Study On ZSM-5 Modified By Zinc For Methanol To Aromatics

Aromatics,especially benzene,toluene,and xylene?BTX?have always been important chemical raw materials.Now more than 85%of aromatics in China are based on petroleum.Methanol to aromatics was proposed in order to enrich the source of aromatics.In this paper,the zinc-modified ZSM-5 catalyst is used in the reaction of methanol to aromatics,and various methods are used to improve the ZSM-5 molecular sieve in order to achieve the desired reaction effect.First,four kinds of Zn/ZSM-5 catalysts were prepared by mechanical physical mixing,impregnation,ion exchange,and in-situ synthesis,and their aromatic selectivity,stability,and regeneration performance in methanol-to-aromatic reaction were compared.It was found that the DS-Zn/ZSM-5 synthesized in situ had the most excellent reaction performance.In the initial reaction,selectivity of DS-Zn/ZSM-5 reached 53.8%and service lifetime reached 150h.Service lifetime of Zn/ZSM-5?Re?reaches 350h,which is more than twice that of fresh catalyst,and aromatic selectivity is also restored to 48%.Next,a series of Zn-ZSM-5 zeolites with different b-axis thicknesses were synthesized by adding ethylenediamine complexed zinc nitrate as a Zn precursor.Followed by hydrothermal synthesis,adding urea and seed crystals with different particle sizes to hydrothermal synthesis.This synthesis method can accurately control the thickness of the catalyst under microscopic conditions with good repeatability.The purpose is to explore the most suitable thickness of catalysts for the reaction of methanol to aromatics.In addition,the complex of ethylenediamine and zinc nitrate can also avoid the formation of Zn?OH?₂ precipitation,which is helpful for the Zn species to enter the pores of the molecular sieve more easily during the crystallization process.100 nm was found to be the best thickness because thinner catalysts suffer from poor stability while thicker ones performed insufficient selectivity.Finally,The Zn/ZSM-5-50 with a thickness of 50 nm was modified through increasing Zn content and coating S-1,respectively,to improve its insufficient aromatization ability.Adding an excessive amount of Zn precursor to the mother liquor will made crystallization for the molecular sieves hindered and the crystallinity will be seriously reduced.The method of using TPAOH solution for secondary crystallization worked out well,indicated by an obvious improvement on selectivity from 35%to 63%.