Acid sites development on Cr3+/SiO2 catalysts obtained by the sol–gel method and hydrothermal treatment: Effect of calcination temperature

Abstract

Cr/SiO2 catalysts (4–8% Cr, w/w), obtained from Cr(III) salts and tetraethylortosilicate (TEOS), were prepared by the sol–gel method. After TEOS hydrolysis in ammoniacal solution (pH: 9.5), the gel was treated under hydrothermal conditions at 120–220°C. The solids were characterized by N2 sorptometry, TG–DTA, DRUV–vis, XRD, and FTIR. In vacuum or N2 atmosphere, Cr3+ ions were stable up to 450°C. However, calcination in air promotes the partial oxidation of Cr3+ to Cr6+ as the temperature increases from 200 to 450°C. The amount of Cr6+ formed depends on the temperature of the hydrothermal treatment. 2,6-Lutidine adsorption allows to determine the evolution, with respect to calcination temperature, of both Lewis and Brönsted acid centers. Cr3+ ions were related to the presence of Lewis sites, and Cr6+ ions to the development of weak Brönsted acid sites. Accordingly, the 2-propanol dehydration reaction rate shows an increase when the catalysts are treated in N2 at 450°C, while catalysts calcined in air develop a lower acidity. The dehydration reaction is attributed to the presence of Lewis acid centers.