Modulation of Uptake and Reactivity of Nitrogen Dioxide in Metal-Organic Framework Materials

We report the modulation of reactivity of nitrogen dioxide (NO₂ ) in a charged metal-organic framework (MOF) material, MFM-305-CH₃ in which unbound N-centres are methylated and the cationic charge counter-balanced by Cl^- ions in the pores. Uptake of NO₂ into MFM-305-CH₃ leads to reaction between NO₂ and Cl^- to give nitrosyl chloride (NOCl) and NO₃ ^- anions. A high dynamic uptake of 6.58 mmol g^-1 at 298 K is observed for MFM-305-CH₃ as measured using a flow of 500 ppm NO₂ in He. In contrast, the analogous neutral material, MFM-305, shows a much lower uptake of 2.38 mmol g^-1 . The binding domains and reactivity of adsorbed NO₂ molecules within MFM-305-CH₃ and MFM-305 have been probed using in situ synchrotron X-ray diffraction, inelastic neutron scattering and by electron paramagnetic resonance, high-field solid-state nuclear magnetic resonance and UV/Vis spectroscopies. The design of charged porous sorbents provides a new platform to control the reactivity of corrosive air pollutants.