Spatial control of MOF distribution in mixed-matrix membranes enables CO₂-Separation at low concentration

We demonstrate that spatial control of the dispersion of the Zn-based metal-organic framework (MOF) CFA-1 into Pebax-based mixed-matrix membranes (MMMs) enables significant performance enhancements for direct air capture (DAC)-relevant CO₂ separation. In this work, CFA-1 was incorporated in MMMs via two approaches: (i) homogeneous dispersion through ultrasonication and (ii) controlled sedimentation to produce a MOF-enriched feed-side surface layer. Gas permeation tests with dilute CO₂ feeds (0.1–1% in N2) across 0–100% relative humidity reveal that homogeneous dispersion of CFA-1 enhances both CO₂ permeability (140 Barrer) and selectivity (71) at 0.1% CO₂ under humid conditions compared to pristine Pebax. The layered architecture further increases performance, achieving selectivity up to 109 (∼54% enhancement over homogeneous dispersion), a performance regime that is challenging for most MMMs under ultra-low CO₂ conditions. Maxwell model analysis indicates that CFA-1 behavior deviates from classical diffusion-only transport and is consistent with additional adsorption and hydration-assisted contributions. These results establish that spatial filler distribution provides an effective design principle for CO₂ pre-enrichment, offering a promising membrane-based DAC pathway.