Acidity of sulfated oxides: Al2O3, TiO2 and SiO2. Application of test reactions

Abstract

The acidic properties of different oxide/SO₄²⁻ type superacids have been studied. Test reactions indicated that the acid strength of the Al₂O₃/SO₄²⁻ system changed considerably with H₂SO₄ surface content notably −7.05 < H₀ < −10 (1%), −10 < H₀ < −14 (2–3%), H₀ < −14 (4–9%), while for TiO₂/SO₄²⁻ preparations it was high and less dependent on catalyst composition: H₀ < −14 (≥1%) and finally for SiO₂/SO₄²⁻ catalysts it was low with −0.75 < H₀ < −3.50 (1–9%). Alkane isomerization over Al₂O₃/SO₄²⁻ and TiO₂/SO₄²⁻ catalysts was observed to begin with the formation of an unsaturated intermediate state through the action of redox centres. Further alkene transformation into carbenium ion in the presence of Brønsted sites (Al₂O₃/SO₄²⁻) or secondary Brønsted centres (TiO₂/SO₄²⁻) allows skeletal alkane isomerization to proceed.