Improved CO2 Capture from Flue Gas by Basic Sites, Charge Gradients, and Missing Linker Defects on Nickel Face Cubic Centered MOFs

The adsorptive properties of the isoreticular series [Ni-8(OH)(4)(H2O)(2)(BDP_X)(6)] (H2BDP_X = 1,4-bis(pyrazol-4-yl)benzene-4-X with X = H (1), OH (2), NH2 (3)) can be enhanced by postsynthetic treatment with an excess of KOH in ethanol. In the case of X = H, NH2, 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)-(H2O)(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)(H2O)(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)(H2O)(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 CO2 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.