Metal-organic frameworks (MOFs) are novel, highly porous 3-dimensional nanostructured coordination polymers. They consist of metal cations and anions of organic linkers, which are potential binding sites for molecules. The majority of MOFs are solids, which are able serve as robust matrices for sorption of various molecules in different phases including water, biomedical drugs, and environmental pollutants. Aluminum MOFs (Al-MOFs) have large porosity, high chemical and thermal stability, structural versatility to accommodate molecules, and low toxicity. Al-MOFs find emerging applications in purification of air and water from hazardous compounds, small-scale production of potable water from moist air, purification of foods, and delivery of drugs and cosmetics. MIL-160(Al) is an unusual Al-MOF with environmentally “green” linker of dianion of 2,5-furandicarboxylic acid 2,5-FDCA. Due to favorable nanopore structure and high affinity of linker to polar molecules, MIL-160(Al) is suitable for environmental and biomedical applications. 2,5-FDCA is a plant-derived natural chemical produced from carbohydrates, which classifies it as a renewable sustainable compound. However, using 2,5-FDCA as a building block (linker) in MIL-160(Al) is challenging since it is hard to remove an excessive 2,5-FDCA, due to very low vapor pressure and low solubility in most solvents. For emerging biomedical applications, pure MIL-160(Al) is highly desired. Many papers claim purity of MIL-160(Al), but they rely on X-ray diffraction, thermal analysis, and electron microscopy which only reveal the most abundant phase. Spectroscopic methods are more suitable for detection of impurities, but their use in studies of MIL-160(Al) is very limited. We used FTIR spectra to assess the amount of 2,5-FDCA impurity in MIL-160(Al) via characteristic peaks e.g. at 1700 cm-1 due to–COOH group. Next, we tested several organic and organic-free solvents to fully remove 2,5-FDCA. This includes assignments of multiple FTIR peaks of MIL-160(Al) and major 2,5-FDCA impurity to vibrational modes. Finally, we studied sorption of water vapor by MIL-160(Al) from ambient air at room temperature. Sorption proceeds in tens of minutes to adsorption complex with four water molecules per MIL-160(Al) unit. In the complex, water is bound to at least two functional groups in MIL-160(Al). The ex-situ and in-situ FTIR spectroscopy under ambient conditions is a powerful tool to study reactions of MOFs with polar molecules.

Ex-situ FTIR spectroscopic control of purification of aluminum metal-organic framework MIL-160(Al) and in-situ study of water sorption reaction

ACS Spring Meeting 2021

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