Reversible adsorption of nitrogen dioxide within a robust porous metal-organic framework

Nitrogen dioxide (NO₂) is a major air pollutant causing significant environmental^1,2 and health problems^3,4. We report reversible adsorption of NO₂ in a robust metal-organic framework. Under ambient conditions, MFM-300(Al) exhibits a reversible NO₂ isotherm uptake of 14.1 mmol g^-1, and, more importantly, exceptional selective removal of low-concentration NO₂ (5,000 to <1 ppm) from gas mixtures. Complementary experiments reveal five types of supramolecular interaction that cooperatively bind both NO₂ and N₂O₄ molecules within MFM-300(Al). We find that the in situ equilibrium 2NO₂ ↔ N₂O₄ within the pores is pressure-independent, whereas ex situ this equilibrium is an exemplary pressure-dependent first-order process. The coexistence of helical monomer-dimer chains of NO₂ in MFM-300(Al) could provide a foundation for the fundamental understanding of the chemical properties of guest molecules within porous hosts. This work may pave the way for the development of future capture and conversion technologies.