Zinc-zirconium Oxide Solid Solution Catalyzed CO₂ Oxidative Dehydrogenation Of Propane To Propylene

The oxidative dehydrogenation of propane with carbon dioxide(CO2-ODP)is characterized as an energy-saving and environmentally benign process for the production of propene,which has continuously been attracted much worldwide attention.Although intensive works have been done,the activity and stability of available catalysts for CO2-ODP are still poor.Thus,the development of a high-performance catalyst for the titled reaction is the critical issue for the industrialization of the green process.The analysis of the available reports indicate that CO2-ODP may follow the Mars-van-Krevelen redox mechanism,of which the defective sites of oxides can be catalytic centers.Thus,in this thesis,we investigated the zinc-zirconium oxide(ZnO-ZrO2)solid solution as catalysts for CO2-ODP.By using different preparation methods,ZnO-ZrO2 catalysts with varied compositions were obtained.The catalysts were comparatively evaluated for CO2-ODP under the conditions of P at 1 atm,T at 550?,WHSV(C3H8)=1.89 h-l,and CO2 to propane molar radio of 5,leading to the high-performance catalyst.The textural and structural properties of the catalysts were characterized by N2 adsorption/desorption at-196?,XRD and Raman.The redox property and the content of defect sites over the catalyst were investigated by H2-TPR and XPS techniques.The amount and properties of the carbonaceous deposits over the used catalysts were evaluated by TG-DSC and Raman analyses.By correlating the reaction results with the characterization data of different catalysts,the key factor affecting the catalytic performance of CO2-ODP over ZnO-ZrO2 catalysts was clarified,and the possible mechanism was proposed.The detailed experimental and main conclusions are summarized as follows.(1)20%ZnO-ZrO2 catalyst with a ZnO molar content of 20%was prepared by co-precipitation,sol-gel and carbon template method,respectively.Characterization results indicate that the textural and structural properties of 20%ZnO-ZrO2 are closely related to the preparation method.The single-phase tetragonal ZnO-ZrO2 solid solution was formed over the 20%ZnO-ZrO2 catalysts prepared by both co-precipitation and sol-gel method.In contrast,a mixed phase of crystalline ZnO and tetragonal ZnO-ZrO2 solid solution was present over the catalyst prepared by the carbon template method.CO2-ODP reaction results indicate that the pure ZrO2 showed poor activity with a propane conversion of less than 3%,irrespective of preparation methods and crystal structure.However,the catalytic activity of 20%ZnO-ZrO2 catalyst was remarkably enhanced with the introduction of ZnO,and the reactivity order of the preparation methods for 20%ZnO-ZrO2 catalysts following co-precipitation method>sol-gel method>carbon template method.By correlating the reaction results with the characterization data of 20%ZnO-ZrO2 catalysts prepared by different methods,it can be concluded that the catalytic activity of the 20%ZnO-ZrO2 catalyst is closely related to the structure of single-phase solid solution and oxygen defect sites.The 20%ZnO-ZrO2 prepared by co-precipitation method with a single-phase solid solution and the richest oxygen defects showed the highest catalytic activity for CO2-ODP.(2)A series of xZnO-ZrO2 catalysts(x is the molar content of ZnO and x=0,5%,10%,20%,45%,100%)were synthesized via the co-precipitation method.Characterization results indicate that a mixed phase of monoclinic phase and tetragonal phase was present over the pure ZrO2.Adding ZnO(0 to 45%)to ZrO2 led to the phase change of ZnO-ZrO2 from monoclinic to tetragonal.The solid solution structure was formed over 5%ZnO-ZrO2,which was mainly in the tetragonal phase mixed with some in monoclinic phase.With increasing the content of ZnO,only tetragonal solid solution structure was formed in the case of 10%ZnO-ZrO2 and 20%ZnO-ZrO2,however,a mixed phase of tetragonal solid solution and crystalline ZnO was present over 45%ZnO-ZrO2.CO2-ODP reaction results indicate that the activity of xZnO-ZrO2 catalysts for CO2-ODP was firstly increased and then decreased with increasing the content of ZnO from 0 to 100%,leading to the highest propane conversion over 20%ZnO-ZrO2 catalyst.The correlation of the characterization and catalytic results of the xZnO-ZrO2 catalyst,it is found that the activity and stability of the ZnO-ZrO2 catalysts were closely related to the structure of single-phase solid solution and the amount of oxygen defect sites.The 20%ZnO-ZrO2 with a single-phase solid solution and the richest oxygen defects showed the highest catalytic activity for CO2-ODP,meanwhile,both the coke depositing rate and the promoting effect of CO2 for the titled reaction were responsible for the stability of ZnO-ZrO2 solid solutions.Moreover,the CO2-ODP very probably performed via the Mars-van-Krevelen redox mechanism over the ZnO-ZrO2 solid solution.The rich oxygen defects and high mobility of lattice oxygen in the ZnO-ZrO2 solid solutions can efficiently catalyze the C=O bond in the CO2 and selectively activate the C-H bond in the propane,which are responsible for matching the oxidation and reduction process,leading to the high-performance catalyst for CO2-ODP.