Study On The Fe Active Species In Ethylbenzene Dehydrogenation With CO₂

Styrene,basis product of chemical industry,is mainly produced by the dehydrogenation of ethylbenzene.The main disadvantage of the existing technology is the energy consumption, and the conversion rate constrainted by the thermodynamic equilibrium.Recently,CO₂ is taken as a moderate oxidant instead of water vapour,which is an effective way to reducing enegy consumption and decreasing CO₂ pollution.The key to this process is the preparation of high-performance catalyst,and the research of active species can provide theoretical and experimental basis for the development of efficient catalysts.In present work,three catalysts of Fe₂O₃-MgO/γ-Al₂O₃,Fe₂O₃/MgAl₂O₄ and MgFe_0.1Al_1.9O₄ with the same percent of Mg,Fe and Al were prepared by impregnation and sol-gel methods respectively and were tested in dehydrogenation reaction of ethylbenzene with CO₂ at 580℃.The structure and surface properties of the catalysts were characterized by X-ray diffraction(XRD),energy dispersive analysis X-ray(EDX), tempreture-programmed reduction(H₂-TPR),and temperature programmed desorption of CO₂(CO₂-TPD).Several significant results and conclusions were showed as follows:1) The preparation methods of the catalysts had effect on the dispersion state of Fe.Large particles ofα-Fe₂O₃ were found in Fe₂O₃-MgO/γ-Al₂O₃ prepared by impregnation method. Super-dispersed Fe₂O₃ formed on Fe₂O₃/MgAl₂O₄ prepared by impregnation method.All Fe³⁺ existed in spinel framework for MgFe_0.1Al_1.9O₄ which is prepared by sol-gel method.2) The dispersion state of Fe influenced the activity and stability of the catalysts.Large particles ofα-Fe₂O₃ on Fe₂O₃-MgO/γ-Al₂₃ have shown poor catalytic performance. Super-dispersed on Fe₂O₃ Fe₂O₃/MgAl₂O₄exhibited very high initial activity,but it deactivated rapidly during the time on stream of 20 h.Fe³⁺ existing in spinel framework of MgFe_0.1Al_1.9O₄ showed both high activity and good stability simultaneously during ethylbenzene dehydrogenation with CO₂.3) The deactivation of Fe₂O₃/MgAl₂O₄ catalyst is mainly caused by reduction of Fe³⁺.Fe³⁺ incorporating in spinel framework,which can resist sintering and reduction,results in the high stability of MgFe_0.1Al_1.9O₄.Moreover,the weak surface acidity of MgFe_0.1Al_1.9O₄ is beneficial to prevent the carbon formation.