Capturing the concentration dependence of trans-2-butene diffusion in silicalite-2 zeolite with a jump diffusion model

Molecular dynamics simulations of the diffusion of trans-2-butene in zeolite type MEL at 623 K have revealed an initial increase of the self-diffusivity with increasing loading, in contrast to simulation data collected for the other butene isomers. This is usually the signature of repulsive guest-guest interactions. At higher loadings, however, the concentration dependence was shown to decrease in a way consistent with attractive guest-guest interactions (Jousse, F.; et al. J. Phys. Chem. B 1997, 101, 4717). The initial rise reaches a maximum of about 25% for a loading of 1.5-2 molecules per unit cell. A jump diffusion model with parameters deduced from the molecular dynamics simulations accurately reproduces the diffusivity simulated at infinite dilution. For higher loadings guest-guest interaction parameters must be included in the jump diffusion model. Two simple models are presented, in order to rationalize the simulation results with a small number of parameters. The simpler model considers spherically symmetric adsorption sites and uses only two parameters; it is shown, however, that this model is unable to account for the simulated concentration dependence of the trans-2-butene self-diffusivity. The second model, with three parameters, includes the channel-like structure of the adsorption site and fits very effectively the concentration dependence of the diffusivity. Repulsive interactions between two molecules in the same site are responsible for the initial increase of the diffusivity. Attractive interactions arise when more than two molecules are in the same site, leading to a steeper decrease of the diffusivities with increasing loading.