Sol–gel silica modified with phosphate and sulfate ions

Abstract Sol–gel silica was synthesized using TEOS as a precursor and phosphoric or sulfuric acid as a source of phosphate and sulfate ions. The textural properties increase with the addition of these ions in comparison with unmodified SiO 2 . The solid, SiO 2 –H 3 PO 4 presents a surface area of 634 m 2 /g, greater than the solid modified with sulfate ions. FTIR spectra of the solids show Si–OH vibrations bands that remained even after annealing the samples at high temperature. It is observed that the relative abundance of Si–OH substitution strongly depends on the hydrolysis catalyst used. Slight Lewis acidity can be seen in SiO 2 –HNO 3 sample. The density of Lewis and Bronsted acid sites increased with the incorporation of phosphate and sulfate ions. These acid sites are stable, maintaining strongly adsorbed pyridine at 400 °C.