Ketonization of Acetic Acid over Iron Oxide: Intertwined In Situ Changes of Active Sites and the Contradictory Role of Oxygen Vacancies

Iron oxides are catalytically active for the ketonization of acetic acid in bio-oil upgrading. However, due to the complex reaction pathway and phase evolution of iron oxide, the underpinning catalytic mechanism is still far from fully understood. Through purposely designed experiments, advanced characterization, and density functional theory (DFT) modeling, this work confirmed a higher activity of magnetite than hematite for a lower activation energy and a higher reaction order with respect to acetic acid vapor pressure. For pristine hematite, the oxidation of acetic acid causes a gradual loss of lattice oxygen and an in situ phase change of hematite into magnetite. The α-H abstraction step on the (001) surface of hematite is the most energy-intensive, limiting the whole ketonization reaction rate. In contrast, once magnetite forms, its (111) surface becomes active, with β-keto acid decarboxylation as the rate-limiting step for a lower energy barrier. Additionally, on the surface of pristine hematite, the ketene species could be a precursor for the formation of β-keto acid, which, however, was not observed on the magnetite surface. The oxygen vacancy plays a controversial role in the two different oxides. Its presence on hematite facilitates α-H abstraction from the acetate adsorbate. In contrast, its presence on magnetite is detrimental, as it removes the tricoordinated lattice oxygen─the most active site for the initial α-H abstraction. These fundamental results provide practical insights into the optimization of iron oxide catalysts for prior H₂-reduction and the controlled manipulation of oxygen vacancies to mitigate in situ acetic acid oxidation and associated coke deposition apart from the aldol condensation.