Synthesis, Characterization And Catalytic Activity Of Me-MCM-41 For Oxidative Dehydrogenation Of Ethane To Ethylene With CO₂

Ethylene is one of the most important substances for chemical industry. It is usedmainly for the synthesis of fine chemicals, plastics, chemical fiber and rubber etc.,which are applied in many areas of national economy. Industrially, ethylene isobtained predominantly from steam cracking of naphtha or natural gas and from fluidcatalytic cracking of liquid petroleum products. Recently, with the increase in need forethylene and the decrease in storage of petroleum, it is necessary to developalternative routes for the ethylene production. Oxidative dehydrogenation of ethane toethylene with CO2 is one of new methods for ethylene production. As a weak oxidanttaking part in the reaction, CO2 can enhance the ethane conversion and ethylene yield.Under suitable reaction conditions, a high ethane conversion and ethylene yield can beachieved over the appropriate catalysts. This project as a new method of utilizing CO2and natural gas is a green chemical process with low energy consumption. The subjectis of great importance for utilizing ethane in natural gas and oil gas to produceethylene with high ethylene selectivity, yield and ethane conversion, on the other hand,taking advantage of CO2, which let out from petrochemical plant mitigating thegreenhouse effect, and the byproduct CO is usually utilized as raw material in F-Tsynthesis. If this process is applied successfully and widely, it has great social andeconomic benefit.A series of mesoporous Me (Co, Cr, V, Ni, Fe)-MCM-41 molecular sieves with ahexagonal and well-ordered structure were synthesized and characterized by XRD,FT-IR, 29Si MAS-NMR, DTA-TGA, DRS UV-Vis, XPS, TEM, BET, washingtreatment with NH4Ac, NH3-TPD, in-situ IR, Py-IR and TPR techniques. Actualamount of metal incorporated into MCM-41 framework, valence of Me, coordinationenvironment, acidity/basicity and radox ability were analyzed to understand thestructure of the synthesized Me-MCM-41 catalysts. The results indicate that a part ofMe incorporates into the framework of MCM-41, and the other part of Me existsoutside the MCM-41 framework. Ion radius and ion valence have great influence uponincorporation of metallic atoms into the MCM-41 frame structure. The smaller theradii of Me and Me valence trending to 4+, the more the amount of Me incorporatesinto the MCM-41 framework.Catalytic properties of Co-MCM-41 and Co-MCM-48 catalysts for oxidativedehydrogenation of ethane to ethylene with CO2 were systemically investigated. Thecatalytic performance of these catalysts is greatly related to their structure.Co-MCM-41 catalyst exhibits good stability to heat and maintains its framework at973K. Coke is the main reason for catalyst deactivation. Based on the experimentaldata, we analyzed the main reactions under the experimental conditions and assumedthe mechanism of CO2 with C2H6 over the mesoporous Co-MCM-41 catalyst in detail.CO2+C2H6→C2H4+CO+H2O reaction is predominant at low temperature. However,C2H6→C2H4+H2 is main reaction at high temperature. Main reactions over themesoporous Co-MCM-48 catalyst is CO2 reforming of C2H6 to synthesis gas.Different valence of Co ions causes different catalytic properties between theCo-MCM-41 and Co-MCM-48. We assume that Co (II) is the active species for theCO2-dehydrogenation of ethane, while Co (III) is the one for the CO2 reforming C2H6to synthesis gas.Catalytic properties of Cr-MCM-41 for oxidative dehydrogenation of ethane toethylene with CO2 were also studied. It was shown that the valence of Cr ions andthe acidity/basicity of the catalyst much effect on the catalytic activity of thecatalysts. Cr6+ and Cr3+ are the active centers. The optimal reaction conditions areinvestigated by comparing the reaction performance under different reactionconditions. Deactivated Cr-MCM-41 catalyst can be regenerated by O2, but can notbe regenerated by CO2.According to catalytic properties, the Me-MCM-41 catalysts series can bedivided into three types: the first type is the catalyst (Cr-MCM-41, V-MCM-41 andFe-MCM-41) with excellent catalytic properties for the reaction of ODE with CO2 toethylene;the second type is the catalyst (Ni-MCM-41) with excellent catalyticproperties for the reaction of CO2 reforming ethane to synthesis gas;the third type isthe catalyst (Co-MCM-41) with excellent catalytic properties for the reaction ofCO2-dehydrogenation of ethane to ethylene.The experimental results show that at T=973K, P=0.1MPa, GHSV=4000h-1,VCO2/VC2H6=2 the ethane conversion, ethylene selectivity and yield over the3%Co-MCM-41 are 39.5%, 98.6% and 39.0%, respectively;over the5%Cr-MCM-41 is 43.3%, 86.7% and 37.5%, respectively;the ethane and CO2conversion over the 3%Ni-MCM-41 is 78.9% and 83.7%, respectively. In additionthere exists a relationship between the catalytic activity of Me-MCM-41 catalystsand the electronic affinity energy of the corresponding metal atoms. However, as forthe difference in catalytic selectivity of these catalysts should be investigated in moredetail to make it clearly understood.