Optimal Pore Chemistry in an Ultramicroporous Metal-Organic Framework for Benchmark Inverse CO₂ /C₂ H₂ Separation

Isolation of CO₂ from acetylene (C₂ H₂ ) via CO₂ -selective sorbents is an energy-efficient technology for C₂ H₂ 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 C₂ H₂ . We report an effective strategy to construct optimal pore chemistry in a Ce^IV -based ultramicroporous metal-organic framework Ce^IV -MIL-140-4F, based on charge-transfer effects, for efficient inverse CO₂ /C₂ H₂ separation. The ligand-to-metal cluster charge transfer is facilitated by Ce^IV with low-lying unoccupied 4f orbitals and electron-withdrawing F atoms 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 /C₂ H₂ 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.