Non-oxidative Dehydrogenation Of Ethane To Olefins And Aromatics On The Cobalt-based Zeolite Catalysts

Ethylene（C₂H₄）,as an important chemical raw material in industry,is mainly produced by steam cracking and fluidized catalytic cracking of naphtha.Since the ethylene molecule contains unsaturated carbon-carbon double bonds,important organic materials such as polyethylene,ethylene oxide,and ethanol can be synthesized by polymerization,oxidation,hydration and other methods.On the other hand,aromatic hydrocarbons（benzene,toluene,xylene,etc.）are chemical raw materials second only to low-carbon olefin market demand.They can be used as polymer materials and pesticides,as well as dyes,detergents,and paint And other related chemicals.Therefore,both ethylene and aromatics have very important economic value and become a necessity for social development.However,with the increasing demand for ethylene and aromatics and the increasing scarcity of petroleum resources,ethane dehydrogenation has become an important way to increase the production of ethylene and aromatics.At present,ethane dehydrogenation is mainly divided into three methods:steam cracking,oxidative dehydrogenation（ODH）and catalytic dehydrogenation（EDH）.Steam cracking to produce ethylene refers to passing ethane steam under high temperature conditions to produce ethylene through gas phase cracking.This method has a complicated preparation process and high energy consumption,which is easy to cause environmental pollution.On the other hand,oxidative dehydrogenation is difficult to control due to the deep oxidation reaction,and the selectivity of ethylene is low.In contrast,non-oxidative ethane dehydrogenation（EDH）can avoid most of the problems associated with ODH.Therefore,constructing a suitable catalytic dehydrogenation catalyst,maximizing the single-pass conversion rate of ethane and reducing energy consumption are the keys to ethane dehydrogenation.In this paper,the method of liquid-phase ion exchange is used,that is,the Co metal cation is exchanged to the zeolite by using the proton exchangeable characteristics of the Br（?）nsted acid site of zeolite,and a highly dispersed Co-based catalyst with a special electronic state and a clear structure is constructed.It provides convenience for correlating active species with catalyst activity and product selectivity.Ion exchange catalysts have good application prospects in many heterogeneous reactions,but there are few reports in EDH reactions.（1）Various Co/SAPO-34-IE and Co/SAPO-34-IWI catalysts were prepared by ion-exchange method and impregnation method.Those catalysts were characterized by XRD,BET,SEM,TEM,NH₃-TPD and H₂-TPR methods,and the influence of the Co species state in the catalyst on the catalytic dehydrogenation performance of ethane was explored.The results show that the selective cleavage of ethane C-C bonds and C-H bonds is closely related to the state of Co species on the catalyst.The metal Co species on the catalyst and the easily reduced Co O_x clusters on the outer surface facilitate the simultaneous cleavage of the ethane C-C bond and the C-H bond,resulting in higher carbon deposition and methane selectivity;and the Co（II）and a small amount of Co O_x clusters that are difficult to be reduced in the pores are beneficial to inhibited C-C bond cleavage and deep C-H bond cleavage,thus get close to 100%olefin selectivity.（2）In addition,HZSM-5 zeolite was used as the carrier to prepare Co/HZSM-5-IE and Co/HZSM-5-IWI catalysts with different silicon-to-aluminum ratio zeolite by the same method,and the same characterization method was used to study those catalysts.When HZSM-5 zeolite is used as the carrier,the physical and chemical properties of Co/HZSM-5catalyst such as phase composition,acidity and structural composition are investigated,and its catalytic performance in the reaction is investigated.The results show that the Co（II）species exchanged and anchored by the Br（?）nsted acid site of zeolite in the Co/HZSM-5-IE catalyst is the main active site of the reaction,which can activate C₂H₆ to form C_xH_y species,and then C_xH_y species Oligomerization and cyclization at the Br（?）nsted acid site finally form aromatic hydrocarbons.Compared with the Co/SAPO-34-IWI catalyst prepared by the impregnation method,the Co/SAPO-34-IE catalyst exhibits higher and stable reaction activity and lower carbon deposit due to the presence of a large number of Co（II）species.