Atmospheric water harvesting (AWH) has emerged as a potential solution to overcome the increasing global water stress. Adsorption-based water harvesters capture water from the atmosphere in an adsorption-desorption cycle. The performance of an AWH device, thus, 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 hydrophobic materials, such as all-silica zeolites.
Al-rod-based MOFs, such as MOF-303, have been posed as sorbents suitable for AWH in extremely arid conditions with high hydrolytic stability. In our previous work, we attributed the high hydrophilicity of MOF-303 to strong hydrogen bonding of primary water molecules to alternating hydrophilic patches in the MOF, constituted by opposite pairs of linker pyrazole functionalities and neighboring µ₂-OH groups in the inorganic rods. Insights into the role of linkers in constituting the primary binding sites were used to tune the low RH water adsorption isotherm behavior in linker-variate isoreticular analogues of MOF-303. However, to further increase the maximum water uptake in these MOFs (~0.45 g/g), new Al-rod-based MOFs are sought with larger pore volume without compromising the favorable hydrophilic characteristics of MOF-303. The conventional strategies to extend linkers in MOFs, using either polycyclic aromatic linkers or appending additional aromatic rings to the linker, generated either hydrophobic, less porous, or large-pore hydrolytically labile frameworks.
Here, using an integrated computational-experimental approach, a suitable linker extension strategy of appending a vinyl group to the pyrazole linker in MOF-303 is deployed to design a new isoreticular MOF, MOF-LA2-1, with ~50% increased water uptake capacity as compared to MOF-303 with a modest increase in the water adsorption isotherm step to higher RH, but still suitable for AWH in arid conditions. MOF-LA2-1 also exhibits other improved sorbent characteristics for AWH compared to MOF-303. Furthermore, force-field-based Monte Carlo simulations in the NpT-Gibbs ensemble are used to refine the partial disorder in the crystal structure of MOF-LA2-1.

Ephraim Neumann

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