Study On Catalyst For The Ammoxidation Of Propane To Acrylonitrile

Since petroleum price keeps up, nowadays, new technologies related to producing chemicals with light alkanes instead of light alkenes have drawn more and more attentions. Among them, the ammoxidation reaction of propane is currently investigated as a possible alternative to the ammoxidation reaction of propene to produce acrylonitrile in commercial application, in which the design and preparation of highly efficient catalystsis a key problem, resulting in further increase in the yield of acrylonitrile. Recently, Sb-V and Mo-V mixed oxides are two promising catalyst systems for the ammoxidation reaction of propane in industry.In this paper, Sb-V and Mo-V mixed oxides were studied as model catalysts. In order to enhance the yield of acrylonitrile, the effects of promoters and preparation conditions on the catalytic performances of these catalysts were studied in detail. The results obtained in this paper can be used as the guidance for designing, preparing and optimizing catalysts for the ammoxidation of propane. The main results are obtained as follows:The dependence of Sb-V mixed oxide catalytic activity on different molar ratios of Sb/V was studied. The results show that, when the catalyst with Sb/V molar ratio of<3was used the conversion of propane is increased and the selectivity in acrylonitrile is decreased significantly, resulting in a deep oxidation of propane and more formation of COX by-products. Using the catalyst with Sb/V molar ratio of>3, higher selectivity of acrylonitrile can be obtained and the conversion of propane is decreased significantly. The highest yield of acrylonitrile was obtained when Sb/V molar ratio reaches to3in this catalyst. In this VSb3mixed oxide catalyst, there are rutile-type SbVCU and ct-Sb2O4, SbVC>4is key active phase, and the valence state of Sb and V could be kept unchangeable during the ammoxidation reaction. When Sb/V is lower or higher than3in this catalyst, V2O5or cc-Sb2O4phases would be increased in the catalyst.The dependence of Sb-V mixed oxide catalytic activity on promoters (oxalic acid, Al, W, Mo) was studied. The results show that, an addition of oxalic acid in preparation process helps to keep V in a low valence state and generate SbVC>4active phase. When adding Al in the catalyst, distributed Al (as alumina) inside the catalyst plays a role of support, increasing the dispersion of active sites and the conversion of propane. The presence of W in VSb3mixed oxide can promote the formation of rutile-type Sb(W,V)C>4, resulting in further improvement of the acrylonitrile yield. If Mo was added in this catalyst, the yield of acrylonitrile is decreased unexpectedly. It has been found that adding oxalic acid, Al and W in the Sb-V mixed oxide catalyst could increase the number of non-stoichiometric rutile-type SbVO4on the catalyst surface and accelerate reduction and re-oxidation rate of the catalyst, resulting in an increase in both the conversion of propane and the selectivity of acrylonitrile. Using the ViSbaA^Wi5mixed oxide catalyst,30.2%yield of acrylonitrile could be achieved.The dependence of Mo-V mixed oxide catalytic activity on Mo, V, Te and Nb modifier was studied. The results show that, using the MoVo.31Teo.23Nbo.24oxide catalyst.35%yield of acrylonitrile was achieved. The effect of adding La, Ce and Pr on the catalytic performance of MoVo.31Teo.23Nbo.24was investigated. It has been found that adding Ce can promote the formation of Ml and M2key active phases, which can further improve the efficiency of the catalyst.45.3%yield of acrylonitrile was obtained over the catalyst with molar ratio of Ce/Mo of0.05. This can be explained that the amount of Ml active phase in this catalyst increases to maximum, the existence of Ce+/Ce+redox pair accelerates the lattice-oxygen migration, and the reduced active site can be instantaneously re-activated or re-oxidized during the ammoxidation reaction.