Finely Tuning Metal Ion Valences of [Fe_{3- x}M_x(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 Fe_3-xM_x(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.