(465a) Metal-Organic Frameworks As Suitable Candidates for Atmospheric Water Harvesting

Atmospheric water harvesting (AWH) has recently emerged as a potential solution for the rising global water crisis.^1,2 Adsorption-based water harvesters capture water from the atmosphere in an adsorption-assisted step while a second thermal desorption step regenerates the sorbent for the next cycle. The performance of the AWH device critically depends on the water affinity and water adsorption characteristics of the adsorbent amongst several other factors. Metal-organic frameworks, porous and crystalline materials built from inorganic metal oxide nodes connected by organic linkers, are suited as sorbents for AWH owing to their tunable water adsorption properties ranging between extremely hydrophilic sorbents, such as aluminosilicates and silica gels, and hydrophobic materials, such as all-silica zeolites and polymer gels. MOF-303, a rod-based MOF in which infinite Al(OH) rods are connected by 1-H-pyrazole-3,5-dicarboxylate (PZDC^2–) linkers, is posited as a state-of-the-art sorbent for AWH with a reported water uptake of 0.7 Lkg^–1day^–1 in the arid climate of the Mojave desert (upto 10% R.H. (relative humidity), 10 ).¹

In our recent work,² we elucidated the molecule-by-molecule water uptake in MOF-303, concomitant with its water adsorption isotherm, using a combination of periodic density functional theory (DFT) calculations and single-crystal X-ray diffraction. MOF-303 exhibits an initial small S-shaped step in its isotherm followed by a larger steep step in the isotherm at R.H. = 13% where the initial step can reduce the water output from the harvester by 20%. The higher hydrophilicity of MOF-303 is attributed to the presence of alternating hydrophilic cavities in the MOF, where each hydrophilic cavity is constituted by opposite pairs of linker pyrazole functionalities and their three neighboring µ₂-OH groups in the inorganic rods. The hydrophilic cavities were found to strongly bind upto three water molecules to the framework through multiple H-bonds with the neighboring H-bond donors and acceptors. Water molecules adsorbed into the primary sites were found to further seed other water molecules to form isolated clusters in the MOF cavities, which subsequently formed interconnected water networks in the MOF pores at the saturated water loading. The role of linkers in strongly binding the initial water molecules to the framework was used to modulate the “initial” water affinity of the MOF by replacing the PZDC^2– linkers with 2,4-furandicarboxylate (FDC^2–) linkers, forming isoreticular MOF-333, to suppress the initial S-shaped step in its isotherm. The elimination of the S-shaped step in the isotherms of MOF-333 with a moderate shift in its step to R.H. = 22% is expected to increase the water output from a MOF-333-based AWH device.

The design of the next-generation sorbents for AWH further necessitates an accurate prediction of the water adsorption properties of the MOF. While DFT-based water binding energies allow to gauge the water affinity of a given MOF for water adsorption, the water uptake in the MOF under different water partial pressures can be predicted using force-field-based Monte Carlo (MC) simulations. However, MC simulations for water adsorption in MOFs are challenging due to the choice of force field parameters which can accurately describe the MOF-water interactions, the different water models available for describing water-water interactions, and the structural flexibility of MOFs in presence of guest molecules. Here, MC simulations in the NpT-Gibbs ensemble (GEMC) are used to predict the water adsorption isotherms in MOF-303 and MOF-333.

References:

(1) Hanikel, N.; Prévot, M. S.; Fathieh, F.; Kapustin, E. A.; Lyu, H.; Wang, H.; Diercks, N. J.; Glover, T. G.; Yaghi, O. M. Rapid Cycling and Exceptional Yield in a Metal-Organic Framework Water Harvester. ACS Cent. Sci. 2019, 5, 1699–1706.

(2) Hanikel, N.; Pei, X.; Chheda, S.; Lyu, H.; Jeong, W.; Sauer, J.; Gagliardi, L.; Yaghi, O. M. Evolution of Water Structures in Metal-Organic Frameworks for Improved Atmospheric Water Harvesting. Science 2021, 374, 454–459.

Note: This material is based upon worked supported by the Defense Advanced Research Projects Agency under Contract No. HR0011-21-C-0020. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Defense Advanced Research Projects Agency.