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

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.