| The efficient activation and directional conversion of methane is considered to be the"holy grail"reaction in the catalysis community.Among them,oxidative coupling of methane（Oxidative coupling of methane,OCM）to ethylene is one of the important ways for the direct and efficient conversion of methane to light olefins.However,there are still many problems in the current OCM reaction,such as the selectivity of the product is not high,and the reaction needs to be carried out at a very high temperature,etc.In addition,due to the introduction of O2 in the OCM reaction,the problem of over-oxidation is inevitable,which makes it difficult for the sum of methane conversion and product selectivity to exceed 100%.Therefore,it is necessary to develop a new method to reduce the reaction temperature while increasing the selectivity.In view of the above problems,this paper proposes a new method for oxidative coupling of methane by microwave catalysis.High-efficiency microwave catalysts that can realize oxidative coupling of methane by microwave catalysis are developed.The catalysts were characterized by H2-TPR,XRD,CO2-TPD,XPS,Raman,etc.The mechanism of microwave catalysis was discussed.The specific research contents are as follows:（1）Mn2O3-TiO2-Na2WO4/Si O2+SiC microwave catalysts were successfully prepared by ball milling and wet impregnation,and both methods were found to have good activity at low temperature.And the effect of Mn2O3 and TiO2 loadings on the OCM reaction was investigated.When the Mn2O3 and TiO2 loadings were 12.4%,the catalyst could maintain stability under microwave for 20 h.12.4%Mn2O3-12.4%TiO2-10%Na2WO4/Si O2+SiC microwave catalyst in microwave catalytic reaction mode（MCRM）at 700°C,CH4 conversion was 26.6%,C2-C3 selectivity was 76.5%,C2-C3yield 20.4%.While under the conventional reaction mode（CRM）at 700°C,the conversion of CH4 was 12.3%,the selectivity of C2-C3 was 61.9%,and the yield of C2-C3 was 7.5%.The experimental results show that microwave can not only reduce the reaction temperature,but also improve the product selectivity.（2）Li Mn2O4/TiO2+SiC microwave catalysts were prepared by solid-state reaction method,and the effects of Li Mn2O4 content and active metals were explored.It was found that the catalytic performance of OCM increased with the increase of Li Mn2O4content,the activity in MCRM was higher than that in CRM,the CH4 conversion and C2-C3 selectivity were 26.5%and 67.5%in MCRM at 750°C,and the C2H4 selectivity and C2-C3 yield were 41%and 17.9%,respectively.（3）La2O3+SiC-based microwave catalysts were developed,and the effects of loading Sr and Ce on the reaction were investigated.It is found that the catalytic activity and selectivity of the La2O3 catalyst under the loading of Sr are better and higher than those of the conventional reaction mode.In MCRM at 700°C,the CH4 conversion and C2-C3 selectivity were 23.4%and 68.3%,the C2H4 selectivity was 35%,and the C2-C3yield was 16%.（4）The mechanism of microwave catalysis of OCM reaction was discussed.It was found that on 12.4%Mn2O3-12.4%TiO2-10%Na2WO4/Si O2+SiC microwave catalyst,the activation energy of OCM reaction was reduced from the conventional 173 k J/mol to 27.5 k J/mol,showing the direct microwave catalytic effect.It is found that microwave irradiation can improve the selectivity of OCM reaction,showing microwave selective catalytic effect. |
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