Facile aqueous synthesis of ionic liquid/phosphotungstic acid co-functionalized MOF-808@silica composite for efficient mixed-mode liquid chromatography.

BACKGROUND: Metal organic frameworks (MOFs) are eminently suited for liquid chromatographic separation. MOF-808 is a type of MOF assembled through the coordination of zirconium clusters with 1,3,5-benzenetricarboxylic acid, which possesses an ultrahigh specific surface area and excellent stability. These features make MOF-808 an ideal modifier for HPLC stationary phase. Moreover, further functionalization represents a key strategy for advancing MOF@SiO2 stationary phase, as it offers a direct route to superior separation performance and broader utility. Therefore, a facile functionalization strategy for MOF-808@SiO2 is essential to enable green synthesis and practical application of high-performance MOF-based stationary phase.

RESULTS: In this study, a room-temperature aqueous synthesis method was employed to co-immobilize phosphotungstic acid (PTA) and ionic liquid (IL) onto the MOF-808@SiO2. The coordination bonds between the Zr clusters and the IL/PTA modifiers, together with the electrostatic attraction between IL and PTA, jointly enable the stable anchoring of the IL/PTA modifiers within MOF-808. Additionally, PTA, characterized by strong electronegativity and high acidity, enhances electrostatic interactions and serves as a rich source of hydrogen bond acceptors, and IL introduces cationic sites, while MOF-808 offers a stable framework with an abundant pore structure. The MOF-808@SiO2 is combined with PTA and IL to achieve a unique synergistic effect, improving the separation performance and application range. Experimental results show that the IL/PTA/MOF-808@SiO2 exhibits good separation performance for various analytes, including positional isomers, nucleosides, sulfonamides, vitamins, and benzoic acid compounds, with its applicability further demonstrated through the analysis of food samples.

SIGNIFICANCE: In this work, PTA and IL have been sequentially introduced onto MOF-808@SiO2via a facile and green stepwise strategy to fabricate a mixed-mode stationary phase. Benefiting from the complementary retention mechanisms provided by anionic PTA and cationic IL, the as-prepared IL/PTA/MOF-808@SiO2 exhibits favorable separation performance. This IL/PTA co-functionalization strategy shows good application prospects for constructing high-performance mixed-mode stationary phase and provides a feasible reference for the performance regulation of MOF-based separation material.