Low-temperaure Oxidative Coupling Of Methane:MO_x-doped Mn₂O₃-Na₂WO₄/SiO₂ Catalysts And MO_x-doping Effect

The oxidative coupling of methane?OCM?is one of the most important and challenging subject in catalysis,industry and energy field,which is a significant way to convert methane directly and efficiently into low-carbon olefins.Ethylene is the basic raw material of petrochemical industry,and its output is an important symbol to measure the level of petrochemical industry in a country.The production of ethylene is still largely dependent on petrochemical industry at present.In view of the shortage of petroleum resource shortage and the surplus of methane resource in the today's era,if the oxidation coupling of methane process can be applied industrially,it will change the whole petrochemical industry.However,at present,the oxidative coupling process of methane still faces the problems of high reaction temperature and low product yield.Therefore,to develop low-temperature,highly active and selective catalyst for the OCM reaction is the direction of joint efforts for the researchers all over the world.To accomplish this goal,the most promising Mn₂O₃-Na₂WO₄/SiO₂ catalyst was modified using metal oxides and the MOx-doping effect on their OCM performance was investigated.The main research contents are presented as follows:Firstly,a series of titanium-containing carriers,including titanium-silicon molecular sieve?Ti-MWW and TS-1?,TiO₂?anatase?,Ti₂O₃ and CaTiO₃,were selected to prepare the Mn₂O₃-Na₂WO₄/Support catalysts by impregnation method.In comparison with the Mn₂O₃-Na₂WO₄/SiO₂ base catalyst,the Ti-MWW and TS-1 supported catalysts achieved excellent low-temperature performance for the oxidative coupling of methane reaction.In particular,the catalyst supported on Ti-MWW could deliver²6%CH₄conversion and⁷6%C₂-C₃ selectivity when decreasing the reaction temperature?catalyst bed temperature?from 800 to 720 ^oC for a feed of CH₄/O₂/N₂?5/1/4,molar ratio?with a gas hourly space velocity?GHSV?of 8000 mL?g^-1_cat.?h^-1.However,when the reaction temperature was lower than 750^oC,the OCM performance for the TiO₂,Ti₂O₃,CaTiO₃,and SiO₂ supported catalysts decreased sharply.Secondly,the Mn₂O₃-Na₂WO₄/Ti-MWW,Mn₂O₃-Na₂WO₄/TS-1 and Mn₂O₃-Na₂WO₄/Ti-MWW catalysts were carefully characterized by means of XRD,Ramam,XPS,SEM,EDX,H₂-TPR,ICP-AES and BET.It was found that the low-temperature OCM performance of the Mn₂O₃-Na₂WO₄/TS-1 and Mn₂O₃-Na₂WO₄/Ti-MWW catalysts was due to the generation of MnTiO₃ phase in the OCM reaction process.The formation of[MnTiO₃?Mn₂O₃]chemical looping induces the quick transformation between Mn²⁺and Mn³⁺to active CH₄ and O₂ at lower temperature.In addition,Na₂WO₄ has synergistic catalysis with[MnTiO₃?Mn₂O₃]chemical looping,which improves the target product selectivity to obtain a good low-temperature OCM performance.For Mn₂O₃-Na₂WO₄/SiO₂ catalyst,the transformation between Mn²⁺and Mn³⁺depends on[MnWO₄?Mn₂O₃]chemical looping to activate CH₄ and O₂,but this chemical looping can only work above 800 ^oC.In other words,the Mn₂O₃-Na₂WO₄/SiO₂ catalyst will have better OCM performance only when the reaction temperature is above 800 ^oC.Moreover,the transition rate from Mn²⁺to Mn³⁺was calculated quantitatively,and the criterion of threshold value of lattice oxygen transition rate of catalyst was proposed.The[Mn₂O₃-Na₂WO₄]-based catalyst performance is greatly dependent on the Mn²⁺-to-Mn³⁺rate no matter what temperature is,and a transition rate of 10%/min seems critical for a better OCM conversion/selectivity.Because of the high price of Ti-MWW molecular sieve,the TiO₂ doped Mn₂O₃-Na₂WO₄/SiO₂ catalyst was prepared by ball milling method