Life History of the Metal–Organic Framework UiO-66 Catalyzing Methanol Dehydration: Synthesis, Activation, Deactivation, and Demise

• Published in: Chemistry of materials 2022-04, Vol.34 (7), p.3395-3408
• Main Authors: Conley, Edward T, Gates, Bruce C
• Format: Article
• Language: English
• Subjects: Chemistry; Materials Science
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.2c00242

The life cycle of a metal–organic framework (MOF) catalyst, UiO-66, which incorporates Zr6O8 nodes and 1,4-benzene dicarboxylate linkers and is known for its stability, was investigated with methanol dehydration as a test reaction. Catalyst performance was determined with a once-through flow reactor at atmospheric pressure and temperatures of 473–573 K. The products were analyzed by online gas chromatography, and catalyst samples removed from the reactor after various times on stream were characterized by X-ray diffraction crystallography, surface area measurements, scanning electron microscopy, and 1H NMR spectroscopy of samples digested in NaOH. The MOF particles underwent marked changes in composition and structure during catalysis, being activated initially as node formate and acetate ligands formed in the syntheses were removed by reaction with methanol to make methyl esters that desorbed and opened catalytic sites. From the beginning, the catalyst underwent deactivation, becoming completely deactivated after times on stream of 10–20 h, depending on the initial MOF composition. The deactivation was caused by methanol reacting to form node methoxy ligands and to break node–linker bonds, unzipping the MOF and creating amorphous material preferentially near the MOF particle surfaces. As the interior particle structure became degraded and inaccessible, the catalysis shut down. The tactics implemented in this work are suggested to be of value for assessing the strengths and limitations of MOFs as practical catalysts.