Preparation And Optimization Of ZSM-5-based Catalyst In Methane Dehydroaromatization

Methane dehydroaromatization(MDA)is a potential technique in converting natural gas into value-added aromatics directly,and developing efficient catalyst is the key for the industrialization of MDA reaction.Mo/ZSM-5 is the most studied MDA catalyst.However,severe coking of catalysts and Mo-induced dealumination of ZSM-5 during regeneration hinder the industrial application of this catalyst.Besides,disputes still exsit about the coking mechanism of MDA,making optimization of coking resistance being the Achilles’Heel of this reaction.Since the coking mechanism is highly associated with active phase,pore structure and reaction pathway,improving the coking resistance through the identification of active phase,determination of reaction pathway and discrimination of coke type is of critical significance to MDA reaction.For the sake of highly coking-resistant catalyst,in this study,the coking mechanism of MDA reaction is established via identifying the function of metal and acid sites in Mo/ZSM-5 catalyst.Based on the mechanism study,the structure and composition of this catalyst is further regulated to obtain a MDA catalyst with ideal methane conversion,high selectivity to aromatics and strong coking resistance,which will provide more theoretical support to the design of MDA reaction.To optimize coking resistance,the coking mechanism of MDA reaction on Mo/ZSM-5 catalyst surface is studied by investigating the role of Bronsted acid site(BAS).A step-by-step impregnation is used to introduce Mo and Na to ZSM-5.Characterizations show that BAS is efficiently regulated without obvious change in Mo dispersion under same Mo loading.Then the coking mechanism is proposed by studying the influence of BAS variation on MDA performance.The decrease in BAS results in barely affected selectivity to benzene,validating that after dehydrogenation on Mo sites,methane can be converted to benzene directly inside zeolite channels without the aid of BAS.Besides,the amount of carbonaceous deposits within zeolite channels reduces after the decrease of BAS,indicating a restricted growth of hydrocarbon pools confined in the channels by the steric constraint of Na~+,which leads to the decrease in both methane conversion and selectivity to naphthalene.These prove that the hydrocarbon pools act as the precursors for both carbonaceous deposits within zeolite channels and aromatic products,hence the formation of inner coke follows the hydrocarbon pool mechanism.On the basis of coking mechanism,the influence of inner-diffusional path on MDA performance especially the coking resistance of Mo/ZSM-5 is investigated.A novel grinding synthesis method(GSM)is applied to synthsize ZSM-5.Intra-crystalline mesopores are generated by simply regulating the amount of Na OH in the precursor gel without adding any mesoporogen templates.With increasing Na OH amount,the alkalinity of synthesis system increase,resulting in the dissolution-recrystallization of some meta-stable crystallite,thus generating intra-crystalline mesopores.Further increasing Na OH amount will lead to the disappearance of intra-crystalline mesopores as well as larger crystal size.On the preparation basis,the inner-diffusion path of Mo/ZSM-5 is regulated.Results show that the amount of carbonaceous deposits inside channels decrease after introducing intra-crystalline mesopores and reducing crystal size,but the methane conversion and selectivity to aromatics of Mo/ZSM-5 are also reduced,proving that the channel carbonaceous deposits is beneficial to MDA reaction.However,shortening the inner-diffusion path leads to the increase of external coke,which barely improves the coking resistance of MDA catalyst.To further optimize MDA performance,the active component is tailored and Fe/ZSM-5catalyst with shortened induction period and high coking resistance is prepared via the GSM approach.It is found that iron species in the obtained catalyst are presented mainly as isolated or low-polymerized sites located in ZSM-5 channels,generating strong metal-support interaction.These iron species are highly resistant to the reduction and carburization of methane,resulting in rapid formation of active iron sub-oxides,thus considerably shorten the induction period.Owing to the strong metal-support interaction,the formed iron sub-oxides could also withstand sintering/agglomeration and be stabilized inside the zeolite channels,significantly improve the catalytic stability.While for Fe/ZSM-5 catalysts prepared by conventional impregnation method,the iron oxides remain principally at external surface as big clusters/particles with weak metal-support interaction,which then undergo complete reduction and carburization during MDA reaction,forming iron carbides as the active sites.The larger size of iron oxides on the external surface results in a much slower formation of iron carbides as well as hydrocarbon pool species,leading to a longer induction period.Besides,the big iron carbides catalyze the complete dehydrogenation of methane,generating much more external coke than GSM catalysts.Hence,enhancing Fe dispersion,thus suppressing the coke on the outmost catalyst surface is the key to boost the coking resistance of MDA reaction,which will provide more thinking to the design of MDA catalysts.