Study On V-based Catalysts For Oxidative Dehydrogenation Of Ethylbenzene To Styrene With CO₂

Styrene（ST）is one of the most important materials in industry,which is widely used for synthesizing plastic and rubber.It is commercially produced mainly by the dehydrogenation of ethylbenzene（EB）in presence of a large amount of superheated steam at high reaction temperature（600⁶50℃）.However,the main defects of the traditional process are high reaction temperature and large energy consumption because latent heat of superheated steam is difficult to recycle.The reaction of ethylbenzene dehydrogenation in CO₂ instead of superheated steam not only improves styrene yield but also reduces reaction temperature and energy consumption.What’s more,it can consume the greenhouse gas effectively.Therefore,the oxidative dehydrogenation of ethylbenzene with CO₂（CO₂-ODEB）to styrene has been extensively investigated as an efficient,energy-saving,and environmentally benign process.A challenge with the green process of CO₂-ODEB is the catalyst deactivation,due to the coke deposition.In this paper,controlled synthesis of the Vanadium-based catalysts is made by tuning the amount of vanadium and promotor,physical and chemical properties of support,as well as the metal-support interacting,in order to improve catalytic performances of the catalyst,especially the coke resistance performance.Herein,we adopted silica,ordered mesoporous alumina,nano alumina as supports respectively to prepare V-based catalysts.The catalytic performances of the catalysts in CO₂-ODEB were investigated in a fixed bed reactor.The results for XRD,N₂ adsorption,H₂-TPR,CO₂-TPD and TGA showed the relationship between catalyst structure and catalytic performances,the decisive factor of catalyst deactivation and the promoting effect of CO₂ on EB dehydrogenation.The results also provided a theoretical basis for the controlled synthesis of V-based catalysts with an excellent resistance to coke deposition in the CO₂-ODEB.The detailed results showed as the following:（1）The novel VO_x/Al-m catalysts and VO_x/Al-n catalysts led to higher catalytic activity and more stable catalytic performance than VO_x/γ-Al₂O₃catalysts under the same conditions.It was found that the catalytic performances of VO_x/Al-m catalysts,VO_x/Al-n catalysts and the structure of surface VO_x species were closely related to the vanadium loading.Only monomeric and/or polymeric VO_xspecies were present with lower vanadium loadings.The catalytic activity increased with the increase of vanadium loadings.It was well established that the polymerization degree of the surface VO_xspecies increased with the increasing of the VO_xsurface densities,and the crystalline V₂O₅ was formed at higher vanadium loadings.The catalytic activity decreased as vanadium loading was increased.V（0.8）/Al-m catalysts and V（0.8）/Al-n catalysts were the optimum composition.The ST yield of 51.9%for V（0.8）/Al-m catalysts and the ST yield of 59.7%for V（0.8）/Al-n catalysts respectively were achieved after time on stream of 1 h,the catalytic activity remained about the same after time on stream of 12 h.The ST yield could still reach 40.9%and 49.6%respectively after time on stream of 24 h.（2）The main reason for catalyst deactivation is the coke deposition and the deep reduction of surface vanadium species(V⁵⁺→V³⁺).There are different reasons for different vanadium loadings.Under the vanadium loadings of 0.8mmol/g over VO_x/Al-m catalysts and the vanadium loadings of 1.2 mmol/g over VO_x/Al-n catalysts,loss of yield for ST increased as the amount of carbon deposition increased,indicating that the main reason for catalyst deactivation is the coke deposition.Above the vanadium loadings of 0.8 mmol/g over VO_x/Al-m catalysts and the vanadium loadings of 1.2 mmol/g over VO_x/Al-n catalysts,the coke deposition of VO_x/Al-m catalysts and VO_x/Al-n catalysts changed little with the increase of vanadium loadings.However,Loss of yield for ST increased rapidly,indicating that the main reason for catalyst deactivation is the deep reduction of surface vanadium species.（3）The modification of VO_x/SiO₂ catalysts with appropriate amounts of Cr led to higher catalytic activity and more stable catalytic performance.V（0.8）Cr（0.25）/SiO₂ catalyst was the optimum composition.The EB conversion of 58.2%and ST selectivity of 98.7%were obtained.The catalytic activity remained about the same after time on stream of 12 h.The dispersion of active VO_xspecies was improved,the basicity of V/SiO₂ and CO₂ adsorption were enhanced,and coke formation was inhibited by the addition of Cr,all these may contribute to the high catalytic activity and stability of V-Cr/SiO₂ in CO₂-ODEB.（4）ST yield in CO₂ was higher than that in an atmosphere of N₂ over VO_x/Al-m catalysts,VO_x/Al-n catalysts and V-Cr/SiO₂ catalysts,especially V-Cr/SiO₂ catalysts,indicating that CO₂ markedly promoted the EB dehydrogenation,which was attributed to that CO₂ as a soft oxidant can effectively keep the surface vanadium species in a high oxidation state during the dehydrogenation reaction.