Study On The Preparation Of Rare Earth And Vanadium-based Composite Oxides And Their Catalytic CO₂ Oxidation Of Ethylbenzene For Deoxidation

As an alternative to the direct dehydrogenation of ethylbenzene(EB)to styrene(ST),the oxidative dehydrogenation of EB witih CO2(CO2-ODEB)has attracted extensive attention,and much work has been done.However,the severe deactivation of the commonly investigated catalysts such as Fe or V oxides is a bottleneck for the industrialization of CO2-ODEB.Based on the previous work in our group and the possible Mars-van Krevelen redox mechanism of CO2-ODEB,in this dissertation,the relationship between the defective properties of Ce1-xZrxO2 solid solutions and the catalytic performance of CO2-ODEB is rigorously revealed.Then,the effect of rare earth oxides on the catalytic performance of CO2-ODEB over V2O5-RexOy-Al2O3(Re=Ce,Pr,Nd,La,and Yb)is comparatively investigated.Finally,the order mesoporous V2O5-Ce0.5Zr0.5O2-Al2O3 synthesized by combining the urea-hydrolysis assisted hydrothermal with the evaporation-induced self-assembly(EISA)method was systematically optimized for CO2-ODEB,and the function of CO2 during the reaction process and the possible deactivation mechanism of catalyst are proposed.The experimental and main conclusions are summarized as follows.(1)A series of Ce1-xZrxO2 catalysts(x=0,0.1,0.25,0.5,1.0)were synthesized via the two-step hydrothermal method.As revealed from the characterization results of XRD and Raman,the pure cubic solution of Ce1-xZrxO2 without any phase separations was obtained irrespective of the compositions.CeO2 was the most active catalyst while zirconia was inactive for CO2-ODEB.With the increase of Zr content,the initial conversion of EB of Ce1-xZrxO2 gradually decreased,however,the redox cycle rate of Ce3+/Ce4+was consequently increased,and the the stability of Ce1-xZrxO2 for the CO2-ODEB reaction was improved.(2)Another series of Ce1-xZrxO2(x=0,0.1,0.3,0.5,0.9,1.0)catalysts were synthesized via the urea-hydrolysis assisted hydrothermal method.The uniform cubic Ce1-xZrxO2 solution was also observed when x?0.5.However,separated tetragonal and monoclinic phases were presented in Ce0.1Zr0.9O2,just as pure ZrO2.With the increase of x from 0 to 0.5,the increase of relative content of defects,as well as the oxygen vacancies over the surface of Ce1-xZrxO2 was observed.The catalytic evaluation in the CO2-ODEB reaction showed that Ce0.5Zr0.5O2 gave the best initial EB converstion.The activity of Ce1-xZrxO2 was determined by both the amount of the reducible Ce4+and the difficulty of oxidation of Ce3+by CO2.The coke on the surface of catalyst can both inhibit the oxidation of Ce3+back to Ce4+by CO2 and promote the CO2-ODEB reaction.The dual effect may be the key ingredient responsibility for the stability of the catalyst for CO2-ODEB.(3)The order mesoporous V2O5-RexOy-Al2O3(Re=Ce,Pr,Nd,La,and Yb)were synthesized by combining urea-hydrolysis assisted hydrothermal with the evaporation-induced self-assembly(EISA)method for the first time.And the catalytic performance for CO2-ODEB was investigated.The addition of different rare earth showed different effects on the reduction temperature and the amout of reducible vanadium species,the anti-carbonation ability and the graphitization degree of carbonation.Among the catalysts investigated,V2O5-CeO2-Al2O3 exhibited better catalytic performance of CO2-ODEB,due to the promotion of the redox recycle of V5+/V4+by the faster redox recycle rate of Ce3+/Ce4+,so as to inhibit the deep reduction of V5+.(4)V2O5-Ce1-xZrxO2-Al2O3(x=0,0.5,1)mixed oxides with a highly ordered mesoporous structure,a high specific BET surface area,and a large total pore volume were successfully synthesized by combining the urea-assisted hydrothermal and modified EISA methods.The addition of Ce0.5Zr0.5O2 can not only increase the disperse of vanadium species,reduce the reduction temperature of V5+ and increace the amount of reducible V5+,but also inhibit the deep reduction of V5+due to the faster redox cycle rate of Ce3+/Ce4+,so as to maintain the activity and stability of the catalyst.With the increase of vanadium content,the redox rate of Ce3+/Ce4+ did not match the redox rate of V5+/V4+.V4+ can not be oxidized into V5+by CO2 in time,and the high relative V5+ratio was hardly to be maintained.The deactivation,thereafter,became faster.Although the deposited coke formed in the reaction would cover the active site,block the pore channel and decrease the specific surface area,it was not the main reason for the deactivation of the catalyst.(5)CO2 can eliminate the coke generated during the reaction and reoxidize the reduced V4+.The possible mechanism of CO2-ODEB reaction on V2O5-Ce0.5Zr0.5O2-Al2O3 is described as follow:after adsorption and activation on the catalyst surface,EB reacts with reactive oxygen species in V2O5 to remove water and to generate ST,and the active component V5+is reduced to V4+.At the same time,the mobile oxygen in Ce0.5Zr0.5O2 solid solution can oxidize V4+to V5+.The reduced Ce3+in Ce0.5Zr0.5O2 solid solution is converted into Ce4+through the oxygen provided by CO2,and CO2 is released in the form of CO.