Aliphatic Ligand Design Principles for Rigid Pore-Space-Partitioned Metal-Organic Frameworks for Gas Separation

Aliphatic ligands are often sidelined in the design of framework materials because their conformational flexibility can contribute to problems such as difficult crystallization, low porosity, and stability. Attempts to boost porosity by ligand elongation usually worsen these problems. Here we propose an expanded bioisosteric replacement (eBIS) concept capable of both scaling up and rigidifying aliphatic ligands. We demonstrate one example realized via linking two cyclohexyl rings in series, which restricts ligand flexibility through intramolecular non-covalent interactions providing an alternative to the π-conjugation-based rigidity. The resulting ligand displays consistent rigidity across multiple MOF platforms. On the pacs platform, it can realize extreme pore geometry with the highest hexagonal c/a ratio and new metal-cluster chemistry such as the first synthesis of nickel-titanium oxocluster. It can boost the BET surface area to as high as 2810 m² g^-1, likely the highest among aliphatic-dicarboxylate MOFs. Furthermore, it leads to possibly largest C₂H₆/C₂H₄ uptake differences (88 cm³ g^-1, uptake ratio of 1.83, 273 K) among rigid MOFs, a desired property for C₂H₆-selective separation, which is confirmed by breakthrough experiments. The remarkably low adsorption enthalpies for C₂H₆ (14.7 kJ mol^-1) and C₂H₄ (15.1 kJ mol^-1) enables low-energy adsorbent regeneration benefitting practical separation.