Self-diffusion and permeability of methane, carbon dioxide, and their mixture in zeolite: Cage effect, fluid redistribution, and nanosegregation

Equilibrium and non-equilibrium molecular dynamics simulations are used to decipher the mechanisms that rule the diffusion and permeability of pure fluids and mixtures inside a cage-like nanoporous material. In more detail, we consider the case of methane, carbon dioxide and their mixture within the RHO zeolite – which is made up of nanometric cages connected by narrow subnanometric windows. The fluid molecular structure is found to significantly affect both the self and collective dynamics; while carbon dioxide (CO 2 ) transport slows down in a monotonous fashion upon increasing the fluid adsorbed amount n a , methane (CH 4 ) transport displays a non-monotonous behavior upon increasing n a . We identify that such differences arise from the fluid organization within the nanoporosity which strongly affects the characteristic caging time (or equivalently the transfer rate through the narrow windows). In particular, we show that the increase in the self-diffusivity for CH 4 at large n a arises from the fluid redistribution within the zeolite cages and windows. Upon investigating mixture transport as induced by a chemical potential gradient (or equivalently a pressure gradient), we find that local segregation of the mixture components significantly affects the induced flow. While a homogeneous flow is observed in the case of fully miscible mixtures, immiscible mixtures exhibit a marked different behavior as the two fluid components respond differently to the applied driving force. We show that such finding can be rationalized using a simple thermodynamic model based on the Gibbs-Duhem equation combined with Stokes equation for momentum conservation. • Molecular simulation is used to investigate the dynamics of gases and gas mixtures in zeolite. • Dependence of transport coefficients on adsorbed amount is either monotonous or non-monotonous. • Fluid redistribution and mixture miscibility drastically impact both self-diffusion and permeability of nanoconfined fluids. • Cage effects and its impact on transport are rationalized by considering residence times and free energy profiles.