Stable Ultramicroporous Metal-Organic Framework with Hydrophilic and Hydrophobic Domains for Selective Gas Adsorption

Herein, we report the thermal and chemical stability, and the gas adsorption behavior, of a mixed-linker phosphonate MOF, [Cu(4,4'-bpy)_0.5(1,4-NDPAH₂)], namely TUB41 (where bpy = bipyridine and NDPAH₄ = naphthalenediphosphonic acid). TUB41 demonstrates remarkable chemical stability across a wide pH range (1-11) and retains its structural integrity after 2 years of repeated adsorption cycles and activation at 80 °C under ambient humidity. Cryogenic adsorption experiments reveal that TUB41's pores selectively exclude gases with larger kinetic diameters, such as N₂ and Ar, while accommodating smaller molecules like CO₂ and H₂O at elevated temperatures. The enthalpies of adsorption for CO₂ at a loading 0.01 mmol g^-1 and H₂O at a loading of 0.7 mmol g^-1 are -41 and -38 kJ mol^-1, respectively, reflecting their strongly attractive interactions with TUB41 under different conditions. Molecular dynamics simulations reveal that CO₂ molecules adopt ordered arrangements in the central hydrophobic regions of the pores, guided by strong nonbonding interactions, while H₂O molecules preferentially bind to the hydrophilic secondary building units. Mean-squared displacement analyses confirm that both gases remain spatially constrained within the pores. These findings highlight TUB41 as a chemically robust and highly selective MOF, with potential for applications in gas separation, photocatalytic water splitting, and CO₂ reduction under challenging conditions.