Acid sites development on Cr(3+)/SiO(2) catalysts obtained by the sol-gel method and hydrothermal treatment: Effect of calcination temperature

• Published in: Applied catalysis. A, General Vol. 457; pp. 26 - 33
• Main Authors: Cuesta Zapata, Pablo M, Parentis, Monica L, Gonzo, Elio E, Bonini, Norberto A
• Format: Journal Article
• Language: English
• Published: 24-04-2013
• Subjects: Calcination; Catalysis; Catalysts; Chromium; Dehydration; Hydrothermal treatment; Silicon dioxide; Sol gel process
• ISSN: 0926-860X

Cr/SiO(2) 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 degree C. The solids were characterized by N(2) sorptometry,TG-DTA, DRUV-vis, XRD, and FTIR. In vacuum or N(2) atmosphere, Cr(3+) ions were stable up to 450 degree C. However, calcination in air promotes the partial oxidation of Cr(3+) to Cr(6+) as the temperature increases from 200 to 450 degree C. The amount of Cr(6+) 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 Bronsted acid centers. Cr(3+) ions were related to the presence of Lewis sites, and Cr(6+) ions to the development of weak Bronsted acid sites. Accordingly, the 2-propanol dehydration reaction rate shows an increase when the catalysts are treated in N(2) at 450 degree C, while catalysts calcined in air develop a lower acidity. The dehydration reaction is attributed to the presence of Lewis acid centers.