MetalâOrganic Frameworks for Thin-Layer Chromatographic

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Metal-Organic Frameworks for Thin-Layer Chromatographic Applications Claudia Schenk, Christel Kutzscher, Franziska Drache, Stella Helten, Irena Senkovska, and Stefan Kaskel ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13092 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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ACS Applied Materials & Interfaces

Metal-Organic Frameworks for Thin-Layer Chromatographic Applications Claudia Schenk, Christel Kutzscher, Franziska Drache, Stella Helten, Irena Senkovska, and Stefan Kaskel* Inorganic Chemistry I, Technische Universität Dresden, Bergstr. 66, 01062 Dresden, Germany KEYWORDS: TLC, metal-organic frameworks, zirconium based MOFs, DUT-67, planar chromatography

ABSTRACT. Preparation of thin-layer chromatographic (TLC) plates based on metal-organic frameworks (MOFs) as porous stationary phases is described. DUT-67 (DUT = Dresden University of Technology), a zirconium based MOF, was used in combination with a fluorescent indicator as stationary phase for analyzing a small selection of a wide spectrum of relevant analytes. The successful separation of benzaldehyde from trans-cinnamaldehyde and 4aminophenol from 2-aminotoluene is reported as a model system using optimized eluent mixtures containing acetic acid.

Thin-layer chromatography (TLC) is a well-established technique used for separation and identification of chemical compounds1–3 amongst other in food analysis, pharmaceutical and forensic applications. The majority of stationary phases employed in different types of

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chromatography are based on porous silica gel which is the most important material involved in this technique.3 Latest developments, such as high-performance thin-layer chromatography (HPTLC)6, ultrathin-layer chromatography (UTLC),7,8 forced-flow layer chromatography (FFLC),9 and innovations in detection methods (e.g. TLC-MS10 and densitometry6,11) led to an increase of efficiency and more reproducible performance due to smaller particles, thinner layers and therefore to lower achievable level of detection and faster separations.3,11 Preferred materials for stationary phases in HPTLC/UTLC are also often based on silica gel (i. a. chemically bonded silica gel for RP-TLC and monolithic silica gel for UTLC).3,8 However, development of new materials such as porous polymers, electrospun polymeric nanofibers, or carbon materials is ongoing.12–16 In recent years, metal-organic frameworks (MOFs) are explored as stationary phase for gas and liquid chromatography, since they provide on a molecular level well defined structure, high accessible inner surfaces with adjustable properties, and variable pore sizes. To the best of our knowledge, MOFs have not been reported for TLC applications yet, although their properties fit to the requirements on those materials.15, 17–20 An usage of MOFs as stationary phase in TLC can also offer new input in various other applications. Results obtained from separation and identification of organic compounds can be used to determine a proper solvent system as it is important for transferring TLC data into retention times for column chromatography separations.21 In the following, we report the first successful application of a MOF (DUT-67) as stationary phase in TLC for the separation of two different organic mixtures. With those initial investigations, we demonstrate the qualification of this material for further chromatographic applications.


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DUT-67 is a white, porous, hydrolytically stable, zirconium based MOF in which eightconnected clusters are linked by 2,5-thiophene-dicarboxylate (tdc2-) into a 3D open framework structure (Figure S1 ESI).22,23 Formate anions cover the residual sites of the cluster and may be used to tune the polarity of the MOF. DUT-67 is chosen because of its good chemically stability against water and organic solvents, which is an important requirement for application in TLC separations (for synthetic details see ‡).24–27 The particle size of the DUT-67 crystals (truncated octahedral) as estimated by SEM is 0.76 µm in average (Figure 1). A narrow crystal size distribution resulting from the optimized synthesis protocol is essential for using the material as stationary phase and for reproducibility of TLC separations. The slurry containing DUT-67, hydroxypropyl cellulose, styrene-butadiene-rubber, and F254 in ethanol was spread on the aluminum plate using automatic spreading device (for more details see ‡ and ESI). The addition of F254 indicator allows to detect compounds absorbing UVlight at 254 nm. The indicator content was optimized in a series of experiments (Figure S2 ESI). The thickness of the coating was 125 µm in average.

Figure 1. SEM image of as synthesized DUT-67 (left) and composite material (right). Powder X-ray diffraction (PXRD) proofs a high degree of crystallinity for DUT-67 after composite drying (Figure 2). However, the specific surface area and pore volume decreased from


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1000 m2g-1 and 0.42 cm3g-1 (for pristine DUT-67) to 500 m2g-1 and 0.20 cm3g-1 (for composite), due to the additional amount of non-porous components (Figure 2). Taking the reduced active mass into account, the specific surface area and pore volume for the DUT-67 fraction in the composite amount to 781 m2g-1 and 0.31 cm3g-1, respectively. This is an excellent result, considering porosity drop by the binders typically causing partial pore blocking. Most remarkably, the chromatographic films show a high chemical stability in the presence of organic solvents as it is required for TLC (e.g. ethyl acetate and iso-hexane) as well as in eluents used for separations (benzene – acetic acid (20:1) and acetonitrile – water – acetic acid (1:1:4)). It is evidenced by PXRD control measurements and visual inspection of the plates after soaking in the selected solvents for one hour (Figure S4 ESI) or after separation performed (Figure S5 ESI).

