Generation of Brønsted acid sites on Al2O3-supported Ta2O5 calcined at high temperatures

An alumina-supported tantalum oxide (Ta2O5/Al2O3) calcined at 1223 K promoted both Friedel–Crafts alkylation of anisole with benzyl alcohol and isomerization of α-pinene, and exhibited Brønsted acidity despite the high temperature calcination. Among the Ta2O5/Al2O3 catalysts tested, 33 wt% Ta2O5/Al2O3 calcined at 1223 K showed the highest activity. A monolayer of tantalum acid-like compound, which has distorted octahedral symmetry, was stabilized over 33 wt% Ta2O5/Al2O3 catalyst calcined at 1223 K. The two-dimensional Ta–O–Ta network of stabilized tantalum acid-like compound probably accounts for the generation of Brønsted acid.