Dynamics of n-butane-methane mixtures in silicalite, using quasielastic neutron scattering and molecular dynamics simulations

The transport of n-butane-methane mixtures in the zeolite silicalite was st udied using molecular dynamics simulations and quasielastic neutron scatter ing experiments over a range of loadings and compositions at 200 K. Self-di ffusivities are seen to decrease monotonically with loading of either speci es. Self-diffusivity values calculated from the dynamics simulations are in excellent agreement with the experimental measurements from quasielastic n eutron scattering if one lakes into account the errors associated with both techniques. We also studied the detailed dynamical behavior of the system. The dependence on the wave vector of the halfwidth at half-maximum of the incoherent dynamic structure factor, describing self-correlations in the mo tion of sorbate molecules, is indicative of a jump diffusion process. By mo nitoring and analyzing the molecular motion in the simulation, we confirmed that diffusion takes place through successive jumps between the interiors of adjacent channel segments. Precise and quantitative calculations mapping the MD trajectories onto a coarse-grained jump model reveal mechanistic as pects of the motion. Distributions of jump lengths and rate constants are a ccumulated for the various jump types executed by each sorbate species. Jum p lengths are widely distributed between 0 and 15 Angstrom, the mean jump l ength being a decreasing function of the loading. The mean time between jum ps is actually smaller at higher occupancies, because there short jumps pre vail, occurring back and forth in a highly correlated fashion.