Size Effect of Unsupported CuOx on Propylene Epoxidation by Oxygen

Propylene epoxidation by oxygen over Cu-based catalysts is a most ideal and economical process. The unsupported CuO x nanoparticles were synthesized by liquid phase reduction method. The crystalline phase and size of CuO x nanoparticles were characterized by XRD and TEM, respectively. The catalytic performances of CuO x with different pretreatment temperatures and atmospheres were also investigated. The as prepared unsupported CuO x nanoparticles were composed of Cu and Cu 2 O. With elevating the pretreatment temperatures under N 2 atmosphere, the size of CuO x nanoparticles increased gradually from 25 nm to 58 nm. However, the relative content between Cu and Cu 2 O remained almost unchanged, indicating that the valence state of Cu species in CuO x was kept nearly constant. The C 3 H 6 epoxidation activities showed a first increases and then decreases trend with enhancing the particle size of CuO x . The highest activity of propylene epoxidation was achieved when the particle size of CuO x was 41 nm. The conversion of propylene was 0.14% and the selectivity of propylene oxide was 30% respectively under the reaction temperature of 150 °C. The results indicated that the appropriate CuO x particle size was a key factor for propylene oxide formation on Cu-based catalysts. Graphic Abstract The C 3 H 6 epoxidation activities showed a first increases and then decreases trend with enhancing the particle size of CuO x . When the CuO x particle size was 41 nm, the formation rate of PO reached maximum. The appropriate particle size of Cu species with low valence state was crucial to the generation of PO for C 3 H 6 epoxidation reaction by O 2 on Cu-based catalysts.