Sol–gel silica modified with phosphate and sulfate ions

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 SiO2. The solid, SiO2–H3PO4 presents a surface area of 634 m2/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 SiO2–HNO3 sample. The density of Lewis and Brönsted acid sites increased with the incorporation of phosphate and sulfate ions. These acid sites are stable, maintaining strongly adsorbed pyridine at 400 °C.