Molecular Dynamics Study of Diffusion and Surface Permeation of Benzene in Silicalite

Molecular dynamics simulations have been carried out to determine the uptake and release rates for benzene in an idealized crystal of silicalite (the pure silica form of ZSM-5) with two external surfaces perpendicular to the straight channel [010]. A realistic model has been developed to simulate a system in which the kinetics are controlled by the combined effects of surface resistance (pore blocking at the external surface) and intracrystalline diffusional resistance. The system has been treated using periodic boundary conditions and contains a finite reservoir in which (for uptake calculations) the benzene molecules are initially located. For the calculation of the release, the benzene molecules are all located initially within the crystal, and the periodicity along the length of the reservoir is removed so that the molecules are released at zero pressure. The effect of a surface barrier has been investigated by considering three systems with different degrees of channel entrance blocking (0, 50, and 87.5%). The resulting calculations of uptake and release make it possible to estimate the relative importance of surface resistance (pore blocking) and intracrystalline diffusion in determining the sorption rate. It is shown that the classical model based on the formal solution of the one-dimensional diffusion equation, taking account of the finite rate of permeation through the crystal surface (i.e., surface resistance) via the boundary condition, provides a good representation of the kinetic behavior. For comparison, self-diffusion and tracer exchange are also simulated for the same system under comparable conditions.