A comparative study of C₂H₂ adsorption properties in five isomeric copper-based MOFs based on naphthalene-derived diisophthalates

Abstract

In this work, five positional isomeric ligands consisting of two peripheral isophthalate moieties attached to the central naphthyl core in different ways, namely, 5,5'-(naphthyl-1,3-diyl) diisophthalate (H₄L1), 5,5'-(naphthyl-1,4-diyl) diisophthalate (H₄L2), 5,5'-(naphthyl-1,5-diyl) diisophthalate (H₄L3), 5,5'-(naphthyl-1,6-diyl) diisophthalate (H₄L4) and 5,5'-(naphthyl-2,6-diyl) diisophthalate (H₄L5), have been used to generate five copper-based MOF isomers. As revealed by single-crystal X-ray diffraction studies, they adopted two different types of topologies depending on the organic ligands: ssa topology for the MOFs ZJNU-71 and ZJNU-74 based on the ligands H₄L1 and H₄L4, respectively, and nbo topology for the MOFs ZJNU-72, ZJNU-73 and NOTT-103 derived from the ligands H₄L2, H₄L3 and H₄L5, respectively. Furthermore, their C₂H₂ adsorption properties were systematically investigated, revealing that their different C₂H₂ uptake capacities can be mainly related to their different pore sizes since they possess the same chemical compositions and gravimetric densities of open metal sites. In particular, among these five MOF compounds investigated, ZJNU-71 exhibits the highest gravimetric C₂H₂ uptake of 208.1 cm³ (STP) g^-1 at 295 K and 1 atm. The value is also among the highest reported for MOF compounds under the same conditions. This work provides a fundamental understanding of the impact of the positional isomerism of the organic ligands on the structures as well as gas adsorption properties of the resulting MOFs.