Tomoyuki Kitano

Effect of High-Temperature Calcination on the Generation of Brønsted Acid Sites on WO3/Al2O3

The acid properties of a series of alumina‐supported tungsten oxide (WO3/Al2O3) catalysts with loadings of 5–50 wt % WO3 calcined at various temperatures were investigated by acid‐catalyzed reactions (benzylation of anisole and isomerization of α‐pinene) and FTIR spectroscopy. The relationships between acid properties, structures, and catalytic performances were evaluated. Both the catalytic activity and amount of Brønsted acid sites depend on the calcination temperature and WO3 loading. High‐temperature calcination (1123 K) generated Brønsted acid properties, and 20 wt % WO3/Al2O3 calcined at 1123 K exhibited the highest activity among the catalysts tested. The activities for the benzylation of anisole and α‐pinene isomerization over WO3/Al2O3 calcined at 1123 K were proportional to the Brønsted acidity, which indicates that these reactions occurred on the Brønsted acid sites. Tungsten oxide, which has distorted octahedral symmetry, was loaded as 2 D monolayer domains below 20 wt %, and these domains covered most of the alumina surface at 20 wt %. If the WO3 loading was sufficient to form 2 D tungsten oxide monolayer sheets (>20 wt %), some of the Brønsted acid sites on WO3/Al2O3 were obscured by monoclinic WO3 that has no Brønsted acid sites, which resulted in a decrease of the catalytic activity. This suggests that Brønsted acid sites are generated at the boundaries between tungsten oxide monolayer domains.

Characterization of Thermally Stable Brønsted Acid Sites on Alumina‐Supported Niobium Oxide after Calcination at High Temperatures

Thermally stable Brønsted acid sites were generated on alumina-supported niobium oxide (Nb2O5/Al2O3) by calcination at high temperatures, such as 1123 K. The results of structural characterization by using Fourier-transform infrared (FTIR) spectroscopy, TEM, scanning transmission electron microscopy (STEM), and energy-dispersive X-ray (EDX) analysis indicated that the Nb2O5 monolayer domains were highly dispersed over alumina at low Nb2O5 loadings, such as 5 wt%, and no Brønsted acid sites were presents. The coverage of Nb2O5 monolayer domains over Al2O3 increased with increasing Nb2O5 loading and almost-full coverage was obtained at a loading of 16 wt%. A sharp increase in the number of hydroxy groups, which acted as Brønsted acid sites, was observed at this loading level. The relationship between the acidic properties and the structure of the material suggested that the bridging hydroxy groups (Nb-O(H)-Nb), which were formed at the boundaries between the domains of the Nb2O5 monolayer, acted as thermally stable Brønsted acid sites.