Figure 2. Left: PXRD patterns of DUT-67: calculated (blue), as made (black) and of composite (grey). Right: Nitrogen physisorption isotherms of as synthesized DUT-67 (black) and composite material (grey). Initial separation trials using the novel MOF-based TLC plates were performed for example using benzaldehyde and trans-cinnamaldehyde as analytes. Up to now, the majority of natural


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benzaldehyde is mostly produced via hydrolysis of trans-cinnamaldehyde from natural cinnamon oil, and thus the separation of these two compounds is also of industrial relevance.28–30 Furthermore, the aldol condensation reaction of benzaldehyde and acetaldehyde results in transcinnamaldehyde and TLC is frequently used to track the reaction process.31 In addition, separation of benzaldehyde from trans-cinnamaldehyde is challenging due to their similar physical properties and new separation methods are still under development.32 On commercial non-functionalized silica plates the separation of benzaldehyde and transcinnamaldehyde is performed using a mixture of benzene, ethyl acetate, and acetic acid as mobile phase.5 On DUT-67 plates a reasonable separation with a solvent mixture of benzene and acetic acid (20:1) could be achieved (Figure 3a). The retardation factors (Rf) for benzaldehyde and trans-cinnamaldehyde are 0.17 and 0.55, respectively. Good results were obtained using 2,4-dinitrophenylhydrazine as staining reagent (resulting in the formation of the corresponding coloured hydrazones).5

a)

b)

Figure 3. a) Separation of benzaldehyde (green circle) and trans-cinnamaldehyde (red circle) on DUT-67 TLC plates with a mixture of benzene and acetic acid (20:1) as eluent; b) Separation of 4-aminophenol (green circle) and 2-aminotoluene (red circle) on DUT-67 based TLC plates with acetonitrile, water, and acetic acid (1:1:4) as eluent.


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As a second model separation to evaluate MOF TLC plates, we chose 4-aminophenol and 2-aminotoluene as analytes, two molecules varying only slightly with respect to the substitutional pattern (Figure 3b). For separation of these two compounds on silica containing TLC plates, various compositions of the eluent mixture are recommended (e. g. dibutyl ether, ethyl acetate, and acetic acid (10:10:1) or benzene and methanol (19:1)).5 However, none of these mixtures led to the separation of 4-aminophenol from 2-aminotoluene on the DUT-67 TLC plates. Moderate separation results could be achieved using an eluent mixture containing acetonitrile, water, and acetic acid (1:1:4) (Figure 3b). As staining reagent a solution of vanillin and sulfuric acid5 was used resulting in an Rf value of 0.16 for 4-aminophenol and 0.24 for 2aminotoluene. These relatively low Rf values point out the strong interactions between the analytes and the stationary phase. These can be explained by the presence of coordinating sites in the cluster of DUT-67 saturated by monocarboxylic acid or solvent molecules. The amino groups of both analytes as well as the hydroxyl group of 4-aminophenol represent potential concurrent ligands, retarding the elution process and causing an eluent with higher elution power to achieve a transport through the stationary phase. In case of the oxophilic DUT-67 such eluents are acetic acid and water, which are stronger binding ligands as compared to amines, consequently reducing the interaction strength between analytes and DUT-67. Due to small differences in substrate-MOF interactions separation of the isomers is finally achieved. Both separations show the high interaction strength between the chosen analytes and DUT-67. As a result, low Rf values are obtained and eluents with high elution power are needed to achieve separation. An ingredient of both eluent mixtures is acetic acid, which decreases these interactions by coordination to DUT-67 clusters, promoting separation of various analytes.


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In summary, we demonstrated for the first time the integration of a metal-organic framework into homogeneous coatings and its successful application as stationary phase for TLC applications. The high chemical stability of the zirconium based DUT-67 provides reasonable separation performance of aromatic aldehydes and amines. A particular strength of our approach can be seen in the ability to rapidly monitor selective interactions of the MOF with a large number of substrates which has relevance for the development of advanced MOF applications such as catalysis, selective liquid phase adsorption, sensing, or HPLC separation.

ASSOCIATED CONTENT Supporting Information Additional information about the structure of DUT-67, TLC plates fabrication, details of separation procedure, and powder X-ray diffraction measurements for controlling the stability of DUT-67 composite in different solvents and used eluent mixtures. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes ‡ Synthesis of DUT-67: ZrCl4 (1.38 g, 6 mmol) and 2,5-thiophene-dicarboxylic acid (0.66 g, 4 mmol) were solved in 150 mL of DMF/NMP mixture (1:1) by sonication. Formic acid (26.8 mL, 0.71 mol) was added and the solution was heated at 120 °C for 3 days. The white product was filtered, washed with DMF and ethanol and dried at 80 °C for 16 h.


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Preparation of TLC plates: The slurry of all components (0.6 g of DUT-67, 13 mg of hydroxypropyl cellulose (HPC), 19.5 mg of styrene-butadiene-rubber (SBR), 0.3 g of F254, and 3 mL ethanol) was shook in a vibration ball mill for 10 minutes and subsequently was spread on aluminum plate using motorized film applicator.

ABBREVIATIONS DUT, Dresden University of Technology; FFLC, forced-flow layer chromatography; HPC, hydroxypropyl cellulose; HPLC, high-performance liquid chromatography; HPTLC, highperformance thin-layer chromatography; MOF, metal-organic framework; PXRD, powder X-ray diffraction; RP-TLC, reversed phase thin-layer chromatography; SBR, styrene-butadiene-rubber; SEM, scanning electron microscopy; tdc2-, 2,5-thiophene-dicarboxylate; TLC, thin-layer chromatography; TLC-MS, thin-layer chromatography – mass spectroscopy; UTLC, ultrathinlayer chromatography.

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MetalâOrganic Frameworks for Thin-Layer Chromatographic

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