| Hydrogen cyanide(HCN),an important basic chemical,is widely used in the fields of synthetic medicine and fine chemicals.However,the technologies for HCN production are usually operated at ultra-high temperature of 1000-1300 oC.Furthermore,noble metal Pt or Pt-Rh mesh catalysts must be used to catalyze C-N coupling reaction.In this paper,HCN and CH3 CN were synthesized by a C-N coupling reaction which was catalyzed by the combination of CH4/NH3 dielectric barrier discharge plasma and supported cheap metal catalysts.The results and conclusions are as follows:1.In a single dielectric barrier discharge quartz tube reactor,the optimal CH4/NH3 molar ratio for HCN synthesis is 1:2,and the optimized length of ground electrode is 5 cm,with a total feed gas flow rate of 60 m L/min.At the temperature range of 260-380 oC,low temperature is beneficial to the formation of CH3 CN,while high temperature is beneficial to the formation of HCN.At 380 oC,the conversion of methane was 17.1%,of which the selectivity and yield of hydrogen cyanide reached 39.4% and 6.7%,respectively.When the CH4/NH3 plasma was packed by particles of fumed Si O2,CH4 conversion increased to 26.3%,meanwhile,the selectivity and the yield of HCN were enhanced to 55.1% and 14.5%,respectively.2.The influence of metal components on the C-N coupling reaction of CH4/NH3 plasma was studied by using fumed Si O2 as catalysts support.At 380 oC,Cu,Cr and Zn metal catalysts selectively produced HCN,while Fe and Co metal catalysts selectively synthesized CH3 CN.At 260 degrees,however,all of the Cu,Cr,Zn,Fe,Co and Ni catalysts generated CH3 CN with higher selectivity than HCN.For HCN synthesis,the loading and the supports of Cu catalysts were investigated.When the CH4/NH3 plasma was packed by 10% Cu/S-1 catalyst granules,CH4 conversion reached 29.9%,of which 77.7% HCN selectivity and 23.2% HCN yield were achieved.3.The Cu7-Cr3 bimetallic catalyst formed by introducing the second metal Cr into Cu/S-1 catalyst has the best performance in HCN synthesis.At 380 oC,CH4 conversion reached 33.4%,meanwhile,the selectivity and yield of HCN reached 81.5% and 27.2% respectively.The catalyst characterization shows that the improved catalytic performance could be attributed to the formation of Cu Cr O4 alloy phase in the Cu7-Cr3/S-1 catalyst.In addition,NH3-TPD results indicate that the medium acidity of the catalysts may be beneficial to this reaction.4.At 260 oC,Fe,Co,Ni and Mo catalysts are favorable for the formation of CH3 CN.The loadings of Fe and Co catalysts were investigated.The loading of the Fe catalyst with the highest yield of CH3 CN was 20%,while the optimized loading of Co catalyst was 15%.The support material of Fe and Co catalysts for CH3 CN production was also studied.The results show that S-1 supported Fe and Co catalysts exhibited higher activity than those of other supports.Over 10% Co/S-1 catalyst,26.1% CH4 conversion,87% CH3 CN selectivity and 22.7% CH3 CN yield were achieved at 260 oC.5.The mechanism of CH4/NH3 plasma C-N coupling reaction was preliminarily discussed.Methane and ammonia molecules were activated by plasma into CHx and NHx radicals,respectively.These radicals species further combined to each other via C-N and C-C coupling reactions,which may be responsible to the production of HCN and CH3 CN.The presence of catalysts in the CH4/NH3 plasma may mainly promote the adsorption and surface reaction. |
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