Mixed Metal-Organic Framework Stationary Phases for Liquid Chromatography.

Strategic design of the stationary phase in liquid chromatography (LC) is crucial for modern separation science. Herein, a design approach using mixed metal-organic frameworks (MOFs) as tunable LC stationary phases is proposed. Three MOFs with an isostructural pillared-layer structure are employed, with pore sizes tuned by the systematic design of the constituent ligands, using 1,4-benzenedicarboxylate (bdc), 1,4-naphthalenedicarboxylate (ndc), and 9,10-anthracenedicarboxylate (adc). Packed columns filled with the MOFs and their mixed-particle/solid-solution stationary phases are prepared and examined for the retention capability of polyethylene glycol (PEG) in LC. While the MOF-packed columns filled with binary mixtures of different MOF particles provide good control of the retention with respect to the particle mixing ratio, the columns filled with mixed-linker solid-solution MOFs show a significant multicomponent effect on the retention behavior. Specifically, mixed-linker solid-solution MOFs consisting of bdc/ndc binary ligands are found to show a strong retention that surpasses even their parent MOFs, namely, pure bdc- and ndc-MOF stationary phases. The retention behavior on the MOF-packed columns is explained by the specific nanostructures of the solid-solution MOFs, which affects the balance between substrate affinity and adsorption kinetics into the MOF pores, dictating the total retention capability. The results provide an extra dimension for stationary phase design using MOFs as a promising recognition medium for LC.