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

Abstract Isolation of CO 2 from acetylene (C 2 H 2 ) via CO 2 ‐selective sorbents is an energy‐efficient technology for C 2 H 2 purification, but a strategic challenge due to their similar physicochemical properties. There is still no specific methodology for constructing sorbents that preferentially trap CO 2 over C 2 H 2 . 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 CO 2 /C 2 H 2 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 CO 2 . The exceptional CO 2 uptake (151.7 cm 3 cm −3 ) along with remarkable separation selectivities (above 40) set a new benchmark for inverse CO 2 /C 2 H 2 separation, which is verified via simulated and experimental breakthrough experiments. The unique CO 2 recognition mechanism is further unveiled by in situ powder X‐ray diffraction experiments, Fourier‐transform infrared spectroscopy measurements, and molecular calculations.