| In this paper,nickel-cobalt/HZSM-5 catalysts for the dehydrogenation of methane to high hydrocarbons(aromatics)were prepared and characterized.The cobalt-nickel/HZSM-5 catalysts with different molar ratios were prepared by mixing,impregnation,dehydration,drying,calcination and hydrogenation using dipping method and nickel nitrate,cobalt nitrate and HZSM-5 as raw materials.XRD analysis showed that the diffraction peaks at 20(°)at 37.42 and 43.57 were Ni2+and NiO.The diffraction peaks at 8.24,9.16,23.37 and 23.95 were nickel and cobalt alloy NiCo204.And the diffraction peaks of the nickel cobalt and molecular sieve disappeared after the reaction,and 20(°)formed a new diffraction peak near 22.4,36.2,39.8 and 48.6,which was deduced as the formation of carbide on the catalyst surface.TG-DTG results show that the conversion of a before the reaction is 0.87-9.09%from 25? to 300? indicating that the mass loss is present.The conversion rate of the catalyst after reaction is 0.04-1.90%Rose to 600? the conversion rate of a is 9.09-10.36%,weight loss began to slow down.It can be seen that the mass loss of the catalyst before the reaction is 4-5 times that of the reaction,which is due to the loss of impurities or the instability of the molecular sieve adsorbed in the catalyst before the reaction.The volatiles in the molecular sieve are reduced due to the methane entrainment of the catalyst pores after the reaction in the fixed bed.The maximum conversion of the catalyst was 41.43%at 700? and air atmosphere,and the content of non-decomposed carbon was 58.57%.The DSC results show that asymmetric exotherm is observed in air at 450-600 ?,indicating the presence of different overlapping oxidation peaks corresponding to different carbonaceous materials on the surface of the catalyst after the reaction.XPS analysis showed that the two characteristic peaks at 780.9eV and 796.3eV were attributed to Co2p3/2 and Co2pl/2 before reaction,respectively,and the presence of metal Co304(Co3+)was shown at 780.9eV.The binding energy of 798.0 eV refers to the sample Co2p1/2.The increase of binding energy indicates that the increase of valence electron charge produced by oxidation produces a higher oxidation state,indicating that there is d electron transfer between Ni and Co in Ni-Co/HZSM-5,resulting in electron enrichment on element Ni.The main peak of the catalyst is near 855.5eV and 856.7eV,and Ni2p3/2 is the characteristic of Ni2+or NiO.The peak intensity of Ni2p1/2 peak and 879.7eV at 883.OeV is 876.OeV.Before the reaction was significantly reduced,indicating that the activity of Ni2+or NiO almost disappeared.After the reaction,the peak value of Ni2p3/2 was significantly lower than that before Ni2p3/2.The peaks with a binding energy of 880.7 eV were attributed to metal Ni,which was consistent with the XRD data.In addition,XPS measurement showed that the carbonization of the catalyst was due to the polymerization of ethylene,and the formation of ethylene was due to the synergistic effect of lattice oxygen and cobalt-nickel in the molecular sieve.The post-carbon content of the catalyst on the surface of the catalyst Reducing the indirect proof that the oxygen in the catalyst is involved in the dehydrogenation of methane.The results showed that the contents of carbon,oxygen and silicon in the catalyst were 78.0,16.1 and 4.97%,respectively,and the carbon content ratio was higher than that before the reaction.The results showed that the catalytic activity of 9Ni-9Co(1:1mol)/HZSM-5 higher than 490%. |
PDF Full Text Request