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Adsorption Sites and Diffusion Rates of Benzene in HY Zeolite by Force Field Based Simulations

Summary: Adsorption Sites and Diffusion Rates of Benzene in HY Zeolite by Force Field Based
Fabien Jousse,*,, Scott M. Auerbach, and Daniel P. Vercauteren
Computational Chemical Physics Group, Institute for Studies in Interface Science, Faculte´s UniVersitaires
Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium, and Department of Chemistry and
Department of Chemical Engineering, UniVersity of Massachusetts, Amherst, Massachusetts 01002
ReceiVed: October 6, 1999; In Final Form: December 15, 1999
A variety of force field based simulations have been used to study the location and diffusion of benzene
adsorbed in a model zeolite HY (Si/Al ) 2.43), namely: molecular docking; equilibrium and nonequilibrium
molecular dynamics; and Monte Carlo umbrella sampling. Multiple adsorption sites are found, with benzene
facially coordinated to one or two H(1) or H(2) protons in the supercage. Some slight adsorption onto the
12-membered ring windows is also observed, in accordance with infrared measurements. The minimum energy
path at low temperature proceeds via a creeping of the molecule along the zeolite wall between stable sites,
with an activation energy varying between 10 and 20 kJ mol-1. This type of creeping motion is observed
both for intracage and intercage diffusion. Cartwheel jumps between sites are seen to proceed with higher
activation energies of approximatively 30 kJ mol-1. Multiple paths from site to site open as the temperature
increases. This results in a strong temperature dependence of the potential of mean force in the zeolite cage,
as calculated by umbrella sampling. Nonequilibrium molecular dynamics simulations initialized at the transition
state between two states show that the molecules do not relax in a single final state but in a multiplicity of
states; only cage-to-cage jumps keep a sense, as a majority of molecules relax in the final cage. Due to the


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


Collections: Chemistry