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

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 SiO 2, 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 H 2 O 2 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 H 2 O 2 to probe the numbers and nature of the active sites across this series and in a set of related Ti-, Zr-, Hf-, and Ta-SiO 2 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-SiO 2 to enable rational design for next-generation oxidation catalysts.