Abstract:

Metal-organic frameworks (MOFs) are a new class of porous materials, which are synthesized using organic ligands and inorganic metal ions or metal clusters. MOFs possess tunable structures through the self-assembly of a large number of organic linkers and metal nodes, which is beyond the scope of conventional porous materials. In addition, MOFs have excellent properties, including the lowest density (as low as 0.13 g/cm), highest specific surface area (as high as 10400 m²/g), and largest pore aperture (as large as 9.8 nm) among all porous materials reported till date. Because of their high porosity, large surface area, tunable apertures, as well as high chemical and thermal stabilities, MOFs have been widely applied in the fields of adsorption, separation, and catalysis. In addition, MOFs have been successfully applied as stationary phases for isomer separation in gas chromatography (GC). Since the use of the first MOF (MOF-508) packed column for the separation of alkane isomers in GC, several other MOFs (e. g., MIL-47, MOF-5, and ZIF-8) have been employed for the GC separation of isomers. However, packed-column-type separation not only requires gram-scale quantities of MOFs, thereby increasing the analysis cost, but also results in poor separation efficiency. The first MOF (MIL-101) capillary column designed toward cost reduction allowed for the baseline separation of xylene and ethylbenzene isomers within 100 s under constant-temperature conditions. Since then, the capillary-type column has been widely utilized in the MOF-based stationary phase for GC separation.
Alkanes, xylene isomers and ethyl toluene, oxy-organics and organic pollutants are not only important chemicals in industry but also harmful environmental pollutants. Thus, the separation of these analytes is of practical importance environmental monitoring and industrial quality control. However, it is difficult to realize the efficient separation and detection of these isomers or racemates because of their similar boiling points and molecular sizes. In the past decades, GC was utilized as a rapid and efficient technique for the separation of the abovementioned analytes. The stationary phase used in GC plays a dominant role in the separation processes. This review summarizes the MOF-based GC separation of the abovementioned targets based on the different classification of analytes, including alkanes, xylenes, racemates, oxy-organics and persistent organic pollutants.
The separation mechanisms of different analytes are also discussed according to the structural benefits of MOFs. The separation mechanisms mainly involve van der Waals forces between the MOFs and analytes, interactions between the unsaturated metal sites and different functional groups of the analytes, molecular sieve effect or shape selectivity, and hydrogen-bond or π-π interactions. In addition, the chiral recognition abilities of MOFs possibly depend on the interactions between the chiral active sites in chiral MOFs and racemates.
Furthermore, efficient GC separation is influenced by thermodynamic and kinetic factors. The thermodynamic factor is mainly the difference between the partition coefficients of the separated components, which also reflects the properties of the analytes as well as the interactions between the stationary phase and the analytes. The kinetic factor also affects the column efficiency and chromatographic peak shape. Compared with traditional inorganic porous materials, MOFs with tunable structures are more favorable for optimizing the separation of isomers from both thermodynamic and kinetic standpoints. Therefore, this review summarizes the separation mechanism when using MOFs as stationary phases for isomer separation via thermodynamic and kinetic analyses. We hope the review would aid the state-of-art design of MOF stationary phases for high efficient isomer separations in GC.

Key words: gas chromatography (GC), stationary phase, chiral separation, metal-organic frameworks (MOFs)