(395j) 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.

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