(700d) Forced Dynamic Operation of Propylene Selective Oxidation to Acrolein

The selective oxidation of propylene to acrolein (ACO) and acrylic acid (ACOOH) are conventionally carried out under steady state conditions in fixed-bed or fluidized-bed reactors on bismuth-molybdate based catalyst. It is thought that the Mars-Van Krevelen mechanism governs the selective oxidation to ACO or ACOOH; specifically, in the absence of gas phase oxygen, propylene reacts with nucleophilic lattice oxygen. Decoupling the oxidation and reduction stages of the catalyst hence has the potential to improve (ACO and ACOOH) selectivity. In this study, we evaluate forced dynamic operation of a laboratory fixed-bed reactor to identify conditions of ACO and/or ACOOH selectivity/yield enhancement over steady state operation.

The effluent ACO concentration of an FDO and a steady state operation (SSO) with the same reductant to oxidant ratio (ROR) is shown in Figure 2. Because SSO receives more oxygen (11.7% O₂) than FDO does in its primary cycle (5%) at first, its ACO yield is greater than FDO's. With time, however, sustained cyclic operation outperforms the SSO (17% ACO yield in FDO compared to 12% in SSO). Moreover, prolonged FDO had better ACO selectivity than SSO (54% in FDO vs. 41% in SSO). The catalyst's initial high activity is caused by an increase in the amount of oxygen stored there during pretreatment. The amount of oxygen the catalyst has available to convert propylene into acrolein steadily decreases as the catalyst's re-oxidation occurs more slowly than its reduction. By adding more oxygen for a longer period during the secondary re-oxidation cycle of the FDO, the initial high activity of the catalyst can be maintained, which is demonstrated in Figure 3. When catalyst oxidation is rate-limited at low temperatures (below 370 °C), changing the feed composition by re-oxidizing the catalyst without the reductant improves acrolein selectivity and yield in comparison to traditional steady state operation.