Directional Regulation of One-Dimensional Channel Length in Metal-Organic Frameworks for Efficient Xylene Isomer Separation in Gas Chromatography

Abstract

The separation of xylene isomers in capillary gas chromatography (GC) is essential and a significant challenge in analytical chemistry. Metal-organic frameworks (MOFs), as emerging porous materials, exhibit great potential for separation applications. However, the effective utilization of MOF-based stationary phases in GC is heavily constrained by their morphology and particle size. Larger particles lead to uneven coating on the inner wall of chromatographic columns, reducing the mass transfer efficiency and diffusion of analytes, which severely compromises chromatographic separation performance. Reducing the channel length of the MOFs are crucial methods for the development of GC stationary phases. In this study, by optimizing the amount of pyridine modulator, we successfully reduced the length of the MOF-74 nanorods, subsequently reduced the one-dimensional channel length in MOF-74. Compared to the longer hexagonal-shaped MOF-74-1, the nano-MOF-74-3 stationary phase showed a more uniform deposition on the inner wall of the capillary column. The MOF-74-3 column provided high separation performance for xylene isomers, achieving a separation factor of 6.11 for pX/oX, which outperformed both MOF-74-1 and commercial columns such as HP-5MS and VF-WAXMS. The MOF-74-3 column demonstrated excellent separation performance after five injections of xylene isomers, indicating good reproducibility in the separation process. The xylene molecules exhibited a smaller mass transfer coefficient and faster diffusion in nano-MOF-74-3 than in MOF-74-1 column, effectively reducing chromatographic peak tailing. Moreover, the MOF-74-3 column also provided baseline separation for various alkane isomers and substituted benzene isomers. This work successfully decreased the aspect ratio of MOF-74-1 and MOF-74-3 from 2.6 to 1.1 through the addition of pyridine. The high-efficiency MOF-74-3 separation column achieved high-resolution separation of xylene isomers, alkane isomers, and substituted benzene isomers. This method offerd a new direction for the design of high-resolution stationary phases, which were essential for advancing GC-based analytical methods.