Study On The Catalytic Performance And Preparation Of Co/Al₂O₃ Catalysts For Oxidative Dehydrogenation Of Ethylbenzene With CO₂

Styrene is the fourth largest ethylene derivative after polyethylene,polyvinyl chloride and ethylene oxide,and is widely used in the production of resins,plastics and synthetic rubbers.At present,about 90%of styrene in industry is directly dehydrogenated from ethylbenzene at high temperature（600～650℃）under a large amount of superheated steam.The main problems existing in the traditional process are:（1）the reaction temperature is high;（2）the superheated steam liquefies a lot of heat,which is difficult to recover,and the energy consumption is huge.Using CO₂ as mild oxidant instead of superheated water steam to oxidize ethylbenzene dehydrogenation to styrene can not only reduce the reaction temperature and energy consumption of styrene production,but also improve the dehydrogenation efficiency of ethylbenzene and the selectivity of styrene.Therefore,CO₂ oxidative dehydrogenation of ethylbenzene（CO₂-ODEB）,as an energy saving,efficient and environmentally friendly green process,is expected to become an alternative process for the production of styrene.Designing and developing catalysts with high activity and high stability is the key to the industrialization of CO₂-ODEB.Cobalt-based catalysts have good C-H bond activation ability,and Co/Al₂O₃catalysts show good catalytic activity in CO₂-ODEB reaction.However,the effects of different preparation methods and Co loadings on the structure of Co species,the relationship between the structure of Co species and its catalytic activity,and the catalytic behavior of Co/Al₂O₃ and the reasons for its deactivation remain unclear.To this end,the Co/Al₂O₃ catalyst was prepared by sol-gel method and impregnation method in this thesis.The effects of preparation method and Co loading on the surface Co species structure and catalytic performance were studied in detail.The structure-activity relationship of Co/Al₂O₃ catalyst and the cause of deactivation as well as the effect of CO₂ were investigated by a variety of characterization techniques providing new ideas for the design and optimization of new high-efficiency Co-based catalysts for the CO₂ oxidative dehydrogenation of ethylbenzene.The main findings are as follows:（1）The catalyst activity is closely related to the preparation method.The activity of the Co/Al₂O₃ catalyst prepared by the sol-gel method is obviously better than that by the impregnation method.After 30 h of reaction,the ethylbenzene conversion of the catalyst prepared by the sol-gel method（60.1%）was 4.85 times that of the catalyst prepared by the impregnation method（12.4%）.The sol-gel method greatly improves the dispersion of the active component Co,and the surface Co species mainly exists in the form of tetrahedral Co²⁺ions,which is beneficial to CO₂-ODEB reaction.（2）The catalytic behavior and Co species structure of Co/Al₂O₃ depend on the Co loading.With the increase of Co loading,the catalyst activity showed a trend of first increasing and then decreasing.When the Co loading was 10 wt.%,the conversion of ethylbenzene reached the maximum.With low Co loading（≤10 wt.%）,tetrahedral Co²⁺ions are preferentially formed on the surface of the catalyst.With the increase of Co loading,the number of Co²⁺sites that can contact the reactants increases,and the catalyst activity increases accordingly.The formation of crystalline Co₃O₄ on the surface of catalysts with high Co loadings（>10 wt.%）resulted in fewer Co²⁺sites exposed to the surface and decreased catalyst activity.The Co²⁺on the catalyst surface has a good correspondence with the catalytic activity,indicating that the tetrahedral Co²⁺ions are the active sites of the Co/Al₂O₃ catalyst.（3）10Co-Al₂O₃ showed the best catalytic performance.The conversion rate of ethylbenzene was up to 64.4%,and the selectivity of styrene was 99.3%.After 30 h of reaction,the catalyst was slightly deactivated.During the dehydrogenation process,the Co²⁺active sites can not only maintain the structural stability,but also inhibit the formation of carbon deposition,thereby improving the catalyst stability.However,the crystalline Co₃O₄ formed on the catalyst surface is easily reduced to metal Co species,which promotes the formation of carbon deposition and leads to the deactivation of the Co/Al₂O₃ catalyst.The 10Co-Al₂O₃ catalyst regeneration test results after 30 h of reaction show that the 10Co-Al₂O₃ catalyst has good regeneration performance.Compared with inert N₂ atmosphere,CO₂ significantly improved the stability of Co/Al₂O₃ catalyst.In the N₂ atmosphere,although the initial activity of the catalyst is high,the deactivation is very fast,and the formation of a large number of carbon nanotubes/filamentous carbon deposition is the main reason for the deactivation of the catalyst;the CO₂ reaction atmosphere can effectively suppress and eliminate the carbon deposition on the surface of the catalyst,Thus,the stability of the catalyst is significantly improved.