Coadsorption of CO2/CH4 onto zeolitic imidazolate frameworks: The role of inner surface polarity, framework flexibility and topology

The ZIF sub-family of metal-organic frameworks (MOFs) possesses considerable robustness, tunability of pores, flexibility. These properties make ZIFs potential materials for selective CO₂ adsorption from mixture of gases. However, the lack of experimental multi-component co-adsorption studies makes it challenging for translating the fundamental understanding towards large-scale separation applications. Hence, we focused on both single component adsorption and CO₂/CH₄ co-adsorption measurements. The selected frameworks include ZIF-8, ZIF-90, ZIF-7, ZIF-11 and ZIF-71, which laid the foundation of understanding the influence of functional groups, structural flexibility, crystal size and topology on CO₂ selectivity in presence of CH₄. The results indicate that polar functional group and gate-opening flexibility are a couple of key parameters to improve CO₂ selectivity for ZIFs. Topology of ZIFs, on the other hand, did not show a significant effect on CO₂ selectivity. The Ideal Adsorbed Solution Theory (IAST) is used to show the compatibility between the theoretical and experimental selectivity. The IAST exhibits mismatch for ZIF-90, indicates limitation of the theory in considering the thermodynamic analysis of framework-guest interactions. The present study highlights the factors influencing CO₂ selectivity in presence of CH₄ through both single and multi-component adsorption isotherms.