Finely Tuning Metal Ion Valences of [Fe3–xMx(μ3–OH)(Carboxyl)6(pyridyl)2] Cluster-Based ant-MOFs for Highly Improved CO2 Capture Performances

Solvothermal reactions of different trinuclear precursors and 5-(pyridin-4-yl)isophthalic acid (H2L) successfully led to four anionic ant topological MOFs as Fe3–xMx(μ3–OH)(CH3COO)2(L)2·(DMA+)·DMF [M = Mn(II), Fe(II), Co(II), x = 0, 1, 2 and 3], namely, NJTU-Bai79 [NJTU-Bai = Nanjing Tech University Bai's group, Mn3(μ3–OH)], NJTU-Bai80 [Fe2Mn(μ3–OH)], NJTU-Bai81 [Fe3(μ3–OH)], and NJTU-Bai82 [Fe2Co(μ3–OH)], which possess the narrow pores (2.5–6.0 Å). NJTU-Bai80–82 is able to be tuned to the neutral derivatives [NJTU-Bai80–82(-ox), ox = oxidized] with M2+ ions oxidized to M3+ ones in the air and the OH– ions coordinated on M3+ ions. Very interestingly, selective CO2/N2 adsorptions of NJTU-Bai80–82(-ox) are significantly enhanced with the CO2 adsorption uptakes more than about 6 times that of NJTU-Bai79. GCMC simulations further revealed that neutral NJTU-Bai80–82(-ox) supplies more open frameworks around the −CH3 groups at separate spaces to the CO2 gas molecules with relatively more pores available to them after the removal of counterions. For the first time, finely tuning metal ion valences of metal clusters of ionic MOFs and making them from electrostatic to neutral were adopted for greatly improving their CO2 capture properties, and it would provide another promising strategy for the exploration of high-performance CO2 capture materials.