A family of ssa-type copper-based MOFs constructed from unsymmetrical diisophthalates: synthesis, characterization and selective gas adsorption

Abstract

In this work, three porous MOFs (ZJNU-74, ZJNU-75 and ZJNU-76) were constructed via solvothermal reactions of unsymmetrical naphthalene-derived diisophthalate ligands and Cu²⁺ ions. As revealed by single-crystal X-ray diffraction analyses, they consist of dicopper [Cu₂(COO)₄] paddlewheels connected by the organic ligands to form three-dimensional networks with the same ssa topology, thus representing a MOF platform which allows understanding of the structure–property relationship. The gas adsorption properties of the three MOF compounds with respect to C₂H₂, CO₂ and CH₄ have been systematically investigated. They exhibit very high C₂H₂ uptakes varying from 179.2 to 188.6 cm³ (STP) g⁻¹ and CO₂ uptakes varying from 88.1 to 99.2 cm³ (STP) g⁻¹ at 298 K and 1 atm. The predicted IAST selectivities range from 24.9 to 29.7 for an equimolar C₂H₂/CH₄ gas mixture and from 4.8 to 5.4 for an equimolar CO₂/CH₄ gas mixture at 298 K and 1 atm. These results reveal that they possess very promising potential for C₂H₂/CH₄ and CO₂/CH₄ separations. In particular, compared to the unmodified parent MOF ZJNU-74, the alkoxy group-functionalized MOFs ZJNU-75 and ZJNU-76 exhibit comparable and even higher C₂H₂ and CO₂ uptake capacities as well as better C₂H₂/CH₄ and CO₂/CH₄ adsorption selectivities under ambient conditions, which might be attributed to the optimized pore sizes as a result of ligand modification.