Inverse CO₂-C₂H₂ Separation on the Low-Silica CHA Zeolite Through Cooperative Cation and Gas Molecule Migration...

Inverse CO₂/C₂H₂ separation is promising for direct C₂H₂ purification; however, designing cost-effective and CO₂-selective stable porous materials remains challenging. Herein, by precise Si/Al ratio design and inorganic cation regulation in low-silica CHA zeolites, we achieve excellent inverse CO₂/C₂H₂ separation based on the trapdoor effect via a cooperative cation and gas molecule migration mechanism, distinct from the transient and reversible cation deviation previously reported. The designed K-CHA exhibits high CO₂ capacity (3.51 mmol g^-1) and much lower C₂H₂ uptake (0.62 mmol g^-1) at 298 K and 1 bar, achieving an ideal adsorbed solution theory (IAST) selectivity of 4350, outperforming most metal-organic frameworks (MOFs) and zeolites. Breakthrough experiments confirmed the exceptional one-step C₂H₂ purification ability of K-CHA, yielding a productivity of 662.9 mmol kg^-1. Rietveld refinement located cation positions within CHA. Density functional theory (DFT) calculations and ab initio molecular dynamics simulations (AIMD) elucidated the separation mechanism that CO₂ interacts more strongly with K-CHA compared to C₂H₂, and the diffusion barrier for CO₂ passing through K⁺-gated 8-rings is lower than C₂H₂. AIMD further revealed distinct trajectories and synergistic migration of the door-keeping K⁺ ions and CO₂/C₂H₂ molecules during diffusion. This work provides new insights into the trapdoor mechanism, advancing our fundamental understanding.