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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 loadings. Even if both systems present low-energy barriers to the diffusion (less than 10 kJ/mol), an essential difference appears between the two zeolite types. On one hand, TON presents unidirectional straight channels, and therefore there is almost no change of entropy during the migration of a guest molecule in the channel, so that their most probable position at 623 K corresponds to their minimum energy position. On the other hand, MEL presents intersecting straight channels, and while the minimum energy positions are located in the channels, the most probable positions are at the intersections, due to entropy effects which are larger than the energy change at 623 K. Using transition rate constants for siteto-site jumps estimated from the molecular dynamics trajectories, we have modeled the behavior of the four isomers of butene by a jump diffusion model (JDM). This appears to reproduce reasonably well their meansquare displacement in zeolite MEL, both at infinite dilution and at nonzero loading, due to the high free energy barriers attributed to the entropy effects. In zeolite TON, the self-diffusivities computed from a JDM are systematically underestimated as compared to those computed by molecular dynamics, due to the insufficient thermalization of the molecules. To better represent this diffusion mechanism, we have introduced a correlated jump diffusion model that accounts 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 model reproduces well the diffusivity of cis-2-butene and isobutene in zeolite TON, but not that of trans-2-butene and 1 -butene. The difference probably originates in the "fitting" of the guest molecules in the channels, as well as the guest-guest interactions.
|Number of pages||16|
|Journal||Journal of physical chemistry B|
|Publication status||Published - 12 Jun 1997|