Improved CO_2 Capture from Flue Gas by Basic Sites, Charge Gradients, and Missing Linker Defects on Nickel Face Cubic Centered MOFs

摘要

The adsorptive properties of the isoreticular series [Ni_8(OH)_4(H_2O)_2(BDP_X)_6] (H_2BDP_X = 1,4-bis(pyrazol-4-yl)benzene-4-X with X = H (1), OH (2), NH_2 (3)) can be enhanced by postsynthetic treatment with an excess of KOH in eth-anol. In the case of X = H, NH_2, this treatment leads to partial removal of the organic linkers, deprotonation of coordinated water molecules and introduction of extraframework cations, giving rise to materials of K[Ni_8(OH)_5(EtO)-(H_2O)_2(BDP_X)_(5.5)] (1@KOH, 3@KOH) formulation, in which the original framework topology is maintained. By contrast, the same treatment with KOH in the [Ni_8(OH)_4(H_2O)_2(BDP_OH)_6] (2) system, enclosing the more acidic phenol residues, leads to a new material containing a larger fraction of missing linker defects and extra-framework cations as well as phenolate residues, giving rise to the material K_3[Ni_8(OH)_3(EtO)(H_2O)_6(BDP_O)_5] (2@KOH), which also conserves the original face cubic centered (fcu) topology. It is noteworthy that the introduction of missing linker defects leads to a higher accessible pore volume with a concomitant increased adsorption capacity. Moreover, the creation of coordinatively unsaturated metal centers, charge gradients, and phenolate nucleophilic sites in 2@KOH gives rise to a boosting of CO_2 capture features with increased adsorption heat and adsorption capacity, as proven by the measurement of pulse gas chromatography and breakthrough curve measurements of simulated flue gas.