Enhancing CO₂/N₂ and CH₄/N₂ separation performance by salt-modified aluminum-based metal-organic frameworks

The energy-saving separation of CO₂/N₂ and CH₄/N₂ in the energy industry facilitates the reduction of greenhouse gas emissions and replenishes energy resources, but is a challenging separation process. The trade-off between adsorption capacity and selectivity of the adsorbents is one of the key bottlenecks in adsorption separation technologies' large-scale application in the above separation task. Herein, we introduced a series of fluoroborate or fluorosilicate salts (Cu(BF₄)₂, Zn(BF₄)₂ and ZnSiF₆) into the open coordination nitrogen sites of aluminum-based metal-organic frameworks (MOF-253) to create multiple binding sites to simultaneously enhance the adsorption capacity and selectivity for the target gas. By the synergistic adsorption effect of metal ions (Cu²⁺ or Zn²⁺) and fluorinated anions (BF₄^- or (SiF₆)^2-), the single-component adsorption capacity and selectivity of salt-modified MOF-253 (MOF-253@Cu(BF₄)₂, MOF-253@Zn(BF₄)₂ and MOF-253@ZnSiF₆) for CO₂ and CH₄ were effectively improved when compared to pristine MOF-253 at 298 K and 1 bar. In addition, the salt-modified MOF-253 has a moderate adsorption heat (<30 kJ mol^-1) which could be rapidly regenerated at low energy by evacuation desorption. As confirmed by the ambient breakthrough experiments of MOF-253 and MOF-253@ZnSiF₆, the real separation performance for both CO₂/N₂ (1/4) and CH₄/N₂ (1/4) was obviously improved. This work provides a feasible post-modification strategy on uncoordinated sites of the framework to improve adsorption separation performance and promote the development of ideal adsorbents with a view to realizing their application in the energy industry.