Preparation and characterization of SO42-/TiO2 and S2O82-/TiO2 catalysts

Abstract

Nanosized solid superacids SO₄^2-/TiO₂ and S₂O₈^2-/TiO₂, as well as MCM-41-supported SO₄^2-/ZrO₂, were prepared. Their structures, acidities, and catalytic activities were investigated and compared using XRD, N₂ adsorption-desorption, and in situ FTIR-pyridine adsorption, as well as an evaluation reaction with pseudoionone cyclization. The results showed that SO₄^2-/TiO₂ and S₂O₈^2-/TiO₂ possess not only nanosized particles with diameters <7.0 nm, a BET surface greater than 140 cm²/g and relatively regular mesostructures with pores around 4.0 nm, but also a pure anatase phase and strong acidity. Different from the Lewis acid nature of SO₄^2-/ZrO₂/MCM-41, SO₄^2-/TiO₂ and S₂O₈^2-/TiO₂ exhibit mainly Br¢nsted acidities. The strongest Br¢nsted acid sites were produced on SO₄^2-/TiO₂ promoted with H₂SO₄, while Lewis acid sites on S₂O₈^2-/TiO₂ even stronger than those on SO₄^2-/ZrO₂/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO₄^2-/TiO₂ and S₂O₈^2-/TiO₂ exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO₄^2-/ZrO₂/MCM-41. It can be concluded that the existence of more Br¢nsted acid sites was favorable for proton participation in the cyclization reaction.