Adsorption Of Xylene Isomers In MOF UiO-66 By Molecular Simulations

ADSORPTION OF XYLENE ISOMERS IN MOF UiO-66 BY MOLECULAR SIMULATIONS

 

Miguel Angelo Granato¹, Vanessa F. Duarte Martins¹, Alexandre Filipe P. Ferreira¹ and Alírio E. Rodrigues¹

 

¹ LSRE - Laboratory of Separation and Reaction Engineering – Associate Laboratory LSRE/LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal

Abstract

This work presents results of molecular simulations on adsorption of the four xylene isomers in the porous zirconium terephthalate UiO-66(Zr). The grand-canonical Monte Carlo simulations (GCMC) are compared to multi-component adsorption equilibrium data obtained by breakthrough experiments (Bárcia et al., 2011). Additionally, the adsorption behavior of each pure isomer was evaluated, in order to better understand the multicomponent adsorption. The adsorbate molecules were simulated using four different models:

-          TraPPE-UA (Wick et al., 2000): In this model, the aromatic pseudo-atoms [CH(aro) and R‑C(aro) for the link to aliphatic side chains] are treated as a single interaction center as well as the CH₃‑sp³ and the CH₂‑sp³ aliphatic pseudo-atoms.

-          OPLS (Jorgensen et al., 1993): It is an all-atom (AA) model used for the substituted benzenes except the CH₃-sp³ aliphatic groups which are treated as united atoms centered on the carbon.

-          Extended TraPPE (Wick et al., 2002): This new TraPPE–UA representation of the arenes contains a positive partial charge (q_center = +2.42 e), placed at the center of the ring, and two negative partial charges representing the p‑electron clouds (q_pi = -1.21 e), placed at a distance of 0.785 Å from the ring center and on a line perpendicular to the arene plane.

-          Charged‑AUA model introduced by Nieto-Draghi et al. (2007a, b). This model describes the interactions inside the p‑cloud of electrons by a positive partial charge (q_center = +8.13 e), and two negative partial charges (q_pi = -4.065 e), located at 0.4 Å above and below the ring plane on its hexagonal symmetry axis.

 The simulations confirm that the experimentally observed ortho-selectivity is preferential in relation to the other isomers. Access to the small cages seems to be favored for ortho-xylene due to its more compact bulky structure that provides more advantages in terms of rotation degrees of freedom. Furthermore, it was found that there is a competition between the other three isomers (para-xylene, meta-xylene and ethylbenzene) for adsorption in the big cages. Molecular simulations allow prediction of single component adsorption isotherms, heats of adsorption, and selectivity properties of xylene isomers mixtures which are essential for the development of adsorption based separation processes.

 

Figure1 .Example snapshot of four components adsorption in UiO66. Green: accessible surface; light blue: o-xylene; blue: p-xylene; dark blue: m-xylene; dark red: ethylbenzene.

References

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