Mechanistic Insights into Gas Capture and Diffusion During the Co2 Separation of Mixed Matrix Membranes Using Zif Materials as Nanofillers

Abstract

The incorporation of zeolitic imidazolate framework (ZIF) as nanofillers in polyimide (PI)-based mixed matrix membranes (MMMs) offers a promising strategy to overcome the trade-off between CO2 permeability and selectivity in gas separation. However, the mechanistic insights into the role of ZIF crystal structures in enhancing CO2 separation performance remains unclear. Herein, a series of ZIF materials with tailored ligands were synthesized and incorporated into PI membranes to systematically investigate their impact on CO2 capture and diffusion. The results reveal that ZIF crystal structures establishes π-π conjugation with CO2 molecules, leading to strong CO2 capture. In contrast, only weak interactions, such as van der Waals forces, exist between the materials and N2 molecules. Molecular dynamics (MD) simulations indicate that the π-π conjugation between ZIF crystal structures and CO2 molecules is stronger than hydrogen bonding and amino bonding interactions. The incorporation of ZIF materials can significantly enhance the CO2 separation performance of the corresponding MMMs. The optimal CO2/N2 selectivity of MMMs reaches 29.2, which is 4.3 times higher than neat PI membrane. However, the mechanism by which the incorporation of ZIF materials enhances the CO2 separation performance of MMMs differs from the mechanism of CO2 adsorption by ZIF materials. The strong CO2 adsorption performance of well-distributed ZIF materials leads to high CO2 solubility selectivity of the corresponding MMMs. Additionally, π-π bonding interactions with CO2 molecules can facilitate CO2 diffusion in MMMs. However, adding materials with excessively strong adsorption energy for CO2, such as Zn-N-60, can weaken CO2 facilitated diffusion due to the difficulty of gas desorption from Zn-N-60.

Keywords: ZIF materials, CO2 separation, mixed matrix membranes, π-π interactions, adsorption-diffusion synergy