Bronsted Acid Property of Alumina-Supported Niobium Oxide Calcined at High Temperatures: Characterization by Acid-Catalyzed Reactions and Spectroscopic Methods

The acidic properties of alumina-supported niobium oxide (Nb2O5/Al2O3) calcined at high temperatures, with Nb2O5 loadings of 5-30 wt % as Nb2O5, were investigated using acid-catalyzed reactions (benzylation of anisole, cumene cracking, and isomerization of a-pinene), and Fourier-transform infrared spectroscopy. Structural characterization was carried out using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS) analysis. The relation between acidic properties and structure is discussed. The addition of niobium to gamma-Al2O3 generates Bronsted acid sites, where acid-catalyzed reactions take place. The catalytic activity depends on calcination temperature and niobium loading. In a series of Nb2O5/Al2O3 catalysts, 16 wt % Nb2O5/Al2O3 calcined at 1123 K exhibited the highest activity. Nb2O5/Al2O3 showed significant catalytic activity in the isomerization of a-pinene to camphene and limonene, even after evacuation at 1173 K, indicating high thermal stability of the Bronsted acid sites. XPS and XAFS revealed that a niobic oxide monolayer with distorted octahedral NbO6 units was formed and stabilized on alumina. Bronsted acid sites are probably generated at boundaries between two niobic oxide monolayer domains. When the Nb2O5 loading was sufficient to form two-dimensional niobium oxide overlayers (>16 wt %), some of the Bronsted acid sites on Nb2O5/Al2O3 were deposited by AlNbO4 having no Bronsted acid site, resulting in a lowering of the catalytic activity.