Propylene/Ethylene Separation via a Pressure-Induced Flexible Ultramicroporous Metal-Organic Framework.

Efficient separation of propylene/ethylene mixtures remains a formidable challenge because of the very similar physicochemical properties of the two olefins, particularly in methanol-to-olefins product streams. Herein, we report a pressure-responsive flexible ultramicroporous metal-organic framework, Co-PPA, constructed from pipemidic acid ligands and Co(II) ions, featuring pearl-strand-like one-dimensional channels and reversible guest-induced gate opening. At 298 K and 0.1 bar, Co-PPA exhibits a high C3H6 uptake of 25.21 cm3 g-1, whereas the uptake of C2H4 is only 0.26 cm3 g-1, affording an exceptionally high C3H6/C2H4 uptake ratio of 96.96. In situ single-crystal X-ray diffraction analyses together with GCMC simulations and electrostatic potential mapping suggest that the preferential adsorption of C3H6 is associated with multiple synergistic host-guest interactions, including close contacts with the pore surface, dispersion interactions, and favorable electrostatic complementarity within the confined channel environment. Dynamic breakthrough experiments further demonstrate that Co-PPA enables direct production of polymer-grade ethylene (≥99.95%) with a productivity of 28.4 cm3 (STP) g-1 from equimolar C3H6/C2H4 mixtures. Additionally, Co-PPA exhibits excellent thermal stability, chemical robustness, and superior cycling reversibility. This work highlights flexible ultramicroporous frameworks as a promising platform for energy-efficient olefin purification and provides useful insight into the design of gate-opening adsorbents for challenging light-hydrocarbon separations.