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 CO 2 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 CO feeds (0.1–1% in N 2 ) across 0–100% relative humidity reveal that homogeneous dispersion of CFA-1 enhances both CO 2 permeability (140 Barrer) and selectivity (71) at 0.1% CO 2 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 CO 2 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 CO 2 pre-enrichment, offering a promising membrane-based DAC pathway. • Pebax-based MMMs are designed and tested under direct air capture–relevant dilute CO 2 conditions. • CFA-1 incorporation enables high performance, reaching 160 Barrer and CO 2 /N 2 selectivity of 109. • A MOF-enriched surface layer formed via controlled sedimentation boosts CO 2 permeation. • Spatial control of MOF distribution delivers ∼54% selectivity enhancement over uniform dispersion. • Demonstrates filler spatial placement as a design principle for ultra-dilute CO 2 separations.