Kinetics of adsorption of pure and mixtures of linear and branched C6 alkanes onto silicalite by non-equilibrium molecular dynamics

The separation of saturated branched and linear alkanes may be investigated by selective adsorption on microporous organophilic MFI type zeolite. From the literature, adsorption equilibria of mixtures show a favourable selectivity for less branched alkanes [1]. However the separation process is significantly influenced by the transport properties. The knowledge of the transport coefficients of alkanes in the zeolite is then useful to optimize the separation process. The literature values of intracrystalline diffusion coefficients of hydrocarbons in MFI type zeolite are spread over a wide range of order of magnitude [2]. They depend on the experimental techniques, origin of zeolites... In this study we investigated kinetics of adsorption of n-hexane (HEX) and 2 methyl-pentane (2MP) on aggregates and infinite membrane of silicalite at a temperature of 440K using transient non equilibrium molecular dynamics simulations (TNEMD) [3]. This type of simulation gives access both to the kinetics of adsorption and to the equilibrium state. In a recent article [3] we presented a TNEMD study of the adsorption kinetics of n-butane on aggregates of silicalite. The same procedure is applied for HEX and 2MP on aggregates and extended to the study of infinite membranes to get both the Fick’s diffusion coefficients and the self-diffusion coefficients along the three crystallographic orientations. In the following, the directions a, b and c will refer to the crystallographic directions respectively [100], [010], [001] commonly used for the MFI type zeolite [3]. b is the direction of the straight channels, a and c are in the direction of zigzag channels. Details of the simulations are described in ref. [3].