To address water scarcity on earth, new methods of obtaining clean drinking water are needed. One way to obtain clean drinking water is through atmospheric water capture, which can occur in arid regions with low relative humidity (RH). Metal-organic frameworks (MOFs), a class of materials that exhibit large surface areas and high porosity, have been shown to be promising materials to harvest water from the air. Here, we present extensive molecular dynamics (MD) simulations of water adsorption in ZIF-90, a prototypical MOF that exhibits a well-defined step in the adsorption isotherm at ~30% RH. Simulations carried out with our data-driven, many-body MB-pol model, which correctly predicts the properties of water from the gas to the condensed phase, reveal that this increase in water adsorption is induced by the formation of hydrogen bonds between the water molecules and the aldehyde groups of the framework. To gain molecular-level insights into the structural, thermodynamic, and dynamical properties of water adsorbed in ZIF-90, we monitor the evolution of the water hydrogen-bond network as a function of RH through the calculation of the corresponding infrared (IR) spectra. Specifically, employing quantum centroid molecular dynamics (CMD) simulations and explicitly accounting for Fermi resonances, the coupling between the OH stretch fundamental and HOH bend overtone, allows us to achieve good agreement with the experimental IR spectra. We further demonstrate that the agreement with the experimental spectra becomes quantitative after tuning the interaction between the water molecules and the aldehyde groups of the framework. Finally, by combining the results of our MD simulations with experimental sum frequency generation (SFG) spectra, we demonstrate that water adsorbs in ZIF-90 one pore at a time, rather than occupying all pores simultaneously. Besides providing fundamental insights into the behavior of water in heterogeneous confinement, our study indicates that combining advanced MD simulations with vibrational spectroscopy allows for elucidating the molecular mechanisms of water adsorption, which is key to the design of new MOFs with improved adsorption capacity of water for applications in atmospheric water capture.

Elucidating water adsorption mechanisms in metal-organic frameworks

ACS Spring 2022

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