The accessibility of nitrogen sites makes a difference in selective CO₂ adsorption of a family of isostructural metal–organic frameworks

Abstract

By using three rigid diisophthalate organic linkers incorporating different numbers and orientations of Lewis basic nitrogen atoms into the spacers between two terminal isophthalate moieties, namely, 5,5′-(quinoline-5,8-diyl)-diisophthalate, 5,5′-(isoquinoline-5,8-diyl)-diisophthalate, and 5,5′-(quinoxaline-5,8-diyl)-diisophthalate, a family of isostructural copper-based metal–organic frameworks, ZJNU-43, ZJNU-44 and ZJNU-45, were successfully solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction. The three MOFs, after activation, exhibited almost the same porosities but distinctly different CO₂ adsorption properties. At room temperature and 1 atm, the adsorption capacities for CO₂ reached 103, 116 and 107 cm³ (STP) g^-1 for ZJNU-43a, ZJNU-44a and ZJNU-45a, respectively. Furthermore, Ideal Adsorbed Solution Theory (IAST) and simulated breakthrough analyses indicated that ZJNU-44a bearing much more easily accessible nitrogen sites is the best among the three MOFs for the separation of the following two binary gas mixtures at 296 K, i.e. 50/50 CO₂/CH₄ and 15/85 CO₂/N₂ gas mixtures, indicating that the accessibility of nitrogen sites plays a much more crucial role, which is further confirmed by comprehensive quantum chemical calculations. The work demonstrates that the CO₂ adsorption properties of MOFs depend not only on the number of Lewis basic nitrogen sites but also more importantly on their accessibility.