Brønsted 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 (Nb 2 O 5 /Al 2 O 3 ) calcined at high temperatures, with Nb 2 O 5 loadings of 5–30 wt % as Nb 2 O 5, were investigated using acid-catalyzed reactions (benzylation of anisole, cumene cracking, and isomerization of α-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 γ-Al 2 O 3 generates Brønsted acid sites, where acid-catalyzed reactions take place. The catalytic activity depends on calcination temperature and niobium loading. In a series of Nb 2 O 5 /Al 2 O 3 catalysts, 16 wt % Nb 2 O 5 /Al 2 O 3 calcined at 1123 K exhibited the highest activity. Nb 2 O 5 /Al 2 O 3 showed significant catalytic activity in the isomerization of α-pinene to camphene and limonene, even after evacuation at 1173 K, indicating high thermal stability of the Brønsted acid sites. XPS and XAFS revealed that a niobic oxide monolayer with distorted octahedral NbO 6 units was formed and stabilized on alumina. Brønsted acid sites are probably generated at boundaries between two niobic oxide monolayer domains. When the Nb 2 O 5 loading was sufficient to form two-dimensional niobium oxide overlayers (>16 wt %), some of the Brønsted acid sites on Nb 2 O 5 /Al 2 O 3 were deposited by AlNbO 4 having no Brønsted acid site, resulting in a lowering of the catalytic activity.