Optimal Pore Chemistry in an Ultramicroporous Metal-Organic Framework for Benchmark Inverse CO /C H Separation.

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