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 CeIV -based ultramicroporous metal-organic framework CeIV -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 CeIV 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.