Direct epoxidation of propylene on NaCl-modified CU/SiO2 catalysts using air as oxidant

Propylene oxide (PO) is an important chemical intermediate, but its traditional production processes cause serious environmental problems. Direct epoxidation of propylene with molecular oxygen has attracted both industrial and academic interests. A series of NaCl-modified Cu catalysts were prepared by impregnation method, and their catalytic performance for epoxidation of propylene with air as the oxidant was investigated. The results showed that SiO2 is a suitable support and NaCl is an effective promoter for the epoxidation. The unmodified Cu/SiO2 has no catalytic activity, while the modified Cu/SiO2 with 20%NaCl gives 17.7% selectivity for PO. The unreduced Cu/SiO2 is not active for this reaction, while the catalyst reduced with H-2 shows higher activity, which indicates that the reduced Cu species is very important for the reaction. Under the conditions of n (C3H6): n (O-2) : n(H-2) = 10:1:1, SV = 1.5 x 10(4) h(-1) and theta = 215 degreesC, the propylene conversion is 0.16% and PO selectivity reaches up to 44% on 10% Cu-2% NaCl/SiO2 catalyst. The addition of hydrogen in the feedstock can remarkably increase the PO selectivity and stabilize the catalyst. It is proposed that H-2 reacts with oxygen species on the catalyst surface, thus keeps the catalyst from being oxidized. The results of temperature-programmed reaction showed that there is a reduction peak at about 400 degreesC for Cu/SiO2, when it was modified with NaCl, the peak shifts to 300 degreesC, which implies that the interaction between CuO and NaCl takes place. The reduced copper species with low valence may be the active sites for epoxidation.