A Flexible Phosphonate Metal-Organic Framework for Enhanced Cooperative Ammonia Capture

Ammonia (NH₃) production in 2023 reached 150 million tons and is associated with potential concomitant production of up to 500 million tons of CO₂ each year. Efforts to produce green NH₃ are compromised since it is difficult to separate using conventional condensation chillers, but in situ separation with minimal cooling is challenging. While metal-organic framework materials offer some potential, they are often unstable and decompose in the presence of caustic and corrosive NH₃. Here, we address these challenges by developing a pore-expansion strategy utilizing the flexible phosphonate framework, STA-12(Ni), which shows exceptional stability and capture of NH₃ at ppm levels at elevated temperatures (100-220 °C) even under humid conditions. A remarkable NH₃ uptake of 4.76 mmol g^-1 at 100 μbar (equivalent to 100 ppm) is observed, and in situ neutron powder diffraction, inelastic neutron scattering, and infrared microspectroscopy, coupled with modeling, reveal a pore expansion from triclinic to a rhombohedral structure on cooperative binding of NH₃ to unsaturated Ni(II) sites and phosphonate groups. STA-12(Ni) can be readily engineered into pellets or monoliths without losing adsorption capacity, underscoring its practical potential.