Study On Synthesis Of Nano-sized ZSM-5 And Catalytic Performance In The Methanol-to-aromatics Reaction

Aromatics are very important raw materials for the chemical industry,which are mainly produced by petro-chemical processes such as the cracking and reforming of petroleum.The development of renewable sources to generate aromatics,especially benzene,toluene,and xylene（BTX）is gaining importance due to the limited availability of fossil fuels.In recent years,aromatization of methanol has received intensive attention since methanol is currently produced from a wide variety of resources such as fossil-fuel-based syn-gas,natural gas,and biomasses.The ZSM-5 zeolite is often used as catalysts for the aromatization of methanol.And its crystal size plays an important role in catalytic performance and product selectivity.Compared with the micro-sized ZSM-5 zeolite,the nano-sized ZSM-5 zeolite has a larger specific surface area,a larger number of out surface active center and a shorter pore channel,which are beneficial to improve the diffusion and reactivity of the reactants and products in pores,showing better catalytic stability and resistance to coke deposition.In this paper,the synthesis of nano-sized ZSM-5 zeolite was studied and the effect of ZSM-5 zeolite crystal size,silica-alumina ratio and concentrations of NaOH modified the zeolite on the aromatization of methanol was investigated.When the ZSM-5 zeolite is synthesized with tetraethylorthosilicate as the silicon source,aluminum nitrate nonahydrate as the aluminum source,the ratio of tetramethylammonium hydroxide in the synthesis system has a great influence on crystal size of the ZSM-5 zeolite.In a certain range,the more the templating agent is added,the smaller crystal size of the ZSM-5zeolite,but when the crystal size is reduced to 300⁴00 nm,the effect is no longer obvious.When adding high ratio of tetramethylammonium hydroxide,the low temperature aging stage can effectively promote to synthesize the regular and uniformly distributed nano-szied ZSM-5zeolites.The results of methanol aromatization reaction of ZSM-5 zeolites with different crystal size show that the average conversion of methanol,aromatics selectivity and BTX selectivity are improved with the decrease of crystal size.The selectivity of BTX was 37%for the 99.1±7.0 nm nano-sized zeolites with the highest aromatic selectivity（above 42%）.The coke content of the deactivated catalyst decreases with the decrease of the crystal size of the catalyst.The nano-sized ZSM-5 with different SiO₂/Al₂O₃ molar ratio are successfully synthesized by adjusting the ratio of silicon source and aluminum source in the synthesis system.The results of methanol aromatization reaction of nano-sized ZSM-5 with different SiO₂/Al₂O₃ molar ratio show that the nano-sized ZSM-5 with low SiO₂/Al₂O₃ molar ratio has higher aromatics selectivity and the nano-sized ZSM-5 with high SiO₂/Al₂O₃ molar ratio has higher light olefin selectivity.The most suitable catalyst for methanol aromatization is the nano-sized HZSM-5catalyst with low SiO₂/Al₂O₃ molar ratio.The studies of scale-up test of nano-sized ZSM-5 zeolite with low SiO₂/Al₂O₃ molar ratio are carried out.The nano-sized ZSM-5 zeolite is synthesized with tetraethylorthosilicate（industrial grade）as a silicon source,aluminum hydroxide（industrial grade）as aluminum source in 600 L crystallization reactor to effectively reduce the cost.The characterization of nano-sized ZSM-5 modified by different concentrations of NaOH solution and the results of aromatization of methanol show that NaOH solution with high concentration destroys the skeleton of ZSM-5 seriously and its catalytic performance is poor.NaOH solution with low concentration destroys the skeleton of ZSM-5 slightly and makes a large number of secondary mesoporous structures in zeolite,promoting the contact and diffusion of the reactants significantly improving the catalytic activity of the catalysts.