A flexible two-dimensional layered metal–organic framework functionalized with (trifluoromethyl)trifluoroborate: synthesis, crystal structure, and adsorption/separation properties

Abstract

Flexible porous materials have great potential for adsorption/separation of small molecules. In this study, a highly CO₂-selective two-dimensional (2D) layered metal–organic framework (MOF) showing gate-type adsorption properties was synthesized and fully characterized by single-crystal X-ray diffraction (XRD), in situ powder XRD, thermogravimetric analysis, inductively coupled plasma atomic emission spectroscopy, and gas adsorption/separation analyses. The MOF named ELM-13 is a 2D layered material functionalized with (trifluoromethyl)trifluoroborate to control interlayer interactions and exhibits dynamic guest accommodation/removal properties. In a gas adsorption study, the MOF showed no adsorption at low pressure followed by abrupt uptake and sudden desorption at a pressure lower than that of adsorption, i.e., gate adsorption, in the N₂, O₂, Ar, NO, and CO₂ isotherms at the boiling/sublimation temperature of each gas. The structure of the MOF in the CO₂ adsorption was influenced by both the amount adsorbed and the adsorption process. High-pressure adsorption experiments at 273 K indicated that, out of N₂, O₂, Ar, and CO₂, only CO₂ was adsorbed on the MOF below 900 kPa. The CO₂ selectivity from an equimolar CO₂/N₂ mixture with a total gas pressure of 1 MPa at 273 K was experimentally evaluated and compared with the CO₂ selectivities of other selected porous materials theoretically, suggesting that ELM-13 is a good candidate for CO₂ separation.