Pathways, mechanisms, and kinetics of propylene oxidation on reducible metal oxide catalysts

Abstract

We report the pathways, mechanism, and kinetics for the formation of side products during propylene oxidation to acrolein on two metal oxide catalysts: Bi2Mo3O12 and a molybdenum-based mixed metal oxide doped with nickel and cobalt. This work showcases the utility of combining kinetic measurements, product rank and stability analysis, co-feed experiments, isotopic tracer studies, and kinetic modeling in the investigation of complex reaction networks during oxidation reactions on reducible metal oxide formulations. The trends of the yields of all C1 – C3 products of propylene oxidation on Bi2Mo3O12 at 623 K as a function of conversion show that acrolein, acetaldehyde, acetone, and acetic acid are direct oxidation products of propylene while acrylic acid and ethylene are secondary products. Co-processing acetaldehyde, acetone, acrylic acid, and acetic acid, separately, with propylene, oxygen, and water revealed (i) the existence of over-oxidation 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, acetic acid from acetaldehyde and acrylic acid, and (iii) the inhibitory effects of water on the decomposition of acetic acid to COx and acrylic acid to acetaldehyde and ethylene. We employ isotopic tracer experiments with allyl alcohol-13C3 and acrylic acid-13C3 to show that the carbon backbone of propylene is preserved in the sequential oxidation of propylene to allyl alcohol, acrolein, and acrylic acid during propylene oxidation at 623 K on a molybdenum-based mixed metal oxide catalyst promoted with cobalt and nickel used in the commercial production of acrolein. 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 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. We develop a kinetic model to quantitatively capture the kinetic behavior of all C1-C3 products during propylene oxidation on Bi2Mo3O12 using pseudo-first-order rate expressions in the oxygenate 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. We expand this model to include all C1 – C6 byproducts and surface species including lattice oxygen, vacancy site, surface hydroxyl group, and carbon-containing intermediates in propylene oxidation to acrolein at 623 K on a molybdenum-based mixed metal oxide catalyst promoted with cobalt and nickel. The model reproduces experimental molar amounts of 19 C1 – C6 products as well as of propylene, oxygen, and water reactants as a function of propylene conversion assessed in 30 independent experiments. The model is further validated by comparing the model output to the measured isotopic distributions of C5 products during isotopic tracer experiments with acrylic acid-13C3 as the probe molecule. The surface coverages of relevant species are investigated as a function of propylene conversion to assess the involvement of lattice oxygen, surface hydroxyl, and lattice oxygen vacancies as well as the major and minor pathways for the formation and consumption of the allyl and ethenyl surface species. The molecular level understanding of the mechanistic pathways and intrinsic active sites for byproduct formation during the partial oxidation of propylene will provide a fundamental assessment of catalysts for industrial acrolein production and guide future development of metal oxide catalysts.