Synthesis Of HCN From Methane And Ammonia Driven By Synergy Between Plasma And Pt Catalysts

Hydrocyanic acid?HCN?,an important chemical,is widely used in pesticides,medicines,metallurgy,fuels,and polymers.Currently,the industrial production of HCN are mainly achieved by the Andrussow process and BMA process,which require high temperature of1000 ^oC and 1300 ^oC,respectively.Our previous researches show that HCN can be synthesized at 400 ^oC with the help of plasma.In this thesis,firstly the influence of plasma packing materials,CH₄/NH₃ molar ratio and gas hourly space velocity?GHSV?on HCN synthesis were studied.After that,the effects of Pt catalysts on HCN synthesis were investigated.Finally,a possible reaction mechanism was inferred.The results and conclusions are shown as follows:?1?Packing materials have remarkable influence on HCN synthesis.Among the ten packing materials,which have been studied in this thesis,Hbeta,TiO₂,and?-Al₂O₃ are beneficial to generate CH₃CN,while TS-1,S-1 and fumed SiO₂ are conducive to producing HCN.TS-1 is the best packing material to synthesize HCN,and 12.2%HCN yield can be obtained at 400 ^oC through the combination of CH₄/NH₃ plasma and TS-1.According to the results of NH₃-TPD and N₂-physisorption,acid strength and acid amounts of packing material may not be the determining factors of synergistic effect between plasma and packing materials.While the external surface area of packing materials has a remarkable effect on this synergistic effect,and the bigger the external surface area,the better the reaction performance.Besides,when CH₄/NH₃ plasma is packed by TS-1,the optimal CH₄/NH₃ molar ratio of the feedstock is found to be around 1:2,under which the CH₄ conversion and HCN selectivity reach 19%and 61%,respectively.?2?Pt/TS-1 catalysts significantly improve both CH₄ conversion and HCN selectivity.By using Pt as the active component of catalysts,TS-1 is found to be the optimal support.Thus,25%CH₄ conversion and 80%HCN selectivity were obtained at 400 ^oC when combining Pt/TS-1 catalysts with plasma.Furthermore,the optimal loading is found to be 1%.The most possible reason could be that the Pt/TS-1 catalyst with 1.0%loading has the acid center with appropriate strength,which is conducive to the adsorption of the active species and the desorption of the reaction intermediates.?3?The products selectivity can be regulated by introducing a second metal into Pt/TS-1catalysts.Among the ten metals,which have been investigated in this thesis,Ni,Fe and K are in favor of CH₃CN production,while Cr,Cu and Mg are conducive to the formation of HCN.The Pt-Cr/TS-1 catalysts exhibit the optimal catalytic activity in CH₄/NH₃ plasma for HCN synthesis?20.6%HCN yield at 400 ^oC?.?4?By combining catalyst characterization,OES diagnostic results and literatures,a possible mechanism for the formation of HCN in CH₄/NH₃ plasma is suggested.The reaction process mainly includes three steps:firstly,CH₄ and NH₃ are dissociated by plasma to generate active species such as CH_x and NH_y.Second,these active species are adsorbed by Pt catalysts,and then the adsorbed species react with each other to produce the intermediate of CH_xNH_y.During the second step,Pt?111?plane is capable of adsorbing CH_x species to generate the Pt-CH₂ species,which is an active intermediate and it can react rapidly with NH_y to form the CH_xNH_y species.Finally,through dehydrogenation reaction,the CH_xNH_y species are transformed into H₂C=NH and C=NH*,then HCN is subsequently produced through the reactions H₂C=NH?HCN+H₂ and C=NH*?HCN.