Low-Temperature Light-off MnO_x-Na₂WO₄ Based Catalysts For The Oxidative Coupling Of Methane（OCM） And Their Reaction Light-off Relevant Chemistry

Ethylene is one of the most important petrochemical raw materials,and its output is an important indicator to measure the development level of a country’s petrochemical industry.For a long time,China’s ethylene production has been overly reliant on petroleum resources.Although China’s ethylene production capacity ranks the second in the world,China is also a big ethylene consumer and with a huge ethylene equivalent gap.Therefore,the low carbonization and diversification of ethylene raw materials are the inevitable trend of ethylene development in China in the future.Methane is the major component of natural gas,and its direct activation and conversion into high value-added chemicals is a major challenge for catalysis and industry.Oxidative coupling of methane（OCM）is a promising process for the direct conversion of methane to light olefins.Since the oxidative coupling of methane was first reported in 1982,researchers have been committed to promoting the industrialization of the OCM process and have developed thousands of catalysts,among which the Mn Ox-Na2WO4/Si O2catalyst is considered to be the most promising for industrial applications due to its excellent catalytic performance and stability.However,the optimum reaction temperature of this catalyst is still as high as 800-900 ^oC,which largely restricts its industrial application.Therefore,the development of low-temperature and high-activity OCM catalyst is the crux of the OCM research.Although researchers have carried out extensive studies on the Mn Ox-Na2WO4/Si O2 catalyst,the active sites,the catalytic mechanism and the role of each component are still unclear.In view of the above problems,this thesis focuses on the Mn Ox-Na2WO4 based catalyst system,and investigates the influence of Si O2 and different oxide supports on the low-temperature OCM light-off of the Mn Ox-Na2WO4 based catalysts.Moreover,the reaction chemistry of the supports involved in the redox cycles of Mn Ox species was studied,and the chemical nature of the redox cycle light-off of Mn Ox species affected by the physicochemical properties of the supports was revealed.Furthermore,the correlation relationship between the low-temperature light-off of the catalysts and the light-off of Mn Ox redox cycles was established.Finally,the nature of the chemical regulation of Na2WO4 on the Mn Ox redox was elucidated.These findings provide a new idea for the development of low-temperature OCM catalysts with industrial application prospects.The main research contents of this thesis include the following three parts:（1）Study on low-temperature catalytic oxidative coupling of methane with different Si O₂ supported Mn O_x-Na₂WO₄/Si O₂ catalystsMn O_x-Na₂WO₄/Si O₂ catalyst is a classical catalyst for the oxidative coupling of methane.Generally,it is believed that the Si O₂ is only an inert support.Through the thermodynamic analysis of the reduction processes of Mn species by CH₄,it is speculated that the“Mn₇Si O₁₂?Mn Si O₃”redox cycle(Mn₇Si O₁₂+6Si O₂?7Mn Si O₃+1.5O₂),in which Si O₂ participates,may realize the low-temperature light-off of OCM reaction.The Mn O_x-Na₂WO₄/Si O₂ catalysts（Mn O_x-Na₂WO₄/A-Si O₂ and Mn O_x-Na₂WO₄/S-Si O₂）supported on different Si O₂ were prepared.Under the same reaction conditions,the Mn O_x-Na₂WO₄/A-Si O₂ catalyst exhibited a better low-temperature OCM performance,i.e.,23%of CH₄ conversion and 72%of C_2-3 selectivity at 680 ^oC,whereas the Mn O_x-Na₂WO₄/S-Si O₂ catalysts shows a high OCM light-off temperature of 800 ^oC.In-situ and ex-situ XRD results show that both catalysts showed a phase evolution process of“Mn₇Si O₁₂?Mn Si O₃?Mn WO₄”.Compared to Mn O_x-Na₂WO₄/S-Si O₂,the Mn O_x-Na₂WO₄/A-Si O₂ catalyst holds a fast“Mn₇Si O₁₂→Mn Si O₃”reduction half-cycle rate.Additionally,the H₂-TPR,O₂-TPD and O₂-TPO results show that the Mn O_x-Na₂WO₄/A-Si O₂ catalyst owns better redox properties,which can lead to an easier activation of CH₄ and O₂ in the reaction.The comparison experiments showed that the Si O₂ is essential for the“Mn₇Si O₁₂?Mn Si O₃”redox cycle.The ²⁹Si MAS NMR characterization results show that the structure of Si O₂ in the Mn O_x-Na₂WO₄/Si O₂ catalyst has an important influence on the redox rate of“Mn₇Si O₁₂?Mn Si O₃”:the catalyst with more Si O₂-Q² units is easier to form Mn Si O₃ and drives the running of“Mn₇Si O₁₂?Mn Si O₃”redox cycle,resulting in better low-temperature OCM activity.Furthermore,DFT calculation results show that the self-reduction reaction between Mn₇Si O₁₂ and Si O₂ containing Q² units has a lower Gibbs free energy change（ΔG）,which verifies that the Si O₂ containing Q² units is favorable to react with Mn₇Si O₁₂ to form Mn Si O₃ at low temperature.