Low-temperature Chemical Chaining Ethylbenzene Oxidative Dehydrogenation Over Iron-based Spinel Catalyst

Styrene is an important basic organic monomer in the chemical industry and is widely used in the synthesis of polymer products,such as plastics,rubber and resin.With the continuous development of new polymer chemical products,the demand for styrene has significantly increased.However,the current ethylbenzene dehydrogenation process for styrene production,still has many pressing challenges,such as thermodynamic limitations in the dehydrogenation reaction process,high energy consumption,catalyst deactivation,low catalyst activity,and high operating temperature（>600 ^oC）.These problems restrict the green,low-carbon,and high-quality development of the styrene industry.Therefore,achieving low-temperature dehydrogenation of ethylbenzene for efficient styrene production is an attractive but challenging research topic.In this study,a series of Fe-based spinel oxygen storage catalysts were prepared using a citric acid-assisted sol-gel method in combination with impregnation method.The efficient dehydrogenation of ethylbenzene to styrene via chemical looping process was proposed.The reaction characteristics of iron-based spinel catalysts with ethylbenzene were investigated.The effect of K surface modification on the conversion performance of iron based spinel catalyst,and the cyclic stability of iron based spinel catalyst in redox reactions were also explored.The catalytic conversion mechanism of ethylbenzene to styrene over iron based spinel catalysts was revealed through characterizations（such as XRD,H₂-TPR,O₂-TPD,Raman,TEM-EDS,XPS）and molecular dynamics simulation.According to the XRD results,a series of iron-based spinel catalysts MFe₂O₄（M=Co,Cu,Mn,Ni,Zn）are successfully synthesized by citric acid assisted sol-gel method.The reductive trend of the obtained spinel catalyst is as follows:Cu Fe₂O₄>Mn Fe₂O₄>CoFe₂O₄>Ni Fe₂O₄>Zn Fe₂O₄.The results of EB-TPR as well as 450 ^oC thermostatic reaction showed that the five iron-based spinel catalysts are potential alternative materials for the process of ethylbenzene dehydrogenation for styrene preparation.The sequence of dehydrogenation performance for catalysts is as follows:CoFe₂O₄>Zn Fe₂O₄>Ni Fe₂O₄>Mn Fe₂O₄>Cu Fe₂O₄.However,the ethylbenzene conversion performance of CoFe₂O₄ can’t be sustained.After K modification,the iron-based spinel catalyst（5 wt.%K/CoFe₂O₄）can achieve77.0%ethylbenzene conversion and 96.62%styrene selectivity at 450 ^oC,while the reaction temperature is lower than 650 ^oC of the existing process.After 50 cycles of EB/O₂ redox reaction,the conversion of ethylbenzene over 5 wt.%K/CoFe₂O₄ can still be maintained at 60%,indicating that 5 wt.%K/CoFe₂O₄ has good structural stability in the cyclic reaction.In addition,the catalyst has good resistance to carbon accumulation during the cyclic reaction,which ensures its high activity in the conversion of ethylbenzene to styrene.XPS and In-situ DRIFTS results indicate that the excellent performance of 5wt.%K/CoFe₂O₄ at low temperatures is mainly attributed to the fast electron transfer ability of Cometal oxides.The peroxidation inhibition on catalyst,and the increase of Co³⁺,O_ads,O_surf,and O_latt active species by K modification could enhance the dehydrogenation activity of the catalyst.XRD and DFT calculations suggest that formation of K₃Fe O₄ structure could also be the active site for reducing the energy barrier of styrene preparation.The ethylbenzene conversion pathway is followed direct and oxidative dehydrogenation.The two H atoms are continuously dissociated from the branched methylene（-CH₂）and methyl（-CH₃）of EB on the surface of catalyst to produce styrene.Subsequently,the dissociated H atoms would combine with each other or with the O_surf,O_latt of the catalyst to form H₂ and H₂O.This work provides a new pathway for the production of styrene,which is important for the dehydrogenation of alkanes to olefins.