Heterogeneous selective ammoxidation over bismuth molybdate catalysts

Abstract

The ammoxidation of propene and of acrolein has been studied over two catalysts, one (UBM4) with Bi:Mo = 0.73:1 similar to commercial bismuth molybdate catalysts and the other (GS1) consisting of the koechlinite phase Bi 2O 3 · MoO 3. The rates, reaction orders, Arrhenius parameters, and selectivities of the ammoxidations and of several selected reactions which appear in routes proposed by other workers were studied in a static vessel at 400 °C. The results show that oxides of nitrogen are not realistic intermediates in the ammoxidations, nor is the main route for ammoxidation of propene over UBM4 through gaseous acrolein, even though the dissociative adsorption of propene is probably the rate determining step. The ammoxidations over GS1 show resemblances to the reactions over UBM4 in rate laws, and in the rates per unit area of the catalysts at 400 °C. However, the selectivities are much higher, being about 95% for both over GS1 compared with 50% for propene and 70% for acrolein over UBM4. The rate of ammoxidation over GS1 is twice the rate of selective oxidation, suggesting that the adsorbing propene molecule reacts immediately with a surface species containing nitrogen, rather than reacting first with the oxide lattice to form allyl and hydroxyl radicals as it seems to do on UBM4.