Development of Heterometallic Annular Tetranuclear Clusters in Metal–Organic Frameworks for Methane Purification and Storage

Annular tetranuclear cluster based metal–organic frameworks (MOFs) have displayed unique advantages in gas adsorption and separation due to their highly connected robust architectures. Herein, two novel heterometallic tetranuclear motifs, [Y₂Cd₂(μ₃-O)₂(COO)₈(H₂O)₂] and [Y₂In₂(μ₃-O)₂(μ₂-O)₂(COO)₈(H₂O)₂], were successfully explored, which were further extended by 1,3,5-tris(4-carboxyphenyl)benzene (H₃BTB) tritopic linker to give isostructural MOFs (SNNU-326 and -327). SNNU-326 and -327 both exhibit the abilities to remove impurities (C₂-hydrocarbons and CO₂) in natural gas (NG) and excellent CH₄ storage capacities at high pressures. SNNU-326 shows better CH₄ purification and storage performance than SNNU-327 owing to different framework charges, in which only one counter ion is needed in SNNU-326 but two of them are necessary for SNNU-327, thus resulting in an obvious decrease of surface area. Dynamic breakthrough experiments demonstrate that SNNU-326 can effectively separate CH₄ from equimolar C₂H₂/CH₄, C₂H₄/CH₄, C₂H₆/CH₄, and CO₂/CH₄ mixtures with breakthrough interval times of about 40.6, 35.1, 54.2, and 10.2 min g^–1 (273 K, 1 bar, 2 mL min^–1), respectively. At the same time, SNNU-326 exhibits excellent CH₄ storage capability with total and working uptakes of 154.3 cm³ (STP) cm^–3 (80 bar) and 103.4 cm³ (STP) cm^–3 (5–65 bar) at 273 K on account of the collaborative impacts of adequate apertures, high surface areas, and multiple open metal sites.