Molecular Trapdoor in HEU Zeolite Enables Inverse CO 2 ‐C 2 H 2 Separation

The adsorptive separation of molecules with very similar physical properties is always a challenging task. Reported herein is the design and tailoring of zeolite adsorbent for the precise discrimination and separation of CO 2 ‐C 2 H 2 mixture through the pronounced trapdoor effect. Typically, Sr exchanged K‐type clinoptilolite, namely Sr/K‐HEU, is developed as a robust zeolite adsorbent for inverse CO 2 ‐C 2 H 2 separation, showing the‐state‐of‐the‐art dynamic CO 2 /C 2 H 2 selectivity of 48.0 and sustainable CO 2 dynamic uptake of 0.96 mmol/g at the same time. The perfect recyclability and the intrinsic low‐cost nature of Sr/K‐HEU make it a promising candidate for practical applications. Three‐dimensional electron diffraction determines the precise structure of Sr/K‐HEU and density functional theory calculations reveal the intricate interplay between guest molecules and the gate‐keeping extraframework cations. Briefly, extraframework Sr 2+ cations from the ten‐membered rings of HEU zeolites act as the molecular trapdoor, allowing the entry of CO 2 molecules while excluding C 2 H 2 . This work presents a new example of molecular trapdoor in zeolite and its successful application in the challenging inverse CO 2 ‐C 2 H 2 separation, which not only expands the scope of molecular trapdoor concept but also improves current understanding on the nature of molecular trapdoor.