The Key Structure-activity Relationship And Reaction Path Control Of The ZSM-5 Catalyst For Methanol To Aromatics

The effects of zinc chemical state distribution on Zn/HZSM-5 surface,microporous dispersion and acidity on the key reaction steps in the catalytic system of methanol to aromatics were revealed.Key structure-activity relationship of cognition,and based on the above load by ion exchange method physicochemical properties appropriate nano flake zinc species modified HZSM-5 as catalyst,the catalytic efficiency of aromatization and product aromatic molecules and methanol secondary alkylation two parallel reactions,to a great extent,realizes the effective control of the catalytic system path.The selectivity and yield of BTX aromatics of the target product were improved,and the life of the catalyst was greatly prolonged.The specific content is as follows:（1）Both the zinc species loading method and the zinc loading amount significantly affect the chemical state distribution of zinc on the surface of HZSM-5,and affect the reaction path of methanol to aromatics catalytic system in different degrees.The zinc species on the surface mainly exist in two forms:Zn OH⁺species and Zn O nanoclusters.The former catalyzes the dehydrogenation step in the aromatization reaction and promotes the generation of aromatic hydrocarbons together with protic acid,which is beneficial to the improvement of the selectivity of aromatic hydrocarbons.The latter catalyzed the methanol cracking side reaction to generate CO,CO₂,etc.,which was unfavorable to the generation of aromatic hydrocarbons.The Zn O species on the surface will be reduced to zinc metal during the stable phase,which will lead to more severe side reaction of cracking.Compared with the impregnation method,the ion exchange method is more favorable to the existence of zinc species in the form of Zn OH⁺.Excessive loading would lead to an increase in the proportion of Zn O nanoclusters in the surface zinc species.（2）The effect of ZSM-5 microporous diffusion on the secondary alkylation of methanol and benzene/toluene in the methanol aromatization system was revealed by using conventional HZSM-5 and nano-sheet HZSM-5 with similar acidity distribution,SiO₂-modified conventional HZSM-5 as catalyst,and methanol and toluene as the co-feed as probe catalyst.Compared with the other samples with longer micropore diffusion path or smaller micropore pore size,the nano-sheet short-porous HZSM-5micropore molecule has higher diffusion efficiency,and the concentration imbalance caused by the difference of methanol and aromatic molecules diffusion rate in micropore is effectively weakened,which can effectively ensure the efficient alkylation of methanol and toluene molecules.Therefore,the use of nano-sheet HZSM-5 as the main material of the methanol-to-aromatics catalyst can not only catalyze the aromatization reaction,but also efficiently catalyze the secondary alkylation of aromatics products and methanol,which helps to improve the selectivity of BTX aromatics.（3）A number of HZSM-5 catalysts with similar crystal size and morphology but different acid distribution were used to catalyze the reaction with methanol and toluene as the probe.It was found that reasonable acidity distribution on the surface of the catalyst was a necessary condition for the efficient secondary alkylation of products aromatic molecules with methanol in the catalytic reaction system of methanol to aromatic hydrocarbons.The higher B/L acid ratio on the surface of the catalyst is beneficial to the alkylation of methanol and aromatic hydrocarbons.Too high or too low acid density or acid strength is not conducive to efficient alkylation.The outer surface of HZSM-5 is also an important catalytic site for alkylation reaction.Because the outer surface space limitation effect is obviously weaker,it is more conducive to catalyze the alkylation of methanol and aromatic molecules.（4）Based on the understanding of the above key structure-activity relationships,the methanol-to-aromatics catalyst was prepared by ion exchange method supported with zinc modified SH-HZSM-5 with a crystal thickness of 5-10 nm.First of all,the zinc species on the surface of nano-sheet molecular sievers are mainly Zn OH⁺species,and they have a high enough medium intensity acid potential density.Therefore,Zn-L acid promotes the dehydrogenation step and co-catalyzes with protic acid to significantly improve the efficiency of methanol aromatization reaction.Secondly,Zn/SH-HZSM-5 has high ratio of weak+medium strong acid,high ratio of B/L acid and good microporous diffusion property,which effectively guarantees the efficient secondary alkylation reaction between methanol and aromatic products benzene/toluene to generate BTX aromatic hydrocarbon.In addition,the short intracrystalline micropore diffusion path of nano-sheet catalyst significantly reduces the pore diffusion retention time of light aromatic products generated by parallel reactions of aromatization and alkylation,which effectively avoids the deep alkylation conversion into C₉₊heavy aromatic byproducts or even coking species.At the same time,the high surface area and mesopore volume of molecular sieves also make it has high carbon capacity.The prepared 0.804 wt%Zn/SH-HZSM-5 zeolite catalyzes efficient aromatization and the secondary alkylation of aromatics products with methanol,and effectively controls the reaction path in the methanol to aromatics catalytic system to a large extent.The methanol conversion rate was higher than99.90%,the BTX yield was higher than 54.56%,and the C₉₊aromatic yield was lower than 5.12%after continuous one-way operation for 200 h.