Specific K⁺ Binding Sites as CO₂ Traps in a Porous MOF for Enhanced CO₂ Selective Sorption

Metal-organic frameworks (MOFs) can be fine-tuned to boost sorbent-sorbate interactions in order to improve gas sorption and separation performance, but the design of MOFs with ideal structural features for gas separation applications remains a challenge. Herein it is reported that unsaturated alkali metal sites can be immobilized in MOFs through a tetrazole based motif and that gas affinity can thereby be boosted. In the prototypal MOF of this type-NKU-521 (NKU denotes Nankai University), K⁺ cations are effectively embedded in a trinuclear Co²⁺ -tetrazole coordination motif. The embedded K⁺ sites are exposed to the pores of NKU-521 through water removal, and the isosteric heat (Q_st ) for CO₂ is boosted to 41 kJ mol^-1 . The nature of the binding site is validated by molecular simulations and structural characterization. The K⁺ cations in effect serve as gas traps and boost the CO₂ -framework affinity, as measured by the Q_st , by 24%. In addition, the impact of unsaturated alkali metal sites upon the separation of hydrocarbons is evaluated for the first time in MOFs using ideal adsorbed solution theory (IAST) calculations and column breakthrough experiments. The results reveal that the presence of exposed K⁺ sites benefits gas sorption and hydrocarbon separation performances of this MOF.