Study On Reaction Mechanism Of N₂O Oxidizes CH₄ To Methanol Over Zeolite

CH4,as the main feedstock of methanol,is converted into methanol through syngas route in industry.The process of producing syngas needs high temperature and pressure condition,so the syngas route consumes energy a lot.Compared with traditional industrial route,the reaction condition of direct oxidation of methane to methanol（DMTM）is more mild,which only requires low temperature and ordinary pressure.As theα-O species generated via N₂O reacting with zeolite catalysts can activate CH₄,N₂O is a fantastic single oxygen oxidant for DMTM reaction system.For the moment,develop catalysts with high-performance to raise the methanol yield and selectivity is still a great challenge for N₂O oxidizes CH₄ to methanol（N₂O-DMTM）.This paper focused on N₂O-DMTM reaction system and preliminary screened different structures zeolites modified by Fe,Cu and Ni,and the results manifested that BEA and FER zeolites possess better performance.After that,the reaction conditions of Cu-,Fe/Cu-,Fe-BEA and Cu-FER catalysts were optimized.At last,the reaction mechanism and active sites of N₂O-DMTM over Cu-,Fe/Cu-,Fe-BEA and Cu-FER catalysts were investigated systemically by means of In-situ FTIR,MS,EXAFS and other characterization techniques as well as DFT,AIMD and other theoretical calculation means.The main research contents and conclusions are as follows:（1）Under 320 ℃,10 vol%vapor content and 0.6 wt%Cu content,Cu-BEA catalysts have the best performance and the methanol yield and selectivity is 242.9μmol·g·h and 71.6%,respectively.It has been discovered that Cu-BEA-0.6%catalyst pretreated by He under high temperature exists[Cu]⁺and[Cu]⁺--[Cu]⁺two kinds active sites,which can react with N₂O forming containingα-O species([Cu-O]⁺and[Cu-O-Cu]²⁺,respectively),by in-situ UV-vis,in-situ FTIR,EPR and other characterization methods.Furthermore,the proportion of[Cu-O-Cu]²⁺is 23.8%and the micro-structure of[Cu-O-Cu]²⁺was analyzed by EXAFS.The thermodynamic stability of reactive sites and reaction intermediate species were analyzed via ab initio thermodynamics（AIT）method.The results indicated that[Cu-O-Cu]²⁺is more stable than[Cu]⁺and[Cu]⁺--[Cu]⁺and the vapor from reaction system can’t damage the structure of[Cu-CH₃]⁺,[Cu-OH]⁺,and[Cu-O-Cu]²⁺.Based on multiple kinds characterization and theoretical calculation methods,H₂O proton transfer reaction mechanism in[Cu]⁺--[Cu]⁺active site was proposed for the first time.H₂O proton transfer reaction mechanism revealed the reason why vapor promotes methanol formation and the reaction energy barrier of H₂O proton transfer reaction mechanism is the lowest than other four reaction routes.Micro-kinetic modelings were established for the two kinds active sites and the results showed that H₂O proton transfer reaction route possesses the highest reactive rate(8.58×10³ s^-1)among four reaction route under actual reaction condition.（2）Under 270 ℃,10 vol%vapor content,0.6 wt%Fe content and 0.68wt%Cu content,Fe/Cu-BEA catalysts have the best performance and the methanol yield and selectivity is 259.1μmol·g·h and 71.7%,respectively.It has been proved that[Fe]--[Cu]is the major active site for Fe/Cu-BEA-0.6%catalyst and[Fe]--[Cu]reacts with N₂O will generate[Fe-O-Cu]²⁺species which possessesα-O via in-situ FTIR,H₂-TPR-MS and other characterization.Based on AIT method,the thermodynamic stability of different active sites was compared.The results showed that[Fe-O-Cu]²⁺is more stable than[Fe]--[Cu]and single copper active sites.Via TPSR-MS,in-situ FTIR testing,it has been certified that H₂O proton transfer can also happen in[Fe]--[Cu]active site and Fe_0.6%Cu_0.68%-BEA catalyzes N₂O-DMTM following the radical mechanism.AIMD and DFT were employed to calculate reaction mechanism as well as the micro-kinetic modelings were used to compare reaction rate.The results indicated that the energy barrier of the process that[Fe]--[Cu]reacts with N₂O formingα-O species is 0.18 e V which is 0.73 e V lower than that of the process that[Cu]--[Cu]does and H₂O proton transfer reaction mechanism（in[Fe]--[Cu]active site）possesses the highest reaction rate(3.64×10⁹s^-1)among three reaction route.Besides,it has been verified that the synergistic effect of Fe and Cu cations from Fe/Cu-BEA is derived from strong electric field effect of[Fe]--[Cu]active site through the theoretical simulations including electronic density difference,Bader charge analysis and density of state（DOS）.（3）Under 250 ℃,20 vol%vapor content,and 1 wt%Fe content,Fe-BEA catalysts have the best performance and the methanol yield and selectivity is227μmol·g·h and 72.9%,respectively.It has been found that Fe-BEA-1%catalyst has[Fe-O-Fe]²⁺active site during reaction process and[Fe-O-Fe]²⁺will transform to[Fe-OH]⁺--[Fe-OH]⁺species contacting with vapor by EXAFS and in-situ FTIR。Based on serval kinds characterization,the fact that Fe-BEA-1%catalyst catalyzes N₂O-DMTM also following the radical mechanism and H₂O can participate in N₂O-DMTM reaction as well as inhibit methanol over oxidation has been discovered.The thermodynamic stability of different active sites was compared via AIT method,and the stability order is[Z-Fe₂O-2OH]>[Z-Fe₂-O]>[Z-Fe₂-2OH]>[Z-Fe-O]>[Z-Fe-OH]>[Z-Fe₂]>[Z-Fe].Besides,the energy barriers of elementary reaction path from different reaction route were calculated via DFT simulation and micro-kinetic modelings of different reaction mechanism were established,and the results showed that[Fe-OH]⁺--[Fe-OH]⁺reaction mechanism route’s reaction energy barriers is lowest and reaction rate(6.37×10⁴ s^-1)is fastest than the other routes,the carbon deposition and deactivation of[Fe]and[Fe]--[Fe]active sites is due to large amounts of CH₃ covering the surface of the active sites and H₂O can not only change the active site,but also take part in forming methanol process through OH.（4）Under 330 ℃,10 vol%vapor content,and 0.31 wt%Cu content,Cu-FER catalysts have the best performance and the methanol（including dimethyl ether）yield and selectivity is 2736μmol·g·h and 80%,respectively.In addition,Cu-FER-0.31%catalyst remained active after 100 h using.It has been discovered that Cu-FER-0.31%catalyst exists[Cu₂-O]²⁺species and the process of Cu-FER-0.31%catalyzes N₂O-DMTM following radical mechanism via CO probe testing and In-situ FTIR.According to the results of AIT and DFT calculation,di-copper active site is more stable than mono-copper active site,and[Cu₂]²⁺active site has the highest efficiency in the three active sites.