Comparative Study of Nb-Incorporated Cubic Mesoporous Silicates as Epoxidation Catalysts

Niobium in varying amounts was successfully incorporated into SBA-16 Material via a one-pot direct synthesis technique. Complementary small angle X-ray speCtroscopy (SAXS), wide-angle X-ray diffraction (XRD), N-2 sorption isotherms, and high resolution transmission electron microscopy (HR-TEM) imaging results confirm the structural features and integrity of the prepared Materials. The Nb-SBA-16 materials poSsess high surface areas (829=100 m(2)/g) and or volumes (0.69-0.82 cm(3)/g) that decrease with incteasing niobium cOntent. Nicibium exist S mostly' as tetrahedral NbO4 and oligomeric NbO4 species. NH3-TPD results reveal that Nb insertion imparts Lewis acidity to the SBA-16 supports, with the acidity increasing with Nb loading. The Nb-SBA-16 materials show good cyclohexene epoxidation activity with H2O2 as oxidant,, with the epoxide selectivity decreasing at higher Nb loadings due to ring-opening and other side-reactions of the epoxide, attributed to increased acidity. The Nb-SBA-16 materials generally show superior performance compared to either Nb-KIT-6 or Nb-KIT-5 materials at lower Nb loadings and similar reaction conditions. Unlike the other :catalysts, both the cydohexene conversion and epoxide selectivity' go through a maximum in the case Nb-SBA46 materials, suggesting that the surface Nb oxide species that dictate the overall acidity are different in the three catalysts.