Structure and Kinetic Selectivity of Trapdoor Zeolites in Hydrogen Isotope Separation

The potential of two families of trapdoor zeolites with chabazite (CHA) and merlinoite (MER) structures have been explored for selective hydrogen isotope separation by investigating the influence of framework structure and the presence of different trapdoor cations (K⁺, Rb⁺, and Cs⁺) on the thermal and kinetic responses. By leveraging the distinct porous structures of these frameworks, the separation of H₂ and D₂ gases is facilitated. The energy required for hydrogen to displace the door-keeping cations and fully access the inner zeolite porosities is identified for each sample, with frameworks containing heavier cations requiring higher critical temperatures. At 77 K, both MER and CHA frameworks show D₂ adsorption amounts nearly double those of H₂. Kinetic studies indicate slower diffusion for D₂ molecules through these porous structures compared to H₂, particularly at pressures below 100 mbar. This research suggests that D₂ is more likely to access a broader range of diffusion pathways than H₂, due to the presence of the trapdoor cations restricting certain pathways during gas sorption. This leads to differing kinetic responses, increased adsorption amounts and prolonged equilibration times for D₂, indicating potential for effective separation of H₂ and D₂ using the molecular trapdoor mechanism.