ANALYSIS OF MD TRAJECTORIES AS A JUMP DIFFUSION PROCESS - BUTENE ISOMERS IN ZEOLITE TYPES TON AND MEL

We have studied, by molecular dynamics, the self-diffusion of the four butene isomers in zeolite types TON and MEL at 623 K, for several loa dings. Even if both systems present low-energy barriers to the diffusi on (less than 10 kJ/mol), an essential difference appears between the two zeolite types. On one hand, TON presents unidirectional straight c hannels, and therefore there is almost no change of entropy during the migration of a guest molecule in the channel, so that their most prob able position at 623 K corresponds to their minimum energy position. O n the other hand, MEL presents intersecting straight channels, and whi le the minimum energy positions are located in the channels, the most probable positions are at the intersections, due to entropy effects wh ich are larger than the energy change at 623 K. Using transition rate constants for site-to-site jumps estimated from the molecular dynamics trajectories, we have modeled the behavior of the four isomers of but ene by a jump diffusion model (JDM). This appears to reproduce reasona bly well their meansquare displacement in zeolite MEL, both at infinit e dilution and at nonzero loading, due to the high free energy barrier s attributed to the entropy effects. In zeolite TON, the self-diffusiv ities computed from a JDM are systematically underestimated as compare d to those computed by molecular dynamics, due to the insufficient the rmalization of the molecules. To better represent this diffusion mecha nism, we have introduced a correlated jump diffusion model that accoun ts for insufficient thermalization by supposing that a given molecule has a larger probability to jump in the same direction as its previous jump than in the reverse direction. This correlated jump diffusion mo del reproduces well the diffusivity of cis-2-butene and isobutene in z eolite TON, but not that of trans-2-butene and 1-butene. The differenc e probably originates in the ''fitting'' of the guest molecules in the channels, as well as the guest-guest interactions.