12-Phosphotungstic acid (PWA)-based complex catalysts for propylene epoxidation were prepared by modification with nitrogen-containing heterocycles such as imidazole, pyrazole and 1,2,4-triazole. Heterocycles played an important role in promotion of the catalytic activity and selectivity to propylene oxide, mostly due to the strong electronic interactions between heterocycles and terminal tungsten-oxygen on PWA. Propylene epoxidation over the complex catalyst seriously depended on the pressure and the temperature, favoring high pressure and low temperature for improved conversion and yields, respectively. The complex catalysts maintained good thermal stability up to 450 degrees C as well as recycling performance for 5 reaction runs without any significant loss of catalyst, suggesting that they are promising heterogeneous catalysts on propylene epoxidation. (c) 2013 Elsevier B.V. All rights reserved.