(395j) Adsorption Of Xylene Isomers In MOF UiO-66 By Molecular Simulations

Authors: 
Granato, M. A., LSRE - Laboratory of Separation and Reaction Engineering - Associate Laboratory LSRE/LCM, Faculdade de Engenharia - Universidade do Porto
Martins, V. F. D., LSRE - Laboratory of Separation and Reaction Engineering – Associate Laboratory LSRE/LCM, Faculdade de Engenharia - Universidade do Porto
Ferreira, A., LSRE - Laboratory of Separation and Reaction Engineering - Associate Laboratory LSRE/LCM
Rodrigues, A. E., University of Porto - Faculty of Engineering



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 (qcenter = +2.42 e), placed at the
center of the ring, and two negative partial charges representing the p‑electron clouds
(qpi = -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 (qcenter = +8.13 e),
and two negative partial charges (qpi = -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.

UIO66-4comp_Pat_2.5_398b.jpg

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

Bárcia, P.S.; Guimarães, D.; Mendes,
P.A.P.; Silva, J.A.C.; Guillerm, V.; Chevreau, H.; Serre, C.; Rodrigues, A.E. ?Reverse
shape selectivity in the adsorption of hexane and xylene isomers in MOF
UiO-66.? Microporous and Mesoporous Materials, 139, 67‑73
(2011
).

Jorgensen, W.L.; Laird, E.R.; Nguyen, T.B.;
Tirado-Rives, J. ?Monte Carlo Simulations of Pure Liquid Substituted Benzenes
with OPLS Potential Functions.? J. Comput. Chem., 14, 206‑215
(1993).

Nieto-Draghi, C.; Bonnaud P.; Ungerer, P. ?Anisotropic
United Atom Model Including the Electrostatic Interactions of Methylbenzenes.
I. Thermodynamic and Structural Properties?. J. Phys. Chem. C, 111,
15686‑15699 (2007a).

Nieto-Draghi, C.; Bonnaud P.; Ungerer, P. ?Anisotropic
United Atom Model Including the Electrostatic Interactions of Methylbenzenes.
II. Transport Properties?. J. Phys. Chem. C, 111, 15942‑15951
(2007b).

Wick, C.D.; Martin, M.G.;
Siepmann, J.L. ?Transferable Potentials for Phase Equilibria. 4. United-Atom
Description of Linear and Branched Alkenes and Alkylbenzenes.? J. Phys.
Chem. B.
104, 8008‑8016 (2000).

Wick, C.D.; Siepmann, J. I.; Klotz, W. L.; Schure. M.
R. ?Temperature effects on the retention of n-alkanes and arenes in
helium?squalane gas?liquid chromatography Experiment and molecular simulation?.
J. Chromatogr. A, 954, 181?190 (2002).

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