High performance liquid chromatography of substituted aromatics with the metal-organic framework MIL-100(Fe): Mechanism analysis and model-based prediction
Introduction
Metal-organic frameworks (MOFs), also called porous coordination polymers (PCPs), are an emerging class of crystalline porous materials constructed with inorganic metals ions and organic functional linkers [1], [2]. A key fascinating feature of MOFs is that their structures (pore sizes, geometrics and chemical properties) can be rationally designed for specific targets [3], a possibility which does not exist in other porous materials to the same degree. The versatility of different MOFs leads to a wide range of promising applications in gas storage [4], separation [5], [6], catalysis [7], biomedicine [8], and so on [9]. Besides their tunable structures, the absence of dead volume, high specific surface area, high loading capacity of guest species and inherent porosity make MOFs promising materials as chromatographic stationary phases [10], [11], [12]. Recently, several MOFs have been successfully employed as high-performance liquid chromatography (HPLC) stationary phases to separate dissolved molecules. For example, HKUST-1 packed columns showed good separation performance of chemical isomers [13], [14]. Yan and co-workers [15], [16], [17], [18], [19] demonstrated the use of different MOFs, especially MIL-101(Cr), for HPLC of substituted aromatics. In the last two years, the application of MOFs from the UiO series in HPLC has been brought into special attention due to their excellent thermal and chemical stability [20], [21]. However, most of the previous studies are focused on MOF synthesis and present the chromatographic performance as “proof-of-concept”. What is often missing is the fundamental information about the adsorption interaction between the dissolved solutes and the MOF-based stationary phase, especially the equilibrium isotherms and diffusion parameters. However, for practical applications and model based optimization of the use of MOF based columns in HPLC systems these parameters are of great importance and must be known for in-depth understanding. Instead of the broad size and shape distribution of MOF powders fabricated by traditional solvothermal methods, the recently developed layer-by-layer liquid-phase epitaxy (LPE) process [22], [23] is a promising method to prepare homogenous MOF coatings, so-called surface-mounted MOFs (SURMOFs), with controllable thickness on the surface of a suitable functionalized substrate [24], [25].
In present work, MIL-100(Fe) was selected from the wide range of possible MOFs due to its suitability to generate SURMOFs and its stability in the presence of water. The reaction of iron(III) chloride (FeCl3) and 1,3,5-tricarboxylic acid (BTC) leads to a rigid 3D crystal structure with large pores, named MOF-100(Fe) by Horcajada et al. in 2007 [26]. Compared to other metal ions, Fe(III) is an ideal candidate to be used as the metal nodes in the assembling of MOFs, because of its low toxicity, easy accessibility, and most importantly, its robust Lewis acidity, which gives rise to strong coordinate covalent bonds with the organic linkers to form more stable MOFs. The MIL-100(Fe) structure (Fig. 1, inset) consists of 2.5 nm and 2.9 nm mesoporous pores with window diameters of ca. 0.47–0.55 nm and 0.86 nm, respectively. Moreover, MIL-100(Fe) possesses large accessible and permanent pores, and exhibits a remarkable thermal (>270 °C) and chemical stability (organic solvents or water). Additionally, the presence of the accessible coordinately unsaturated metal sites in the walls of MIL-100(Fe) structure allows the strong coordination of guest molecules. MIL-100(Fe) has been intensively studied for applications in separation [27], [28], catalysis [29] and drug delivery [30].
In this work, we present the synthesis of homogenous MIL-100(Fe) coatings onto the surface of magnetic microparticles (MPs) by a LPE process in order to yield a core–shell architecture (Fig. 1). The core–shell structured MIL-100(Fe) MPs were used as HPLC stationary phase to illustrate the MOF separation performance and mechanism of mixed aromatic hydrocarbons. The interaction between the organic linkers of the inner surface of the MOF shell and aromatic compounds results in their different affinities toward the stationary phase. First, these affinities were determined independently by single component HPLC experiments. Afterwards, the separation of mixtures of three different aromatic compounds was demonstrated using isocratic and stepwise elution modes. In this context, the capability of a recently published chromatographic software ChromX [http://mab.blt.kit.edu/chromx.php] to predict the chromatographic behavior of mixtures on the basis of single component equilibrium and kinetic data is shown. This allows an in-silico optimization of the separation as well as an in-depth understanding of the limiting parameters, pointing the way for future developments of MOF based stationary phases.
Section snippets
Chemicals and reagents
Carboxyl-functionalized magnetic silica particles (SiO2-MAG-S1964-COOH), with narrow size distribution of 4.7 ± 0.14 μm were purchased from microParticles GmbH, Berlin, Germany. Empty stainless steel HPLC columns (200 mm long × 1.0 mm i.d., column volume 180 μL, order code: N1910 0000) were bought from VDS optilab Chromatographie Technik GmbH, Berlin, Germany.
All reagents were at least analytical grade and used without further purification. Iron(III) chloride hexahydrate (FeCl3·6H2O, 99%),
Characterization of synthesized MIL-100(Fe) MPs
The as-synthesized MIL-100(Fe) MPs were characterized by XRD and SEM experiments. The successful growth of a MIL-100(Fe) shell onto the MPs is validated by the good agreement between the XRD pattern of the obtained MIL-100(Fe) MPs and the simulated one (Fig. 3a). The SEM images of the raw MPs and the prepared MIL-100(Fe) MPs are shown in Fig. 3b and 3c, respectively. Fig. S1 shows the images of these uniform uncoated and coted MPs with more numbers under a lower magnification. Fig. 3c displays
Conclusions
Crystalline MIL-100 (Fe) SURMOF thin films were coated onto magnetic microparticles having a COOH-terminated surface using the process of liquid-phase epitaxy (LPE). Chromatographic columns packed with the as-synthesized MIL-100(Fe) magnetic particles offer improved separation performance for aromatic hydrocarbons with increasing water content of the mobile phases (MeOH/H2O), due to the change of the affinity strength between the analytes and the crystalline MOF-based solid phase. Additionally,
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