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.