Kinetic modeling to estimate fundamental yield bounds for selective propylene oxidation over bifunctional catalysts

The selective oxidation of propylene via heterogeneous catalysts is one of the most successful commercial processes to produce acrylic acid (CH2CHCOOH). For this catalytic oxidation process where multiple products are possible, it is desirable to understand how a catalyst can control the reactivity of the system toward desired products. In this work, an elementary-step kinetic mechanism for the catalytic oxidation of propylene was constructed based on information in the literature and analogous surface and gas-phase reactions. This model was used in several distinct ways: to fit experimental data, to see if there are fundamental limits on the achievable yield, and to explore how the catalyst parameters would have to change to allow better yields. Such information can provide guidance to experimentalists designing new real-world catalysts with improved yields and selectivities, a process which is often expensive and time-consuming. From our modeling results, some byproducts are inevitable, but it appears that there is no fundamental reason that high (∼99%) yields of acrylic acid could not be obtained with the right catalyst, in either the one-step or the two-step process through acrolein. Some properties of any high-yield catalyst for acrylic acid production are outlined.