摘要
本文报导了La_2O_3/BaCO_3催化剂上甲烷氧化偶联反应如果,详细地考察了催化剂配比、反应温度、原料气组成及空速、催化剂制备方法的改变等对反应结果的影响,在反应温度T=780℃,CH_4/O_2=5:1和GHSV=1.8×10~4h^(-1)的反应条件下,甲烷转化率和乙烯选择性分别为25.5和71.4%,同时利用XRD,FTIR及EDS等技术对此催化剂进行了结构表征,其结果证明La_2O_3和BaCO_3两相间的协同效应可能是此催化剂体系具有高反应活性的原因.
La2O3/BaCO3 catalysts are evaluated catalytically in the oxidative coupling of methane to higher hydrocarbons. BaCO3 is known to be selective for the title reaction, but its activity is low. On the other hand, La2O3 is active,but the selectivity is only about 50%. However,we find 8-10wt% La2O3/BaCO3 catalysts exhibit high activity and selectivity for the conversion of CH4 to C2H4 and C2H6, giving 25.5% methane conversion and 71.4% C2 selectivity at 780℃, 1.8×104 GHSV and 50% air in methane.
The effect of process conditions on the oxidative coupling of methane has been studied; factors examined include the effect on the product distribution of oxygen concentration in the reactor feed,gas hour space velocity ( GHSV ) and reaction temperature. It appears that gas-phase reactions after the catalyst bed result in the dehydrogena-tion of C2H6 to C2H4, an increase of the oxygen concentration in the reactor feed gives an adverse effect on C2 selectivity, C2H4 and C2H6 are susceptible to further oxidation. Selectivity for methane conversion to higher hydrocarbons rises to over 75.8% as CH4/O2 is raised to about 7 : 1 when reaction temperature is const ant. However, selectivity declines as GHSV and temperature are decreased. Here, optimal reaction conditions give C2 yields of nearly 20%.
To understand the process of methane activation more deeply, we characterized 8wt% La2O3/BaCO3 catalyst using XRD, FT-IR and EDS techniques. The results suggested that the catalyst only consisted of La2O3 and BaCO3. No formation of either solid solution or new compound is observed. La2O3 and BaCO3 are well mixed and the particles of different phases are in good contact. There is a spectacular tendency to segregate in the two phases.The catalytic synergy persists, or even becomes more important, after segregation of two phases. The origin of the synergy is a specific cooperation between La2O3 and BaCO3 phases. BaCO3, as a controlling phase, activates gaseous oxygen into a mobile oxygen species, called spillover oxygen. These oxygen species can migrate from BaCO3 onto the surface of La2O3 where they create active sites to activate methane. As a further study of this mechanism, the catalyst was prepared by coprecipitation and mechanical mixing respectively. The sample prepared by the former method, with good contacting and mixing, shows high activity and selectivity. Of course,it is necessary for us to give further proofs of oxygen spillover from BaCO3 onto La2O3.
出处
《分子催化》
EI
CAS
CSCD
1989年第3期181-189,共9页
Journal of Molecular Catalysis(China)
