Simulation of adsorption and diffusion of hydrocarbons in zeolites

Molecular simulations are used to investigate the energetics and siting of linear and branched alkanes in the zeolite silicalite. The calculated heats of adsorption of the branched alkanes are in good agreement with the experimental data. The simulations show a striking difference between the behaviour of linear and branched alkanes. The linear alkanes are relatively free to move in all channels of the zeolites. The branched alkanes are trapped with their CH group in the intersection of the zig-zag and straight channels of silicalite. This trapping of the branched alkanes suggests that diffusion of these molecules is an activated process; most of the time the molecule is located in the intersection but, occasionally, it hops from one intersection to another. The straight and zig-zag channels form a barrier for the diffusion. We present some preliminary calculations of this hopping rate, from which the diffusion coefficient can be calculated. These preliminary results are in reasonable agreement with experimental data.