Study On Fe-based Zeolite Catalysts For Direct Conversion Of CH₄ Into Oxy-organics By N₂O

Natural gas(mainly methane)is one of the most abundant clean energy in the world.The conversion of methane into valuable oxy-organics has been an attractive research topic for many years.Currently,methane is usually converted into syngas(CO+H2),which can be used to produce methanol in industry.However,this process is quite a complex and energy-intensive one,which is conducted in wild condition.Notably,N2O can be used to activate methane via iron-based zeolites at a relative low temperature,which can reduce the deep oxidation of methane.Here,we prepared different iron-based zeolites catalysts(FeZSM-5 and FeSAPO)for the partial oxidation of methane by N2O,and the relationship between preparation methods,iron composition,zeolite property and catalytic performance is investigated.Besides,the active centers and reaction mechanisms have also been studied.The main contents of the dissertation are as follows:Firstly,we prepared FeZSM-5 catalysts,and studied the cffect of calcination temperature on the iron species,zeolite property and catalytic performances.With the increase of calculation temperature in inert atmosphere,some ferric iron in octahedral coordination will be gradually converted into active centers(a-sites)and large a-Fe2O3 particles.Meanwhile,framework aluminum(Bronsted acids)will decrease.When the calcination temperature is 900?,the amount of a-sites reaches the maximum,whereas the amount of Bronsted acids reaches the minimum,which leads to the best catalytic performance with an oxy-organics selectivity of 13.3%and yield of 0.12%.Both a-sites and Bronsted acids favors the CH4 conversion,while more a-sites and fewer Bronsted acids favor the better selectivity.The coke formed by the deep reaction of oxy-organics accounts for the catalyst deactivation.Based on the optimized calculation temperature,we studied the effect of iron dispersion on the structure and catalytic performance of FeZSM-5.Traditional deactivation evaporation drying and novel freeze drying are used after the impregnation to change the iron dispersion.Compared with the evaporation-drying catalyst,a more uniform iron distribution is obtained in freeze-drying catalyst,and fewer large a-Fe2O3 particles are found.Besides,in freeze-drying catalyst,more ?-sites(50%higher than evaporation-drying catalyst)and fewer Bronsted acids are obtained,which leads to a higher oxy-organic selectivity(23.6%).and yield is 0.21%,which is 60%higher.Based on the results of 27Al MAS-NMR,N2O titration and NO-FTIR,a-sites are found to be formed by the interactions between iron species and framework aluminum,which contains the Fe2+-O-Al structure.As freeze drying and framework aluminum favors the formation of a-sites,we prepared FeZSM-5 catalysts via freeze drying with different Si/Al molar ratios,in order to increase the amount of a-sites.With the increase of aluminum content,more a-sites,fewer large a-Fe2O3 and oligomeric FexOy are found in the catalyst.The catalyst with the most a-sites(31.8%)possesses the highest oxy-organics yield(0.32%),while the catalyst with the fewest a-sites(2.6%)possesses the highest oxy-organics selectivity(29.6%)and medium yield(0.21%).Combined with 27 Al MAS-NMR and NH3-TPD,it is supposed that weak acids in the catalyst account for the super performance,which can greatly reduce the deep oxidation of oxy-organics.Inspired by the above results,silico-alumino-phosphate zeolites(SAPO-11 and SAPO-34),rather than the traditional aluminosilicate zeolites,are chosen as the new supports for their higher aluminum content and weaker acid strength.When FeSAPO catalysts are calcinated at 550? in air,Fe3+-Al-Si will be formed due to the interaction between iron species and framework aluminum.And then the catalysts are pretreated in inert gas at 900?,some Fe3+-Al-Si will be transformed into a-sites,and the amount of a-sites is 2-3 times of FeZSM-L in the sixth chapter.However,the catalysts without 900? pretreatment also exhibit good performance,which is quite differerent from previous catalyst.Therefore,He,N2O and CH4 are introduced into the catalyst in turn under reaction temperature(300?)to simulate the reaction.It is found that some Fe3+-Al-Si will be transformed into a-sites and there is a redox cycle(Fe2+?Fe3+?Fe2+)similar to catalyst with 900? pretreatment.In traditional catalyst,a-sites is generated in high temperature,usually more than 500?.It is supposed that the existence of P decreases the generation temperature of a-sites.The highest oxy-organics yield(0.37%)is obtained in FeSAPO-34 with the selectivity more than 40%,which exhibits the best performance in our research.