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

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 (HL1), 5,5'-(naphthyl-1,4-diyl) diisophthalate (HL2), 5,5'-(naphthyl-1,5-diyl) diisophthalate (HL3), 5,5'-(naphthyl-1,6-diyl) diisophthalate (HL4) and 5,5'-(naphthyl-2,6-diyl) diisophthalate (HL5), 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 HL1 and HL4, respectively, and nbo topology for the MOFs ZJNU-72, ZJNU-73 and NOTT-103 derived from the ligands HL2, HL3 and HL5, respectively. Furthermore, their CH adsorption properties were systematically investigated, revealing that their different CH 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 CH uptake of 208.1 cm (STP) g 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.