SIMULATION OF ACTIVATED MOLECULAR PASSING EVENTS IN ZEOLITE NANOPORES
Amit Gupta and Randall Q. Snurr <http://catalysis115.cqe.nwu.edu/ >
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
. 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  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  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.
 Karger, J.; Ruthven, D.M., Diffusion in Zeolites and Other Microporous
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 Coppens, M.-O.; Bell, A.T.; Chakraborty, A.K., Chem. Eng. Sci. 1998,
 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.
 Theodorou, D.N. in Diffusion in Polymers, edited by P. Neogi, Marcel
Dekker, New York, 1996; pp. 67-142.