Study On The Catalysts For Catalytic Dehydrogenation Of Ethane

Ethylene is a basic chemical in the petrochemical industry and is used as a raw material for the production of a variety of chemicals.Today’s world has put forward higher requirements for the production process and capacity of ethylene.Ethane dehydrogenation to ethylene has attracted much attention because of its low cost and environmental protection.Ethane dehydrogenation mainly includes catalytic dehydrogenation and oxidative dehydrogenation of ethane.The introduction of an oxidant in the oxidative dehydrogenation of ethane can break the limit of thermodynamic equilibrium and increase the conversion rate.However,the oxidation depth is difficult to control after the introduction of oxidant,so oxidative dehydrogenation has the problem of low ethylene selectivity.As far as the catalytic dehydrogenation of ethane is concerned,the harsh reaction conditions put forward higher requirements on the stability of the catalyst structure and the ability to resist carbon deposition.Based on this,the development of a new type of ethane dehydrogenation catalyst,with improved stability and delayed coking deactivation under high temperature conditions,is the current research focus.The main content of this thesis is to investigate the performance of Co-based and Sn-based catalysts for ethane dehydrogenation,and to establish the structure-activity relationship of the catalysts using a variety of characterization methods.Studies on the ethane dehydrogenation over Co/Al₂O₃ catalyst show that the four-coordinated Co²⁺on Co/Al₂O₃ catalyst is the active species for ethane dehydrogenation to ethylene,while the aggregated metallic Co catalyzes the cleavage of C-C bonds.The calcination temperature possesses a great influence on the dehydrogenation performance of the Co/Al₂O₃catalyst.After high-temperature roasting,the catalyst activity is reduced,and both the yields of methane and coke are significantly deceased,while the ethylene selectivity is greatly improved.This is because after increasing the calcination temperature,the main Co species changes from aggregated Co₃O₄ to Co Al₂O₄.As a result,the dispersion of the Co species on the catalyst surface is improved,consequently inhibiting the occurrence of side reactions such as cracking and coke deposition,and promoting the formation of the target product ethylene.The support screening of Sn-based catalysts shows that Sn/SiO₂ catalyst exhibits the best ethane dehydrogenation performance.The loading of Sn species has an important impact on the catalytic activity for ethane dehydrogenation.When the loading of Sn species is 5 wt%,the reaction performance reaches the best.The optimized reaction conditions are:the reaction temperature is 700 ^oC,and the mass space velocity of ethane is 0.4 h^-1.In order to further improve the stability of the Sn-based catalyst for ethane dehydrogenation,the deactivation reason have been investigated.The results show that the deactivation of the catalyst is caused by coke deposition during the reaction and the loss and agglomeration of Sn species on the catalyst surface.Different methods are used to improve the stability of the catalyst.Firstly,the complex impregnation method is used to improve the dispersion of the active components.The experimental results show that the dehydrogenation performance of the catalyst prepared by the conventional impregnation method decreases significantly after 7 h reaction,while the conversion and ethylene selectivity remained stable for 9 h over the catalyst with complexing agent reacts.In addition,Sn/MCM-41 catalyst was prepared by the in-situ synthesis method to disperse Sn species in the pores of MCM-41 molecular sieve.The results show that the deactivation index and coke content of the catalyst with in-situ synthesis method is significantly reduced in contrary to the catalyst prepared by the conventional impregnation method.To inhibit coke deposition,an attempt was made to introduce CO₂ and water vapor in the reaction atmosphere for ethane dehydrogenation.The reaction results show that both CO₂ and water vapor can react with the coke generated in the dehydrogenation process and improve the stability of the catalyst.