Bromine Oxidation Of Methane To Catalysts

Higher hydrocarbons are obtained in the process of petroleum refinery.It is a green and clean fuel.In Recent years,the major process for the conversion of nature gas to liquiud fuels is initiated from syngas,which is called Fischer-Tropsch process.However,this is an consumingly energy release process.More than one fourth of natural gas must be burned to generate heat for the steam reforming of natural gas.Here,we develop a non-syngas process to synthesize higher hydrocarbons.In our process,the first step is to convert the methane to methyl bromide by the reaction of methane with hydrogen bromide and oxygen,then higher hydrocarbons are produced by the condensation of methyl bromide in the second step.With this route, methane could be high efficiently converted to higher hydrocarbons.In this paper,we focus on study the first step that the oxidative bromination of methane(OBM)over active catalyst.In the OBM reaction,methane reacts with oxygen over catalyst to form methyl bromide when HBr/H₂O exist,with largely energy released. We investigate several factors,such as metal supported,calcination temperature,calcinations time,doped other metals,HBr/H₂O rate,material gas ratio,and reaction temperature,which are important for the conversion of methane and the selectivities of products.As a result,we find that the catalyst 0.4%Rh_0.8Ru_0.2/SiO₂-900-10 is optimal to the oxidative bromination of methane.When CH₄(20.0 mL/min),O₂(5.0 mL/min),N₂ (5.0 mL/min),and HBr/H₂O(6.0 mL/h,40%liquid)are fed into the above catalyst bed,at 700℃reaction temperature,the methane conversion reaches to 41.8%,and the selectivities of CH₃Br,CH₂Br₂,CO,CO₂ are 85.3%,3.1%,11.3%,0.3%respectively.In the products,there is a small quantity of hydrogen formed.Otherwise,we are aware of a disciplinarian, that a longer time and higher temperature of calcination,more metal quantity supported,higher material gas ratio(CH₄:O₂),faster HBr/H₂O rate and higher reaction temperature are facilitatory for CH₃Br and CH₂Br₂ formed,but they will make against the formation of CH₃Br and CH₂Br₂ when excessively provided.The investigation of catalyst life indicates that the catalytic activity has not decline after 40 hours continuously reaction. The XRD characterization shows that there is no change in catalyst structure after reaction(only poorly crystallized cristobalite phase observed).The BET measurement indicates that the specific surface areas are all smaller than 1.0 m²/g,which of catalysts under high temperature calcinations.This result is beneficial to the formation of aim-products(CH₃Br and CH₂Br₂),and inhibits the deep oxidation of methane.On the side,for the optimization of the OBM process,this paper also has studied the mechanism of the OBM reaction.In conclusion,the OBM reaction occurs both in the gas phase and over the surface of the catalyst Rh/SiO₂,but the surface reactions contribute the major part in the formation of CH₃Br and CH₂Br₂.The possible pathway is that HBr react with oxygen to form bromine radicals and adsorbed bromine species.The bromine radicals and adsorbed bromine species might react with methane to form CH₃Br.CH₃Br could react with Br·radicals to form CH₂Br₂ in the gas phase.CH₃Br could also adsorb onto the Rh/SiO₂ surface to form CH₂Br^* species,which could react with Br·radicals and/or Br^* species to form CH₂Br₂.The major part of CO is formed from the steam reformation and the oxidation of CH₃Br.The gas phase oxidation of CH₃Br only produces CH₂Br₂ and CO.CO₂ is formed in the oxidation of CO over the surface of the catalyst at temperature higher than 560℃.The presence of HBr inhibits the deep oxidation and the steam reformation of methane.