Identifying UiO-67 Metal-Organic Framework Defects and Binding Sites through Ammonia Adsorption

Ammonia is a widely used toxic industrial chemical that can cause severe respiratory ailments. Therefore, understanding and developing materials for its efficient capture and controlled release is necessary. One such class of materials is 3D porous metal-organic frameworks (MOFs) with exceptional surface areas and robust structures, ideal for gas storage/transport applications. Herein, interactions between ammonia and UiO-67-X (X: H, NH₂ , CH₃ ) zirconium MOFs were studied under cryogenic, ultrahigh vacuum (UHV) conditions using temperature-programmed desorption mass spectrometry (TPD-MS) and in-situ temperature-programmed infrared (TP-IR) spectroscopy. Ammonia was observed to interact with μ₃ -OH groups present on the secondary building unit of UiO-67-X MOFs via hydrogen bonding. TP-IR studies revealed that under cryogenic UHV conditions, UiO-67-X MOFs are stable towards ammonia sorption. Interestingly, an increase in the intensity of the C-H stretching mode of the MOF linkers was detected upon ammonia exposure, attributed to NH-π interactions with linkers. These same binding interactions were observed in grand canonical Monte Carlo simulations. Based on TPD-MS, binding strength of ammonia to three MOFs was determined to be approximately 60 kJ mol^-1 , suggesting physisorption of ammonia to UiO-67-X. In addition, missing linker defect sites, consisting of H₂ O coordinated to Zr⁴⁺ sites, were detected through the formation of nNH₃ ⋅H₂ O clusters, characterized through in-situ IR spectroscopy. Structures consistent with these assignments were identified through density functional theory calculations. Tracking these bands through adsorption on thermally activated MOFs gave insight into the dehydroxylation process of UiO-67 MOFs. This highlights an advantage of using NH₃ for the structural analysis of MOFs and developing an understanding of interactions between ammonia and UiO-67-X zirconium MOFs, while also providing directions for the development of stable materials for efficient toxic gas sorption.