Metal Modified ZSM-5 Zeolite Catalyzed Co-Aromatization Of Methane With Methanol And Reaction Mechanism

Methane is a cheap carbon-based clean energy sourced from natural gas and shale gas,and its conversion into high value-added fuels or chemicals is an urgent need of the world’s energy economy.In order to overcome the high reaction energy barrier caused by stable chemical structure of methane,the conversion temperature of methane is usually above 600℃.However,the increased reaction temperature will accelerate catalyst deactivation rate,and increase by-products and energy consumption.The co-reaction of methane with other reactant can realize the effective conversion of methane under mild conditions.Methanol,as a cheap and easily available chemical raw material derived from coal chemical industry and biomass,its co-reaction with methane not only further reduces the conversion temperature of methane,but also can convert methane directly into high value-added aromatics and improve methanol aromatization.Meanwhile,co-reaction of methane with methanol can neutralize the reaction heat of methane activation and methanol aromatization.Therefore,it is of great significance to carry out study on the co-aromatization of methane and methanol for the efficient use of C1 resources and the activation and conversion of light alkanes.Among the various catalysts,the metal-modified ZSM-5 zeolite shows excellent catalytic reactivity.Nevertheless,the current research focuses on the improvement of the co-reaction performance with methane as the main body,while ignoring the mechanism of the metal active site,the conversion path of methane in the co-reaction,etc.This makes the development and application of the methane co-reaction process severely hindered.To realize the efficient conversion of methane in the co-reaction of methane with methanol,the relationship between loaded-metal on metal-modified ZSM-5 zeolite catalyst and methane conversion is systematically studied,and the reaction path of methane in the co-reaction is investigated in detail from the aspects of catalyst reaction active center and co-reaction mechanism.The main work and research results are as follows:(1)The co-aromatization reactivity of methane with methanol over ZSM-5 zeolite catalysts modified by different metals was investigated.Studies have shown that the number and strength of medium-strong acid sites formed by the combination of loaded-metals and strong acid sites on the zeolite are related to methane conversion in the co-reaction,and zinc metal was selected as the optimal modified metal.Furthermore,the physical and chemical properties of the modified ZSM-5 zeolite with different zinc loading and its reaction and deactivation were investigated.Meanwhile,the loaded-state of zinc metal in the ZSM-5 zeolite is closely related to zinc loading:When the zinc loading is less than 1.5 wt.%,loaded-zinc mainly exists on the catalyst in the form of ZnOH+,and the methane conversion activity increases with the increase of metal loading.When the zinc loading content is high,the loaded-zinc on the zeolite changes to the oxidation state,which leads to metal sintering during the reaction,which causes methane conversion and co-reaction reactivity to decrease,and the presence of methane accelerates the sintering deactivation of the high zinc loading catalyst.In contrast,the strong interaction of zinc with low metal loading and strong acid sites avoids sintering and deactivation of the catalyst.(2)The effects of different modification and preparation methods on the state of loaded-zinc species on ZSM-5 zeolite were investigated,and various characterization revealed the relationship between various loaded-zinc species and the methane conversion reactivity in the co-reaction.The results-show that the zinc species introduced by the physical mixing method existin the form of crystalline zinc oxide and inhibit the methane conversion in the co-reaction.However,the zinc species introduced by ion exchange and impregnation mainly exist on the catalyst in the form of ZnOH+,and the active site promotes the methane conversion and the methanol aromatization.In addition,the calcination atmosphere has an important influence on state of loaded-zinc species on the ZSM-5 zeolite:hydrogen calcination is beneficial to the formation of high-valent Zn2+ species that promote methane conversion,thereby enhancing the methane conversion reactivity in the co-reaction.But nitrogen calcination inhibited the transformation of zinc species to ionic state on the catalyst,which in turn led to a decrease in the reactivity of catalyst.(3)The influences of reduction-oxidation pretreatment on the state conversion of zinc oxide on the ZSM-5 zeolite and co-reaction were studied,and the methane co-reaction performance of the catalyst was optimized.Various characterizations were used to explore the affection of reduction pretreatment time on the loaded state of zinc oxide.It was found that reduction-oxidation pretreatment can effectively promote the transformation of zinc oxide species to ionic zinc species on the catalyst,and obtain quantitative relationship between methane conversion and the number of medium strong acid sites on catalysts.Moreover,the effect of acid sites in the zeolite on the formation of ionic zinc species was investigated,and it was proved that the strong acidic sites are beneficial to the state transformation and dispersion of loaded-zinc oxide.At last,the reduction-pretreatment is used to optimize the performance of catalyst,and effectively improves the reaction performance of the cataly st in the methane co-reaction.(4)The conversion path of methane in the co-reaction with methanol was systematically investigated by various methods such as temperature-programmed surface reaction-mass spectrometry(TPSR-MS)and the addition of a small amount aromatics or olefins to adjust the reaction path of MTA.The results showed that methane was converted by co-feeding with methanol on Zn/ZSM-5 under mild conditions of 300℃ and 1 atm,and it was proved that it was not methanol but the reaction product or intermediate formed by the "aromatic hydrocarbon cycle" reaction path in the methanol to aromatics reaction(MTA)The body played a vital role in the conversion of methane.Subsequently,methane conversion was promoted and the apparent activation energy of methane conversion was reduced by enhancing the"aromatics cycle" reaction path.(5)The method of online mass spectrometry and catalyst pre-coking was used to study the reason for the formation of methane conversion in the coupled methanol reaction,and the relationship between coking and methane conversion was further explored by changing the pre-coking conditions.The results show that the reason for the formation of the methane conversion induction period is caused by carbon deposition of the catalyst,and the linear relationship between the conversion of methane and the amount of carbon deposition in the co-reaction meets.Finally,a long-period co-reaction experiment with aromatics was added to find that the promotion of methane conversion is beneficial to reduce the carbon deposition of the catalyst.