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 studied using molecular dynamics simulations and quasielastic neutron scattering experiments over a range of loadings and compositions at 200 K. Self-diffusivities are seen to decrease monotonically with loading of either species. Self-diffusivity values calculated from the dynamics simulations are in excellent agreement with the experimental measurements from quasielastic neutron 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 motion of sorbate molecules, is indicative of a jump diffusion process. By monitoring 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 aspects of the motion. Distributions of jump lengths and rate constants are accumulated for the various jump types executed by each sorbate species. Jump lengths are widely distributed between 0 and 15 Angstrom, the mean jump length being a decreasing function of the loading. The mean time between jumps is actually smaller at higher occupancies, because there short jumps prevail, occurring back and forth in a highly correlated fashion.