Chromatographic separation through confinement in nanocages Stijn Van der Perre a , Tim Duerinck a , Pieterjan Valvekens b , Dirk E. De Vos b , Gino V. Baron a , Joeri F.M. Denayer a,⇑ a Vrije Universiteit Brussel, Department of Chemical Engineering, Brussel, Belgium b Katholieke Universiteit Leuven, Centre of Surface Chemistry and Catalysis, Leuven, Belgium article info Article history: Available online 24 August 2013 Dedicated to Dr. Michael Stöcker on the occasion of his retirement as Editor-in-Chief of Microporous and Mesoporous Materials. Keywords: Chromatography Adsorption Metal-organic frameworks Confinement Nanocages abstract Porous materials such as metal-organic frameworks offer great potential for separation technologies. Over the past decade the synthesis of a large number of new structures has been reported but relatively little attention has been given to the application of these novel adsorbents as stationary phases in gas or liquid chromatography. In this work, we report on the potential of the metal-organic framework UiO-66 for the separation of cyclic isomers. The extremely high preferential retention of cyclic hydrocarbons of the aromatic’s and functionalized cycloalkane’s type is exploited to achieve difficult separations. Confine- ment effects play a crucial role in fine-tuning the adsorbent’s properties. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Metal-organic frameworks (MOFs) have received interest in modern analytical chemistry for the use of this novel advanced class of microporous crystalline materials as stationary phase in chromatography. Because of their unique properties, such as a uni- form pore size distribution, an ordered pore structure and more variable pore functionality in comparison with the conventional (functionalized) silica or alumina stationary phases, they look promising as a chromatographic separation medium in analytical applications. Due to their hybrid inorganic–organic character, MOF-based stationary phases show different interactions with the analytes, opening more possibilities to separate different mix- tures. Several MOFs, such as MOF-5 (or IRMOF-1) [1–3], IRMOF-3 [2], ZIF-8 [3,4], MOF-508 [5], CUK-1 [6], MIL-100(Cr/Fe) [7], MIL- 101(Cr) [3,8], MOF-CJ3 [9], UiO-66 [10], JUC-110 [11], a charge- polarized MOF [12], HKUST-1 (or [Cu 3 (BTC) 2 ]) [13] and homochiral MOFs ([Cu(sala)] n [14] and D-camphoric acid based MOFs [15,16]), were successful tested as a stationary phase in gas chromatogra- phy (GC). Mostly, MOF crystals were deposited on the inner layer wall of a capillary column to fabricate the so-called PLOT columns (porous layer open tubular) [1–4,7–10,13–16] with the intention to obtain high-resolution GC separation(s) of target analytes in com- plex samples. These MOF-coated capillaries overcome the poor res- olution on MOF packed GC columns due to significant diffusion resistance of the bulky packing [2,4,8]. Important targets in indus- try and environmental sciences (alkane and aromatic positional isomers) were chosen as analytes. For example the separation of linear alkanes from branched isomers is a very important process in the petroleum refining industry, including the improvement of the gasoline octane number [17–22]. ZIF-8 showed strong power to sieve linear alkanes from branched alkanes owing to the narrow pore windows (3.4 Å) [3,4], while MOF-5 [3], MOF-508 [5] and MIL-100(Cr/Fe) [7] follow the order of the boiling points. The MIL-100(Fe) coated capillary column yields even better results for the separation of C6–C7 alkane isomers than the commercial GC-Gaspro and HP-5MS capillary columns, while the MIL-100(Cr) coated capillary gave a poor separation performance. Stronger energetic interactions between the alkanes and MIL-100(Cr) and loss of more freedom during separation lead to longer retention times, tailing chromatographic peaks and lower resolution on the Cr(III) variant [7]. Although MOF-CJ3 [9] does not display a molec- ular sieving effect, isooctane (bp: 372.2 K ) elutes earlier than n- heptane (bp: 371.4 K ) which is not in agreement with their boiling point. Probably supramolecular interactions (mainly CH–p interac- tions) with alkanes are involved, where the benzene rings are al- most parallel to the pore direction, which favors these supramolecular interactions. Unlike MOF-5, MOF-508 and MIL- 100(Cr/Fe), branched C6 isomers are more strongly retained in UiO-66 than their linear ones [10]. Its specific structure displays re- verse shape selectivity, which means that the adsorption of these alkane molecules is dictated by their rotational freedom inside the small cavities [23]. The selectivity for the GC separation of lin- 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.08.010 ⇑ Corresponding author. Tel.: +32 2 629 17 98; fax: +32 2 629 32 48. E-mail address: joeri.denayer@vub.ac.be (J.F.M. Denayer). Microporous and Mesoporous Materials 189 (2014) 216–221 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso