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 Brønsted 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.