High-Connected Ternary Metal–Organic Framework Platform: Synthesis, Structure, and Methane Storage Capacity

Design and synthesis of a single metal–organic framework (MOF) that simultaneously achieves high gravimetric and volumetric working capacities for methane storage are crucial for advancing the use of natural gas as a vehicular fuel. However, this presents a significant challenge due to the inherent trade-off effect between the gravimetric and volumetric methane adsorption capacities of a single porous material. Herein, we initially synthesized a novel pyridine-carboxylic acid ligand and combined it with a trimeric iron cluster along with a series of dicarboxylic acid ligands of varying lengths or functionalities. Employing a dual-solvent system and dual-modulator solvothermal principles, we successfully constructed a 9-c ternary MOF platform. X-ray diffraction analysis reveals that the structures all feature an ncb-type topological network with a cage-channel biporous hierarchy. Through a multistep solvent exchange followed by supercritical carbon dioxide drying method, we successfully activated this series of materials, achieving substantial porosity, with pore volumes exceeding 1.90 cm³ g^–1, gravimetric surface areas surpassing 4800 m² g^–1, and volumetric surface areas greater than 1600 m² cm^–3. High-pressure methane adsorption tests at 80 bar demonstrated that the series of materials exhibited a high total gravimetric and volumetric methane adsorption capacity. Notably, when the testing temperature was lowered to 273 K, these materials showed significant increases in total gravimetric and volumetric methane adsorption. Particularly, the Fe-ncb-TPDC-II constructed using the longest dicarboxylate linker achieved gravimetric and volumetric methane storage working capacities of 0.533 g g^–1 and 232 cm³ (STP) cm^–3, respectively, performing exceptionally well compared to reported porous materials under similar conditions.