Adsorption Sites and Diffusion Mechanism of Alkylbenzenes in Large Pore Zeolite Catalysts as Predicted by Molecular Modeling Techniques

Abstract

In this work, we demonstrate the efficiency of force field energy minimization technique to study the adsorption and diffusion behavior of large molecules inside the micropores of zeolites. Molecular modeling studies for diffusion of alkylbenzene molecules, namely, ethylbenzene,p-diethylbenzene, isobutylbenzene, ando-,m-, andp-isobutylethylbenzene in various zeolites, such as faujasite, zeolite L, mazzite, and mordenite, indicate that mordenite is a good catalyst for selective synthesis ofp-isobutylethylbenzene. The periodic variations of interaction energy between the molecules and zeolite framework in the calculated diffusion energy profiles are used to predict the energy barrier for diffusion. Force field energy minimization calculations for the cage to cage diffusion of the alkylbenzenes in faujasite show no significant diffusional energy barrier for any of the molecule. Zeolite L shows a very small selectivity towardp-isobutylethylbenzene which is due to a rapid change in minimum energy configuration as the molecules diffuse along the pore. In the case of mazzite, a high diffusional energy barrier is observed foro-isobutylethylbenzene compared tom- andp-isomers. Calculations of the diffusion energy profiles for the molecules in mordenite show that there is negligible energy barrier for the diffusion ofp-isobutylethylbenzene, whereas an energy barrier of 17.95 kJ/mol exists for diffusion ofm-isobutylethyl-benzene and a significantly large energy barrier of 95.69 kJ/mol exists foro-isobutylethylbenzene. Thus, the efficiency of shape selective production ofp-isobutylethylbenzene in these zeolites will be in the order faujasite ∼ zeolite L<mazzite<mordenite. The adsorption of the molecules in general are energetically favorable when the alkyl groups have maximum interaction with the surface of the zeolite pores.