Surface-Layer Pore Aperture Engineering in a Metal-Organic Framework for High-Capacity CO2/C2H2 Molecular Sieving

Molecular sieving in metal-organic frameworks (MOFs) is commonly achieved by narrowing the entire pore network, which enhances size exclusion but often sacrifices adsorption capacity. Here we report surface-layer pore aperture engineering as a spatially decoupled strategy for high-capacity CO2/C2H2 molecular sieving. Through surface-confined coordination modification of a copper-based MOF, the external pore apertures were selectively regulated while the internal adsorption space was largely preserved. Microscopy, elemental mapping, depth-resolved XPS, and X-ray absorption spectroscopy support the formation of a surface-confined functional shell at the pore aperture. The modified framework converts a nonselective, high-capacity adsorbent into a CO2/C2H2 molecular sieve. BUT-169-NH2-TRZ retains a high CO2 uptake of 90.3 cm3 g-1 at 298 K and 1 bar while suppressing C2H2 adsorption to 2.7 cm3 g-1, representing a new benchmark for CO2/C2H2 sieving separation. Further optimization with hexylamine affords faster CO2 transport and dynamic CO2/C2H2 separation, delivering a high productivity of 1.13 mol kg-1 for C2H2 (purity > 99.99%) over repeated breakthrough cycles. This work establishes surface-layer pore aperture engineering as a route to overcome the conventional selectivity-capacity trade-off in molecular sieving separations.