K-doped Mo-V-Sb-O crystalline catalysts for propane selective oxidation to acrylic acid

Mo-V-Sb-O complex metal oxide catalysts were synthesized hydrothermally and potassium was doped to the catalysts either during the hydrothermal synthesis or by ion-exchange method. The obtained catalysts were characterized by ammonia TPD and tested for propane selective oxidation to acrylic acid (AA) in order to investigate relationship between surface acid property and catalytic properties. The K-doped catalysts showed higher selectivity to AA than the un-doped catalyst but poorer in the propane conversion. It was observed that potassium clearly decreased the ammonia adsorption capacity on the surface without affecting bulk structural phases and furthermore a linear relationship was obtained between the reaction rate of propane and the ammonia adsorption capacity. The results strongly suggest that surface protonic acid sites involve in the oxidative activation step of propane which is regarded as the rate-determining step. On the basis of a demonstrative test using the K-doping by ion-exchange and the grinding procedure, it is concluded that the selectivity increase by the K-doping was due to the less influence of potassium on the rate of propene selective oxidation. We discuss a multi-functional character derived from high dimensional structures of the catalysts and mechanism of the selective oxidation of propane.