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

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 multiplicity of possible cage-to-cage paths, the temperature dependence of the cage-to-cage rate constants is deeply non-Arrhenius.