Abstract
Metal–organic frameworks (MOFs) have demonstrated great utilizability in separation applications, as in the separation of small volatile compounds via gas chromatography (GC). In the present work, HKUST-1 (Hong Kong University of Science and Technology), one of the best investigated MOFs, is used as a stationary phase for the gas chromatographic separation of various analytes possessing different modes of interaction due to their differences in polarity and the presence of free electron pairs. The system was investigated by inverse gas chromatography (IGC) to demonstrate in general how MOF materials can be quantitatively and qualitatively characterized in respect to their Lewis basic and acidic properties. Applying IGC theory, the investigation of the separation problem of benzene and its completely hydrogenated analogue cyclohexane was used to the determine the donor properties of the MOF linker benzene-1,3,5-tricarboxylic acid and the separation of diethyl ether, diisopropyl ether, tetrahydrofuran, and di-n-propyl ether to determine the acceptor properties of the coordinatively unsaturated sites of the copper(II)-secondary building unit (SBU), i.e. the nodal points of the MOF lattice.
Highlights
IntroductionThe synthesis of a very large number of metal– organic frameworks with different and partially exotic topologies, that display, if the inapplicability of the standard BET analysis for microporous systems is ignored, BET values of up to 10 000 m2 g−1, is still the main objective of many research groups.[1,2,3,4,5,6,7] Beside synthesis, many potential applications of this class of materials connected to the structural variability in combination with crystallinity and permanent porosity as well as the possibility to tune pore size and geometry for specific applications are conceivable and were intensively discussed in recent years.[8,9,10,11,12,13,14,15,16,17] A specific field of Metal–organic frameworks (MOFs) application that allows to utilize these features are the various types of chromatography, such as liquid chromatography[16,17] and gas chromatography.[18,19,20]Recently, the separation of different analyte mixtures, such as those of xylene isomers,[19,21] n-alkanes,[20,22,23] polychlorinated biphenyls,[24] polycyclic aromatic hydrocarbons,[24] and branched alkanes[23,25] by MOF based capillary GC was Molecules, like aromatic compounds, can be coordinated by weak interactions such as π–π stacking and C–H–π interactions as largely explored in the field of supramolecular438 | CrystEngComm, 2015, 17, 438–447Paper chemistry.[38,39] The interaction between analytes and the crystalline lattice can be classified in several ways depending on the properties of the analyte
Characterization of the HKUST-1 coatings inside a capillary In contrast to the published procedures to deposit a thin film of a metal–organic framework in capillaries made from fused silica by other authors, we chose the approach to coat gas chromatography (GC) capillaries with HKUST-1 based on the controlled secondary building unit (SBU) approach (CSA)
In order to track the decrease of the film thickness, the amount of HKUST-1 was determined as described earlier by acid treatment and Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements
Summary
The synthesis of a very large number of metal– organic frameworks with different and partially exotic topologies, that display, if the inapplicability of the standard BET analysis for microporous systems is ignored, BET values of up to 10 000 m2 g−1, is still the main objective of many research groups.[1,2,3,4,5,6,7] Beside synthesis, many potential applications of this class of materials connected to the structural variability in combination with crystallinity and permanent porosity as well as the possibility to tune pore size and geometry for specific applications are conceivable and were intensively discussed in recent years.[8,9,10,11,12,13,14,15,16,17] A specific field of MOF application that allows to utilize these features are the various types of chromatography, such as liquid chromatography[16,17] and gas chromatography.[18,19,20]Recently, the separation of different analyte mixtures, such as those of xylene isomers,[19,21] n-alkanes,[20,22,23] polychlorinated biphenyls,[24] polycyclic aromatic hydrocarbons,[24] and branched alkanes[23,25] by MOF based capillary GC was Molecules, like aromatic compounds, can be coordinated by weak interactions such as π–π stacking and C–H–π interactions as largely explored in the field of supramolecular438 | CrystEngComm, 2015, 17, 438–447Paper chemistry.[38,39] The interaction between analytes and the crystalline lattice can be classified in several ways depending on the properties of the analyte. Analytes possessing a permanent dipole moment are characterized in addition to the nonspecific part of the van der Waals interaction (interactions with a 1/r6 dependence), which is the London- or dispersion interaction,[40] by the so called specific part which should be in the case of HKUST-1 mainly the Debye-interaction.[41] This type of interaction is defined as the angle averaged interaction between a permanent and an induced dipole. The other contribution to the specific part of the van der Waals interaction is the so called Keesom interaction,[42] which results from averaged interaction between two permanent dipoles
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