Study On Hydrothermal Stability Of ZSM-5 Zeolite For Aromatization

ZSM-5 as an important zeolite plays a great significant role to increase the economic benefits of petrochemical industry for the production of aromatic hydrocarbons in FCC gasoline.Due to the process of regeneration need high temperature hydrothermal environment,dealumination of the zeolite is favored,resulting in a loss of Bronsted acid and the decrease of the aromatization performance of the catalyst.Therefore,in order to understand the active center of the aromatization reaction and the effect of hydrothermal aging on aromatization of ZSM-5 zeolite.In this paper,seven sets of zeolites were designed.The effects of hydrothermal treatment on zeolite structure,active center and aromatization were investigated by various characterization methods and reaction evaluation of 1-hexene,methylcyclohexane.We improved the hydrothermal stability of aromatization of ZSM-5 zeolite by zinc and phosphorus modification.The aromatization properties of ZSM-5,ZSM-11 zeolites with Si-Al ratio of 60?SiO₂/Al₂O₃,molar ratio?and ZSM-5,Beta zeolites with Si-Al ratio of 60 were investigated with 1-hexene in a fixed bed microreactor.The yields of aromatic hydrocarbons were 34.6 wt%,33.7 wt%,30.2 wt%,10.3 wt%,and the hydrogen transfer indices were 7.09,4.07,6.15 and 9.39.The results show that the three-dimensional pore structure and the larger pore size of the zeolite are beneficial to the hydrogen transfer reaction,and the aromatization performance of ZSM-5 zeolite was the best.With the ion exchange method and impregnation method to modified the same amount of zinc,the results show that the introduction of zinc can improve the aromatization performance of the zeolite,and the aromatic hydrocarbon yield of Zn?ex?-ZSM-5 catalyst obtained by ion exchange is the highest,reaching 37.1 wt%.The aromatics of H-ZSM-5,ZnNa-ZSM-5 and ZnH-ZSM-5 were 24.7 wt%,20.4 wt%and35.6 wt%,the hydrogen transfer indices were 6.62,0.55 and 5.59,and the amount of hydrogen produced was 0.12 wt%,0.69 wt%,1.09 wt%.The results show that the hydrogen transfer reaction plays an important role in the formation of aromatic hydrocarbons,the Zn-L acid center plays an important role in the dehydrogenation reaction,the metal and acid bifunctional catalysts can maintain high hydrogen transfer ability and promote the dehydrogenation reaction,thereby promoting the aromatization reaction.The catalyst of ZSM-5,Zn-?ex?-ZSM-5,Zn-?im?-ZSM-5?prepared by impregnation method?was subjected to hydrothermal treatment at 650?for 4 h.Py-IR characterization revealed that hydrothermal treatment reduced the B acid content of the sample.We evaluated the aromatization reaction with 1-hexene,the results showed that the yield of aromatic compounds decreased from 24.7 wt%,37.1 wt%,30.2 wt%to 6.1wt%,9.8 wt%and 8.9 wt%,and the hydrogen transfer index decreased from 6.62,6.12,6.15 to 0.49,0.45,0.44.The above results demonstrate that an important reason for the decrease in aromatization performance is that hydrothermal treatment reduces the hydrogen transfer reaction.Evaluation of the aromatization of methylcyclohexane for ZSM-5,Zn?ex?-ZSM-5,ZSM-5-st,Zn?ex?-ZSM-5-st,the results showed that the aromatics selectivity of Zn?ex?-ZSM-5 was almost constant before and after hydrothermal aging,and the toluene content in aromatics increased obviously of the hydrothermal catalyst,and the formation of toluene is mainly related to the Zn-L acid center,so it indicated that hydrothermal treatment has little effect on the dehydrogenation performance of Zn-L acid.The results of ¹H and ²⁷Al NMR characterization show that the high temperature hydrothermal process mainly destroys the bridged hydroxyl groups of the zeolites,resulting in a decrease of B acid.The presence of zinc species can stabilize the skeletal aluminum structure on the zeolites.The XPS characterization revealed that hydrothermal had little effect on the chemical environment of zinc species.Zn?ex?-ZSM-5,Zn?im?-ZSM-5 zeolites was subjected to phosphorus-assisted modification,and the hydrothermal zeolites were evaluated by 1-hexene aromatization.It was found that the yield of aromatics increased significantly from 9.8 wt%,8.9 wt%to 13.6 wt%and 13.1 wt%,and the hydrogen transfer index increased from 0.45 to 0.44to 0.83 and 0.80.It indicated that the introduction of phosphorus is more to improve the B acid center of the hydrothermal stability,and reduce the hydrothermal process on the hydrogen transfer capacity of zeolites.