Atmospheric water harvesting (AWH) has emerged as a potential solution to overcome the depleting availability of freshwater. The performance of adsorption-based AWH devices critically depends on the water affinity and water adsorption characteristics of the adsorbent. Metal-organic frameworks, porous materials built from inorganic metal-oxo nodes and organic linkers, are suited as AWH sorbents owing to their tunable water adsorption properties. MOF-303, MOF in which Al(OH) rods are connected by pyrazole-based (PZDC^2–) linkers, is posited as a promising sorbent for AWH. We elucidated the water uptake mechanism in MOF-303, concomitant with its adsorption isotherm, using periodic density functional theory calculations which was corroborated by X-ray diffraction measurements. MOF-303 exhibits an initial S-shaped step in its isotherm followed by a larger steep step at RH = 13%, where the initial step can reduce the water output from the AWH device by 20%. Hydrophilic patches in MOF-303, constituted by opposite pairs of linker pyrazolegrousp and adjacent µ₂-OH groups, were found to strongly bind upto 30% water molecules to the MOF through multiple H-bonds. These water molecules further seeded other water molecules upon saturation. The role of linkers in constituting strong water adsorption sites was used to modulate the water affinity of MOF-303 by replacing the PZDC^2– linkers with furan-based linkers, forming isoreticular MOF-333, to suppress the initial S-shaped step in its isotherm which is expected to increase the water output from a MOF-333-based AWH device. This understanding of the MOF-water interactions was further leveraged to design new MOFs for AWH exhibiting enhanced water uptake with optimal hydrophilicity suitable for arid conditions. Hypothetical MOFs, built by varying the structural properties of existing MOFs, were screened for their water adsorption characteristics using force-field-based MC simulations in the NpT-Gibbs ensemble. Although MC simulations for water adsorption in MOFs are challenging due to choice of force field parameters which accurately describe MOF-water and water-water interactions, and the structural flexibility of MOFs in presence of guest molecules, water isotherms are predicted here in very good agreement with measured isotherms. In this computationally-supported materials-discovery effort, a MOF which could increase the AWH performance by upto 50% relative to MOF-303 was predicted and experimentally validated.

Bojun Feng

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