A comparative study of the effect of functional groups on C2H2 adsorption in NbO-type metal–organic frameworks

Abstract

Investigation of the effect of functional groups on gas adsorption in a MOF of a given structure is undoubtedly very important because it facilitates targeting porous MOFs with enhanced storage capacities by ligand functionalization. In this work, we chose an NbO-type MOF platform to evaluate the impact of organic functional groups on C₂H₂ adsorption. Correspondingly, we synthesized five diisophthalate ligands with the same length but different organic functionalities (NH₂, CH₃, NO₂, F, and CF₃), and successfully incorporated them into MOFs with underlying NbO topology. C₂H₂ adsorption experiments reveal that these functional moieties can enhance the affinity towards C₂H₂, but not all these compounds outperform the unfunctionalized parent compound NOTT-101 in terms of C₂H₂ uptake. At 298 K and 1 atm, the C₂H₂ uptake varies from 153.7 to 193.8 cm³ (STP) g⁻¹, depending on the attached organic functional groups. Particularly, the amine group-functionalized compound ZJNU-34(NH₂) exhibits the maximum C₂H₂ uptake of the six compounds evaluated, reaching as high as 203.6 cm³ (STP) g⁻¹ at 295 K and 1 atm. Such a 10% increase of C₂H₂ uptake compared to the parent compound might be attributed to acid–base and/or hydrogen-bonding interactions between the NH₂ groups with adsorbed C₂H₂ molecules. The fundamental understanding of the impact of functional groups on C₂H₂ adsorption demonstrated in this work provides valuable information for future designing of porous MOFs with enhanced acetylene-storage capacities.