Molecular Trapdoor in HEU Zeolite Enables Inverse CO₂-C₂H₂ 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 CO₂-C₂H₂ 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 CO₂-C₂H₂ separation, showing the-state-of-the-art dynamic CO₂/C₂H₂ selectivity of 48.0 and sustainable CO₂ 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 Sr²⁺ cations from the ten-membered rings of HEU zeolites act as the molecular trapdoor, allowing the entry of CO₂ molecules while excluding C₂H₂. This work presents a new example of molecular trapdoor in zeolite and its successful application in the challenging inverse CO₂-C₂H₂ separation, which not only expands the scope of molecular trapdoor concept but also improves current understanding on the nature of molecular trapdoor.