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Capturing the Concentration Dependence of trans-2-Butene Diffusion in Silicalite-2 Zeolite with a Jump Diffusion Model

Summary: Capturing the Concentration Dependence of trans-2-Butene Diffusion in Silicalite-2 Zeolite
with a Jump Diffusion Model
Fabien Jousse,*,, Scott M. Auerbach,,§ and Daniel P. Vercauteren
Departments of Chemistry and Chemical Engineering, UniVersity of Massachusetts,
Amherst, Massachusetts 01003, and Computational Chemical Physics Group,
Faculte´s UniVersitaires Notre-Dame de la Paix, Rue de Bruxelles, 61, B-5000 Namur, Belgium
ReceiVed: January 23, 1998; In Final Form: June 2, 1998
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


Source: Auerbach, Scott M. - Department of Chemistry, University of Massachusetts at Amherst


Collections: Chemistry