The accessibility of nitrogen sites makes a difference in selective CO₂ adsorption of a family of isostructural metal–organic frameworks
2015
Song, Chengling | Hu, Jiayi | Ling, Yajing | Feng, Yunlong | Krishna, R. | Chen, De-li | He, Yabing
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.
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