Regulation Of Reaction Pathway For Methanol Stepwise Aromatization By Optimizating The Configuration Of Dual-component ZSM-5 Catalysts

Light aromatics,such as benzene,toluene and xylene,are important chemical raw materials and play an important role in national economic production.Based on the mature technology of methanol production from syngas and the catalytic conversion of methanol to aromatics,it can get rid of the dependence on petroleum resources and realize the clean and efficient utilization of carbon resources such as coal.However,the complex reaction network of methanol to aromatics（MTA）results in various hydrocarbon products,including olefins,alkanes,aromatics and other hydrocarbons.Multiple secondary reactions such as polycondensation,hydrogen transfer and deep alkylation of these species on acid ZSM-5 grains not only reduce the light aromatic selecivity,but also significantly affect the coke deposition and catalyst deactivation.Notably,methanol can be converted to light hydrocarbons over high Si/Al ratio ZSM-5,which can be further converted into light aromatics on low Si/Al ratio Zn/ZSM-5.The methanol stepwise aromatization via light hydrocarbons can be realized by coupling above two ZSM-5catalysts.The formation of polycyclic aromatics and other coke species can be signally inhibited for such reaction mode,which is conducive to catalytic stability.The appropriate coupling mode of catalysts is the key to achieve efficient collaboration.At present,the physical mixing of metal oxide and molecular sieve catalyst can show a good synergistic effect in the CO/CO₂ hydrogenation reaction,which significantly improves the CO/CO₂ conversion and product selectivity.In contrast,the reaction pathway of methanol stepwise aromatization catalyzed by two-component ZSM-5 is extremely complicated.Both methanol and hydrocarbon products can be converted on two ZSM-5 catalysts,which makes it more difficult to control the order of step reaction.How to effectively control the material migration process and adjust the microscopic reaction pathway are the difficulties of this study.In this work,the catalyst packing mode and the nanocomposite structure of two kinds of ZSM-5 were finely regulated for the efficient methanol stepwise aromatization,and the methanol feed method was also optimized to optimize the product distribution.First,two kinds of ZSM-5 was coupled by physical mixing method,and the synergistic catalytic mechanism and the stepwise conversion behavior of methanol were investigated.Then,the influence of catalyst spatial configuration on the synergistic catalytic performance of two-component ZSM-5 catalysts was studied by adopting various packing modes of above two ZSM-5 catalysts.Furthermore,the methanol feed method of dual packing mode with excellent reaction stability was finely tuned to the control the alkylation depth of hydrocarbons in the process of methanol aromatization.And the effect of methanol on the aromatization of light hydrocarbons was explored.Finally,two kinds of ZSM-5 catalysts were integrated at the nanoscale and the integrated structure was optimized to strengthen the diffusion-transformation of hydrocarbon molecules for effective methanol stepwise aromatization process.The effects of the morphology of nano-integrated ZSM-5 on product distribution and catalyst deactivation were discussed.The specific research results and conclusions of this work are as follows:（1）After mixing the high Si/Al ratio ZSM-5 into the low Si/Al ratio Zn/ZSM-5catalyst system,the microscopic reaction process of methanol stepwise aromatization via light hydrocarbons was enhanced.It is found that the high proportion of ZSM-5 with high Si/Al ratio could promote the conversion of methanol to light hydrocarbons,and thus strengthen the aromatization process of light hydrocarbons on the Zn/ZSM-5 grains.The coke deposition of Zn/ZSM-5 grains was significantly inhibited because the methanol-induced aromatic aggregation and transformation.However,the backmixing of aromatics to the high Si/Al ratio ZSM-5 grains accelerated the surface coke deposition and catalyst deactivation,even faster than the aromatization catalyst.It is worth noting that moderately increasing the acid density of mixed ZSM-5 could enhance the dealkylation of heavy aromatics and the cracking of macromolecular coke precursor species,and significantly improve the coke resistance and overall catalytic stability.When the SiO₂/Al₂O₃ ratio of ZSM-5 was 120,the selectivity of light aromatics was increased by 1.5 times and the catalyst lifetime was prolonged by nearly 20 times.（2）The configuration of Zn/ZSM-5 and high Si/Al ratio ZSM-5 could change the proximity betwean two active components,and then affect their synergistic effect.In powder mixing mode,the space beween catalyst particles was at the micron level.Although the light hydrocarbons could rapidly diffuse and transform into aromatics,it also aggravated the backmixing of aromatics onto the surface of high Si/Al ratio ZSM-5 grains,reduceing the catalytic stability.In the two-stage stratified mode,the centimeter-level distance between two catalysts significantly inhibited aromatics backmixing and improved catalytic stability,but the aromatization reaction was inhibited due to the limited diffusion and conversion of light hydrocarbons between ZSM-5 particles.For this reason,the spacing between the two catalysts was controlled within millimetres by alternately packing two catalysts in multiple layers to promote the diffusion of light hydrocarbons and inhibite the backmixing of aromatics.For such specific packing mode,the catalytic life and light aromatic selectivity could be improved by30%and 26%than that of dual-stage packing mode,respectively.（3）It was found that the introduction of a small amount of methanol into the lower Zn/ZSM-5 bed could promote the generation of active hydrocarbon pool species and strengthen the aromatization of light hydrocarbons.The in-situ and model experiments confirmed that C₅cycloalkenes such as methylcyclopentene and dimethylcyclopentene are important intermediate species in the aromatization of light hydrocarbons.The introduction of methanol promoted the expansion of C₅ ring structure into C₆ ring structure,which could further transform into toluene,xylene and other aromatics after multi-step dehydrogenation.However,reducing the introduction position of methanol or increasing the amount of methanol in the aromatization bed would intensify the alkylation of aromatics products,promote the coke accumulation and catalyst deactivation.After incrducing a part of methanol into the middle position of lower Zn/ZSM-5 layer,the two-component catalyst presented the best catalytic performance.Compared with that of conventional dual-stage packing mode,the selectivity of total aromatics in shuch packing mode increased significantly from 24.3%to 40.2%,the xylene selectivity in aromatization increased from 37.1%to 44.0%,the selectivity of toluene decreased from 27.9%to 20.8%,and the catalytic life was extended from 75.0 h to 97.5 h.（4）A series of dual-ZSM-5 composite catalysts with different nanoscale integrated structure were constructed to strengthen the synergistic effect for improving the catalytic stability.The structure of aluminum-rich shell&silicium-rich core or silicium-rich shell&aluminum-rich core could be obtained by adjusting the ratio of Si/Al of ZSM-5 seed and the ratio of Si/Al of silica-alumina gel for the secondary growth during the synthesis of dual-ZSM-5 composite catalysts.In contrast,the latter structure can promote the stepwise aromatization of methanol,and showed a more stable activity with a lifetime of 37 h.The former was further treated by desilication-recrystallization.By introducing Na⁺and finely controling basicity,the hollow ZSM-5 double-shell with cavity between the shells could be obtained.Such structure can strengthen the stepwise reaction and promote the rapid diffusion of aromatics formed on the inner shell,significantly inhibit coke deposition,and the catalytic life was increased to 53h.Moderately increasing the acid density of the external high Si/Al ratio shell can strengthen the coordination between the two shells,increasing the catalyst lifetime to 93 h.When the overgrown ZSM-5 seed with low Si/Al ratio was used for secondary growth in the high Si/Al ratio silica-aluminium sol,the high Si/Al ratio ZSM-5 with stacked structure was formed on its surface,which was more conducive to the stepwise reaction and the diffusion of macromolecular products,improving the catalytic life to 99 h.