Preparation of sp2c-COF functionalized silica gel material as chromatographic stationary phases for their high-resolution separation of geometric isomers

Abstract

Exploration and development of novel chromatographic stationary phases is an effective way to improve the separation efficiency of geometric isomers with similar physicochemical properties. Covalent organic frameworks (COFs) are a new class of porous organic polymers that show a wide range of applications in the field of separation science due to their tunable geometries and functionalities, infinitely extended network structures, and abundant interaction sites. However, the common imine-bonded COFs are poorly resistant to hydrolysis in HPLC. In this work, 2,4,6-trimethyl-1,3,5-triazine (TMT) and 1,3,5-tris (4-formylphenyl) triazine (TFPT) were used as raw materials, a sp2 carbon-conjugated covalent organic framework (sp2c-COF) was synthesized through self-assembly monolayer-assisted surface-initiated Schiff-base-mediated hydroxyl-aldehyde condensation reaction, and loaded on the surface of silica substrate with a CN bond to obtain a new segregated material (SiO2@sp2c-COF). SiO2@sp2c-COF was used as a high-performance liquid chromatography (HPLC) packing material for the separation of geometric isomers. Benefitting from its superb in-plane π-conjugation, highly ordered and robust framework structure, high chemical and thermal stability of sp2c-COF, and the unique hydrophilic covalent triazine groups, hydrophobic benzene rings and other groups that can provide a variety of interactions such as hydrophobicity, π-π stacking, hydrogen bonding and hydrophilicity of the prepared SiO2@sp2c-COF stationary phases, they exhibit excellent molecular shape selectivity and resolution in separating geometrical isomers. These geometric isomers include isomers such as polycyclic aromatic hydrocarbons (PAHs), tocopherols, carotenoids, diethylstilbestrol, 1,4-cyclohexanediol and astaxanthin. Compared to commercial C18 columns, this column has more flexible selectivity and higher separation performance. In addition, due to the introduction of hydrophobicity, π-π stacking action and hydrophilic triazine components, the SiO2@sp2c-COF stationary phase also has RPLC/HILIC mixed mode characteristics. Baseline separations of monosubstituted benzenes, alkylbenzenes, positional isomers, sulfonamides, benzoic acid, anilines, nucleosides and nucleobases compounds were achieved on SiO2@sp2c-COF packed columns. This successful application highlights the great potential of sp2c-COF for the separation of geometrical isomers, and provides a way to overcome the stability of common imine-bonded COFs materials in HPLC, as well as to compensate for the shortcomings and deficiencies of a single chromatographic mode in the separation of complex samples. Furthermore, this is the first report of a practical separation of important geometrical isomers using sp2c-COF materials.