Tubular metal–organic framework-based capillary gas chromatography column for separation of alkanes and aromatic positional isomers
Highlights
► MOF-CJ3 was chosen as a stationary phase to prepare a capillary GC column. ► The column offered good separation of linear and branched alkanes. ► The column offered good separation of aromatic positional isomers.
Introduction
Metal–organic frameworks (MOFs) are a new class of porous materials with large surface area, high crystallization degree, diverse structures, and tunable pores, which can be applied for discrimination, exchange, adsorption, and separation of certain molecules [1], [2], [3], [4], [5]. Recently, research focus of MOF materials has gradually evolved from structure and property studies to real potential applications in liquid and gas chromatography, which have drawn extensive attention [6], [7], [8], [9], [10], [11], [12].
Based on organic ligands, MOFs used as stationary phases in chromatography can be divided into two types. The first is constituted by 1,4-benzenedicarboxylate (BDC)-derived ligands such as MOF-5 [13], [14], MIL-47 [15], [16], MIL-53 [16], [17], [18], [19], MIL-101 [20], IRMOF-3 [21], MOF-508 [22], and a MOF including Cd2+ ions [23]. Aromatic analytes and natural gas components, especially ethyl benzene (EB) and xylene isomer, have been well separated on these stationary phases. Another type of MOFs comprises imidazole-based ligands, i.e., ZIF-8, which showed good ability to separate alkanes via molecular sieving mechanism [24], [25]. Similar to BDC ligands, 1,3,5-benzenetricarboxylate (TBC) ligands contain a conjugate benzene ring and multi-carboxylate groups for coordination and hydrogen bonds, which are suitable for constructing various MOFs. However, MOFs with TBC ligands have rarely been studied as stationary phases for chromatography, except for the notable HKUST-1 that well-separated small hydrocarbons in gas chromatography [14], [26].
Many MOFs used as stationary phases have a cubic crystal system [13], [14], [16], [24]. In these crystals, same pores exist in three directions (3D channel), and mobile analytes in these pores cannot be restrained in a certain direction. In comparison, tubular MOFs contain one-dimensional (1D) channels in which the pore's wall is filled with organic ligands and the sizes of the pore's entrance window and inner diameter are equal. This structural peculiarity endows tubular MOFs suitable for chromatographic separation. The analytes in a tube may easily move in and out, and their movements are restrained in one direction, which can speed up the analytes’ separation processes. In addition, the conjugate planes in organic ligands are nearly parallel to the pore direction in tubular channels and may be favorable for π⋯π and C
H⋯π interactions between analytes and the pore wall.
Based on the aforementioned points, a tubular MOF, namely, MOF-CJ3 with TBC ligands [27] was selected as gas chromatography stationary phase. As shown in Fig. 1A and B, MOF-CJ3 possesses a 1D tubular channel extending along the c direction. The pore wall is composed of TBC ligands, which can provide benzene rings and carboxyl groups to form supramolecular interactions. All the selected analytes in this work, for example, linear and branched alkanes, xylene isomers and EB, and aromatic positional isomers (such as cresol, hydroquinone, and dichlorobenzene) play significant roles in modern industry [28], [29], [30], [31]. The commercially common detection and separation methods for these compounds usually need long analysis time or lack of environmental concern [20]. Therefore, it is still very urgent to develop new GC stationary phases for separating them. Herein, the capillary GC columns with MOF-CJ3 as stationary phases offer baseline separation for most of them.
Section snippets
Chemicals and reagents
All chemicals and reagents were at least of analytical grade. Ultrapure water (18.2 MΩ cm) was obtained from a WaterPro water purification system (Labconco Corp., Kansas City, MO). Zn(NO3)2·H2O, glacial acetic acid, N,N-dimethylformamide (DMF) and 1,3,5-benzenetricarboxylic acid (Aladdin Chemistry Co. Ltd., Shanghai, China) were used to prepare MOF-CJ3. All the analytes and solvents such as xylene, dichlorobenzene, bromobenzonitrile, hydroquinone, cresol, chloronitrobenzene and nitrotoluene
Characterization of MOF-CJ3 and GC capillary column with MOF-CJ3
The experimental XRD pattern of MOF-CJ3 crystals was in good agreement with the simulated one [27] (Fig. 1C). TGA curve reveals that the MOF-CJ3 crystals are stable up to 250 °C [24]. The activated MOF-CJ3 gave a BET surface area of 525 m2 g−1. Fig. 1D shows SEM images of the cross section of the fabricated open tubular column and the coating on its inner wall. The micrographs demonstrate that small particles of MOF-CJ3 were coated on the inner wall of the capillary column. The MOF-CJ3 crystals
Conclusions
In general, MOF-CJ3, as a representative for MOFs with tubular structures, shows some effective and distinctive applications as a GC stationary phase for separation of alkanes and aromatic positional isomers. The MOF-CJ3-based GC column gave baseline separations of linear and branched alkanes with carbon number over 6, and a series of substituted aromatics, including cresol, hydroquinone, dichlorobenzene, bromobenzonitrile, chloronitrobenzene, and nitrotoluene isomers. Elution sequence of most
Acknowledgments
This work was financially supported by the National Natural Science Foundation of PR China (grants no. 21003053 and 21171059), the Natural Science Foundation of Guangdong Province (grant no. 10451063101004667), and Science and Technology Department of Guangdong (grant no. 2010B090300031).
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