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

The presence of active sites in metal-organicframework (MOF) materials can control and affect theirperformance significantly in adsorption and catalysis. However,revealing the interactions between the substrate and active sites inMOFs at atomic precision remains a challenging task. Here, wereport the direct observation of binding of NH3in a series of UiO-66 materials containing atomically dispersed defects and openCu(I) and Cu(II) sites. While all MOFs in this series exhibitsimilar surface areas (1111-1135 m2g-1), decoration of the-OHsite in UiO-66-defect with Cu(II) results in a 43% enhancement of the isothermal uptake of NH3at 273 K and 1.0 bar from 11.8 inUiO-66-defect to 16.9 mmol g-1in UiO-66-CuII. A 100% enhancement of dynamic adsorption of NH3at a concentration level of630 ppm from 2.07 mmol g-1in UiO-66-defect to 4.15 mmol g-1in UiO-66-CuIIat 298 K is observed.In situneutron powderdiffraction, inelastic neutron scattering, and electron paramagnetic resonance, solid-state nuclear magnetic resonance, and infraredspectroscopies, coupled with modeling reveal that the enhanced NH3uptake in UiO-66-CuIIoriginates from a {Cu(II)middotmiddotmiddotNH3}interaction, with a reversible change in geometry at Cu(II) from near-linear to trigonal coordination. This work represents thefirstexample of structural elucidation of NH3binding in MOFs containing open metal sites and will inform the design of new efficientMOF sorbents by targeted control of active sites for NH3capture and storage

Automated summary

This article reports the direct observation of the binding of NH3 in a series of UiO-66 materials containing atomically dispersed defects and open Cu(I) and Cu(II) sites. Experimental and modeling studies reveal that the interaction between Cu(II) and NH3 leads to an enhanced NH3 uptake. This work is significant for the design of new efficient MOF sorbents.