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

Ammonia(NH3) is a widely used toxic industrial chemical which causes severe respiratory ailments. Therefore, understanding and developing materials for its efficient capture is necessary. 3D porous metal-organic frameworks(MOFs) with high surface areas and robust structures are ideal for gas storage/transport. Herein, interactions of NH3 and UiO-67-X(X: H, NH2, CH3) zirconium MOFs are studied, under Ultra-High Vacuum(UHV) conditions, using temperature programmed desorption mass spectrometry(TPD-MS) and in-situ temperature programmed infrared(TP-IR). NH3 is observed to interact with μ3-OH groups present on the nodes of UiO-67-X via hydrogen bonding. TP-IR studies reveal, under UHV, UiO-67-X to be stable to NH3. Interestingly, an increase in intensity of C-H stretching mode of MOF linkers is detected upon NH3 exposure, attributed to NH-π interactions with linkers. These same binding interactions were observed in GCMC simulations. Based on TPD-MS, binding strength of NH3 to the MOFs was determined to be ca. 60 kJ/mol, confirming physisorption. In addition, missing linker defect sites consisting of H2O bound to Zr4+ were detected through formation of nNH3⋅H2O clusters, characterized by TP-IR. Structures consistent with these assignments were identified using DFT calculations. Tracking these bands through adsorption on thermally activated MOFs gives insight into dehydroxylation process of UiO-67-X. This highlights an advantage of using NH3 for structural analysis of MOFs and provides directions for the development of stable materials for efficient toxic gas sorption.