Observation of binding of carbon dioxide to nitro-decorated metal–organic frameworks

Metal–organic frameworks (MOFs) functionalised with amine, amide and hydroxyl groups show great promise for CO₂ binding due to their ability to form hydrogen bonds to CO₂. Herein we report the adsorption and selectivity of CO₂ in four iso-reticular MOFs adopting the NbO topology. Functionalisation of the parent MOF, MFM-102, with –NO₂, –NH₂ and alkyl groups leads to an enhancement of CO₂ adsorption of up to 36% for the NO₂-decorated MOF and with raised selectivity. MFM-102-NO₂ shows the highest adsorption capacity for CO₂ (184 cm³ g⁻¹ at 273 K and 1.0 bar) within this series, comparable to the best-behaving iso-reticular MOFs. At 298 K and 1.0 bar, MFM-102-NO₂ shows a CO₂/CH₄ selectivity of 5.0. In situ inelastic neutron scattering and synchrotron FT-IR micro-spectroscopy were employed to elucidate the host–guest interaction dynamics within CO₂-loaded MFM-102-NO₂. Neutron powder diffraction enabled the direct observation of the preferred binding domains in MFM-102-NO₂, and, to the best of our knowledge, we report the first example of CO₂ binding to a –NO₂ group in a porous MOF. Synergistic effects between the –NO₂ group and the open metal sites lead to optimal binding of CO₂ molecules within MFM-102-NO₂via hydrogen bonding to C–H groups.