Metal organic frameworks as sorbents for volatile organic compounds
Metal organic frameworks (MOFs) are a class of highly porous materials with large surface areas, large pore volumes, and chemical tunability. These features make MOFs desirable as sorbents for applications such as gas storage, gas separation, and gas sensing. In this work, MOF thin films of UiO-66-R, where R = -H, -NH2, and-NO2, were fabricated onto Au-coated Si wafers and Au-coated quartz crystal microbalance surfaces using a vapor-assisted conversion method. The films were then characterized by scanning electron microscopy, powder x-ray diffraction, x-ray photoelectron spectroscopy, reflection absorbance infrared spectroscopy, and Raman spectroscopy. The spectroscopy reveals that the films of UiO-66-H, UiO-66-NH2, and UiO-66-NO2 are polycrystalline and 1 – 3 µm thick. The diffraction patterns reveal that the UiO-66-NO2 film potentially has the most missing linker defects. The UiO-66-R films grown on quartz microbalance crystals were activated by heating under high vacuum and exposed to a known pressure of benzene, toluene, ethyl benzene, and the xylene isomers (BTEX). The Henry’s constant, which describes the adsorption capacity for each MOF, was calculated from the mass change during the adsorption isotherm at 30°C, 25°C, and 20°C. The enthalpy of adsorption and entropy change was determined by plotting the logarithm of Henry’s constants versus the reciprocal of temperature. The results reveal the Henry’s constant for BTEX increased in the following order: UiO-66-H < UiO-66-NH2 < UiO-66-NO2. The results suggest that the functional groups on the organic linker and missing linkers influence adsorption behavior. The Henry’s constant of the films UiO-66-H were an order of magnitude smaller than those obtained for UiO-66-NH2 and UiO-66-NO2 films, largely due to the large pore size and lack of any functional group. The results suggest that UiO-66-NO2 films contain more missing linker defects than UiO-66-NH2 films. As a result, the heat of adsorption and entropy change for BTEX molecules in UiO-66-NH2 films is more negative than UiO-66-NO2. In contrast, due to a large pore size caused by the missing linkers, the adsorption capacity of UiO-66-NO2 films is larger than UiO-66-NH2 films.
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