Abstract: The gas chromatographic separation of C₈ aromatic isomers, including ortho-xylene (oX), meta-xylene (mX), para-xylene (pX), and ethylbenzene (EB), remains a significant and persistent challenge in analytical and separation science. This difficulty primarily arises from their identical molecular formula and extremely similar physicochemical properties. Most notably, their boiling points differ by less than 1 K, resulting in nearly indistinguishable volatility. As a consequence, conventional gas chromatography often suffers from severe co-elution and peak overlap, which compromises both resolution and quantitative accuracy in industrial quality control. Traditional stationary phases, such as polysiloxanes and polyethylene glycols, predominantly rely on non-specific van der Waals interactions or broad polarity-based partitioning to separate analytes. These mechanisms, governed largely by thermodynamic equilibrium, often prove inadequate to amplify the subtle molecular differences among the xylene isomers. Metal-organic frameworks (MOFs) have recently emerged as a promising alternative for addressing this challenge. MOFs are crystalline porous materials constructed from metal nodes and organic linkers through coordination self-assembly. Their key advantage lies in their exceptional structural and chemical tunability. Pore size, shape, dimensionality, and internal surface chemistry can be precisely tailored at the angstrom scale, enabling rational control over separation behavior. In this review, we summarize the advantages of MOFs as stationary phases for xylene separation from three perspectives: separation mechanisms, metal-node-dependent framework design, and column fabrication strategies. First, separation mechanisms play a central role in determining the performance of MOF-based stationary phases. Owing to their exceptional design flexibility, MOFs can integrate multiple synergistic separation mechanisms within a single material. Therefore, this review highlights representative mechanistic case studies. The superior separation performance of MOFs mainly originates from three complementary mechanisms. (1) Molecular sieving and shape selectivity arising from well-defined pore apertures enable the discrimination among isomers based on kinetic diameter and molecular geometry. (2) Specific intra-pore interactions caused by different functional groups, aromatic linkers, and open metal sites on the pore walls introduce targeted non-covalent interactions, such as π-π stacking, hydrogen bonding, and weak coordination. These interactions magnify subtle differences in adsorption energies among xylene isomers, leading to the separation. (3) Kinetic mass-transfer modulation is essential for achieving high column efficiency and sharp peak shapes. This can be realized by reducing particle size to shorten diffusion paths, introducing hierarchical pore structures, or constructing two-dimensional layered architectures to regulate diffusion dynamics. Second, MOFs constructed from different metal nodes exhibit distinct separation behaviors. A wide variety of MOFs based on diverse metal centers have been explored for C₈ aromatic separations. Zirconium-based MOFs, such as UiO-66 and PCN-608, have been extensively investigated due to their outstanding thermal and chemical stability, with separation performance further improved through morphology control and layer-stacking engineering. Aluminum-based MOFs, including MIL-160 and Al-TCPB-Me₂, benefit from low-cost precursors and typically feature one-dimensional channels with precise size and shape recognition. Copper-based MOFs have evolved from early hydrolytically sensitive materials to ultra-stable frameworks, combining high separation resolution with long column lifetimes. In addition, titanium-, zinc-, and iron-based MOFs, such as MIL-125-NH₂, MCF-50, and MIL-142, have also been reported to exhibit good xylene separation capability. Third, column fabrication plays a decisive role in determining the final separation performance. This review further discusses different capillary pretreatment strategies and stationary-phase coating methods, highlighting their influence on coating uniformity, stability, and chromatographic separation performance. Finally, we provide an outlook on future development directions of MOF-based gas chromatographic stationary phases for xylene separation. Future efforts should focus on improving column stability and durability, alongside developing cost-effective and scalable synthesis routes. Additionally, enhancing trace-level impurity analysis in complex matrices and facilitating integration into miniaturized and portable GC systems are crucial for practical application. This work aims to offer guidance for the rational design of high-performance MOF-based stationary phases and their practical application in the separation of complex aromatic hydrocarbons.

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