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.