7% RH, the differences in the percentages of the 0D-0A topology indicate that the water molecules tend to form a more connected hydrogen-bond network in the presence of CO 2 compared to when CO 2 is absent. The small difference in the percentage of the 0D-0A topology observed at 74.5% RH can be attributed to the overwhelmingly higher concentration of water with a ratio of H 2 O to CO 2 equal to 22:1.…”

Ultrafast Water H-Bond Rearrangement in a Metal–Organic Framework Probed by Femtosecond Time-Resolved Infrared Spectroscopy

“…Although fundamental physical studies of water in MOF pores have led to the development of more efficient MOFs for AWH, − there are significant barriers to understanding water H-bond networks and dynamics in MOFs, as water molecules may not behave the same way when confined over nanometer length scales as they do in bulk. − These challenges are rooted in the characterization methods for MOFs. Diffraction-based methods boast atomic precision but are limited to water that is close to crystalline ,, and are insensitive to liquid-phase water dynamics.…”

Computational insights into the interaction of water with the UiO-66 metal–organic framework and its functionalized derivatives

“…62,69,70 This strategy has proven successful in previous MD simulations incorporating MB-pol water model to study water-MOF interactions. 62,70,73 In addition, Buckingham force field parameters describing the van der Waals (vdW) interactions between the oxygen atoms (OW) of the water molecules with m 3 -OH, and the hydrogen atoms (HW) of the water molecules with m 3 -O and O linker interaction sites were specifically fitted to DFT reference energies using the GA to obtain a more accurate representation of these interactions. To this end, we first optimized cluster model structures with a water molecule hydrogen-bonded to the m 3 -OH, m 3 -O and O linker sites (Fig.…”