High dispersed 10% MoO3/SiO2 catalysts were prepared by the sol gel method using a precipitant (ammonium hydroxide) with different pH values, and investigated by XRD, FT-IR spectroscopy of pyridine adsorbed and Raman spectroscopy techniques, and so on. The results show that the catalytic performance of MoO3/SiO2 for the epoxidation of propylene with cumene hydroperoxide (CHP) is affected by pH value of precipitant, and MoO3/SiO2 prepared with precipitant of pH 9 exhibits the highest yield of propylene oxide (PO). It has been found that the weak Lewis acidic sites on MoO3/SiO2 are the active sites of the propylene epoxidation with CHP, total amount of Lewis acid sites on the catalyst surface is related with the CHP conversion, and the weaker Lewis acid sites is in favor of the propylene epoxidation. When the amount of Lewis acid sites on the catalyst surface is more and their acid strength is higher, the CHP degradation and PO acid-catalytic hydrolysis would be speeded up, resulting in a reduction of the PO selectivity. The concentration and strength of the Lewis acid sites on MoO3/SiO2 are affected by pH of precipitant, and the catalyst prepared with precipitant of pH 9.0 possesses the most weakly Lewis acidic sites and the highest selectivity to PO (91.5%). Besides, the addition of certain amount of Nd can increase the weakly acidic sites to enhance CHP conversion and reduce the Lewis acidity of the catalyst thus suppress PO hydrolysis. (C) 2013 Elsevier B.V. All rights reserved.