Acidity and Characterization of 12-Tungstophosphoric Acid Supported on Silica-Alumina

This work deals with preparation and characterization of H3PW12O40 (H3PW) supported on silica-alumina. Impregnation of H3PW (15, 20, 30 and 40 wt.%) on commercial silica-alumina support in acidic aqueous solution is effective for preparing this catalyst keeping its Keggin structure, according to different methods of characterization. The catalysts were tested in a model reaction of acetic acid with ethanol and 30 wt.% H3PW/SiO2-Al2O3 had the highest activity under the conditions: catalyst calcination at 300 degrees C, temperature of 100 degrees C, acetic acid: ethanol molar ratio of 2: 1 and catalyst: acetic acid mass ratio of 10 wt.%. The reaction yield was 79 and 100% selectivity for ethyl acetate over three reutilizations, for reaction time of 2 h. The calculated total acid site distribution was 0.299 mmol g(-1) (97% of the theoretical probed by pyridine), and most of these (0.236 mmol g(-1)) were Bronsted weak-medium strength (pyridine desorption between 300 and 500 degrees C).