Exceptional Packing Density of Ammonia in a Dual-Functionalized Metal-Organic Framework

We report the reversible adsorption of ammonia (NH₃) up to 9.9 mmol g^-1 in a robust Al-based metal-organic framework, MFM-303(Al), which is functionalized with free carboxylic acid and hydroxyl groups. The unique pore environment decorated with these acidic sites results in an exceptional packing density of NH₃ at 293 K (0.801 g cm^-3) comparable to that of solid NH₃ at 193 K (0.817 g cm^-3). In situ synchrotron X-ray diffraction and inelastic neutron scattering reveal the critical role of free -COOH and -OH groups in immobilizing NH₃ molecules. Breakthrough experiments confirm the excellent performance of MFM-303(Al) for the capture of NH₃ at low concentrations under both dry and wet conditions.