The temperature dependence of single-file separation mechanisms in one-dimensional nanoporous materials

Abstract

The mobility of fluids adsorbed in nanoporous materials with one-dimensional porous networks can be exploited to effect kinetic separations of small hydrocarbons. These nanopores provide an environment in which fluids, with similar bulk diffusivities, exhibit drastically different mobilities in the adsorbed phase. The basis of this difference in mobilities is linked to the two different modes of motion within the nanopore: ordinary uni-directional diffusion and single-file motion. This transport phenomenon has no bulk analog; it is a novel characteristic of fluids confined in nanoscale channels. The transition between these two fundamentally different modes of motion is a sensitive function of pore size, adsorbate size, density of the adsorbed phase, and the temperature. In this work, we investigate the temperature dependence of the transition from ordinary diffusion to single-file motion and apply it to a model separation system. Using molecular dynamics simulations of methane and ethane in the one-dimensional molecular sieve, AlPO 4-5, this work demonstrates, first, that temperature can be used to control whether a pure component undergoes ordinary or single-file motion. Second, we examine the temperature dependence of the diffusivities of components in a binary adsorbed mixture. Third, we demonstrate that the operating temperature places limitations on whether the separation of a mixture can be successfully obtained using this technique.