Synthesis−Structure–Function Relationships of Silica-Supported Niobium(V) Catalysts for Alkene Epoxidation with H2O2

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• 作者：Nicholas E. Thornburg ; Scott L. Nauert ; Anthony B. Thompson ; Justin M. Notestein
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：September 2, 2016
• 年：2016
• 卷：6
• 期：9
• 页码：6124-6134
• 全文大小：629K
• 年卷期：0
• ISSN：2155-5435

文摘

Many industrially significant selective oxidation reactions are catalyzed by supported and bulk transition metal oxides. Catalysts for the synthesis of oxygenates, and especially for epoxidation, have predominantly focused on TiO_x supported on or co-condensed with SiO₂, whereas much of the rest of Groups 4 and 5 have been less studied. We have recently demonstrated through periodic trends using a uniform molecular precursor that niobium(V)-silica catalysts reveal the highest activity and selectivity for efficient utilization of H₂O₂ for epoxidation across all of Groups 4 and 5. In this work, we graft a wide range of Nb(V) precursors, spanning surface densities of 0.07–1.6 Nb groups nm^–2 on mesoporous silica, and we characterize these materials with UV–visible spectroscopy and Nb K-edge XANES. Further, we apply in situ chemical titration with phenylphosphonic acid (PPA) in the epoxidation of cis-cyclooctene by H₂O₂ to probe the numbers and nature of the active sites across this series and in a set of related Ti-, Zr-, Hf-, and Ta-SiO₂ catalysts. By this method, the fraction of kinetically relevant NbO_x species ranges from ∼15% to ∼65%, which correlates with spectroscopic evaluation of the NbO_x sites. This titration leads to a single value for the average turnover frequency, on a per active site basis rather than a per Nb atom basis, of 1.4 ± 0.52 min^–1 across the 21 materials in the series. These quantitative maps of structural properties and kinetic consequences link key catalyst descriptors of supported Nb-SiO₂ to enable rational design for next-generation oxidation catalysts.