Ligand Substitution Engineering for Modulating the Pore Microenvironment of MOFs to Enhance CO2 Gas Separation Performance in Mixed Matrix Membranes

Defect engineering allows for precise control of the microporous structure and coordination defects in metal-organic frameworks (MOFs). This approach offers an innovative pathway to optimize the gas separation performance of MOF/polymer mixed matrix membranes (MMMs). Herein, UiO-PzDC nanoparticles with gradient defects were synthesized by substituting terephthalic acid with 2,5-pyrazinedicarboxylic acid (PzDC) as a linker in a novel UiO-66 derivative, while modulating the defect concentration through the addition of 2-pyrazinecarboxylic acid (2-PZC). The material was dispersed within a highly permeable polymer matrix (e.g., PIM-1), and the influence of defect concentration on the interfacial properties and mass transfer behavior of mixed matrix membranes (MMMs) was systematically investigated. Combined analysis of experimental results and molecular dynamics simulations demonstrated that the incorporation of PzDC ligands significantly enlarged the MOF pore size, while the 2-PZC-induced defects further enhanced the microporous connectivity. Owing to the synergistic effect of MOF defects and the PIM-1 matrix, the composite membranes demonstrated superior filler-matrix interfacial compatibility, and the defect concentration exhibited no significant influence on interfacial stability. Gas transport studies revealed that the defect-engineered MOF facilitated an ultrafast transport channel for CO 2 through enhanced diffusion selectivity. MMMs containing a high defect concentration of UiO-PzDC (defectivity: 3.03) exhibit breakthrough separation performance, achieving a CO 2 permeability of 18,553 Barrer (a 384% improvement over pure PIM-1) and a CO 2 /N 2 selectivity of 23. This performance combination surpasses the 2019 Robeson upper bound. This study elucidates the universal principle of ligand substitution and defect engineering in synergistically modulating the mass transfer mechanism of metal-organic frameworks (MOFs), establishing a novel paradigm for designing next-generation membrane materials that simultaneously exhibit high permeability and selectivity.