Study On Performance Of Tandem Catalyst Based On HZSM-5 Molecular Sieve For Methanol To Aromatics

Aromatics are important basic raw materials in the field of organic chemicals,and they are widely used in the production of rubber,resin,ink,etc.At present,most aromatics on the market come from petroleum industry,but my country’s oil resources are scarce,and the demand for aromatics is increasing year after year.And the contradiction between the two has become increasingly prominent.Therefore,it is urgent to find a new production method to increase the production of aromatics.Based on the basic national conditions of“rich coal,lean oil,and less gas”,the methanol-to-aromatics（MTA）process based on coal-to-methanol technology has attracted wide attention from researchers once it was launched.One of the keys to this technology is to develop a catalyst with high selectivity and high stability.In response to the above problems,this paper adopts the idea of coupling reaction to construct a tandem catalyst in order to improve the stability of the reaction and the selectivity of BTX in one step,and preliminary explore the mechanism of its catalytic MTA reaction.The research content and results of this article are as follows:1.The construction of tandem catalysts and the study of their catalytic performance in MTA reaction.The nano-mesoporous high-silica ZSM-5 zeolite（SZ-170）and the Zn-modified low-silica ZSM-5 zeolite（Zn/NZ）were prepared by the hydrothermal synthesis method and the equal volume ultrasonic impregnation method,respectively,and the acidity,crystal form,pore structure and other properties of the catalysts were characterized by XRD,N₂ adsorption-desorption,SEM,TEM,NH₃-TPD,Py-IR and other characterization methods.The HZSM-5（Si/Al>100）--Zn/HZSM-5（Si/Al<25）tandem catalyst built in a reactor effectively coupled the methanol to olefin and olefin aromatization reactions at 0.1 MPa and 450°C.The experimental results show that the tandem catalyst can efficiently convert methanol to light aromatics BTX.The catalyst has a longest active life of 260 h,and the highest selectivity of BTX in the product is 54.32%.By comparing the product distribution of the MTA reaction catalyzed by different catalysts,it is found that a large amount of low-carbon olefin intermediates are indeed generated on the upper bed catalyst in tandem catalysts,and these low-carbon olefins are converted into a large amount of light aromatics BTX after passing through the lower bed catalyst.At the same time,it is also found that the physical and chemical properties of the upper bed zeolite have an important influence on the product distribution of the tandem catalyst.For example,when the silicon-aluminum ratio is high,it is very beneficial to improve the BTX selectivity of the tandem catalyst.Based on the above research,the activity life of different catalysts in catalyzing MTA reaction was further investigated,and the results showed that the tandem catalyst significantly have higher activity life than traditional catalysts.By comparing and analyzing the experimental results,it can be found that the upper bed catalyst in tandem catalysis has a protective effect on the catalytic activity of the lower bed catalyst,and it is the main factor affecting the stability of the tandem catalyst.Combining the respective product distributions,the author believes that oxygenic carbon is the main reason for the rapid deactivation of the catalyst in MTA reaction,and the presence of high silica zeolite in tandem catalyst can inhibit the formation of this carbon deposition to a certain extent.This is also the key to the high stability of the tandem catalysts to convert methanol.2.The effect of Zn in tandem catalysts on the performance of HZSM-5 molecular sieve for MTA reaction.A series of Zn/HZSM-5（Si/Al<25）catalysts with different mass fractions were prepared by the isometric ultrasonic impregnation method,and their acidity,crystal form and other properties were characterized by XRD,FT-IR,NH₃-TPD,ICP and other characterization methods.Based on the form of the HZSM-5（Si/Al>100）--Zn/HZSM-5（Si/Al<25）tandem catalyst,the role of Zn in MTA reaction,especially the aromatization process of olefins,was specifically investigated.The experimental results show that the introduction of Zn does increase the selectivity of BTX in the product,which increases the selectivity of BTX from39.64%to 50.66%,but high content is not better.The presence of Zn species not only improves the dehydrogenation aromatization activity in the reaction,but also inhibits the hydrogen transfer reaction,thereby increasing the selectivity of BTX in the product.However,the introduction of zinc species will reduce the amount of strong acid in the zeolite,and when the zeolite strong acid content is very small,it is adverse to the reaction,and ultimately affect the overall catalytic performance of the reaction.