Effect of arrangement of functional groups on stability and gas adsorption properties in two regioisomeric copper bent diisophthalate frameworks

Abstract

The structure–property relationship investigation is of prime importance to development of porous MOFs exhibiting better performance. In this work, two regioisomeric diisophthalate ligands, with the only difference being the position of methyl and amino groups with respect to isophthalate moieties, were rationally designed and synthesized, and their corresponding dicopper paddlewheel-based MOFs were successfully fabricated under suitable solvothermal conditions. Single-crystal X-ray diffraction studies showed that they displayed the same network topology of ssa type, thus providing a platform to investigate the impact of the substituent's arrangement on gas adsorption properties. As revealed by gas adsorption studies, the pore textural properties and gas uptake capacities with respect to C₂H₂, CO₂, and CH₄ of ZJNU-11 are significantly higher than those of its isomer ZJNU-12 despite their same structures, indicating that the arrangement of functional groups makes a difference to the framework stabilities against desolvation and thus to the gas adsorption properties. In addition, ZJNU-11 is also capable of selectively adsorptive separation of C₂H₂ and CO₂ from CH₄, with high C₂H₂/CH₄ (v/v, 1/1) and CO₂/CH₄ (v/v, 1/1) adsorption selectivities of 36.2 and 5.9 at 298 K and 1 atm. This work not only reported a porous MOF material for highly selective C₂H₂/CH₄ and CO₂/CH₄ separations, but also demonstrated that the rational arrangement of substituent groups will be favorable to improve the MOF's stabilities and thus gas adsorption properties, which needs to be taken into account upon design and construction of porous MOFs.