Creating Brønsted Acidity at the SiO2-Nb2O5 Interface

Abstract Catalytically active acid sites associated with the silica-niobia interface were probed with a series of overcoated SiO2 on Nb2O5 (SiO2/Nb2O5) mixed oxide materials prepared by deposition of tetraethyl orthosilicate onto niobic acid (Nb2O5⋅nH2O) or calcined niobia (Nb2O5). NH3 TPD and pyridine DRIFTS studies indicated that the speciation of acid sites in the materials evolved as a function of SiO2 loading, impacting the quantity and stability of Bronsted sites. Catalyst activity was highly dependent on SiO2 loading in the liquid phase hydroalkoxylation of dihydropyran with n-octanol. At SiO2 surface densities corresponding to approximately 1 Si per 2 surface Nb, the activity of these catalysts passed through a maximum approximately 20 times higher than the activity of calcined Nb2O5. Apparent reaction barriers measured over the most active SiO2/Nb2O5 catalysts were 10 kJ/mol lower than those measured over niobic acid, suggesting that the OH features unique to the SiO2-Nb2O5 interface were slightly more reactive than those on niobic acid.