Acid property of Nb2O5/Al2O3 prepared by impregnation method by using niobium oxalate solution: Effect of pH on the structure and acid property

•The acid property of Nb2O5/Al2O3 depended on the pH of the impregnating solution.•Increase of the pH caused increase of the size of niobium oxide monolayer domains.•Brønsted acidity was superior on Nb2O5/Al2O3 having large niobium oxide domains.•A strong distortion of Nb(OH)Nb bond is necessary to generate Brønsted acid sites.

5 and 16 wt% Nb2O5/Al2O3 catalysts were prepared by impregnation method by using niobium oxalate solution with various pH values. The acid property of Nb2O5/Al2O3 was examined by benzylation of anisole and pyridine adsorbed Fourier transform infrared (FT-IR) spectra. The structure of domains of niobium oxide monolayer on 5 and 16 wt% Nb2O5/Al2O3 were characterized by means of X-ray diffraction (XRD), Raman spectroscopy, and pyridine adsorbed FT-IR spectra. The acid property of 16 wt% Nb2O5/Al2O3 strongly depended on the pH values of niobium oxalate solution. As increase of pH value of niobium oxalate solution, the size of domains of niobium oxide monolayer increased. Brønsted acidity on Nb2O5/Al2O3 increased with increasing in the domain size of niobium oxide monolayer. The structural analysis strongly suggests that the stress triggered by generation of the boundaries between domains is higher, as the domain size of niobium oxide monolayer increases. We proposed that the strong distortion of Nb(OH)Nb bond at the boundaries between domains of niobium oxide monolayer is necessary to generate Brønsted acid sites on Nb2O5/Al2O3 catalysts.

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