Synthesis and research of new superacid ZrO2— SiO2—SnO2 oxide

Abstract

The superacid ternary ZrO₂—SiO₂—SnO₂ oxide has been synthesized by the sol-gel method with different atomic ratios Zr : Si : Sn. It is determined that superacid sites are formed at the ion content: 10 ⩽ Zr⁴⁺ ⩽ 35, 50 ⩽ Si⁴⁺ ⩽ 53, 5 ⩽ Sn4+ ⩽ 40 %. The highest strength of acid sites (H₀ = –14.52) is observed in the interval: 21 ⩽ Zr⁴⁺ ⩽ 29, 60 ⩽ Si⁴⁺ ⩽ 67, 11 ⩽ Sn⁴⁺ ⩽ 20 %. According to the acid sites strength distribution on Zr₂₉Si₆₀Sn₁₁ surface, there are 10 % of superacid sites (−14.52 ⩽ H₀ ⩽ −12.14) and 40 % of strongly acid sites with −12.14 ⩽ H₀ ⩽ −8.2 at the total acidity of 1.5 mmol/g. The total concentration of acid sites on ZrO₂—SiO₂—SnO₂ surface is 1.1–1.7 mmol/g. All samples are characterized by a highly developed surface (200–400 m²/g) and an average pore diameter of ≈ 3 nm. Samples with Sn ⩽ 25 and Zr ⩽ 45 % have amorphous structure. It is shown that, at the tin content up to 25 % (at.), SnO₄ and SnO₆ species are observed in the structure of ZrO₂—SiO₂—SnO₂. At increasing the tin content, the formation of SnO₂ is observed. From the electron diffuse reflectance spectra of ZrO₂—SiO₂— SnO₂ samples, the limiting band gap of 3.8–4.3 eV is calculated, that is a criterion of the synthesis of superacid ZrO₂—SiO₂—SnO₂. According to the ¹¹⁹Sn and ²⁹Si MAS NMR spectra of ZrO₂—SiO₂—SnO₂, a partial shift of the electron density from zirconium atoms to silicon and tin atoms is observed that causes the formation of supera cid Lewis sites (H₀ = −14.52). It is shown that superacid Zr₂₉Si₆₀Sn₁₁ catalyst efficiently catalyzes the acy lation of toluene with acetic anhydride at 150 °С in a flow reactor with 45 % conversion of anhydride at 100 % selectivity towards n-methylacetophenone.