SIMULATION OF ACTIVATED MOLECULAR PASSING EVENTS IN ZEOLITE NANOPORES

Department of Chemical Engineering

Northwestern University, Evanston, IL 60208

Pore blockage in zeolites is important in deactivation by coking, in

shape-selective catalysis, and for the interpretation of diffusion data

[1]. Most modeling of pore blockage to date has employed lattice-model

Monte Carlo simulations, where the rate constants for hopping between sites

were fit to experimental data [1-4]. Similarly, rates of a small molecule

"passing" a larger molecule were taken as parameters. To provide a more

detailed picture of these passing events, as well as predictive

capabilities, we use atomistic molecular simulations [5] to study the

effect of the nature and location of pore blockage on the diffusion of

smaller molecules in silicalite. Pore blockages of various natures are

modeled as hexane, cyclohexane, and benzene, while the diffusing species is

chosen to be methane.

On the timescale of a molecular dynamics (MD) simulation benzene and

cyclohexane do not diffuse, enabling us to "place" them at certain

locations (channels or intersections) in silicalite and then study the

diffusion of methane as a function of the degree and position of the

blockage. This provides interesting information, which can be compared

against experimental data. However, these MD simulations are quite time

consuming. To focus attention on the rare passing events themselves, we

calculate minimum-energy and free-energy paths [6] for a methane molecule

approaching and passing a blockage molecule. We constrain the methane to a

series of planes along the path and perform either energy minimizations or

MD simulations of the constrained methane/blockage/zeolite system; the

blockage molecule is free to adopt any position or configuration during the

minimizations or MD but is essentially restrained by the zeolite framework

from moving too far from its initial position. The framework topology

provides necessary information on the general direction of the diffusion

paths, thus avoiding the necessity of first locating the transition states

for the passing events.

[1] Karger, J.; Ruthven, D.M., Diffusion in Zeolites and Other Microporous

Solids; Wiley: New York, 1992.

[2] Theodorou, D.; Wei, J., J. Catal. 1983, 83, 205-224.

[3] Tsikoyiannis, J.G.; Wei, J., Chem. Eng. Sci. 1991, 46, 233-253.

[4] Coppens, M.-O.; Bell, A.T.; Chakraborty, A.K., Chem. Eng. Sci. 1998,

53, 2053-2061.

[5] Theodorou, D.N.; Snurr, R.Q.; Bell, A.T., in Comprehensive

Supramolecular Chemistry, Vol. 7, edited by G. Alberti, T. Bein, Pergamon,

Oxford, 1996; pp. 507-548.

[6] Theodorou, D.N. in Diffusion in Polymers, edited by P. Neogi, Marcel

Dekker, New York, 1996; pp. 67-142.