in order to explore the industrialization for the oxidative coupling of methane process.As-obtained catalyst showed a good low-temperature OCM performance at 650 ^oC,with²2%CH₄conversion and⁶2%C₂-C₃ selectivity,and was stable for at least 500 h using a feed of CH₄/O₂/N₂?5/1/4,molar ratio?and a gas hourly space velocity?GHSV?of 8000mL?g^-1_cat.?h^-1,showing a good prospect of industrial application.In addition,TiO₂-doped Mn₂O₃-Na₂WO₄/SiO₂ catalyst was prepared by solution combustion method.The effects of preparation parameters?TiO₂ precursor,fuel/oxidant ratio???,calcination temperature and active component loading?and OCM reaction conditions?CH₄/O₂ molar ratio,gas space-time velocity?GHSV?and reaction temperature?were systematically investigated on the catalyst performance for the OCM reaction.The catalyst obtained under the optimum preparation conditions delivered a good low temperature OCM performance,with²0%CH₄ conversion and⁷0%C₂-C₃ selectivity at 700 ^oC using a feed of CH₄/O₂/N₂?5/1/4,molar ratio?and a gas hourly space velocity?GHSV?of 8000 m L?g^-1_cat.?h^-1,and was stable for at least 250 h.XRD and Raman showed that the good low-temperature OCM performance for the TiO₂-doped Mn₂O₃-Na₂WO₄/SiO₂ catalyst prepared by solution combustion method is also attributed to the generation of MnTiO₃ and the formation of[MnTiO₃?Mn₂O₃]chemical looping during the OCM reaction.Finally,inspired by the above interesting finding of the MnTiO₃-driven low-temperature[Mn³⁺?Mn²⁺]chemical looping for the improvement of the low-temperature OCM performance of the TiO₂-doped Mn₂O₃-Na₂WO₄/SiO₂ catalyst,we asked ourselves whether some other metal oxides such as MgO,Ga₂O₃ and ZrO₂ can react with MnO_x to form mixed-oxides similar to MnTiO₃,by using as additives,would show the ability to improve the low-temperature OCM performance of the Mn₂O₃-Na2WO4/SiO2.To seek answer,a series of MO_x?M=Ti,Mg,Ga,Zr?-modified Mn₂O₃-Na₂WO₄/SiO₂ catalysts to be used in the OCM reaction were prepared by impregnation method.In comparison with the non-modified Mn₂O₃-Na₂WO₄/SiO₂ catalyst,the TiO₂-modified shows attractive improvement of the low-temperature activity/selectivity and the MgO-modification almost does nothing whereas the Ga₂O₃-and ZrO₂-modification show negative effect.The TiO₂-modified catalyst achieved a high CH₄ conversion of²3%with a high C₂-C₃ selectivity of⁷3%at 700 ^oC for a simulated feed gas of 50vol%CH₄ in air?CH₄/O₂/N₂=5/1/4?using a gas hourly space velocity?GHSV?of 8000mL?g^-1_cat.?h^-1.The MO_x-modified Mn₂O₃-Na₂WO₄/SiO₂ catalysts after OCM reaction were systematically characterized by means of SEM,BET,XRD,Ramam and ICP-AES.The formation of the MnTiO₃ phase and MnTiO₃-dominant catalyst surface definitely brings birth of the interesting improvement of the low-temperature catalyst activity/selectivity for the OCM reaction.The newly generated Mg₂MnO₄ maybe play an equal role as the Mn₂O₃ with achieving the[MnWO₄?Mg₂MnO₄]chemical looping instead of the high-temperature[MnWO₄?Mg₂MnO₄]chemical cycle in the MgO-modified catalyst;that is why the MgO-modified catalyst provides OCM performance comparable to the non-modified catalyst.The Ga₂O₃-and ZrO₂-modification facilitate the reduction of Mn₂O₃ and subsequent combination with Na₂WO₄ to form relatively inactive MnWO₄ while absolutely suppressing the transformation of SiO₂ into the?-cristobalite,which is the main cause for their deteriorated OCM performance.Although the introduction of Mg,Ga and Zr into the Mn₂O₃-Na₂WO₄/SiO₂ catalysts is not working to improve their low-temperature OCM performance,we clearly understand why did not and in a sense our exploration is useful and helpful for OCM catalyst development.