Designing ultrathin mixed matrix membranes for CO2 separation with composite porous nanosheets

Ultrathin mixed-matrix membranes (MMMs) have emerged as promising candidates for efficient membrane-based carbon capture, owing to the combination of the high performance of mixed-matrix materials, and the low mass-transfer resistance of ultrathin membranes. However, incorporating fillers into ultrathin separation layers remains challenging due to the strict requirements on filler dimension and dispersion. Herein, we report a Pebax-based ultrathin MMM incorporating ZIF-8@COF composite porous nanosheets for efficient CO 2 /N 2 separation. Ultrasmall ZIF-8 nanoparticles (∼4 nm) were grown in situ on COF nanosheets, yielding a composite porous filler (∼11 nm thick) that combines the high aspect ratio of COFs with the superior interfacial compatibility and CO 2 affinity of ZIF-8. The resulting ZIF-8@COF/Pebax membrane exhibits 63% and 41% enhancements in CO 2 permeability and CO 2 /N 2 selectivity, respectively, over the pristine Pebax membrane, which can be attributed to abundant CO 2 adsorption sites from ultrasmall ZIF-8, well-defined molecular transport pathways from the porous nanosheets, and hydrogen-bond-mediated interfacial chain rigidification. Remarkably, the separation layer remains defect-free even when thinned to ∼100 nm, owing to the ultrathin architecture of the ZIF-8@COF nanosheets and their excellent interfacial compatibility. The resulting ultrathin mixed matrix membrane delivers an attractive CO 2 permeance of 1365 GPU and a CO 2 /N 2 selectivity of 38.4 under mixed-gas conditions, along with robust operational stability and pressure resistance. This work demonstrates that two-dimensional porous composites hold great promise as fillers for fabricating high-performance ultrathin MMMs toward practical carbon capture applications.