Optimizing mass-transfer and oxygen components to enhance toluene oxidation for MnOx/SAPO-34 and revealing mechanism by in-situ DRITFS

Abstract

A series of MnO_x/SAPO-34 was synthesized using deposition-precipitation method with acid, alkali, saline to modify their physicochemical property enhancing the performance for toluene oxidation. The structure-activity relationship was explored in depth using variant instruments. It was found that the SAPO-34 was etched by desiliconization with oxalic acid as precipitant and converting its micropores into mesopores as well as macropores, which is not only conducive to anchoring and dispersion of more active MnO_x, but also promote the mass transfer of reactants. Meanwhile, the catalyst derived from oxalic acid possesses abundant redox sites with the strongest low temperature reducibility. As the results, the Mn/S34-OA exhibits the best performance of toluene oxidation with T₉₀=237 °C and ∼100 % CO₂ selectivity. Besides, the catalyst maintained long-term stability even in presence of H₂O at 260 °C. Furthermore, the reaction mechanism is revealed by in-situ DRIFTS experiments. It is demonstrated that lattice oxygen directly participates in oxidation of toluene as the crucial active component in path of toluene to benzyl alcohol to benzaldehyde to benzoic acid to CO₂ finally. In addition, the consumed lattice oxygen is replenished by adsorbed gaseous O₂, which is the key to maintain continuous reaction cycle.