Hydrolytically Stable Phosphonate-Based Metal-Organic Frameworks for Harvesting Water from Low Humidity Air

Harvesting water from air offers a promising solution to the global water crisis. However, existing sorbents often struggle in arid climates due to limitations such as low sorption capacities, hydrolytic instability, slow mass transport, high desorption enthalpy, and costly operation. Phosphonate-based metal-organic frameworks (MOFs), known for their exceptional water stability, have not been extensively explored for water harvesting. This study systematically investigates the performance of STA-12 (M═Co, Ni, Mg) and STA-16 (M═Co, Ni), a series of stable phosphonate-based MOFs, as water sorbents. STA-12 MOFs demonstrate remarkable adsorption at ultra-low humidity (<10%), while STA-16(Co) exhibits a high water uptake capacity of 0.54 g g^-1 at 10-50% relative humidity (RH) and 0.72 g g^-1 at 34% RH. Molecular simulations and solid-state NMR identified liquid-like water, critical for harvesting applications, as the key contributor to the superior sorption performance of STA-16(Co). Scalable aqueous synthesis methods are developed, producing tens of grams of MOFs per batch without high-pressure equipment. A prototype device incorporating STA-12(Ni) demonstrated the feasibility of these materials for real-world water harvesting, showcasing their potential to address water scarcity in arid regions.