(174bk) Revisit the Molecular Sieving Behavior in Zeolite LTA for High-Performance Gas Separation

Chemical separation plays a vital role in the societal development, accounting for 15% annual total energy consumption globally. An efficient alternative technology to conventional distillation would bring huge benefits though reducing energy use, emissions, and pollution. Molecular sieving represents the most desirable approach for separation, where it discriminates molecules by size/shape and exclusively allows for certain component to enter, ideally achieving absolute separation. Zeolite LTA is the most prominent example of molecular sieves. For example, people have been long believing that zeolite 3A (K form) excludes molecules (e.g., CO₂ being 3.3 Å in kinetic diameter) larger than its pore aperture size of 3 Å, while zeolite 4A (Na form) with pore aperture size of 4 Å admits both CO₂ and N₂ (3.64 Å). Prominent researchers in this field have fine-tuned the aperture size by adjusting the relative composition of Na and K to be seemingly between CO₂ and N₂, realizing sieving between these two gases. Despite the exciting separation results, such a size-based sieving may not be the true underlying mechanism. Our recent discovery of molecular trapdoor mechanism in another small-pore zeolite molecular sieves, chabazite, should account for this scenario in LTA. In molecular trapdoor mechanism, different molecules are discriminated based on the ability of guest molecules to temporarily and reversibly move the “door-keeping” cations and thus to enter, rather than size-match. With this in mind, we revisited zeolite LTA and strikingly observed significant CO₂ admission in 3A although the kinetics is relatively slow, which is counter-intuitive from the conventional perspective of molecular sieving. We further proved that K-form LTA with reduced cation density (Si/Al = 2 vs. conventional Si/Al = 1) showed consistent results. Establishing the new understanding of non-size based sieving will allow for the developing next-generation molecular sieving adsorbents for highly efficient separation currently not possible.