（2）Study on Mn O_x-Na₂WO₄ catalysts supported by different metal oxides and their catalytic oxidative coupling of methaneThe Mn O_x-Na₂WO₄ catalyst can achieve the low-temperature OCM reaction through the“Mn₇Si O₁₂?Mn Si O₃”or“Mn₂O₃?Mn Ti O₃”redox cycle.In order to explore the possibility of“Mn³⁺(or Mn⁴⁺)?Mn²⁺”redox cycle to reduce the light-off temperature of the OCM reaction,the Mn O_x-Na₂WO₄ catalyst was supported on four different metal oxides（Sn O₂,Ti O₂,Mg O and Zn O）and applied to the OCM reaction.When using a feed of CH₄/O₂（5/1,molar ratio）and a gas hourly space velocity（GHSV）of 4000 m L·g^-1_cat·h^-1,the OCM light-off temperature of the four catalysts ranked as follows:Mn O_x-Na₂WO₄/Sn O₂（660 ^oC）<Mn O_x-Na₂WO₄/Ti O₂（670 ^oC）<Mn O_x-Na₂WO₄/Mg O（690 ^oC）<Mn O_x-Na₂WO₄/Zn O（720 ^oC）.The Mn O_x-Na₂WO₄/MO_x（M=Sn,Ti,Mg and Zn）catalyst was treated with CH₄ and O₂ respectively at 680-720 ^oC and then analyzed by XRD,showing that the redox cycle of Mn species on each catalyst was:Mn₂O₃?Mn O?Mn₂Sn O₄;Mn₂O₃?Mn Ti O₃;Mg₆Mn O₈?Mn O-Mg O,and Mn₃O₄?Mn O and Zn Mn₂O₄?Mn O.The XRD and XPS results showed that the reduction half-cycle rate of“Mn³⁺(or Mn⁴⁺)→Mn²⁺”and the proportion of the surface lattice oxygen to the overall surface oxygen atoms of each catalyst were in the following order:Mn O_x-Na₂WO₄/Sn O₂>Mn O_x-Na₂WO₄/Ti O₂>Mn O_x-Na₂WO₄/Mg O>Mn O_x-Na₂WO₄/Zn O.The O₂-TPD experiment results showed that the essence of different metal oxides to regulate the active temperature of OCM reaction is to establish different“Mn³⁺(or Mn⁴⁺)?Mn²⁺”redox cycle,and the easier it is to achieve“Mn³⁺(or Mn⁴⁺)?Mn²⁺”redox cycle over the catalyst,the easier it is to release weak bonding surface lattice oxygen,and thus achieving better low-temperature OCM performance.（3）Study on catalysis roles of Na₂WO₄ in the Mn O_x-Na₂WO₄/Si O₂ catalystThe role of Na2WO4 in the Mn Ox-Na2WO4/Si O2 catalyst is still unclear,especially its effect on the redox cycle of Mn species and whether it provides lattice oxygen in the reaction.Three comparative catalysts were prepared by slurry mixing method and washing treatment:Pristine catalyst containing cristobalite,Mn7Si O12 and Na2WO4;Na2WO4/Si O2 catalyst containing cristobalite and Na2WO4;Washing-2nd catalyst containing cristobalite and Mn7Si O12.The results of the fixed bed experiments and CH4/O2 temperature-programmed surface reaction show that Mn7Si O12 is favorable for CH4 activation;Na2WO4 can improve the C2-3 selectivity;the coexistence of Mn7Si O12and Na2WO4 is beneficial to achieve good OCM performance because of the synergistic catalysis between them.O2-TPD,XRD,Raman,and other characterization results showed that the Na₂WO₄ catalyzed the reaction of“Mn₇Si O₁₂+6Si O₂?7Mn Si O₃+1.5O2”,reducing the forward and reverse reaction temperatures.Pulse experiments of CH4 and CH4/O2 showed that Na2WO4/Si O2 catalyst was inert for activating CH4 in the absence of O2,but could activate CH4 and O2 in the presence of O2,possibly because of the Eley-Rideal（E-R）mechanism rather than the Mars-van Krevelen（Mv K）mechanism.By coupling reduction-reoxidation treatment with washing treatment of the Pristine catalyst,¹⁷O selective labeling of Mn7Si O12 and/or Na2WO4 was achieved.The¹⁷O ss-NMR and Raman characterization results indicate that,when the reaction temperature was low（700 ^oC）,the main redox cycle is“Mn₇Si O₁₂?Mn Si O₃”and Mn7Si O12 provides the lattice oxygen,whereas the lattice oxygen in Na2WO4 exists stably in the crystal;when the reaction temperature is high（750 ^oC）,the main redox cycle is“Mn7Si O12?Mn WO4”,in which Mn7Si O12 still serves as lattice oxygen supplier rather than Na2WO4.It should be noted that the lattice oxygen in Na2WO4could be cyclically exchanged into Mn7Si O12 through“Td-WO4?Oh-WO6”at 750 ^oC.Even so,the lattice oxygen in Na2WO4 was not involved in the activation of CH4directly.