Optimal Pore Chemistry in an Ultramicroporous Metal-Organic Framework for Benchmark Inverse CO₂/C₂H₂ Separation

Isolation of CO₂ from acetylene (C₂H₂) via CO(2)selective sorbents is an energy-efficient technology for C₂H₂ purification, but a strategic challenge due to their similar physicochemical properties. There is still no specific methodology for constructing sorbents that preferentially trap CO₂ over C₂H₂. We report an effective strategy to construct optimal pore chemistry in a Ce^IV-based ultramicroporous metalorganic framework Ce^IV-MIL-140-4F, based on charge-transfer effects, for efficient inverse CO₂/C₂H₂ separation. The ligandto-metal cluster charge transfer is facilitated by Ce^IV with lowlying unoccupied 4f orbitals and electron-withdrawing Fatoms functionalized tetrafluoroterephthalate, affording a perfect pore environment to match CO₂. The exceptional CO₂ uptake (151.7 cm³ cm^-3) along with remarkable separation selectivities (above 40) set a new benchmark for inverse CO₂/C₂H₂ separation, which is verified via simulated and experimental breakthrough experiments. The unique CO₂ recognition mechanism is further unveiled by in situ powder X-ray diffraction experiments, Fourier-transform infrared spectroscopy measurements, and molecular calculations.