Study On Chemically Modified ZSM-5 For Methanol To Aromatics

Aromatics?benzene,toluene,xylene,BTX?are important raw materials for organic chemical industry.Among them,PX is an important raw material for the industrial production of dimethyl terephthalate,and it is an essential intermediate in petrochemical production.At present,the production of commercial PX is mainly based on petrochemical industry,such as the production of p-Xylene by toluene alkylation,toluene disproportionation and the separation of xylene isomers.In order to meet the increasing demand of aromatics,it has become the main research direction of aromatics production technology to develop new processes and technologies for aromatics production from non petroleum routes,increase the proportion of p-xylene in aromatics and reduce the energy consumption for production.However,considering the resource structure of"lack of oil,less gas and rich coal"and the excess methanol production capacity and output,the use of cheap methanol to produce aromatics has been widely concerned and studied by researchers.In this paper,Zn,Si,P,Mg and other elements were introduced into ZSM-5 zeolite by equal volume impregnation,and the effect of these elements on the reaction performance of methanol to aromatics was studied in a fixed bed reactor.The catalysts were characterized by SEM,TEM,XRD,BET and NH₃-TPD.It is found that the introduction of Zn can effectively improve the reaction activity of HZSM-5,reduce the strong acid strength and quantity of molecular sieve,form the active center of Zn OH⁺dehydrogenation,and improve the aromatics selectivity of MTA reaction products.When the loading amount of Zn is 3%,the aromatics selectivity reaches 58%;the deposition of Si O₂can overlap the acid sites on the surface of molecular sieve,reduce the strong acid quantity on the surface of molecular sieve,and inhibit the formation of aromatics The side reactions such as isomerization and alkylation of p-xylene can improve the selectivity of p-xylene in the product.With the increase of Si O₂ deposition,the selectivity of p-xylene gradually increases.When the deposition reaches about 6%,it reaches the extreme value.The introduction of P element can reduce the volume of inner surface and micropore of molecular sieve,increase the curvature of pore,and increase the expansion of m-xylene and o-xylene in the pore With the increase of P₂O₅ loading,the selectivity of p-xylene is also increasing;Mg O loading can further reduce the specific surface area and pore volume of molecular sieve,neutralize the acid center of pore and surface,and further improve the selectivity of p-xylene.The ZSM-5 zeolite modified by Zn,Si,P and Mg was used to study the reaction conditions of methanol to aromatics.The results show that the selectivity of p-xylene in xylene is 79.3%,aromatics is 30.3%,and C₂-C₄ is 54.2%when the reaction conditions are atmospheric pressure,reaction temperature is 425?C,methanol mass space velocity is 1 h^-1,nitrogen flow rate is 15 ml/min.At the same time,the selectivity of aromatics in the product can be significantly improved by coupling the aromatization of olefins with methanol or the further aromatization of low-carbon olefins with high yield in the product.The effects of temperature and carrier gas flow rate on aromatization of olefins were studied.The results showed that the aromatics selectivity increased with the increase of temperature,but decreased with the increase of carrier gas flow rate,but the selectivity of p-xylene increased to some extent.