Study On The Control Acidic Properties Of ZSM-5 Catalyst For The Improved Step Conversion Of Methanol To Aromatics

The synthesis of aromatics from coal via methanol was an important way to realize efficient and clean utilization of coal resources.However,the reaction process of methanol to aromatics catalyzed by ZSM-5 zeolite was complex and had a series of side reactions,which led to low aromatics selectivity.Meanwhile,the diffusion of coke precursors were restricted by micropore and proned to form coke via polymerization reaction,leading to catalyst deactivation.Aiming at the problems of low aromatics selectivity and poor catalytic stability in methanol to aromatics,the methanol to aromatics could be divided into two processes,methanol to light hydrocarbons and light hydrocarbons to aromatics,which could significantly improve the catalytic performance.The reaction process and catalytic technology of methanol to light hydrocarbons were mature,but the aromatization of light hydrocarbons process still need to be studied.Considering that the acid density and type played an important role of ZSM-5 in the catalytic aromatization of methanol,in this paper,the acidic properties of ZSM-5 were changed by adjusting the Si O₂/Al₂O₃ ratios and introducing the Zn species to clarify the internal influence of ZSM-5 on the aromatization of light hydrocarbons.Based on the internal factors affecting the stability of one-step and two-step methanol conversion,four ZSM-5 catalysts with reduced acid density were loaded in a fixed bed from top to bottom.The mechanism of methanol conversion with high-performance over step-by-step catalysts was studied by combining with the staged loading experiments.The main results and conclusion acquired were as follows:（1）Adjusting the acid density by changing the Si O₂/Al₂O₃ ratio of the aromatization micro-ZSM-5 zeolite can significantly affect the catalytic performance of methanol aromatization via light hydrocarbons.When the Si O₂/Al₂O₃ratio increased from 36 to 70,the acid density decreased from 1.23 mmol g^-1 to 0.67 mmol g^-1 and the two-step reaction lifetime increased from 46.5 h to 190.8 h.When the Si O₂/Al₂O₃ratio was 60,the aromatics selectivity reached to 33.7%,which was also significantly higher than 27.9%of 36.In contrast,the catalytic stability and product selectivity of micro-ZSM-5 catalyzed by methanol one-step conversion to aromatics were not significantly affected of acid density.（2）The ratio of Br（?）nsted acid sites to Lewis acid sites was adjusted by introducing the Zn species to further improve the aromatics selectivity of the two-step reaction.When the mass fraction of Zn was 0.5%,the ratio of Br（?）nsted acid sites to Lewis acid sites was reduced from19.0 to 1.7 and the enhanced dehydroaromatization promoted the aromatics selectivity from27.5%to 31.4%.It was found that the acid density of parent zeolite should be fully considered when adding Zn species and low Zn content should be added to ZSM-5 with low acid density,which could better promote the catalytic synergy of Br（?）nsted acid sites and Lewis acid sites and enhance the aromatics selectivity（3）In the direct aromatization of methanol,the early conversion of methanol in the upper catalyst inhibited the over-alkylation of aromatic components in the lower reaction,which maked the coke content and coke formation rate of the lower catalyst was 62%and 60%of that in the upper catalyst respectively.By loading the aromatization catalyst on the top of the alkene catalyst,it was found that the macromolecular hydrocarbon components produced by the aromatization reaction would damage the lower catalyst lifetime.If the aromatization functional zone moved down in the catalytic bed,the entire catalytic stability would be improved.（4）Nano ZSM-5（Z480/Z240/Z120/ZnZ60）with gradually increasing acid density was sequentially filled in the fixed-bed catalyst bed from top to bottom,which promoted the step-by-step aromatization of methanol.As the number of packing stages increasing,the degree of aromatization gradually increased and the aromatics selectivity increased from 15.6%for Z480to 32.6%for the four-stage catalyst.When methanol passed through ZSM-5 with increasing acid density,it could be converted more effectively in advance,which suppressesed the rapid coke deposition in the lower aromatization zone due to excessive alkylation.The aromatization zone of ZnZ60 was located at the bottom of the bed,which avoided the damage of macromolecular hydrocarbon components generated by aromatization to other catalysts.The catalytic lifetime was 238 h,which was much higher than the 81 h in the two-step methanol conversion process.