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

The transport of n-butane-methane mixtures in the zeolite silicalite has be en studied. We have used long molecular dynamics simulations for the calcul ation 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-but ane from 2 to 9 molecules per unit cell causes the diffusivity of methane t o 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 inter iors and occupy the intersection regions. A conformation analysis indicates that, at high methane concentrations, n-butane molecules are forced to pop ulate preferentially the gauche conformation. We have identified an anomalo us diffusion regime for both species at higher loadings. Interestingly, ano malous effects are more pronounced for methane than for n-butane in all thr ee directions, but most strongly in the z-direction, along which no direct channel pathway exists. Crossover to normal "Fickian" diffusion occurs at t imes on the order of nanoseconds. Visualization of trajectories from the dy namic simulations reveals a jumplike character of intracrystalline motion. We have studied the interaction energies for each species in each of the th ree silicalite environments. Sorbate-sorbate energy distributions show a st rong concentration dependence.