Selective Oxidative Dehydrogenation Of Ethane Over Mo And Zn Catalysts In CO₂ Atmosphere

In recent years,with the improvement of exploitation technology,huge reserve of shale gas has been discovered all over the world.Ethane,as the second largest component of shale gas,has brought more attention.Oxidative dehydrogenation of ethane to ethylene in CO₂atmosphere（CO₂-ODHE）is a new approach with great development prospect due to its energy saving,economy,environmental protection and free from thermodynamics.However,the main problem facing this process is to design and prepare a catalyst that can oxidize ethane to ethylene efficiently and selectively.Therefore,this research is focuses on the exploration of catalysts which can tailor ethane efficiently for CO₂-ODHE reaction.In this study,a series of LDO-supported molybdenum oxide catalysts were synthesized and the surface species were regulated by altering the load.Combined with the results of Raman,UV-vis and other characterization methods,there were three Mo species on the surface of the catalyst:[Mo O₄]^2-,[Mo₇O₂₄]^6-,[Mo₈O₂₆]^4-.And based on the results of catalytic performance test,[Mo O₄]^2-species had the strongest activation ability for ethane,but preferred the non-selective pathway.Poly[Mo₇O₂₄]^6-species promoted the selective pathway,and the existence of[Mo₈O₂₆]^4-reduced the ability to activate ethane.The changes of the valence state before and after the reaction suggested that the reaction followed the redox mechanism.It was found that the catalytic performance of Mo based catalysts was closely related to the Mo species on the surface,and the selective cracking of ethane could be realized by adjusting Mo species on the catalyst surface,resulting in the improvement of the reaction activity.In this study,ZnO catalysts with different morphologies were prepared,and the influence of morphology on the catalytic performance of CO₂-ODHE reaction was investigated.In CO₂-ODHE reaction,ZnO-AC₂-4 catalyst with porous nanosheet structure exhibited the best catalytic performance with the 11.48%ethylene yield.The physicochemical properties of ZnO catalysts with different morphologies were investigated by various characterization methods.The results of XRD,SEM and TEM inferred that wurtzite ZnO deposited along the direction of[011（?）]formed the nanosheet structure.According to XPS and CO₂-TPD,the oxygen vacancy ratio and the number and strength of alkaline sites on the surface of the catalysts changed with the transformation of morphology.Combined with the results of catalytic performance test,the ZnO active sites exposed by the porous nanosheet structure were conducived to the activation of ethane,but also preferred the non-selective pathway.The deactivation of ZnO catalysts were mainly caused by catalyst sintering and carbon deposition.The presence of CO₂ can react with the carbon deposition e to improve the stability of ZnO catalysts.In addition,the regeneration experiment showed that ZnO catalyst possessed well regeneration performance,and the ethane conversion rate can still maintain at 7.5%after two regenerates.