Mass transfer resistance of noble gases in KFI zeolite: Insights from molecular simulations

Noble gases are commonly applied in chemical industry, working substance, and energy storage fields. Understanding how mass transfer resistance for noble gases inside zeolites forms and functions is beneficial for their extraction and utilization. Molecular simulations were performed to provide unique insights into the transport resistance of noble gases (Ne, Ar, Kr) within KFI zeolites. A monotonically increasing relationship is observed between the corrected diffusivities of the three noble gases and zeolite thickness, and these diffusivities gradually converge to their respective maximum values in infinitely long zeolites. Though the average aperture and cross-sectional area of the eight-membered ring window are slightly smaller in the flexible KFI zeolite compared with its rigid counterpart, the three noble gas species diffuse more rapidly, attributed to the framework's local outward expansion. The corrected diffusivity of Ne is the largest, followed by that of Ar, with Kr being the smallest. Among three gases, Ne possesses the smallest kinetic diameter, the lightest molecular mass, the weakest host-guest interactions and the lowest activation energy, which endows it with the minimum mass transfer resistance. At higher temperatures, Ne, Ar and Kr receive enhanced kinetic energy, which helps them overcome energy barriers and accelerates their diffusion processes. The critical membrane thicknesses for both Ne and Ar fall within the range of several tens of nanometers, which is nearly half the value observed for Kr. This study conducts a detailed analysis of mass transfer resistance of noble gases in KFI zeolites, and advances the design and development of membrane technologies.