Temperature-programmed desorption of NH3 (NH3-TPD) was used to measure the total amount of acid sites on the silica-alumina catalysts. For this, samples (100 mg) were dried in N2 flow at 600 °C (10 K min -1 , 3 h) and subsequently loaded with NH3 (2 vol% NH3 in N2, 20 mL min -1 ) at 140 °C. When the physisorbed NH3 was desorbed, the samples were heated to 600 °C (10 K min -1 ). The amount of chemisorbed NH3 was detected by FT-IR.