Study On The Controllable Preparation Of Chromium-based Oxides And Their Catalytic CO₂ Oxidized Ethylbenzene Dehydrogenation

The oxidative dehydrogenation of ethylbenzene to styrene with CO₂ as a weak oxidant（CO₂-ODEB）has received considerable attention.Compared with the traditional dehydrogenation process,CO₂-ODEB not only overcomes the shortcomings of low equilibrium conversion rate and high energy consumption,but also has the prominent advantages of high efficiency and effective utilization of green house gases.However,the development of CO₂-ODEB catalysts with high activity and high stability is still particularly important.Based on the previous work in our group and the basis of the redox properties between Ce_xZr_1-xO₂ solid solution and chromium oxide,a series of Cr₂O₃/（-）Ce_xZr_1-xO₂ were prepared by different methods.Considering the solid phase reaction natrue of interaction between different solid oxides,Cr₂O₃-CexZ1-xO2-OMA were synthesised to increase the interface area between Ce_xZ_1-xO₂ and chromium oxide,which may improve its catalytic performance for CO₂-ODEB.Main research contents and conclusions are as follows:（1）The Cr₂O₃/（-）Ce_xZr_1-xO₂ catalysts were prepared by one-pot EISA and impregnation methods,respectively.The supported Ce_xZr_1-xO₂ used in the impregnation method was prepared by EISA and hydrothermal method.Effects of catalyst preparation method,Ce_xZr_1-2O₂ composition and Cr₂O₃ loadings were investigated for CO₂-ODEB.The results showed that 9%Cr₂O₃-Ce_0.5Zr_0.5O₂-E prepared by EISA method exhibited higher catalytic activity,which may attributed to the better dispersion of chromium species,larger specific surface area and higher surface Cr⁶⁺ content.In the impregnation method,the support Ce_xZr_1-xO₂-H prepared by hydrothermal method has larger surface area and lower crystallinity,which made the catalytic activity of Cr₂O₃/Ce_0.5Zr_0.5O₂-H better than that of Cr₂O₃/Ce_0.5Zr_0.5O₂-E.Therefore,the surface interfacial properties between highly dispersed chromium species and Ce_xZr_1-xO₂ with large surface area and low crystallinity may be the key to influence its catalytic activity and stability.（2）A series of Cr₂O₃-based catalysts with ordered mesoporous structrue were prepared by EISA method.The effects of Cr₂O₃ loadings,Ce_xZr_1-xO₂ composition and cerium-zirconium ratio on the catalytic performance of CO₂-ODEB were investigated.The results showed that 12%Cr₂O₃-Ce_0.6Zr_0.4O₂-OMA presented the high reactivity and stability（the initial EB conversion rate was 77 wt.%,and 42 wt.%EB conversion was kept until 50 h）.XPS,Raman and N2 adsorption/desorption were carried out to ascertain the relationships between the catalytic performances and structural properties of fresh and used catalysts.It shows that the Ce_xZ_r-xO₂ can stablilize Cr⁶⁺and the interfacial area CexZrq-xO2 and between Cr₂O₃ is large.However,further characteriztion results revealed that the active species Cr⁶⁺ decreased continuously in the reation and the coverage of carbonaceous species over the active sites may be the reason for the catalyst deactivation.（3）Rare earth elements were introduced into 12%Cr₂O₃-Ce_0.6Zr_0.4O₂-OMA by EISA method and the effects of different additives（La,Pr,Nd,Sm,Y）on the catalytic activity and stability were investigated.It was found that the Y-doped catalyst 12%Cr₂O₃-Ce_0.6Zr_0.3Y_0.1O₂-OMA has the best catalytic performance of CO₂-ODEB（the initial EB conversion rate was 78 wt.%,and 47 wt.%EB conversion was kept until 50 h）.The results of XPS,Raman and other characterization indicated that the introduction of the rare earth oxide may produce more lattice oxygen defects,which is more favorable for stabilizing Cr⁶⁺.