Revisit the molecular sieving mechanism in LTA zeolites: does size really matter?

Abstract

“Molecular sieving”-based separation of similar-sized gases (e.g., CO₂, N₂, and CH₄) is both desirable and challenging due to the difficulty of obtaining adsorbents with pore sizes that permit exclusive admission. The “molecular trapdoor effect” offers a promising solution, focusing on the difference in gases’ ability to dynamically open a “door” via interaction with the “door-keeper” in adsorbents, rather than relying on size-sieving. In this study, we studied Na and K-exchanged zeolites with Si/Al ratios ranging from 1 to 2.2 and demonstrate that potassium form zeolite LTA with a Si/Al ratio of 2.2 (referred to as r2KLTA) exhibits the molecular trapdoor mechanism, as evidenced by CO₂/N₂ separation, gas adsorption, and in situ powder X-ray diffraction experiments. The K⁺ ion, acting as the door-keeper, is situated at the eight-membered ring (8MR) pore aperture of LTA, enabling the exclusive separation. Notably, this separation mechanism diverges from the traditional static sieving model and suggests that gas molecule admission is regulated by dynamic door-opening. In contrast to previous reports showing negligible CO₂ adsorption in r1KLTA (3 A zeolite), our findings reveal a significant CO₂ uptake, which points to the trapdoor mechanism as the key factor. This study offers new insights into the classical zeolite molecular sieve (3 A) for gas separation, where gas selectivity is governed by dynamic door-opening rather than static interactions. The demonstrated molecular trapdoor effect in r2LTA zeolites opens new possibilities for designing adsorbents with high selectivity and enhanced kinetics at optimal temperatures.

Graphical Abstract