Study On Doping Of Iron-based Perovskite Catalysts For Chemical Chaining Ethylbenzene Oxidative Dehydrogenatio

Styrene is a commonly used intermediate in the chemical industry and is widely used in the production of various downstream polymers.Currently,90%of styrene is produced by direct dehydrogenation of ethylbenzene.But this process requires the introduction of a large amount of superheated steam,which consumes a large amount of energy.The Oxidative dehydrogenation method of in-situ oxidation of hydrogen gas using oxidizing atmosphere has gradually been applied to the the research of styrene production.Currently,it has undergone research on gasous oxidants such as O₂ and CO₂.In recent years,due to the advantages of chemical looping method that are not limited by equilibrium and the oxygen activity of oxygen carriers can be regulated,they have been widely used in the study of selective oxidative dehydrogenation of alkanes.Applying chemical looping technology to the oxidative dehydrogenation of ethylbenzne to styrene will greatly reduce energy consumption,and the key lies in the design of oxygen carriers.In this study,two types of Fe-based perovskites were modified by A/B site doping in order to maintain the original structure of perovskite.XRD showed that all samples maintained a good perovskite structure.In La perovskite,a styrene yield of 65%was finally achieved on La_0.5K_0.5Fe_0.6Mn_0.4O₃ at 450°C.The same A/B site doping was also carried out in alkaline earth series perovskites,and ultimately a 73%of styrene yield was achieved on Sr_0.8Ba_0.2Fe_0.2Mn_0.8O₃at 550°C.During the doping process,it was also found that the doping of A/B sites would have two effects:on the one hand,the increase in lattice oxygen activity in the catalysts;on the other hand,the change of conversion caused by the change of active sites.After the performance comparison of La-and alkaline earth-perovskites,Sr_0.8Ba_0.2Fe_0.2Mn_0.8O₃ was selected as the material for subsequent study of redox cycle stability.By optimizing the reaction temperature and time,the temperature for the cycle were determined to be 550°C and the time on stream to be 20 minutes.Within 40 cycles,the catalyst gradually deactivates as the number of cycles increases,reaching its lowest performance point around the8th cycle.Through XRD and XPS analysis,it was found that the deactivation of the catalyst was due to the formation of carbonates during the cycles,leading to the destruction of the perovskite structure.At the same time,the increase in surface valence states of Fe and Mn also corresponded to an increase in CO₂ in the product distribution.Through the comparison of another 10 cycles,it was found that heating and decarbonization treatment can restore the perovskite structure and slightly improve performance,stabilizing at a styrene yield of 77%.Combining XRD,TEM,and XPS analysis,the reason for the improved performance may be better crystallinity,dispersibility,and more reasonable surface state.The role of adsorbed oxygen,lattice oxygen,and lack-oxygen-catalyst and reaction type were demonstrated through mass spectrometry pulse experiments.The promotion effect of A/B site doping on the CL-ODH of ethylbenzene was demonstrated through in-situ infrared spectroscopy,and the reason for deactivation was also analyzed to some extent:the deactivation is likely due to the adsorption of CO₂ by perovskite to form carbonate.