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

Abstract

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.