Metal-Organic Frameworks And Its Composites For Liquid Chromatographic Separation

Metal-organic frameworks (MOFs), also called porous coordination polymers (PCPs), are a new class of hybrid inorganic-organic microporous crystalline materials self-assembled from metal ions with organic linkers via coordination bonds. Owing to their fascinating structures and unusual properties, such as large surface area, structural diversity, good thermal and chemical stability, uniform and regulatory pore size, and the availability of framework functionality, MOFs have great potential for separation applications. In the past decade, MOFs have been widely used as stationary phase for gas chromatography. However, researches on the application of MOFs as stationary phase for high performance liquid chromatography (HPLC) have lagged behind. This dissertation focused on the expanding the scope of MOF-based stationary phase and the analytes, and exploring the potential of MOFs composites as separation media for HPLC. The main contents are summarized as follows:(1) Metal-organic frameworks (MOFs) with open metal sites have great potential for enhancing adsorption separation of the molecules with different polarities. However, the elution and separation of polar compounds on such MOFs packed columns using nonpolar solvents is difficult due to too strong interaction between polar compounds and the open metal sites. Therefore, the coordination status of the open metal sites in MOFs was controlled by adjusting the content of methanol (MeOH) in the mobile phase for fast and high-resolution separation of polar compounds. To this end, HPLC separation of nitroaniline, aminophenol and naphthol isomers, sulfadimidine, and sulfanilamide on the column packed with MIL-101(Cr) possessing open metal sites was performed. The interaction between the open metal sites of MIL-101(Cr) and the polar analytes was adjusted by adding an appropriate amount of MeOH to the mobile phase to achieve the effective separation of the polar analytes due to the competition of MeOH with the analytes for the open metal sites. Fourier transform infrared spectra and X-ray photoelectron spectra confirmed the interaction between MeOH and the open metal sites of MIL-101(Cr). Thermodynamic parameters were measured to evaluate the effect of the content of MeOH in the mobile phase on the separation of polar analytes on MIL-101(Cr) packed column. The universality of the present approach for the control of the open metal sites of MOFs with MeOH was demonstrated using another MOF containing open metal sites, MIL-100(Fe). This approach provides reproducible and high performance separation of polar compounds on the open metal sites-containing MOFs.(2) Metal-organic framework MIL-100(Fe) was explored as a novel stationary phase for both normal-phase and reverse-phase HPLC. Two groups of analytes (benzene, toluene, ethylbenzene, naphthalene and1-chloronaphthalene; aniline, acetanilide,2-nitroaniline and1-naphthylamine) were used to test the separation performance of MIL-100(Fe) in the reverse-phase mode, while the isomers of chloroaniline or toluidine were employed to evaluate its performance in the normal-phase mode. The MIL-100(Fe) packed column gave a baseline separation of all the tested analytes with good precision. The separation was controlled by negative enthalpy change and entropy change in the reverse-phase mode, but positive enthalpy change and entropy change in the normal-phase mode. The relative standard deviations of retention time, peak area, peak height, and half peak width for eleven replicate separations of the tested analytes were0.2-0.7%,0.5-3.6%,0.6-2.3%and0.8-1.7%, respectively. The mesoporous cages, accessible windows, excellent chemical and solvent stability, metal active sites and aromatic pore walls make MIL-100(Fe) a good candidate as novel stationary phase for’both normal-phase and reverse-phase high performance liquid chromatography.(3) The unique features of high porosity, shape selectivity and multiple active sites make MOFs promising as novel stationary phases for HPLC. However, the wide particle size distribution and irregular shape of conventional MOFs lead to lower column efficiency of such MOFs-packed columns. Monodisperse SiO2@MOF core-shell microspheres were fabricated as the stationary phase for HPLC to overcome the above-mentioned problems. Zeolitic imidazolate framework-8(ZIF-8) was used as an example of MOFs due to its permanent porosity, uniform pore size, exceptional chemical stability. The unique carboxyl modified silica spheres were used as the support to grow ZIF-8shell. The fabricated monodisperse SiO2@ZIF-8packed columns (5cm long×4.6mm i.d.) show high column efficiency (23000plates m-1for bisphenol A) for the HPLC separation of the endocrine-disrupting chemicals (bisphenol A,β-estradiol and p-(tert-octyl)phenol) and the pesticides (thiamethoxam, hexaflumuron, chlorantraniliprole and pymetrozine) within7min with good relative standard deviations for eleven replicate separations of the analytes (0.01-0.39%,0.65-1.7%,0.70-1.3%and0.17-0.91%for retention time, peak area, peak height, and half peak width, respectively). The SiO2@ZIF-8microspheres combine the advantages of the good column packing properties of the uniform monodisperse silica microspheres and the separation ability of the ZIF-8crystals.(4) Metal-organic framework UiO-66has been incorporated into porous poly methylarylic acid-co-ethylene dimetharylate (MAA-co-EDMA) monolith to enhance HPLC separation of small molecules in an isocratic mode. Four groups of small molecules:neutral polycyclic aromatic hydrocarbons (benzene, naphthalene, fluorene, pyrene, chrysene), basic aniline series (acetanilide,4-fluoroaniline,2-nitroaniline,1-naphthylamine), acid phenol series (resorcinol, m-cresol,2,6-dimethylphenol,2,6-dichlorophenol) and naphthyl substitutes (1-naphthol,1-methylnaphthalene,1-chloronaphthalene) were used to test the chromatography separation enhancement on the prepared monolith. Baseline separations of all the tested analytes on UiO-66incorporated monolith were achieved with improved column efficiency compared with the parent monolith. The relative standard deviations of retention time, peak area, peak height, and half peak width for eleven replicate separations of the tested analytes were0.02-0.27%,0.33-1.49%,0.35-1.85%, and0.19-1.09%, respectively. The intrinsic characteristics such as aromatic rings-based structures, carboxylate functional groups and Zr active sites make UiO-66a good candidate as a novel nanomaterial to afford monolithic column for enhancing chromatographic separation of small molecules.