Ammonia Hydration in a Cu(II)-Pyrazolate Framework for Efficient Trace Capture.

Ammonia (NH3) emissions from industrial and agricultural activities pose severe environmental and health issues. Trace NH3 capture typically relies on chemisorption at Lewis acid sites or physisorption on porous adsorbents but usually suffers from irreversible binding, energy-intensive regeneration, and structural degradation. In this work, for the first time, we demonstrate a new hydration pathway as a promising solution. In a Cu(II)-pyrazolate framework, BUT-64(H2O), the bridging water molecules between adjacent Cu(II) ions serve as Brønsted acid sites to hydrate ammonia, achieving a remarkable NH3 packing density of 0.27 g cm-3 at 0.1 kPa and an adsorption capacity of 1.51 mmol g-1 for 1000 ppm NH3 under 80% relative humidity, among the leading adsorbents. The reversible hydration mechanism combines enhanced NH3 affinity with facile regeneration and mitigated moisture co-adsorption, overcoming the inherent trade-off. The remarkable alkaline stability of this material also highlights its potential as an energy-efficient sorbent for trace NH3 capture.