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 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.