Abstract:

Chromatographic separation stands as a cornerstone technique in modern analytical chemistry， playing an indispensable role in critical areas such as chemical purification， environmental monitoring， and pharmaceutical analysis. With the increasing complexity of analytical samples， particularly regarding the efficient separation of structurally similar isomers， higher requirements have been imposed on chromatographic stationary phases. Traditional porous materials such as zeolites and silica show limited tunability in pore size distribution and surface chemistry， making it difficult to meet the urgent need for high-resolution and rapid separation of complex mixtures. Therefore， the exploration of novel ordered porous materials with precisely tunable pore structures and functionalities has become a frontier research direction in separation science. Covalent organic frameworks （COFs）， as a category of crystalline porous polymers， possess great application potential in separation science. Their prominent advantages mainly include designable periodic frameworks， easily modified functional groups， and precisely tunable pore sizes. These intrinsic merits render COFs promising candidates for next-generation chromatographic separation media. In this work， a novel strategy is proposed to regulate the pore structure of COFs via rational topological design， aiming to improve their gas chromatographic performance toward challenging isomer separation. This is achieved by constructing heteroporous COFs that integrate distinct pore size distributions within a single， ordered framework. To realize this strategy， a heteroporous COF designated as ET-COF was successfully constructed. It was synthesized via a reversible imine condensation reaction between a D_2h-symmetric tetraamine monomer， 4，4′，4″，4′′′ -（ethene-1，1，2，2-tetrayl）tetraaniline （ETTA）， and a C₂-symmetric linear dialdehyde linker， terephthalaldehyde （TPA）. This assembly affords a two-dimensional framework with a Kagome topology. The porous structures of material were rigorously characterized. It has been demonstrated through nitrogen adsorption-desorption experiments that ET-COF exhibits a distinct dual-pore structure， including triangular micropores of approximately 1.1 nm and hexagonal mesopores of 2.89 nm， with a specific surface area of 724 m²/g and a pore volume of 1.26 cm³/g. The rich pore structure and high specific surface area confer multiple size-exclusion effects. By virtue of these structural merits， an ET-COF-bonded capillary chromatographic column was further prepared via a facile in-situ growth method， which enables stable and uniform strong chemical bonding of the COF layer to the inner wall of a fused-silica capillary. Finally， the chromatographic performance of this column was systematically evaluated. The heteroporous ET-COF column demonstrated excellent separation performance across a wide range of test analytes. Baseline separation was successfully achieved for diverse mixtures， including homologous n-alkanes （C5-C10）， alkylbenzenes （from benzene to butylbenzene）， n-straight-chain fatty acid esters， and critically a set of C9 aromatic hydrocarbon isomers （n-propylbenzene， isopropylbenzene， and mesitylene）， with resolution values ranging from 2.0 to 15.3， suggesting outstanding separation efficiency. The improved chromatographic behavior can be rationalized by the uniquely hierarchical pore structure originating from rational topological design. The triangular micropores afford a confined microenvironment that imposes prominent size- and shape-selectivity， enabling discrimination of isomers with subtle differences in molecular configuration. Meanwhile， the hexagonal mesopores act as rapid transport pathways to promote fast mass transfer and avoid pore blocking. Such a synergistic effect of selective micropores and high-flux mesopores within one crystalline framework represents a distinct merit of the heteroporous topology. In summary， this work establishes a feasible topological strategy for constructing high-performance COF-based stationary phases and investigates the impact of hierarchical pore structures （synergistic effects of microporous shape selectivity and mesoporous mass transfer） on separation performance. By rational topological assembly， precise pore engineering and integration of hierarchical channels are readily realized， which greatly boost the separation selectivity toward challenging isomers. This study not only develops a promising heteroporous COF stationary phase for gas chromatography， but also provides a new design paradigm for extending the application of COFs in analytical separation science.

Key words: gas chromatography （GC）, stationary phase, Kagome topological structure, heteropore covalent organic frameworks, isomer separation