Effects of catalyst crystal structure on the oxidation of propene to acrolein

Abstract

Bismuth molybdate is known to be active for the oxidation of propene to acrolein and its activity can be altered by substitution of other elements (e.g. Fe, V, W) into the scheelite phase of α-Bi2Mo3O12. This work has further revealed that the apparent activation energy for acrolein formation correlates with the band gap of the catalyst measured at reaction temperature. It is, therefore, of interest to establish to how the crystal structure of the catalyst affects the activation energy. We report here an investigation of propene oxidation conducted over Bi, Mo, V oxides having the aurivillius structure with the composition Bi4V2-xMoxO11+x/2 (x=0−1) and compare them with oxides having the scheelite structure with the composition Bi2-x/3MoxV1-xO12 (x=0−1). The aurivillius-phase catalysts again show a correlation between the apparent activation energy and the band gap of the oxide, and the only difference being that for a given band gap, the apparent activation energy for the aurivillius-phase catalysts is 1.5kcal/mol higher than that for the scheelite-phase catalysts. This difference is attributed to the lower heat of propene adsorption on the aurivillius-phase catalysts. A further finding is that for catalysts with band gaps greater than ∼2.1eV, the acrolein selectivity is ∼75% for the conditions used and independent of the propene conversion. When the band gap falls below ∼2.1eV, the intrinsic selectivity to acrolein decreases rapidly and then decreases further with increasing propene conversion. This pattern shows that when the activity of oxygen atoms at the catalyst surface becomes very high, two processes become more rapid – the oxidation of the intermediate from which acrolein is formed and the sequential combustion of acrolein to CO2.