Formation of Local Defects and Mesopores in a Structure of UiO-66-NDC Metal-Organic Framework

Abstract— A UiO-66 metal-organic framework is synthesized using the solvothermal method. The framework of the obtained compound consists of Zr6O4(OH)4 clusters connected by 1,4-naphthalenedicarboxylate linkers. During synthesis two organic acids are introduced to control alterations in the properties of the material. It is shown that adding acetic acid results in a material with local defects, whereas benzoic acid in the compound structure leads to the creation of mesopores. The samples are comprehensively characterized via various experimental techniques. According to X-ray powder diffraction data, all of the samples possess a single-phase UiO-66 cubic structure. The thermal stability of the samples is studied via thermogravimetry and in-situ X-ray diffraction upon heating. Local defects are found to exert no significant influence on the thermal stability; furthermore, the samples retain their structure unchanged at temperatures of up to 500°C. The presence of mesopores lead to framework failure already at 300°C. The use of thermogravimetric analysis (TGA) and X-ray absorption spectroscopy along with evaluation of the degree of porosity makes it possible to monitor the formation of defects at both local and large scales.