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

• Published in: Journal of the American Chemical Society Vol. 138; no. 8; pp. 2739 - 2748
• Main Authors: Malonzo, Camille D, Shaker, Sammy M, Ren, Limin, Prinslow, Steven D, Platero-Prats, Ana E, Gallington, Leighanne C, Borycz, Joshua, Thompson, Anthony B, Wang, Timothy C, Farha, Omar K, Hupp, Joseph T, Lu, Connie C, Chapman, Karena W, Myers, Jason C, Penn, R. Lee, Gagliardi, Laura, Tsapatsis, Michael, Stein, Andreas
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 02.03.2016; American Chemical Society (ACS)
• Subjects: catalysis (heterogeneous), materials and chemistry by design, synthesis (novel materials)
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.5b12688

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.