Hammett Acidity Function for Mixed ZrO2–SiO2 Oxide at Elevated Temperatures

Abstract

The mixed ZrO₂‑SiO₂ oxides with different 4 ≥ Zr/Si ≥ 0.25 atomic ratios have been prepared using a sol-gel method. It was shown that the concentration-strength acid site distributions on the surface of ZrO₂‑SiO₂ are caused by Zr⁴⁺/Si⁴⁺ ratio. The ZrO₂‑SiO₂ with Zr/Si = 0.5 is characterized by the highest strength of acid sites (Н₀ = –11.35). According to the IR-spectra of adsorbed pyridine, the B- and L-acid sites are present in the ZrO₂‑SiO₂ samples with Zr/Si<1 whereas at Zr/Si>1 only L‑sites are formed. Basing on the UV-Vis diffuse reflectance spectra of adsorbed indicators, the H₀ values for ZrO₂‑2SiO₂ have been determined at elevated temperatures. The Hammett acidity function reaches the superacid value of Н₀ ≤ –14.5 at Т ≥ 85 °С.