Study Of Metal-Modified ZSM-5 Molecular Sieve For The Preparation Of Light Aromatics By Coupling Methanol With N-Hexane

Aromatics are basic products and basic raw materials of petrochemical industry,among which light aromatics such as benzene,toluene and xylene have important uses in synthetic resins,dyestuffs,synthetic fibers,medicines,insecticides and other industries.For the past few years,the demand for aromatics in China has been increasing,and the aromatics produced by traditional petroleum routes cannot meet the production demand,and the supply and demand are tense.At present,the technology of using coal to make methanol in China is very mature,resulting in overcapacity of methanol.The development of high value-added methanol downstream products is an effective way to solve this problem.At the same time,there are more light hydrocarbon resources in China’s petrochemical plants,such as hydrocoker naphtha,reforming plucked oil and aromatics extraction residual oil,etc.These low carbon hydrocarbon by-products have light components,low octane value and high saturated vapor pressure,and are generally used as cheap fuels,which have not been reasonably utilized.The use of inexpensive methanol and low value-added light hydrocarbons for co-aromatization to prepare light aromatics not only solves the problem that light hydrocarbon resources are difficult to use efficiently,but also solves the current situation of low economic efficiency of methanol,and realizes the rational use of resources,and provides a feasible way for the production of high value-added chemical products.In this study,a series of hollow ZSM-5 molecular sieves were synthesized by treating conventional ZSM-5 molecular sieves with a mixture of inorganic base（NaOH）and organic base(C₈H₁₈NBr)solution,and the effect of different molar ratio mixtures of inorganic and organic bases to modify ZSM-5 molecular sieves was investigated on the physicochemical properties of the catalysts.The effects of mixed solutions of NaOH and different types of organic bases(C₈H₁₈NBr,C₄H₁₁N)on the acidity and pore weave properties of the catalyst were investigated.Then,ZSM-5catalysts with different pore structures were synthesized by different alkali treatments to investigate the effect of pore structure on the yield of BTX.Finally,lamellar ZSM-5 molecular sieves and lamellar mesoporous molecular sieves were synthesized,and the weaving properties of the catalysts were adjusted by metal modification,and they were used for the aromatization reaction of methanol and n-hexane co-monomerization,and the reaction performance of the catalysts was evaluated.The main results of the study are as follows:1.Hollow ZSM-5 molecular sieves were prepared by treating conventional ZSM-5 molecular sieves with a mixture of inorganic（NaOH）and organic(C₈H₁₈NBr)bases of different molar ratios.All hollow ZSM-5 catalysts showed a significant increase in the number of strong and weak acids,with the NaOH-treated catalysts showing the largest increase in the total number of acids but a lower catalyst lifetime.The ZSM-5 catalysts prepared by treatment with a 3:1 molar ratio of NaOH and C₈H₁₈NBr significantly increased the BTX selectivity from 37.3%to 48.5%.2.By examining the treatment of ZSM-5 molecular sieves with different NaOH and different structural organic bases(C₈H₁₈NBr,C₄H₁₁N)mixes.It was found that the weaving parameters of the catalysts treated with NaOH mixed with different organic bases changed significantly.the catalyst BTX selectivity of the hollow ZSM-5 prepared with NaOH and C₄H₁₁N base solution was lower（44.8%）,the conversion of methanol was lower than 85%when the reaction was carried out for 24 h and the lifetime was shorter,which was related to the chain structure of the organic bases and the resulting spatial site resistance.3.Catalysts with different pore structures were prepared by different treatments using a mixed base solution at a molar ratio of 3:1 between NaOH and the organic base(C₈H₁₈NBr).The effects of microporous,hollow and mesoporous structures of ZSM-5 catalysts on the performance of methanol and n-hexane co-aromatization reactions were investigated.It was shown that the BTX yields of microporous and mesoporous structures of ZSM-5 were lower,with BTX yields of 37.3%and 42.2%,respectively,while the BTX yields of hollow structure ZSM-5 were as high as 48.5%,which was attributed to the number of mesopores and the cavity structure.4.The lamellar ZSM-5 molecular sieves and lamellar mesoporous ZSM-5 molecular sieves were synthesized,and then the Zn/ZSM-5 catalysts were prepared by impregnation method and compared with the catalytic performance of Zn-loaded hollow molecular sieves or ordinary molecular sieves.The results showed that the BTX yield of the lamellar ZSM-molecular sieve was39.4%,which was higher than that of the conventional ZSM-5（37.3%）,which was related to the shorter diffusion path and higher acid amount of the lamellar ZSM-5 catalyst.In addition,the Zn-loaded catalysts significantly improved the BTX selectivity,which was attributed to the addition of Zn changing the pore structure and acidity of the catalyst.