Three-Dimensional Covalent Organic Framework for the Capture of Nitrogen Dioxide

We report the first adsorption isotherm of nitrogen dioxide (NO2) in covalent organic frameworks (COFs). Leveraging the dihedral angle (43.6°) of the biphenyl unit in biphenyl-3,3',5,5'-tetracarbaldehyde, upon reaction with p-phenylenediamine, a three-dimensional COF, PKU-800, has been synthesized. Powder X-ray diffraction, hyperpolarised solid-state nuclear magnetic resonance spectroscopy, and modeling collectively reveal that PKU-800 exhibits a triply interpenetrated qtz-type framework structure, which incorporates missing-aldehyde defects (∼8%) and thus pendant amine termini (∼16%). At 298 K, PKU-800 shows remarkable adsorption of NO2 (4.8 and 18.8 mmol g-1 at 0.01 and 1.0 bar, respectively), representing a new benchmark for NO2 sorbents. Dynamic breakthrough experiments confirm efficient capture of NO2 (from 2500 to < 0.1 ppm) by PKU-800 under both dry and wet conditions, with dynamic capacities of 4.97 and 5.27 mmol g-1, respectively. In situ spectroscopic and modeling studies reveal that PKU-800 combines a robust three-dimensional topology, fully accessible channels, and strong binding interactions at both the imine and amino sites to synergistically trap NO2, which can be converted readily to nitric acid upon reaction with water in air.