Molecular dynamics simulation of n-butane-methane mixtures in silicalite

The transport of n-butane-methane mixtures in the zeolite silicalite has been studied. We have used long molecular dynamics simulations for the calculation of diffusion tensor components for both species over a wide range of loadings and compositions at 300 K. Self-diffusivities are seen to decrease monotonically with loading of either species. Raising the loading of n-butane from 2 to 9 molecules per unit cell causes the diffusivity of methane to drop by a factor of 60. The spatial distribution of molecules of the two co-adsorbed species was investigated, showing that, at high occupancies, n-butane molecules force methanes to partially abandon straight channel interiors and occupy the intersection regions. A conformation analysis indicates that, at high methane concentrations, n-butane molecules are forced to populate preferentially the gauche conformation. We have identified an anomalous diffusion regime for both species at higher loadings. Interestingly, anomalous effects are more pronounced for methane than for n-butane in all three directions, but most strongly in the z-direction, along which no direct channel pathway exists. Crossover to normal "Fickian" diffusion occurs at times on the order of nanoseconds. Visualization of trajectories from the dynamic simulations reveals a jumplike character of intracrystalline motion. We have studied the interaction energies for each species in each of the three silicalite environments. Sorbate-sorbate energy distributions show a strong concentration dependence.