Anchoring metal ions in iCOF to enhance the charge density for boosting CO2 separation in mixed matrix membranes

Ionic covalent organic frameworks (iCOFs), with electrostatic microenvironment suitable for CO₂ separation, are regarded as ideal materials for gas membrane separation. However, the improvement of CO₂/CH₄ selectivity remains challenging because of low charge density of iCOFs. This study proposes the strategy for efficient CO₂ separation by anchoring metal ions (Cu²⁺, Al³⁺ and Zr⁴⁺) between iCOF interlayers to regulate the charge density of iCOF in mixed matrix membranes (MMMs). The polarizability of different metal ions regulates their charge transfer with iCOFs, leading to charge densities that follow the increased order: Cu-COF ​< ​Al-COF ​< ​Zr-COF. Compared to metal-COFs with low charge density, the Zr-COF featuring high charge density exhibits a more favorable positively electrostatic microenvironment for CO₂ separation in MMMs, primarily attributed to the fact that it enhances interaction with the negatively charged oxygen atoms in CO₂ molecules, thereby improving CO₂ transport in MMMs. Pebax/Zr-COF MMM exhibit the optimal CO₂ separation performance with enhanced permeability (∼66 ​%) and selectivity (∼93 ​%) than that of pure Pebax membrane, surpassing the Robeson upper bound. Therefore, anchoring metal ions in iCOF interlayers to enhance charge density offers a strategy for designing efficient CO₂ separation in MMMs.