Direct Observation of Ammonia Storage in UiO-66 Incorporating Cu(II) Binding Sites

The presence of active sites in metal-organic framework (MOF) materials can control and affect their performance significantly in adsorption and catalysis. However, revealing the interactions between the substrate and active sites in MOFs at atomic precision remains a challenging task. Here, we report the direct observation of binding of NH₃ in a series of UiO-66 materials containing atomically dispersed defects and open Cu(I) and Cu(II) sites. While all MOFs in this series exhibit similar surface areas (1111-1135 m² g^-1), decoration of the -OH site in UiO-66-defect with Cu(II) results in a 43% enhancement of the isothermal uptake of NH₃ at 273 K and 1.0 bar from 11.8 in UiO-66-defect to 16.9 mmol g^-1 in UiO-66-Cu^II. A 100% enhancement of dynamic adsorption of NH₃ at a concentration level of 630 ppm from 2.07 mmol g^-1 in UiO-66-defect to 4.15 mmol g^-1 in UiO-66-Cu^II at 298 K is observed. In situ neutron powder diffraction, inelastic neutron scattering, and electron paramagnetic resonance, solid-state nuclear magnetic resonance, and infrared spectroscopies, coupled with modeling reveal that the enhanced NH₃ uptake in UiO-66-Cu^II originates from a {Cu(II)···NH₃} interaction, with a reversible change in geometry at Cu(II) from near-linear to trigonal coordination. This work represents the first example of structural elucidation of NH₃ binding in MOFs containing open metal sites and will inform the design of new efficient MOF sorbents by targeted control of active sites for NH₃ capture and storage.