Self-diffusion of n-alkanes in MFI-type zeolites: A Molecular Dynamics study and a comparison to Quasi-Elastic Neutron Scattering experiments

MFI-type zeolites [1] are compounds defined by the chemical formula NaxAlxSi96xO192. Sodium cations are free while Aluminium is substituting Silicium of the structure. When the ratio Si/Al>1000, the zeolite is named silicalite, while below, one calls it NaZSM5 zeolite. These MFI-type zeolites can be represented by a network of interconnected channels. The distance between two intersections is roughly 1.0nm while the diameters of channels are about 0.55nm. There are two groups of channels: Straight and sinusoidal ones. This channels' diversity leads to a highly anisotropic diffusivity. Zeolites should be considered as open framework materials, and they are gaining increasing importance in industrial applications such as catalysis, but also separation by sieving when the dimensions of the pores are close to those of the molecules. A crucial role is played in these processes by adsorption and transport of the guest molecules. We present a study of the equilibrium diffusion of a set of n-alkanes (methane, nbutane, n-hexane, n-octane and n-decane) in MFI-type zeolites at 300K. The work can be divided in two parts: A Molecular Dynamics (MD) simulation of the self-diffusion coefficients of the n-alkanes set in the purely siliceous (ratio Si/Al infinite) silicalite zeolite. A comparison of MD computations to Quasi-Elastic Neutron Scattering (QENS) experiments.