Cu/SiO2 catalysts modified with six kinds of Na salts respectively are synthesized through an incipient wetness impregnation method,and their crystalline phase,particle size and Cu species valence state are characterized by means of XRD,TEM and XPS.The influences of the variety and mole fraction of Na salts on the activity of Cu/SiO2 catalysts in propylene epoxidation is investigated in detail.It is found that Cu2+species may be reduced to varying degrees by six kinds of Na salts,which can produce different amount of Cu0 species or Cu+species.All these six kinds of Na salts can improve the activity of Cu/SiO2 for propylene epoxidation,and Na2CO3 is the most favorable for the formation of propylene oxide.The appropriate amount of Na2CO3 is able to promote the formation of Cu+species and facilitate the epoxidation of propylene,but too much amount of Na2CO3 results in the generation of Cu0 species and the aggregation of Cu species,which is not conducive to the formation of propylene oxide.The higher the Cu+species mole fraction,the higher the activity of Cu/SiO2 in propylene epoxidation and the higher the formation rate of propylene oxide.Cu+species with small size have the most activity for propylene epoxidation and are the main active sites for propylene epoxidation reaction.
propylene epoxidationCu/SiO2Cu+speciesNa2CO3Na salt promoter
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