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

Nanosized solid superacids SO42−/TiO2 and S2O82−/TiO2, as well as MCM-41-supported SO42−/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 SO42−/TiO2 and S2O82−/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 SO42−/ZrO2/MCM-41, SO42−/TiO2 and S2O82−/TiO2 exhibit mainly Bronsted acidities. The strongest Bronsted acid sites were produced on SO42−/TiO2 promoted with H2SO4, while Lewis acid sites on S2O82−/TiO2 even stronger than those on SO42−/ZrO2/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO42−/TiO2 and S2O82−/TiO2 exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO42−/ZrO2/MCM-41. It can be concluded that the existence of more Brønsted acid sites was favorable for proton participation in the cyclization reaction.