The Synthesis Of ZSM-5 Catalysts And Reaction Optimization For Rational Conversion Of Methanol To Aromatics

As a conversion process of methanol to aromatic-based hydrocarbons catalyzed by ZSM-5 acidic zeolite,methanol to aromatics(MTA)could promote the downstream conversion of coal based methanol and realize the efficient and clean utilization of coal.MTA reaction involved many complex reactions and many kinds of products could be obtained,resulting in a low aromatic selectivity.Besides,the macromolecules could be trapped in micropores of ZSM-5 due to the strong diffusion limitation,which results in the deep aromatization reaction and the easy formation of coke.Coke could cover the acid sites and even block the micropores of ZSM-5,leading to a rapid catalyst deactivation.Aiming at the aforementioned problems in MTA reaction,this work optimized the acid properties and pore structure of ZSM-5 by regulating the H₂O/Al ratio of hydrothermal growth solution and the hydrothermal reconstruction of silicalite-1,respectively,and investigated the relationship between the catalyst structure and MTA reaction.Besides,based on the methanol to light hydrocarbons and aromatization of light hydrocarbons in MTA reaction,the corresponding catalysts were prepared and coupled to improve the catalytic stability and aromatic selectivity through the regulation and optimization of the reaction packing modes.The two-step conversion reaction mechanism of methanol to aromatics via light hydrocarbons as intermediates were also researched.The main results and conclusions are listed as follow:(1)Nano-ZSM-5 with strong acid sites were successfully synthesized by regulating the concentration of hydrothermal growth solution of pure-silica nucleus.It was found that the formation of nano sized ZSM-5(60?90 nm)could be promoted when the H₂O/Al ratio increased from 200 to 2600.Simultaneously,more Al distributed at the exterior of crystals,which enhanced the acidity accessibility and was beneficial to the catalytic stability.Increasing the H₂O/Al ratio could enhance the acidity of HZSM-5 and the total acidic amount of ZSM-5increased from 0.59 mmol g^-1 to 0.81 mmol g^-1,which promoted the generation of aromatics for MTA reaction.When the reaction condition was 400°C,1.0 MPa and 4.7 h^-1,the catalytic lifetime and aromatic selectivity in liquid hydrocarbons reached 155 h and 66.0%;(2)A series of rich mesoporous nano-ZSM-5(?75 nm)with large external surface area(>186 m² g^-1)were successfully synthesized by regulating the hydrothermal reconstruction degree of nano sized silicalite-1(?60 nm)in TPAOH solution containing NaAlO₂.It was found that,at the early stage of hydrothermal reconstruction,the microporous structure of silicalite-1 have been significantly damaged and a large number of small-sized mesoporous structure(6?9 nm)have been formed.The obtained HZSM-5 possessed a large external surface area,which reached to 287 m² g^-1.With the proceeding of hydrothermal reconstruction,small-sized mesopores coalesced and connected to each other to form large-sized mesopores(15?35 nm)and even hollow structure.During hydrothermal reconstruction process,five-coordinated Al and six-coordinated Al could be converted into four-coordinated framework Al,and the total acid amount reached0.59 mmol g^-1.Although,the acidic amount increased gradually,the rich mesopores of nano-ZSM-5 facilitated the diffusion of reaction moleculars.Thus,the catalytic lifetime increased to 164 h and the aromatic selectivity in liquid hydrocarbons increased to 74.5%.The regenerated catalysts possessed significantly decreased acidity but retained rich mesopores and large external surface area.The regenerated catalyst exhibited a 2.5-fold longer lifetime as compared to the fresh catalyst;(3)Physically mixing the high Si/Al ratio HZSM-5 alkenes catalyst and the low Si/Al ratio Zn/ZSM-5 aromatization catalyst,methanol could be firstly converted into light hydrocarbons over alkene catalyst,and then to aromatics on neighboring aromatization catalyst efficiently.Aromatic selectivity was increased to 39.9%and much higher than 30.3%of packing aromatization catalyst solely.However,the aromatics generated from aromatization catalyst accelerated the deactivation of neighboring alkene catalyst,which was not beneficial to the catalytic stability.When the mixing ratio of alkene catalyst and aromatization catalyst was 1.6:0.4,catalytic lifetime was only 39 h.Increasing the Si/Al ratio of aromatization catalyst from 30 to 50,it reduced the generation of aromatics and the catalytic lifetime of mixing catalysts was increased from 39 h to 56 h.In addition,decreasing the mesh of two catalysts from 60?100 to 20?40 could reduce the contact opportunity of aromatics on the active sites of alkene catalyst,which increased the catalytic lifetime to 112 h.Decreasing the proportion of aromatization catalyst also could reduce the generation of aromatics,which improved the catalytic lifetime.When the mixing ratio of alkene catalyst and aromatization catalyst was 1.8:0.2,the catalytic lifetime was 219 h and much higher than 22 h of packing aromatization catalyst solely;(4)Packing HZSM-5 alkene catalysts above Zn/ZSM-5 aromatization catalysts could obtain a higher catalytic stability as compared with the mixing mode of two catalysts,which was attributed to the decreased negative effect of aromatics from aromatization catalyst on alkene catalyst.When alkene catalyst was packed above aromatization catalyst with equal quantity,the catalytic lifetime could reach 205 h and was much higher than 36 h of physical mixing mode of two catalysts.Light alkenes generated from upstream alkene catalyst could promote“alkene-based cycle”in the downstream catalyst and suppress the generation of heavy aromatics,which increased the BTX selectivity from 15.5%of packing aromatization catalyst solely to 19.1%.Decreasing the packing ratio of above two catalysts could optimize the coupling efficiency of dual cycle reactions via increasing the corresponding catalytic capacity of alkene catalyst and aromatization catalyst.When the weight ratio of two catalysts was decreased from1:1 to 0.25:1.75,aromatic selectivity and BTX selectivity was increased from26.7%and 19.1%to 48.2%and 34.7%,respectively.