(555f) Mechanistic Origins of Unselective Products in Acrolein Synthesis on Mixed Metal Oxide Catalysts

Linh Bui, Aditya Bhan

University of Minnesota – Twin Cities, Minneapolis, MN 55455

The selective oxidation of propylene to acrolein is the first step in the production of high value acrylate-based chemicals. The formation of aldehydic and acidic organic byproducts, even in small quantities (less than 10% yield), poses significant challenges to the down-stream processing of acrolein and its derivatives. We describe in this talk the mechanistic origins of over-oxidation and C-C bond scission products in the partial oxidation of propylene to acrolein on mixed metal oxide catalysts. We combine transient kinetic studies, co-feed experiments of aldehydes and carboxylic acids formed as byproducts in propylene oxidation, and isotopic-labeling studies to elucidate the reaction mechanisms, identify the existence and the involvement of relevant surface intermediates, and develop an extensive reaction network describing the formation of all C₂ – C₆ products (> 20 C₂-C₆ products are formed in this chemistry at carbon selectivity as low as 0.001%), and illustrate the underlying mechanisms for C-C bond cleavage and formation reactions. The carbon backbone of propylene is preserved in sequential oxidation of propylene to allyl alcohol, acrolein, and acrylic acid, as well as propylene oxidation to acetone and propanal via water-mediated pathways. Transient kinetic measurements in conjunction with co-feed experiments of C₂ and C₃ aldehydes and carboxylic acids show that decarbonylation and decarboxylation reactions, reactions of organic compounds with surface-adsorbed oxygen species, and total combustion reactions are the three mechanisms for C-C bond cleavage. C-C bond formation reactions that result in C₄ – C₆ byproducts is shown to occur via: (i) the addition reaction of a propylene-derived surface allyl (C₃H₅) species with formaldehyde to form C₄ products and with propylene and/or allyl alcohol to form C₆ products, or (ii) the addition reaction of an acrolein (acrylic acid)-derived surface ethenyl (C₂H₃) intermediate with propylene to form pentadiene and with acrolein to form C₅ cyclic oxygenates. These mechanistic insights provide guidance for process conditions and catalyst development to minimize the formation of undesired products.