Isostructural Transformation From a Hydrogen-Bonded Metal-Complex Framework to a Metal-Organic Framework for Enhanced...

Metal-organic frameworks (MOFs) are promising sorbents for ammonia (NH₃) storage and capture. However, rationally synthesizing target MOFs with NH₃ tolerance and reversible uptake still remains challenging. Here, we present a hydrogen-bonded metal-complex framework (ALP-HOF-1) that can isostructurally transform to a porous MOF (ALP-MOF-4) via a ligand substitution strategy to dramatically enhance NH₃ tolerance and reversibility. The remarkable NH₃ tolerance was unambiguously confirmed by the retention of NH₃ adsorption capacities after consecutive adsorption/desorption cycles and breakthrough experiments. At 298 K and 1.0 bar, ALP-MOF-4 exhibits a high NH₃ packing density comparable to liquid NH₃. X-ray photoelectron spectroscopy and computational studies ascertain the binding domains of adsorbed NH₃ molecules. Open metal sites, Brønsted basic carbonyl (─C═O), and acidic ─NH groups in ALP-MOF-4 act cooperatively as preferred anchoring sites for NH₃ capture, resulting in excellent trace NH₃ capture performance under mixed NH₃, CO₂, and N₂ streams with effluent NH₃ below 50 ppm. The structural integrity and recyclability of ALP-MOF-4 demonstrate its potential as a durable NH₃ sorbent. This work provides design principles for rational transformations from HOFs to MOFs featuring active-site environments for selective gas capture.