Construction Of Coated ZSM-5 And Its Reaction Performance Of Methanol To Aromatics

The demand for aromatics,especially benzene,toluene and xylene,as an important organic chemicals,has increased dramatically.The methanol-to-aromatics reaction technology can realize the synthesis from methanol to aromatics products,which can not only reduce the dependence on the petroleum route,but also promote the efficient and clean conversion of coal,and has broad development prospects.Although some progress has been made in methanol-to-aromatics technology,the reaction process is complex,and it is impossible to achieve complete directional control of the reaction process on a single catalyst,and the aromatics selectivity is low.By mixing two catalysts with low Si/Al ratio ZSM-5 and high Si/Al ratio ZSM-5,through regulation and optimization,the coupling of two-step reaction processes of methanol to olefins and olefin aromatization can be realized,and the aromatics selectivity can be improved.Based on this,this work fully considered the advantages of the stepwise conversion of methanol from low-carbon hydrocarbons to aromatic hydrocarbons,and constructed a coated catalyst integrated with high silicon-aluminum ratio ZSM-5 and low silicon-aluminum ratio ZSM-5.Combined with its morphology,changes in acid properties and product distribution of catalytic methanol to aromatics reaction,the conversion mechanism of methanol on the coated catalyst was explored.The main conclusions are as follows:（1）Using the secondary hydrothermal method,the nano-ZSM-5 with low silicon-aluminum ratio was placed in the growth liquid of different silicon-aluminum ratio for coating growth.It was found that the parent powder was partially dissolved during the growth process and surrounded on its surface.The raw material liquid first forms small crystal grains,and the coating of the parent powder is completed by gradually accumulation.The particle size distribution of the samples in the feed solution with low Si O₂/Al₂O₃ ratio is not uniform,but with the increase of Si O₂/Al₂O₃,the coating tends to be uniform and the surface presents rough protrusions structure.The secondary growth significantly changed the surface acid properties and aromatization properties.The total acid content in the feed solution with a higher Si/Al ratio（220）could be increased to 194.9μmol g^-1,which was significantly higher than the 169.7μmol g^-1 of the original powder.It is worth noting that the proportion of strong acid on the catalyst surface also increased significantly from 37%of the original powder to 53%.Further combined with the performance evaluation,it was found that the change of acidity not only promoted the methanol aromatization conversion and improved the aromatics selectivity,but also strengthened the dealkylation process of aromatics and improved the BTX selectivity,so that the aromatics selectivity and BTX selectivity were respectively increased by 16.1%and8.2%increased to 23.8%and 13.5%.（2）The xerogel steam-assisted solid-phase conversion method is used to grind the xerogel with low silicon-aluminum ratio ZSM-5 and high silicon-aluminum ratio ZSM-5,and then steam-assisted crystallization to obtain a coated catalyst.Under the condition of low coating amount,the silica-alumina species in the gel can be successfully coated on the surface of the original powder,and the sample particles grow up.In the catalytic methanol conversion process,under the catalysis of high silicon-aluminum ratio ZSM-5,low silicon-aluminum ratio ZSM-5and high silicon-aluminum ratio ZSM-5 in turn,the process of methanol step-by-step conversion to prepare aromatics is promoted.When the silicon coating ratio is 1:0.25 and 1:0.5,the silicon-alumina species in the gel can be successfully coated on the surface of the original powder,and the sample particles grow up.The aromatic selectivity increased from 12.9%of the parent powder to 14.5%and 13.9%,respectively.When the silicon cover ratio is increased to 1:1 and 1:2,some Si-Al species could grow on the surface with low Si-Al specific ratio,and there were small particles formed by independent nucleation of Si-Al species.On the ZSM-5catalyst with aluminum distribution gradient,the step-by-step conversion of methanol from low-carbon hydrocarbons to aromatics can be realized,and the total aromatics selectivity reaches 15.2%.However,the initially formed aromatics are further alkylated under the catalysis of small particles formed by independent nucleation,resulting in an increase in the proportion of heavy aromatics in the aromatics.