Synthesis-structure-function relationships of silica-supported niobium(V) catalysts for alkene epoxidation with H2O2

Abstract

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 TiOx 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 S. 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 NbOx species ranges from similar to 15% to similar to 65%, which correlates with spectroscopic evaluation of the NbOx 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.