Discriminative Separation of Gases by a “Molecular Trapdoor” Mechanism in Chabazite Zeolites

Abstract

Separation of molecules based on molecular size in zeolites with appropriate pore aperture dimensions has given rise to the definition of "molecular sieves" and has been the basis for a variety of separation applications. We show here that for a class of chabazite zeolites, what appears to be "molecular sieving" based on dimension is actually separation based on a difference in ability of a guest molecule to induce temporary and reversible cation deviation from the center of pore apertures, allowing for exclusive admission of certain molecules. This new mechanism of discrimination permits "size-inverse" separation: we illustrate the case of admission of a larger molecule (CO) in preference to a smaller molecule (N(2)). Through a combination of experimental and computational approaches, we have uncovered the underlying mechanism and show that it is similar to a "molecular trapdoor". Our materials show the highest selectivity of CO(2) over CH(4) reported to date with important application to natural gas purification.

Structural changes of synthetic paulingite (Na,H-ECR-18) upon dehydration and CO2 adsorption

“…The CO 2 uptake indicates that the CO 2 gas molecules must interact strongly with Na + cations in the 8R windows of ECR-18, drawing them out of their positions blocking the windows and creating an opening large enough for other CO 2 molecules to pass through the 8Rs into the neighbouring cage. This mechanism for gas separation has been called a trapdoor mechanism [8,9,11]. It permits adsorptive discrimination based on the magnitude of the interactions between different gas molecules and the gating cations, rather than the relative sizes of the molecules.…”

“…Typically, moderate selectivity for CO 2 over N 2 or CH 4 can be achieved over cationic zeolites such as Na-X [4] or Na-A [5], which result from the quadrupole moment of CO 2 and its polarisability. Recently, however, small pore zeolites of specific structures and compositions have been found to show very high selectivity for CO 2 over less polar gases such as CH 4 and N 2 due to a molecular 'trapdoor' effect that makes them strong candidates as sorbents for carbon capture [6][7][8][9][10][11][12].…”

“…[6] and K-, Rb-and Cs-chabazite [11,12] are examples of the former, Na-, K-, and Cs-forms of zeolite Rho [7][8][9][10] are examples of the latter. The presence of D8R windows in zeolite Rho, together with the ability of its framework to distort strongly, allows the D8R sites to bind the cations more closely, and permits trapdoor behaviour to occur up to higher gas pressures.…”

Cation Control of Molecular Sieving by Flexible Li-Containing Zeolite Rho

“…[14][15][16] Recent computational studies have demonstrated that extraframework cations can move in the presence of CO 2 , 17 making cation gating effects possible. [18][19][20][21][22][23][24] In the postulated 'trapdoor' mechanism, cations occupying window sites permit the passage of molecules such as CO 2 or H 2 O that interact strongly with cations but block the windows to molecules that interact more weakly (such as CH 4 or N 2 ). These 'trapdoor' 3 zeolites adsorb CO 2 from mixtures of CH 4 or N 2 with very high selectivity.…”

“…These 'trapdoor' 3 zeolites adsorb CO 2 from mixtures of CH 4 or N 2 with very high selectivity. Zeolites A 18,19 and chabazite 20,21 with particular cation compositions have been suggested to demonstrate such behaviour, as have certain forms of zeolite Rho [22][23][24] and structures related to Rho. 25,26 In zeolite Rho, 27 large lta cages are linked by double 8-ring structural units, which act as windows and are also the favoured sites for large univalent cations such as Na + , K + and Cs + (8-ring refers to a ring of 8 framework metal atoms and 8 oxygen atoms).…”