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Isoreticular functionalization is a well‐elucidated strategy for pore environment tuning and the basis of gas separation performance in extended frameworks. The extension of this approach to discrete porous molecules such as metal‐organic cages (MOCs) is conceptually straightforward but hindered by synthetic complications, especially stability concerns. We report the successful isoreticular functionalization of a zirconium MOC with tetrazole moiety by bottom‐up synthesis. The title compound (ZrT‐1‐tetrazol) shows promising C2H2/CO2 and C2H2/C2H4 separation performance, as demonstrated by adsorption isotherms, breakthrough experiments, and density functional theory calculations. The design analogy between MOFs and highly stable MOCs may guide the synthesis of novel porous materials for challenging separation applications. [ABSTRACT FROM AUTHOR]

Isolation of CO2 from acetylene (C2H2) via CO2‐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 CeIV‐based ultramicroporous metal–organic framework CeIV‐MIL‐140‐4F, based on charge‐transfer effects, for efficient inverse CO2/C2H2 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/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. [ABSTRACT FROM AUTHOR]