High Ammonia Adsorption in MFM-300 Materials: Dynamics and Charge Transfer in Host–Guest Binding

Ammonia
(NH3) is a promising energy resource owing to
its high hydrogen density. However, its widespread application is
restricted by the lack of efficient and corrosion-resistant storage
materials. Here, we report high NH3 adsorption in a series
of robust metal–organic framework (MOF) materials, MFM-300­(M)
(M = Fe, V, Cr, In). MFM-300­(M) (M = Fe, VIII, Cr) show
fully reversible capacity for >20 cycles, reaching capacities of
16.1,
15.6, and 14.0 mmol g–1, respectively, at 273 K
and 1 bar. Under the same conditions, MFM-300­(VIV) exhibits
the highest uptake among this series of MOFs of 17.3 mmol g–1. In situ neutron powder diffraction, single-crystal
X-ray diffraction, and electron paramagnetic resonance spectroscopy
confirm that the redox-active V center enables host–guest charge
transfer, with VIV being reduced to VIII and
NH3 being oxidized to hydrazine (N2H4). A combination of in situ inelastic neutron scattering
and DFT modeling has revealed the binding dynamics of adsorbed NH3 within these MOFs to afford a comprehensive insight into
the application of MOF materials to the adsorption and conversion
of NH3.