Manganese-based Oxides Catalyze The Dehydrogenation Of Ethylbenzene By CO 2 Oxidation

Styrene （ST） is one of the most important monomers for the syntheses of valuable polymers such as rubber, plastic, and resin, etc. Among current various production teachnologies, the oxidative dehydrogenation of ethylbenzene （EB） with carbon dioxide （CO₂-ODEB） has received considerable attention, due to its high efficiency, energy-saving, and effective utilization of the greenhouse gas of CO₂. However, the poor catalytic activity and stability for CO₂-ODEB of reported catalysts limit its industrial application.It is well known that manganese oxides （MnOx） are characteristic of their unique stuctures, crystal phases, mixed-valences of Mn (e.g. Mn²⁺ and Mn³⁺), and thus high catalytic activity. In our work, a series of MnOx with various physicochemical properties were prepared by different synthetic methods such as the thermal decomposition method, the sol-gel method, and the precipitation method. The obtained MnOx catalysts were evaluated for the CO₂-ODEB reaction. XRD, H2-TPR, XPS and TG/DSC measurements were carried out to ascertain the relationships between the catalytic performances, synthetic methods, and structural properties of MnOx. Meanwhile, the deposition of carbonaceous species over theses catalysts was investigated. In view of redox cycles of Ce⁴⁺/Ce³⁺ and Mn³⁺/Mn²⁺ electron couples, a set of Ce/Mn composite oxides catalysts were prepared. Moreover, it was reported that Zr-based oxides displayed excellent activity to catalyze CO₂-ODEB. Here, various Mn/Zr composite oxides catalysts were also designed and their catalytic activities were assessed.Main research contents and coulusions were described as follows:（1） The MnOx catalysts with different structures were prepared by the precipitation method, the complex-decomposition method, and the thermal decomposition method, respectively. Subsequently, their catalytic activities for CO₂-ODEB were evaluated. Preliminary experiment results indicate that the catalytic activity of the MnOx is closely correlated to their structural properties which depend on the synthesis methods and parameters. A Mn3O4 catalyst prepared by the precipitation method show much higer catalytic activity （40% EB conversion under the optimum conditions of T= 550℃, P= 0.1 MPa, CO₂/EB （molar ratio）=5, and flow rate of EB= 0.006 mL·min-1）. It may be attributed to the presence of Mn³⁺/Mn²⁺ redox couple in Mn3O4 catalyst, lower crystallinity, smaller grain size, larger specific surface area and harsh reduction temperature. Moreover, further characterization and evaluation results reveal that the CO₂-ODEB over MnOx may follow the redox mechanism. The redox cycle of Mn³⁺/Mn²⁺ redox couple in MnOx and the carbonaceous deposition are key factors in determining the activity and durability of the MnOx for CO₂-ODEB.（2） The Mn/Ce composite oxides catalysts were prepared by the co-precipitation method and their catalytic activities for CO₂-ODEB were evaluated. It was found that all theses Ce/Mn composite oxides show higher catalytic activity in contrast to their individual oxides. Among them, the Mn/Ce= 4/6 catalyst displays the highest conversion of ethylbenzene （up to 60%） but poor stability. Higher activity of Mn/Ce= 4/6 catalyst maybe result from its higher specific surface area and the solid solution structure which promotes the redox cycles of Ce⁴⁺/Ce³⁺ and Mn³⁺/Mn²⁺ electron couples. The disruption of solid solution structure as well as irreversible reduction of high valence manganese may lead to activity deterioration. The coverage of carbonaceous species over the active sites may also be the reason for the catalyst deactivation.（3） The Mn/Zr composite oxides were prepared by the co-precipitation method and were assessed as catalysts for CO₂-ODEB reaction. Results indicate that all theses catalysts exhibite high catalytic activity. Among them, the Mn/Zr= 3/7 catalyst show the highest conversion of ethylbenzene （50%）. It may be attributed to the formation of a solid solution structure and the highest specific surface area. The Mn/Zr= 3/7 catalyst also show superior catalytic stability, which mainly due to the transition of monoclinic ZrO₂ into the cubic ZrO₂ during the reaction process without the new phase MnO. Furthermore, irreversible reduction of high valence manganese and the deposition of carbonaceous species may result in catalyst deactivation.