Robust Metal–Organic Framework with An Octatopic Ligand for Gas Adsorption and Separation: Combined Characterization by Experiments and Molecular Simulation

A newly designed octatopic carboxylate ligand, tetrakis­[(3,5-dicarboxyphenyl)­oxamethyl]­methane (TDM^8–) has been used to connect a dicopper paddlewheel building unit affording a metal–organic framework (MOF), Cu₄(H₂O)₄(TDM)·xS (PCN-26·xS, S represents noncoordinated solvent molecules, PCN = porous coordination network) with novel structure, high gas uptake, and interesting gas adsorption selectivity. PCN-26 contains two different types of cages, octahedral and cuboctahedral, to form a polyhedron-stacked three-dimensional framework with open channels in three orthogonal directions. Gas adsorption studies of N₂, Ar, and H₂ on an activated PCN-26 at 77 K, 1 bar, reveals a Langmuir surface area of 2545 m²/g, a Brunauer–Emmett–Teller (BET) surface area of 1854 m²/g, a total pore volume of 0.84 cm³/g, and a H₂ uptake capacity of 2.57 wt %. Additionally, PCN-26 exhibits a CO₂/N₂ selectivity of 49:1 and CO₂/CH₄ selectivity of 8.4:1 at 273 K. To investigate properties of gas adsorption and the adsorption sites for CO₂ in activated PCN-26, theoretical simulations of the adsorption isotherms of CO₂, CH₄, and N₂ at different temperatures were carried out. Experimental results corroborate very well with those of molecular simulations.