Effect Of Transition Metal And Molecular Sieve Structure On Catalytic Dehydrogenation Of Ethane

Ethylene and aromatic hydrocarbons are important raw materials for modern chemical industry,have become important pillars of social life,and the demand for them is increasing worldwide.The traditional production methods of ethylene and aromatics use steam cracking and catalytic cracking of naphtha,but the energy consumption is high and their pollution is serious.With the decreasing oil reserves and the falling price of shale gas,the non-oxidative dehydrogenation of ethane to produce ethylene and aromatics has attracted the attention of more researchers.The green and pollution-free production process and its ideal target product selectivity are very consistent with the concept of sustainable development under the background of“Two Carbons”in China.At present,the non-oxidative dehydrogenation of ethane to ethylene still suffers from poor catalyst durability,which is far to meet the demand of a process avoiding frequent catalyst regeneration.The main reason is due to catalyst deactivation,resulting from coke formation and metal sintering.On the other hand,the non-oxidative dehydroaromatizaiton of ethane to aromatics is more complicated,the active site structure and detailed dehydrogenation mechanism of metal-sopported HZSM-5 molecular sieve catalysts are still controversial,and there is a lack of understanding of the different acid centers of ZSM-5.The complete functions of surface acid sites should be profoundly realized.To solve the above problems,the design of the catalyst is the key factor.Enriching the types of metal active centers in molecular sieves and clarifying the composition and structure of catalysts are of great significance for the optimization of catalyst performance and the understanding of the specific functions of key components.This paper has carried out research from the following aspects:(1)Based on a large number of previous screening work of transition metal and DFT calculation verification,this paper found that the highly dispersed Co(II)species constructed on SAPO-34 molecular sieve by ion exchange method has the ability to selectively break C-H bonds of ethane,and has 95.3%ethylene selectivity.The same Co(II)species were constructed on aluminum-containing MCM-41 and ZSM-5 zeolites,which had the highest selectivity for ethylene(99.5%)and aromatics(40%),respectively.(2)In order to make up for the disadvantage that the ion exchange method is limited by the Pair Al content of aluminum-containing molecular sieves,a one-pot method was used to incorporate Co atoms into the MCM-41 molecular sieve framework,and the framework structure was used to regulate and stabilize a large number of tetrahedral-coordinated Co(II)species.While effectively inhibiting the reduction of Co(II)species,the reaction performance is stable at 600°C for 20 h,and the ethylene selectivity is as high as 99.5%,which has a good industrial application prospect.During this process,Co O_x cluster species were found to promote the decomposition of ethane into methane and carbon deposits.(3)By changing the solvent composition during the preparation of the metal-supported ZSM-5 catalyst,the mechanism of water-assisted Na~+(Lewis acid)ion exchange and the generation of additional H~+(Bronsted acid)was found when Na ZSM-5 participated in ion exchange.Based on this discovery,a series of Co-ZSM-5 catalysts with a single active site and a well-defined acid center were successfully prepared.The synergistic effect between different acids(Lewis acid,Bronstd acid)of ZSM-5 zeolite and a single Co species(Co(II)species or Co O_x cluster)on the ethane conversion and formation of aromatics was investigated.The Co-HZSM-5-W catalyst,which can stably catalyze the non-oxidative dehydroaromatizaiton of ethane to aromatics,was successfully prepared,and the aromatics selectivity was close to 40%.In this paper,the catalyst preparation methods(ion exchange,one-pot method,impregnation,solid phase grinding and solvothermal method)and corresponding characterization methods are rationally used.Based on the characterization and evaluation results,specific countermeasures were proposed to solve the deactivation problem in the process of ethane dehydrogenation to ethylene.At the same time,this paper contributed its own insights into understanding the complete roles of different acid centers of ZSM-5 in the process of ethane dehydroaromatizaiton.