Tuning Open Metal Site-Free <b>ncb</b> Type of Metal–Organic Frameworks for Simultaneously High Gravimetric and Volumetric Methane Storage Working Capacities

Abstract

The design and synthesis of a single metal-organic framework (MOF) material with simultaneously high gravimetric and volumetric methane storage working capacities are still a great challenge. The open metal site (OMS) in MOFs is generally regarded as an advantage to enhance host-guest affinity. However, it is detrimental to the methane storage working capacity to some extent due to the resulting high low-pressure uptake. Moreover, the reported methane storage MOFs are predominately focusing on edge-transitive or low-connected mixed-linker networks. In contrast, high-connected mixed-linker MOFs have been less investigated for methane storage. Herein, three isoreticular nine-connected trinuclear iron-based Fe-<b>ncb</b>-MOFs without OMSs have been judiciously designed and successfully constructed by means of the mixed-linker approach associated with the fixing amide-functionalized pyridyl-carboxylate ligand L^P (4-(pyridin-4-ylcarbamoyl)benzoate) and three differing sized dicarboxylate ligands. High-pressure methane adsorption measurements show that, with the isoreticular extension from BDC (1,4-benzenedicarboxylate) to BPDC (4,4'-biphenyldicarboxylate) and ABDC (azobenzene-4,4'-dicarboxylate), three Fe-<b>ncb</b>-MOFs exhibit gradually increasing not only gravimetric but also volumetric storage capacities because of their balancing gravimetric surface area and volumetric surface area, hierarchical pore system, and modest CH₄ heats of adsorption. Among them, the Fe-<b>ncb</b>-ABDC demonstrates a rare combination of simultaneously high gravimetric and volumetric CH₄ storage working capacities of 0.302/0.37 g g^-1 and 196/240 cm³ (STP) cm^-3 at 298/273 K and between 80 and 5 bar, respectively, which outperform the 8-c Fe-<b>8T18</b>-ABDC assembled from a shorter pyridyl-carboxylate ligand IN (isonicotinate) and ABDC, due to its limited pore volume, the presence of OMSs, and more confined pore spaces, and place Fe-<b>ncb</b>-ABDC among the best performing MOFs.