Efficient ammonia capture via reversible single crystal structural transformation of a simple Ni metal-organic framework

The environmentally friendly batch preparation of metal-organic framework (MOF) adsorbents is a crucial step toward their application in ammonia capture and separation. Simultaneously, directly observing the adsorption structure of ammonia poses a highly challenging issue for the design of efficient ammonia adsorbents. Here, we report the green batch preparation of NiCl₂(pyz)₂ from waste Ni MH batteries for efficient ammonia adsorption and separation. It is found that the NH₃ uptake of NiCl₂(pyz)₂ is as high as 21.7 mmol/g at 25 °C and 1.0 bar. This adsorbent has excellent low-concentration ammonia adsorption capacity and shows potential for use in protective equipment to reduce ammonia concentration in important scenarios, such as ammonia leakage. The binding of NH₃ in the adsorbent was directly observed by the transition from a single crystal of NiCl₂(pyz)₂ to a single crystal structure of NiCl₂(NH₃)₆. PXRD, UV-DRS, XPS, and FT-IR results showed that competitive coordination is the main adsorption mechanism. This work has become a typical example of environmentally friendly batch preparation of MOF adsorbents from waste Ni MH batteries for ammonia adsorption, and provides guidance for the design of new and efficient MOF adsorbents through the observation of single crystal structures.