Investigating carbon dioxide sorption properties of two isostructural Co ^II and Zn ^II mixed-ligand metal–organic frameworks containing imide-based ligands

Abstract

Metal-organic frameworks (MOFs) are porous materials composed of metal ions or clusters coordinated to organic ligands.Due to their tunable structures and porosity, MOFs have attracted attention for a variety of applications, including gas storage, separation, sequestration, drug delivery, and catalysis.Flexible MOFs or soft porous crystals, can undergo dynamic structural transformations in response to external stimuli.This structural flexibility can enhance their performance, particularly in selective gas adsorption and sensing applications.In this study, we present the synthesis, characterization, and carbon dioxide sorption studies of two isostructural, flexible MOFs:Where L1 is 2,7-bis(pyridin-3-ylmethyl)benzo [lmn][3,8]phenanthroline-1,3,6,8(2H,7H)tetraone and HOBZ is 4,4-oxybis(benzoic acid).The two MOFs were synthesized using the solvothermal method and characterized using single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA).Carbon dioxide sorption experiments were conducted up to 20 bar at 298 K. SCXRD analysis revealed that both MOFs are three-dimensional structures with 12% potential solvent-accessible volume.PXRD shows a structural change upon activation, which indicates structural flexibility.The CO sorption isotherms of the two MOFs differ significantly.MOF 1 displays a type IV isotherm with pronounced hysteresis upon desorption.The MOF adsorbs 0.6 mmol g -1 CO up to 15 bar after which the structure opens and adsorbs and additional 1.3 mmol g -1 , resulting in a total of 1.9 mmol g -1 at 20 bar.In contrast, MOF 2 shows a typical type I isotherm, reaching a maximum CO uptake of 1.2 mmol g -1 at 20 bar.