Mechanistic Origins of Unselective Oxidation Products in the Conversion of Propylene to Acrolein on Bi₂Mo₃O₁₂

A reaction network detailing the formation and consumption of all C₁-C₃ products of propylene oxidation on Bi₂Mo₃O₁₂ at 623 K is developed to show that acrolein, acetaldehyde, acetone, and acetic acid are direct oxidation products of propylene while acrylic acid and ethylene are secondary products. Coprocessing acetaldehyde, acetone, acrylic acid, and acetic acid, separately, with propylene, oxygen, and water revealed (i) the existence of overoxidation reactions of acrolein to acrylic acid and ethylene and oxidation pathways from acetone to acetaldehyde and acetic acid, (ii) the promotional effects of water on the synthesis rates of acetaldehyde from acrolein, acetone from propylene, and acetic acid from acetaldehyde and acrylic acid, and (iii) the inhibitory effects of water on the decomposition of acetic acid to CO_x and acrylic acid to acetaldehyde and ethylene. A kinetic model is developed to quantitatively capture the kinetic behavior of all species using pseudo-first-order rate expressions in the organic reactant for all reaction pathways; additional promotional and inhibitory dependencies on water pressure were added to describe the kinetics of reaction rates affected by water. Based on the proposition that multiple types of active sites exist on the mixed metal oxide surface during propylene oxidation, a detailed mechanistic network is postulated that describes all molecular transformations with relevant surface intermediates and provides critical insights into the underlying pathways involved in overoxidation and C-C bond scission reactions.