Synthesis-Structure–Function Relationships of Silica-Supported Niobium(V) Catalysts for Alkene Epoxidation with H ₂ O ₂

Journal Article · Fri Aug 12 00:00:00 EDT 2016 · ACS Catalysis

DOI:https://doi.org/10.1021/acscatal.6b01796· OSTI ID:1298281

Thornburg, Nicholas E.; Nauert, Scott L.; Thompson, Anthony B.; Notestein, Justin M.

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.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• OSTI ID: 1298281
• Journal Information: ACS Catalysis, Vol. 6, Issue 07, 2016; ISSN 2155-5435
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY