Catalytic And Recycle Mechanisms Of Lattice Oxygen In The Ethylbenzene Oxy-dehydrogenation With CO₂

Styrene,as one of the significant raw materials of petrochemical industry,is mainly produced by the dehydrogenation of ethylbenzene with steam,which accounts for more than 90%of the worldwide capacity.However,the process has several problems,such as huge energy consumption and high reaction temperature.Employing CO₂ as a soft oxidant instead of steam in the oxy-dehydrogenation?ODH?of ethylbenzene?EB?is considered as a promising alternative technology for styrene?ST?production.However,the main shortcoming of the new process is lack of an efficient and stable catalyst.Numerous experimental results showed common used metal-oxide catalysts suffer gradual deactivation for several hours along with the phenomena of lattice oxygen loss or carbon deposition.In this paper,different metal-oxide catalysts were used as model catalysts and density functional theory?DFT?method was used to systematically investigate the reaction mechanism of ethylbenzene dehydrogenation,the role of CO₂ in the reaction process,and the catalytical and recycle mechanisms of lattice oxygen in the metal-oxide catalysts during the ethylbenzene dehydrogenation process.The main research contents and conclusions in this paper are described as follows:?1?The V₂O₅?001?and CeO₂?111?surface were used as model to explore the catalytic mechanism of lattice oxygen in the ethylbenzene oxy-dehydrogenation,results show that among three lattice oxygen site on V₂O₅?001?:the vanadyl oxygen O?1?,the bridge oxygen O?2?,and the three-coordination oxygen O?3?,the O?1?site possesses the highest activity for the ethylbenzene oxy-dehydrogenation.The breaking of first C-H bond of ethylbenzene molecule is more favorable to take place through the radical mechanism.In the next C-H bond breaking,H can be captured by adjacent lattice oxygen and can also be captured by newly formed OH groups,they have almost the same reaction barriers.On the CeO₂?111?surface,only one kind of lattice oxygen exist.The breaking of first C-H bond of ethylbenzene molecule can take place through the oxygen-insert mechanism and radical mechanism.In addition,the H atoms produced by the ethylbenzene oxy-dehydrogenation can easily combine with the lattice oxygen to form H₂O,and the dissociation of H₂O from the catalyst surface is the main cause of the loss of active sites-lattice oxygen.?2?The clean and defect V₂O₅?001?and CeO₂?111?surfaces were used as model to explore the mechanism of CO₂ supplementation and maintenance of lattice oxygen.Results show that the capacity of CO₂ to dissociate and supply O on oxygen vacancy site is limited due to its chemical stability??G=-396kJ/mol?.In comparison with the defect O-termination of Fe₂O₃?0001?and V₂O₅?001?,the defect CeO₂?111?possesses the lowest reaction barrier of CO₂repairing the oxygen vacancy?CeO₂?111?<Fe₂O₃?0001?<V₂O₅?001??,besides,increasing the number of oxygen vacancy sites can accelerate the ability of CO₂ activation and dissociation.In addition,on both clean V₂O₅?001?and CeO₂?111?surfaces,CO₂ reaction with H through the RWGS route is not dominant,which results in the H atoms on the catalyst surface cannot be removed in time to avoid the combination of H and lattice oxygen and the generation of oxygen vacancy on the catalysts surface.?3?The?-Ce₂Zr₂O₈?111?surface with high oxygen storage capacity and oxygen diffusion ability was used as model to investigate the effects of lattice oxygen migration on the catalytic activity and stability of the catalysts.Results show that the catalytic activity of lattice oxygen and the formation of oxygen vacancy can be improved by increasing the diffusion ability of lattice oxygen in the metal-oxide catalyst.In addition,due?-Ce₂Zr₂O₈?111?possesses special oxygen diffusion characteristics,when the surface lattice oxygen?O_c and O_c'?release from the catalyst,the sub-surface or bulk-phase lattice oxygen will spontaneously or overcome a lower reaction barrier than C-H bond activation to migrate to the surface reaction sites,thereby maintaining the catalyst activity and stability during the ethylbenzene oxy-dehydrogenation.Compared with gas phase CO₂,the lattice oxygen supplied by the bulk phase is more efficiently.?4?The VO_x/CeO₂?111?was used as model to investigate the recycling mechanism of lattice oxygen in the catalyst.Results show that the main catalyst is VO/CeO₂?111?.When VO/CeO₂?111?is reduced to V/CeO₂?111?,the lattice oxygen from the support CeO₂?111?and CO₂ can both rapidly re-oxidize the supported V to VO with lower reaction barriers compared with the C-H bond breaking?1.25 eV?of ethylbenzene molecule,the corresponding reaction barriers are 0.79 eV and 1.13 eV,respectively.