Adsorption of xylene isomers in MOF UiO-66 by molecular simulation Miguel Angelo Granato ⇑ , Vanessa Duarte Martins, Alexandre Filipe P. Ferreira, Alírio E. Rodrigues 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 article info Article history: Received 23 May 2013 Received in revised form 24 January 2014 Accepted 2 February 2014 Available online 14 February 2014 Keywords: Molecular simulation Adsorption Monte Carlo Xylenes separation UiO-66 abstract This work presents results of molecular simulations on adsorption of mixtures of the four xylene isomers in the porous zirconium terephthalate UiO-66. The grand-canonical Monte Carlo simulations (GCMC) are compared to multi-component adsorption equilibrium data obtained by breakthrough experiments. Four different force fields for the xylenes were evaluated. The simulations confirm that the experimentally observed ortho-selectivity is preferential in relation to the other isomers. Additionally, it was found that there is a competition between the other three isomers (para-xylene, meta-xylene and ethylbenzene) for adsorption in the UiO-66 structure. Molecular simulation is applied as a powerful research tool in pre- dicting adsorption equilibrium properties of potential adsorbent candidates for xylene isomers mixtures that are essential for the development of adsorption based separation processes. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction The recent synthesis of a new family of zirconium-based Metal Organic Frameworks (MOFs) with high surface area and excep- tional stability opened a wide range of possibilities on separation applications. The UiO-66 is based on a ZrO 6 (OH) 2 octahedron, and 1,4-benzene-dicarboxylate (BDC) linkers. Its cubic 3D-pore structure consists of an array of octahedral cavities of diameter 1.1 nm, and tetrahedral cavities of diameter 0.8 nm. The other two members of the family are UiO-67 with 4,4 0 -biphenyl-dicar- boxylate (BPDC) as linker, and UiO-68 with terphenyl-dicarboxyl- ate (TPDC) as linker [1]. Repetitive hydration/dehydration tests have revealed that the dehydroxylated and hydroxylated versions of UiO-66 are fully reversible. Molecular simulations and X-ray powder diffraction data have been applied to elucidate the struc- ture of dehydroxylated form of UiO-66, as well as its adsorption properties of CH 4 , CO, and CO 2 [2,3]. Further addition of substituents to the aromatic linker gener- ated a new series of isoreticular MOFs, based on the parent UiO- 66 structure, from three different linker ligands H 2 N-H 2 BDC, O 2 N-H 2 BDC, and Br-H 2 BDC. It has been demonstrated that this class of functionalized MOFs can retain their high thermal and chemical stabilities [4]. However, studies on application of UiO-66 for separation of mixtures are still incipient. The first experimental study conducted for hexane and xylene isomers mixtures shows that the adsorption order of structural isomers in UiO-66 is opposite to the one ob- served in conventional adsorbents [5]. This reverse shape selectiv- ity was also observed for the xylene isomers adsorption in liquid phase, using n-heptane as eluent [6]. 2. Molecular simulation methodology The Monte Carlo technique in the grand-canonical (lVT) ensem- ble (GCMC) has been extensively applied for calculations of adsorption properties, such as isotherms and heats of sorption. De- tailed explanation of this simulation technique can be found else- where [7]. The UiO-66 framework was considered rigid, and periodic boundary conditions were applied in all directions. The model was built from the X-ray diffraction data CIF file, deposited at the Cambridge Crystallographic Data Centre under code 733458 [1,8]. This structure represents the dry form of UiO-66, obtained after the activation procedure, which is described by Bárcia et al. [5]. The potential parameters for the non-metallic atoms were ta- ken from the DREIDING force field [9]. The Zirconium parameters were taken from the UFF force field [10] since they are not avail- able in the DREIDING force field. The adsorbate molecules were simulated using four different approaches. The first two are well known from the literature: 2.1. Model 1 The Transferable Potentials for Phase Equilibria-United Atom (TraPPE-UA) model [11] (Wick et al.). In this model, the aromatic pseudo-atoms [CH(aro) and R-C(aro) for the link to aliphatic side http://dx.doi.org/10.1016/j.micromeso.2014.02.014 1387-1811/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. Tel.: +351 22 508 1578; fax: +351 22 508 1674. E-mail address: mgranato@fe.up.pt (M.A. Granato). Microporous and Mesoporous Materials 190 (2014) 165–170 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso