Size Effect of Unsupported CuOx on Propylene Epoxidation by Oxygen

Abstract

Propylene epoxidation by oxygen over Cu-based catalysts is a most ideal and economical process. The unsupported CuOx nanoparticles were synthesized by liquid phase reduction method. The crystalline phase and size of CuOx nanoparticles were characterized by XRD and TEM, respectively. The catalytic performances of CuOx with different pretreatment temperatures and atmospheres were also investigated. The as prepared unsupported CuOx nanoparticles were composed of Cu and Cu2O. With elevating the pretreatment temperatures under N2 atmosphere, the size of CuOx nanoparticles increased gradually from 25 nm to 58 nm. However, the relative content between Cu and Cu2O remained almost unchanged, indicating that the valence state of Cu species in CuOx was kept nearly constant. The C3H6 epoxidation activities showed a first increases and then decreases trend with enhancing the particle size of CuOx. The highest activity of propylene epoxidation was achieved when the particle size of CuOx 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 CuOx particle size was a key factor for propylene oxide formation on Cu-based catalysts. The C3H6 epoxidation activities showed a first increases and then decreases trend with enhancing the particle size of CuOx. When the CuOx 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 C3H6 epoxidation reaction by O2 on Cu-based catalysts.