Thermal Stabilization of Metal–Organic Framework-Derived Single-Site Catalytic Clusters through Nanocasting

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• 作者：Camille D. Malonzo ; Sammy M. Shaker ; Limin Ren ; Steven D. Prinslow ; Ana E. Platero-Prats ; Leighanne C. Gallington ; Joshua Borycz ; Anthony B. Thompson ; Timothy C. Wang ; Omar K. Farha ; Joseph T. Hupp ; Connie C. Lu ; Karena W. Chapman ; Jason C. Myers ; R. Lee Penn ; Laura Gagliardi ; Michael Tsapatsis ; Andreas Stein
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：March 2, 2016
• 年：2016
• 卷：138
• 期：8
• 页码：2739-2748
• 全文大小：673K
• ISSN：1520-5126

文摘

Metal–organic frameworks (MOFs) provide convenient systems for organizing high concentrations of single catalytic sites derived from metallic or oxo-metallic nodes. However, high-temperature processes cause agglomeration of these nodes, so that the single-site character and catalytic activity are lost. In this work, we present a simple nanocasting approach to provide a thermally stable secondary scaffold for MOF-based catalytic single sites, preventing their aggregation even after exposure to air at 600 °C. We describe the nanocasting of NU-1000, a MOF with 3 nm channels and Lewis-acidic oxozirconium clusters, with silica. By condensing tetramethylorthosilicate within the NU-1000 pores via a vapor-phase HCl treatment, a silica layer is created on the inner walls of NU-1000. This silica layer provides anchoring sites for the oxozirconium clusters in NU-1000 after the organic linkers are removed at high temperatures. Differential pair distribution functions obtained from synchrotron X-ray scattering confirmed that isolated oxozirconium clusters are maintained in the heated nanocast materials. Pyridine adsorption experiments and a glucose isomerization reaction demonstrate that the clusters remain accessible to reagents and maintain their acidic character and catalytic activity even after the nanocast materials have been heated to 500–600 °C in air. Density functional theory calculations show a correlation between the Lewis acidity of the oxozirconium clusters and their catalytic activity. The ability to produce MOF-derived materials that retain their catalytic properties after exposure to high temperatures makes nanocasting a useful technique for obtaining single-site catalysts suitable for high-temperature reactions.