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

Abstract

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.

Molecular simulations of the adsorption of cycloalkanes in MFI-type silica

“…50 For molecular dynamics simulations, it is generally believed that the influence of the flexibility is rather small for molecules that are small compared to the pore of the zeolite, 51,52 but much larger for hydrocarbons that fit tightly into the channels of the zeolite 53,54,56 or in cation-containing zeolites where vibrations of the framework, cation, and adsorbate are strongly coupled. 55 To investigate the influence of the flexibility of the zeolite framework on the adsorption properties of these rather bulky molecules we performed additional simulations in which the zeolite atoms where allowed to move. The results are presented in Fig.…”

Molecular origin of decreased diffusivity with loading for benzene/HY

“…Faujasite (FAU) is a solid acid zeolite that serves as the main active component of fluid catalytic cracking catalysts; the function of this zeolite is greatly affected by the diffusion properties of large aromatic molecules in it . Numerous experimental studies, as well as kinetic Monte Carlo (KMC) and molecular dynamic (MD) simulations, focused on the transport behavior of aromatics in FAU. In particular, studies concerning the coverage dependence of aromatic diffusion in FAU have been conducted widely.…”

Computer Simulation of Sorption and Transport in Zeolites

“…• Distinguishing explicitly Si from Al atoms by randomly distributing them in the frame up to the desired ratio Si/Al in such a way that Löwenstein's rule is fulfilled [21][22][23][24].…”