Molecular Simulation Insights on Xe/Kr Separation in a Set of Nanoporous Crystalline Membranes.

Separation of xenon and krypton is highly relevant to several applications such as spent nuclear fuel processing. Molecular simulation has been extensively used to understand the Kr/Xe separation performance of nanoporous materials for adsorption-based technologies but less frequently for membrane-based technologies. Motivated by recent experimental reports on krypton-selective membranes, herein, we present grand canonical Monte Carlo and biased molecular dynamics simulations (using adaptive biasing force) to elucidate the nature of adsorption- and diffusion-based Kr/Xe separation mechanisms in a set of nanoporous materials: SAPO-34, ZIF-8, UiO-66, and IRMOF-1. Xenon is found to preferentially adsorb on all materials, but diffusion selectivity for krypton is found to dominate the overall membrane separation selectivity. To increase adsorption selectivity for krypton, large pore cages are found to be desirable. To increase diffusion selectivity for krypton, stiff pore windows with a diameter smaller than xenon (but larger than krypton) are found to be desirable. No perfect molecular sieving was found, but the relatively rigid SAPO-34 was more effective at excluding xenon than the more flexible ZIF-8. Indeed, during xenon "window crossing," the SAPO-34 window opened to only 3.8 Å, while the ZIF-8 window opened to 4.1 Å, resulting in a lower free energy "diffusion" barrier for xenon in ZIF-8. Therefore, an ideal membrane material for Kr/Xe separation should be rigid and have large pore cages and small pore windows. Temperature was found to have opposite effects on adsorption and diffusion selectivity, but because of the dominance of diffusion selectivity, our simulations indicate that it is preferable to operate membranes for Kr/Xe separation at lower temperatures than at higher ones.